Structural changes for using film coverings

[Dennis Toth]

Guys,
With good balsa getting hard to get and expensive we need to start using thinner wood (heavier) to keep costs down and film coverings to keep weight down. Doing this means that we need to think about some structural changes to keep wings and fuses ridged. For fuse structure we can add more formers and use molded top and bottom blocks. I have used simple box fuse construction then used blue foam to shape the top and bottom covered with 2oz fiberglass and finishing resin.

Wings need some thinking. On most normal wing designs like a D tube the sheeting and spars take the bending and the covering handles the twisting. With film coverings there is significant stretch with can lead to the ship doing funny things in windy conditions.

So the question is what kinds of stiffening can be incorporated in the wing structure to add the needed stiffness in twist?

Best,     DennisT

[Steve Berry]

Trailing edge sheer webs between ribs.

Main spar sheer webs between ribs.

Perhaps a geodetic rib layout behind the spars?

Absolutely cap strips for all ribs.

Biggest point may be ensuring all joints are tight so that everything locks together with minimal slop.

At that point, though, the most effective way, assuming you're building on a job and not "in the hand" or pinned down to the building surface, is to fully sheet the wing. Done right, once the glue is dry, everything is locked in and there should be no twisting.

Of course, fully sheeting the wing leads to just using a foam wing.

Steve

[EricV]

Guys,
With good balsa getting hard to get and expensive we need to start using thinner wood (heavier) to keep costs down and film coverings to keep weight down. Doing this means that we need to think about some structural changes to keep wings and fuses ridged. For fuse structure we can add more formers and use molded top and bottom blocks. I have used simple box fuse construction then used blue foam to shape the top and bottom covered with 2oz fiberglass and finishing resin.

Wings need some thinking. On most normal wing designs like a D tube the sheeting and spars take the bending and the covering handles the twisting. With film coverings there is significant stretch with can lead to the ship doing funny things in windy conditions.

So the question is what kinds of stiffening can be incorporated in the wing structure to add the needed stiffness in twist?

Best,     DennisT

To be honest Dennis, I always preferred a fully sheeted wing, even if it was a millennium wing, just because it was way easier to finish and maintain in the long run, no worries about changing flight conditions causing inconsistent performance, flex, sags, wrinkles, or a stiff blade of grass or your ham-fisted launch buddy poking a hole. Buff all you want and don't worry about a split in the fragile paper. etc.

I agree, we need some alternatives for lightweight structures, not sure what they are, and am curious to see what those smarter than I are going to add here.

One thing I've often wondered, is if we forgo the balsa on a sheeted wing, what other "structural covering" might exist other than balsa that is light enough for our use? Would something like a modern version of really thick butchers paper work like we used to build kite's from, and eliminate the need for silk-spanning or filming the wing? Or some other poly-razmataz composite sheeting or veneer in uber thin light sheets be available?

It would be interesting to have access to some warehouse that supplies these kinds of things to walk the isle's and look, touch, weigh and see what is available in this day and age... (Scotty could always give you the formula for transparent aluminum. Hello... Computer? Oh, a keyboard, how quaint...) you would think there would be something out there that we could afford and hijack to make use of for our purposes. There will always be issues to overcome, like compound curves in areas like wingtips etc., but it's not like film or paper go around those without some effort and complications to make them look good either. We had to learn how to deal with it, and would have to learn to work with new materials just the same. It's easy to say it doesn't exist, but every so often, it probably wouldn't hurt to check and see what's new out there.

EricV

[fred cesquim]

i have been using monokote for a decade or more, never added anything to the structure, all models built light as designed, even my own design i-beam
so far so good.
as long as itīs built without twists, will stay like that after covering with monokote.
maybe for IC bigger engines there will be some issues but i have a Tgazer e-powered and monokoted for years now
as for the fuselages i always paint them ( carbon veil underneath) si o have no knowleadge about twisting with monokote.

[Tim Wescott]

A true D-tube wing, with shear webbing between the spars, is rigid in torsion.

If you can find any articles by Allen Brickhaus see what his thoughts were -- he covered in film because he couldn't tolerate dope fumes, so he would have had some experience at this.

[Dave Hull]

If you are going to eliminate most of the balsa in the wing by using a foam core, then simply add carbon ribbon in an X-pattern from Root LE to Tip TE and vice versa. Top and bottom. Then apply magical covering that (a) costs next to nothing, (b) goes on easily, (c) weighs less than MonoKote, (d) looks as good as painted-in-mold composite wings. Or just build glass/foam/glass skins....

Foamy bouncing slope gliders used fiberglass strapping tape over foam cores. It functioned as a stiffener, and may or may not have needed wood spars.

Some old control line designs (VECO?) had small stringers, perhaps only 1/8" square balsa(?) running diagonally thru the wing if vague memories are true...? but they need to be paired and have vertical separation or they are just going to act as a drag/anti-drag member.

Dave

[Crist Rigotti]

What Fred said!

[M Spencer]

Baron put 1/4 tip ribs , & tips ? in em , so he could haul the covering unwrinkeled . On a Tape . Foist ones . Was the first to get No 1 on plastic in appearace ?? or wassit .

[Ty Marcucci]

On "most", if not all, of Tom Morris builds, he uses 1/16th balsa every where and the wings, usually millenial structures, are super light and do not twist or warp. He covers them with sheeting or Poly span.

I often wonder if many of you have a secret stash of good stuff like I do.  I learned the hard way to do this in the 70's when we had a balsa shortage due to cryogenic tankers using 12" balsa insulation and these ships were 800 feet long.  That is a LOT of balsa.

Also pure luck allowed me to buy the entire balsa stock from a local hobby shop going out of business..a full SIG box. But a lot of it is now in models and club members come begging.

[Steve Helmick]

A true D-tube wing, with shear webbing between the spars, is rigid in torsion.

By definition, a D-tube wing has a vertical full depth spar, so no shear webbing is possible. You are thinking of a "C-tube" wing. Adding full depth diagonal ribs inside the D-tube or C-tube wing will increase the torsional rigidity even more. I'm not sure how much it would hurt to butcher holes in the diagonals for the front leadout, but of course, the smaller the better.

I'm a fan of vertical grain webbing on a built-up TE also. Diagonal rear ribs don't impress me a lot, because they are too long, too flexible, and too little angle. Making small triangles is better structurally than large triangles. Adding a small spar or two between the main spar and the TE will fix some of that, however.  Steve

[John Park]

By definition, a D-tube wing has a vertical full depth spar, so no shear webbing is possible. You are thinking of a "C-tube" wing. Adding full depth diagonal ribs inside the D-tube or C-tube wing will increase the torsional rigidity even more. I'm not sure how much it would hurt to butcher holes in the diagonals for the front leadout, but of course, the smaller the better.

I'm a fan of vertical grain webbing on a built-up TE also. Diagonal rear ribs don't impress me a lot, because they are too long, too flexible, and too little angle. Making small triangles is better structurally than large triangles. Adding a small spar or two between the main spar and the TE will fix some of that, however.  Steve

A long time ago, I knew some free-flight duration fliers who had found that diagonal REAR ribs contributed little or nothing to the torsional stiffness of their thin-section wings, but that diagonal riblets inserted between the ribs forward of the mainspar did add stiffness - nothing like a D-tube, of course, but quite noticeable and for little added weight.  It might be worth trying on Twister-type wings.  (By the way, as far as I'm concerned, adding shear webbing between the mainspars of a C-tube wing converts it into a D-tube wing with an I-beam full-depth spar.  As I build my wings on jigging rods, that's the technique I use - a GMA-style sheet mainspar with alternating over-and-under slots for the ribs is possible but very fiddly to assemble.)