control horn widths

I've never really had a problem that I know of with this, but I am mainly curious. Is there any reason flap horn wires are generally just wide enough to clear the sides of the fuselage and elevator horn wires just wide enough to fit near the inside edges of the elevator? It seems that (especially with flaps) it leaves a lot of room for twisting near the outside of the control surfaces. Has anyone ever made a wire that runs from the center of the control surface rather than the inside edge? What, besides weight and having to notch more of the leading edge of the control surface, problems might this cause? I guess one problem would be the PIA of having to make one rather than use an already assembled control horm assembly.

The reason I use the minimum width is to lessen the suseptibility twisting under load the longer wire would allow.

What twists the most: The control wire or the balsa flap? Anybody ever measured them? I'm guessing the long flap would twist more than tempered (music) wire. Dunno for sure, though...
jd

First obvious problem is that plastic hinges coudn't be used inside the flap horn leg. You'd have to use cloth style hinges which, although perfectly fine, complicates wing finishing a bit. I suspect that a horn would have to be no wider than about six inches on a tapered flap for maximum torsional effectiveness. Past that diminishing returns in the form of wire flex could set in. A wider horn might help on constant chord flaps but those are relatively uncommon.

Weight. A flap horn going to the center of the flap would probably be 4 to 5 times longer, and a lot heaver then balsa.
Jerry

It is surprising how much tortional twist you can get with plain old Music wire. Chrysler corporation used tortion bars for a long time on the front of their cars instead of springs. And they were thick! Just bend the ends on a full length piece of music wire and then try to twist it. Hmmm.

Now, a wood flap can, also, twist. Covering it with PlastiKote doesn't help much. Covering it with silkspan helps a bunch! If you build the plane but put on all the finish before attaching the flaps, you can see (and feel) the difference very easily Using carbon fiber strategically in the construction or wrapping the carbon fiber around the flap before finishing, also helps a bunch. Using a little heavier wood in the construction, also, helps, but the "skin" is what is going to help the most.

Just be advised there will be some twisting affect on the flaps. But most people taper their flaps towards the tip, so the twisting forces will be lessened in the process.

How about this one...

Why not make the entire leading edge of the flap from a continous CF tube like we make pushrods from??? shucks, given enough patience you could even slot it for the RC style hinges.

The writer makes a good point. I remember Al Rabe spent considerable time (and weight) covering his flaps with numerous layers of fiber glass to resist twisting. If the torque was distributed along the entire span by an untorqueable tube you could mke the flaps from tissue...well, almost.

I like Ted's idea, but most carbon tubes are uni-directional, not spiral wound, so their torsional stiffness would not be as good as it could be. Which is not to say it would not be good enuf. Tying the fairing onto the tubes may prove to be a problem. I have often thought and sometimes stated, that we should perhaps ponder how many superior designs were junked as bad ideas when all that was lacking was stiffer flaps, stiffer pushrods or better hingeline sealing. Things that really bother me in structures are sheet flaps and tail surfaces with the center cut out and ribs/tissue filling the cutout. Another is a sheet core with ribs and tissue added. Bad concepts. Frankly, the I-beam wing is not a great structure, but can certainly be made to work well with some modern technology helping out. Stiff wings, flaps, tail surfaces, fuselage and controls all are going to be leaps forward in performance. Steve

>How about this one...
>
>Why not make the entire leading
>edge of the flap from
>a continous CF tube like
>we make pushrods from???
>shucks, given enough patience you
>could even slot it for
>the RC style hinges.
>
>The writer makes a good point.
> I remember Al Rabe
>spent considerable time (and weight)
>covering his flaps with numerous
>layers of fiber glass to
>resist twisting. If the
>torque was distributed along the
>entire span by an untorqueable
>tube you could mke the
>flaps from tissue...well, almost.
>

I think I mentioned this before, but I have considered making linkages to drive the flaps from both ends, and the middle. It would use stuff like R/C aileron horns.

Cool idea, Brett, especially if you did it with cables. It might be a useful application for Kevlar.

Ted, I made some superlight, superstiff carbon torque tubes for flap LEs using the Lost Aluminum method (lay up the parts on an Al tube and dissolve the tube in HCl). I shall try to remember to take you a sample. Indeed, the hinge slots will be the tough part.

Having said what I said above and with Ted's and Brett's and Howard's input following, I am going to ask a question now that may sound a bit like heresy.

We need to keep the control system solid--bulletproof. If the pushrods bend or the horns twist, we will lose our leverage and lose our turn. But...

Given the fact that it is possible to have too much flap (either area or movement) -- Matthew has had to increase his elevator response (or ratio) in relation to the flaps on his last three airplanes -- and given the fact that it is generally accepted practice to have less flap area (or effect) at the tips than we build in at the center, what is wrong with a "little" twist in the flap. As long as we build our flaps of matched wood or other construction practices to allow them to twist equally, a "little" twist would allow a little less deflection at the tip with less flap effect, the same as using less area as is our general practice.

Does anyone know how much our flaps do twist in the air? I don't. But maybe we are getting a little more twist than we realize and it is not hurting.

And, Howard, what is different in the carbon fiber tube you constructed with the "lost aluminum" construction over the standard carbon fiber tubes that are available "over the counter"? Just curious.

>As long as we build
>our flaps of matched wood
>or other construction practices to
>allow them to twist equally,
>a "little" twist would allow
>a little less deflection at
>the tip with less flap
>effect, the same as using
>less area as is our
>general practice.
>
>Does anyone know how much our
>flaps do twist in the
>air? I don't.
>But maybe we are getting
>a little more twist than
>we realize and it is
>not hurting.

Maybe. I have a vague plan on how to figure out how much it twists that I may get to after the team trials. Getting it to twist equally is probably a lot easier said than done.

But, I think anything springy is bad. If you made it perfectly stiff, it may very well require less flap or more taper, but we will have removed the compliance from the system, and improved the control response.

>
>Maybe. I have a vague plan on how
>to figure out how much it twists that I may
>get to after the team trials. Getting it to twist
>equally is probably a lot easier said than done.
>
>But, I think anything springy is bad. If you made
>it perfectly stiff, it may very well require less
>flap or more taper, but we will have removed the
>compliance from the system, and improved the control
>response.
>
> Brett

I am agreeing with both comments, Brett. (I told you it may be heresy.) It is that hoped for "equal" springiness (that may not be equal) that may be what gives some people fits in trying to trim properly. Matt has found on his last several ships the need to reduce the amount of flap movement in relation to elevator movement. This may, in part, be due to increased stiffening during construction. We was planning on decreasing the area of the flaps a bit on his next ship, but, after conversations with several others is now thinking there may be an advantage in getting the needed amount of lift with somewhat larger flaps with less deflection over smaller flaps that need to be deflected more (and thus could produce more drag.)

If you do come up something to test the flexing, I would be interested in learning your results.

I spiral wound two plies of high-modulus .002" prepreg at 45 degrees in each direction. I cooked it under pressure at the requisite 350 degrees. It is very light and very weak in bending.

You have an interesting point about flexibility, Leonard. What's best for flaps? What's the right flexibility for a wing? These are just more Mysteries of Stunt.

LAST EDITED ON Aug-21-01 AT 05:58 AM (CDT)

This is how I rebuilt the Elevator for the Heavy Twister. The CF tube was set into the LE of the flap--extended about 6" for a 18" span. I used two right angle solid carbon sticks going into holes in the ele(like the ends on a wire joiner.) They were 3/32 carbon as well. Then I wrapped a couple of tows around the LE and out the chord of the ele to help transfer the torque to the skin of the solid balsa elevator (silkspan and dope skin)

This elevator is MUCH stiffer (in torque and chord bending) than the original with its 3/32 wire joiner epoxied into the edge of the elevator. The original had 4 oz carbon over the areas where the wire was inserted and the horn attached (surface mounted horn) in an attempt to stiffen up the coupling between the wire and the ele. And it had carbon veil over the entire surface as well. It was stiff chordwise but the wire would still twist.

The new one came in 1-1/2 oz lighter than the original.

I would think this might be an interesting application for flats as well. THe slots can be easily cut for hinges but care would need to be taken to prevent splitting of the linear strand tubes.

Steve is correct about the woven tubes being much better in torsion but I'm not aware of any small ones commercially. Actually, a square tapered section covered with a braided tube and vacuum bagged would be the cats meow! But is flap twist really a major problem?

Regards

I had considered using the CF tube/rod like Curt used, but putting a lever on it would be a little dicey for me. I also thought about molding a one piece unit, more like a wire horn. Getting pivot support at the center would be difficult, as it probably wouldn't like even Teflon running direct on CF. Sleeving over the fabricated or molded horn/joiner with brass would be a good idea. Slotting a tube for hinges would not be on my list of good things. I would use the old "tube & wire, but the pivot location cannot then be the same as for the center. Won't work. Cloth hinges have the same problem. If you ran the tube full span, you could put brass tube sleeves on in hinge areas, and slot for some kind of blade hinge. Randy Powell did something like this on his high aspect ratio model seen at Roseburg, but I'm not sure of the details. He said it was a lot of work! Steve

What about using a Dave Brown fiberglass pushrod? Fairly light, fairly strong and I wouldn't be afraid to use a worn down Dremel cutting disk to slot one. Even with the slots, maybe filled with a little epoxy on where the hinges are, would be stronger and stiffer than just a horn and balsa flaps. I may try this on the Nobler I'm building. If it doesn't work, I can easily cut two new flaps.

The Dave Brown rods arent really very stiff compared to (even smaller diameter) carbon fiber rods.

I've been thinking of how to attach the horn to the rod along with Steve, but havent come up with a real good setup yet. The best is probably molding the horn right into the rod with tow into a mold.

A square section keyed into the horn might work as well.

For hinging the joiner, a vertical piece of 1/16 x 1/4 attached to the ends of the joiner and configured as a pin hinge might be easier than wrapping teflon or delrin arount the carbon tube.

I continue to be very pleased with the elevator joiner and carbon horn on the Twister and am doing another for a larger profile plane now. I'm still going to use a 1/8 wire adustable system for the flaps however.

The other responder is quite right--very time consuming.