All valid points. I was not originally intending to keep the wing area the same, I wanted it slightly larger than the Speedster to make up for the added weight, however even with the reduced chord and wingspan the wing area is dramatically more, but not double.
I did not change the tail size and I'm not convinced I need to. It displays some spiral instability which would indicate more directional stability than roll stability. However I do think the short wingspan is creating some roll instability and the short fuselage in relation to the wing chord is providing for a short moment arm for the tail to compensate which would only exaggerate the problem.
. . . and the spin you stalled into is probably more from the small vertical stabilizer than anything else -- From the description (I can't watch the vid from here
) it sounds like you tip-stalled and the tail came out from behind you. That's typical spin behavior. You likely don't have much yaw stability, and that's what it tends to look like. Make the wing longer, induce more dihedral, and your spin-stall probably still won't go away -- it's the V-stab that needs to grow to improve yaw stability.
If you feel the elevator wasn't pitchy, then the horizontal may be enough. A bit bigger would give you more room to tolerate nose-heaviness, but if you've got a good balance that's not necessary.
As always, your build your call.
As for the dihedral, I just "winged" it. I didn't really have any good valid reasons for more or less so I went with what looked about right. At least I have a starting point to work with. It turns very easily so I don't think I need more for a rudder plane, however I think by adding some dihedral to the top wing may help with the stability issues. I may add ailerons on version two which may help with a lot of these issues, but I would rather not to keep weight and simplicity down.
Fair enough. As you know, that dihedral contributes to roll stability, of which you need extra when you're flying R/E (If all the plane has is coupled roll, it needs help to spring-back-to-level it can get when on neutral rudder). Good rule of thumb for a R/E mono-plane with only the center dihedral break is 1-2" per foot of span (1:12 - 1:6 slope, works out to ~5-10 degrees). For a bip, It's probably higher if it's only one wing (not sure of that either), so I'd lean toward the high side of that. Again, I don't know how much you have there, so not sure how your experience is measuring up against the RoTs.
As for a dihedral break on the top wing . . . I'd be more concerned with creating a crease in a seamless/sparless wing panel. For strength, probably best to leave it unbroken . . . and it looks nicer that way too
I think I probably will shrink the wing spacing a bit with a narrower chord. That said I don't have much knowledge on the subject of biplane wing spacing vs chord length. Any further info on this subject would be appreciated!
To give you a feel for the physics, there's a weird venturi effect that happens between biplane wings, and a cord-length is a good breakpoint for the behaviors it creates. The tighter the venturi is (smaller the spacing) the less lift/more drag the wing generates, but the stronger the stabilizing effects are in relation to it -- roll and pitch stability increase as spacing shrinks.
a weird twist to this . . . stagger acts somewhat like spacing -- every mm of stagger (back of forward) will give you that much impact in spacing -- 3/4 cord spacing with 1/4 cord stagger acts like 1 cord spacing.
BTW, there are some improved stall behaviors you can create with decalage between the main wings (relative incidence), but that's your call. Right now it looks like it's at 0 (both wings are parallel to each other), so both wings stall roughly at the same speed/attitude, but if you were to get the top wing to stall first, your bottom wing (the one with all the dihedral) will continue to fly as the nose begins to involuntarily drop . . . and you keep your roll stabilization deeper into the stall. A degree or two of decalage here could make an impressive difference in the tendency of center v. tip stall.
