What is Wrong?

[DLC]

Here are a couple of pictures of a plane I am having problems (or should that be 'issues'?) with. The problem is an abrupt snap stall into a left spin. The pictures are of the second version. The fuselage of the first version was wiped out as the result of such a spin. In the first version the horizontal tail, for some reason, shifted while the epoxy was drying and ended up clearly skewed in the horizontal plane relative to the wing and fuselage. I thought that might be the problem, so took great care to make sure the horizontal tail was aligned on Version 2. The results were the same - an apparent snap stall into a left spin from which I couldn't recover. Again the body was demolished. The wing and tail are undamaged, and the body is a quick build. However, before trying again I would like to understand the problem.

The wing is two sheets of sturdyboard bent over 6 foam airfoils and connected with a balsa strip leading edge - no spars, no trailing edge. The airfoil is the WASP from Profili. I have used the WASP for several other planes and all have had very benign stall characteristics. The fuselage is essentially a box of sturdyboard. The tail surfaces are sturdyboard with a hardwood spar at the hinge line. Span is 30" and chord is 6". Wingloading with the battery being used at the time of the crash is 8.4 oz/sq.in. Power is a BP-21 brushless with 8-4.3 prop.

I double checked alignments before and after the crash and the only thing I could find was a warp in the last third of the right wing which made the tip have 1 or 2 degrees trailing edge down misalignment. If anything I would have expected thsi to cause a tip stall to the right, since the tip was at a higher angle of attack then the root.

Does the configuration suggest anything I might look at? Anything I should look at?

[space_case]

Looks like the vertical tail may be under sized. What are the areas of the flying surfaces and the length from say, the leading edge of the wing to the leading edge of the tail?

[DLC]

space_case

Thanks for the input. I had the same thought after the crash of version 1, so I increased the size of the vertical tail. The vertical tail volume of Version 1 was close to the lower limit of the typical tail volume range. The tail volume of version 2 was around the middle of the typical range. I don't have the numbers you mentioned immediately available. I will dig them up.

[Batmanwpg]

< double checked alignments before and after the crash and the only thing I could find was a warp in the last third of the right wing which made the tip have 1 or 2 degrees trailing edge down misalignment.>

This is most likely the culprit. Think about it this way: for left turn you input up aileron on the left wing and down on the right. The lift generated on the right increases and lift on the left drops. Also you have even more lift cause by this extra angle of attack on the right tip so your plane is being snapped over by this lift differential.
Never fly with un-wanted warps. They will cause nothing but grief and confusion :eek:

[Sparky Paul]

I've fixed such wash-ins with Gurney Flaps.
A short piece of 1/8" square balsa glued to the -top- of the washed-in portion, about the same span as the wash-in.