Sorry for resurrecting this thread. But it was a beautiful day today and I finally managed to get over my nerves..
Haha, it is quite unnerving isn't it? Looking at it, thinking "I'm not supposed to SEE you..."
I went and did a deadstick with our CFI afterwards from overhead the field, and I don't recall that we had any problems getting it to stop? Maybe we pulled back to nearly a stall, I can't recall.
At FL55, class E and a block altitude of 3000" to FL55 assigned by ATC, with an emergency field in sight, with 2000 RPM, carb heat on, fuel pump on, and maintaining 80 knots, I finally decided to pull ICO to see what would really happen. As expected, the prop windmilled. Restoring the mixture got it going almost immediately. Good.
Next test. ICO and then try to stop the prop by reducing airspeed. Deep into the stall, almost spinning, but still a windmilling prop. Restore mixture, climb back to FL55. Good. I now know that the engine will keep windmilling whatever the airspeed, unless there is a massive internal failure or unless I do something really stupid.
Try again. ICO, reduce airspeed to about 60 knots, then pull up into a half-g pushover to get the speed well below Vs without getting into a spin. Finally, the engine stops completely. Stationary prop in-flight. Funny thing is, it did not scare me one bit because there still was a lot of wind noise. Restore speed to about 80 knots, key to start position. Engine roars into life after three or four blades. Perfect.
Back to altitude, now for the real test. ICO, Pull up, half-g pushover to get the prop stopped again. Restore the mixture and now dive to see if the airspeed can get it working again. I lost a good 1500" to get to 140 knots but once I got to 140 knots: one blade, two blades and then the engine roars into life again.
So what did I learn today about the R2160? First, the engine on this particular bird will windmill until the airspeed drops below about 40 knots, well below Vs. Restoring the mixture will almost immediately bring it back to life. Second, a stationary prop only needs a small nudge of the starter button to get it windmilling again. And third, a stopped prop on this bird will need about 140 knots to get going again. That requires a 45 degree or so dive and an altitude loss of about 1500".
Of course in another aircraft the numbers will be different but this greatly boosted my confidence in this aircraft. Particularly because it doesn't have an inverted flight system but we do push negative g's in it regularly.