Skin hinging ala Composite ARF
 

I just got back from spending the day at the Triple Tree Jet Invitational, and was able to get up close and personal with Bob Violett and his new "BobCat Composite". (BVM's "BobCat" design, now being built by Composite ARF to BVM's specifications)

Guys, it just doesn't get any cleaner than this... The image in the photo is of the right wing. The flaps are skin hinged at the bottom, and "lock into place in the up position, leaving an almost invisible seam... certainly nothing you'd call a "gap". The ailerons are skin hinged at the top, and very precisely cut at the bottom (second photo) so that the aileron leading edge slides up under the cut "wiper"... NICE!

For me, the highlight of the event was simply getting to meet and chat with Bob Violett about design, fabrication, etc.. He is the consumate gentleman, and it was a real pleasure to hang out and see his excellent work and excellent flying.

I've started to post more photos of the event, at http://homepage.mac.com/mikejames/rc.../rc_album.html

RE: Skin hinging ala Composite ARF
 

Thanks for the pics Mike,

I have tried this, but realised I know to little, since mine came out to stiff. I used kevlar for the hinges, maybe to much epoxy.
Anybody tried silicon hinges on a powerplane?

Regards

Coligny

RE: Skin hinging ala Composite ARF
 

Hey, these aren't that hard, with a little practice!

Here's a test I did, using a sample layup of just the rear part of the vertical fin/rudder. I used a piece of nylon release film, cut on the bias, and sandwiched between the glass and balsa. After curing, i lightly scored the outside, (inside was pre-scored) and... Voila! Nice clean hinge line, and easily 90 degrees of control movement.

We're vacuum bagging the actual parts as I write this, and the fuselage mold will be done in a few days. Havin' some fun now!

RE: Skin hinging ala Composite ARF
 

Here's the first skin-hinged test of the vertical fin. (Fin halves here are just taped together, not properly joined.) I left the primer and paint off, so you could see the interior. You're looking at the right side of one of the vertical fins. The hinge material is the same nylon release ply (cut on the bias) used for the hinge.

It was lightly scored on this side after pulling it from the mold, and an 8mm gap was routed on the opposite side, to allow the leading edge of the rudder to move under the trailing edge on the left side. It easily deflects to a little past 30 degrees in both directions, which is plenty for this application. Single-sided versions, such as for flaps, allow as much deflection as you'll ever need... 90 degrees if you like.

As I refine these, I'll post more photos. Right now, I'm still new to this process, and am working out the production bugs. We're close, though.

RE: Skin hinging ala Composite ARF
 

Nice Mike,

Any chance of a section drawing of this layup?

Thanks

RE: Skin hinging ala Composite ARF
 

Ok...

First, I have to give credit where it's due, and say that I make no claims of inventing this. Glider pilots and others, including Composite-ARF, have been doing it, and doing it very well, for a long time. I'm just learning the process, and sharing my results.

The first image is a general reference drawing I did some time ago, as it appears on my site, at http://homepage.mac.com/mikejames/rc...sandparts.html (I'll be updating this page soon, as a few items have changed or improved.) It shows the method for using the outer skin itself as the hinge. The second image is how we're doing things today.

Note that when you create the balsa wing skins, you leave a space, (at our scale, about 8 to 10 mm wide) where the hinge line will be. The balsa is beveled toward the outside glass, to keep resin from pooling up there and filling the space. After the skins have cured, and before joining the two halves in the mold, I score this lightly on the inside, and make sure there's no epoxy there to prevent movement. (A tiny bit won't stop the movement, but a "puddle" will keep the hinge from working right.)

Join the two mold halves, and let them cure. Then, lightly score through the outer layer of glass on the side of the surface that's got the hinge material. Obviously, be careful not to cut the hinge! Some people will use an X-Acto blade, some people hand-hold a dremel cutoff wheel, and others will use a small triangular file. Whatever works for you.

On the opposite side, cut a relieved area approximately 8 to 10 mm wide, which allows the control surface to move freely toward that side. You can easily get the normal 30 degrees or so of movement, depending on the width of the relieved area, and it's very easy to get a full 90 degrees or more of movement on the hinge side, which makes it handy for (bottom-hinged) flaps. See my earlier photos of BVM's Composite Bobcat for an example of how this should look, when it's done really well. If you're really clever with this geometry, you can get the relieved side to slide under the trailing edge of the wing when deflected, like a sort of "wiper".

One other note:
At first glance, you might think that the nylon release material is a stupid choice, because after all, it's designed so that epoxy won't stick to it. (!) BUT... It's also designed so that epoxy passes through it. So, the epoxy on both sides of the material goes through the fabric weave, and bonds to itself. I see very little chance of delamination, backed up by the fact that a lot of people use this material. But, if it spooks you, just use some lightweight (say 1.8 oz.) Kevlar instead.

RE: Skin hinging ala Composite ARF
 

I've been doing exactly what Mike shows, but have been using 2 oz glass (cut on bias) as the hinge material. Works pretty slick.