Is Lower AR really better in the junk?

I have been hearing for some time now the Lower AR wings are better able to handle junky air conditions. I know it is true in real aerodynamics numbers for sure but at our level of changes it is such a small amount comapritively, from desing to design, is it really the higher AR that is giving up in the junk or the motor. ?

I just finished up Werwage's latest article in MA and he brings up a good point. When we get a strong motor we build a bigger plane. Then we go through the motor thing again. An the cycle continues. By bigger it could be span or thickness or everything on the plane but bigger will cause the motor to work harder.

I am beginning to think it is more the power to wing ratio that handles the junk better than that of the AR.

Look at the Berringer plane. People ohh and ahh over those things all the time and the AR on those is pretty high for stunt. If the power is there and the loading is low it shrugs off the winds easily.

I think the maneuver performance of the higher AR wings is so much more that it isnt really even a consideration. The lift produced lets you really settle sown your engine run. My opinion there.

Anyway just wondering what your ideas on it were?

What will happen when the 80 and up comes along????

Oh I cant wait!!

Better start saving now..

When the .80 comes, I will waiting for Al Rabe to do a really good P.47 bubble top!! No one can do the warbirds like him!!

>I have been hearing for some time now the Lower AR wings are
>better able to handle junky air conditions. I know it is
>true in real aerodynamics numbers for sure but at our level
>of changes it is such a small amount comapritively, from
>desing to design, is it really the higher AR that is giving
>up in the junk or the motor. ?

I think the high AR has less drag, so it can do more with less engine. The lower AR "appears" to handle bumps better, and tames burst of excess speed with drag.

Put a huge motor in a little plane and it will rip through any condition. That would be a positive when the chips are down. Will it perform the best? Maybe not... I do not know. Give me a couple more years.

If I can can submit simple anecdotal evidence, and also with a humble mass produced sport profile, I do notice that my Scientific Giant Stuntmaster (Fox), which has an AR of only about 3.8:1 or so, handles windy conditions better than other comparable models. It also has a moderately blunt LE, and a not so thick foil. Currell

Gee Brad,

Are you advocating a .65 in a Satona??

Just kidding, but think about it anyway!

>Gee Brad,
>
>Are you advocating a .65 in a Satona??
>
>Just kidding, but think about it anyway!
>
>Jim Pollock

No, but I might advocate a 77. Actually, Matt very seriously considered going to about 630 square inches on his next plane with a 65. Isn't that about what the Satona is? He was flying the 53 this fall on a 600 square inch plane and was liking it.

But, back to the subject matter, I am not so sure that directly into or out of the wind the aspect ratio would make so much difference, (drag would) but in turbulence, the longer the wing span, the more chance of one wing going up while the other goes down. Here, at times--in turbulent conditions--the smaller plane or lower aspect wing might be better (while keeping the same power.)

AR.....Brain is on safety mode. What is AR?....Al Rabe...lol

Scott(got alot od things on my mind)Riese

Gee Scott, maybe it meant Aspect Ratio.

>Gee Scott, maybe it meant Aspect Ratio.

OH........silly me

People comment on the Sotona's size often. However the span on that plane is right in line with the 60 size models of today. Somehwere around 58", I think. I had the plans for it at my house for a while and the wing area is not that of an Impact or Geo-bolt but the span is darn close. I didnt measure but I would suspect it has a fairly high AR for stunt.

65 in a Sotona. I thought that too. But then I remembered what Randy told me once about 3 years ago. He was still running a 40 and everyone else was using the larger motors. He said the larger motor means more weight. It needs a larger tank, the motor itself, the pipe is larger, the prop is larger, the cg gets messed up per the design, he told me it really adds up. He even said it could be as much 4.5 to 5 oz. after it was all said and done.

Cramming a larger motor in the smaller plane works. I should know I flew Bears for years and they were often if not always the smallest planes on the circle. Still running the largest motors available at the time. But there is just something not there like there is with the larger longer wings as long as you can power it.

I put a Geo-bolt wing in a the 56-72 sized model and it was really awesome. The wing loved the power and it was rock solid.

Randy,

You commented on tip vortex issues. Cant you cure that with tip styles. Say a flat plate or a round blunted tip. I know I have a Fancher type tip on my current Nats plane. It is just like what is on his Purple plane. Tappered and sharp at the TE. It grabs every whip and dip in the air it can find. I had the same wing with blunted rounded tips, on Furias, carved balsa, not Morris style, and it was a much more stable flying model.

However, the Berringer wing comes to mind. I was just looking at Steve's new one a few minutes ago. The wing tappers from the sadlle to the tip at a point. The wing follows a straight tapper al the way down. It not a wing like we build with a tip rib then a rounded tip. The wing tip rib is so small it fits in the palm of your hand.

I am sure there is a happy medium in there somewhere. the Impact has it I think and the Geo-bolt wing. They are practicaly identicle in the span, within an inch, and the AR is really close also. Of course that is just my opinion as of now and everyone knows I havent been at this game too long. It could change. I like the discussions.
Oh well just thoughts.

(snip)
>65 in a Sotona. I thought that too. But then I remembered
>what Randy told me once about 3 years ago. He was still
>running a 40 and everyone else was using the larger motors.
>He said the larger motor means more weight. It needs a
>larger tank, the motor itself, the pipe is larger, the prop
>is larger, the cg gets messed up per the design, he told me
>it really adds up. He even said it could be as much 4.5 to
>5 oz. after it was all said and done. (snip)

I don't argue with Randy on much, but this is one area where we do disagree.

The difference in weight of the motors is what, 2 or 3 tenths of an ounce? (I guess I would have to weigh a 40 against a 65 for the exact number. But, remember, they are in the same case.)

As to prop, well, it is like putting bigger tires on a car. This is where we get our drive. But the hub is the same, we are just adding another blade (or extending them if using a 2-blade). The 13.5 inch 3-blade that Matt is currently using is under an ounce and a half. A 12 inch 2-blade is about an ounce. So we gain a half ounce here.

We can use the same motor mount.

The pipe also gives us power. The header is the same, the coupler is the same, the mount at the back is the same, the length of the carbon pipe is the same. We add only diameter to the carbon pipe. I don't have the two pipes here to compare, but what is the difference, half an ounce? It can't be any more.

On the tank, the front and back plates are the same. We are only adding length to the body and the tubing. This is thin tin. I just weighed two Brodak uniflow tanks at Random, a 5 ounce and a 6 1/2 ounce--in their packages (bigger bag on the 6 1/2 ounce tank). And they weighed the same. There has got to be some difference, but they weighed the same. So lets just say that soldering can have more of an effect on weight than body length. But let's add 2 tenths of an ounce just for the fun of it.

The last item is fuel. Matt is currently using 5 1/2 ounces on his PA 61 with the 65 pipe and the 13.5 inch 3-blade prop. How much are you going to use on your 40? I know, some guys use 8 ounces on their 61s and 65s. Some guys use more on their 40s. A lot depends on how you run them. But, frankly, the fuel load is greatly overstated. When you begin the pattern, the maneuvers aren't as bothered by the extra weight. Level flight, wingovers, loops, etc. When you get to the end of the pattern where you get into the verticals and overhead where weight becomes more critical, most of the fuel load has already been burned off. Even if we set up our engines for the same type of run, there will probably be at best half an ounce of fuel difference at this point in the pattern. And if you are running your 40 slobbery rich, you may be using more fuel than Matt is using on his 65. Let's add a half ounce here, just to be fair, considering only the amount of fuel left for when it counts.

So what do we have here?
engine? 3/10 ounce
prop? 1/2 ounce
pipe?? 1/2 ounce
tank?? 2/10 ounce
fuel? 5/10 ounce

OK, we have add an ounce and a half to the flying weight of the plane going from a 40 to a 61 or 65. One lousy ounce and a half! And we are assuming a metal tank. Put in a carbon fiber tank and the weight difference against a 40 with a metal tank would be nil!

But I can tell you that even if you added several ounces, the difference in power more than makes up for it. We have personally taken an overweight plane that was a dog, put in a more powerful engine, and suddenly brought it to life. A 50% increase in power for 2 ounces? I will take that. The 4-stroke guys are giving up a lot more than that.

So, I am still wondering, if a Satona size plane with a 65 might not be a good combination. You can detune the bigger engine if it is too much (venturi, prop, whatever). But you can't milk more out of the smaller engine if it is not there.

Bring on the 77.

>I don't argue with Randy on much, but this is one area where
>we do disagree.

I am just telling yu what the man who puts the motors together told me was the reason for the smaller motor in the plane he was flying. And he was flying it really good too!

>
>The difference in weight of the motors is what, 2 or 3
>tenths of an ounce? (I guess I would have to weigh a 40
>against a 65 for the exact number. But, remember, they are
>in the same case.)

I dont think they are in the same case. The 61 and the 65 are in the same case. I called Randy but there was no answer. The weight difference I think is more than just a couple of tenths.

>
>As to prop, well, it is like putting bigger tires on a car.
>This is where we get our drive. But the hub is the same, we
>are just adding another blade (or extending them if using a
>2-blade). The 13.5 inch 3-blade that Matt is currently
>using is under an ounce and a half. A 12 inch 2-blade is
>about an ounce. So we gain a half ounce here.
>

Half an ounce on the front of your stunter with an extra 1.5" of diameter...you really dont think there will be any trim issues with that??? Come on....

>We can use the same motor mount.
>
>The pipe also gives us power. The header is the same, the
>coupler is the same, the mount at the back is the same, the
>length of the carbon pipe is the same. We add only diameter
>to the carbon pipe. I don't have the two pipes here to
>compare, but what is the difference, half an ounce? It
>can't be any more.

The 65 pipe will not fit where the 40 pie goes if you enclose it. So mods will be made to the plane for that pipe. Could add more weight.

>
>On the tank, the front and back plates are the same. We are
>only adding length to the body and the tubing. This is thin
>tin. I just weighed two Brodak uniflow tanks at Random, a 5
>ounce and a 6 1/2 ounce--in their packages (bigger bag on
>the 6 1/2 ounce tank). And they weighed the same. There
>has got to be some difference, but they weighed the same.
>So lets just say that soldering can have more of an effect
>on weight than body length. But let's add 2 tenths of an
>ounce just for the fun of it.

What if that extra length keeps it from fitting? More mods.

>
>The last item is fuel. Matt is currently using 5 1/2 ounces
>on his PA 61 with the 65 pipe and the 13.5 inch 3-blade
>prop. How much are you going to use on your 40? I know,
>some guys use 8 ounces on their 61s and 65s. Some guys use
>more on their 40s. A lot depends on how you run them. But,
>frankly, the fuel load is greatly overstated. When you
>begin the pattern, the maneuvers aren't as bothered by the
>extra weight. Level flight, wingovers, loops, etc. When
>you get to the end of the pattern where you get into the
>verticals and overhead where weight becomes more critical,
>most of the fuel load has already been burned off. Even if
>we set up our engines for the same type of run, there will
>probably be at best half an ounce of fuel difference at this
>point in the pattern. And if you are running your 40
>slobbery rich, you may be using more fuel than Matt is using
>on his 65. Let's add a half ounce here, just to be fair,
>considering only the amount of fuel left for when it counts.

I dont really care to much about fuel loads either. But in this case you are off a bit here. I have a model that fits two different piped engines. One uses 6 oz and the other uses 8 and sometimes quits before the clover. The less fuel model is Alot easier to get the WO to look real hard and snappy at the bottom. You know the kind that lands you on top of the heap at the end of the day. Also it tears through Triangles with more ease also. .5 of fuel at take off is not much of a difference but 2 oz is.

>
>So what do we have here?
>engine? 3/10 ounce
>prop? 1/2 ounce
>pipe?? 1/2 ounce
>tank?? 2/10 ounce
>fuel? 5/10 ounce
>
>OK, we have add an ounce and a half to the flying weight of
>the plane going from a 40 to a 61 or 65. One lousy ounce
>and a half! And we are assuming a metal tank. Put in a
>carbon fiber tank and the weight difference against a 40
>with a metal tank would be nil!
>
>But I can tell you that even if you added several ounces,
>the difference in power more than makes up for it. We have
>personally taken an overweight plane that was a dog, put in
>a more powerful engine, and suddenly brought it to life. A
>50% increase in power for 2 ounces? I will take that. The
>4-stroke guys are giving up a lot more than that.

Not if the model is near its weight envelope. I have spent many years flying with Bob Gieseke and flying his designs and they are targeted to fly at a certain weight or under. If you go over EVEN with more power the plane still has poor flight characteristics. I know I have been there several times.

>
>So, I am still wondering, if a Satona size plane with a 65
>might not be a good combination. You can detune the bigger
>engine if it is too much (venturi, prop, whatever). But you
>can't milk more out of the smaller engine if it is not
>there.
>
>Bring on the 77.

Like you Leonard I think the bigger motor would be better also. However there is a reason Curt and Randy worked this plane out for the 40. I would like to know what it was before I go and cause myslef alot of heartache.

Anyway I was just telling you what he said. I didnt think it would add up to that much either BUT i have since laid it all up and there is more of increase than you say and the trim issues with props as large as you say would be very noticable.

I think Curt was flying on less than 12" in Diameter anyway. So the effect would be even more so.

Doug,

AAyeee Vayeee

Vaat hav I started?

Jim Pollock

P.S. I think judicious use of carbon components could bring weight issues into an acceptable realm close to the .40's.

LAST EDITED ON Jan-29-04 AT 03:41 PM (CDT)
 
>I think Curt was flying on less than 12" in Diameter anyway.
> So the effect would be even more so.

I think that there is lot to be said for diaeter no matter the size of the engine or plane.

I really like the larger props up to a point, as they open my PERSONAL comfort zone. I must, however, acknowledge that to win the Nats style contests, a large prop may not be the best choice. With some caveats.

I have watched very closely that Ted, Brett, David, Bob G., Bill Rich, Randy Smith (who chooses not to run a huge engine and prop) and even Doug (there are many others icluding Howard Ruch, Richard Oliver, etc) do not run as large a diamater as could be supported by the engine. I believe now (I used to think different) that properly setup this smaller diameter is a huge advantage in terms of maintaining constant energy in the airplane in the modern aft CG high energy airplanes with high enery wings (read that higher aspect ratio).

To some extent the prop acts as a wing, and a large wing tends to pick up energy very easily from the wind. Also, there are noticable effects on trim from the larger diameters (yaw). There is also a direct correlation between turn radius and disc (even though I believe some of this can be overcome with airplane design, this is evident from what Al has done with his BBQB) which is premium to impress anyone at our Nats.

I think that maybe there is a correlation between the two elements of the wing and prop, for optimized wind flying. For a larger diamter a smaller low aspect wing might be better choice, and the reverse that a smaller diameter might be necessary to control the potential energy of a large, high AR wing. The idea being that something has to help control the hills and valleys of the speed as the wind blows. The combination of a high energy wing and a high enery prop would, I feel be less than ideal. I have indeed proven this to myself.

So, to that end, I think that the facts bear out that these two philosphies have been successful. The high energy airplane/lower energy prop has certainly been successful in the form of the West Coast setup and what Doug is doing now with his Geo Bears and 12" 4 blades. I also think Al has proven the other end of the spectrum is also appealing with a small winged, high drag approach. However, there are few examples of Nats winners with huge, high energy airplanes and huge props. The combination, as I have proven to myself is simply too volatile in the high winds.

Maybe it is no mystery that even Paul Walker has been able to icrease diameter since going to the smaller, higher drag wing of the P-51. The Impact style wing I feel might always work best with a smaller diameter prop.

As always, this is simply just a matter of conjecture, and even convincing myself will take years of airplane testing, but I think there is some common sense and history to the the logic.

Everything is smaller, including the spinner. Before we began the project, Randy put the Satona 10 ounces lighter than the full sized Katana. Both of our Katana's weigh in at 58.5, the Satona is 48.5 ounces.

Smaller 2 blade prop, less wood, less paint, smaller spinner, smaller tank, less tip weight, smaller pipe, it all adds up.

Actually I wasn't arguing the plane Curt. I agree that the smaller plane builds lighter. Same thought Bob Hunt had with his plane about the same size. The plane will build smaller and lighter. And it flies well. I, for one, didn't even think of it as being a smaller plane when I saw it. My only thought was to fly the same plane with the bigger engine. Very little weight gain by bolting on a 61 or 65, the larger pipe, prop and tank. You should be able to still use the 1 3/4 inch spinner. and everyting else.

Hey, and you proved that the plane (or the pilot with the plane) could do the job I am just wondering if maybe some of our planes are getting a little too big.

>Actually I wasn't arguing the plane Curt. I agree that the
>smaller plane builds lighter. Same thought Bob Hunt had
>with his plane about the same size. The plane will build
>smaller and lighter. And it flies well. I, for one, didn't
>even think of it as being a smaller plane when I saw it. My
>only thought was to fly the same plane with the bigger
>engine. Very little weight gain by bolting on a 61 or 65,
>the larger pipe, prop and tank. You should be able to still
>use the 1 3/4 inch spinner. and everyting else.
>
>Hey, and you proved that the plane (or the pilot with the
>plane) could do the job I am just wondering if maybe some
>of our planes are getting a little too big.

But by going to a larger motor, we would have to carry more fuel and swing a bigger prop, use fatter, heavier lines, carry more tip weight, and we have started the whole sequence over again. Now we need a longer tail to balance things out, and more tail to keep things straight when we swing the bigger prop, with a fatter fuse to house the larger pipe. May as well go back to the thicker airfoil too since everything else has grown.

The Satona was designed to be optomized with a piped PA 40. If we wanted to run a PA61 or 65, we would use a model that was optomized for it.

Many have commented about the Satona's size and how it does not appear to be a small model. Well, it is not when compared to a Nobler, but next to an SV-22, it is obviously smaller. When it sits alone, the placement of the canopy and smaller spinner when combined with the paint scheme, make it look as large as a 60 sized plane. It is only 637 square inches, while the larger SV's push 690.

Have they gotten too big? It is all personal opinion. Smaller is in some ways easier to fly, but larger is more impressive. Both work, it is what best suits your style and ability that should determine which you use.

>Everything is smaller, including the spinner. Before we
>began the project, Randy put the Satona 10 ounces lighter
>than the full sized Katana. Both of our Katana's weigh in at
>58.5, the Satona is 48.5 ounces.
>
>Smaller 2 blade prop, less wood, less paint, smaller
>spinner, smaller tank, less tip weight, smaller pipe, it all
>adds up.

Doug's Blue Bear weighs 52 oz with a PA 65 in it. That was Bob's standard weight for a Bear when he was flying the little one with the PA 65.

>>Everything is smaller, including the spinner. Before we
>>began the project, Randy put the Satona 10 ounces lighter
>>than the full sized Katana. Both of our Katana's weigh in at
>>58.5, the Satona is 48.5 ounces.
>>
>>Smaller 2 blade prop, less wood, less paint, smaller
>>spinner, smaller tank, less tip weight, smaller pipe, it all
>>adds up.
>
>Doug's Blue Bear weighs 52 oz with a PA 65 in it. That was
>Bob's standard weight for a Bear when he was flying the
>little one with the PA 65.

Yep ole blue hangs in there pretty good. It has an area 675 or 665 depends on how you measure it, swept tips. It has an 18.75% tail section. Very real looking compared to the Big Bear.

PSSSSST....dont tell anyone...it has soaked up oil over the years and now it weighs 53.5... SSSSSSHHHHHHHH....!!!! I dont want to hurt it's feelings...

>Gee Brad,
>
>Are you advocating a .65 in a Satona??
>
>Just kidding, but think about it anyway!
>
>Jim Pollock

No I think Bob G. did. It's called a Bear...

Doug,

I suspect it has more to do with tip votex issues. With a high AR, the tip are narrower and I believe they generate a bunch more vortex. I know the very high AR planes I've built are quite sensitive to turbulence. My experiment with an elliptical plane had the opposite effect. They thing seem to care little about turbulence. An interesting juxtaposition.

LAST EDITED ON Jan-29-04 AT 00:16 AM (CDT)
 
>I suspect it has more to do with tip votex issues. With a
>high AR, the tip are narrower and I believe they generate a
>bunch more vortex.

LESS vortex, actually. Hence the lower induced drag.

>Why do gliders have high AR?

See above.

LAST EDITED ON Jan-29-04 AT 00:28 AM (CDT)
 
Randy-

>I suspect it has more to do with tip votex issues. With a high AR, the tip are narrower and I
>believe they generate a bunch more vortex.

I disagree here. The energy put into tip vortices creates induced drag. With less tip chord and more span for lift, you have less vortex loss, because less of the high pressure air under the wing can round the tip toward the upper surface - which explains your lower induced drag with higher A/R.

FWIW:

As Howard points out, the slope of the Lift Coefficient (CL) vs angle of attack (aoa) curve is greater with higher A/R. This means that, for any particular wing area, higher-A/R wings generate a given lift at lower aoa's. This also means that at high A/R, it takes less change in aoa to produce greater changes in lift, and stall occurs at lower aoa. Therefore, a slight upset produces a greater lift change, making the higher A/R plane more difficult to correct in gusts. The more efficient high-A/R wing will also lose or gain more lift in changing airspeeds. This all puts a higher premium on pilot reflexes and aircraft stability in turbulence as well as a greater change in flight characteristics as the plane changes orientation to the wind during each lap. I suspect these are the roots of Randy's turbulence effects.

As posted, gust-induced roll and coupled yaw effects should be affected more for high-A/R planes due to the greater leverage of the longer half-spans. I think that the high-A/R wings then need more taper to control or alleviate this effect. The Mean Geometric Chord (thus lift center for each wing) then is also closer to the fuselage (limit is 1/3 halfspan for pointed wing), reducing the root bending moment. That's probably why planes like the Australian "Firecracker" that fly in heavier wind are more tapered. That's quite possibly why Randy's elliptical wing was less affected, and why Bob Palmer's elliptical/semi-elliptical designs were advertised as "windy weather" flyers. I gave my high-A/R (.6+) flapless experiment a taper ratio of only .6; 'wonder whether that's going to help much...but like Randy, I like the high-A/R types enough to spend some time on them.

I agree that if you want to take advantage of the high-A/R plane's efficiency in turns, you probably have to put up with the turbulence effects too, unless you go with higher wing or span loading on a powerful stunter. Then there's always the calm days!

SK

LAST EDITED ON Jan-29-04 AT 00:59 AM (CDT)
 
>As Howard points out, the slope of the Lift Coefficient (CL)
>vs angle of attack (aoa) curve is greater with higher A/R.
>This means that, for any particular wing area, higher-A/R
>wings generate a given lift at lower aoa's. This also means
>that at high A/R, it takes less change in aoa to produce
>greater changes in lift, and stall occurs at lower aoa.

Aha.. That's what Howard meant. I wasn't sure what he meant by "wing lift curve slope". And this makes good sense as to why, if it's true, the high AR wing is more prone to turbulence.

Serge,

>>I agree that if you want to take advantage of the high-A/R plane's efficiency in turns, you probably have to put up with the turbulence effects too, unless you go with higher wing or span loading on a powerful stunter. Then there's always the calm days!<

Not being an engineer, I can only speak to experience I've had with these (I've built 12 high to very high AR stunters). They are very sensitive to turbulence. There are ways to moderate the sensitivity to some extent, but as you note, you just sort of have to put up with it. And that can be very difficult in areas where there is either a lot of wind and obstructions near the circle or it's dead calm and hot with thermals. I'd note also, this may or may not be coupled to flying in the wind. My experience has been that the amount of wind per se really doesn't make that much difference, it's the turbulence that kills you. I've flown them in very windy conditions, but without obstructions (large open fields with no buildings, trees or other nonsense around for quite a distance) and had relatively little difficulty. They are sure killer planes when it isn't turbulent.

Doug, let us reason:

Why do gliders have high AR? They must be sensitie to the smallest breeze.

now, thing of a Lever. The longer the arm is from the fulcrum, the stronger it is.

I ain't sayinh anytinhg, this is just food for thought.

GBY!

Up-and-down gust response is proportional to wing area/weight and to the wing lift curve slope, which increases with aspect ratio. I'll try to attach a picture:

LAST EDITED ON Jan-29-04 AT 00:03 AM (CDT)
 
Howard,

Which is why my very high AR planes could carry the weight (I built a couple of heavy ones) and turn on a dime, but were too sensitive to turbulence to fly at Salem. I get nervous when I would turn a square corner in the wind near those trees and be able to read my ama number on the top of the outboard wing. Clearly...and upright. {shudder }

>I just finished up Werwage's latest article in MA

What issue is this in? I can't find a recent Werwage article?

SK

Serge,

March 04 model aviation.

LAST EDITED ON Jan-30-04 AT 00:49 AM (CDT)
 

>I think the maneuver performance of the higher AR wings is
>so much more that it isnt really even a consideration. The
>lift produced lets you really settle sown your engine run.
>My opinion there.

Maybe. The thing that really doesn't go away is the rolling moments, at least that's what always got me. You got enough ponies and control thereof to make up the micro-teeny difference in performance between 5.25:1 and 5.75:1. I don't think anybody is seriously considering anything much outside that range. Or, to put it another way, I think it will make a lot less difference than the random variation in everything else in the system. But only one way to find out.

As far as the size goes, I don't really think much of the old scaling rules, at least not anymore. We are running piped 61's and 65s in models some would have considered too small for a (rather anemic) ST46. They weigh more than the old 46 planes, but 650-660 was on the small side for 46s back in the 70's. It really used to matter a lot whether or not you got the wing area right, but now you can switch from a 40 to a 65 and not really even have to do much re-trimming, and the performance really isn't that different.

These engines just don't seem to care what they are bolted to, within reason. You just have to know how to get them to run the way you want, without yanking the handle out of your hand in the corners. Brad noted my approach above - smaller props than the max possible. An 11.5-3.7 UCT 3-blade is not a lot of prop for a PA or RO-Jett 61 - but it's an awfully large prop for a 40VF. So I claim it's just like running the world's most macho and controlled 40, instead of a relatively wimpy 61 setup. And no trim issues like there would be with a "big meat" 13+ inch 61 prop.

In any case, I just don't see the airplanes getting any bigger (motor rules aside). They are already somewhat like driving Corvettes on a go-kart track. Curt, Phil, and Bob built small airplanes - or at least a little bit smaller that the more usual 650-660 square inch planes, and very close to the grandaddy of most of the West Coast airplanes, the Imitation. When I talked to Bob about smaller airplanes I was amazed when it finally dawned on me that his "small airplane" was actually bigger than my regular plane! I think his may have gotten a little smaller afterwards, and after flying Phil's a lot. But that's just a guess.

To the list of "smaller" planes I might add Dee's Oriental Plus. I've not flown many, if any, truly high performance, well-built, well-trimmed stunt plnes. Except the Plus. Besides all the convenience of being easy to transport, this plane just goes where it's pointed, never yanking your arm off and never losing line tension. Having flown this plane, I know finally what it is that we are striving for. Best regards,

Brett,

In 01 and 03 Bob had his Geo Bears with him and they use the Geo Bolt wing with the rest being Bob's numbers. So yes they are kind of big. This is what I modeled my plane after EXACTLY!

Up until 01 it was the usual Bears with 640-650 squares before the wing was installed. Thin foils with moderately sharp LEs. Small flaps. 20% tail section with rudderlets. He went back to this in 02 but then returned with a new big bad boy in 03 and now he is building another.

I think he still prefers the smaller model but he really likes the geo wing and how it performs.

>Up until 01 it was the usual Bears with 640-650 squares
>before the wing was installed.

You mean, counting the area under the fuse? That's the way I add mine up, too.

I had no idea they were that size - I guessed more like 700.

Hi

I just signed up.

Good to see some sound aerodynamic advice but also there are some misconceptions comming out. To consider the effects of AR in isolation, we first have to hold some other things constant. It is probably best to hold wing area and mass constant and allow span and chord to change with the different ARs we are considering. That way we can consider models of similar power and wing loading.

Any question of AR relates to tip vortex and it is important to understand, each vortex is around a half span in radius. This is the promary cause of downwash effect which extends quite some distance behind the plane and to some extent ahead of the wing due to viscosity forces between air molecules.

The effect is increases with low AR and high angle of attack. So we will see a difference between aerodynamic angle of attack (AAOA) and geometric angle of attack (GAOA). Ever watched a delta wing jet land?
The wing may be inclined at 40 deg (GAOA), this is a low AR wing operating near stall so there is a very strong downwash ahead of the wing and the (AAOA) may only be 10 degrees or so. From this, you can see that angular changes in airflow over a low AR wing will produce very little change in AAOA and therefore little change in lift coefficient. That means the plane will respond less to turbulance and to elevator control.

While on this, consider speed. There is a horizontal and vertical compant to turbulance. As airspeed increases, only the horizontal componant of turbulance is increased. This produces an apparant reduction in the GAOA as the wing passes. Combined with reduced time of encountering turbulance, the plans response to turbulance is greatly reduced with increased air speed.

Drag:
Higher AAOA and low AR increase induced drag. Low AR reduce profile drag because the chord is longer. So from a drag perspective, it's a matter of minimising the combined effecter of these 2 types of drag. However induced drag rises exponentially with AAOA so is far more important when using high lift sections like on gliders and when pulling heigh G loops. You may have noticed planes that slow down a lot in loops and 8s etc. So if you want a plane that will perform stunts with the most consistant air speed then a heigh AR is needed. To minimise heigh G drag for an aerobatic CL plane, an AR of 8 - 10 is about right. That will be at the expense of speed in level flight and smooth flight in turbulance.

There is no best AR, all is compromise.

While away this weekend, I thought some on what I wrote earlier. We seem basically agreed that if you want the the low-induced-drag benefit of high-aspect ratio (-A/R) wings in tight maneuvers, then you have to live with the compromise of greater susceptability to gust upsets (c.f. post #13).

However, as posted above, I believe that you can reduce the (coupled) yaw and roll effects of gusts by increasing wing (plan-view) taper. In particular, for conventionally (mildly) tapered wings, it should be possible through increased taper ratio to increase the A/R while maintaining the same area and the same spanwise position of the Mean Aerodynamic Chord (MAC). By keeping the spanwise center of lift inboard, this would avoid giving the higher-A/R wing additional rolling leverage for asymmetrical gust loads. You can check this out by playing with wing parameters on this site:

http://www.palosrc.com/instructors/mac.htm

I checked out their equations a couple years ago and found them to be valid. Using their formula for the spanwise distance of the MAC from the root ('d' below), I derived simple equations that allow conversion from a wing of given Area, A/R, and taper to a wing of different A/R and taper, but having the same area and position of its MAC. I'll try to illustrate these within the constraints of e-mail editing.

First some definitions:

Cr = Root Chord
Ct = Tip Chord
b = Span
A = Area
R = Aspect Ratio = b^2/A
d = spanwise location of MAC
t = Taper Ratio = Ct/Cr

More Equations:

1) Area of Tapered Wing: A = b(Cr + Ct)/2 = b(Cr)(1+t)/2

2) MAC distance d = b(1 + 2t)/{6(1 + t)} (derived from formula on Palos Verde site)

Using subscripts "1" and "2" for two different wings and letting A1 = A2 and d1 = d2 (well, they don't look much like subscripts), we can derive formulas relating span (b), aspect ratio (R), and taper ratio (t) of two wings having equal area and the same MAC position (length from root), but differing aspect ratios.

From Aspect Ratio definition and setting A1 = A2:

3) b2/b1 = (R2/R1)^.5 (square root of quotient of aspect ratios)

From the expression for d, setting d1 = d2, and combining with #3:

4) b2/b1 = {(1 + t2)(1 + 2t1)}/{(1 + t1)(1 + 2 t2)} = (R2/R1)^.5

You can solve for t2 to get this:

5) t2 = (X-1)/(1 - 2X), where X = (b2/b1)(1 + t1)/(1 + 2t1) = {(R1/R2)^.5}{(1 + t1)/(1 + 2t1)

Example:

Suppose you have wing #1, with R1 = 5.2, t = .75, and A1 = 630 in^2, and you'd like to replace it with a wing of equal area, but an aspect ratio of 6. You could keep the MAC position the same if you increased the taper ratio. But what would the new dimensions be?

First you have to know the dimensions of your original wing. Solving the aspect-ratio equation for b, you get b1 = 57.236". Using this in the wing area equation (1), you get Cr1 = 12.58". Taper ratio of .75 gives Ct1 = .75(12.58") = 9.435". For reference, you can use equation (2) to get d1 = 13.63" or about 47.6% of the half-span out.

Now, the new wing: A2 = 630, R2 = 6.0. Solving the aspect ratio equation, b2 = 61.48". Equation (5) then gives t2 = .49238. You can check now with equation (2) to see that d2 = 13.63", the same as for wing #1 (except of course that it is now at 44% of the new span). Since that's OK, use equation (1) and the computed taper t2 to compute root and tip chords:

Cr2 = 13.73" and Ct2 = 6.76", both of which should keep the wing of your stunter above the critical Reynolds Number.

Of course you'll either have more spanwise flow, or your t.e. won't be the most efficient - from what I've read. I haven't looked into elliptical wings yet...

SK

Serge

Perhaps George, and Curtis purused these formulas before designing the Magnum and Sky Dancer?? That's of course George Aldrich and Curtis Comer. Both of these planes exhibit a large amount of sweep in the leading edge, and I have actually built and flown a Sky Dancer.
It is pretty stable in the wind and I would guess that overall it would be considered slightly higher aspect ratio.

Jim-

The Mackey "Lark", with its above-average taper, looks good too in the wind.

SK

Hi Serge

I admitt I have not looked too closely at your maths but I doubt the concept because any formulae concerning tip vortex effects take wing plan form into account. That means that tapering will have the exact same downwash effects as a wing of equivilant apparant AR. The main reason for taper is to ensure a more even spanwise lift distribution. From what I have seen, spanwise disrtibution of lift force tends to follow an eliptical curve regardless of taper. That means that a constant chord wing will produce little lift but much drag at the tips.

For this reason an eliptical plan form allows most of the wing to work at the same AAOA and therefore lift coefficient. The Spitfire, wing planform was designed around a straight line aerodynamic centre (AC, 25% of chord) this results in a swept forward TE that encourages the tip vortices to centre more inboard as in a lower AR wing. The best solution therefore is a half eliptical LE and a straight TE.

I am not convinced that yaw and roll are aerodynamic issues for CL.
In my limited experience, roll is more related to engine run and torque reaction variations than to turbulance. After rotating the spray bar some, roll and yaw problems seem to have disappeared. Funny how it used to be worse on windy days, maybe the turbulance or G forces were upsetting the airflow entering the venturi. However, you may be correct. I only fly for fun these days.

I think a lot of CL use way too much tip weight. One model had a tendancy to come in on slack lines during the transition of an fig 8. The numbers showed correct lead out postition, rudder size and offset more than adequate. The problem was the inclined angle of the wing in a loop. Reversing elevator made the model loop inside the circle and loose line tension. Now I just make sure the tip weight is just a little more than required to counterballance the lines and I don't extent the inboard wing any more for the same reason.

Ken-

You have kind of lost me on some of this. From what you said in your earlier post, you seem to have a lot of experience in this, but some of the things below go contrary to what I've read or think I've seen. Also, you may have misunderstood the main point above, which was to be able to change the aspect ratio with slightly less compromise to stability in gusts.

>I admitt I have not looked too closely at your maths but I
>doubt the concept because any formulae concerning tip vortex
>effects take wing plan form into account. That means that
>tapering will have the exact same downwash effects as a wing
>of equivilant apparant AR.

I think that's really my intent - to change these effects with a greater aspect ratio. However, I'm not clear on the meaning of "same downwash effects". My understanding was that the closer a wing comes to elliptical lift distribution, the more uniform the downwash is across the span. Is this what you mean here? My purpose was to get effects of a higher A/R with less compromise - i.e. reduce vortex losses while maintaining the position of the lift center.

>The main reason for taper is to
>ensure a more even spanwise lift distribution. From what I
>have seen, spanwise disrtibution of lift force tends to
>follow an eliptical curve regardless of taper.

My understanding was/is that taper can be employed to more or less closely approximate elliptical lift distribution or even go past it. Even more taper of higher aspect ratio wings can produce a wing with the maximum lift for a given root bending moment, another kind of efficiency. Past that, it's just reduction of vortex losses.

Basically, the texts I've read espouse the idea that the longer the chord at a given inboard station, the greater the local lift. In other words, the further from the tip you get, the more two-dimensional the flow. They then shape the spanwise lift distribution somewhat according to the shape of the wing. That is at least consonant with the idea of placing the MAC where you want it.

>For this reason an eliptical plan form allows most of the
>wing to work at the same AAOA and therefore lift
>coefficient.

I'll have to think about this.

>The Spitfire, wing planform was designed
>around a straight line aerodynamic centre (AC, 25% of chord)
>this results in a swept forward TE that encourages the tip
>vortices to centre more inboard as in a lower AR wing. The
>best solution therefore is a half eliptical LE and a
>straight TE.

I did read something in the NACA literature indicating that spanwise straight trailing edges gave greatest efficiencies. However, won't spanwise flow from more than mildly swept leading edges cause boundary layer difficulties, especially at the tips? This is what I have read is a problem in back-swept flying wings and their pitch/roll controls near the tips. Several historical designs have developed peculiar habits due to this shape, when too severe. Not that I doubt your description, but I am having trouble visualizing tip vortices centering "more inboard"; I'm trying to visualize that. Do you mean that they can swirl more inboard without obstruction BEHIND the trailing edge?

>I am not convinced that yaw and roll are aerodynamic issues
>for CL.

They seem to be a trim issue for the top flyers (a SSW search will probably find some relevant analysis; try 'hingeing' - sp? - for instance), and I have had some interesting roll and yaw adventures with my "SkyRay" on windy and turbulent days. They are of course related, when one wing is faster than the other or when dihedral effects from sweep enter in. In fact, this coupling seems to be the chief stumbling block for Bill Netzeband in his efforts to refine his swept-leading-edge "Fierce Arrow" design.

>In my limited experience, roll is more related to engine run
>and torque reaction variations than to turbulance.

Certainly that too, although any asymmetrical aerodynamic effect is liable to upset stability about more than one axis, and unless care is taken, any action about one axis is liable to influence motion about the others. IMO.

Overall, the control and stability of models constrained to flight in a hemisphere (or for me, slightly more than the hemisphere ) is deceptively complex. To get it right is a real challenge, since, unlike other modes, the plane flies in constantly changing relative wind (all three directions) and other externally applied forces. The mathematics is complicated - not just for the dynamics, but for the paths and manuevers.

>I think a lot of CL use way too much tip weight.

May be true, since this is an inertial as well as gravitational influence. No doubt it can induce yaw AND roll, whenever a manuever is begun. Any yaw will induce a roll tendency, and vice versa, since lift varies as the square of the speed for parts of the wing.

One model
>had a tendancy to come in on slack lines during the
>transition of an fig 8.

Complex moment, when accelerations are reversed.

The numbers showed correct lead out
>postition, rudder size and offset more than adequate.

The rudder offset affects the model differently at different speeds. This also yaws and thus rolls the plane as speed changes, subject to control line tension influences, including their varied application point as line tension alternates.

>The problem was the inclined angle of the wing in a loop.
>Reversing elevator made the model loop inside the circle and
>loose line tension. Now I just make sure the tip weight is
>just a little more than required to counterballance the
>lines and I don't extent the inboard wing any more for the
>same reason.

These are valid concerns.

I'll have to admit that, as I try to analyze the "simple" controlliner, I continue to be awestruck at its complexity. There's a whole world of physics right here.

SK

P.S. I know there are better ways of saying yhese things and that I'll probably regret not having edited more, but I've GOT to get some sleep and get up and build some tomorrow. I may revisit this one sometime later.

Hi Serge

We may well be saying much the same thing, difficult to explain just with words.

>Ken-
>
>You have kind of lost me on some of this. From what you said
>in your earlier post, you seem to have a lot of experience
>in this, but some of the things below go contrary to what
>I've read or think I've seen. Also, you may have
>misunderstood the main point above, which was to be able to
>change the aspect ratio with slightly less compromise to
>stability in gusts.

Only private study and RC comp glider design.
I think I do understand your main point.
While induced drag (Cdi) is proportional to AR, so is induced incidence. It is induced incidence that is responsable for pitch sensativity. So by playing with planform to effectively reduce vortex effects, you will reduce induced drag but also increase pitch sensativity just like a model of higher AR.

>>I admitt I have not looked too closely at your maths but I
>>doubt the concept because any formulae concerning tip vortex
>>effects take wing plan form into account. That means that
>>tapering will have the exact same downwash effects as a wing
>>of equivilant apparant AR.
>
>I think that's really my intent - to change these effects
>with a greater aspect ratio. However, I'm not clear on the
>meaning of "same downwash effects". My understanding was
>that the closer a wing comes to elliptical lift
>distribution, the more uniform the downwash is across the
>span. Is this what you mean here? My purpose was to get
>effects of a higher A/R with less compromise - i.e. reduce
>vortex losses while maintaining the position of the lift
>center.

OK By downwash effect, I an trying to refere to induced incidence in more lay terms. I think you need to distinguish between geometric AR and apparent AR. Aparent AR is a term I coined to so as not to get into an involved explanation of the plan form correction factor used in induced drag and induced incidence calculations.

Basically, taper can make a wing act like a slightly higher AR wing. That is what I mean by apparant AR. You reduce in induced drag but also reduce the induces incidence so the pitch sensativity to turbulance will be the same as for the higher aparant AR wing.

>
>>The main reason for taper is to
>>ensure a more even spanwise lift distribution. From what I
>>have seen, spanwise disrtibution of lift force tends to
>>follow an eliptical curve regardless of taper.
>
>My understanding was/is that taper can be employed to more
>or less closely approximate elliptical lift distribution or
>even go past it. Even more taper of higher aspect ratio
>wings can produce a wing with the maximum lift for a given
>root bending moment, another kind of efficiency. Past that,
>it's just reduction of vortex losses.

I was talking of lift force there, not lift coefficient
We have to be clear on the difference here or will completey
misunderstand and seem very wrong.

>Basically, the texts I've read espouse the idea that the
>longer the chord at a given inboard station, the greater the
>local lift. In other words, the further from the tip you
>get, the more two-dimensional the flow. They then shape the
>spanwise lift distribution somewhat according to the shape
>of the wing. That is at least consonant with the idea of
>placing the MAC where you want it.
>
>

Yep I think you have the right idea.
Take a constant chord wing. The tip vortex produces a downwash effect ahead of the wing due to viscosity. This extends the entire length of the wing but is more pronounced near the tip. I am calling this effect induced incidence. This is in effect an aerodynamic twist or wash out despite the wing being geometrically straight. So the aerodynamic angle of attack (AAOA) is higher at the root than near the tip. Therefore Cl at the root is above wing mean Cl and reduces to 0 at the tip. The spanwise distribution of Cl is shown to be eliptical in texts and as the chord is constant, spanwise lift force distribution is also eliptical.

Now if we take an eliptical wing planform, texts show that the spanwise distribution of CL is constant almost the the tip. That means, there is almost no erodynamic twist along the span, the whole wing is working at the same Cl. It also means that spanwise lift force distribution is again eliptical due to the local chord in our eliptical plan form.

So in both cases the spanwise lift force distribution is eliptical.
Wing taper has not changed this but has improved induced drag due to the whole wing operating at the same AAOA and CL. So I agree that local chord does not effect local lift force and therfore will not influence moment loading about the root.

We can see than that an eliptical plan form is the ideal. A tapered wing may be employed as a practical approximation though excessive taper can result in tip stalling and should be avoided.

Not at all sure what you mean by bending moment. Do you mean that since the tips of a tapered wing are loaded better, any rolling forces will be greater?

By MAC, do you mean wing aerodynamic centre?

Wings are often designed of convenience with all the chords AC in a straight line but you don't have to do that. You can gain the wing AC by chord weighted means of the staggered sections. Just the same as summing moment forces.

Does this seem a better, more accurate explanation?

>>For this reason an eliptical plan form allows most of the
>>wing to work at the same AAOA and therefore lift
>>coefficient.
>
>I'll have to think about this.

See above.

>
>>The Spitfire, wing planform was designed
>>around a straight line aerodynamic centre (AC, 25% of chord)
>>this results in a swept forward TE that encourages the tip
>>vortices to centre more inboard as in a lower AR wing. The
>>best solution therefore is a half eliptical LE and a
>>straight TE.
>
>I did read something in the NACA literature indicating that
>spanwise straight trailing edges gave greatest efficiencies.
>However, won't spanwise flow from more than mildly swept
>leading edges cause boundary layer difficulties, especially
>at the tips? This is what I have read is a problem in
>back-swept flying wings and their pitch/roll controls near
>the tips. Several historical designs have developed peculiar
>habits due to this shape, when too severe. Not that I doubt
>your description, but I am having trouble visualizing tip
>vortices centering "more inboard"; I'm trying to visualize
>that. Do you mean that they can swirl more inboard without
>obstruction BEHIND the trailing edge?
>

The exact centre of the vortex is usually slightly inboard of the tip on most wings, special tip shapes being mostely ineffective.
I am not sure if the vortex centre starts at the tip then drifts inboard on if it is centred inboard from the start. Just something I read in a text without detailed explanation.

Swept foward TE encourages the vortex to centre more inboard than usuall. So a straight TE has a higher apparent AR (more efficient). I don't know the basis of your boundary layer concerns unless it is based on crossflows changing the pressure gradient from the designed profile. Surely that is not a huge concern on a model plane and on such a wing, only gets serious near the tip where the vortex will cause similar effects anyway. It seems to me that sweep imposes an inboard or outboard cross flow upon the usuall vortex cross flows.
So a foward sweep pulls the vortex inboard, reducing aparant AR while a back sweep pushes the vortex outboard increasing the aparant AR.
It's not like the whole wing is extremely swept so I would not expect
to have a large cumulative crossflow problem.

I think the main problem with this plan form for CL is that the AC and therefore CG are so far behing the root LE, it becomes difficult the get the engine back for enough for good ballance without having to add tail weight.

>>I am not convinced that yaw and roll are aerodynamic issues
>>for CL.
>
>They seem to be a trim issue for the top flyers (a SSW
>search will probably find some relevant analysis; try
>'hingeing' - sp? - for instance), and I have had some
>interesting roll and yaw adventures with my "SkyRay" on
>windy and turbulent days. They are of course related, when
>one wing is faster than the other or when dihedral effects
>from sweep enter in. In fact, this coupling seems to be the
>chief stumbling block for Bill Netzeband in his efforts to
>refine his swept-leading-edge "Fierce Arrow" design.

Yes but I am not sure it is necessarily an aerodynamic problem.
I am suggesting it may be mostely an enertia, ballance, engine run and leadout position problem. For CL, I think these are way more important for pitch and roll than any aerodynamic forces. As far as diferential speed across the span goes, given the radius of the circle, just how significant is that? In say a 2.5cc class on 15.9m lines, thats only +- 3% in velocity and +- 6% in aerpdynamic forces at the extreme tips and less at other stations along the span so the combined effect is even smaller. That is why I think it a mistake to extend the inboard wing or add much tip weight. I think the real problem lies in the tradition of overcompensating.

If the pilot can see the top or bottom of the wing in a loop, I think it's overcompensated. It would be interesting to do the math on it.

>>In my limited experience, roll is more related to engine run
>>and torque reaction variations than to turbulance.
>
>Certainly that too, although any asymmetrical aerodynamic
>effect is liable to upset stability about more than one
>axis, and unless care is taken, any action about one axis is
>liable to influence motion about the others. IMO.

True but not equal effects.

>Overall, the control and stability of models constrained to
>flight in a hemisphere (or for me, slightly more than the
>hemisphere ) is deceptively complex. To get it right is a
>real challenge, since, unlike other modes, the plane flies
>in constantly changing relative wind (all three directions)
>and other externally applied forces. The mathematics is
>complicated - not just for the dynamics, but for the paths
>and manuevers.

Yes but I think You only need be concerned with the major issues and so can simplify the math. After all engineering is just to get you in the ball park, after that, it's down to experimintation and refinement. The trick is to identify the major issues and factors in physics so you can ignore the trivial. I guess I just go for the first 1 or 2 significant figures.

>>I think a lot of CL use way too much tip weight.
>
>May be true, since this is an inertial as well as
>gravitational influence. No doubt it can induce yaw AND
>roll, whenever a manuever is begun. Any yaw will induce a
>roll tendency, and vice versa, since lift varies as the
>square of the speed for parts of the wing.

Yaw only has a significant roll effect where there is wing dihedral. In that case, the yaw causes a diferential incidence effect on dihedral wings. Flat wings = no differential incidence = no roll, except a slight wind shadow along the fuselage wing junction but that is so small and inboard, the roll is very slow indeed by compasison. That is why all rudder controlled RC models have loads of dihedral while aleron ones have very little.

> One model
>>had a tendancy to come in on slack lines during the
>>transition of an fig 8.

>Complex moment, when accelerations are reversed.

Yes the model was rolled in the loop because the CG was too far outboard. Then it could not roll back the othe way quickly enough.
the solution was to remove most of the tip weight to prevent rolling either way.

> The numbers showed correct lead out
>>postition, rudder size and offset more than adequate.
>
>The rudder offset affects the model differently at different
>speeds. This also yaws and thus rolls the plane as speed
>changes, subject to control line tension influences,
>including their varied application point as line tension
>alternates.

Interestingly aerodynamic formula reveal forces are proportional to velocity squared as are centrapetal forces. So you would expect all the physics to be in ballance regardless of velocity. However what does change is the Reynolds number (Re) which explains reduced aerodynamic efficiency at low speed and why the lines go slack when the engine runs out.

>>The problem was the inclined angle of the wing in a loop.
>>Reversing elevator made the model loop inside the circle and
>>loose line tension. Now I just make sure the tip weight is
>>just a little more than required to counterballance the
>>lines and I don't extent the inboard wing any more for the
>>same reason.
>
>These are valid concerns.
>
>I'll have to admit that, as I try to analyze the "simple"
>controlliner, I continue to be awestruck at its complexity.
>There's a whole world of physics right here.
>
>SK
>

Indead aircraft are the ultimate example of physics.

>P.S. I know there are better ways of saying yhese things and
>that I'll probably regret not having edited more, but I've
>GOT to get some sleep and get up and build some tomorrow. I
>may revisit this one sometime later.
>

Me too so much to say, so little time.
This is getting too big for the net.
Mabe just pull out a few main points for reply.

LAST EDITED ON Feb-05-04 AT 12:03 PM (CDT)
 
Well, reading what I typed below, I see that I have really not at all succeeded in my attempt at brevity. You have made some interesting points, some of which we agree on. I need to state though that I think my original point about tapering appropriately so that aspect ratio can be increased without moving the wing's aerodynamic center is still valid. It certainly concurs with R. T. Jones' classic Soaring Magazine article on plan forms for reduced induced drag. While he dosen't give a derivation, his conclusions and illustration indicate that what I suggest is possible and desirable. Here's what I typed tonight. I'm a bit amused at myself for getting so into it, but rather than erase all that effort, I'll just post the message:

Yeah, I'll need to just address a few main (for me) points. On a limited time budget, here goes...

>Not at all sure what you mean by bending moment. Do you mean that since the tips of a
>tapered wing are loaded better, any rolling forces will be greater?

No, I was outlining a way to avoid some of the unwanted rolling moments (see below). I mean that they can be reduced - even made the same - for greater spans at the same Cl, by increasing taper. If the taper ratios are the SAME, then the longer spanned wing will definitely incur greater rolling forces from gust induced speed or aoa differentials between wings. This is separate from the problem I outlined in previous posts of more efficient wings being more subject to gust upsets from increased speed or aoa - a problem that I think we have to live with as a trade off in choosing higher A/R wings.

Root bending moment is just the moment exerted by the summed lifts of the wing about the root position; it's the leverage from all the infinitesimal lifting forces, each multiplied by its spanwise distance from the root and summed (integrated) across the wing span.

>By MAC, do you mean wing aerodynamic centre?

I'm not sure of that expression, but from what you say later, I see that you are familiar with the MAC concept (see below). Just to nail it down...

"MAC" is Mean Aerodynamic Chord, the specific chord length at a certain spanwise position at which the results of all aerodynamic forces of the half span may be assumed to be concentrated for computation of aircraft moments. Of course the simplifying assumption is that each unit of area has the same aerodynamic lift. While this may seem crude, it is usually found to be a good starting point in aircraft design, with results that don't seem to miss by an awful lot. The quarter-chord point of the MAC is the wing's AC (aerdynamic center) and is used to determine initial c.g. positions. My equations' "d" is the outboard spanwise distance of the MAC of the half span.

>Wings are often designed of convenience with all the chords AC in a straight line but you
>don't have to do that. You can gain the wing AC by chord weighted means of the staggered
>sections. Just the same as summing moment forces.

>Does this seem a better, more accurate explanation?

This is really the method of determining MAC. Understood and used in my work/play with these things.

>We can see than that an eliptical plan form is the ideal. A tapered wing may be employed as
>a practical approximation though excessive taper can result in tip stalling and should
>be avoided.

That's why I commented about critical RN at the conclusion of my example.

> I don't know the basis of your boundary layer concerns unless it is based on crossflows
>changing the pressure gradient from the designed profile. Surely that is not a huge concern
>on a model plane and on such a wing, only gets serious near the tip where the vortex will
>cause similar effects anyway.

The theory was that the spanwise flow (outward) on the upper surface encountered the flow rounding the tip from below, thickening the boundary layer and stagnating the flow. This sounds like a potential hazard at low RN's. Anyway, whether or not it is a correct explanation, flying wings with too much rearward sweep seemed historically to be subject to control/stability problems due to this as well as when control surfaces were placed outboard and encountered tip vortices. These and the aft c.g. were the classic early problems.

>It seems to me that sweep imposes an inboard or outboard cross flow upon the usuall
>vortex cross flows. So a foward sweep pulls the vortex inboard, reducing aparant AR while a
>back sweep pushes the vortex outboard increasing the aparant AR. It's not like the whole
>wing is extremely swept so I would not expect to have a large cumulative crossflow
>problem.

Forward sweep seems to be winning out in sailplanes. Whatever the mechanism, forward sweep has been shown to inhibit tip stall, in some cases obviating the need for washout. Apparently it inhibits outward flow on the bottom near the tips. Anyway, the net result is positive enough that top sailplanes are now employing forward sweep for more reasons than that the root stalls first. I believe from what I heard in John Roncz' Oshkosh Forum back in 1996 that Jim Marske had finally convinced him that the twist in the Genesis II wing was unnecessary and inefficient. That said, it is also true that the raked back tip is being used on these planes now; so the combined local effect of a larger inward spanwise flow over the major part of the wing and the raked tip's effective inhibition of local spillage is win-win.

>I think the main problem with this plan form for CL is that the AC and therefore CG are so
>far behing the root LE, it becomes difficult the get the engine back for enough for good
>ballance without having to add tail weight.

That can be a problem, but except for finding fuel tank space for large engines, I don't see it as a net negative. It allows longer tail moments and gives a good c.g. compromise for flapped planes. At least that's the argument given by a couple of the world class flyers on this list, and it is one thing that can be modeled mathematically without that much trouble.

>Yes but I am not sure it is necessarily an aerodynamic problem. I am suggesting it may be
>mostely an enertia, ballance, engine run and leadout position problem. For CL, I think these
>are way more important for pitch and roll than any aerodynamic forces.

I don't know the relative amounts, but it is easy for one to translate into the other. What you have mentioned are main trimming areas. If the plane has an aerodynamic problem (faulty alignment of wing/stab/thrustline, excessive rudder offset, flap/elevator coordination to suit whatever c.g. position and wing loading are chosen, side area concerns, etc.) and all other trim remedies have been optimized, the plane can be suboptimal to an extent deemed unsatisfactory by the pilot. IOW, I don't think that you can divorce the two categories

>As far as diferential speed across the span goes, given the radius of the circle, just how
>significant is that? In say a 2.5cc class on 15.9m lines, thats only +- 3% in velocity
>and +- 6% in aerpdynamic forces at the extreme tips and less at other stations along the
>span so the combined effect is even smaller. That is why I think it a mistake to extend the
>inboard wing or add much tip weight. I think the real problem lies in the tradition
>of overcompensating.

I agree about overcompensating being a real danger. However, there is a significant calcuatable effect of speed differences across the span in flight around 60'-70' circles. Martin Hepperle has a nice integration across the span of the airspeeds and lift. I cannot currently get by browser to access his site, but to the best of my memory of his (or my?) results, the center of lift moves outboard somewhere between 1/2" and 1". The true problem comes not so much from asymmetrical compensation per se, but from giving the thrust line a ccw moment (ccw flying - into circle) about the c.g. when it is moved outboard without further compensation. The general expert opinion here (from experience and theory) is that more weight is required (for some optimists, allowed) for symmetrical ships than for asymmetrical ones. I have seen more than one claim that certain asymmetrical planes fly without tip weight. The other extreme is the observation by several that extreme asymmetry, as in the All American causes the plane to come into the circle on takeoff, until sufficient flying speed is attained.

>Yes but I think You only need be concerned with the major issues and so can simplify the
>math. After all engineering is just to get you in the ball park, after that, it's down to
>experimintation and refinement. The trick is to identify the major issues and factors in
>physics so you can ignore the trivial. I guess I just go for the first 1 or 2 significant figures.

I agree with the philosophy, but not the degree. Based not on my flying skills, but on commonly posted material from some of the best on SSWF, I believe that some of the issues you might see as more trivial are important to them. Some of these seem aerodynamically related. Of course, the degree I need to trim my planes to enjoy CL is not quite as high as necessary for these guys. Ultimately, as I said above, it is not ALWAYS possible to agree on relative merits. There have been some big arguments on this site. However, it's all basically modeling of dynamics. The best model predicts results most accurately.

>Yaw only has a significant roll effect where there is wing dihedral. In that case, the yaw
>causes a diferential incidence effect on dihedral wings. Flat wings = no differential
>incidence = no roll, except a slight wind shadow along the fuselage wing junction but that is
>so small and inboard, the roll is very slow indeed by compasison.

Understood and considered over long period. But I think speed differential is important, since some yawing in highly accelerated maneuvers can be violent. There are also questions of dynamic stability - divergence effects. Most such I agree are rare, but I don't think that the little pieces should be ignored, when they may add up to a big chunk somewhere.

>Interestingly aerodynamic formula reveal forces are proportional to velocity squared as are
>centrapetal forces. So you would expect all the physics to be in ballance regardless of
>velocity. However what does change is the Reynolds number (Re) which explains reduced
>aerodynamic efficiency at low speed and why the lines go slack when the engine runs out.

Yes they are both V^2 proportions. Point taken. However, there is still the very noticable change in airspeed with direction while incurring changes in groundspeed at differing rates. Prop loading changes during transitions, and there can also be a significant related change in engine run characteristics in my planes starting/going upwind vs. downwind (I had not taken this seriously, before getting back into flying, but there are more than one cause of this very real phenomenon, not all of which can be eliminated). Results include increased propwash speed across the rudder going upwind. Finally, rudder offset is an aerodynamic solution to a problem seemingly better handled with speed. Its consequent drag and outward thrust vectoring interfer with line tension up high by slowing the plane. I confirmed this early on by decreasing yaw through removing some rudder offset and moving the leadouts forward from their stock position on a SkyRay. This resulted in immediately increased - and USEABLE - tension overhead. Things get complicated, and I don't think everything happens in balance on the V^2 front.

Wow. My wing tip didn't get finished and installed tonight. I agree, we need to perhaps shorten things.

SK

Hi Serge

Yes I really believe we are on the same page.
Mainly the differences seem to have centred around the degree of accuracy require and I take your point about what is important for fun is different to that for comp.

I have been around a number of sports, some at hight levels of comp and I think that there is one thing in common. That is that people follow fashon beyond reason. Maybe I'm a bit cynical in believing the top competitors look too far into the trivial and miss the bigger issues. I can live with that.

OK so I have come back to CL after 30 odd years so I'm not up with current fashons in comp and don't intend going there either but I have yet to see a CL design that I think is well ballanced in respect of the major issues of physics.

>No, I was outlining a way to avoid some of the unwanted
>rolling moments (see below). I mean that they can be reduced
>- even made the same - for greater spans at the same Cl, by
>increasing taper. If the taper ratios are the SAME, then the
>longer spanned wing will definitely incur greater rolling
>forces from gust induced speed or aoa differentials between
>wings. This is separate from the problem I outlined in
>previous posts of more efficient wings being more subject to
>gust upsets from increased speed or aoa - a problem that I
>think we have to live with as a trade off in choosing higher
>A/R wings.

OK I did misunderstand, I thought your aim was to reduce Cdi while keeping similar induced incidence. I agree taper will reduce Cdi while retaing the same span and therefore similar rolling moments since lift force distribution along the span is not much affected by taper but Cdi is.

>>By MAC, do you mean wing aerodynamic centre?
>
>I'm not sure of that expression, but from what you say
>later, I see that you are familiar with the MAC concept (see
>below). Just to nail it down...

Clear on that now.

>The theory was that the spanwise flow (outward) on the upper
>surface encountered the flow rounding the tip from below,
>thickening the boundary layer and stagnating the flow. This
>sounds like a potential hazard at low RN's. Anyway, whether
>or not it is a correct explanation, flying wings with too
>much rearward sweep seemed historically to be subject to
>control/stability problems due to this as well as when
>control surfaces were placed outboard and encountered tip
>vortices. These and the aft c.g. were the classic early
>problems.

Understood though the suggested planform is not that swept and would not have as strong cross flow as a fully swept back.

>Forward sweep seems to be winning out in sailplanes.
>Whatever the mechanism, forward sweep has been shown to
>inhibit tip stall, in some cases obviating the need for
>washout. Apparently it inhibits outward flow on the bottom
>near the tips. Anyway, the net result is positive enough
>that top sailplanes are now employing forward sweep for more
>reasons than that the root stalls first. I believe from what
>I heard in John Roncz' Oshkosh Forum back in 1996 that Jim
>Marske had finally convinced him that the twist in the
>Genesis II wing was unnecessary and inefficient. That said,
>it is also true that the raked back tip is being used on
>these planes now; so the combined local effect of a larger
>inward spanwise flow over the major part of the wing and the
>raked tip's effective inhibition of local spillage is
>win-win.

The main reason for forward sweep on full size training gliders has been instructor pilot visability. I agree there are some well known aerodynamic advantages but some stall stability disadvantages as well.
If the rearward root section stall first, centre of lift moves forward and more difficult to recover.

>I don't know the relative amounts, but it is easy for one to
>translate into the other. What you have mentioned are main
>trimming areas. If the plane has an aerodynamic problem
>(faulty alignment of wing/stab/thrustline, excessive rudder
>offset, flap/elevator coordination to suit whatever c.g.
>position and wing loading are chosen, side area concerns,
>etc.) and all other trim remedies have been optimized, the
>plane can be suboptimal to an extent deemed unsatisfactory
>by the pilot. IOW, I don't think that you can divorce the
>two categories

I agree the 2 can't be completey divorced, I just think that too often the cause of problems is misidentified, often from experience. Like in the case I outlined earlier, most people would probably have increased rudder size and offset or put the leadouts further back.

>I agree about overcompensating being a real danger. However,
>there is a significant calcuatable effect of speed
>differences across the span in flight around 60'-70'
>circles. Martin Hepperle has a nice integration across the
>span of the airspeeds and lift. I cannot currently get by
>browser to access his site, but to the best of my memory of
>his (or my?) results, the center of lift moves outboard
>somewhere between 1/2" and 1". The true problem comes not so
>much from asymmetrical compensation per se, but from giving
>the thrust line a ccw moment (ccw flying - into circle)
>about the c.g. when it is moved outboard without further
>compensation. The general expert opinion here (from
>experience and theory) is that more weight is required (for
>some optimists, allowed) for symmetrical ships than for
>asymmetrical ones. I have seen more than one claim that
>certain asymmetrical planes fly without tip weight. The
>other extreme is the observation by several that extreme
>asymmetry, as in the All American causes the plane to come
>into the circle on takeoff, until sufficient flying speed is
>attained.

I agree with all this.
I would put the CG on the spanwise centre of lift. If you wanted to go further, I think the correct compensation for line weight would be 1/4 of the total weight of lines, 1/2 supported by pilot and 1/2 by plane, 1/4 being a mean between level and wingover flight. Prop torque is probably best compensated for by very slight wing twist to roll againt torque regardless of attitude. The roll force will not always = torque of course so an average for full power flight speeds would have to do. As drag is proportional the V^2, same as lift, you could put the thrust line in the same place as CG. However line drag and take off safety might lead to a more symetrical location. I can live with either. I have just seen too many cases of exagerated asyemetry especially in smaller planes.

>Finally, rudder offset is an
>aerodynamic solution to a problem seemingly better handled
>with speed. Its consequent drag and outward thrust vectoring
>interfer with line tension up high by slowing the plane. I
>confirmed this early on by decreasing yaw through removing
>some rudder offset and moving the leadouts forward from
>their stock position on a SkyRay. This resulted in
>immediately increased - and USEABLE - tension overhead.
>Things get complicated, and I don't think everything happens
>in balance on the V^2 front.

Agree that rudder is a poor solution. Correct leadout position and speed seem better. Rudder really requires a force couple to work well so a deep side force generating fuselage can help at lower speeds, that requires less rudder offset. I did some calcs a while back and found a large flat bottom section fin more efficient than a smaller rudder or cambered fin, given equal turning effect.
Low fin AR generally means loads of induced incidence and drag
so you want to avoid high Cl. I just balance rudder moment with line drag moment to ensure a straight flight path if the lines go slack bu t even that much is probably unnecessary if the leadouts are correctly placed.

It's sure been an interseting discussion despite a few misunderstandings at first.

>Maybe I'm a bit
>cynical in believing the top competitors look too far into
>the trivial and miss the bigger issues.

I didn't want to mislead though. More than one top flyer has posted about how the effects of certain trimming or design ideas are overshadowed or masked by more important effects.

>The main reason for forward sweep on full size training
>gliders has been instructor pilot visability. I agree there
>are some well known aerodynamic advantages but some stall
>stability disadvantages as well.
>If the rearward root section stall first, centre of lift
>moves forward and more difficult to recover.

That effect is probably reasonably handled by the horizontal tail on conventional designs with the very mild forward sweep used. It could really be a problem with sweeps of, say, >15-20 degrees. The reason tailless designs can employ mild forward sweep is that the increased "tail" moment achieved by placing the elevator at the (aft) center is advantageous, while the elevator encounters separated flow and loses effectiveness, as the root approaches stall. Neat thing.

>Low fin AR generally means loads of induced incidence and
>drag
>so you want to avoid high Cl.

Drag in the empenage isn't always a bad thing. It is stabilizing. Ted has posted and written several times that his horizontal stab/elevators are designed with lower than optimum aspect ratios because of this (and I suppose perhaps to delay stall).

>It's sure been an interseting discussion despite a few
>misunderstandings at first.

'been fun.

Serge

Hi Serge

I did a cetre of lift calc in Excel thought you might be interested.
It's a crude mean of stations based calc but I think good enough for the purpose. You can adjust line length to wing centre, wing area and AR. It then calcs the spanwise distance between geometric and lift centres.

For the size of model I fly these days, it comes up with around 8mm but would be larger for comp stunt models. If you want to take a look at it and compare with the intrigal solution you mentioned, how can I get it to you? I'd appreciate your evaluation too.

Been thinking of the rolling problem. It seems that for line tension reasons, many flyers want their planes to roll in a manouver so put the CG outboard of Centre of Lift. The resulting normal componants of, lift, enertia and gravity form a moment couple to roll the wing until brought into equalibrium with the opposing line tension moment (centrapetal/centrafugal force couple).

In a loop, the increased enertia/lift moment couple increase the (desired) roll efect. Sudden gusts etc would have to have the same roll effect and if violent enough the rolling enertia might cause ossilation around equilibrium (undesirable). Since both the desired manouver rolling and unwanted turbulance rolling both have the same root cause, I don't see how you could get one without the other.

Don't get me wrong though. I think this rolling as a monouver response is unnecessary and line tension is better handled in other ways. It also causes transition problems in fig 8 as I mentioned in earlier post. For me, it is definatly undesirable.

What is current majority opinion on trimming to cause a rolling response to manouvers, desirable \ or undesirable?

What we sometimes interpret as differential velocities in turbulance may in fact not be so differential as we think. The above enertia/lift moment couple may well be the major cause. If we align CG and centre of lift, then we have removed the enertia/lift rolling moment couple, both as aresponse to manouvers and to turbulance. Of course turblance and gusts will still cause some differential velocity along the span causing minor spanwise shifts in center of lift.

Wondering if you have differing thoughts on trimming?

Really interesting. I have flown a USA-1 (low AR) and a Saturn (not as low AR), and A geo Bolt wing (higher AR). The more I fly, the more I think that the power train is the deciding factor in the "junk". Ask the guys who have witnessed my "into the wind" patterns. All of those set ups have had more than "adequate" power trains.
Bill <><

Hi Bill

If you mean use a more powerful motor and fly faster, you
are correct. As I stated before, speed will reduce the variations in angle of attack that the wing experiences, reducing G forces and also reduce the time the plane experiences these G forces.

Speed is one of the major factors here. AR is the another.

Regards,
Ken

>Really interesting. I have flown a USA-1 (low AR) and a
>Saturn (not as low AR), and A geo Bolt wing (higher AR).
>The more I fly, the more I think that the power train is the
>deciding factor in the "junk". Ask the guys who have
>witnessed my "into the wind" patterns. All of those set
>ups have had more than "adequate" power trains.
>Bill <><
>
>Disclaimer: I am neither a NATS Champion (yet), professional
>engine reworker, engine producer, column writer, Hobby Shop
>owner, *rockeet* scientist, nor otherwise. Just an old
>Advanced flier.

LAST EDITED ON Feb-05-04 AT 00:33 AM (CDT)
 
Well, Ernesto Cuevas' of Mexico might have thought so back in 1981, evidenced by his 7:1 High A/R Stunter, "RADICAL" (some how I want to put the word "DUDE" afterwards...) and it's powered by a ST46 no less. I recalled this was in one of Hunts FM articles in 81 and dug it up and scanned it for your viewing pleasure. Heh... Does anyone remember this ship, did it leave an impression? Looks cool enough on the ground that is for sure.

Hunt did go on to praise the tighter rulebook patter Dennis "Eclipse", (a 7:1 job also) was flying, but that it did sacrifice some smoothness. He predicted at the time the higher A/R would be seen much more in the future....

>Well, Ernesto Cuevas' of Mexico might have thought so back
>in 1981, evidenced by his 7:1 High A/R Stunter, "RADICAL"
>(some how I want to put the word "DUDE" afterwards...) and
>it's powered by a ST46 no less. I recalled this was in one
>of Hunts FM articles in 81 and dug it up and scanned it for
>your viewing pleasure. Heh... Does anyone remember this
>ship, did it leave an impression? Looks cool enough on the
>ground that is for sure.
>
>Hunt did go on to praise the tighter rulebook patter Dennis
>"Eclipse", (a 7:1 job also) was flying, but that it did
>sacrifice some smoothness. He predicted at the time the
>higher A/R would be the wave of the future....
>

I had a scratch-built that was almost coincident with the Eclipse, and was probably within a pencil-line-width of the same dimensions. Completely convergent evolution. It flew pretty well in clean air with an ST G21/35. It, and its antecendants, all flew respectively well and turned really well for the day. But every single one of them suffered in turbulence, and to a lesser extent, from whip-up. The whip-up you could solve today, the rocking and rolling, you couldn't.

I finally convinced myself in the mid-80's. I had the last of this line, a very large 7:1 aspect ratio profile airplane with a ST Bull Ring 46 (and later a G21/46). Flew well for the most part, but still rocked and rolled an awful lot. I ended up trying to load it up with ballast to pretty high wing loading, but it didn't really help. I put it away, and flew Imitiation derviatives that maybe didn't turn as tight, but did everything better. One day I got tired of the thing just sitting around, so I just cut 4" off the tip of both panels, took out the ballast, fixed the Monokote, and flew it. It turned almost as well, and also cruised around like it was on rails. That was the end of the high-aspect ratio experiments for me.

Most everybody has gone down this road at one time or another - been there, done that.

Until someone comes up with a 1/2 oz gyro-controlled roll stabilizer, I think thethe correct answer is somewhere between 5 and 5.5. I wager this encompasses 99% of very successful airplanes. In that range, I would be amazed if you could really tell the difference between 5.1 and 5.3, given the random variation of other parameters.

What frequency response would one need for the roll gadget?

>What frequency response would one need for the roll gadget?

I don't know for sure - pretty ripping high, I would think. A corner takes about 200 milliseconds, and that's 5 Hz. I would guess it would have at least that 10hz. to avoid making it worse rather than better. And that's bandwidth - not sample frequency.

It looks I am going to design a small device processing inputs from elevator to tell to gyro what is wantend roll

>Until someone comes up with a 1/2 oz gyro-controlled roll stabilizer,...

What about a little tab-like gizmo, like Charles Mackey published in his Carousel article? This would be a hinged surface with a weight extended forward past the hinge line, inside the wing. Mass would be chosen so that level flight aero forces would overcome its weight and keep the tab close to horizontal, but when the wing was subjected to an upward jolt, for instance, the mass would be deflected "downward" in the wing, thus deflecting the tab upward to push the wing down against the gust. In maneuvers, such a tab placed only on the outside wing would roll the plane some to tighten lines (Mackey's purpose), but placed on both wings, it would work symmetrically (unfortunately decreasing lift) but would tend to always counter roll. Too jumpy? Workable at all?

SK

Brett,

>>The whip-up you could solve today, the rocking and rolling, you couldn't.

I agree. It is better without flaps (as to turbulence sensitivity), but doing so gives up turn, so what's the point?

LAST EDITED ON Feb-09-04 AT 08:49 PM (CDT)
 
I'd think that turbulence sensitivity is greater without flaps. You don't need as big a wing on an airplane with flaps. That's one of the primary advantages of flaps.

Howard,

Strangely, it didn't turn out that way. Though the plane had a very high aspect ratio (about 9.5 to 1), it seemed a good deal smoother in the junk. This could be due to the long narrow flaps I'd previously used with a similar design twisting in the wind, as it were, or perhaps I just managed to build a stiffer wing, but it seemed happier than previous attmepts in the turbulence. I didn't get many flights on that first flapless, very high aspect ratio plane. It sort of ran into a non-flying accident. Long story. To bad, too. It was about 600 square inches and weighed about 42oz (a semi-profile). So I'm planing another attempt later this summer.

Don't worry, you won't see it at a contest. It's just an experiment to finally prove to myself that the pursuit is a colossal waste of time.

Hi Doug

When you asked about the connection between AR and the plane responding to turbulance, I assumed you meant vertical or pitching movement of the plane. Some have talked about roll and yaw. I don't think these two are essentially an AR issue. Can you explain the motion you are most concerned with:

Pitch
roll
yaw
vertical displacement, no rotations.

Some of these may be able to be reduced independantly of AR.

Regards,
Ken.

>Hi Doug
>
>When you asked about the connection between AR and the plane
>responding to turbulance, I assumed you meant vertical or
>pitching movement of the plane. Some have talked about roll
>and yaw. I don't think these two are essentially an AR
>issue. Can you explain the motion you are most concerned
>with:
>
>Pitch
>roll
>yaw
>vertical displacement, no rotations.
>
>Some of these may be able to be reduced independantly of AR. \

I think the roll (and to a lesser extent, the yaw) is the effect I have found to be most troublesome. It's really not hard to see - more wing span for a given size = more likely to get significantly different local air velocity from tip to tip, and also, more moment arm over which it acts. It's not that difficult to handle the pitch effects since it's by far the most stable axis. I think you will, if you do the analysis, that the higher aspect ratio also couples yaw into roll much more strongly than lower aspect ratio. Add that to a lower over all roll and yaw natural frequencies (roughly the same line tension, but much larger moment of inertia), and it's easy to see how they get in trouble.

That's all just analysis. Experience seems to bear it out. One or two examples of roll/yaw turbulence sensistivity might be explained away as apocryphal, but almost everyone that has tried it has come to the same conclusion.

Brett-

I agree with your analysis, and my limited, but eventful (!) flying experience in windy Cleveland also supports your conclusions. While the effects you've mentioned can't be eliminated on high-A/R models, I think that a couple can be diminished.

>more wing span for a given size = more likely to get significantly different local air velocity
>from tip to tip, and also, more moment arm over which it acts.

The first can't be avoided with higher A/R, but the moment can be diminished some, if the A.C. is moved inboard to the position it would have on an equal-area, conventional-A/R wing, computed as in post #35. Narrower tips have to help (with large enough RN's). I wonder whether that technique used in moderation might even allow conventional wings another inch or two in span without increasing gust sensitivity. The wing area could then be slightly diminished for the same maximum lift. Resulting higher wing loading might then reduce other gust effects marginally, although I haven't computed anything. Perhaps the sum of such small effects could be made significant. Of course, this all begs the questions of whether present planforms with abundant power aren't really already fully optimized or a tighter corner actually desired?

>Add that to a lower over all roll and yaw natural frequencies (roughly the same line tension,
> but much larger moment of inertia), and it's easy to see how they get in trouble.

Tapering should reduce the moment of inertia some, at least proportionally to the area distribution, if not the cube of dimensions. Yes?

If I can keep it out of the ground, I'm planning to play with these ideas a little with a new wing in appropriate conditions at the Cleveland field this coming season. Unfortunately, I am such a slow builder that I am committing the sin of trying more than one thing at a time, and tail arm considerations may well obscure the results. But, hey, it's a hobby.

SK

What is current wisdom.

Do most top stunt flyers want the plane to roll in a manouver or not?