Bend Flap Horn to Trim?

[Peter in Fairfax, VA]

What is the best way to trim a stunter so that it hangs evenly on the lines, meaning that the outer wingtip is at the same height right side up or inverted?  Bend the flap horn?  If so, how?

For combat planes without flaps, there were two popular methods:  Either taking out any wing warp by heating the covering, or, more rarely, adding a bendable metal trim tab.

The stunter I have has built-up flaps.  They are neither straight along their length nor at the same height as viewed from the rear of the plane.  The flap horn is either drilled or slotted into a small section of the flat which is solid balsa.  One idea is to clamp plywood over the balsa and then bend.  Another idea is to remove the warp, as well, with steam, boiling water or a heat gun.  Yet another idea is to go with solid C-grain flaps, with significant 1/16" plywood glued on over the flap horn, so as to ease field adjustment.

Thoughts?  This topic is concerning me, as the first plane I flew in about 25 years had a warp, so that it came in on me during the first loop.  While it didn't crash (I was reaching out and also ran back), it wasn't the flying experience I was after.  Confined myself to practicing inverted flight, low, and doing long lazy eights.

thanks,

Peter

[Tim Wescott]

Yes, bend the flap horn.  Then on your next plane build one of these.

You need flaps that are stressed to bend the horn.  I would worry about the horn just going into the balsa, but it sounds like your flaps need some serious work anyway.  I would tweak them using the plywood plates over the horn areas as you suggest to get the plane flying, but at the same time I'd get some C-grain balsa and build new flaps as you describe.

[Larry Fernandez]

I build my planes with removable flaps. If they need to be tweaked, I simply remove the flaps and tweak the flap horns.
Another plus with removable flaps, is that if you find you need larger or smaller flaps, you can change them out in a couple of minutes

Larry, Buttafucco Stunt Team

[Jim Svitko]

You say that the flaps are built-up and that they are not straight along their length.  You do not say how much they are out of line/crooked but no matter how small it is, it does not take much to mess things up.  I would fix this crookedness/misalignment first.  Unless, of course, you just want to get in the air as soon as possible.  Then, tweaking something might help to some extent but it does not fix the cause of the problem.

[Peter in Fairfax, VA]

Great ideas.  Couple of questions:

The idea of inserting the flap horn into a drilled plywood inset piece instead of balsa makes sense.  In that case, about how large would the inset be, and how would it be attached to the rest of the flap?

Following a link above regarding an adjustable horn rig, I saw a trimming flowchart.  I understand some of the basics, to a degree, like keeping the wings level, setting the CG, setting the leadout position.  I understand the terms roll and yaw.  However, what is "hinging?" 

[pmackenzie]

Hinging is rolling during turns.

[Tim Wescott]

The idea of inserting the flap horn into a drilled plywood inset piece instead of balsa makes sense.  In that case, about how large would the inset be, and how would it be attached to the rest of the flap?

This is one way.  It worked out easily because Twister flaps aren't tapered, and I didn't much care if it showed a bit.  The plywood plates are generously sized because I wanted to be able to grab the surface directly, without making a tool (which meant that when someone gave me a nicer plane, one of the dumba** things I did to it was break the flap).

You could also just butt-glue the plywood piece to the end of the flap, if you think it gives you enough room to grab things.  Or you could cut a "U" out of the flap and inset the plywood into it.  Or, or or -- there's a lot of ways to skin this cat.

[Ken Culbertson]

Great ideas.  Couple of questions:

The idea of inserting the flap horn into a drilled plywood inset piece instead of balsa makes sense.  In that case, about how large would the inset be, and how would it be attached to the rest of the flap?

Following a link above regarding an adjustable horn rig, I saw a trimming flowchart.  I understand some of the basics, to a degree, like keeping the wings level, setting the CG, setting the leadout position.  I understand the terms roll and yaw.  However, what is "hinging?"

I slip a brass or aluminum tube over the horn about 1/4" form the bend and 1/8" longer than the horn.  I sandwich that between two 1/16" basswood ribs and fill in with epoxy on built up flaps or drill out and press fit it into a 1/4" basswood end cap for solid flaps.  1/32" plywood cover plates on both.  Don't glue the horn - two reasons.  #1 it allows the horn to slide inside the tubing which takes pressure off of the joint to the wing and #2 it makes the flaps removable.

Ken

[Howard Rush]

Peter, welcome back.  You must be really old now.

If the wing is warped, dewarp it if you can.  Use hot water, same as for combat planes.  Take Jim's advice about straightening the flap.  If it won't go straight, replace it.  C-grain sheet works fine.  The cross section doesn't need to be tapered. 

Inevitably, you'll need to give the flaps a tweak.  Your plan to clamp (I just squeeze) plywood blocks over the balsa where the control horn is should work.

Paul Walker's trim process, to which Tim sent you, is the standard.  A heap of flying went into its development.