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Design => Stunt design => Topic started by: steven yampolsky on December 31, 2013, 09:59:03 AM
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I am looking for feedback on an idea:
If using molded leading edge extending all the way to the top of the airfoil, do I really need a spar extending all the way to the wing tip? I am thinking of stopping the spar at the rib that has the landing gear mount and that's it.
Am I loosing anything? What are the potential implications?
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Kinda yes, kinda no -- if it's thick enough the sheeting replaces the spar, but unsupported sheeting is much more prone to buckling under compression.
Having an inch wide foam "spar", or 1/16" shear webs, etc., may prevent the buckling.
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Steve,
This would depend on the size and weight of the ship. For a small ship say <36" a spar about half the span would work. For larger ships I think putting 3/32" vertical sheer webs between the ribs is a good alternative to top and bottom spars. I like to put in the sheer webs even with the top and bottom spars just use 1/16".
The sheer webs form a box between each rib and the top and bottom of the molded sheeting to prevent the wing from beer canning under hard corners (take an empty beer can and grab the end and bend), the sheer webs act as a full beer can and resist the cave-in.
Best, DennisT
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The sheer webs form a box between each rib and the top and bottom of the molded sheeting to prevent the wing from beer canning under hard corners
Dennis's "beer canning" is my "buckling under compression". In case you were wondering.
Hey Dennis -- why not "Cinnamon chai tea with organic lemon canning"?
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There's always one there to laugh at you when you put a couple of unnecessary plies of carbon on something, but where's a stress man when you need one?
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Steven I would just put the spars in. They don't have to weigh much but might save your bacon. The wing gets a great deal of load in manuevers that will in time manifest itself in flexing. This can only get worse as the flexing damages whatever skin you were counting on. I have , and have seen other, airplanes WITH softer spars that will show flexing to someone outside the circle. Bounced or rough surface landings also put quite a lot of stress on the wings that isn't accounted for if trying to calculate flight loads. What happens here is some unseen damage occurs causing perhaps a catastrophic failure 30 flights later.
About the stub spars: when you stop the spars mid span you create a hard spot in the structure. This is now a fulcrum where stress concentrates rather than spreads out along the span. This then is your potential failure point. A tapering spar from root to tip is more ideal. This puts the greatest strength where the greatest load is yet trimming weight (and strength) from where its not needed. In short, more pain than gain by removing wing spars. Didn't even talk about warp resistance....
Dave
The fact that its molded gains nothing. In fact if you are then eliminating the leading edge it's worse. That soft thin skin won't give you the strength you might get in other materials.
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I've considered this approach, using an EGG CRATE construction, where the angled ribs intersected at least twice, with ribs angled in the opposite direction. At least one competitive FF design used this approach, which resulted in a straight warp resistant large wing. H^^
I'm adding a view of a constant chord wing I've been working on for a profile stunter I've been thinking about.
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Why not mold a web in so a D-tube is formed that just needs minimal closure? The edge is the weakest part, but you can choose the edge to be under minimal loading so it can be self supporting by locating it more to the center of the wing.
Phil
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In my own designs, the spars (usually 1/4 square) serve to build a straight wing! Wing jig is steel rods, but the spars, once glued in, assist in holding everything steady while the sheeting, or T.E., cap strips, etc. are put on.
I can'r see any good reason to eliminate spars. If weight is a concern, realize that balsa spars are the least of your worries.
F.C.
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Tim,
I guess the Cinnamon chai tea with organic lemon canning would be more proper. But how about we compromise on "Craft IPA Beer canning".
Steve,
How much weight do you think would be saved by not having a spar? There are a couple ships I know of that are D tube with only leading edge sheeting and a sheer sheet spar. They are OTS ships the "Guided Whistle" and Red's "International Stunt Winner". I don't think the weight reduction will be worth the structural weakness.
Best, DennisT
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How much weight do you think would be saved by not having a spar?
Quite a bit, the way I've been making spars lately.
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I am looking for feedback on an idea:
If using molded leading edge extending all the way to the top of the airfoil, do I really need a spar extending all the way to the wing tip? I am thinking of stopping the spar at the rib that has the landing gear mount and that's it.
Am I loosing anything? What are the potential implications?
Steven
Check your stunt history. Besides being the location of creation of the I beam wing, the Detroit area was the location of the development of the sparless stressed skin wing. The Adamisin family developed it and and it was used in several designs.
The V tailed Swee Pea was published In AAM and I think Archie had something published in AAM in the early 70's. They both used this kind of construction. I don't know what .the advantages or disadvantages were, but Big Art showed me a plane that was built by Don Bambrick(?) that was super light. You might want to check out the articles or get a hold of Dennis.
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here is my version of the Adamisin wing. the winf is sparless.
molded shells, laser cut ribs, exterior surface kevlar. Dont use Kevlar for exterior suface if any sanding is required.
Jose Modesto
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Photos of reduced spar with ribs and a sparless and ribless wing.
#1 and 2 reduced spar lengh. wing built as plugin wing, built in lost foam jig
#3 and 4 sparless, ribless shell wings panels. the extent of construction after the shells are molded is the addition of movable leadouts and tip weight box. glue top shell to bottom shell done.
Sinas model at the 2013 Nat's used this type of wing.
Photo #3 painted in the mold 1/4 panel
Jose Modesto
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Dont use Kevlar for exterior suface if any sanding is required.
That's for sure. I've been there. You see a little bump, reach for the sanding block, and have to restrain yourself. It goes against all your instincts.
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Jose,
Every time I see that molded shell wing I am blown away. That is fabulous work, and the ultimate expression of the sparless.wing concept.