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Building Tips and technical articles. => Building techniques => Topic started by: John G. Park on August 30, 2012, 11:00:02 AM
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Heard about this and understand the concept.
How do you do it, though?
Specifically, how to "blow up"a print of a regular rib, rules for amount of trusses, is 45 degrees essential, what about center of wing and bell crank area, etc?
Is there a place to turn to for information?
Thanks
John Park
AMA 29443
Flint MI
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Webber Truss? I know the Warren Truss, but not this.
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Well, if Randy doesn't know, then I guess I don't feel so bad.
I assume you mean a wing that has diagonal ribs meeting at the LE and TE? Or do you mean a wing that has chordwise ribs with diagonals in between?
At any rate, if the chord is constant then you just draw a rib with the desired thickness and a length that gets you from point A on the LE to point B on the TE.
If the wing is tapered then you do the same thing, except that you need to measure or calculate all your rib lengths individually. If you're going with all diagonal ribs, and you don't mind the angles not matching up quite, you can use evenly tapered ribs and just stick them where they fit. No matter what, unless you are a numero-uno computer whiz, it's probably sensible to plan on cutting the ribs a bit oversize and plan on sanding the wing to shape with a long bar sander.
There's no magic in 45 degrees.
I'm not sure what you mean by "amount of trusses", but if you mean "number of ribs" then your biggest problem is going to be covering sag, so eyeball things, decide how much sag you can live with, and go. I'd be inclined (if I did this at all) to use straight ribs from spar to LE, and a truss in back. You lose some strength, but you ribs where you need 'em to reduce covering sag.
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Diagonal bracing instead of shear webs does not have to be at 45°, just attach the braces to the spars beside the ribs. Here is the way I do the centre bays where the bending moment is maximum and the bellcrank gets in the way.
The shear web is made from two layers of 1/32" sheet laminated with the grain at 90° to each other. In this particular design, there is a piece of 3/8" square bearer between the ribs, glued to the ribs and spar doubler, carrying the bellcrank pivot and the loads. A similar shear web with clearance for the bellcrank is fitted in the open bay in the pic and then diagonal braces to about half span on each side.
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ill be doing somthing like that for my fancher twister. what keeps the rod from falling through the wing?
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...... is 45 degrees essential.........
Its more an ideal angle to aim for as sheer failures exhibit themselves at that angle between two parallel load bearing spars.