Locking control surfaces

[bjaffee]

I know some people are foregoing flaps on their DS planes, or else using small control surfaces to prevent flutter.

What I've been wondering for a while (aside from the idea of balancing the flaps), is whether you could have your cake and eat if you could lock the flaps in the centered position when you needed to.

For example, if you drove a single steel pin into the center of the flap from the sub trailing edge of the wing to lock it in place, would that help prevent flutter? Or, would you have to drive in multiple pins to keep the flap from twisting at the tips too?

I figure you could use a pretty tiny servo, mounted near the existing flap servo bay, to drive the pin in and out (it wouldn't really need much torque to accomplish). The flap linkage itself could be set up to be pretty sloppy, to prevent any binding of the flap servo if the locking pin were engaged with the flap not perfectly aligned. Since the flaps would only be used and unlocked at lower speeds, the slop shouldn't matter too much.

[Tim Bigley]

Properly installed surfaces will not flutter! I would be willing to bet if you built a plane to go fast that other problems would arise before you had problems with flaps. I also think you would sacrafoce the structure of the wing with all the moving parts.

KISS (keep it simple stupid)

[bjaffee]

Hmmm...given a stiff enough structure, I think the moving parts are always going to be the weakest link, no matter how stiff you make everything (many full scale aircraft have their top speeds limited soley by when the control surfaces will start to flutter, for example). If you go fast enough, I think you can always flutter something, especially if it's a large, unbalanced control surface.

What I'm proposing shouldn't really weaken the structure, and would actually be a fairly simple setup.

[Ollie]

A simpler albeit more expensive solution would be to use digital servos which all but lock in neutral with holding torque several times the torque of an analog servo of the same size.

[bjaffee]

Ollie,

Are you saying that normal digital servos already do that, or would this be a special feature? Lots of of people already use digitals on their control surfaces, at least on molded planes that get DS'ed. You'd still have to worry about servo slop though, unless you regularly maintained the servos (changed out the gears, cases, etc).

[Ollie]

If an analog servo is loaded to its rated torque, the servo output will be deflected 20 or 30 degrees from neutral. The deflection of all digital servos is much, much less because the motor is pulsed several hundred times per second instead of 30 times per second. All analog servos are comparitively spongy under heavy loads because of the low pulse rate. Of course you do need robust mechanicals such as Multiplex or Voltz for best results. There is a big difference in strength between plastic, aluminum, brass and bronze gears. There is a big difference in metal gear backlash depending on manufacturing tolerances, lubrication and wear. There is a big difference in backlash due to case design, gear pin support and case material deflection for servos operated under heavy loads.

BTW, all that holding torque under load causes heavy battery drain for sustained loads.

[ShredAir]

Originally posted by bjaffee
Hmmm...given a stiff enough structure, I think the moving parts are always going to be the weakest link, no matter how stiff you make everything (many full scale aircraft have their top speeds limited soley by when the control surfaces will start to flutter, for example). If you go fast enough, I think you can always flutter something, especially if it's a large, unbalanced control surface.
That's exactly why I think that dedicated DS planes will have as few as possible control surfaces which are as small as possible. I too am a KISS proponent.

Dieter Mahlein
http://shredair.com

[Craig Toutolmin]

A successful solution that I have used for a few years now is to not let the flaps reflex by installing stops along the sub-trailing edge. The flap servo at center trim is slightly loaded into the stops - eliminating any gear slop. Under flight loads there is no additional load on the servo and very little battery drain. The bottom hinge design must be strong.

A surface that benifits from locking is a rudder. This would be for a sport plane that ventures deep into the DS realm. I used tape in the past. It would be nice to have a clean locking mechanism built into the fuse.

The nose of Barry's Icon, the nose of Tom's Mojo and the many planes that had their joiner boxes ripped from the center panel were all failures of non-"moving parts".

[hkrussell]

I think that one of the major causes of flutter is terrible geometry. I remember looking at an Acacia that was set up to factory specs and had 1/8" or more movement in the flaps. That was caused because it uses the really short ball joints for control horns instead of a proper control horn. The short horns allow for 90' of throw, but that isn't necessary for most slope planes. Using longer horns and dealing with less throw help the flutter problem.

On the other hand, heavy control surfaces cause a bunch of problems also. I ran into this with a homebuilt. Heavy flaps were bad.

Craig's greenie has little tiny ailerons, with little bitty servo's. No flutter and good control. Very fast two meter plane.

I'm intrigued by the idea of the pin, but if it was 'loose' enough to engage while flying, would that be so much that it would still flutter?

By the way, I personally prefer all my planes to have full sized control surfaces with lots of throw. I will use the largest servo I can fit into the servo bay and put a fairing over the part that sticks out. 75+ ounces of torque is not unreasonable for the ailerons and 100 is good for flaps. That plane went 140 or so. My next DS plane will have 140/160 on the ailerons and flaps. I bet a bunch of flutter is caused by people using servo's that aren't strong enough for the size of the surface and the speed they fly.

Like putting the HS5125 into the wing of a 120" DS plane. I think that servo would be too weak for F3F flying, much less DS'ing.

Ken

[bjaffee]

Originally posted by hkrussell

I'm intrigued by the idea of the pin, but if it was 'loose' enough to engage while flying, would that be so much that it would still flutter?

I'm not saying that the pin would be loose...this would be very stiff...hopefully enough so that the surface essentially becomes a solid part of the wing. However, I was imagining possible problems of bind occuring if the pin tried to engage when the flap didn't happen to be centered EXACTLY right. Therefore, I suggest that the link between the servo and the surface be sloppy, so that there is some give, allowing the pin to engage and self center the flap without causing servo to load up. Once the pin is in, though, the servo linkage is not even part of the equation.

That said, I like Craig's "passive lock" idea of preventing any up upward travel in the flap.

[Ollie]

A slightly tapered pin can engage a hole with matching taper when the control surface is slightly out of position. As the pin is forced home, the control surface is locked into the correct position. The pin and hole have to be lubricated for best results.

[Craig Toutolmin]

A plane that has very poor flap control geometry is the Tragi. The first few degrees of movement down or up rotate the pushrod instead of pushing or pulling on the servo.

Another problem is that many F3F planes are, well, design for F3F and have their airleron servos mounted inboard to reduce roll inertia instead of being mounted near the center of the surface to minimize surface twist. The most common surface to flutter (after a sloppy flap) is the upper aileron.

[F3X]

[QUOTE]Originally posted by Craig Toutolmin
[B]A plane that has very poor flap control geometry is the Tragi. The first few degrees of movement down or up rotate the pushrod instead of pushing or pulling on the servo.

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I don't know what this opinion is based on. With a rigidly installed control horn, a straight pushrod and a clevis connected to a servo are there is no twist.
You may be confusing someone’s building error as poor design geometry. The Top hinged Tragi's are controlled exactly like most other Euro ships, Top hinged bottom driven. Some bottom hinged 701/702 are bottom hinged and bottom driven, this config needs the control horn to be about 1/2 long from the lower surface to provide proper support of the surface, longer for DS.

[Craig Toutolmin]

Hi Tom,

This is based on the many Baskin Tragis, Tim Cone's Tragi and Steve Situm's Tragi. My opinion is based on those 5+Tragis. This would be every Tragi that I have looked over at a race. A simple bend in the control horn would remedy the problem.

[F3X]

Sorry Craig I must be missing something your tyring to say.

A straight flap linkage will not rotate and a bent one will.

How is the Tragi flap linkage different from so many other models?