Glide Ratio

[Forrest Barber]

Re: Glide Ratio

Wow I better read this tomorrow , IMHO and after doing about 300-400 engine out landings at airshows, I stop the prop at 5000 ft AGL and do aerobatics on the way down ala Duane Cole ( my hero) ..... sometimes the prop starts up again usually passing through 100 mph, I can feel the drag. the last part is always with a stopped prop, I DO go furthur that way!! How about the term "zero thrust", that is about 1000 to 1100 RPM. lower RPM , the prop pushes back against the thrust washers.... thus more drag , I Think

[Ron Coleman]

Re: Glide Ratio

Hi Bill
Explain to me why we went to so much trouble to make folding propellers on our rubber powered competition models, please.

Ron C
N96995

[N95179]

Re: Glide Ratio

Forrest has the key: Zero Thrust

This figure changes for different airspeeds as the drag curves move.

There is a difference in drag between a windmilling prop on a dead engine and a prop simply idling on a running engine. With a dead engine the relative wind must push through the blades forcing them to turn (creating drag), while an idling engine still produces thrust (although minimal), especially at extremely low airspeeds like after touchdown where the air being blown back is at a faster velocity than the forward airspeed. The prop can act as a puller or a brake, it just depends on the airspeed and power setting.

This is why killing the engine on touchdown keeps you from running up the bank at the other end of the lake.

[flyguy]

Re: Glide Ratio

That was pretty funny, I just wrote almost exactly the same thing as above after coming back from the rotating prop experiment link without checking this second page. Just deleted it. Very well put Chris. Nice straightforward explanation.
My comment on the experiment was that it proves that different pitch props rotate at different speeds and that a fast rotating prop absorbs or redirects more energy and therefore produces more drag than a slow rotating one.
Does that surprise anyone?
I would give the guy a C for working so hard.
DC

[Dan Mahalak]

Re: Glide Ratio

now that is cool.

[Bobdog]

Re: Glide Ratio

Guy's, Remember, your prop is nothing but a rotating wing...when it's turning, it's creating lift AND drag. When it's stopped, it's just a 1X6X72 in the wind. Another example, if you tow a Gyroglider with the blades stopped there's no more drag than pulling a kid's wagon, but start spinning the blades, and it will take a car to get you up to 10 MPH.

Bob

[N74DV]

Re: Glide Ratio

and a helicopter with a failed engine will fall MUCH faster with the rotor blades stopped than rotating

[Robert Lees]

Re: Glide Ratio

...and that's the truth, unfortunately, but it qualifies the discussion exactly.

[Ron Coleman]

Re: Glide Ratio

Hi again Bill..
Don't bother answering my question about the folding prop, it was meant for a different Bill and a different subject. Cut and paste can have unintended concequences.

Ron C

[N74DV]

Re: Glide Ratio

Hi again Bill..
Cut and paste can have unintended concequences.
Ron C

no doubt..... just ask John Bobbit!

[FredK]

Re: Glide Ratio

The EAA CAFE members devised a crankshaft endplay position sensor to determine zero thrust durning glide testing. FK

[Dusty]

Re: Glide Ratio

May I suggest a practical experiment:

Find an airport, gravel bar, frozen lake, or other suitable largish place to land.

Climb well above said largish landing area.

Yank red knob on dash all the way out.

Observe prop spinning away.

Observe ground approaching.

Slow till prop stops spinning.

Observe ground still approaching, but a little slower.

Push nose over. Notice not a lot of anything happening, if you're running a flat prop. Push nose WAY over. Promise self to make DAMNED sure prop does NOT stop turning close to ground in absence of suitable landing areas.

Dan: there's a lot more to landing short than getting slowed down. I fly short landings as slow as I can and use power to adjust my glide path. "Short" almost always means "precise" - who gives a damn if you can land in 300' - if it happens on the wrong half of a 500' strip. Slips make a whole bunch more drag than anything any prop could do, and I have absolute control over where I hit with power available.

If you shut down on skis/glare ice you're just along for the ride. Keep that thing running! I would have broken a couple airplanes without some power to dodge stuff. I can turn her sideways with rudder and no power, but that doesn't change my trajectory much. If you _really_ need to get shut down in a hurry, rumor has it that full power and full left rudder will do it - either by getting you going backwards, or peeling the gear off.....

Kicking a door open makes some difference. Or just makes me feel better, maybe. Gives the passengers something to do, anyway.

Kicking the rudder around seems to make some difference. Full aileron makes some difference, but does goofy things on really slick ice.

A couple bolt through your skis (heads lower than your skegs) make great brakes for not much drag on takeoff, but they REALLY pick up overflow.

[whake]

Re: Glide Ratio

I remember back in the days when I flew Corsairs in the navy I could slow a Corsair in a verticle dive by comming back on the Throttle. that big 15 ft 6 inch prop acted as a brake. Note the picture I put wheel pants on the T-Craft, so far no difference in performance that I can detect, buit it sure is purty..

[mulwyk]

Re: Glide Ratio

Walter:

Sure is.

[DannyDot]

Re: Glide Ratio

Yes it is purty. I like your paint job as well as your pants.

Danny Deger