Hey guys,

Here's a question that I can't get my head around.

An aircraft has a tendency to stay in a spin. This shows:
A. A lack of directional stability
B. Too much directional stability compared to lateral stability
C. A tail fin which is too small
D. That ventral strakes and/or a dorsal fin extension is required
E. That the aircraft needs anhedral or less dihedral

Apparently the correct answer is B.

I thought that too much directional stability relative to lateral stability would make the aircraft more susceptible to spiral dives, rather than spinning. I went with A since B, D, and E are sort of the same thing.

Thanks.

Hi,
In the spin, the aircraft rolls in the direction of the most deeply stalled wing which causes a yaw in that direction. As the aircraft yaws, it further increases the angle of attack on the more stalled wing and the cycle continues. The spin will be aggravated by a lack of lateral stability and by strong directional stability.
Option B is the answer because the lower lateral stability means the aircraft will drop a wing quite readily and the subsequent yaw (directional stability) will be quite strong thus aggravating any stalled condition on the wings as the aircraft autorotates.
Options A, C and D infer the problem is due to the aircraft having too little directional stability. Reduced directional stability would actually inhibit the spin characteristics not worsen them.
Option E is essentially saying the aircraft must be too laterally stable which would make it harder to spin, not easier.

Cheers,

Rich