Setting the wing angles
Now that the leading edge is no longer on the bottom of the wing, I had some design questions about how to align the top and bottom wings in the fuselage. My first question was: which wing line to do I pick for aligning the wings? Leading edge to trailing edge? Flat bottom? Top or bottom of trailing edge. I asked this on the FliteTest Facebook group and received an insightful answer from Dan Crews.
He said:
"ideal AOA on the wing and decalage between both wings and the wings and the tail will depend on the airfoil's characteristics itself, and your goals as a designer . . . but there is no data for that airfoil to guide you in that . . . so . . .
The next best thing you can do is realize the air doesn't care as much about the fuselage as you do. It cares what the relative angles between the all the primary surfaces are (decalage), and it cares how that relates to the thrust-line. The fuse does contribute to the forces, but little enough you can file that away as "I've got bigger problems to deal with".
So how do you align the wings to each other? to the tail? Without data, the line you pick for the airfoil is only a reference, so nearly any will do. Even with data, it's only a reference, and you'd use the data to set the wing to behave how you'd like it in various airspeeds and attitudes. For a reference, I'd go with the LE-TE line, personally, since it's generally closer flat at a cruising attitude . . . generally.
Once you've picked the line, decalage between the main wings should be such that the forward wing stalls first (lower wing, for the staggerwing). You always want the nose to drop in a stall. To do this, the forward wing should be just a few degrees nose up from the trailing wing. Just a few (<5). This is a subtle effect. Don't be tempted to make it pronounced. Increasing the decalage between the wings too much will push at least one if not both of them out of optimum for any given attitude. We just want to make sure the leading wing barely falls out of normal flight first.
Ideal decalage between the tail and wings is a bit more tricky, and again, without data becomes an eyeball mechanism. In the end, the up/down elevator will act as a change in camber of the tail surface bringing the effective Stab/elevator AoA back into line with the ideal decalage for cruise -- in effect, you'll trim that out . . . so long as you have enough throw to be happy with that trim. Spitballing, I'd set the decalage between the shallowest wing (most aft wing -- upper for the staggerwing) and the tail to zero (again, assuming the line is from farthest point LE to farthest point TE), since this is likely close to cruise attitude."
Based on that advice and the fact that I wanted the top wing to be flush with the top of the fuselage, I went with "leading edge to top of trailing edge" as my reference line for alignment. I have no idea if that is correct aerodynamically, but it's a good guess.
I then created a template for the
airfoil shape of the wing (see picture below with explanation in linked thread), measured out a 5 degree increase in AOA for the bottom wing, level for the top wing, and used the template to trace the aligned wing shape onto the two halves of the unfolded fuselage.
Here is a picture of the result with the top wing attached and the fuse resting (unattached) on the bottom wing. Note that this is *not* the final alignment of the bottom wing because the wing isn't fully closed up. The final alignment has the leading edge buried up in the fuselage to create the small increase in AOA Dan suggests.
Also I've seen Hildaflyer have good success with bent wing shapes like that (and folded ones too) with a thin layer of fiberglass over the top for skin strength.
