Glide Ratio

[Scott Todd]

I may be a bit rusty on the math but a statue mile is 5280 feet. So from 1000' above ground, that works out to about 5:1. If the prop stops, it gets better, not worse. A windmilling prop at idle is more drag than a stopped prop.

[Skippydiesel]

Thanks for all the feedback.

Sonex has written to me with a 11:1 glide ratio.

After reading all of your suggestions I have decided to go with a conservative 8:1. The reasoning (hopefully sound) is two fold:

My EFB "OzRunways" has a glide distance ring, around the aircraft icon, that relates to the ratio that I have entered.

The above ring would add to my decision making, achievable landing sites, in the event of an engine out forced landing.

[gammaxy]

Sounds reasonable. I think you can be reasonably confident it will be at least that good.

I've shut the engine down several times in flight. Early in Phase 1, the propeller stopped pretty solid and I dove to 90+mph and couldn't get it to windmill. More recently, the engine seems to want to windmill and would take some effort to get it to actually stop. So, I'm not sure I have a good way currently to test the engine-stopped configuration to see if that gets me to 11:1.

I imagine the Xenos guys might have some useful insight to the propeller stopped vs windmilling question. I wonder if your longer propeller will be noticeably different.

[sonex892.]

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.

[N190YX]

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?

[GraemeSmith]

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?

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 1299 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)

[ldmill]

I may be a bit rusty on the math but a statue mile is 5280 feet. So from 1000' above ground, that works out to about 5:1. If the prop stops, it gets better, not worse. A windmilling prop at idle is more drag than a stopped prop.

Yep - that's what I get for multi-tasking and not paying attention to my typing fingers... 2 miles for every 1000' above ground, or somewhat above 10:1... I also agree with you on the stopped prop vs. windmilling, I just tend to plan ubber conservative when pondering things going quiet up front.
Thanks Scott!
Lorin

[gammaxy]

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.

So... you inspired me to go out and try 80mph today (thanks for the excuse to fly). First, I flew a calibration triangle at about 75mph so I could attempt to determine a correction for my IAS. I then flew 70, 75, and 80mph IAS descents. I got the best glide ratio at 70mph IAS, but when I came back to crunch the numbers I am getting a ~10mph error after also accounting for wind. It's possible my 70mph IAS is really 80mph TAS (which might agree with your observation), but I probably need to investigate further. I am pretty confident that my IAS is reading low, though, just not sure I believe it's really 10mph low.

So my best glide today seems like it was 627fpm at 80mph TAS which works out to a glide ratio of 11.2. I kindof feel bad for advocating for lower numbers if this is correct.

I also tried to glide with the engine off, but couldn't easily stop the propeller, so don't have any data there.

[GraemeSmith]

A "Quick and Dirty" air test this morning in cold hard and smooth air. Basically one shot at each speed. To look for a trend. High concentration to stay on speeds - so way better than a "real world in anger" number.

The "Rule of Thumb" to put the chord line parallel to horizon was at about 65-70KIAS. These are quite different numbers than I got in summer heat and much better. This was in KNOTS not mph. Calculated as such.

Aircraft was at ~950lb Gross

VgTest.jpg (27.27 KiB) Viewed 1137 times

[Skippydiesel]

Very good Graeme. very close to Sonex official 11:1. Achieving this in nice cold/dense air will likely result in slightly lower performance in your summer months.

I assume Sonex derived their glide ratio with the aid of optimum climatic conditions, a light aircraft & a "top gun" pilot .

My glide ration is for entering in to my POH & EFB and will have to accommodate all seasons, pilot detractions, aircraft loading & prop turning/not situations, so I think I will stick with the ultra conservative 8:1, at least until I can demonstrate to myself that I can easily better this.