wsmalley

My Feedback: (6)

Bought a sailplane needing lots of TLC. Stripped the covering off the wings thinking they were flat bottomed and discovered a healthy undercamber. Obviously, the way they were originally covered did not seal the covering to the low portion. Having never done it before, how do you do it?

probligo

wsmalley,

First up, bear in mind here that I am a freeflighter and I don't use the kinda films that RCers seem to like.

Second, I guess that this is something a bit biggerr than your normal ff glider so lets take a look at the kind of techniques I use on F1A models.

Covering - mylar attached with latex (balsarite and such)

Clean up all old surfaces, get the old glue crap off and sand lightly, vaccuum and tack rag to get the rubbish out.

Apply adhesive along all spars, ribs, te (the whole width) and le. If there is a D-box I would not skin the whole thing with glue, but do "rib" it at about 50mm gaps.

Cut mylar about 25mm too big all round.

Lay on undersurface, and iron down mainspar/rear of D-box. Use just enough heat to attach the mylar, not shrink it.

Leave to cool.

The next is the tricky bit because you get no second chances.

I generally tack the four corners of the panel, making sure that I get the tension in the mylar equal all round. You want this stage to be smooth and even tension, not tight because ironing the mylar into the curve is going to give all the tension you want.

Then tack the middle te and le, again making sure that the tension in the film is as even as possible.

Tack the midpoints (1/4span).

Repeat, until the tack points are 100mm or less apart, then iron the te and le edges down hard along the full length. Again - no shrink!!!

Now you should be able to start running the iron along the ribs and attaching the film. Do the centre of the panel first, then the root/tip then "join the dots". If you have done the previous steps right it should come out well. Again this is attach not shrink.

Trim the edges so that you have got at least 6mm all round to wrap (as with monokote etc). Apply adhesive to the film, allow to dry and then iron down using a piece of photocopy paper to protect the iron. (how do I know that? )

Leave pinned down for a day or so to allow the film to creep to let out any warps.

Cover top surface. (Pretty much a repeat)

Then shrink the film very lightly. If laying the film has been done right there should be no room for more than that.

I have used the same method with polyester film (the kind that has the threads in it. Lay the thread grain spanwise for best results.

That help?

wsmalley

My Feedback: (6)

Thanks for the treatise on covering! This is my first attempt at this type of wing. Obvious the fellow before me didn't ask the question! I've steered away from anything with undercamber since it looked impossible to do, short of full sheeting. Thanks again, I'm sure there are some others who will also benefit. Bill S.

probligo

Bill,

I am not a lawyer, but please read the caveats I put on it.

I set it out in full because I do not know how you rc guys handle the films that you use. So I dont know if what I have writ is totally foreign or not...

No trouble for me because I am used to working with the B- series airfoils (5-7% camber and 6% thick) and Thomanns foils.

John M. Bigelow

I have planked part of the mid section the heavy part of the camber with balsa first glued from the top then covered it's hard
nothing to tack to. CU