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Building Tips and technical articles. => Building techniques => Topic started by: mccoy40 on October 18, 2019, 07:28:27 AM
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All,
I was looking at a set of plans for a cosmic wind - a profile with about a 42 inch span. What I am questioning is that there were two spars 5/16 by 1/4 and the spars twisted as it went further out on the wing.
Is this advisable? I mean would you be stressing the wing and basically building I a warp possibly? :o
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All,
I was looking at a set of plans for a cosmic wind - a profile with about a 42 inch span. What I am questioning is that there were two spars 5/16 by 1/4 and the spars twisted as it went further out on the wing.
Is this advisable? I mean would you be stressing the wing and basically building I a warp possibly? :o
I would replace them.
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All,
I was looking at a set of plans for a cosmic wind - a profile with about a 42 inch span. What I am questioning is that there were two spars 5/16 by 1/4 and the spars twisted as it went further out on the wing.
Is this advisable? I mean would you be stressing the wing and basically building I a warp possibly? :o
I had a Cosmic Wind a long time ago and I don't remember a twisted spar. Any chance you can take a shot of those plans?? If it is a true "Twist", I am with Motorman but if it is "bent" then that is a different issue. Balsa is the Alzheimer's of the wood world. It has little to no memory. If bent and left bent it will assume that shape eventually especially if some moisture is involved. The Cosmic Wind does not have an elliptical wing or a tapered airfoil (other than length) so I see no reason for anything but a straight spar.
Ken
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Do you mean twisted, like the driveshaft on a '32 Ford that's had its 85 horsepower engine hot-rodded to 200 horsepower?
Or do you mean bent, like a guardrail that stopped a hot-rodded 32' Ford with a powerful engine but with the original mechanical brakes?
And are the two spars placed symmetrically, so that one is twisted clockwise while the other is twisted counter-clockwise?
No matter what, a single 5/16x1/4 spar doesn't have nearly the torsional rigidity to affect the wing of a 42-inch wingspan CL plane. So there's something else in the wing structure -- either wing sheeting, or possibly the wing covering -- that will completely overpower any twist imparted by the spars.
If the wing has LE sheeting and shear webs, that "D" tube will hold the wing in position once the glue dries, even if individual pieces of wood are held in stress (so be sure it's straight on the building board!) Even if the wing has no sheeting, it'll be floppy in torsion until you cover it -- then the covering will hold the wing in place (and let the wing be adjusted later, which is nice).
Finally, if there are two spars twisted in opposition, then as long as you choose matched pieces of wood, either by selecting spars that seem evenly matched to you or by cutting them out of a single sheet that does not vary much across its width, (which is overkill but what I would probably do) then they'll cancel each other out.
Post a picture of the plans, or a link, or tell us the name of the designer so we can do a search, but I think you're fine.
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I was looking at a set of plans for a cosmic wind - a profile with about a 42 inch span. What I am questioning is that there were two spars 5/16 by 1/4 and the spars twisted as it went further out on the wing.
This does not compute. D>K
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All,
I mean twisted. SO the spar starts out in the middle as parallel to the leading edge and trailing edge faces. like so | | | - top spar.
| | | - lower spar
then as it gets to the tip the spars twist, and are not parallel to the leading edge and trailing edge faces - like so | \ | - top spar at tip
| / | - lower spar at tip
Hope the diagram helps
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Looks bent, not twisted. If it bothers you get rid of it, it's probably a 25 cent piece of wood! Or less.
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Steam is a friend
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If the spar wood is warped or twisted, replace it but I think your question is different. If you place the center rib next to the tip rib are the sides of the spar notch parallel to each other or is there a different angle.
Which Cosmic Wind do you have? If memory serves me (and it doesn't more that I would like), the Goldberg Cosmic Wind that I had as a kid had the spars well behind the high point of the wing. This had the effect of cocking them in so that they were flush with the ribs. I think they were behind the high point at the center as well. The Brodak version is probably the same since it came from Goldberg.
Let's say for the sake of argument that they were at the High Point in the center and did indeed twist at the tips. As long as the wood is matched and you make sure that they fit the notches properly then the twists will offset each other and not cause any warping. By the time you get to covering the wing, any tension in the spars will have relaxed anyway. Balsa has virtually no memory. That is why it molds so well. If it really bothers you dampen the spars and twist them into shape before you glue them in.
Ken
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Ken
You are correct - the spar twists as it moves out on the wing - the spar twists - it is straight but twists from perpendicular. I will wet the spars and twist them into shape. I need to get a photograph of the ribs - It was plan that was done by Mike Garmon.
There are two spars, upper and lower both twist - I use an Adjusto jig so all my ribs are held straight as I build and I can attach both spars without taking the wing off of the jig
Thanks,
Mccoy40