A (warning, very long!) response to Doug from the "Farther" threa...

This response to a question from Doug in Al's Farther of Stunt thread got way too long to justify putting it there. So I thought I'd start a new one on the subject he addressed.

Doug wrote:


>
>Is it not true when you add a larger tail section to the model
>you actually move the Center of Lift more rearward as well?
>Thus meaning that when you move the CG further back you are
>only actually getting it back to where it originally was to
>begin with. Then you end up with the same moment between the
>two but further back on the airframe.
>

Hi Doug,

You're sort of confusing some of the terminology here. To answer your question directly, no, the size and distance aft of the tailplane has no effect whatsoever on the center of lift of the wing itself. Each lifting surface has a unique "center of lift". Thus the center of lift of the wing (which varies somewhat with angle of attack and camber when the flaps are deflected) isn't effected by the size or location of the tail.

What does change with size and location of the tail is the aerodynamic center of the entire aircraft. Technically, every square centimeter of the total airframe contributes to that AC location inasmuch as every bit of an object moving through air affects that air mass and the subsequent effect on the object's path through the air mass.

Stability of that object is the result of the location of the center of gravity to the Aerodynamic Center of the whole object. Thus, spears with heavy points but no lifting surfaces whatsoever fly more or less straight when thrown because the longitudinal (lengthwise) center of gravity is forward of the "center" of the entire object. Thus the heavy end goes first because the center of gravity is forward of the "center" of where the airmass "impacts" the object. Ergo, it is "STABLE" and will fly a goodly distance, always with the pointy end first (has little to do with the point, by the way, but a lot to do with the relationship between the CG and the AC).

Take the heavy point off so it is just a stick and throw it and it'll flail all over the place and go almost nowhere.

Thus, stability of our airplanes has only a modest amount to do with the center of lift of the mainplane (wing ... the use of “mainplane” instead will become clear later) relationship to the CG but a lot to do with the relationship of the CG to the AC (just like the spear).

This distance (between the CG and the AC of the entire aircraft) is known as the "static margin". As long as the CG is forward of the AC the object will be technically "stable", either the spear or our stunters. Thus, the larger the area of the vehicle aft of the CG the greater the static margin will be. With a given amount of total area (most of which is "lifting surfaces", wings and tails, on our stunters) the stability of the ship will be the same as long as the static margin remains the same.

Thus, as we make our tails for a given wing larger (or to follow the example, borrow some of the wing area and use it in the tail surfaces), the AC of the ship will move aft. When this happens we can move the CG aft as well to regain an identical static margin and resulting identical stability.

This is why, as Brett has commented, our modern ships with "aft" CGs are every bit as stable as earlier designs with much smaller tails and further forward CGs. As you well know, modern ships aren't the least bit "squirrely", yet respond aggressively and controllably to control inputs. Inputs which, by the way, don’t need to be very large and don’t require a lot of muscle to apply.

To address Al's approach with the more forward CG for a minute. It is absolutely true that his ships are stable. They have more than adequate static margin. As he and Brett have commented they are also very competitive fliers and in the right hands have proven to be champions. Al's choice to use the "strong arm" approach to flying the maneuvers has certainly proven valid and the following isn't intended to imply otherwise; only to take the discussion a step further.

There is, however, a limit to how far one can go with the "forward CG and a strong arm" concept.

The static margin on any aircraft can be "too" long if aerobatics is the purpose of the design. You can stretch the static margin by adding nose weight or by extending the nose moment on an original design while retaining roughly the same wing loading. If taken too far however, it is entirely possible to make the ship so "stable" that the tail isn't powerful enough to pitch the ship enough to fly even the simplest maneuvers (given enough excessive static margin, you could make a ship that couldn't get airborne at all.

Think of adding a pound of weight to the nose of a thirty ounce Flite Streak. Even if you could launch it instantaneously to normal flight speed it would likely dive fairly rapidly until impact. You won't have enough tail authority to maintain level flight let alone maneuver. (Disclaimer, I don't know if a pound is enough to do this on a Flite Streak. I expect it is but you get the idea, I'm sure)

If you place that same pound of lead at the CG, you'll compromise the overall performance of your maneuvers but you'll be able to fly and pitch the airplane (now at 46 oz total -- heavy but flyable) more or less as before.

The big advantage (in my opinion, as well as a few others), of the ability to move the CG further aft and still retain an appropriate static margin is the ability to get the CG of the vehicle close to the location (on average) of the Center of Lift of the mainplane (wing). This is solely because doing so minimizes the negative pitch moments associated when the lift which supports the wing occurs aft of the weight it is supporting.

When the CG is forward of that point the tail "must" produce a download to keep the nose from falling. This tail download is a fundamental aspect of Al's approach to smaller tails, forward CGs and their commensurate heavier control feel.



It must be noted that this tail download is exactly the same thing to the wing as more weight, i.e. the further forward the CG the greater the wingloading when airborne – which becomes important as we discuss maneuvering.

Anyhow, it should be obvious that the greater the static margin (further forward the CG on a given airplane) the more deflection is required for a given wing load situation. As we’ve (I think) agreed, the nose loaded Flite Streak will require every bit of tail force available to even attempt to maintain level flight. Maneuvering is likely not an option.

What happens during maneuvering amplifies any such effects due to the G loads imposed by that maneuvering. Thus, if you are flying a loop with a two pound airplane that imposes, let’s say, 10 Gs on the airframe, it now “weighs” 20 pounds and the wing must be positioned/configured (angle of attack/flap deflection) so as to provide at least that amount of lift PLUS the lift necessary to carry any additional load from the download required of the tailplane to pitch airplane to the necessary angle of attack. If the weight of the vehicle (its CG) is located in the same place as where the lift is centered the tail forces required to achieve that angle of attack will be “smaller” and if the CG is forward of that point the additional forces will be “larger”.

Under a given set of conditions a wide variety of CG/CL relationships can be configured to provide championship level performances. IN MY HUMBLE OPINION ONLY, a configuration which keeps the CG and CL in close proximity during the pitch changes associated with our maneuvers will perform more predictably and with a more consistent control force input over a broader range of wind and weather conditions than will one which uses significant amounts of separation between the two.

This means only what it says. As has been amply demonstrated over the years, a lot of further forward CG designs have whupped butt in our biggest meets over the event’s history, certainly including Al’s which were prime examples of the breed. In recent decades, however, that hasn’t been the case. (and, I well understand the Berringers’ fine performances in Europe and the WCs. To forego yet another novel, I’ll only mention the tiny size of the flaps and the purposely flexible flap horns on those airplanes and direct anyone interested to earlier comments regarding negative pitching moments associated with flap deflection in other of my posts and/or publications)

To me, the discussion from an aerodynamic perspective is akin to sealing hingelines. Except for specific reasons (such as Al’s very understandable desire to fly scale like airplanes for his own personal satisfaction) there is no really good reason not to take advantage of this aerodynamic reality. Especially in the era of bigger motors in “normal” sized airplanes where we can have longer and larger tails without getting tail heavy as a result of doing so.

I’d like to take credit for this concept but I’d be tooting a horn that isn’t mine to toot. I came to this conclusion after getting soundly whipped in a gale at a Lincoln nats Walker flyoff when I was using all the muscle I had available to keep from running into the ground while doing the tricks and Mr. Walker went out and flew in the same air and put in a pattern that looked like the wind had disappeared (hint, it hadn’t). I may be slow but I’m no fool. After congratulating Paul on his flight (the only flight I’ve ever seen that had half the observers standing and applauding when he landed), I went over and picked up his ship, as I usually do, with my fingers under the wing halfway out the span. The first time I tried to pick it up in fell on the tail wheel. Only after moving my fingers well aft of where I “expected” the CG to be was I able to pick it up.

Think of a cartoon with a light bulb suddenly shining over the guy’s head. It was like a slap in the face that immediately affected everything I’ve done with design and trim since that day. I urge readers to think about the concept (really not much more complex than understanding the physics of a play ground teeter totter with one big kid and one little ked) and decide if their own desires in a stunt design are unique enough in their demands (as are Al’s) to justify ignoring the value of the large tail/aft CG type of ship.

You can ignore the following unless you’re interested in classic sort of illustration regarding static margins.

As alluded to above, the relationship of the CG to the AC of an airborne object is pretty consistent in terms of “stability”. What isn’t consistent is where that CG will be located in reference to the nose or tail of the entire aircraft. Also not consistent is the relationship of the CG to what we think of as the “wing”. This is why I spoke earlier of the “mainplane” instead of the more common “wing”.

There are two primary surfaces on a “normal” aircraft. One (the mainplane) providing lift to support the vehicle and one providing lift to control the pitch attitude of that mainplane which we call the stabilizing surface. When this stabilizing surface is in the back like a stunter we call it a tail or stab/elevator or empennage, etc. It’s the classic looking airplane.

When that stabilizing surface is forward of the mainplane we call it a canard and designs with them are known by the same name.

In either case, the smaller stabilizing surface does its job by creating lift. In terms of doing so for “stability” of the vehicle, its purpose is to keep the “stable” vehicle from diving (by convention, stability means the nose wants to pitch down. If it wants to pitch up it is unstable and very difficult to fly as anyone who has flown a truly tail heavy stunter will verify)

To do so, the classic tail must produce a download and the canard an upload to prevent the nose from dropping (not unlike, by the way, the stable spear we discussed earlier which is stable with the right CG/AC relationship and a floundering stick without it. If you drop a spear its “inherent stability” will cause the heavy end to precede the rest of it and will impale the poor guy standing at the bottom of the building every time – not so if the spear isn’t stable)

The thing that is worth noting in terms of our discussion is where the CG has to be relative to the Center of Lift of the “mainplane” in order for the vehicle to be stable on our two types of airplane. Let’s take the more uncommon, the canard, first. Doing so will also help us understand why you don’t see any canard stunters in the Walker Cup or at among the Extras and Yaks at the full scale aerobatic comps.

With a canard the Center of Gravity MUST, I repeat, MUST be forward of the Center of Lift of the Mainplane. By definition the Aerodynamic Center (see above to refresh your memory) of the entire vehicle must be forward of the mainplane to one degree or another depending on how large the canard surface is, how far forward of the mainplane it is and how much “other” wetted surface (body, gear, wheel pants, etc. etc.) there are forward.

Since the CG must be forward of the AC it is clear the CG must always be a little to a lot forward of where the lift to support the vehicle is largely produced, i.e. at the Center of Lift of the mainplane.

Granted, stability requires that the canard produce positive lift to stabilize that surface. Sounds good on the surface and is good from a fuel efficiency standpoint but …

When maneuvering in pitch the G loads associated with the maneuver will still be centered at the CG but will greater by a factor equal to that G load. Thus, your two pound stunter at 10 Gs will weight 20 pounds with the whole twenty being well ahead of the wing that is supporting that weight. The additional “wing”loading on the canard will be dramatically increased, the pitch change will become more and more difficult to maintain let alone accelerate to a higher angle and the canard will stall long before the mainplane (a design characteristic that is of some value in a person carrying ship whose only purpose is to cover ground safely).

This scenario is kind of the ultimate examination of the desirability of keeping the CG in proximity to the Center of Lift of the surface providing the lion’s share of the lift necessary. Note that there is no really legitimate way to overcome this problem while retaining stability in the vehicle.

The conventional layout is another animal entirely.

Remembering the discussion of Aerodynamic Centers and Static margins it is clear to see that, on a conventional planform, we can continue to move our CG aft and still retain stability as long as we move the aerodynamic center aft as well. We do that when we use larger and/or longer “tails”.

If we wanted to (and I don’t advocate this) we could make the tail large enough to move the CG well aft of the Center of Lift of the conventionally located mainplane (our wing) and still have a stable configuration. Gone to extremes we could get a flyable, stable airplane with the center of gravity behind the entire wing itself (again, of little – in fact, no -- value from an aerobatic standpoint)

You can, I’m sure, visualize what is happening in the above example. We can keep enlarging the “tail” until it becomes larger than the “wing” and “voila” we now have a canard instead of a conventional ship and we start thinking of how forward of the “mainplane” the CG must be to retain stability. This instead of discussing where the CG will fall with respect to the Mean Aerodynamic Chord (MAC) of the mainplane which is what we do with any remotely conventional stunter (or most full sized aircraft).

Again, way too much stuff here. Hope it hasn’t bored everyone but, on the other hand, reading this far has always been an option!



Fly Stunt

Ted Fancher

OH NO THAT LOOKED ALL WRONG!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

When I delete off a post I always put a Z in there. I guess I should not do that.

I AM VERY SORRY!! I WAS NOT TRYING TO BE RUDE I PROMISE!!!

Read below.

I didnt mean that to be a Z like I was sleeping. I made a response before I finished the whole thing. Then I took it down. Well, because my little ones got up from their naps, then we had snacks, then we had play time, then we had dinner, and then we had bedtime, then the wife and I cleaned the kitchen and the house, then she and I watched a movie, that we both thought was terrible, and we wished we had that 2 huors back, and then she went to bed, and now here I am finally getting to finish reading this.

I had the post up all afternoon. And never refreshed the page and I nibbled through it as I could throughout the day. Then I sat back down here and read through it 2 more times. I never saw the other replies until just now. I didnt know I had come off that way and I truly apologize for it. I hope you can see my honest mistake.

It is very appearant to me and many others that Ted spent a great deal of time writing this post. THANK YOU FOR THE POST!! BUT what is more appearant is that one, I cant spell, and two he has done a whole lot more research and time spent learning this stuff than he has in writing it.

Fact is through many conversations some years ago on private emails and reading alot of his posts over the years I already knew what he wrote. I do actually totaly understand what he wrote and it is very clear to me how it works. I dont understand all of the ins and outs that go along with it but I get the general concept very good. What I cant do is write it down properly when I need to, example Al's thread when I used CL instead of AC.

BUT I will tell you all it is only through Ted's , and others on this board, willingness to spend countless hours typing and talking about this stuff that helps me and others to understand it better.

I dont know if you remember a few years ago I was having a dog of time with a stunt ship called the Furias with a 72 in it. Turns out it was the Inertia causing the whole loaded control issues. Ted was one of the proponents saying that was the issue or the main cause of the severly loaded controls. Others said it was the large prop that was causing the problems and others said this and that and the other. But it was the inertia in this case and I porved it to be so and some at my field still didnt believe me even after I showed them point blank how it worked with a motor replacement right there in front of them.

I say this to tell you all that I read Ted's and others post with great interest in learning about aerodynamics and for pleasure and for gain in the competition circles.

I also feel the aft AC/CG combination of the alrger tailed stunt ship is truly at advantage simply because TO ME across the board the plane acts more consistanly through many different weather conditions and that my freinds is the key to success in stunt, CONSISTANCY.

Once again please do not take my initial response as a slight on Ted or his efforts. That was simpiy a misunderstanding. Like I said before, I started reposne before I finished reading and then I deleted it and I usually put a Z because the board wont let you update a post with nothing in it and I cant delete posts but I guess I really need to not do that because that comes off really rude. Again sorry about that. I had no idea.

deleted by Ted

Doug,

Take what Ted explained to you (very well if I say so. This is as good as some of the Aero classes I had at the USAF Academy) and put it in the bank. This is the very thing aerodynamicists use to make modern fighter aircraft (F-15, F-16, F-18, F-22, and F-35) maneuver as well as they do. Static Margin is reduced to the minimum possible (Some taken to the negative) to get the maneuverability that is needed to perform the necessary mission. All the horizontal tails of these aircraft are huge to assist maneuverability as well as augmented by computer assisted flight control systems.

Ted has hit a home run with this one. Kelly Johnson couldn't have done it better.

Thanks, Doug. I appreciate your clarification.

Hope to see you in Tucson.

Ted

I'll need to read that a few times in order to get my head around it, but it does sound logical

Charles

[photo not recovered: .gif]Attachment #1, ( file)

Ted,
Thanks for the lesson in Aero Engineering. Since my major was Mechanical, and I spent my time with racecars, I found it enlightening. Please feel free to continue to educate those of us who didn't study aeronautics.

Pat King
Monee, IL

>Ted,
>Thanks for the lesson in Aero Engineering. Since my major was
>Mechanical, and I spent my time with racecars, I found it
>enlightening. Please feel free to continue to educate those of
>us who didn't study aeronautics.
>
>Pat King
>Monee, IL

You're welome, Pat. Glad you enjoyed it.

Ted

You designed the DOCTOR and MEDIC about ten years ago. In light of what you said here, would you change (increase) the tail volume coeffecient on either design? I didn't see anything in your post that makes one think the principles would apply only to flapped designs.

You didn't say at what time in your career the "epiphany" occurred to you. Prior to the MED and DOC?

Larry Fulwider

>You designed the DOCTOR and MEDIC about ten years ago. In
>light of what you said here, would you change (increase) the
>tail volume coeffecient on either design? I didn't see
>anything in your post that makes one think the principles
>would apply only to flapped designs.
>
>You didn't say at what time in your career the
>"epiphany" occurred to you. Prior to the MED and
>DOC?
>
>Larry Fulwider

Larry,

That's a terrific question, the answer for which would be a bit embarrassing to me if I thought I had nothing new to learn about aerodynamics in these little planes. The fact is that everything that happens to a vehicle as it passes through the air effects everything else about it to a greater or lesser degree. What that means in our little world is that there is a significant difference between a stunter with flaps and one without them.

The simple answer is the "epiphany" happened well before the Doctor and Medic designs; influenced them greatly and, upon building and flying them, turned out not to be as applicable as I thought it might be because the these planes don't use flaps.

Both airplanes fly very well the way they are but, were I to build another, I would make some modest changes. I'll try to address the reasons for doing so in fewer words than in my last post so I don't put anyone to sleep while they read it.

The Doctor was designed with the prospect of using a CG location at the 25% +/- a fraction of the MAC that I had more or less convinced myself was magic for my competition designs. Turns out that wasn't the best location for the CG and the resulting large area, low aspect ratio tail was bigger than necessary. Were I to build another I'd use a tail that was about 15 to 18% of the wing area instead of the roughly 25% of the original design. I would do that with the expectation of running the CG more in the 15 to 20% MAC range.

I actually experimented with a slightly smaller tail on the original Doctor by slicing a full half inch off the trailing edge of the elevator to reduce aerodynamic loads on the control system that were resulting in a bit of Netzeband wall on the overheads and tops of the vertical maneuvers. The ship is very light and has a very low wing loading which means line tension is not huge. The very wide chord elevator was difficult to deflect adequately for good hourglass corners with the modest line tension. Cutting off some tail area made a significant improvement in those maneuvers.

If anyone cares why I think these changes were appropriate and should have been predictable with just a litte bit more forward thinking, read on. If you're getting tired just take the above for what it's worth.

The "ephiphany" was recognizing the effects of excessive static margins under difficult flying conditions like the high winds at Lincoln that year. The results of designing and trimming to utilize that information worked great on airplanes like my Trivial Pursuit series, the Impacts (pretty much all of Paul's airplanes) and Brett's Infinity along with a number of other modern ships using the large tail aft CG concept (by the way, a big tailed airplane can be flown very well with the CG forward because the powerful tail has no problem providing the lift necessary to do so and, therefore, it can be trimmed to fly well with the CG forward ... under a smaller range of environmental conditions, however, than the same design utilizing the advantages discussed in the first post).

What I failed to consider was the fact that the flapped airplane has a built in "feel" factor in the form of the deflected flaps which provide a negative pitching moment of their own when deflected.

Now what the heck does he mean by that, you ask? I thought the whole purpose of moving the CG back to the Center of Lift was to reduce negative pitching moment to a minimum. Why is Ted now saying good things about the same thing?

First of all, remember that a negative pitching moment means that the wing, in the process of producing the lift necessary for the maneuver wants to pitch in the opposite direction. I.e., when you want to pitch up, a negative pitching moment means the airplane wants to turn down and the tail must overcome that desire before it can start to pitch “up” as requested by the pilot.

The negative pitching moment we discussed before comes from the moment between the CG and the CL of the wing. There is, however, another source of negative pitching moment. That source is the “pitching moment” associated with the shape of the aifoil. Any airfoil that is “cambered” (think not symmetrical, more curved on the top than the bottom or vice versa) inherently wants to pitch in the direction of the lesser curve. Thus, our symmetrical airfoils have no pitching moment until we deflect the flap (nothing more than an aft part of the wing that happens to be movable –either up or down in our case)

As a result, a flapped airplane has two possible sources of negative pitching moment. The first inherent to the flapped configuration itself and the second, if we fail to elinate it as discussed before, from a moment between the CG and the CL.

So what, you ask?

Precise control of any air vehicle requires some “control feedback” to provide feel to the pilot so that he can start and stop the turns in maneuvers confidently and precisely. With flaps, we can move the CG clear back to the vicinity of the CL and the negative pitching moment of the flaps will provide the control load and, therefore, the feel the pilot needs.

Without flaps we lose that source of feedback/feel and precise control becomes less predictable. By moving the CG a little further forward (I use a ballpark figure of 15% when bench trimming unflapped ships now – oddly enough, the same location I used to think of as desirable for a flapped ship prior to the “epiphany”.

Once I realized what a far aft CG cost in terms of feel when used on an unflapped ship, the need for the larger tail became less necessary or desirable. There is little value to a tail volume co-efficient that can’t be utilized to improve performance. In other words, although the Doctor and Medic are “stable” (have an adequate static margin) with the CG at 25% MAC, the flight qualities in maneuvers deteriorate. The solution is to utilize the same (more or less) static margin with the CG at a location that provides for enough negative pitching moment to give the pilot good control feedback. Once that decision is made you then make the tail a size that provides that same static margin – in other words, slightly smaller than would be optimum for the same airplane with “normal” flaps.

Hope this was a little shorter than the first. Hope even more that it makes some sense.

Ted Fancher

>What I failed to consider was the fact that the flapped
>airplane has a built in "feel" factor in the form of
>the deflected flaps which provide a negative pitching moment
>of their own when deflected. . . .
> . . .
> . . .
>As a result, a flapped airplane has two possible sources of
>negative pitching moment. The first inherent to the flapped
>configuration itself and the second, if we fail to elinate it
>as discussed before, from a moment between the CG and the CL.
>
>So what, you ask?
>
>Precise control of any air vehicle requires some “control
>feedback” to provide feel to the pilot so that he can start
>and stop the turns in maneuvers confidently and precisely.
>With flaps, we can move the CG clear back to the vicinity of
>the CL and the negative pitching moment of the flaps will
>provide the control load and, therefore, the feel the pilot
>needs.
>
>Without flaps we lose that source of feedback/feel and precise
>control becomes less predictable. By moving the CG a little
>further forward (I use a ballpark figure of 15% when bench
>trimming unflapped ships now – oddly enough, the same location
>I used to think of as desirable for a flapped ship prior to
>the “epiphany”.
>
>Once I realized what a far aft CG cost in terms of feel when
>used on an unflapped ship, the need for the larger tail became
>less necessary or desirable. There is little value to a tail
>volume co-efficient that can’t be utilized to improve
>performance. In other words, although the Doctor and Medic
>are “stable” (have an adequate static margin) with the CG at
>25% MAC, the flight qualities in maneuvers deteriorate. The
>solution is to utilize the same (more or less) static margin
>with the CG at a location that provides for enough negative
>pitching moment to give the pilot good control feedback. Once
>that decision is made you then make the tail a size that
>provides that same static margin – in other words, slightly
>smaller than would be optimum for the same airplane with
>“normal” flaps.
>
>Hope this was a little shorter than the first. Hope even more
>that it makes some sense.
>
>Ted Fancher

You are more succinct and easier to read than you give yourself credit for. Like Mozart, not too many notes, nor too few.

Would a flying tail, pivoted at (or in front of) the elevator leading edge increase feel on an unflapped design? Or is that a college sophomore / airline stewardess category of a "solution" to a problem already well in hand? My thinking is the stabilizer (normal configuration) somewhat "blankets" the elevator.

I am asking because the U-Key 40 has the same wing area as the Medic, and I would like to experiment with a spare before building the Medic. I had planned on just using your numbers off the Medic plans on the U-Key.

Larry Fulwider


>

>>
>>Ted Fancher
>
>You are more succinct and easier to read than you give
>yourself credit for. Like Mozart, not too many notes, nor too
>few.
>
>Would a flying tail, pivoted at (or in front of) the elevator
>leading edge increase feel on an unflapped design? Or is that
>a college sophomore / airline stewardess category of a
>"solution" to a problem already well in hand? My
>thinking is the stabilizer (normal configuration) somewhat
>"blankets" the elevator.
>
>I am asking because the U-Key 40 has the same wing area as the
>Medic, and I would like to experiment with a spare before
>building the Medic. I had planned on just using your numbers
>off the Medic plans on the U-Key.
>
>Larry Fulwider
>

Hi larry, Glad this is helping out a bit.

I encourage you to do all the experimenting you like because you'll always learn something by doing so. I would suggest, however, that before doing so you look into some of th eflying tail discussions that have been onthe forums fairly regularly. Suffice to say that many have tried (including yours truly) and none have found a flying tail a good substitute for a conventional layout. Rather than start all over on that subject I'll let you do a search on the subject here on Len's site and I'd be happy to respond to any questions you might have after doing that research.

Re "blanketing" of the elevator.

that is a term I think you're better off pretty much taking out of your vocuabulary in terms of aerodynamics of lifting surfaces. The whole subject of moving trailing edges of lifting surfaces is often misunderstood and there's a lot of myths regarding them.

Elevators, flaps, rudders and ailerons are nothing more than parts of a lifting surface that act to change the camber of that surface when they are deflected. As part of a lifting surface they peroform their task in exactly the same way as any lifting surface, by increasing the pressure on one side and decreasing it on the other,thus producing lift. That lift either "supports" the airplane (the mainplane) or acts to change the attitude of the airplane about one of the three axes: roll, pitch or yaw.

That lift is produced by changing the camber of the surface to which they are attached. flaps and ailerons are generally a small percentage of the wing chord because they "work" in a positive angle of attack arena. I.e., when the flaps work to created more lift from the wing they are working in the same direction as the change in angle of attack of the entire wing.

Rudders and elevators are generally a much greater percentage of the chord of the vertical and horizontal stabilizers to which they are attached. This is because their job is to make their lifting surface produce lift in the OPPOSITE DIRECTION than that with which the vehicle is trying to go. (This is a hard thing to say with words. Wish I could do pictures which would make it a lot easier to understand.)

Here's an example of what happens. You want your airplane to climb. To do so you need to increase the angle of attack of the wing. Forgetting for a second how you drive that increase in AOA, you can easily see that deflecting your flaps down when the wing AOA is increased will increase the lift of the wing even more. Sort of a win, win combination.

Now, to drive that wing to that angle of attack the horizontal stabilizer (stab and elevator)has to produce a downward lifting force to drive the wing AOA higher. Unfortunately, when in that attitude the stabilizer (flying through the same airmass as the wing to which it is attached) will be in the same positive AOA as the wing and, if the elevator were not deflected, the tail would be producing positive lift which would drive the tail up and lower the wing angle of attack.

This is why elevators (and rudders to a lesser degree) have the higher chord ratio so that they can be deflected enough to produce a net AOA for the entire surface that produces the required lift in the direction desired even when the vehicle itself is at an opposite relationship to the airmass through which it is flying.

The net effect is kind of like flying inverted in a Piper cub. The required angle of attack to maintain altitude with a flat bottomed airfoil when inverted is very high. Using the large chord ratio elevator allows that sort of lift production to happen even in a less than perfect relationship to the air through which it is moving.

The important thing in response is that the "blanketing" effect you're talking about shouldn't be a huge concern.

By the way, that less than perfect relationship to the airmass is the driving force behind pretty much every stunt guy's desire to try a flying tail which should in theory be clearly superior. Why that isn't so is a question I'm not able to answer but I've flown several and have no desire to try anymore.

Ted

I think that Glen Allison's input would be interesting here, as he has built and experimented with stunters with all-flying tails (stabilators) with servos which increase their camber and provide "feel" (centering force). These can be pivoted rearward of single-surface units.

SK

Like a Cherokee? I thought that was for stick-free stability. I wonder what effect it would have on a control line airplane.

Howard Rush

Howard-

Here's the archived URL for what I remembered Glen telling me:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=59406&mesg_id=59406&listing_type=search#59426

We'll see whether it works after I post.

SK

> . . .
>I encourage you to do all the experimenting you like because
>you'll always learn something by doing so. I would suggest,
>however, that before doing so you look into some of th eflying
>tail discussions that have been onthe forums fairly regularly.
> Suffice to say that many have tried (including yours truly)
>and none have found a flying tail a good substitute for a
>conventional layout. . . .

The lack of models with flying tails in stunt tells a story -- I was hoping it was because they didn't work well with flaps. I don't see any point in doing an "experiment" that only demonstrates what everyone already knows (I used to be a Science Fair judge, seen a lot of that!) It didn't occur to me to check the archives, since I had never seen a stunter with that feature. I will now, tho, just for the fun of it.

Actually, Howard's comment was more convincing. I used to have a Cherokee, and you could trim it so that the rate-of-climb indicator would settle at any number you chose -- except ZERO.

Larry "Airline Stewardess and College Sophomore" Fulwider

Ted,
Thank you for that post. I'm sure many people will find it interesting. I did.

edited to delete comment on Doug's reply.

A comment about Ted's "stick". Several years ago, the IAAF (International Amateur Athletic Federation) changed the design of track and field's javelin. The reason was, the world record was getting so long that they were in danger of having the event moved out sites desgined for it. The world record at the time was nearing 344' and it was becoming too dangerous to have in conventional stadiums. The change they made was to move the center of gravity further away from the center of pressure. This had the effect of causing the javelin to fly with more arc and to cause a downward pitching moment at the top of the arc causing it to "stick" earlier. Point is, this change of moving the CG aft didn't change the stability of the javelin, but the flight path. Most of 30' was cut off the world record with the change.

By the way, a competition javelin only weighs about 800g.

Please see my above edited post. I edited and it came off all wrong.

Nice post Ted. I just lost another response here - a pretty lengthy one on the more elementary concept of *why* the c.g. needs to be ahead of the a.c. I used the old concept of c.p. rather than moments about quarter chords. However, I've no more time this weekend to write it again. Since some responses indicate need for that explanation, perhaps you could expand on that - i.e. on why the a.c. is behind the c.g. and how the stab stabilizes during unintended upsets from the straight and level. Sorry...

SK

P.S., that was therapy after a day of many things, including dumb-a** painting techniques (don't ask - it'll only take a few days to fix):-(

Serge,

I've been losing posts also. Just doesn't show up. I've taken to copying the thing then seeing if it posts. And pasting it to a new post if it doesn't. Hmmm...

>Serge,
>
>I've been losing posts also. Just doesn't show up. I've taken
>to copying the thing then seeing if it posts. And pasting it
>to a new post if it doesn't. Hmmm...

Serge,

I'm sorry to hear you lost that post. Everything you write is worth reading. Its a disappointment that we won't see that one.

During the "bylaws wars" I lost enough long posts that I finally started composing them in WordPad and saving them to a folder. Then, when they were ready to post, I pasted them here. I still do that with anything more than a couple of paragraphs.

Randi

I noticed that Randi posted a message on this thread, so I peeked. I figured she had notified Ted that he lost his landing and pattern points.

Howard Rush

Thanks for that Ted and all the other posts on this thread. I've been flying stunt for 60 years and I never knew that a brick on a string had so many options.

I always wounder what happens after power down in this configuration. 55 mph landing speeds are a must!

>I always wounder what happens after power down in this
>configuration. 55 mph landing speeds are a must!

If the airplane suddenly noses up and slows when the engine quits, you have gone too far. But that's almost tangential to this discussion - the point at which this happens changes, drastically, with tail volume. The bigger the tail volume the further back you can have the CG and not encounter this effect.

Brett

This reply was to the brick on a string. I didn't know you could move the CG around on a brick?

>This reply was to the brick on a string. I didn't know you
>could move the CG around on a brick?

My mistake!

Of course, for the brick to hit at 55 MPH you just have to start high enough for gravity to do the trick! Of course, you have to not care about the angle of descent.

Brett

>>This reply was to the brick on a string. I didn't know
>you
>>could move the CG around on a brick?
>
> My mistake!
>
> Of course, for the brick to hit at 55 MPH you just have to
>start high enough for gravity to do the trick! Of course, you
>have to not care about the angle of descent.
>
> Brett
>
or the condition of the brick after touchdown.

Ted

Would Doug or Ted explain exactly the "inertia" problem to which Doug refers? Inquiring minds want to know.

Thanks,

Jim

>
>Would Doug or Ted explain exactly the "inertia"
>problem to which Doug refers? Inquiring minds want to know.
>
>Thanks,
>
>Jim

HI Jim,

I'll have to let Doug remind me in a little more detail. I'm not sure exactly what the issue was we were discussing.

Ted

>>
>>Would Doug or Ted explain exactly the "inertia"
>>problem to which Doug refers? Inquiring minds want to
>know.
>>
>>Thanks,
>>
>>Jim
>
>HI Jim,
>
>I'll have to let Doug remind me in a little more detail. I'm
>not sure exactly what the issue was we were discussing.
>
>Ted

Well it was a about three years ago I think. I had a model with 26% tail and an 18.25" tail moment and a 8.75" nose moment designed around a Geo Bolt wing. The stab at 26% had stablets to make it even more efficiant. The motor was a Saito 72. I was running a 14.5" Eather 2bl.

The trouble I was getting was a very loaded feel in the corners even with the CG at 25% MAC at its furthest point back. About half way through the corner it would simply give lots of feedback and acted as if it didnt want to turn anymore and it took alot of stick force to finish the turns. It resulted in very inconsistant corners.

I tried cramming in tail weight to move the CG further aft to get more corner with less load up, 25% was as far as I got, I think. Before the proper feel was acheived the barbell effect would come into play and the tail wouldnt stop coming around and the corners REALLY suffered.

Many said to try smaller diameter props. I did try this with many props all the way down to 12.5". Thinking it was porp procession causing it to load up. But when you go smaller in diameter usually the RPMs go up and the effect was identical. Actaully I felt NO difference at all in the corner feedback.

I even tried setting up the controls to have more ele throw than flap deflection. No use. Same thing was happening. It would trun faster but the feedback through the corner would increase as you turned tighter. As the wind blew it would get WAY worse.

Now I have flown Al's BBQB with the 72. His has more stick load in the corners than my typical nats plane but it is even and consistant throughout. It is a steady resistance and never feels that it would bind up on you or want to fly out of a corner. It just takes more pull to make the corner. But you would never feel in trouble in a very tight corner. My problem was different. It was a binding feel as the corner progressed.

We checked the controls many times as well. Nothing wrong there.

Here comes Ted with his idea that it was the moment of inertia causing this issue. He used a bicycle tire as an example. I cant remember all of it but it was very easy to follow. The issue was there is just simply too much weight in the nose. So how on earth am I going to get weight out of the nose. Hmmmmmmm, I thought.

Bob G has always told me if you have to move the CG back always try to remove nose weight and not add tail weight if you can. He says the two are not the same thing. I think I believe him now.

Well my brother had been telling me all along to put in the Saito 56 of Bob Reeves in there and give it a whirl. He had been after me to try it for some time.

You see the 72 weighs 3 more oz than the 56. I knew I was going to have all things equal except for the weight to really see what the culprit was. The 72 was using the UHP manifold with the smallest venturi and would swing the prop at 8800 RPMs. I put the 56 on the test stand with the largest venturi and swung the same prop at 8800 rpms and it LOVED every second of it. The 56 was pounding out the power. It sounded great on that prop!

Without hesitation I set up the plane for the 56. Set the controls back to one to one. Took all the tail weight out. Now the CG was at at about 26% MAC with no tail weight added.

Everything was exactly the same as before except I was minus 3 oz of nose weight. The plane flew better than any stunt plane I had ever built. Even the one that carried me to the Top5 in 2002. It was so easy to fly and I was told by a few people that I could really go far with that one. You see it had the best of both worlds. It had a really aft CG/AC couple. This gave it lightening in a bottle in the corners. BUT it has a short nose moment and I could get away with the 14 to 14.5" props and it would be consistant on the bottoms and inersections. I was WAY more consistant at 5' with that plane then I ever have been in my life. Every time I flew I left the field smiling. It was a simple flip and fly rig!

The problem was cured with the removal of nose weight reducing the moment of inertia. There were some around here that couldnt believe it, but they came around once they saw it in the air after the change was made. I was made a believer that is for sure.

Many people chimed in on that thread and there was tons of great advice. All kinds of things to try and I tried them all. I am still thankful for their help becuase I learned alot. Something like 80 posts I think on that thread about trimming that plane.

MOI was the culprit.

Not too long after that. One evening I was flying at dusk with Bob and my Borther and his son Jake. There were two doves sitting on the edge of the circle for an entire flight. With about 3 laps before shut off one of them thought it would be a good idea to fly straight up as I flew by under power. He hit the lines about 1' inside of the wing. Jerking the model from my hand and the thong pulled tight and dumped the model straight in. The bird's head was cut off and found about 10' from his body. You can see from the pics the devistation to the plane AND the motor was beyong repair. Even for Windy. Stupid bird!

[photo not recovered: 21779.jpg]

Attachment #1, (jpg file)

Doug,

Thanks for the explaination. So, you reduced the wing loading by 3 oz. in the nose and some lesser amount in the tail.

Does that mean your use of he term "inertia" relates to wing loading or simply to the barbell effect of the nose/tail weight involved?

I think I can understand that both factors would be improved by removing the weight.

Thanks,

Jim

Jim,

The definition of inertia and what effect is has on objects:

Inertia and Mass

Newton's first law of motion states that "An object at rest tends to stay at rest and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Objects "tend to keep on doing what they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described as inertia.

Inertia = the resistance an object has to a change in its state of motion.

The more mass (weight/g where g ==32.2 ft/sec2) an object or airplane has, the more it resist a change in its state of motion. So reduce the weight in the nose of a stunt ship and it turns easier.

Another noted about "aft" CG. Several years ago, I was obsessed with high aspect ratio designs. One of the final attempts was an 8 to 1 AR design with a very, very long tail moment and a very large tail volume (about 35% of the wing). The plane actually flew pretty well, but the CG was some 35% MAC. The really big tail along with the very long tail moment allowed me the carry the CG a ridiculous distance aft. It was just one of a number of attempts that were pretty far outside the normal envelope. Point is, the plane was really very stable even with CG very, very far back. This was clearly due to long tail moment and large TVC.

>Jim,
>

>The more mass (weight/g where g ==32.2 ft/sec2) an object or
>airplane has, the more it resist a change in its state of
>motion. So reduce the weight in the nose of a stunt ship and
>it turns easier.

If you take the rest of the plane as "fixed" then the mass in the nose required to balance it at the desired point multiplied by the distance from that point will be a constant.
m*r = C

So heavier motors will require shorter noses than lighter ones. That is easy enough to understand.

However the polar moment of inertia is m*r^2, which based on the above would be r*C, with C fixed by the design of the rest of the plane.

So heavier motors will be closer to the CG, result in smaller polar moments, so the plane will be more responsive.
Conversely lighter motors will have to be mounted farther from the CG, resulting in larger polar moments and less responsive controls.

Of course most people just take the whole design as "fixed" and don't change to nose length to match the engine weight.
Instead they add tail weight to compensate for a heavy motor and end up with the "bar bell" effect.

As far as the bigger tail thing, the ones on my F2D planes are probably 30% larger in area than is the current fashion.
I wonder why I make them that way?
Could it have anything to do with being able to a run a more aft CG, still have eyes off straight line stability, and more control power?

Nope, couldn't possibly

Pat MacKenzie

Thanks Pat, et al,
That helps to get my thinking straight. I was not considering the polar moment--senioritis, maybe.

Jim

>The more mass (weight/g where g ==32.2 ft/sec2) an object or
>airplane has, the more it resist a change in its state of
>motion. So reduce the weight in the nose of a stunt ship and
>it turns easier.

Elementary my dear Watson, You have hit the nail on it's head

EDIT: I forgot to add when using a lighter engine you either have to make the nose long or make the gross weight go down with a short nose.I am all for gross weight down.

Hi Tom,

I understand, a little, about mass and inertia--Physics 101; just wondering if anyone was able to separate the "wing loading" (total weight) factor from the "inertia" (barbell) factor.

Jim

>
>Hi Tom,
>
>I understand, a little, about mass and inertia--Physics 101;
>just wondering if anyone was able to separate the "wing
>loading" (total weight) factor from the
>"inertia" (barbell) factor.
>
>Jim

Jim,

When I used the word load in my explanations it was meant to cover the force on the handle at the time of the corner. It would severly "load" up the controls.

The plane with the 72 in it was 66oz. For 720 sqrs that is plenty light. Removal of 3oz to 63 oz total I am sure didnt play much in the wing loading. But then again you could have a very valid point.

I think it ties in well with this thread. The static margin between the CG and the AC were further apart with the heavier motor in there. Thus needing more control and input to get the nose to move. Well when putting in all that control the Huge flaps 3 5/16" at the root would deflect more and more and negatively pitch the plane more and more and you get a really loaded handle. Change the control ratio and you get a plane that will turn but still loaded in the corner because the CG just isnt correct for the deign.

Now move the CG back with tail weight right? Wrong. You cant get enough TW in it to get the CG where it needs to be in relation to the AC to get the stable corner before you get the barbell effect. But I did in fact get enough weight in the tail to get the corner unloaded but the tail was overactive and wouldnt stop coming around. The plane would jump out of the corners and simply over rotate. On this model you cant do it with tail weight because the begining natural postion of the CG was way off. Incidentally I built this Furias with an 8.75" nose moment. This is way shorter than the common stunter of today. But it wasnt short enough. 7-7.5" would be more like it for the 72 is you ask me. Or a longer tail moment, that helps to naturally move the CG back. I know because I built a profile to test my theory and it worked and it flys with the 72 on it GREAT. It is like a 8.5" nose and a 21" tail. The natural position is WAY back and it flys really good and it is alot of fun!

Only option left is to remove the weight from the nose. In effect moving the CG aft without any addictional weight in the aft area of the plane that would cause issues. Now the CG is naturally closer to the AC. The amount of weight the tail must move is now less meaning it has less inertia and the tail is powerful enough to move it with out the handle load up, due to less input needed to overcome the incorrect CG placement. The CG is in the proper place and the plane is stable in level flight and in maneuvers. The prop is huge to help create more stability. Oh, and the Rabe Rudder is working to keep the mose in the proper direction during certain areas of the pattern. It was very nice for a while. All in all it was simply to much nose weight for the design. With a small tail and more forward AC it may have been great as well. But we will never know.

If I have this all wrong someone please straighten me out.

I do have another one on the building board now for about 2 years. It will probably be a dog because I have taken so long to build it.

>>
>>Hi Tom,
>>
>>I understand, a little, about mass and inertia--Physics
>101;
>>just wondering if anyone was able to separate the
>"wing
>>loading" (total weight) factor from the
>>"inertia" (barbell) factor.
>>
>>Jim
>
>Jim,
>
>When I used the word load in my explanations it was meant to
>cover the force on the handle at the time of the corner. It
>would severly "load" up the controls.
>
>The plane with the 72 in it was 66oz. For 720 sqrs that is
>plenty light. Removal of 3oz to 63 oz total I am sure didnt
>play much in the wing loading. But then again you could have
>a very valid point.
>
>I think it ties in well with this thread. The static margin
>between the CG and the AC were further apart with the heavier
>motor in there. Thus needing more control and input to get
>the nose to move. Well when putting in all that control the
>Huge flaps 3 5/16" at the root would deflect more and
>more and negatively pitch the plane more and more and you get
>a really loaded handle. Change the control ratio and you get
>a plane that will turn but still loaded in the corner because
>the CG just isnt correct for the deign.
>
>Now move the CG back with tail weight right? Wrong. You cant
>get enough TW in it to get the CG where it needs to be in
>relation to the AC to get the stable corner before you get the
>barbell effect. But I did in fact get enough weight in the
>tail to get the corner unloaded but the tail was overactive
>and wouldnt stop coming around. The plane would jump out of
>the corners and simply over rotate. On this model you cant do
>it with tail weight because the begining natural postion of
>the CG was way off. Incidentally I built this Furias with an
>8.75" nose moment. This is way shorter than the common
>stunter of today. But it wasnt short enough. 7-7.5"
>would be more like it for the 72 is you ask me. Or a longer
>tail moment, that helps to naturally move the CG back. I know
>because I built a profile to test my theory and it worked and
>it flys with the 72 on it GREAT. It is like a 8.5" nose
>and a 21" tail. The natural position is WAY back and it
>flys really good and it is alot of fun!
>
>Only option left is to remove the weight from the nose. In
>effect moving the CG aft without any addictional weight in the
>aft area of the plane that would cause issues. Now the CG is
>naturally closer to the AC. The amount of weight the tail
>must move is now less meaning it has less inertia and the tail
>is powerful enough to move it with out the handle load up, due
>to less input needed to overcome the incorrect CG placement.
>The CG is in the proper place and the plane is stable in level
>flight and in maneuvers. The prop is huge to help create more
>stability. Oh, and the Rabe Rudder is working to keep the
>mose in the proper direction during certain areas of the
>pattern. It was very nice for a while. All in all it was
>simply to much nose weight for the design. With a small tail
>and more forward AC it may have been great as well. But we
>will never know.
>
>If I have this all wrong someone please straighten me out.
>
>I do have another one on the building board now for about 2
>years. It will probably be a dog because I have taken so long
>to build it.
>

Hi Doug,

the thing that jumped out at me in this post was your description of the that jumped out at me was your comment about the "huge" flaps.

Combined with a reasonably light wingloading on a big airplane (720squre inches) it is likely that flaps that large may have been contributing mightily to the heavy control loads and not been giving you anything back in return.

The use of large flaps on stunters are just another example of the quest for ultimate light wingloadings or the "zero ounce" stunter. All flaps do for us is make a small wing produce lift like a bigger one. If you've already got a big wing using a lot of flap and/or flap movement can e counterproductive in exactly the way you've described. Especially, broad chord flaps will dramatically increase control hinge l0ads while simultaneously delivering a bunch of negative pitching moment which, like a dog chasing its tail, requires more deflection yet further increasing the load at the handle.

You may have noticed the yellow Cardinal that Kevin used to win Senior this year looked quite a bit different from the stock one ... specifically in the size of the flaps. After flying his airplane we did two things to make the control forces useable. We reduced the movement to the minimum available from the on board horns and, when that proved insufficient, we sent him home and had him cut 3/4 inches off the trailing edge from root to tip.

Instant lighter controls and much tighter corners with much less effort and no tendency whatsoever to stall or do anything funny. This will be thought of as heresy by some but the airplane actually flew better with a heavier wing loading and less lift.

If your new version of the airplane looks to be in the same weight range and isn't painted yet I'd be tempted to cut the flaps down so they are no more than 20% of the chord for their full length. That should be a great plenty.

The original Trivial Pursuit which has plenty of corner weighs at least 68 oz (haven't weighed in a decade or so, so who really knows) on 660 squares and flaps that are partial span like 80% or so, appx. and only 18% of the wing chord at every station.

The controls loads are very modest, change very little in windy weather and airplane response is very consistent in all conditions.

It's easy to fly.

Ted

>I actually experimented with a slightly smaller tail on the
>original Doctor by slicing a full half inch off the trailing
>edge of the elevator to reduce aerodynamic loads on the
>control system that were resulting in a bit of Netzeband wall
>on the overheads and tops of the vertical maneuvers. The ship
>is very light and has a very low wing loading which means line
>tension is not huge. The very wide chord elevator was
>difficult to deflect adequately for good hourglass corners
>with the modest line tension. Cutting off some tail area made
>a significant improvement in those maneuvers.

Ted,
This sounds a bit like the situation with your Tucker Spl explained in Brett's fascinating Designing column in the new SN. One of the most interesting c/l experiments I've heard about.

Have you thought about running the ballast experiment on the Doctor? If I had a thorough grasp of the prinicples so clearly set out for us in your writing, I could possibly infer your answer to that question in advance. yeah, right. in my dreams.

>Were I to build another (Doctor)I'd use a tail that was
>about 15 to 18% of the wing area instead of the roughly 25% of
>the original design. I would do that with the expectation of
>running the CG more in the 15 to 20% MAC range.

Let me know if this is opening another can of worms too big for this thread or too big, period. I've been curious why flapless airfoils maxed out at around 19%, while flapped ones are up to about 27% (sans flaps so comparing apples to apples). Is it just because most flapless designs are either old or variations on old stuff, or is there a, no doubt diabolically complex [photo not recovered: devil.gif] , aerodynamic explanation worthy of a PhD thesis in physics?

Thanks,
Kim Mortimore

snip
>
>Let me know if this is opening another can of worms too big
>for this thread or too big, period. I've been curious why
>flapless airfoils maxed out at around 19%, while flapped ones
>are up to about 27% (sans flaps so comparing apples to
>apples). Is it just because most flapless designs are either
>old or variations on old stuff, or is there a, no doubt
>diabolically complex [photo not recovered: devil.gif] , aerodynamic explanation worthy of a
>PhD thesis in physics?
>
>Thanks,
>Kim Mortimore

Kim

The bigger tails on flapped ships are almost entirely the result of dealing with negative pitching moments. Remember, there are two distinct (and distinctly different) sources of negative pitching during maneuvering. The first, over which we have some control, is any "moment" between the CG and the Center of Lift of the wing; the second, the negative pitching moment produced by the deflected flap (which is nothing more than the effects of the flap changing the airfoil from a symetrical airfoil to a cambered one )

The earliest large tailed designs (mostly those from the northeast flown by guys like Casale, Schaeffer, Hunt, Simons, etc) generally had to overcome both of these factors because they used more forward CGs. It was only when guys like Paul recognized the "traditional" 15%+/- MAC CG locations weren't necessary with such large tails that many of us started to run the CGs back near the CL thus largely eliminating the negative pitching moment due to that factor.

The beauty of the resulting configuration is that you still retain the pitching moment from the camber effect which provides some control feel to the pilot; the aft CG unloads the controls which minimizes the need for large control deflections (allowing much slower, more powerful ) control systems; because the CG and CL are roughly co-located, high and/or changing G loads in maneuvers -- especially in wind, have much reduced effect on control deflection necessary to fly the maneuvers, thus; the airplane is more consistent in response through a wide variety of conditions, thus; the airplane is easier to fly well competitively.

See Brett's post about his goal in designing the Infinity to be as easy to fly as possible. This is probably 50% of the process -- the other 50% being the appropriate engine run and prop.

The flapless airplane lacks the ability to do what we've done with the flapped ship and, therefore, we generally find it advisable to separate the CC and CL by a modest amount to provide the feel neccessary (accomplished by the camber on the flapped ship).

Thus, there is no need for the tail to be as big because running the CG back to the 25% range results in too much loss of feel and precision maneuvering suffers as a result. The problem with having the control surfaces bigger than necessary is pretty much a reflection of the Netzeband wall that's been discussed in depth.

Make sense?

Ted

Ted,

Oops, bad communication citation for me (get it?--"citation"--lame joke groan . )

My question.....

>I've been curious why
>flapless airfoils maxed out at around 19%, while flapped ones
>are up to about 27% (sans flaps so comparing apples to
>apples).

.....was actually about airfoil thickness rather than tail area. The snippet of yours I quoted was for the.....

>Were I to build another (Doctor).....

.....part. Apologies for the poorly worded post. The tail area info definitely makes sense and is very helpful. Thanks!

Kim Mortimore

>Ted,
>
>Oops, bad communication citation for me (get
>it?--"citation"--lame joke groan . )
>
>My question.....
>
>>I've been curious why
>>flapless airfoils maxed out at around 19%, while flapped
>ones
>>are up to about 27% (sans flaps so comparing apples to
>>apples).
>
>.....was actually about airfoil thickness rather than tail
>area. The snippet of yours I quoted was for the.....
>
>>Were I to build another (Doctor).....
>
>.....part. Apologies for the poorly worded post. The tail
>area info definitely makes sense and is very helpful.
>Thanks!
>
>Kim Mortimore
>
>


Aha, the old failure to communicate excuse. All right, let's try again.

With a nod of "all due respect" to my friend, Al your apples and apples comment is a flawed anology.

Despite protestations to the contrary, hinged simple flaps as we employ on most stunt ships are perfectly legitimate flaps inasmuch as they change the geometry of the airfoil as deployed to achieve varying co-efficients of lift from the same wing planform. As a result any popular convention regarding the airfoils to which they are attached considers them a part of the airfoil itself ... simply a part which is not fixed, same as ailerons, for instance.

Thus, it is inappropriate in traditional discourse to make references to the airfoil sans the flap which is part of it. Thus, you'll find that there isn't the great discrepancy you've noted between flapped and unflapped thickness ratios.

There is, indeed, some difference between the lion's share of unflapped designs and the "modern" airfoils used on designs from Al's semi scales to Paul's Impact and other later era designs like the Patternmaster.

I think this is simply a recognition of the massive difference in the thrust utilized to power the airplanes. Part of the reason for very thick wings on some modern stunters is to absorb part of the power delivered by these powerplants to allow using the thrust available while controlling speeds. The Trivial Pursuit, for instance, is only a hundred and ten or so square inches larger in area than the Nobler. The difference between George's old Fox .35 and my PAs and RoJett .61 (to say nothing of David's PA 75 in his) is enormus in comparison.

Compared to the engines generally utilized in the "pre flap" era were talking big block hemi V-8 compared to a Model T four banger. Planes (and attached airfoils) appropriate for one are not for the other.

Ted

I've been curious why
>flapless airfoils maxed out at around 19%, while flapped ones
>are up to about 27% (sans flaps so comparing apples to
>apples).

Ron B.
F4Fguy

Kim,

If you make the comparison that way, you are comparing apples and oranges. On the basis of full chord, most of the flapped 'foils come in at 20-22% or less.

Ron B.

>Thus, you'll find that there isn't the great discrepancy
>you've noted between flapped and unflapped thickness ratios.
>Ted

So...the Doctor airfoil, for example, IS thick. Do you think it's at, or near, the maximum useful thickness for a flapless airfoil, even with more power? Not thinking hemi in a Model T here, more like a modification to a not so very old design--Skyray, Doctor, Medic, etc. If there is an aerodynamic reason that the experiment will fail, then no point in running it.

Thanks for your patience,
Kim

>>Thus, you'll find that there isn't the great discrepancy
>>you've noted between flapped and unflapped thickness
>ratios.
>>Ted
>
>So...the Doctor airfoil, for example, IS thick. Do you think
>it's at, or near, the maximum useful thickness for a flapless
>airfoil, even with more power? Not thinking hemi in a Model T
>here, more like a modification to a not so very old
>design--Skyray, Doctor, Medic, etc. If there is an
>aerodynamic reason that the experiment will fail, then no
>point in running it.
>
>Thanks for your patience,
>Kim
>

Kim,

Sorry, I didn't see this when you first posted.

I don't know that the Doctor is anywhere close to the maximum thickness. The bluntness is more of a concern to me than thickness. I can't quantify it for you but the drag (and load on the engine to pull it) from an increased leading edge radius is almost certainly a bigger deal than a thicker wing with a sharper leading edge.

An airfoil (for low mach purposes) that looks pretty much like a teardrop (nature's natural "low drag" configuration)isn't a very draggy item until you do something with it to create more lift, i.e. bend the leading or trailing edge or pitch it to a higher angle of attack ... or both. Obviously, there's a point beyond which we shouldn't go but I haven't a clue what thickness that would be on a symmetrical airfoil.

I believe the blunt leading edge is of particular value with an unflapped wing because it allows the stagnation point to move up and down without separating which allow higher angles of attack before a stall. I probably wouldn't go that blunt again even on a flapless design, however.

Sorry again for not responding sooner.

Ted

>>I believe the blunt leading edge is of particular value...<<<

Must be why the leading edge of the Trivial Pursuit is so blunt and so BIG.

"Sorry again for not responding sooner. Ted"

Not a problem for sure. It's a pleasure and a privilege.

Kim Mortimore