Mass Balance of Ailerons

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Fenix

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I am in the final stages of construction of a pair of custom tapered wings for my RV-4. (To answer now what is likely to be inevitable questions: I contracted with a structural engineer to do a stress analysis/design but have not done any performance analysis - that will come in flight.) I just have to paint them, balance the painted ailerons, and then do the wing exchange from the original wings to the tapered wings.
My question relates to the mass balancing of the ailerons. The standard RV-4 (and 6,7 and 8) ailerons have a 1/2" galvanized water pipe attached to the leading edge of the ailerons that runs the length of the aileron so the mass balance is all along the the aileron length, not at the outboard end as is typical with "hammer type" balances (such as is used on the elevator of the RV's). This length of pipe gives an approximate balance of the aileron. (I'm sure Vans tried different diameters and wall thicknesses until one was about right). On my plane the trailing edge of an aileron (with the actuator rod disconnected) weighs about 4 ounces. So they are not 100% balanced.

My plan for the tapered aileron was to totally balance them so the trailing edge of the aileron, when hinged and with the airplane in the fight attitude, weighs zero or very near zero when painted.
I will accomplish this by having an "adjustable weight" balance instead of the galvanized pipe that weighs whatever it turns out to weigh. In place of the galvanized pipe, I have installed a hollow aluminum tube, which, like the original galvanized pipe design, also runs the entire length of the aileron. In this tube I will slide a solid mild steel round bar and fix it in place with some set screws. I can adjust the length of the solid bar to get the exact balance I want which, as stated above, is "neutral" (comments by any who suggest that I balance them at something other than neutral are welcome.)

My intent was to locate this "mass balance" near the root end of the aileron instead of near the tip end, as is common, because at the root the distance between the hinge point and the leading edge of the aileron, and therefore my aluminum tube, is greater. The arm (distance between the hinge point and center of the aluminum tube/mild steel bar) gets progressively less as you move toward the tip due to the tapered shape of the aileron of course. Putting the mass at the root of the aileron means the mass can be less, thus saving weight.

However, in watching Sonja Englert's video series on A/C design she states the weight can be along the entire length of the aileron (as in the standard RV 4) or it can be at the tip end (as is common on the aforementioned "hammer type" mass) but she seems to indicate a problem with the mass being at the root end of the aileron. She does not elaborate on this and I'm not clear even if in fact it is a problem to put the mass at the root, and if so, is this true in some arrangements but not in others. So I am seeking some input as to whether or not it is OK to put the mass at the root end of my aileron, or if I need to instead put it at the tip end, which will increase the mass required to balance the aileron. AND of course, being the curious type, WHY?

Thanks for any input!
 

Hot Wings

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Though experiment to illustrate:
Take 2 yard sticks and hinge then together along the length to simulate the wing and aileron.

Cut off the 'aileron' stick by about 6 inches so you can grab the 'wing' stick.

Wave the assembly up and down like a flapping wing.....Lots of aileron flutter.

Now add a single localized counter-balance weight to the 'aileron' stick in such a manner that it can be moved along the 'span'.

Start with the weight near your hand (root of the wing) and flap the assembly. The aileron still flutters. If you move the whole assembly up and down, like an up/down draft then the 'aileron' doesn't move relative to the 'wing'.

Now start moving the weight out along the span in increments and repeat the flapping motion. The 'flutter' will decrease as the weight is moved outboard.
++++++++++
The counter-balance is there to offset the inertia of the aileron around the hinge. If the wing didn't flex, and the aileron didn't twist, then the single localized weight could be located anywhere along the span. But wings bend under load and ailerons are not 100% torsion-ally stiff.

With a torsion-ally stiff aileron the best place for the weight is at the tip. But ailerons are not 100% stiff and the weight at the tip leads to vibration problems along the aileron. By distributing the weight span wise the aileron can be built lighter/more flexible. Finding the sweet spot of weight and complexity varies with each design.
 

BJC

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Flutter is a complex subject. Ailerons can flutter in (at least) a couple of modes, and wings have exhibited at least three modes. In all cases, the wing and the aileron can interact.

In cases where the node line along the span of the wing falls aft of an aileron tip mass balance weight, the weight can drive wing flutter. The Stephens Akro / Laser wing had that problem. The fix was to move the mass balance closer to the center of the [edit - originally wrote “wing”] aileron where it was well behind the node line.
As HW commented, the torsional stiffness of the aileron is a factor also, especially wrt aileron flutter.

There is no definitive answer that can be provided to your question without much more information about the wing and aileron design. However, if you have an aileron that is constructed similar to an RV aileron, I would start with 100% balancing located close to the center of mass (spanwise) or along the entire length in proportion to the spanwise mass distribution. A variation that is used for the Glasair Super II elevators is to divide the balance mass between the two ends of the elevator.


BJC
 
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Fenix

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Thank you guys for your input and HW for your detailed illustration.

I gather that I did correctly interpret the the mass balance at the root of the aileron only is prone to be problematic.

From reading your input I see that putting it at the tip of the aileron, which I was planning on doing after watching Sonja's video, isn't a "sure fire" fix either.

It seems the most conservative/practical options without a lot more information and analysis and/or testing would be to put half of the mass at the tip end and half at the root end. This is easily achievable given the setup. Alternatively the entire mass could be placed near the mid span of the aileron which is also easily achievable.

Once the amount of mass required is determined it would probably be practical to find a material that, running the full length of the aileron, would end up being close to the ideal weight, in other words what Vans has apparently done. Though I don't really have any intention of building more than this one copy. And of course it could have all been calculated in advance, with some estimate for the weight of the paint, but this would have been a pretty lengthy calculation of many parts and their associated weight and arm so it seemed most practical to just build and paint the aileron and have a method of balancing it later.

In general do you see any preference of putting half the weight at the root and half at the tip of the aileron or all of the weight in the middle? I realize there may be no "in general rule" and you would be in no position to comment on this without a lot more details, so if that's the case, I get it.

FYI the construction of the wing and aileron does mimic the standard RV-4 design in almost every detail, except of course the taper. The airfoil is also the same. The fuel tank is "longer" and thus holds more fuel than the RV4 standard of I think 32 gallons. Other than being longer the tank is of the same basic design but incorporates some improvements from the RV-8 tank design. Also each wing is 1 foot longer than the original span (giving a total span increase of 2 feet) in an attempt to recover some of the lost wing area from the taper. The taper is 64% and some "washout" is incorporated.
 

wsimpso1

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I have heard descriptions of why to put weight where on ailerons for mass balance, and they were "unsatisfying". The more I thought about what we are trying to achieve, the more I drifted to this description:

We are trying to make the control surface not rotate on its hinges due to the hinge accelerating. Translation along the axis of the hinge is a non-issue. Translation fore and aft, well, that is never going to be anything but tiny. Rotation in yaw is likewise tiny. That leaves acceleration in the vertical axis, and about the roll and pitch axes. Since we are talking ailerons, let's let the elevators and rudder fend for themselves - you can apply the same logical process to them later if you want.

If you accelerate the whole airplane up and down, and the ailerons and wing were rigid (this does not happen, wings bend and twist), the balance weight could be anywhere along the aileron. Wings are bendy things, so if you get the vertical accel by flying the whole airplane into a gust or by changing pitch attitude, the wings will bend, with the tips tending to move way more than the mid span which moves way more than the root.

Now lets assume the gust or a roll input drives an acceleration in the roll axis. Even if the wings are rigid (does not happen), a roll input will tend to let the ailerons lag behind, and the acceleration is bigger at the tip than at root of the wing. Add wing flexibility, and this effect increases.

Pitch axis rotational accel will end up doing a similar thing to a vertical axis accel, with wing load changes and the tips deflecting at higher rates than the root.

Now let's look at the aileron itself. We want it to be torsionally stiff enough that when we put in control movement, the whole thing changes airflow around the wing. If you have nice short ailerons and only two hinges with spherical bearings, and it is strong enough to stand all this stuff, you could get away with balance weight anywhere. The world is not so simple. We want our ailerons light because WEIGHT IS THE ENEMY, and because we have to put in more weight to balance them. Then if you have much span, you might have three hinge points or a piano hinge. Then the aileron has to be soft enough in bending to follow the bending wing as it deflects under load. Hmm, so now we have to atleast think about part of the aileron doing different amounts of twisting in different places...

So, if you only had rigid body accels and no rotation accels, the balance weight could be anywhere along the aileron span, or even linkage based inside the wing, as is done on some ships. But we have bendy twisty wings that bend and twist more as you go outboard, and twisty ailerons that twist different at different places along their span.

Perfect match of aileron mass balance is to distribute the mass along the span of the aileron per the mass in the aileron... This will always give a fully balanced aileron against the three accels we were considering. Trouble with this mode is that the arm is short, so the mass tends to be high to achieve balance.

If you put the balance weight at the root end, it will be fine with strict vertical translation, but the tips of bendy and twisty wings will still want to amplify accelerations on the tip of the aileron - you are not balanced even though you might have thought you were.

Put the balance weight on an extended horn out at the tip, and you can have a longer arm, and achieve balance with less weight. If accels that roll the airplane or cause tips to deflect more than the roots occur, you are somewhat overbalanced, that is the ailerons will deflect to unload the wing not increase it, which is usually acceptable and maybe even desired as it tends to unload the wing and soften the feel of the gust. It has an additional advantage in that it can also provide aero balance. Seems like a decent solution. One drawback is that a flexible aileron with input at the inboard end can have MORE twist at the aerobalanced tip. That is destablizing in roll, and can be troublesome to fly precisely. This mitigates for making sure tip balanced ailerons are nice and stiff torsionally.

One other way is to pick a spot along the aileron span and put a horn there. A-36/P-51 elevators and rudders are balanced this way, with a horn giving both mass and aero balance. We do not see this much - it requires the drag spar to be further forward than we might like, have a dog leg around the horn, or other complications. If you are applying the input force to the surface near the weight, the weight's mass is probably minimized, but now the drag spar gained some weight in the exchange. Hmm.

So, you can try to lower the mass with a longer arm out at the tip with a price of having to make the aileron structurally pretty stiff, which makes for higher loads when the wing bends. You can accept higher weight for the balance weight and a softer, lighter aileron. You can play around with all the ambiguity on having enough balance and down sides of a midspan balanced aileron.

For my airplane, I went with distributed, but then my ailerons are kind of long and soft with three hinge points. So did Van and Neibauer and Cessna and Thorp/Wieck/Bergey. Al Mooney went with mass balanced tip horns but not aero balance. Other folks have gone mass and aero at the tips.

And you are the designer - you get to make the choice, but you have to choose. Decisions, decisions....

Billski
 

proppastie

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We are trying to make the control surface not rotate on its hinges due to the hinge accelerating.
with the control linkage having very little play would it be torsional stiffness of the control surface that determines the problems?.....very stiff......no balance required?
 

Hot Wings

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.....very stiff......no balance required?

As I understand this:

You still might need counter balance with a torsion-ally stiff aileron. Depends on the overall need to mitigate flutter.
Modern sailplanes are probably a good example. They generally have long ailerons and bendy wings with equally bendy, in span, ailerons. The ailerons are still pretty torsion-ally stiff, just kind of limber in the span to keep everything from binding under load..
If we divide up the system into little span wise chunks and look at each segment in isolation then it is easier to visualize the need for counterbalance......even if it is centralized on a torsion-ally stiff surface.

Even if the structure the control surface is attached to is infinitely stiff there could situations where the movement about the pivot of an unbalanced surface could lead to flutter.
 

wsimpso1

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with the control linkage having very little play would it be torsional stiffness of the control surface that determines the problems?.....very stiff......no balance required?
I did not state some other important points about dynamic behaviour.

Control forces and control stiffness is normally not part of the flutter discussion because changing stiffness generally does not change the need for balance. Flutter is by definition an underdamped amplification, and as such it will grow without an upper bound. The forces in a few oscillations go huge. What we can apply with hands and feet is overwhelmed as the control surface oscillates at the resonant frequency of the system involved. Folks that have experienced it with stiff control systems tell of the stick or pedals oscillating wildly, ballistically knocking extremities off the controls and then battering them - breaking things is likely. With soft control systems, they describe the forces oscillating back and forth rapidly while the elastic deformation of the system allows the human end of the system to see smaller deflections than are visible at the control surfaces. What we try to do with our hands or feet is largely irrelevant if you get full on control surface flutter. The big thing to do is load up the airplane and pull the throttle, in the hope you take the airspeed back into the damped range of speeds. There are some places where the amount of stiffness may matter:
  • Aileron systems have a linkage between ailerons on each wing. Here, if one aileron starts to flutter, the linkage can then excite the other aileron!
  • If the balance weight is on a linkage as is used in some sailplanes, the system between aileron and balance device had better be stiff and have very little slop or the weight will not be suppressing the motion it is intended to significant parts of the oscillation, and could still allow flutter;
  • If the system has almost enough damping to prevent flutter, having hands and feet on controls may add enough damping to prevent amplification;
  • I am sure that in this complicated topic, there are others. Have fun guys.
Low control surface stiffness has several possible tails:
  • A control surface's vibe modes can correspond with the base oscillation frequency, it will amplify both - yes, we need to be stiff enough that the control surface resonant frequency is above the resonant frequencies of the thing it is hung on. This is not usually an issue;
  • If the balance horn moves out of phase with the other end (for instance the aero horn goes aileron up and held there aerodynamically while the control input is aileron down from stick and linkage inputs) the weight is not suppressing flutter. So enough stiffness has to be present (as I alluded to in my earlier post) to make both ends of the control surface with a mass and/or aero balance horn stay in phase;
  • I am sure that in this complicated topic, there are others. Have fun guys.
So, applying what I know from classic vibration theory and arrangement of masses and aero forces, I expect that soft control surfaces might contribute, but in general, they could be completely rigid and still flutter on the hinges...

Now maybe someone with a lot more training and/or experience will weigh in on this topic and finish the knowledge.

Billski
 
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Map

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It is not possible to guess on all those factors, that's why manufacturers (of certified airplanes) do ground vibration tests and flutter analysis and flight tests to double-check.

If you follow a few rules of thumb (see my videos on flutter & GVT) and have an airplane with short wings and a Vne<100 kts, the risk of flutter is fairly low.

You can also use the method described in the "FAA Engineering Report 45 Simplified Flutter Prevention Criteria for Personal Type Aircraft" from 1955 to check your airplane. This report is available for free online.
 

peter hudson

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You can also download Mil-A-8870C [Airplane strength and rigidity, vibration, flutter, and divergence] to get an idea of what has worked best over the years and made it into the acquisition requirements for military aircraft.

to paraphrase a bit...

It discusses location of balance weights:
a. they shall be where the deflections of the critical mode shapes are maximum
b. when possible they should be distributed along the span
c. they should be in the plane of the control surface
d. they should not deflect controls during catapults or RATO (propably not an issue for an RV)

Other things of interest is a 100g design limit load for the balance weight attachments and a 60g fatigue load for 500,000 cycles.
Also a tighter free play limit than most people can achieve.

Lots of other best practice stuff to prevent flutter too, non of which is a real requirement for us, but good to think about never the less.
 

davefried

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Thank you guys for your input and HW for your detailed illustration.

I gather that I did correctly interpret the the mass balance at the root of the aileron only is prone to be problematic.

From reading your input I see that putting it at the tip of the aileron, which I was planning on doing after watching Sonja's video, isn't a "sure fire" fix either.

It seems the most conservative/practical options without a lot more information and analysis and/or testing would be to put half of the mass at the tip end and half at the root end. This is easily achievable given the setup. Alternatively the entire mass could be placed near the mid span of the aileron which is also easily achievable.

Once the amount of mass required is determined it would probably be practical to find a material that, running the full length of the aileron, would end up being close to the ideal weight, in other words what Vans has apparently done. Though I don't really have any intention of building more than this one copy. And of course it could have all been calculated in advance, with some estimate for the weight of the paint, but this would have been a pretty lengthy calculation of many parts and their associated weight and arm so it seemed most practical to just build and paint the aileron and have a method of balancing it later.

In general do you see any preference of putting half the weight at the root and half at the tip of the aileron or all of the weight in the middle? I realize there may be no "in general rule" and you would be in no position to comment on this without a lot more details, so if that's the case, I get it.

FYI the construction of the wing and aileron does mimic the standard RV-4 design in almost every detail, except of course the taper. The airfoil is also the same. The fuel tank is "longer" and thus holds more fuel than the RV4 standard of I think 32 gallons. Other than being longer the tank is of the same basic design but incorporates some improvements from the RV-8 tank design. Also each wing is 1 foot longer than the original span (giving a total span increase of 2 feet) in an attempt to recover some of the lost wing area from the taper. The taper is 64% and some "washout" is incorporated.
I am in the final stages of construction of a pair of custom tapered wings for my RV-4. (To answer now what is likely to be inevitable questions: I contracted with a structural engineer to do a stress analysis/design but have not done any performance analysis - that will come in flight.) I just have to paint them, balance the painted ailerons, and then do the wing exchange from the original wings to the tapered wings.
My question relates to the mass balancing of the ailerons. The standard RV-4 (and 6,7 and 8) ailerons have a 1/2" galvanized water pipe attached to the leading edge of the ailerons that runs the length of the aileron so the mass balance is all along the the aileron length, not at the outboard end as is typical with "hammer type" balances (such as is used on the elevator of the RV's). This length of pipe gives an approximate balance of the aileron. (I'm sure Vans tried different diameters and wall thicknesses until one was about right). On my plane the trailing edge of an aileron (with the actuator rod disconnected) weighs about 4 ounces. So they are not 100% balanced.

My plan for the tapered aileron was to totally balance them so the trailing edge of the aileron, when hinged and with the airplane in the fight attitude, weighs zero or very near zero when painted.
I will accomplish this by having an "adjustable weight" balance instead of the galvanized pipe that weighs whatever it turns out to weigh. In place of the galvanized pipe, I have installed a hollow aluminum tube, which, like the original galvanized pipe design, also runs the entire length of the aileron. In this tube I will slide a solid mild steel round bar and fix it in place with some set screws. I can adjust the length of the solid bar to get the exact balance I want which, as stated above, is "neutral" (comments by any who suggest that I balance them at something other than neutral are welcome.)

My intent was to locate this "mass balance" near the root end of the aileron instead of near the tip end, as is common, because at the root the distance between the hinge point and the leading edge of the aileron, and therefore my aluminum tube, is greater. The arm (distance between the hinge point and center of the aluminum tube/mild steel bar) gets progressively less as you move toward the tip due to the tapered shape of the aileron of course. Putting the mass at the root of the aileron means the mass can be less, thus saving weight.

However, in watching Sonja Englert's video series on A/C design she states the weight can be along the entire length of the aileron (as in the standard RV 4) or it can be at the tip end (as is common on the aforementioned "hammer type" mass) but she seems to indicate a problem with the mass being at the root end of the aileron. She does not elaborate on this and I'm not clear even if in fact it is a problem to put the mass at the root, and if so, is this true in some arrangements but not in others. So I am seeking some input as to whether or not it is OK to put the mass at the root end of my aileron, or if I need to instead put it at the tip end, which will increase the mass required to balance the aileron. AND of course, being the curious type, WHY?

Thanks for any input!
I used the galvanized pipe as on the original design. The taper ratio reduced the inner and outer chords proportionally but still allowed the pipe to fit within the profile. It balanced as per the original.

4 R S S.JPG
 
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Traskel

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Great discussion and explanations and beautiful RV. Two things;

1) I'd like to hear what your performance goals for the tapered wing design on your RV were and how they perform relative to those goals and;

2) Given that flutter is such a complicated and viciously dangerous phenomena and since, given that complexity, there seems such potential for compromise in any static solutions could a practical, easily adjustable, and lightweight solution be to approximate a "low end", (slightly under-damped), control surface mass and aero balance using stated methods and then augment it with a small, adjustable rotary damper at the hinge or control input as close to the hinge as possible?

I imagine this is done on many or most high-end high-speed aircraft I'm just wondering if a low-end solution could be derived using existing adjustable hydraulic rotary steering stabilizers developed for motorcycles.

Thank you in advance for any input.

Traskel
 

Fenix

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I'm not sure I properly used the quote function but I am replying to Traskel's question about performance goals of the taper wing.

In my case it was largely a matter of curiosity. There was enough "forum banter" as to the pros and cons of a taper wing on an RV with one of the major pro's being appearance and one of the major cons being a significant increase in stall speed that I just decided, since I was ready for a new project, to build one and add some data to the debate of opinions. Since my plane has performance data with the rectangular wing and I am changing only the wing (and pitot tube) and keeping the same engine, prop, instruments, etc. I could not resist the temptation to get some "good science".

Once it is completed and flown and the data gathered I will post details of the design, testing, and results on the RV (VAF) forum for the benefit of any who may also have been curious.

Now it appears I could have saved some time and simply asked davefried about the effects of tapering the wing - LOL

So Dave,

I few questions for you:

What is your percent taper (ratio of root chord to tip chord)?
Is the airfoil the same?
Is the span the same?
Any other significant changes other than taper?
I assume you used some washout?

Did you fly the plane with the original wing first and if so what changes did you notice?
If you originally built it with the tapered wing and have flown similar RV's with the rectangular wing you can probably still comment on what you believe the changes in flying characteristics/performance to be.

You stated that your aileron "balanced as per the original". Did you also find that "original" is not quite neutral?

Thanks
 

davefried

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Ahhh... the inevitable questions. I would be happy to go at this in it's own thread. Have a look for Putting a tapered wing on an RV, in Aircraft Design / Aerodynamics / New Technology.

For the this thread and as well as I can recall, the aileron balanced level.
 
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