Glide Ratio

[flyguy]

Out goofin' around at 3k ft so I set up a test using GPS relative to my airport. Started 9 statue out and exactly 3000 ft. Power off, hold 58 to 60 statute indicated. (it's pretty accurate down at that end.) Seven miles and 2000 ft came up exactly together. Figure that gives me 10,650 per 1000 or 10.56:1. I was guessing a little better than 9. Wind was less than 2 statute per the GPS.
Darryl

[N95179]

Re: Glide Ratio

Cool test. I've been wondering if there was an official number written somewhere. I still wonder how much difference it would make with the prop stopped. I'm sure not gutsy enough to test that one out!

What do you guys use for planning? Is 10:1 good enough? I'm looking for a conservative, but realistic number.

[Robert Lees]

Re: Glide Ratio

It will be better with the prop stopped, but IMHO not enough to make a vast difference.

The test needs to be done with reference to pressure setting & temperature...can you recall what these were, Darryl?

[VictorBravo]

Re: Glide Ratio

What you use for "planning" can never be the same thing twice in the real world. The Taylorcraft is very much affected by wind, lift, sink, etc. More than many other airplanes. I have no experience in Alaska, but in my world way down here having a "safety factor" is making course deviations and keeping a very keen eye on where the airplane can be landed without damage or injury. Using a "ten to one" glide as a cut and dried guarantee is not smart in my opinion. If you are on the downwind side of a mountain, you often do achieve a ten to one descent angle... with full power!

Rob, how do you figure the glide is better with the prop stopped? If it makes more drag spinning than stopped, then one would assume the faster it spins the more drag it would make. If that's the case it would make an awful lot of drag at 2350 RPM

The propeller blades on a Taylorcraft are larger than the spoilers on most gliders, and are at pretty much the same angle as fully open spoilers. Now I understand that the spoilers are killing lift on a wing as a primary function, but the drag the spoilers make is very significant. In fact, many "spoilers" on some gliders (Mini-Nimbus, Ventus, Glasflugel 304, Slingsby Vega, etc) are so far aft on the wing that they do not "kill" any lift, but only make ahuge amount of drag.

I would like to see a thrust / drag vs. rpm curve on a propeller from stopped to 2350 rpm at a fixed airspeed like 60 mph. My guess is that the laws of nature SHOULD make it a smooth curve if not a straight line.

That being said, the variable which is not accounted for is the energy it takes to turn the engine against compression. If this energy must be extracted from the aircraft, then that is the only thing which might overcome the fact that the propeller alone makes more drag stopped than when turning. As you say, it's probably a close competition! Perhaps we can have a clutch like the free flight model airplanes competitors that allows the propeller to disengage from the crankshaft

[Ed O'Brien]

Re: Glide Ratio

For me flying twins for 25 years means feathered prop training every bi-annual. I've always been told that an unfeathered prop was as much drag as a piece of plywood the same size as the total prop arc. Meaning that's a bunch of drag.

How would you stop/freeze/feather a Taylorcraft prop? Other than have the passenger jump on the hood and hold it steady. A 200lb. man moving to sit on the engine's top cowling, rearranging the CG, then hand stopping a prop, while you are dead-stick landing... well, that probably not recommended, either. That said, I'm likely not as good a pilot as some of you guys.
With regards;
ED OBRIEN

[flyguy]

Re: Glide Ratio

I just happen to remember the barometric (been checking it regularly since I adjusted the altimeter--working PERFECT) it was 30.19 from our AWOS because I remember checking it at 30.20. Would estimate the temp was 50F.

As to prop drag. You can stop the prop by just putting the nose up and it will stop before you stall. You will have to get moving pretty fast to make it start rotating again. I have read and always heard that a rotating prop does indeed create more drag than a stopped one. Would have to kick that one around quite a bit to figure out why. One thing that surely comes into it is that all engines are first of all air pumps. It takes a significant amount of horse power just to suck the air in and pump it through. That is why 75% power at full throttle will give better specific fuel consumption than 75% power at 75% throttle at a lower altitude. There is a higher pumping loss at part throttle. How that works into the equation for total prop drag is beyond me, at least at this point. The basics of it would say that a turning prop is extracting more energy from the passing air than a stopped one therefore the drag is higher. I have no idea whether that is actually a fact or not.
DC

[dbdhenson]

Re: Glide Ratio

....How would you stop/freeze/feather a Taylorcraft prop? Other than have the passenger jump on the hood and hold it steady. A 200lb. man moving to sit on the engine's top cowling, rearranging the CG, then hand stopping a prop, while you are dead-stick landing... well, that probably not recommended, either. ....
With regards;
ED OBRIEN

I forgot where I found this picture, but I thought it might fit in on this discussion. I'm not sure I would want to be the one doing the hand propping.

[1938BF50]

Re: Glide Ratio

If memory serves, I remember reading an official looking story that the guy was actually changing spark plugs......perhaps during an endurance record?

[Hank Jarrett]

Re: Glide Ratio

Yes, a windmilling prop on a non running engine produces MUCH more drag than a stopped prop. The difference between a disengaged windmilling prop and a feathered prop isn't nearly as much but feathered is still less. NACA (predecessor to NASA) did the math to prove it then ran wind tunnel tests to show the numbers were right back in the 20s and 30s. It may be counter intuitive but there isn't much worse than a windmilling prop driving a dead engine.
Hank
The reports on the research and the wind tunnel tests are in the on line NASA library at http://ntrs.nasa.gov/search.jsp . They just recently re-organized so I can't give you a link to the paper itself but the site can keep a number crunching nerd busy for WEEKS!(;f

[Ed O'Brien]

Re: Glide Ratio

The picture of the fella changing spark plugs, stopping his prop, looking for mice... or whatever he's doing at 3000ft., and without a parachute gives me the willies. There's crazy and then there's Britney Spears Crazy... I think that qualifies as both. With regards;
ED OBRIEN

http://vb.taylorcraft.org/attachment.php?attachmentid=2069&d=1172604141

[Dan Mahalak]

Re: Glide Ratio

Okay then.

to land shorter...prop spinning or not?

I say not, time and time again. Make a prop windmill with engine off in that situation......

Dan

[Ed O'Brien]

Re: Glide Ratio

A windmilling prop stops the airplane faster. Twin guys do this all the time. I've landed with 1 prop feathered and other times with one windmilling. It depends on when the twin instructor pulls your engine power in bi-annual reviews. Sometimes you're on short final and don't have time to feather. Just straighten up the plane, keep the airspeed on the numbers and land. The difference is clear.
Although, I can't tell you what the difference is actual stopping distance was... if I ever knew it. The difference is clear. With regards; ED OBRIEN

[crispy critter]

Re: Glide Ratio

I used to have a 1941 BC12 Deluxe. I used to play with it as a glider just to see what it would do. I could ride the thermals along the ridges to the south of our airport for quite a while.Sometimes I could even gain some altitude if the prop was stopped but not with it windmilling. My altitude limit was 500 feet agl,from that altitude I could either make the runway from the mid point of the ridge line or it still left me enough altitude to dive enough airspeed to get the prop windmilling again and restart the engine.If I were too far down the ridge to make the runway and I could not get the speed to restart then I had 3 fields I could use to land in just in case(never had to use them). In a straight glide,no wind,and no thermals I could glide an extra 2-4 miles from 3000agl with a stopped prop then I could with it windmilling. FYI, If I killed the mags and slowed to 50mph and held it for a few seconds it would stop the prop.....it took about 80-90mph to get it windmilling again.This t-craft had a freshly overhauled A-65 turning a 74/45 McCalley prop,empty weight was around 695lbs.

[Dan Mahalak]

Re: Glide Ratio

let us not talk about feathering here in the taylorcrafts, as we cannot.

Can I see some data for a fixed pitch, rotating prop decelerating an aircraft down, IE more drag, faster than the same fixed pitch prop not rotating?

Only data I have in my limited time above earth was landing short, same wx conditions, on a Really slick piece of ice here in AK on skis. Only way to land short, without overrunning into pavement, is to shut downjust after touchdown. This is my airbrake in the General T. Your telling me otherwise?

[N74DV]

Re: Glide Ratio

I just had this discussion on another forum 2 weeks ago.

I found this

http://www.goshen.edu/physics/PropellerDrag/thesis.htm

and the short answer is... "it depends"