Directional Effect of Elevator Asymmetry

Brett Buck, the national champion, is enthusiastic about elevator (or maybe stabilizer) asymmetry on stunt planes. I forget why or which direction. I wonder if it would work as a Rabe rudder. I talked to Gary Letsinger at our club banquet last month and mentioned to him the idea of moving elevators in opposite directions for roll control of transport airplanes. He said that it would cause a heap of proverse yaw: a left roll input would cause a left yaw moment. What if the elevators move together, as on a conventional airplane, but one is bigger than the other, as Brett proposes (I think)?

Howard Rush

Howard --

Interesting -- I was doing some measurements a few weeks ago, and Dee Rice's original Oriental plans show a longer inboard tailplane than outboard. The Brodak ARF Oriental is symmetrical, both tailplane and wing.

Larry Fulwider

I put an extra one inch on the outboard side of the tailplane of combat jobs for years. The intent was to make 'em roll a little outboard on both inside and outside turns.

Not enough to where you see it roll, just maybe a little more line pull than the other guy. It seems to work OK for me.

You do the same thing with tip weight, but weight is weight. Now that I'm out of combat, the secret can be told.

>I put an extra one inch on the outboard side of the tailplane
>of combat jobs for years. The intent was to make 'em roll a
>little outboard on both inside and outside turns.
>
>Not enough to where you see it roll, just maybe a little more
>line pull than the other guy. It seems to work OK for me.
>
>You do the same thing with tip weight, but weight is weight.
>Now that I'm out of combat, the secret can be told.

Except that Brett's philosophy is just the opposite (and correct, I believe, for our precision stunt purposes where we do not want any roll). We add more flap on the outboard tip. Why? Well there are several competing suggestions, more than one of which may be correct. And if there is more than one force involved, well, they just add. But the effect of putting more flap on the outboard panel (mostly at the tip) is to keep the tip from dropping in the corner.

Brett's idea is to add more elevator (probably stabilizer and elevator, but elevator is what is needed) to the inboard side which would have a similar effect of keeping the plane from rolling to the outside. Since Brett's theory is that some (if not all) of the roll is coming from the tail--especially if we use wing assymetry, because we normally do not move the tail plane in with the wing--offseting the tail inboard would negate or eliminate that effect.

OK, care to try it? Brett only builds a new plane every ten years. We need someone to put several of these together and compare it with adding flap area to the outboard panel.

My take was that is was to neutralize the uneven (lift) forces created over the stab/ele. Full Scale planes do all this through various rigging and everything is adjustable.

Sounds like a good spearment for a take apart tail plane--Hmmmm...I know I've seen something like that ...

Curt

Leonard wrote:

. . . .
>Brett's idea is to add more elevator (probably stabilizer and
>elevator, but elevator is what is needed) to the inboard side
>which would have a similar effect of keeping the plane from
>rolling to the outside. Since Brett's theory is that some (if
>not all) of the roll is coming from the tail--especially if we
>use wing assymetry, because we normally do not move the tail
>plane in with the wing--offseting the tail inboard would
>negate or eliminate that effect.
>
>OK, care to try it? Brett only builds a new plane every ten
>years. We need someone to put several of these together and
>compare it with adding flap area to the outboard panel.
. . . .

It seems like a flapless design would be an ideal test bed, as any effect of flap fiddling would not influence the results. "First try" experiments should be OFAT, not DOE, whenever feasible.

Let's make Ferocious try it, as he is doing a series of flapless experiments anyway. And he'll try anything once. A three position or infinitely locatable tailplane (longer inboard, centered, longer outboard) with a quick- change bolt-on arrangement would make a good test bed.

I think it would be easier go at this using a "reductio ad absurdum" approach and first try a significantly longer OUTBOARD panel tailplane on the movable tailplane testbed. See if that makes things worse. If it does, we are on the right track, and have 95% proven the theory. It is a lot easier to detect "badness" than "goodness"!

As a side note, the original Dee Rice Oriental had both a longer inboard wing and flap, and a longer inboard stab and elevator. Harder to say which effect is greater, or if they cancel, or what. That is why I favor the flapless experiment.

Larry Fulwider

A way to really show what happens is to make a tailplane with seperate flippers and horns on each side. Then you could fly it with the inboard connected, the outboard connected, both, and neither. See what happens. (do neither last).

>A way to really show what happens is to make a tailplane with
>seperate flippers and horns on each side. Then you could fly
>it with the inboard connected, the outboard connected, both,
>and neither. See what happens. (do neither last).

Very clever! However the key to keeping it O-FAT is to not change more than One-Factor-At-a Time. In your experiment, we would be halving effective elevator area, a huge additional factor, so it is now a TWO-FAT experiment. You can't be sure which of the two factors is causing the change.

Larry Fulwider

You could also make them adjustible for throw to play with different percentages.
Bruce

Cool. Differential elevators.

>
>Brett's idea is to add more elevator (probably stabilizer and
>elevator, but elevator is what is needed) to the inboard side
>which would have a similar effect of keeping the plane from
>rolling to the outside. Since Brett's theory is that some (if
>not all) of the roll is coming from the tail--especially if we
>use wing assymetry, because we normally do not move the tail
>plane in with the wing--offseting the tail inboard would
>negate or eliminate that effect.
>
>OK, care to try it? Brett only builds a new plane every ten
>years. We need someone to put several of these together and
>compare it with adding flap area to the outboard panel.

Leonard;

What is the delta in terms of area ratios between the inboard and outboard elevator/stab?

V/r

Bob

Bob, you asked

>Leonard;
>
>What is the delta in terms of area ratios between the inboard
>and outboard elevator/stab?
>
>V/r
>
>Bob

I am not Leonard, but the question is, is it a non-zero delta? We don't know that for sure yet, or if it is, whether it is positive inboard or outboard. I think. Common sense tells us it is non-zero, but that is about it.

You may have asked the most troubling question, tho. If we can prove we can accomplish a Rabe Rudder effect with varying tailplane areas, how do we make trimming adjustments? Not as easily, obviously, and I can imagine a change in props (and many other things) requiring an adjustment.

So, as a practical matter, if this method is as, or more, effective than a Rabe Rudder, applying it is a whole new problem.

Larry Fulwider

Assuming a wingspan of 54" and a tailspan of 24", the asymetry of the tailplane probably doesn't mean much. Moving the Center-of-Lift of the tailplane 1" either way wouldn't be able to roll the whole airplane much.

A little tab out by the wing tip rolls it a whole lot. Maybe they should consider putting the ailerons out there?

(snip)
>So, as a practical matter, if this method is as, or more,
>effective than a Rabe Rudder, applying it is a whole new
>problem.
>
>Larry Fulwider
>

This is not taking the place of a Rabe rudder. The Rabe Rudder is not intended to roll the airplane, but to correct for yaw induced by the effects of the prop. The assymetrical tail plane (and especially elevator) is intended to compensate for the dropping of the outbord wing (or lifting of the inboard) in a hard inside turn (and the opposite on an outboard turn). This dipping of the outboard wing occurs because (insert your own reason here). Brett mentions wing assymetry, where we make the inboard wing longer (actually move the fuselage outboard) because of the increased speed (and therefore lift) of the outboard wing in comparison to the inboard wing. When we have wing assymetry such as this, the tail plane is normally moved outboard with the fuselage and he feels (perhaps correctly) that this influences the trim by helping to induce this dipping tendency.

For those who prefer equal panel wings (or fuselage centered on the wing, and I am one who prefers such) this dipping of the outboard wing still occurs in a hard corner. I blame it on the lagging sweep of the lines, which sweep behind the plane in level flight, but actually pull up (or down) on the inboard tip in a hard turn due to the fact that they are lagging behind the turn.

In any case, adding flap area outboard--especially at the tip, helps to correct this. And moving the stabilizer inboard, as Brett has suggested, may help to accomplish the same thing.

And moving it outboard would just make things worse!

Leonard --

You corrected me, saying:

>This is not taking the place of a Rabe rudder. The Rabe
>Rudder is not intended to roll the airplane, but to correct
>for yaw induced by the effects of the prop. The assymetrical
>tail plane (and especially elevator) is intended to compensate
>for the dropping of the outbord wing (or lifting of the
>inboard) in a hard inside turn (and the opposite on an
>outboard turn).

Read the original post! Howard's original question is clear, and he asked if tailplane assymetry could replace a Rabe rudder! That was not MY suggestion -- it is the question on the table as asked by Howard. If you read further in the original post, you will see that Howard (Brett's press secretary in this case -- we haven't heard from Brett in this thread) is totally focused on YAW, not ROLL.


>. . .In any case, adding flap area outboard--especially at the tip,
>helps to correct this . And moving the stabilizer inboard, as
>Brett has suggested, may help to accomplish the same thing.

>And moving it outboard would just make things worse!

Same problem as before. You are stuck on the roll aspect. That was NOT Howard's question.

Everyone is sure that moving outboard is wrong, but if we prove that -- making both roll AND YAW worse, we have an indicator that the tailplane can also function as a yaw control.

I suspect you snap-rolled your Luscomb once or twice. How did you do it? You pulled the stick back and kicked the RUDDER hard (the YAW control device) to induce a violent ROLL.

I suspect you have landed in a crosswind. Anyone who has learned to control his airplane in a crosswind landing knows that Mr. Yaw and Mr. Roll are on speaking terms, and not too distant of relatives.

I suspect if you ever landed an Ercoupe crosswind, you strained the cable connectors to their stress limit trying to get the airplane to fly the way it is "supposed to." In the Ercoupe, with integrated yaw and roll, you easily see how clumsy it can get.

My comments you quoted were related ONLY to HOW we apply this technique IF we prove it is a valid approach to the yaw precession question.

Larry Fulwider

>Leonard --
>
>You corrected me, saying:
>
>>This is not taking the place of a Rabe rudder. The Rabe
>>Rudder is not intended to roll the airplane, but to
>correct
>>for yaw induced by the effects of the prop. The
>assymetrical
>>tail plane (and especially elevator) is intended to
>compensate
>>for the dropping of the outbord wing (or lifting of the
>>inboard) in a hard inside turn (and the opposite on an
>>outboard turn). (snip)
>
>Read the original post! Howard's original question is clear,
>and he asked if tailplane assymetry could replace a Rabe
>rudder! That was not MY suggestion -- it is the question on
>the table as asked by Howard. If you read further in the
>original post, you will see that Howard (Brett's press
>secretary in this case -- we haven't heard from Brett in this
>thread) is totally focused on YAW, not ROLL.

It is not a matter of who asked. It is a matter of answering the question. We are not trying to induce roll input. We are trying to counter roll input (from whatever source). Brett's suggestion was to keep the wings from wagging, if you will. It was to correct and counter the roll. The Rabe rudder is to correct and counter the yaw. Two different things--but only if done properly. Either, if done to excess could cause other problems.

(Any "solution" that causes the plane to roll right and left could also cause the plane to yaw right and left. This could produce a big "wiggle" at the interesections of the eight.)

>>
>
>This is not taking the place of a Rabe rudder. The Rabe
>Rudder is not intended to roll the airplane, but to correct
>for yaw induced by the effects of the prop.
this is true, however dependent upon the balance of rudder area above and below the CG vertical height (or more simply but less accurate the thrustline) the rudder will introduce some assymetry when acuated due to differential loads above and below the thrustline, so roll can be induced this way, also two channel RC planes create roll by rudder when turning.
FWIW

Mark,

I'd always heard, and trimmed accordingly, that 2 or 3 ch RC (rudder- elevator or rudder-elevator-throttle) rolled on rudder due to dihedral. The 'advancing' wing in effect gains angle of attack, and the 'retreating' wing loses AoA.

There may be some advantage to using the (statically)unequal-panel approach from Martin Hepperle, or even my simplified formula, for unequal panel stab/elevator proportions. However, the airflow aft of the wing is pretty churned up after getting through the prop disk, across the wing, and still with some prop blast rotation, back to the tailfeathers.

A stunter's horizontal tail surfaces are not isolated, ideal, wind-tunnel specimens in clean flow...

To others who fear direct yaw effect from a tail group that measures different inside from outside, when it isn't moving: remember that the 'lift' load does vary across the span. If we can put the compensation into the dimensions right, it should NOT have an effect.

It occurs to me that NOT using a longer inboard stab/elev panel might have the reverse effect - that the 'equal-panel' outers act as though they are larger!

But, again, given the roiled air reaching that end of the model, how much is enough, and does the effect of the fuse centerline not being on the same streamlines (think radius from the handle) as the CG, matter?

>
>>>
>>
>>This is not taking the place of a Rabe rudder. The Rabe
>>Rudder is not intended to roll the airplane, but to
>correct
>>for yaw induced by the effects of the prop.
>this is true, however dependent upon the balance of rudder
>area above and below the CG vertical height (or more simply
>but less accurate the thrustline) the rudder will introduce
>some assymetry when acuated due to differential loads above
>and below the thrustline, so roll can be induced this way (snip)

Yes, if the rudder has considerably more area above the center of vertical gravity (leadouts, usually) then it could induce a rolling effect.

>also two channel RC planes create roll by rudder when turning.
>FWIW

You can do this in reall life with a full size airplane, too. When flying cross country I would often fly with just my feet on the rudder pedals and make slight course corrections which would bank the plane as it turned it. The cause of this, however, is the fact that in the turn the outside wing is going faster and thus creates more lift. In our situation no matter how much rudder we give it, the plane is still going to turn left. It may yaw out (0r in--ouch!) but it is still going to turn left. So the moveable rudder would induce pretty much negligible effects this way.

Lenoard,

Wasn't Brett trying to keep the CL's of the wing and tailplane more perpendicular to each other, i.e., aligned along the longitudenal axis of the airplane....? Seem like a logical want/need to me!

Jim Pollock [photo not recovered: sipcof.gif]

>Leonard,(edited for spelling)
>
>Wasn't Brett trying to keep the CL's of the wing and tailplane
>more perpendicular to each other, i.e., aligned along the
>longitudenal axis of the airplane....? Seem like a logical
>want/need to me!
>
>Jim Pollock [photo not recovered: sipcof.gif]

I think that was his idea. But even a plane with equal panels benefits from a larger outboard flap. As such, one would still need a larger inboard stabilizer (and elevator) to accomplish what Brett is suggesting doing. This is why I still stand by my theory as being at least partially (and perhaps a major) factor in why the outboard wing drops in a hard turn.

Len,

Hmmmm, I thought hinging was caused by using a 1.5 ounce tip weight and doing 15G turns. 1.5 X 15 = 22.5 ounces of tip weight in the turn.
The extra flap is suppose to help hold the tip weight from doing the hinging. Aligning the CL's of the wing and stab should help prevent creating a rolling tourque from applying the elevator. Boy this circular aerodynamics can get complicated!

Jim Pollock

>Len,
>
>Hmmmm, I thought hinging was caused by using a 1.5 ounce tip
>weight and doing 15G turns. 1.5 X 15 = 22.5 ounces of tip
>weight in the turn. The extra flap is suppose to help hold
>the tip weight from doing the hinging. (snip)

If we have an ounce and a half extra, we probably have way too much. The tip weight is supposed to balance things out--let you fly level when you aren't doing maneuvers.

Some people's planes hinge too much and they take tip weight out because of this. But then they aren't balanced. Adding outboard flap enables them to carry more (and needed) tip weight.

My theory is that it is not the tip weight, nor even primarily the offset tail (which could contribute to the problem when you have a lot of assymetry on the wing). I blame the lines. In level flight the lines rake back due to drag and the leadouts are moved back to match this line rake. However, in a turn, the line rake is up (or down) and we can do nothing to match this. As we approach the bottom right hand corner of the square, the airplane is diving straight down towards the earth. The lines are raked behind the path of the plane, which in this case is "up". Then when we make the turn, the lines do not follow the turn as quickly as the plane turns. In other words they are still "up" while the plane is turning towards level flight. It isn't that the outside tip drops, but that the inside tip does not. Adding area to the outboard flap--especially at the tip--adds lift to compensate (in other words, both tips are "pulled" up.)


>Aligning the CL's of the wing and stab should help prevent
>creating a rolling tourque from applying the elevator.
>Boy this circular aerodynamics can get complicated!
>
>Jim Pollock

With added assymetry on the wing, you may create an additional problem with the stabilizer not being offset as well. But consider the equal panel wing set-up. This is what I (and Matt) prefer. All of our designs have equal panel wings.

Now, it has been determined that the outboard wing flies faster than the inboard, so the center of lift is outboard of the fuselage on an equal panel wing (like maybe 3/4 of an inch). This would be true of the tail as well, but not so much (maybe 3/8 of an inch). That means the tail is already leveraging in the opposite direction on an equal panel wing. And we still need that extra tip area to keep the wing from waving in the corner. So, it is not just the tail (or in this case even the tail) that is causing the problem. And it is not tip weight, because that is balancing the plane with the leadouts and half the weight of the lines. But in the corner, the tip weight will increase and the line weight will disappear as it pulls up. And there, as I see it, is the problem. Still, moving the stabilizer inboard might accomplish the same thing as adding area to the outboard flap at the tip, although I think adding the area to the flap at the tip is going to be far more efficient.

Imagine that your plane flies in level with perfectly matched CG with AC of wing and also AC of tail and with firm lines. If you pull up, then model points nose up and no rolling moment comes from controll surfaces, lines etc.

BUT before you started that maneuver, your model HAD some angular speed because model flew on ciccular path with center at pilot (outboard wing quicker then inboar wing). So its mass rotates around vertical axle. If your model after that maneuver points nose up, then that rotation is translated to roll (Brett wrote several times about that transition). Means that the outboard wing is still quicker than inboar wing, but now it is roll as the model points nose up now. So if wing does not make extra lift on outer side of wing, then it must roll during and that maneuvre and the only solution is just add little bit of lift during the transition by larger outboard flap.

igor

Howard,
I haven't tried actual asymmetry of the elevator or stab, although it's an interesting question. I have occassionally in the past used elevator tweeks (outside elevator tweeked down at neutral) to achieve a reduction or elimination of yaw presumably from precession. This usually works as a last resort. It requires a retrimming of the average neutral position of the elevator so it isn't always a "quick fix". This does to some extent act the same as (at least theoretically) asymmetry on the elevator in that it gives more effective control force on one side than the other. If the outside elevator is tweeked down it tends to give some of the same effect as the Rabe Rudder in that the outside elevator then creats a yaw to the outside (right) on down elevator and a yaw to the inside on up elevator. (is that right?)
At any rate in practice it works fairly well if needed.

Edited to correct yaw direction (these things always confuse my "weak mind", think it's right now).

Just for reference:
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=251985&mesg_id=251985&listing_type=search#252006

Derek

I didn't write the original post in this thread very well. I didn't mean that elevator asymmetry could replace the function of the Rabe rudder. What I should have said was that elevator asymmetry might affect the effect on yaw of elevator deflection(cn delta e, with appropriate subscript levels). Beyond that, I hadn't thought much about it, preferring to have you guys do the thinking and tell me what would happen. Thanks for the thinking. Thanks, too, Derek, for finding the earlier discussion.

I'd reckon that adding elevator on the left side would add an increment of right yaw in outside corners and a left yaw in inside corners. This is for a conventional airplane with the vertical stabilizer on top; it wouldn't apply to Geneses or Tucker Specials.

Thanks, Randy, for bringing up elevator tweaks. I've used them, thinking they'd do the same as tiny flap tweaks, but now I'll be wary of the yaw effect. I can imagine that raising the left elevator relative to the right elevator might act like left rudder, but how the rudder effect varies with elevator deflection is too hard for me to figure.

Howard Rush

I used to build Combat planes that way. They flew pretty well, my idea was that if you had more area outboard it would produce more drag and yaw the plane out especially in a turn. Kinda like John Jo's outboard streamer mount. There was one "drawback", however. I used aluminum arrowshafts. Plant the plane in the ground and you'd end up with a bent tail boom. I put "disadvantage" in quite marks because the planes actually seemed to fly BETTER after the alteration.

>Brett Buck, the national champion, is enthusiastic about
>elevator (or maybe stabilizer) asymmetry on stunt planes. I
>forget why or which direction. I wonder if it would work as a
>Rabe rudder. (snip)

Taking this tread in a slightly different direction, I have often thought about (and then rejected) the idea of a purposely tilted stabilizer. We get that now and then by "accident" suring the building process, but I am talking now about setting up a tilted stabilizer on purpose. By tilting the stabilizer you get a little bit of rudder effect, and by tilting it with the right side high you would have (somewhat) of the effect of a Rabe rudder.

The big problem, however, is that with the proper Rabe rudder set up you get more outside movement and less inside movement. With the stab it would be equal both ways. And it would be hard(er) to make it adjustable.

look post #3:
http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=24365&mesg_id=24365&listing_type=search#24370

>The big problem, however, is that with the proper Rabe rudder
>set up you get more outside movement and less inside movement.
> With the stab it would be equal both ways.

Leonard,

Has someone tried combining the stab tilt with an off-square hinge line (outboard end of hinge line - or complete horizontal tail - yawed aft)? On outside turns the effects, theoretically, would be additive. On inside turns these yawing effects would tend to cancel each other.
I suppose trimming would involve finding the right combination of offset angles.

Randy

I actually did just as Howard suggests. In my last years Nat's plane I had a larger outboard elevator than the inboard. Not a lot but full span and about 3/32". I was looking for yaw and roll. I noticed several in this thread have not taken into account that the Rabe rudder will in most cases also induce roll. Most rudders do. I am building this year’s plane with equal wing panels with the same elevator arrangement as last year’s plane. No empirical data as most of us can never put our fingers on what actually does what from plane to plane because of the variables. If someone were to do the test with one plane and come up with some data it might be useful. I felt that the extra area on the flap was doing more harm than good so I felt that the elevator would do the same only in a smaller increment with the added effect of yaw, a bonus on outsides, hopefully canceling out GP a little. The flaps seem to keep me moving LO position and CG based on conditions, density and wind. If I didn't chase it corners got ugly. This last years plane seemed better and I did not chase it as much. I have also felt that with my going to bigger engines and props with the same size plane that this would be something to counter the effects, a little maybe?
I got most of this idea from flying quickie 500 pylon with V-Tail. The V-Tial is operated with seperated servos on each elevator and worked independant of each other and then mixed with in the radio program. We also used this to trim how the plane did a hard high g turn. We would imbalance this throw to bring the nose up or down and keep the 90°turn flat.

Disclaimer: this was all a guess on my part when I decided to try this. Howard is the Aero Engineer!

RO

Randy B, to your Post #25:

IMHO, the less we distort things from the best aligned to the slightly curved airflow the models fly through, the better it is likely to be.

Canting a hingeline, or entire horizontal tail, to get a 'rudder' effect, would have to be done right, at the gluing phase, or have measurable adjustability built-in.

Why would you want a rudder effect, anyway, unless you are building a very unlikely fuselage - like the beautiful, but usually un-stunter-looking planes of Al Rabe?

Consider a swept-back tail hingeline (i.e., inboard and outboard hinges both swept back.) It would seem to have a sort of sea-anchor drag effect to keep the model from yawing, wouldn't it? That would work - if it does at all - on both UP and DOWN control motion.

Linkages get complicated, and what certain benefit are we after? Makes for interesting musing, but between the trained and the amateur experts on aerodynamics and the rest of it, it still takes field test and refinement. And to make it a winner, it takes practice, etc., etc.

I install the stab with the outboard tip slightly(1-2 deg) forward of the inboard tip. Seems to help with changes of direction. The theory being that it lines the stab up with the actual direction of the airflow coming off the wing.

>I install the stab with the outboard tip slightly(1-2 deg)
>forward of the inboard tip. Seems to help with changes of
>direction. The theory being that it lines the stab up with
>the actual direction of the airflow coming off the wing.
>from Phil C

Phil,
What type of model have you done this with?
Would you give more details of the effects?


Randy

I've done it in the other direction on combat planes for the same reason. Perhaps Phil wrote that with the wrong sign.

Howard Rush

As all us Free Flight guys can tell you, we use stab tilt as the primary method of inducing glide turn (on models without moveable rudders). Trouble with your hypothesis, I believe, is that when elevator is deflected the yaw will be in opposite directions when elevator is up compared to down. Depending on which way the tailplane is tilted the yaw would be one way with "up" and the other way with "down" elevator.

Has anyone in recorded history tried moveable ailerons to control lift-induced roll? I can envision a small moveable tab at one tip (probably outside, more effective tip) mechanically connected to the bellcrank or pushrods to oppose the roll force. May be more trouble than it is worth, but this is one of the un-exorcisable demons we have been battling for close to 60 years in this hobby, so perhaps worth a try!

Mike,

Exactly what we are looking for! Helps in countering precession during hard maneuvering.

Jim Pollock

Hi Jim:

Maybe this could work! Lots of hare-brained ideas turn out to be effective and this could be one. If we want yaw in one direction on insides, and in the other direction on outsides, stab tilt could do it. Of course it's not the stab tilt per se but the yaw force created by deflecting the elevators attached to the tilted stab that produces the force. Whether the small force needed to turn a FF model at a few mph just above stall would translate into a bigger force at 55mph sufficient to yaw a 60oz model is anyone's guess, but I think it is worth hacking up a profile Cardinal or similar to find out. As always there is the question of how to quantify and measure the results. A moveable trim tab connected ala a Rabe rudder could be easier to build and be more effective, sitting out there in clean air with lots of moment arm. Hmmm. I DO need a new Cardinal for P40 this season...

The problem with all of these other "solutions" is that we don't need as much push in one direction as we do in the other. Thus a properly set up Rabe rudder turns out more than it turns in. Tilting the stabilizer or moving one leading edge forward of the other would give equal "kick" in both directions.

And it is not adjustable (or at least not easily adjustable). So while we are thinking of all of these other ways of compensating for what a Rabe rudder would do, I wonder why we don't just stick with the Rabe rudder. It works. End of story.

>The problem with all of these other "solutions" is
>that we don't need as much push in one direction as we do in
>the other.

That is true for Rabe rudder (elevator coupled rudder), but not an absolute truth. The gyro moment is both ways the same so the perfect cancellation will also need symmetric deflection.

The Rabe rudder is abble (from its nature) to cancel aproximately half of that effect, becasue the gyro moment is linear to angular speed (pitching), but elevator deflection is NOT linear to that angular speed because of mass of the fuselage (must be accelerated and stopped).

So while the Rabe rudder cancel only fraction of that movement, we have lines in play with asymentric effect and thus also optimal rudder deflection is asymetric.

I did "perfect" solution (means rudder deflection linear to angular speed) ... I wrote it here: http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=63522&mesg_id=63522&listing_type=search

The most important point of that thread is, that optimal deflection was SYMMETRIC.

Lot of great ideas here. I have used this idea a few times and found it to be very useful when adjusted correctly. The best set up I have found was the one that made the plane fly as well as possible. The number of trim combinations available to us now are staggering and it does take a couple of flites to get each plane set up to its' maximum potential. RJ

Igor,

Thank you for reminding us of that excellent experiment. What was the frequency response of the servo?

Howard Rush

It was GWS PICO BB servo, speed 0.16/60 deg ... the deflection was under 15 degrees, so I expect pproximately 0.05s from neutral to end position.

Here is a recent experiment performed by my flying partner. Not only is the elevator asymmetric----so is everything else. In case it is not obvious in the photo: engine is not on centerline, and has a fair amount of out thrust; elevator is entirely outboard of engine, stopping at the apex of the TE; the elevator hingeline is parallel to the trailing edge which is highly tapered. I don't know about tip weight, but I suppose it has some?

How did it fly? Just great! Didn't seem to have any trim problems at all. Kind of like flying a normal airplane...after all the tweaks are in....

Dave Hull
LA_Flyer

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