The reason I ask is:

I'm starting to build a 4-star, And I'm going to install separate aileron servos in the wings. I'm thinking I could save an extension wire (and it's hastles), and a little weigh, by moving the servos inward.

I'm thinking, that theoretically, the aileron could flex during maneuvers. Meaning wind movement wouldn't allow the aileron to move up and down as much at the tips as at the base. I'm also thinking I'd gain a miniscule amount of role rate because the servos' mass would be closer to the fuselage, hence less inertia to overcome.

Most of all, I'm thinking "This isn't rocket science." and even if the aileron flexed a little, and even if the servos' mass effected the role rate of the aircraft, the effects would be so miniscule, that I'd never really know the difference.

I'm here posting this question to thee, oh learned group of builders, if there are other things I haven't thought of. Is there some unwritten rule? Any experience with this?


Thanks in advance,

Actually, you're better off having them to one side

I try to mount my servos in the middle of the aileron or just slightly toward the inboard side of the aileron. If the ailerons are wider inboard, then I will mount the servo more toward the inboard end of the aileron. Many planes, however, have one servo in the middle of the wing and that works well, also.

OK... I'll bite.. why?


Using our beloved US-60s (may they rest in peace...) as an example. The aileron servo would be just outboard of the flaps. The control horn would be near the extreme inside end of the aileron. That'd be a change of 2 or 3 bays, not just a mere slide of a half-inch or so...

If the control horn is off center you have less of a chance of getting harmonic vibration (Flutter)

1/3 the aileron towards the inboard side. Probably won't make a difference though being a 4*. If it's a 40 size, I would just use one, even with a 46 size engine it won't matter much. I used to put mine through hell and back and never had a problem with one. The 60 and 120 size need two servos.

PS-If it is the 60 size-clip a wing bay on each side and change the dihedral to 1 degree-you will get a nice roll rate. You can do that on the 40 size but I would leave the length of the wing the same.

Yes, it is beneficial to have them offset from center, best is either 1/3 or 1/5 of the aileron span from the inboard side to minimize the possibility of flutter. Worst case is in the center.

I've been told that the only stupid question is the one that's not asked. I guess this is true but I also think your doing way too much thinking here. That also can be a good thing. A non-overpowered 4 star will fly just fine either way. I like the idea of moving the servos inboard but you need to have enough servo lead to not only reach the receiver but also enough slack to be able to plug them into the RX. This will become a pain cause you'll have to undo what ever is holding the RX in place to plug in the servo leads, then re-install the RX and put the wing on. It's a lot easier to plug two connectors together when putting on your wing instead of of plugging two servo leads into a RX. Think about it.

[:@]you all get way too technical for a simple question asked. build the dang thing........fly it......learn from it.......everyone here has a different opinion anyway.....its your airplane...your building it...your flying it......

Most folks don't build 4-stars for high performance and unless it's going to be way overpowered I think it probably won't matter where you put the horn on your ailerons VMS. However, if you were concerned with potential flutter problems there would be benefit to moving the horn further outboard. Aileron flutter is caused when a pulse in the airstream (usually tip vortex but not always) matches the natural frequency of the aileron causing it to resonate much like pushing yourself on a swing. While swinging every kid figures out that if you match the natural frequency of the swing/rider with the swing of your legs you swing much higher with every cycle. Unfortunately when an aileron experiences resonance the energy builds so fast that something often breaks before you can slow the plane down.

The only way to raise the natural frequency of an undamped aileron out of the operating range is to stiffen it or reduce the unbalanced mass. locating the horn in the center of the aileron is the easiest way to increase the stiffness. Try twisting a piece of aileron stock holding it at each end. Now try twisting it holding it at one end and at the middle. It just got much stiffer! Trying to stiffen the aileron by using harder (denser) wood often won't work because for every amount you stiffen, you also add unbalanced mass. Edmoor, if you've made it this far into my boring post then you are obviously more curious about the hobby than you realize. VMS asked the question because he wanted to hear what others had to say.

I've read RCU forums for several years. I peruse the Aerodynamics thread several times a week. This is the first thread that I have seen a referenced association to aileron flutter and lateral control horn placement. Does anyone have a source/link to other data on this subject? One can always learn in this hobby.

Airbus, I was having some problems with aileron buzz on one of my planes recently as I added power and reduced airframe drag. From your handle I assume you're a commercial pilot. All the serious work on the net was in regard to full scale where the structures are already very stiff to begin with (alum monocoque, hardened steel linkages). Their solutions usually involved complicated mass balancing or dampers inside the wing. Our balsa/plastic film R/C planes are pretty flimsy so the most practical solution for me was to stiffen. In my earlier post I only mentioned the strip aileron itself but one should also closely examine the linkage, pushrod, torque tube, hinges, and hinge mounting wood in the trailing edge too. Even the servo shaft and mounting should be considered. They can all be a source of reduced stiffness.

Here's a Wiki link that describes aeroelasticity pretty well:

[link]http://en.wikipedia.org/wiki/Aeroelasticity[/link]

Here's another wiki that describes the natural frequency relation with some rather painful mathematics (sorry):

[link]http://en.wikipedia.org/wiki/Vibration#Types_of_vibration[/link]

Whats important is the undamped natural frequency of a system is equal to the square root of the stiffness divided by the mass:

Fn = 0.16 (k/m)^1/2

Fn is natural frequency
k is stiffness
m is unbalanced mass

To increase "Fn" you must either increase "k" or reduce "m".

There was a saying that goes, "I don't think I understand all I know about that." I think the last link above about vibration may have put me into that position. My question above was specifically directed to to control horn placement playing a role in control surface flutter. Certainly many factors(shape, speed, tightness of the linkages, etc.) can allow a control surface to flutter. I was unaware that control horn positioning was one of these factors. Was just curious if anyone had a source that showed a specific control(i.e. aileron) was more prone to flutter when the control horn was in the middle of the surface vs. being moved away from the center position.

PS: CrateCruncher, I'm the guy with the Dave Platt Duelist MKII kit and two YS-45s awaiting a build at some point. However, I'm not sure as I age that my eyes are not directing me more toward bigger and slower planes. Just finished a Four Star 120 with a twin cylinder OS 120. What a sweet old man's plane. Easy to see, easy to fly and that twin four-stroke sounds incredible. Plus it is so easy to takeoff and land that it even makes me look good.[&:]

The main thing I was trying to say is that when it comes to aileron placement you want the stiffest aileron possible. I've never heard of moving horns off center to reduce tip flutter either.

My reasoning for locating the horn in the center was quite simple. The stiffness "k" of a shaft in torsional vibration is inversely proportional to length "L" of the shaft:

k = JG/L

k is stiffness
J is the second moment of area
G is the modulus of rigidity for the material
L is length of shaft

Here is another mind numbing math link for those who want more:

[link=http://wikis.controltheorypro.com/index.php?title=Single_Degree_of_Freedom%2C_Free_U ndamped_Torsional_Vibration]free vibration in torsion link[/link]


So making the free length as short as possible will make it as stiff as possible. Since we have one servo for each aileron in VMS's plane the best horn location for stiffness would be in the center (total length/2). If there were two servos for each aileron the best location would be at 1/3 and 2/3 length and so on. Following this logic a torque tube strip aileron is the worst setup possible for inhibiting high speed tip flutter because the constraint is as far as possible away from the vibration source at the tip.

I remember your rare Duelist kit now. It's a beauty. If you look at those torque tube ailerons on his plan you'll see he was trying to cure highspeed aileron buzz on his prototype by adding more hinges. When you build it I would recommend locating the servos at mid aileron span for the reasons outlined above. It will scream with YS-45's!

Flutter is harmonic vibration.

Think of a guitar string: pluck the string and it vibrates (Fig 1)

Place your finger lightly on the 12th fret and pluck the string. Both sides of the string will vibrate (Fig 2). Any guitar player out there can attest to the fact that this is called "Harmonics"

Now, place your finger lightly on the 10th or 11th fret and pluck the string. The string will give a dull "thunk" and stop vibrating (Fig 3).

The exact same thing happens with the control horn. Placeing it off-center will HELP to avoid flutter.

I agree with this. It's a good example of free vibration. The string will vibrate at it's natural frequency until air friction causes the sound to dissipate. Though I would think the side of the string not exposed directly to the pick's impulse is no longer relevant to the system because mechanical energy can't travel past the fret.

You lost me here. The theory predicts that changing the length of the string just changes the natural frequency (the tone up or down). Maybe there's something else going on in the guitar or ear? Reverb cancellation? ? Do you know of a link that might explain this further?

The string will vibrate exactly as shown, HOWEVER... it will vibrate twice as fast.

In other words, if it were an "A" string, the note, "A" has a frequency of 440 beats per second (That's why it is often refered to as "A-440" )

When plucked as a harmonic, (or if you held the string at the 12th fret) it will vibrate at 880 BPS.

The note will still be an "A", but it will be an octave higher.

I'm not sure that applies to a Four Star aileron because its width varies from wing root to wing tip. In your guitar example, it would be as if the string on one side of your finger was thicker.

Good point.