Wing servo installation

[Joe W]

After reading about and seeing various mishaps due to "wing" flutter (most likely control surface flutter instead), I thought that it might be a good thing to pass along my solutions to increase control system stiffness.

Things to work on:
1) mechanical advantage - use the smallest horn possible on the servo and the largest possible on the control surface that gets you the throw that you need.

2) eliminate any free-play or slop in the linkage

3) use the best servo you can that fits

4) maximize the system stiffness


#1 helps the system stiffness as well as control resolution, and allows you to get the most mechanical advantage (force) out of the servo. You might have to do a few trial & error attempts on the horn lengths, and you might have to do some relief cuts on the clevis to get clearance on the servo around the portion of the horn that pushes on to the spline. It is worth it though.

# 2 gets rid of hysteresis issues, and improves the aircraft response and efficiency. I tend to use a little bit of CA on the threaded rod at the clevis to ensure that there is no slop there, and also put a small drop of CA on to the clevis pin area after attaching to the control horn. After kicking with accelerator, I'll move the control surface. This results in a CA bushing in the control horn hole around the clevis pin. It is amazing as to how much this can tighten up things.

# 3 gets you increased stiffness and resolution, as well as reducing the odds that you will be replacing servos and/or gears. Also, you will get better response in high load situations (launch and higher speeds).

# 4 really helps in reducing flutter. It is painful to see people spending lots of money on the latest planes only to see them shred the plane in flutter due to a sloppy installation. Recently I've seen a few planes get hurt or destroyed due to what I feel is preventable flutter. Most people install the servos by just attaching them to the wing upper surface (the bottom of the servo pocket for servos installed from the bottom of the wing). This is not the stiffest installation, as now you are relying on the wing skin to react a bending load. My solution is to tie the servo to the lower wing skin to eliminate the bending loads on the wing upper skin. See the attached pictures for some details on how I have done it on my Supra. I do something like this on all of the planes that I build, and it is surprising as to how much this improves the stiffness of the control surface.

Joe

[Andy W]

I've experienced "softness" in control surfaces on a molded ship that had me thinking it was slop in the linkage, but it turned out to be flex in the skin (which was reinforced). I solved this by "boxing" the servo mount location, bonding the upper and lower skins with balsa carefully installed around the cutout area. I note that a more expensive airframe I own has this type solution built-in (lower skin molded in a similar fashion, and bonded to upper.)
271195
Another option to #4..
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[Soar_dude]

# 2 gets rid of hysteresis issues, and improves the aircraft response and efficiency. I tend to use a little bit of CA on the threaded rod at the clevis to ensure that there is no slop there, and also put a small drop of CA on to the clevis pin area after attaching to the control horn. After kicking with accelerator, I'll move the control surface. This results in a CA bushing in the control horn hole around the clevis pin. It is amazing as to how much this can tighten up things.

# 3 gets you increased stiffness and resolution, as well as reducing the odds that you will be replacing servos and/or gears. Also, you will get better response in high load situations (launch and higher speeds).
Joe

Joe you use digital servos exclusively right? I am speaking for myself if I tighten up my linkages like you recommend my analog servos will not center or go full travel. This increased stiffness will also cause more drain on the battery pack as the servos try and center themselves overcoming the friction of the tight flight controls shortening flying time. What I have found that works best for me to prevent flutter is a torsionally stiff flying surface with a stiff hinge. The stiff hinge helps with keeping the flight control centered.

Soar Dude

[Joe W]

Soar Dude,

I also advocate no binding for the linkage and control surface. The CA on the clevis, when done correctly, doesn't add any appreciable friction to the drive assembly. The goal with this is to reduce control slop, not add friction to the linkage. Friction is a very bad thing, adds hysteresis, and increases power draw. Your goal should be to minimize friction and mechanical loads on the servo, so that the servo sees only the aero loads.

I agree with the benefits of a torsionally stiff flying surface. Unfortunately, this is difficult to adjust after the fact, whereas the servo and linkage installation can be worked on.

Not sure I've had the same results as you regarding a stiff hinge. Are you meaning that the hinge itself has a high spring stiffness? If so, how much of a workout are you giving the servos at higher deflections? And further, does this mean that you are using stronger servos to account for the extra force required to deflect the stiff hinge?

Looking back on my post, I might be getting into trouble with nomenclature. What I mean by system stiffness is the change in the amount of deflection that occurs with a change in the given input load. In other words, how stiff is the spring equivalent? As to tight linkages, what I mean is to eliminate as much free play as possible. I am not meaning that the linkage becomes difficult to move! A linkage that takes force to move is BAD!

Maybe I should add a #5 which states, minimize linkage friction and binding, or alternately stated, ensure that the control surface moves freely without friction or binding. I'd also suggest that the hinge is not stiff, but that is my experience and preference.

Joe

[JoeSantora]

Another way to tighten up the system is to relocate the servos. Most pockets are at the inboard edge of the aileron which gives the control surface it's full length to flex and or flutter. By relocating the horn to mid aileron you cut that length in half, which might reduce the "flutter factor" by the square of the distance. ( or maybe just half :rolleyes: )

[Soar_dude]

Joe W. what I am talking about is stiff hinge that remains naturally centered. most flutter occurs at a faster speed if you have a hinge that will hold the surface centered it will be less prone to flutter. At least this is what I have found. Yes I take a hit in the my maximum deflection but so far I have not had flutter problems. I came to this conclusion when my spirit 100 elevator would flutter in a dive I tightened up the linkage. but it would still flutter then I replaced the CA hinges with a stiffer ones and my flutter went away. I had the same problem with 100 ARF wings it had really floppy tape hinges and they would flutter like a champ I replaced the tape with stiffer bookend tape and the flutter went away.

Joe S. I agree with your idea about moving the servos the ceter of the surface that goes along way to reducing flutter. I could never understand why manufactures are not doing this with drop in servo size and weight it would make sense to do this.

Soar Dude