sonex892. wrote:Not surprised people are only getting 8:1 glide ratio when using a glide speed of only 60 kts. I found the best glide to be a bit above 70 kts or 80mph. The slower the sonex gets below 70kts the steeper the descent angle becomes.
OK - I'm going to go out and try that again. I found 70KIAS was costing me more than 60KIAS. Wonder how much the stopped prop helps get 11:1? Did Sonex give a suggested speed to get that 11:1 and a loading they did it at?
N190YX wrote:I "believe" the glide ratio is better with a stopped propeller versus a windmilling propeller, because a stopped propeller creates less drag. Anyone know if this is true?
Correct.
The rubber band scale model endurance guys figured this out years ago. When the twisted rubber is expended - stopping the prop got you more time aloft.
There is a reason you feather the prop on a twin when an engine fails. The feathered prop stops spinning and presents a lot less drag than the windmilling prop. It's not JUST about presenting the edge and narroeer profile to the air.
I've measured it in a C150. Comparing glide ratios with a windmilling prop and a stopped prop. You go a lot farther on the stopped prop.
BUT
Getting the prop to stop can be problematic.
- 7,000MSL, Airport below on a quiet day.
- stoppedprop.jpg (48.15 KiB) Viewed 1301 times
Unless the engine destroyed itself in a manner that did it for you. I had to pitch up very aggressively and fly WAY below Vg to get the prop to stop. And during that pitch up - the VSI was unwinding downhill like crazy. At the <10,000ft altitudes we fly these little planes at - stopping the prop will cost you more than you will gain. Above 10,000ft - I think I would want to go out and measure it all again to find when it was worthwhile.
(The exercise of stopping the prop was also to see how much of a dive was needed to get it to spin again and so air-start the engine. Interestingly at higher altitudes it took significantly less of a dive (500ft at 8,000ft) to get the engine to run again compared to lower down (nearly 1,500ft down to 2,000ft MSL). Not quite sure why except perhaps having to compress thicker air in the cylinders required more energy by diving the plane faster)