Rotaryphile

All of my original design, taper-wing, fullly aerobatic biplanes increase their spin rate when "down" elevator is applied after the spin is established. The spin is begun in the normal manner for upright spin (full up elevator), then when full down elevator is applied, the rotation rate increases by about 50%. The same thing happens with inverted spins; application of "up" elevator greatly speeding up the spin. I don't understand the reason for this. It may have something to do with blanketing of the horizontal tail by the two wings, but who knows? I haven't observed this peculiarity with monoplanes to any degree.

Montague

I have no idea what causes this kind of thing.

However, I have had my Cap 21 do someting simular. I never got it down exactly, but once in a while, I'd do a Lomchvack (sp), and hold the controls. Somtimes the plane would fall in to a knife edge spin, but sometimes it would fall in to a scary-fast rotating UPRIGHT spin. (holding full down elevator the whole time here).

I splattered the plane, so I never got to really figure out exactly why it went one way or the other, or how to make it always do one or the other. It also never occurred to me to try an upright spin and then reverse the elevator only to see what would happen.

rmh

watch your bipe spin- as you apply the "down"--I would bet that the nose also drops (tail rises) a bit -
this is akin to a skater who spins (rotates) on a skate tip - then moves arms out -in and the speed of rotation changes
It is called conservation of angular momentum
I won't venture any formulas as to what and how - but basically that is how it works.
A flat spin is easy to change rate-- that's how I saved my plane -

acropilot_ty

It is called an elevator accelerated spin, and it happens because of the conservation of angular momentum as stated above. It's easiest to think of the engine swinging around up front. When you push foreward the engine is brought closer in to the axis of rotation and the result is a faster rate of spin. It's actually a bigger killer in the full size world then the famed flat spins, because you are doing one of the correct recovery inputs, just not in the correct order.

Ty

Rotaryphile

Very interesting observation, Ty. Next time out I will try to observe whether the nose-down attitude is greater with down elevator, although if the nose went down steeper with down elevator in an upright spin, I think that I would have noticed. Reversing the aileron deflection while spinning one of these birds slows the spin rate, and greatly slows the sink rate, as the airplane approaches a flat spin, and I will also test the effect of down elevator in a flat spin. Live and learn, as they say. Aerodynamics has a way of keeping a person humble.

Ben Lanterman

I believe the equations for spin rate are related to the mass moment of intertia as noted. It is a function of mass times distance from the axis squared.

The spin velocity is a function of inputs due to aero moments and forces and the mass times distance squared (some liberties have been taken because I can't remember them all!) As the elevator drives the angle of attack down the distance from the spin axis of mass get lower, the answer is lowered by a function of squared numbers and the aero forces and moments haven't chanced all that much. So you get a faster spin.