Violent inward yaw

LAST EDITED ON May-28-04 AT 04:50 AM (CDT)
 
Guys,

Knowing how hard it is to diagnose via email I have long waited and tried to cure one remaining serious trimming problem with my F2B ship. Which does not want to say that it is the only problem but this one needs most urgent cure. I have some hope that one of you out there can help because it is a very marked problem which occurs during specific maneuvres and is aggrevated under specific wind conditions.

My model exhibits a very marked behaviour in the top right hand corner of outside squares where it displays a tendency to yaw inboard considerably especially when there is very little or no wind. I estimate a yaw angle of 10-15 degrees and since it is pointed inward you can imagine the consequences it has on line tension during a part of the flight where you want the model securely tugging the lines. I must say that the model corrects itself instantly once through the corner, it litterally jumps back to normal flying mode picking up speed. The problem is most marked as I said in -near- still air conditions and offcourse the harder I fly the corner, the more serious it is.

Some specifics:
Model: 'Own design' .35-.40 size flown on 62ft lines C/C.
Equall wingpanels, 1,5oz tipweight. The model is trimmed as neutral as possible i.e. no engine offset, airfoiled rudder no offset, leadout position 4/5 inch behind CG which lies at 20% of the average wingchord. It flies near tangent to the circle. Moderate but consistent line tension throughout the pattern (except see above). Offcourse I tried a number of things allready such as increasing tipweight, rudder offset, more backward position of the leadouts both individually and combined. It never solved the problem, often the overall flying characteristics throughout the pattern worsened.

Perhaps I reached the optimum trim and have to live with this one remaining issue but I thought, why not ask? Competitions upcoming!

Rgds, Bram Anker

Some additional info: Apart from equal panels also equal flap area. From the pilots position you can very clearly see that the relative radius of the outboard wingtip compared to that of the inboard wingtip is mucht to big. The outboard wingtip continues to ascent entering the corner, rolling the aircraft inward as it goes through the corner and combining with the yawing effect. Too much tipweight, not enought outboard flap area??

Only the top right corner? not doing it elsewhere on outside square corners? Like first outside corner of sq 8 ?

If it IS doing it elsewhere, and not being a super trim guy, (yep, there I admitted it! ) I would try LESS tip weight.

More weight will eventually lead to hinging to the inside on outside corners.

Does it hinge to the outside on the inside squares.

Has someone else observed from outside the circle??

FWIW

Curt,

Also on first outside corner of square eight. Regarding vieuwers from outside, sure but not knowledgeable enough to help. They applauded after finishing the pattern which is very flattering but in the land of the blind....

Opposite Curt I believe you need more tip weight. Add tip weight until the outside wing drops in the corner, then back it off a bit. You mentioned an ounce and a half. That, in itself, isn't much. But the real measure is how much more outboard you have than inboard. If you have a gram scale, turn the plane upside down. Then grab it by the prop and tail wheel and weigh the outboard tip on the scale. You may be surprised at how little tip weight you really have. Remember, you have to overpower the weight of the leadouts, slider, etc. There can even be a difference in weight between inside and outside panels due to wood selection. Anyway, that is your measure of tip weight.

With equal panels it will need more tip weight than unequal panels. This is good and bad. We don't like extra weight. We do like extra tension. Tip weight equals tension (among other things). An extra ounce or two for extra tension and a secure feeling on the lines I am willing to give.

It also sounds like a precession problem. But with that size engine I have never seen where that made much difference. However, other things to check in addition to tip weight:

Did you build everything straight?
Are the fuselage sides straight up and down? Is the rudder straight up and down? If there is even a slight twist or tilt here, it will act like a weather vane when the airplane is turning and wiggle-waggle the tail in and out.
Also check to make sure the stabilizer is absolutely flat with the wing. Of course, if the wing is tilted in the fuselage, then the fuselage sides won't be straight up and down in line with it. But if the stab is tilted, it will act like a rudder when in action. It, too, must be absolutely level with the wing.
Now, you probably will need to move the leadouts back a little bit. Of course, that is why you have an adjustable slider. You move them forward and back until the tension is about equal from level flight to about the 45 degree mark. Move them back slightly if you need more tension down below. Move them forward slightly if you need more up above (which will decrease it down below). Adjust until equal.

See what the above does and report back. What engine are you using? Prop? Launch rpm?

Add tip
>weight until the outside wing drops in the corner, then back
>it off a bit.

This is correct for INSIDE corners, yes?

> Add tip
>>weight until the outside wing drops in the corner, then back
>>it off a bit.
>
>This is correct for INSIDE corners, yes?

Either way. On an outside corner it will still drop at the bottom once you have gone too far.

Leonard,

As far as I can measure & see the airplane is straight. Gyroscopic effects past my mind also but seems unlikely. My engine is a Stalker .40 SE using a 11x5 APC. Have no idea about launch RPM but it sure wont be high revving. Approx 9K I gather. An additional problem about the design may be that it probably has little inbuild yaw stability because the rudder trailing edge is at the elevator hingeline making for a kind of short fuselage/small area/short moment arm behind the CG.

>Leonard,
>
>As far as I can measure & see the airplane is straight.
>Gyroscopic effects past my mind also but seems unlikely. My
>engine is a Stalker .40 SE using a 11x5 APC. Have no idea
>about launch RPM but it sure wont be high revving. Approx 9K
>I gather. An additional problem about the design may be that
>it probably has little inbuild yaw stability because the
>rudder trailing edge is at the elevator hingeline making for
>a kind of short fuselage/small area/short moment arm behind
>the CG.
>
>Rgds, Bram

Do you have a heavy spinner or weight in the spinner? The problem of precession is amplified by weight, diameter, and rpm. "P" factor enters in as an opposite force, and one will cancel the other. Which wins out may vary from maneuver to maneuver. You may have hit upon one factor, however, if you have little side area behind the center of gravity. More side area (including rudder) helps mitigate this problem.

Try some of the above suggestions, however, and see if anything helps.

>An additional problem about the design may be that
>it probably has little inbuild yaw stability because the
>rudder trailing edge is at the elevator hingeline making for
>a kind of short fuselage/small area/short moment arm behind
>the CG.
>
>Rgds, Bram

Very interesting... My TwistMaster (twister that looks like a Ringmaster) exhibits this exact same trim abnormality and It also has the rudder's trailing edge at the elevator hinge line.

Over the last several years I have tried just about everything, the flaps look like a jig saw puzzle from trying different inboard/outboard areas. The best I was able to do was trim to minimize it and fly the outside corners a little soft so it wasn't noticeable in front of the judges. Making the outboard flap a little larger than the inboard helped the most.

Wonder if the unconventional rudder adds to the problem.

Bram, I've experienced a similar happening. There are some things to think about or try.

1- yaw in the third corner of the outside square can be due to oscillation of the plane on the lines getting in sync with the corners. At the third corner the lines are hanging down and accentuate the effect of the bow in the lines whipping around. The plane generally has also slowed down considerably, making the effect bigger. Try leading the plane through the corner. That means moving your hand horizontally from right to left as the plane goes through the corner. The whipping adds some energy to the plane and reduces the amount and effects of the lines swinging around.

2- A big rudder/fin reduces the yaw effects.

3- adding nose weight, flying the plane faster, and/or using shorter lines all increase the line pull which makes the plane respond less to yawing.

4- try to fly the corner "smoother". Move the controls less abruptly and try to coordinate the motion of your hand following the plane with the amount of control. Maybe make the corner less sharp.

Hi guys,

If the above fixes don't do the job and rampant P-Factor turns out to be the problem, the addition of a Rabe rudder might be the fix. Many of the top guns have used these with great results.

There are several people on the board who have experience with that technology, and who could chime in if necessary. (Or check the archives.).

Later

Had a discussion with Al on the problem with the TwistMaster and the general conclusion was that a Rabe rudder on a ship with the rudder ending at the hinge line will create more problems than it will cure.

Bob,

Don't misquote me. I suggested that you try a movable rudder for a problem you were having trimming your airplane. You said it would be difficult to install one because of the location of the rudder on your airplane.

Len, "P" effect is a minor aerodynamic effect in a stunt ship and does not noticeably compensate gyroscopic precession. There is no such thing as a stunt ship which is immune to the effects of precession as long as we run any engine which moves a rotating mass. Props are gyroscopes regardless of how small the prop may be or how slow the engine turns. This makes the third recent post regarding airplanes which lose tension on hard outside corners. Certainly there are more forces at work and trimming is required to obtain optimum performance with any airplane. My point is, that I observed exactly the same effect with an early stunt ship, analyzed the problem, and found a solution WHICH WORKS FOR ME. It serves no purpose to ignore physics as well as aerodynamics.

Not all airplanes need movable rudders, particularly airplanes which have a lot of side area and happen to run light weight props. All propeller driven airplanes have gyroscopic precession. Stability in the yaw axis will minimize gyroscopic effects enough for most of our stunt ships to fly well enough without active compensation.

Charles Buffalano actually calculated the force of "P" effect as compared to the force of gyroscopic precession in a sophisticated computer model of stunt ship flight and concluded that "P" effects were minimal. Also, that Gyroscopic effects are strong enough that they should be considered.

In short, if your engine/prop combination forms a substantial gyroscope and your aerodynamic dampening is insufficient, YOU WILL HAVE NOTICEABLY LESS TENSION ON OUTSIDE CORNERS. "Tweaking" the flaps to roll the airplane may help outside corners but creates more trim problems elsewhere as it ignores the basic problem. Gyroscopic precession is a simple physical effect. It is the result of rotating a mass, and will be there in turbine and electric powered airplanes as well. It can be ignored but it won't go away.

>Bob,
>
>Don't misquote me. I suggested that you try a movable
>rudder for a problem you were having trimming your airplane.
> You said it would be difficult to install one because of
>the location of the rudder on your airplane.
>
>Len, "P" effect is a minor aerodynamic effect in a stunt
>ship and does not noticeably compensate gyroscopic
>precession. There is no such thing as a stunt ship which is
>immune to the effects of precession as long as we run any
>engine which moves a rotating mass. Props are gyroscopes
>regardless of how small the prop may be or how slow the
>engine turns. This makes the third recent post regarding
>airplanes which lose tension on hard outside corners.
>Certainly there are more forces at work and trimming is
>required to obtain optimum performance with any airplane.
>My point is, that I observed exactly the same effect with an
>early stunt ship, analyzed the problem, and found a solution
>WHICH WORKS FOR ME. It serves no purpose to ignore physics
>as well as aerodynamics.
>
>Not all airplanes need movable rudders, particularly
>airplanes which have a lot of side area and happen to run
>light weight props. All propeller driven airplanes have
>gyroscopic precession. Stability in the yaw axis will
>minimize gyroscopic effects enough for most of our stunt
>ships to fly well enough without active compensation.
>
>Charles Buffalano actually calculated the force of "P"
>effect as compared to the force of gyroscopic precession in
>a sophisticated computer model of stunt ship flight and
>concluded that "P" effects were minimal. Also, that
>Gyroscopic effects are strong enough that they should be
>considered.
>
>In short, if your engine/prop combination forms a
>substantial gyroscope and your aerodynamic dampening is
>insufficient, YOU WILL HAVE NOTICEABLY LESS TENSION ON
>OUTSIDE CORNERS. "Tweaking" the flaps to roll the airplane
>may help outside corners but creates more trim problems
>elsewhere as it ignores the basic problem. Gyroscopic
>precession is a simple physical effect. It is the result of
>rotating a mass, and will be there in turbine and electric
>powered airplanes as well. It can be ignored but it won't go
>away.
>
>Al

Ron B.
F4Fguy

Hi,Al:

AMEN!!

You are SOOOO right.No matter what the engine speed or prop,the gyro couple is significant.
For example:
(1)10-6 RevUp wide blade @11100 RPM, 60'lines,74.3FPS(5.5 sec lap),5' corner--Gc(Gyro Couple)=9.78 oz./in.Going to a narrow RevUp S2 reduces this to 6.18,but it's still significant.
(2)Going the other way to a 12.5-6 Bolly three blade @ 10200 RPM and 65'lines,the Gc goes up to 24.7 oz./in.
These were calculated using experimentally determined polar moments,not "guesstimates".The examples given are real world .Different conditions can be estimated easily,since everything Except the polar moment is in direct proportion.For instance;increasing RPM by 20% increases the Gc by 20%.
Props,on the other hand are not created equal,regardless of material.Consider a TOP-FLITE 13-6 polar moment of .00234 vs a Rev-Up 13-6 @ .00128.This has something to do with weight(mass),but a lot more to do with mass distribution.Just changing to a lighter prop is not necessarily the answer.

Hi Al,

Wouldn't dream of misquoting you, at least not on purpose. I thought we came to the conclusion that besides a linkage nightmare, all of the rudder area is above the stab (and fuselage) which could create roll problems that might be worse than the original yaw problem. Sorry if I misinterpreted what you were trying to teach me.

LAST EDITED ON May-29-04 AT 07:06 PM (CDT)
 
No problem, Bob.

We agreed that a Ringmaster style vertical fin/rudder would be difficult to adapt to a movable rudder, but for practical, not aerodynamic, reasons. Its location on the top of the fuselage would impart some rolling moment to control applications. Whether this would be enough to negate to positive effects of the active gyroscopic compensation would be hard to say. In any case, locating the vertical fin/rudder forward of the elevator hinge line minimizes the yaw axis stabilizing effect of the fin/rudder area. This is kind of asking for yaw problems on stunt ships. It is always optimal to locate side area as far aft of the CG as possible to provide greater stability in the yaw axis.

\ Gyroscopic
>precession is a simple physical effect. It is the result of
>rotating a mass, and will be there in turbine and electric
>powered airplanes as well. It can be ignored but it won't go
>away.

Technically correct. But I would once again point out that most of the time precession is "in the noise" compared to the other trim issues like leadout position and static rudder offset (see example below). If you don't get the rest of it right, and I mean *really* right, there's no point in trying to add another adjustment to take out precession.

BTW, it's not really an active control element. There's no sensor and thus the system has no way of it sensing its own actions or the effect it's trying to control and adjusting accordingly. It's technically an open-loop compensation. Set correctly, it does provide yaw torque proportional to the equilibrium pitch rate, so it can act correctly to first approximation. It works to only the first of many terms, and the underlying assumption is that elevator deflection is equal to the pitch rate. This is not the correct equation of motion but ignoring the other terms is probably fine to the degree it needs to work.

Of course, I have had the same problem with my Intrepid. I switched from a Bolly prop to a Brian Eather prop and that helped quite a bit. So, I guess the moral of the story is turn the lightest prop that you can (Eathers are lighter than Bolly's near the tips)and as short as will fly the airplane successfully. That will mitigate if not eliminate the problem.

Bram
What about leaving most as you have and try about 2 degrees right thrust and 1 1/2 degrees down thrust to start with. Also helps with grooving. Having the all zero setup can cause your problem as well as hunting.

DH in OZ

If it is really most signifficant at >>>top right hand corner of outside squares<<< I would really take care HOW you fly that corner as feroucious already mentioned in his point 4/. I very often see that pitot try to do that particular corner much tighter than any other because it not needs lot of lift (you can), it is far from ground (no problem to hit it abruptly) and it is direction of gyro force (lower tension in lines - lines are losing feedback).

I have a similar problem pointed out to me by Brett Buck last weekend. Tough to see it from the handle perspective. I knew it had a problem but couldn't tell how bad it was. Brett came over during last weekend's Regionals and made some suggestions. Said it was obvious from the next circle. Now that I have the other trim issues mostly sorted out, I'm turning my attention to this one.

Pretty much as discussed, I have a pretty serious yaw problem, but it mostly occurs on square corners at the tops of loops and eights. Both inside and outside, but much more noticably on outsides. We tried a few things like moving the leadouts and using various props. Each thing mitigated the problem to some extent, but hasn't really solved it. I'm now in the process of installing a movable rudder. Once installed, at least I'll have the ability to move the thing around to further lessen the issue. This was Brett's suggestion and I hope to have it done in time for weekend flying to try it out.

>I have a similar problem pointed out to me by Brett Buck
>last weekend. Tough to see it from the handle perspective. I
>knew it had a problem but couldn't tell how bad it was.
>Brett came over during last weekend's Regionals and made
>some suggestions. Said it was obvious from the next circle.
>Now that I have the other trim issues mostly sorted out, I'm
>turning my attention to this one.
>
>Pretty much as discussed, I have a pretty serious yaw
>problem, but it mostly occurs on square corners at the tops
>of loops and eights. Both inside and outside, but much more
>noticably on outsides. We tried a few things like moving the
>leadouts and using various props. Each thing mitigated the
>problem to some extent, but hasn't really solved it. I'm now
>in the process of installing a movable rudder. Once
>installed, at least I'll have the ability to move the thing
>around to further lessen the issue. This was Brett's
>suggestion and I hope to have it done in time for weekend
>flying to try it out.
>

To expand slightly, the fin has a significant offset (maybe 1/4"), and is pretty much the only lateral surface since the fuse is pretty skinny. The result is that the aerodynamics want to hold the airplane at a significant yaw angle.

The leadouts were set pretty far forward (probably to get the airplane flying closer to tangent to the circle by dragging the nose in). But they weren't way far off the right position in terms of line arc.

The net effect is that on every hard maneuver, the combination of reduced speed (reducing the nose-out yaw torque) and the increase in the tension of the lines as the result of moving the controls (inevitable unless you have a 0-offset handle) caused the nose to yaw in. And I don't mean a little bit - it was very, very significant, and of course once it got going, it oscillated all over the place. Very, very, grossly off. Like a salmon jumping up a waterfall.

And of course all this yawing around caused the line tension to vary around, making the control deflection change, and it also hopped all over the place in pitch making it very difficult to control.

My recommendation was to move the leadouts back a lot to make the aerodynamic and mechanical offsets more compatible. First step was to move them back 3/8" and it was immediately a lot better. Of course, now it was yawed way out all the time - but it really didn't hurt anything that much compared to the way it was before. It could probably have gone another 1/4 or so, based on my observations the next day, but it was definitely a step in the right direction. It was also flying much, much faster on Sunday, and this of course messed up the balance, since the fin offset created much more yaw torque than it had at lower speed.

A much more amenable solution would be to remove the fin offset and then put the leadouts where they were supposed to be. This can get the airplane flying tangent without the leadouts and rudder fighting each other and setting up transient yaw motion while maneuvering. In Randy's case, he will end up hinging the rudder, and it will need to end up slightly *inboard* in order to compensate for the offset fin.

The underlying principle is that the rudder/fin offset solely determines the required yaw angle, and the best you can do with the leadouts is to not perturb it by setting them at a compatible position. If you want the airplane to fly tangent to the circle, you cannot have any net aerodynamic yaw.

Of course it won't every be completely perfect, but the microscopic effects of precession can almost be ignored if everything else is right.

Guys,

Trying to respond to you all but first thanks for the numerous suggestions. As I tried to point out, I am not dissatisfied with the overall trim of the model. No doubt things can be improved further but I feel I have reached a satisfactory level here. (BTW this is the same plane/engine which started off with problems leaning out after takeoff and difficulty finding a consistent setting that I reported b4 winter. That problem has been solved, lighter prop -11x5 iso 11x6-, anti foaming agent and upping the nitro content from 5%>7,5%. From that problem I learned that very often you have to look at the obvious things first b4 you start an agressive approach. For instance, to maintain revs after disconnecting the glowplug I upped the compression ratio, tried 'miljons'of glowplugs but it was simply cured by adding a little nitro. Overheating problems which could not be cured with no matter how many holes I bored in the cowl were cured by adding a little castor. Thanks Ted Fancher)

So now with all of the technical and most of the aerodynamical problems solved I am really enjoying flying this model. To try and solve this yawing problem I tried enlarging outboard flap, increasing and decreasing tipweigt(which now nets 1 1/4 oz on the scale Len), adding rudder offset, moving leadouts back. A number of these things mitigated the problem but all of them created new problems elsewhere in the pattern or enlarged existing problems. Especially noticable was a situation where as Brett discribes the mechanical forces of the leadout position are fighting the aerodynamical forces of the yaw trim (I had an adjustable rudder installed).

I know I am biassed towards a neatral trim but this is simply because I don't like excessive line tension and want to have the idea that the aircraft is flying itself through the manouvres. It is purely subjective I know, and I would have to go out of my way if simply increasing line tension solved all problems but it didn't and this is simply what I like.

The funny thing is that shortly b4 I posted my question I noticed your article on the Twistmaster in an old SN and was wandering if you had the same problems as I experienced Bob. You can imagine my surprise that this was confirmed by your reaction. The subsequent discussion where Al Rabe chimed in set me on the course of rethinking about using a RabeRudder. I say rethinking because I had considered it be4 but rejected it because I feared introducing new problems, at the least it is an additional trim item and there are allready so many other things to optimise. (Apart from my flying style that is, I agree that I am tempted to overstear the upper corners because I am comfortably far away from the ground Igor)

So where am I now in this proces of 'objective' experimentation, subjective tastes and psychological induced actions and preferences? The key factor is that by now I am convinced the fuselage side area behind the CG is insufficient, the fin & rudder are way to far forward and consequently I designed and build an aircraft with insufficient inbuilt yaw damping. My fault, last time I will make this one.

Despite my previous statement about simple solutions I decided on making a RabeRudder. It should provide the kick in the right direction on the right moment whilst all other options influence the entire flight. That proved to be much less the linkage nightmare than I also feared Bob, even though, in my case, it needs an extra 'bellcrank' on the outboard fuse side. In fact I made it up last night in 1 1/2 hrs. It links from the top of the elevator to a little bar which pivots on the fuse side and then links back to the rudder (allready had a horn on that side of the rudder). It looks crap but it works (No concours points to be won in europe no matter how BOM I am, talking about preferences and psychology ). I will post pics once I have mastered that art and report back once the proof of the pudding has been flying the model to tell you that all my problems have been solved???????

Thanks & Rgds, Bram

Hi Bram,

Give us a demonstration in Herentals!
By the way I am found of the Rabe rudder as well. Took me about 15 fligths to get it sorted out. It is great when you feel the rudder working for you. Plenty of room to trim your plane 'chez nous'.

For everybody: Herentals Belgium, control line weekend on June 26th and 27th .

See you there!

Paul,

Sure, will be there and looking very much forward too it.

Bram

Picture of the culprit.

That answers a lot of questions. You have an exaggerated precession problem because of a lack of side area behind the CG. A longer tail moment with deeper sides or more rudder area behind the stabilizer should help immensely. Even splitting the elevator and moving the rudder back between the two sections with an added dorsal fin would probably help. That rudder could also be converted into an Rabe rudder for additional help.

As well as sweepback on wing.

Hmm,

Mine has a rather large rudder that hangs pretty far back past the elevators. And while it doesn't have the slab sides of Brett's Infinity, it certainly isn't skinny behind the CG. And my yaw problem is pretty bad. Brett thought it was a rudder/fin mis-alignment issue. We'll see this weekend (the new rudder is just about done and ready to go on the plane). I'm not convinced, but am deferring to Brett since he saw it from outside the circle (and is certainly is a very good observer). This is why I went ahead and cut the rudder off to make it adjustable. If this doesn't do it, I'm about out of ideas.

But I have high hopes...

LAST EDITED ON Jun-04-04 AT 12:39 PM (CDT)
 
>Hmm,
>
>Mine has a rather large rudder that hangs pretty far back
>past the elevators. And while it doesn't have the slab sides
>of Brett's Infinity, it certainly isn't skinny behind the
>CG. And my yaw problem is pretty bad. Brett thought it was a
>rudder/fin mis-alignment issue. We'll see this weekend (the
>new rudder is just about done and ready to go on the plane).
>I'm not convinced, but am deferring to Brett since he saw it
>from outside the circle (and is certainly is a very good
>observer). This is why I went ahead and cut the rudder off
>to make it adjustable. If this doesn't do it, I'm about out
>of ideas.
>

If geting the rudder straight and setting the leadouts accordingly doesn't do it, then you have some other misalignment - like stabilizer "skew". I looked like the classic leadouts fighting rudder problem since it was about the same problem on insides and outsides (also indicating that the problem has little or nothing to do with precession), but maybe not.

The fuse design minimizes the yaw stability. It has a large deep forward fuse around the engine, and I would still rate the rear of the fuse as "skinny" - more than an Ares, but not a lot more. The fin/rudder is reasonably large. I would guess that the lateral CP is pretty far forward and the yaw damping low. Observation supports this - if you get a Nobler-type fuse as sideways as yours was, it pretty much flops right back straight first chance it gets with relatively little overshoot. Yours appeared to continue to oscillate a while after the perturbation was removed. That's probably not what I would prefer, but it's OK; all that means is that you have to be a lot more careful to trim it so theres no perturbations to begin with.

There's nothing at all unusual about the design that makes it intrinsically not trimmable. Trimming is not magic - it follows predictable rules, and almost anything that is halfway close to reasonable can be trimmed if it's straight and rigid.

Brett,

Yea, I was just perseverating some. I haven't seen it from outside the circle, but I believe you're right on (afterall, I wouldn't have cut off the rudder if I didn't think so) and the ability to move the rudder will allow me to trim the yaw out and get the leadouts back in a reasonable position. Heck, just gluing it back on straight would probably do that, but the ability to adjust it should allow me to make the fine adjustments needed after I get it into gross alignment.

Wow, perseverating really is a word. Looks like Randy's retaliating for our jargon.

Yes Howard, it's really a word.

Perseveration

Psychology--

1. The tendency to continue or repeat an act or activity after the cessation of the original stimulus. Continual re-consideration after the fact.

So to is neuron-hosed, though the words are generally not used together.

It's ok Howard, you're probably in a disassociative fugue.

LAST EDITED ON Jun-02-04 AT 10:15 AM (CDT)
 
>>It could probably have gone another 1/4 or so, based on my observations the next day<<

Tried that, Brett. And it did help...sort of. The yaw was less, but I lost the tracking pretty badly. It hunted all over in level flight. Like everything else in trimming, it's all a compromise.

BTW, it went faster on Sunday becuase I was using a prop with more pitch to drill through the wind a bit better. It was pretty breezy.

Strangely, at the controls I could hardly feel the yaw. I could see it was yawing, but I didn't experience any loss of control or uneven-ness in the turn. It seemed to turn fine and fly a very predictable track. One of the problems of not having a knowledgable observer when trimming a new plane is that things like this are tough to pick up. From the handle it's only seems like it's yawing a little. No big deal. But others at the contest noted that the yaw was obvious and substantial.

>>To expand slightly, the fin has a significant offset (maybe 1/4"), and is pretty much the only lateral surface since the fuse is pretty skinny. <<

After getting back, closer inspection reveals that the fin is actually pretty straight. After cutting off the rudder, I used an incidence meter and a laser level keyed to crankshaft of the engine then the fuse sides. This demonstrated that the fin is offset perhaps 1/64" based on a line drawn right down the center vertically and down each side of the fin. The rudder, however, was in fact offset close to 5/16". Like a lot of things, I developed a way to accurately measure this out of sudden need. So the next plane will get this treatment before finishing when the mess is glued on. (duh).

Figures. The only part of the plane that wasn't built in a jig isn't straight. We'll see how it does after I finish up the rework this week. One thing this showed is that this design wouldn't be hurt by more side area and a bigger fin/rudder.

>>>It could probably have gone another 1/4 or so, based on my observations the next day<<
>
>Tried that, Brett. And it did help...sort of. The yaw was
>less, but I lost the tracking pretty badly. It hunted all
>over in level flight. Like everything else in trimming, it's
>all a compromise.

Of course it lost the tracking, because it was flying sideways! But flying sideways is the only way to satisfy the constraints from the offset rudder. Straightening everything out would remove the need to compromise.

>
>BTW, it went faster on Sunday becuase I was using a prop
>with more pitch to drill through the wind a bit better. It
>was pretty breezy.

But it did exacerbate the yaw problem. The same thing happens when maneuvering and the speed changes. The variation of speed results in a varying torque from the aerodynamic forces, and at the same time you are changing the lne tension. This messes up any equilibrium that you might have managed in level flight, and then it's going every which a way.

>Strangely, at the controls I could hardly feel the yaw. I
>could see it was yawing, but I didn't experience any loss of
>control or uneven-ness in the turn. It seemed to turn fine
>and fly a very predictable track. One of the problems of not
>having a knowledgable observer when trimming a new plane is
>that things like this are tough to pick up. From the handle
>it's only seems like it's yawing a little. No big deal. But
>others at the contest noted that the yaw was obvious and
>substantial.

I thought it was hopping pretty good in the corners.

Being able to see the various trim effects from the handle appears to be a sort of gift. Either you have the ability, or you don't. It's not a matter of piloting skill. Many, many people who are excellent pilots can't see what the airplane is doing in the air. They feel something might be wrong through the handle, note that some things are harder than they should be, but can't see why. I think this is why so many people have trouble with things like rudder offset and leadout position - they simply can't see the effects, even when they are grotesquely off.

LAST EDITED ON Jun-02-04 AT 12:34 PM (CDT)
 
Brett,

I think you are right on here. Some things I can tell easily from the handle. Things that involve pitch and roll are generally not too hard (though roll is harder than pitch). But yaw is something I just can't really tell (other than seeing the yaw). I couldn't, for instance, tell you if the yaw is initially to the inside or outside. I just can't feel it and if it isn't effecting my ability to control the aircraft, it doesn't seem like a big problem until an experienced eye sees it from the outside of the circle comes over and asks if I was concerned that the plane was coming for me <grin>.

I'm hoping that with the new rudder arrangement, the problem can be neutralized or at least substantially moderated. As I noted, the fin is pretty straight but the rudder was WAY off. At least I've developed a method to tell next time. I doubt I'll build another plane without an adjustable rudder.

A couple days ago Rick and I went out and as I had fitted a Hayes 3-ounce tank to right side of my first ARF Flite Streak I wanted to fly it. (A Pylon RST-4 had been used in the past, mounted to *left* side of fuselage.)

And another bought-used O.S. 20FP was bolted in place, wanted to test it as well.

Kept thinking of this thread, even though I was not having same problems stated in initial posting.

Model was a dog. Pretty much a Pukey Profile when last time out it was cool.

Too much tip weight. Easily predictable, I'd move about four ounces from left to right side of fuselage. Easy fix.

Motor seemed low on power, removed the air filter.

Model was both light on the lines and appeared to me as banked in very slightly. Rick said it was flying flat, although I noticed when he flew it later it was slightly tip high. Last time I'll ask him for that sort of information.

Added some rudder by attaching trailing edge stock with double-sided tape. Better, but not right.

Took off 7 grams tail weight. Better.

Took off 7 more grams tail weight. Again, better. But still not right.

Fuselage had been broken and repaired, maybe it was pointing in some. With no one to see me do it, added a washer under forward mounting points for engine. Actually, placed the washers under 1/16" aluminum pads, both to cut down on putting a bind into case and so they cannot easily be seen. Not right.

I even had thoughts about line whip, as in I was getting some at certain points in the pattern. Cripes, I hadn't thought about this, or considered it at all since the days when we had to go to .018 lines in Combat and could no longer bang off really cool square eights without getting the .015s out of storage.

Some things don't change. In rolling up the lines they looked pretty big. Grabbed the vernier caliper. Yep, I'd been flying on 60-foot x .018 lines. Hey, the reel and the storage bag both were clearly labeled as the lines on and in being .015s.

Now that model is all screwed up, at least the motor ought to seem a little stronger.

On a different note, I think Brett is right in that being able to see all of what a model is doing seems to be a gift. One I did not get. Nor have I had much success at developing this ability, although I continue to work on it.

I asked PW about this exact problem when Don McClave poked fun at me for the arm-pump effort I was putting into certain hard corners, the model going all goofy on me. But making a real good corner, don't ya know?

At one point I told Paul I'm watching where model is going, in some cases am looking ahead to a specific landmark, flying model to that point whereupon I pick up the model directly. But still look where it is going. Not what it is doing at any given moment in time.

It's been years since that conversation, but my sense of it was that Paul knew exactly what I was talking about, but that he had been flying so much his situation was the pretty much the reverse: "When I fly, *all* I see is the model and what it is doing."

I don't have any desire to get to that point, but it really is nice when I am comfortable with the model being flown and can take the time to actually watch it as the airborne dance is performed.

It's still not easy, and I cannot always make changes which improve the trim of the model.

My way of compensating for this lack of the gift has been to develop an overt willingness to just change stuff. Not exactly willy-nilly, but almost. Coarse changes, fine changes, stupid changes, that-can't-help changes.

Just to see what happens. For too long I didn't take this approach, instead compensating for models in something less than a Galaxy-class state of trim.

Today I would rather fly a model which is out of trim, but I know what is wrong and how to fix it (NW Regionals on Sunday would be one example) than to fly a perfectly trimmed model, one which I have no idea as to how it got that way.

Promised a pix of my RabeRudder setup so here it is. It provides 0 defelction at full up, 5degr at neutral and approx 15 degr at full down. Don't be to harsh on Pandorra, she does fly the pattern you know.

That was a clean, slick installation. Did the moving rudder help fix the yaw problem??

Phil,

That remains to be seen. Have to fly it yet. I did make up a number of pushbars to be able to adjust the system. RC style clevises would be handy but for the time being this will do.

LAST EDITED ON Jun-09-04 AT 08:48 AM (CDT)
 
To round it all up. I flew my ship today under approximately the same conditions as was mentioned in the original post. Very little or no wind so behaviour can be compared. Only a few flights were made so there sure is more trimming to do be4 the raberudder is optimised.

One thing was marked though and that was the absence of the violent inward yaw during that specific part of the pattern (top right hand corners of any outside square). I am glad with that because now this part of the pattern becomes less hairy.This seems to confirm that my ship has very little inbuild yaw stability.

Overal yaw stability and line tension were down from the original setup though so more playing around with the rudder, leadout exit, tipweight and what have you will be neccesary. It all comes at a price.

So far I am glad I tried this. And Bob Reeves, I would certainly give it a try on your Twistmaster. As can be seen from the photograph, installation can be done relatively simple. Thanks for the assistance.

Thanks for the update, if I ever finish getting my new shop setup and organized I will try your rudder setup on the TwistMaster. Pretty ingenious...

Thanks Bram. Now maybe I'll have to try Al's rudder on may latest creation, or at least a trimmable rudder. It has some yaw in the same places you mention, but most of the time I can keep it under control using placement relative to the wind and judicious handle movement. Whatever the cause(gyro effect or line flailing around) some right rudder at the right time should help.

LAST EDITED ON Jun-09-04 AT 04:28 PM (CDT)
 
Bram,

Unfortunately, this airplane happens to be configured to demonstrate yaw in maneuvers. Not only in the tail moment arm much too short for stability and fuselage side area grossly inadequate, but the airplane also appears to have a constant chord wing. Constant chord (very large) wing tips produce a great deal of lift and are therefore very sensitive to yaw effects which cause a noticeable differential in the relative speed of the two wing tips with a resulting differential in the lift of the two tips which causes a severe roll in addition to (or because of) yaw.

Longer fuselages with more side area aft and tapered wings will minimize yaw and roll to the point that a movable rudder may not be required. In any case this airplane is rather extreme in having little yaw stability. Adding a movable rudder is, at best, an inadequate solution to the airplane's problems. You should expect some, but not necessarily, dramatic improvement.

Movable rudders, properly installed and trimmed to provide correction only, should do no harm to any airplane, no matter how stable it may be in design.

As for likely undesirable roll from the rudder being installed above the fuselage thrust line, Yes, there is a rolling moment imparted with rudder deflections but normal, small, movements of the rudder usually cause no DETECTABLE adverse roll. the BBQB Bearcat has an 11" tall rudder, all of it being located above the airplane's thrust line with absolutely NO OBSERVABLE IMPARTED roll from rudder deflections. Doubtlessly, there is some there but if I can't see it, I don' worry about it.

Al,

You are absolutely right about the designflaws. In the next version the fuselage will be lengthened (moment arm from hingeline to hingeline now is 13,5 inch and will grow with 1 inch) but I will also use a conventional split elevator setup with the rudder trailing edge lined up with the elevator t/e. Also a tapered wing leading edge will be used. All the simplifications in the current aircraft were made with the specific aim to produce a new aircraft fast, now I have it, I can invest more time improving the design.

As of now I have flown the plane again with smaller rudder deflections. I was kind of surprised about the effectiveness of the rudder in removing the unwelcome characteristics I described in my first post but as I said, overall yaw stability and linetension suffered. (Which seems to me to make sense because now the rudder is constantly changing the A/C's yaw caracteristics in a situation where there is little or no inbuild yaw damping).

Now I have the rudder set up between a few degrees inboard/full up, neutral/neutral and some 7 degrees outboard/full down and I think I am getting close to the optimum. Overall stability is better but the specific problem has returned albeit to a much lesser degree. So I am a happy user of the RabeRudder! Indeed I wasn't worrying about the imparted roll due to rudder deflection thinking about your Bearcat design which has a very tall rudder well above the centerline.

Bram,

At 14.5", I think that is still an inch shorter than Nobler which is considered short by modern standards. I'd think more like 15 - 16" with the rudder moved aft until the rudder hinge line is aligned with the elevator trailing edge. And for insurance, build a tapered wing.

On another note, it's decided that the sloped forward trailing edge on my elliptical plane is probably inducing the yaw problems. the rudder and fin are just not big enough to dampen it out. So, I'll live with it, perhaps put in a big of wiggly linkage and try that and the next plane will have a bigger strake and fin/rudder.