Gentlemen,
Your mathematical analysis may well be better than mine, I gave ball park figures but the bottom line is this: A number of BobCats have been lost due to diminished pitch response and the power of the servos used is now in question. The fix is easy, install much more powerful servos and that possibilty is eliminated.

I doubt if the problem is aerodynamic. The high tail of the BC MAY be a problem but I doubt it. The behaviour of the BC at the stall is absolutely fine and although the spin is oscillatory (perhaps due to the gyro on the rudders) recovery is almost instantaneous on releasing the controls.

I trained in the RAF on the Vampire T11, which had a configuration similar to the BC, tailplane blanking or diminished pitch response was never a problem throughout the flight envelope including stalling and spinning, although I should add that control response of the manual ailerons and elevators in roll and pitch was dreadful as Mach crit. was approached. However, I have many years of flying a T tail airliner, the VC10, and after the loss of Tridents and a BAC 111 in deep stall accidents caused by tailplane blanking both in flight test and in service (Trident at Heathrow) we were very careful with the VC10 during stall training because of that high tail, never taking it beyond the stick pusher.


There is another full size precedent, too. The early RAF Hunters suffered from jack stalling on the powered flying controls (mainly ailerons , I believe) at high indicated airspeeds. which frightened quite a few pilots in the '50s. That was fixed with better and more powerful PCUs.

On balance I think servo power is the problem so, if there is even a hint of doubt about servo power, why not just install more powerful servos and eliminate the possibilty that the servo is the problem and perhaps save the aircraft, certainly that's my plan with my XL.

No more ideas, nothing thing more to add, I'm afraid so I'll see how I get on with my XL with 8411s on the elevators. In the meantime if any more BCs are lost because of this problem perhaps owners would keep this list up to date and advise of the servo specification.

Regards,
David Gladwin.

PS If one is running out of pitch control rememeber that extending the gear produces a marked nose up pitch at high speed, that MAY just save the day