ripples and waves?

I have some "ripples" or "waves" in the trailing edge sheeting. It looks like the trailing edge sheeting "cupped" after I put it on except where the ribs kept it flat. The result is fairly large (area wise) depressions. These will show up after the covering and finish is on. How do you fill large depressions like this? These are not small dents or dings that I would normally just fill with microlite filler or some such.

Basically you don't. The only thing I can think of is put small partial ribs under the sheeting to brace it up and keep it from cupping. The other thing you can do is replace the sheeting with stiffer wood. That is most likely your problem from the get go. Too soft a wood. Large spans tend to have the wood cup like you have now.

This usually happens with thin wood when the edge of the sheeting is not straight and is held in position while the adhesive cures, causing lengthwise compression stresses. You can't really fix it without replacing it, but if it's on the bottom surface, leave it alone. If on the top and very noticeable, try Matt's method of extra ribs but it might be better to leave it as-is as a lesson-learned because you'll find the dips are stronger than you think and you'll never completely chase them away. Apart from the visual impact, they don't have much affect on the structure.

Rollin K.

I would install vertical grain shear webs (1/16 balsa) between the ribs in between the forward part of the TE sheeting.

This can also be the result of attaching the covering
to the entire trailing edge as opposed to doping it to
the very front of the leading edge and the very rear of
the trailing edge. Allow the covering to shrink across the
entire bay and length prior to applying any thin dope to adhere
the covering to the substructure.
The cupping will occur wether its is silkspan or polyspan.
Dont ask how I know it will cup with both.
Harder wood will help, but not entirely alleviate the problem.Shear webbing,shrinking the covering TE to LE seems to help greatly.

Busby

Ron,

As others suggest, live with it this time. I recall some experts recommending that we NEVER sand the assembled wing any more than absolutely necessary. That is, very lightly and gently to knock down filler fuzz on large areas, or to finish sections at well reinforced spots -- like LE sheeting to LE piece.

D-tube sheeting sags between ribs more often, and more visibly, if the sheeted sections are sanded too heavily. Most prep sanding for these large areas should be done before mounting the sheeting to the wing, so they say. Thin sheeting can also sag between fuselage formers, and, again, moreso if sanding gets too strong.

One possible process involved: The finish coats in the low zones continue to shrink as further coats are applied - the high spots are sanded down enough to reduce the relative load of shrinking dope.

Bill Netzeband refers to prep sanding -- before assembly -- as getting the pieces to almost final shape and sanded 'satin-smooth' while supported BEFORE assembly. He also refers to his total lack of interest in a front row finish... Still his planes are very respectably done.

Hmm

I have to agree, live with this one. Chalk it up to a learning experience. Thicker sheet or closer ribs next time.

A a kid, I once had this hapen on a 5 foot FF sailplane 1/32 LE sheet.
Tried filling with talc and dope but that only made matters worse.
The more filler I added, the more shrinkage and consequent sagging.

Thin TE are a problem to keep straight and untwistered.
I recently did a repair, replacing a thin moving flap.
The original flap was just light sheet with grain running span
wise so naturally the dope and tissue cupped the flap slightly.

This time around, I used a moderatly stiff strip of balsa along the hinge line and extreme TE but very light cross grain balsa in between.
the cross grain balsa prevents cupping and the harder hinge strip takes the tortional load along the flap. Seems to be working well
with good tortional rigidity.