Glide distance

[FireMedic_2009]

I went out flying yesterday and on the way back to my airport I decided to see what the glide distance is on my RV3. I used 90 kts as my glide speed. It came out to less than 2 nm per 1000 ft and that was with the engine at idle and with a 5-10 kts tail wind so with an engine out it would most likely be less than 1.5 nm per 1000 ft. 6 months ago I did an actual engine out to see if the prop would stop or windmill. It windmilled. Next time I go up I’ll do a better flight analysis and measure the distance in both directions and maybe do it with the engine out.

What are the rest of you getting for glide distance per 1000 ft?

 

[BobTurner]

I think you’re in the right ballpark, near 9:1. Note that this is a complicated problem, with lots of variables. You already mentioned engine idling/engine out/prop windmilling/prop stopped. If you do do the ‘with the wind, against the wind’ test, note that the optimum airspeeds are different for these two cases, with the headwind test requiring a bit more airspeed for best results.

 

[Mconner7]

So for us simple folks, no wind is about 1.5 miles per thousand feet?

 

[plehrke]

Speaking of simple rules of thumb:

I have heard (for a low wing) you can glide to anything that is under your wing, meaning visualize a circle around your airplane at span of wing as that is what you can glide to. That then takes into account aspect ratio and altitude above ground in a rudimentary way. I have it on my to-do list to calculate that and also flight test that but not gotten to it yet. Looks like with some quick measurements on the front view of the RV-6 drawing it would be about a 7:1 glide ratio.

I know Foreflight has a display feature that draws a glide distance overlay on the moving map that accounts for inputted glide ratio, winds, and terrain. I don’t use Foreflight, so can’t check, but it would be easy to use it to check the rule of thumb that you can glide to anything within your wing span.

 

[Webb]

If memory is correct, certified planes would be at best glide when the trim is full up and the engine is not running.

Can anyone report this is correct for a RV? And what speed and ratio was the result?

 

[swjohnsey]

Not for me, RV-4 IO-360 CS. Prop back/course/low RPM about 110 mph indicated (a little faster doesn't seem to matter much). Trim is set about neutral .

 

[wawrzynskivp]

Best #s

Always good to know folks are respecting the 'no-fuel' glide test realities. Numbers that float around derived from an engine idling are always troublesome to me.

CS or fixed makes a big difference, the next offering here is all about CS props.

If you are CS then what you do with the knob makes an even bigger difference providing your engine/propcontrol/prop combo allows for control at expected glide RPMs. Some combinations out there will lose coarse pitch pressure at no fuel windmilling #s at best glide. That is a big deal if you don't know it ahead of time. A 9.2:1 glide at coarse pitch will fall apart very quickly if the oil pressure isn't sufficient to maintain coarse pitch. This means that best glide for some aircraft has as the most limiting factor the speed to maintain coarse pitch control. In that scenario you have to know you'll hit the wall somewhere in the landing transition.

To really finesse those #s play with manifold pressure. Doesn't sound logical but think it through: At really low MP the engine is working to create vacuum. At High MP the engine is doing a lot of work on the compression strokes. Somewhere in the middle you will find the engine is absorbing the least energy. This means that once you get the prop slowed down as much as is practicable with prop control, then let it speed up again by adjusting MP. It isn't prop speed that causes drag it's work, energy is energy after all.

This isn't a theory of mine. This is what I discovered by a good bit of 'no-fuel' glide testing. In my aircraft the delta between idle position and about 16" MP is 50 ft/min. Not huge, but some way some day it could matter.