Ross's Video on Dark Aero

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Jay Kempf

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Depends on how you want to define the word "spar." Comes from the naval world. Implies something high aspect ratio that supports a lot of surface area to the wind. When you get into composite structures it is hard to use several hundred year old definitions. And if you do you get limited to metal or wood thinking in molded structures made of string and glue. Hard to build something lighter and stronger than a Pitts but it can be done. It isn't easy though. You have to account for the tension in every single string in the glue just like rebar in concrete. You have to reconcile compression and tension in weird 3D shells with odd shear flows. Not for someone who doesn't like math. Computers help. It is possible to design a hollow wing with nothing that might look like a traditional spar. And that brings it right back to Dark Aero. They have designed a wing that uses a lot of shear webs but no traditional spar per se. What they have figured out is you need a lot of bond area if you don't have a wide cap. so they made what is a whole bunch of I or C beams in close proximity to each other. Probably not the most optimized structure but they were focused on CNC and premade materials and they are making it work. It will be as strong as the edge bonds of the panels to the skin. Bonding edges of honeycomb panels like that normally means routing out the edges and either bonding in a rectangular structural member or filling it with resin and some filler and then sanding smooth. I think they are using the cake bag gob it on technique instead and just distributing the bond area across a lot of webs instead. If they can get a FOS of say 3.0 in peel they are probably good. Dick Schreder, Michel Col0mban, John Monnet and others have done similar with foam ribs and mostly it all worked out.
 
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berridos

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I like their construction in several ways. Infusing the skin in one shot allows for constant thickness of the skin, and it would be a very easy infusion without potential quality issues. With constant thickness skin its very easy to cnc spars and ribs with high accuracy and insert them precisly. Its really a very cnc tailored construction method and the cnc doesnt need to be so big. Just build a big honeycomb panel and cut it in elements in the cnc.
However i dont understand the discontinuos spar elemnts. Wouldnt the intersections with the ribs be a natural longitudinal breaking point of the wing? Their cnc wasnt so big to cut longitudinal continuos spar elements.
 

Jay Kempf

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RE: discontinuous spar elements...

... In order to understand how the shear flow works in this sort of composite structure you have to visualize a bunch of I beams living next to each other. If you separate the skin into spanwise pieces cut halfway between the spar webs you have each i beam. It doesn't matter if the webs have vertical splits every so often because they are only providing stabilization for the skins for buckling. Then the skins provide all the reaction against torsion. Each little narrow ibeam is right next to the other joined by the skin (cap) and so the sum of all of them makes a large multicell spar with all the compression or tension due to bending running in the skins. To do it properly you would have ply drops along the span in the skin. But taper of the shear elements along the span can do the same. This is probably not the lightest way to build a wing with carbon composites. But for a kit it is pretty straight forward to build with a high probability of getting it right assuming the bond area of all those elements to the skin has a huge FOS.
 

berridos

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Therefor would the skins have unidirectional cloth at 0º maybe 2 layer on the upper skin and one layer on the bottom skin, plus the usual 45º for torsion? As I understand the whole skins are the spar caps?
 

Jay Kempf

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Frequently in those sorts of multicell wings there are 0, 90 and 45 layers. I don't know what their ply schedule is but I would guess equal layers of 45's and 0 but all structural cloth not UNI. Makes for a pretty easy layup on a large part. They should have ply drops but they probably don't to keep it simple. It could be lighter. You have to do the math on the interaction between plies. Recently I ran into one where UNI spar caps were being pulled apart by the 45 layers because the torsion strain in the 45 layers was pulling apart the UNI layers perpendicular to their layup direction. The answer was to dump the UNI and go with cloth at zero angle so the 90 fibers could support the zero fibers. This is what I find fascinating about composites. You don't know until you go through it one string at a time and then there's the contribution of the resin at the end.
 

berridos

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What is surprising is that at each station they keep the distance between the spar segments even. Shouldnt the spars be spaced more closed where the skins suffer the highest pressure or suction to keep buckling of skins in check there? Doesnt look optimized in that aspect.
 

EzyBuildWing

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Dark-Aero.....how much better performance will it have than the old Questair-Venture"??
10%.....20%......50% ????
Looks to have similar 6-cylinder engine, drag etc.
 

Kyle Boatright

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Dark-Aero.....how much better performance will it have than the old Questair-Venture"??
10%.....20%......50% ????
Looks to have similar 6-cylinder engine, drag etc.
Ventures often have 300+ hp from their IO-550's. That's 100+ HP over the engine spec'd for the Dark Areo. Ventures are often 300mph airplanes in level flight, which is ~10% more than what Dark Aero projects.
 
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