There’s been a bit of discussion (argument) recently regarding the difference between glide ratios at idle power vs with prop stopped. I’ve long known from experience that the glide ratio with prop stopped is less than at idle, but decided to do real measurement tests to settle the argument.
With a Rotax engine there’s no option to keep the prop spinning; it’ll stop and stay stopped very quickly after switching off at glide speed. In an 80kt dive the prop will creep slowly from one compression stroke to the next, but no way spin.
In this test, the idle speed static on the ground was 1670rpm, at 50kts glide speed it was 2350rpm.
All tests done by timing with stopwatch the descent from 3000 to 2000 QNH, early early morning before any thermal activity. Three attempts at idle and three at prop stopped, with the average used for calculations. Just watching a VSI is not nearly accurate enough for this exercise.
For the Savannah, 50kts is the speed for best glide ratio, as determined by earlier testing.
Times recorded for these tests are - at idle 101 seconds, at prop stopped 94 seconds.
At idle - 101sec/1000ft = 594fpm descent rate = 8.5:1 glide ratio @ 50kts.
Prop stopped - 94sec/1000ft = 634fpm descent rate = 7.9:1 glide ratio @ 50kts.
So for each 1000 feet of descent, the aircraft will glide 8,500ft at idle or 7,900ft with prop stopped. So that’s 600ft (182m) less distance at prop stopped than at idle.
That may not seem like much until you’re on a final with prop stopped and find that you’re falling short of you’re aiming point….. So need to aim farther from the intended touchdown point to allow for that difference. Quite a bit farther in practice, and then slip off height to hit the chosen point.
That’s if you’re practiced at idle power approach already, rather than using some power to maintain a glide slope. If you’re using power regularly then you aren’t at all prepared for a deadstick landing…. I always pull power to idle at the start of base leg and don’t change that right to touchdown – good practice. I only need to change that when someone ahead is tying up the circuit with a long powered final….
I’ve done dozens of prop-stopped landings and really value the experience and the confidence that builds. I realize that we’re supposed to only do that with an instructor on board, but I know from early training what cautions are required and follow those lessons carefully. I reckon everyone should get that experience. A convenient way to do that would be when you next do a BFR to ask the instructor to let you do a couple of dead stick landings – you could learn a lot.….. If at first you can’t hit your chosen spot then do it again and again until you know how the picture should look on approach.
But one tip: As soon as the prop fully stops after switch-off, throw the switches back ON to be ready for a re-start. If you don’t, you’re liable to get a heart attack when you need power again and crank the engine and it doesn’t start right away…..