Vortex ring

[JohnDixson]

LW, Re Post #25: ( a bit off subject-sorry )

Chris ( G-12 ) was correct re 80 being a bit higher than min auto ROD for the S-70, but that isn't the only factor going into the recommended, and therefore the auto Nr chart. Selecting the speed for the flare is critical: too slow and all you do is change the pitch attitude without killing off enough of the vertical speed; too high and with what the average pilot would use for a flare in terms of pitch rate, and you get a balloon, which leaves you with plenty of Nr, but too high,and you'll lose that excess Nr coming out of the flare and well before the collective has to be applied for the landing. For the S-70 models ( UH and SH ) 80 was the best compromise. Similar considerations led to the values selected for the 76 and 92 ( and prior to that, for the 64 and 65 models ).

We had a textbook case of what happens when the speed is too slow one day in Stratford. For reasons I cannot recall, a full auto was thrown onto the end of a very different test card. It was a quite windy day, with a cold front having passed thru previously. Gust spread was significant. The non flying pilot was perhaps the test pilot most experience with autos at SA ( ouch ). But not to worry, the other pilot wasn't exactly an amateur either, right? (Not me )

The approach had to be to the west over the river next to the SA heliport and there was a slight ridge opposite the field. Later data from the tower showed that at the point of flare, the wind dropped 15 kts, so that the flare was performed at 65, not 80. You may have seen the film-it has been on the Internet. Aircraft flared, and all those associated data traces look exactly like the successful full autos done previously, but the tail wheel hit the ground at around 42 fps. Broke the wheel and crunched the aft 2 inches of the stabilator. Very embarrassing. In the video, it is clear that the flare did little except change the pitch attitude of the machine, and the data said the same.