Wing spar measurements - RC Groups
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Nov 07, 2017, 11:43 PM
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Miraj's Avatar

Wing spar measurements

Working on a small project with a plan that doesnt have much info on it, basic cad.
I'm trying to work out where to cut my carbon tube holes through the wing ribs as the plan calls for a 3 degree dyhedral on each side, but I want to use an alloy rod as the spar with out bending the alloy. I could bend the alloy bar, but from what I have read it weakens the alloy.
So my question is, do I draw out the details on the wing 1:1 scale and work out the measurements that way, or is there some maths involved to get the answer with out drawing it?
I've popped together a small scratch up of the front view, its rough
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Nov 08, 2017, 01:10 AM
B for Bruce
BMatthews's Avatar
Well, angles are just angles. So nothing to scale there. Your idea for the sketch looks perfect. If that angle is the proper and you've allowed for the ID and OD of all the parts then you're laughing. Just need to figure out how to bond the slip tubes for the joiner rod into place.

If the sketch isn't exactly accurate then re-do it and accurately draw the ID and OD of the tubes as best you can. Then draw in the sizes for the joiner and the slip tubes you'll be using... Or will it be a one piece joint? You want this to be 1:1 or even do a twice size enlargement of the root area of the tubes so you don't get much error from the pencil lines. Adjust the length of the joiner a little if required to allow it to work for you.
Nov 08, 2017, 04:48 AM
Sonoran Laser Art
I have done several by hand drawing in CorelDraw but I have nice software now that does it for me. I used reference lines, basically line up all the ribs and plot vertical lines for the center of the circles on the ribs. Using the angle with the ribs aligned to the bottom edge determine the height at each rib from the bottom of the rib to the bottom of the circles. Once you know the circle heights the ribs can be layed out horizontally and you can plot reference lines for the bottom of the circles. Your easiest method may depend on the software you use and how familiar you are with it. Once you have the reference lines in place applying the circles is easy. Then just delete all the ref lines.
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Nov 08, 2017, 06:19 PM
Registered User
Drawing it out is a good way to make sure you aren't missing something. However, if you want to know how much vertical space is necessary between the spar caps, you CAN calculate it with simple trig.

To a first approximation, which ought to be pretty close, multiply the sine of the angle by the distance the rod penetrates into the wing, then add the diameter of the rod. If you want to be really persnickety, use the distance in from the TOP of the rod and the cosine of the diameter. But that's going to be a tiny difference.

Anyway, for 3 degrees, 3 inches, and a 1/4 inch rod, we have:
.250 rod dia + (sin 3degrees)*(3 inches) = 0.41 inches

But maybe we need to add double the wall thickness of any tubing you use to hold the rod. In that case, if the wall thickness is 1/64", we get .438. Allow a bit more for any error drilling the hole. If you're going to make the hole an exact fit, and the root rib is thick, you might want to drill the hole at an angle. If it was me, I'd make things with a little bit of slop, use epoxy and a filler/thixotropic additive*, then jig up the wings while the epoxy sets.

If weight is critical, you might look into making some of the filler from wood with the grain running vertically, which will be lighter than epoxy. If you've got a drill press, you could make up a block the thickness of the shear web and drill a hole in it at an angle, but this requires precision. I think there's more about this on the Allegro Lite page at, or maybe at the Allegro Lite group on Yahoo.

Straight wing rods are nice, but a lot of times metal gets stronger when you bend it. Can't say for sure, since I don't know what alloy you're using and I'm not a metallurgist. If you want to experience this, take some thick, soft copper wire and you'll find it gets harder to bend after the first bend or two in the same place. I think this falls under the "work hardening" category. An extreme example is music wire, which can get above 200,000 psi yield for sizes 1/4" and under.

Microballoons, fumed silica, flour, wood dust, etc. If strength is critical, use fumed silica and don't inhale. It's nasty for your lungs and hard to sand, but it's very nice for making epoxy stay put.
Nov 09, 2017, 05:10 PM
Are you serious?
Miraj's Avatar
Thanks for the comments and suggestions guys. With it being only 3 degrees I think I could get away with making the holes in the supporting ribs a little bigger like Lincoln suggested, then with my laser lines make a small jig. If I build the inner wing panels up first and have the tube supporting ribs just tacked in, i can wiggle things around up against the jig and then secure each rib in.
Thanks again

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