Sheet flap treatment

HI folks,

OK, I'm finally starting my USA-1. The plan called for 1/4" sheet flaps. So, I really don't want to end up with a flexy flyer. Any thoughts on methods to stiffen these up. I usually use either epoxy and .50oz glass or epoxy and CF veil on thin sheet flaps. Any other methods out there.

Bill, any suggestions? How did you do the flaps on your USA-1?

Hey Randy!
'bout time.
I had some very good "C" grain, kept the grain running parallel to the TE of the flap, and covered them with .2 oz. CF veil applied with copious amounts of nitrate.. Mine didn't flex.
Gonna start the replacement soon.
Bill <><

Thanks Bill.

I'll see what I come up with. I know that this is an airframe that simply cannot be heavy and I'm already thinking about keeping the structure as light as possible. I'm already replacing the 1/16" fuse doublers with CF and a few other tricks. I'll keep the Louisville Slugger designed main spar, but other areas need some lightening.

>Hey Randy!
>'bout time.
>I had some very good "C" grain, kept the grain running
>parallel to the TE of the flap, and covered them with .2 oz.
>CF veil applied with copious amounts of nitrate.. Mine
>didn't flex.

Polycrylic is even better than the nitrate.

Big Lizard,

I had already built the USA-1 before I found out about that. I am using polycrylic on the rebuild of the SV-11, but it's flaps are over 10 years old! So far, so good.

Bill <><

Randy: My method over the years has also been, .2 oz CF Veil and Nitrate. But, after trying Polycrylic; I've got to agree with Brad, Polycrylic is the way to go. It's fast and easy to work with.

>Randy: My method over the years has also been, .2 oz CF Veil
>and Nitrate. But, after trying Polycrylic; I've got to agree
>with Brad, Polycrylic is the way to go. It's fast and easy
>to work with.
>
>Steve

The super killer for strength is CF veil and Polycrylic.

How do you keep the flaps from warping since Poly is water-based?

>How do you keep the flaps from warping since Poly is
>water-based?

Lay them flat while they dry. Weight them down after they tack off over a non stick surface. Let fully dry that way.

I'd use a carbon torque tube in the leading edge.

>I'd use a carbon torque tube in the leading edge.

Interesting you should say that.

I am not really sure the most effective way to use carbon reinforcement would be in the leading edge. Using that method the internal moment of the flap (measured perpindicular from the flap hinge line) would still rely on a balsa moment arm. It seems to me that a tube in the CENTER of the flap with maybe a couple smaller tubes bored perpindicular through that tube, supported at opposite ends of the flap would reduce the twist spanwise most effectively.

Using that method the carbon tune would truly act as a torque tube, and the forces would be translated by carbon moment arms and not balsa. Also, the support would be at the furthest most tips of the flap spanwise.

Also the steel leg of the control horn should go through the tube also.

Anyway, it is a very cool method what you have done. Got us all thinking around here.

This appears to be a corollary to your belief that prop gyroscopic moment decreases as the the prop gets closer to the CG.

>This appears to be a corollary to your belief that prop
>gyroscopic moment decreases as the the prop gets closer to
>the CG.

Huh?

>This appears to be a corollary to your belief that prop
>gyroscopic moment decreases as the the prop gets closer to
>the CG.

Ouch.

I think I put the tube at the LE because they had a lot of 3/8" aluminum tubing at the surplus store. I made the carbon tubes by winding prepreg around aluminum tubes. That resulted in .40"-OD carbon tubes, which were too big to go elsewhere in the flap. Besides, it made for a nice, round LE. Having the tube farther aft would avoid hinge slits, but would also cause torsion stress to go up for the same amount of material. One could also compare the tube-to-rib stresses, I reckon, for a little tube surrounded by rib to that of a big tube with rib on just one side. Paul's the stress guy, but if you ask him where to put the carbon tube, he'd probably tell you to put it in the elevator pushrod, rather than the flap.

Howard,

Only quesitons I have are, how do you cook the CF under pressure after wraping it around the tube? And what sort of respirator do you wear when dumping HCl down the thing to melt the aluminum?

>Only quesitons I have are, how do you cook the CF under
>pressure after wraping it around the tube? And what sort of
>respirator do you wear when dumping HCl down the thing to
>melt the aluminum?

Wrap it in Fascal, which shrinks when you heat it.
Working outdoors, dip the result in a tall, skinny "bucket" of acid.
Let it sit for a while.