Glide Ratio

[aerobat95]

Hey guys one more question.  The search function on here has not been working for me and I am trying to find some glide info.  Does anyone know what the glide ratio is in a M20F?  I am looking for how many miles per 1000 feet.  In the T-6A that I fly for every 1000 feet we get 2 miles clean. 

 

Thanks

Ray

[N601RX]

About 13:1.   With a full feathering prop (rocket) it increases to 16:1.

[aerobat95]

So our plane will glide 13 miles for every 1000 feet of altitude???  That sounds pretty good...almost too good.

[aerobat95]

I think I understand now......more like around ~2.5 miles per 1000 feet of altitude.  Anyone know what a Cessna or Piper will do???

[DaV8or]

Cessnas and Pipers are about 8:1 IIRC. I thought the Mooney was 12:1.

[Hank]

I'm sure it varies by model, and will also vary by weight. My Owner's Manual says at gross, clean, it's 12.7:1 with the prop stopped, or 10.3:1 windmilling.

[201er]

Quote: aerobat95

So our plane will glide 13 miles for every 1000 feet of altitude???  That sounds pretty good...almost too good.

[Shadrach]

Quote: 201er

No, 13 miles for every 5280 feet (1 mile) of altitude = 13:1

[Hank]

It has also been reported that in addition to loss of more altitude while slowing enough to stop the propellor, lowering the nose to Best Glide will start the prop windmilling again. Unless there are internal mechanical problems with the engine, expect the prop to windmill. If you run out of oil and it seizes, then you get to glide further. Pulling the prop all the way back will reduce windmilling drag and put you somewhere between 10.3 and 12.7:1. At any rate, 10,300 feet is 1.95 miles, or 1.7 nm per 1000' above ground at Best Glide speed, clean, and gross weight. Vary from Best Glide, not be perfectly clean, you'll go down faster. Under gross, you'll glide a little further. The key seems to be pitching for Best Glide right away.

[jetdriven]

I shut down the engine with the mixture and the prop stabilized at about 2000 RPM.  Pulling the prop back fully did nothing.   It makes a huge difference in an A36 though.  

[Ned Gravel]

Quote: Hank

It has also been reported that in addition to loss of more altitude while slowing enough to stop the propellor, lowering the nose to Best Glide will start the prop windmilling again. Unless there are internal mechanical problems with the engine, expect the prop to windmill. If you run out of oil and it seizes, then you get to glide further. Pulling the prop all the way back will reduce windmilling drag and put you somewhere between 10.3 and 12.7:1. At any rate, 10,300 feet is 1.95 miles, or 1.7 nm per 1000' above ground at Best Glide speed, clean, and gross weight. Vary from Best Glide, not be perfectly clean, you'll go down faster. Under gross, you'll glide a little further. The key seems to be pitching for Best Glide right away.

[jlunseth]

The POH for my 231 puts it at 12.5, and I read 12.7 somewhere, but for planning purposes I round down to 12:1 .  So if I am at 18,000 (3 nm), I am going to get 36 miles.  There are enough variables, one being that if you are ever confronted with an engine out you are going to be doing some "cockpit simple" calculations, so that is close enough for me.  A major variable is wind.  If you are over Lake Michigan at 20000 and the winds aloft are westerly at 65 knots, with a glide speed in the mid-high 80's you are not going to get very far trying to go west no matter how high you are.  You had better be thinking in terms of being able to make it to the east side the minute you break the west shore.

[Hank]

Quote: edgargravel

Hank:

I know the books say this, but it is not intuitively obvious to this old engineer why a windmilling prop, that should be providing less drag, results in a better glide ratio than a stopped prop, that should be providing more drag and resulting in a poorer glide ratio.

Anyone have any ideas?

 

[Bolter]

Quote: edgargravel

Hank:

I know the books say this, but it is not intuitively obvious to this old engineer why a windmilling prop, that should be providing less drag, results in a better glide ratio than a stopped prop, that should be providing more drag and resulting in a poorer glide ratio.

Anyone have any ideas?

 

[Ned Gravel]

I also use the prop forward approach to slow the airplane down when on downwind.  But not during a cruise descent.  One other corrollary to this discussion is the fact that, I believe, the prop governor will return the prop to full fine if the engine quits.  That means that it may not be possible to make the pitch any coarser than full fine if the engine is not driving the bits attached.  I could be wrong about that and a windmilling engine should certainly impart some motion to the attachments connected to the accessory case. 

[jwilkins]

A friend from the FAA told me once that we need to be careful when reading the manufacturer's  specs for speed, glide ratios, fuel burn, landing & take off distances, etc. He said the brand new airframe with a brand new engine, usually no extra antennas and flown by a factory test pilot who can 'feel' changes that the instruments cannot even detect, is not representative of what a typical owner piliot will get. Seeing what you get with your plane and technique is better than counting on 'book' figures.

During one of the MAPA Safety foundation recurrent training flights in my former M20F we did best glide speed with engine at idle and the prop full forward, then prop full back. When the prop was full back the rate of descent was significantly less than when the prop was forward.

 I have never tried engine off simulated emergency procedures, just engine to idle, and 477T was one of those 'really good' airframes that had better than typical glide ratio and speeds. The difference on that plane was significant. I haven't tried it in the K yet.

Jim

 

 

[georgeb]

I would use 1.6nm per 1000ft to build in a fudge factor.

Also, It's been said by many more experienced pilots than I that as a rule of thunb, slowing enough to stop the prop at lower altitudes would likely provide diminished returns as the the time spent at less than best glide speed and the resultant pitch down (and altitude loss) to reattain best glide would likely mean that it's better to glide with the fan turning at anything below ~7K.  Mooneys may be somewhat better because of the airframe's willingness to gather speed quickly.

[Shadrach]

Quote: Hank

It has also been reported that in addition to loss of more altitude while slowing enough to stop the propellor, lowering the nose to Best Glide will start the prop windmilling again. Unless there are internal mechanical problems with the engine, expect the prop to windmill. If you run out of oil and it seizes, then you get to glide further. Pulling the prop all the way back will reduce windmilling drag and put you somewhere between 10.3 and 12.7:1. At any rate, 10,300 feet is 1.95 miles, or 1.7 nm per 1000' above ground at Best Glide speed, clean, and gross weight. Vary from Best Glide, not be perfectly clean, you'll go down faster. Under gross, you'll glide a little further. The key seems to be pitching for Best Glide right away.

[Shadrach]

Quote: georgeb

This is pretty much right on, speaking from experience....oh and by the way, the prop was windmilling.

[Hank]

Quote: Shadrach

  L/D ratio determines glide distance. Weight does not decrease glide distance. Best glide speed goes up with weight, however the distance that can be traveled does not. Minimum sink rate does increase with weight.  So the net effect is, if you're at best glide speed for your weight, you will go just as far at MGW as you will at 300lbs under gross. It will just take longer to travel the distance at the lighter weight.

http://books.google.com/books?id=V3SZXFWuCIgC&lpg=SA3-PA17&ots=HlLIHgjHmB&dq=Best%20Glide%20Speed%20Airplane%20flying%20handbook&pg=SA3-PA16#v=onepage&q&f=false

[georgeb]

 So did you make the North/South taxiway? You're here to post about it, so you must have been able to affect a reasonable outcome.

[Shadrach]

Quote: Hank

The extra time to glide the "same distance" will be of greater importance if the air is not dead calm . . . depending on whether you want to go upwind or downwind, the difference may become critical.

This is why it pays to think these things through ahead of time. Neither page of section "V EMERGENCIES" mentions what to do, or if it is advisable to stop the prop, or if I should go faster or slower flying solo with only reserve fuel vs. climbing out at gross when the engine dies. There is a bland, generic disclaimer that it will be more accurate to note actual performance than to rely on the information in the book. (!) So let's all go be temporary test pilots at the times of our choosing, than have the need to conduct a first test under Emergency conditions.

[Shadrach]

Quote: georgeb

Ha!, actually made the Runway...I was aiming for the taxiway though...Had enough speed to make a steep turn, pull gear down, and it was all over...as soon as that gear dropped, came down like a rock....point the nose down (really hard to do 50' up in they air by the way), flare right at the end....hard landing and rolled about 50' before coming to stop...shut down ELP  8R-26L for about 35 minutes while they got a tug to get me off the runway and Hazmat emergency crew gathered their stuff....Fun times . Everyone and everything was fine though....

[xftrplt]

Why does a stopped prop create less drag than a windmilling one?

-------------

The prop blades are airfoils.  When windmilling and not rotating fast enough they are aerodynamically stalled.

A stalled airfoil makes less lift than one that is not.

A windmilling prop's "lift" is drag to the airframe. 

Clear as mud?

[jetdriven]

Yes.  The windmilling prop turns the engine at 1500 or 2000 RPM, which takes work. A stopped prop has the aerodynamic drag of its associated flat plate area.  But it is less than a windmilling prop.

I think Flying magaizing did a test of a 172 and stopped the prop. It glided further with the prop stopped but it required almost stalling the airplane to stop it. Any speed above Vg and it started windmilling again. I think their consensus was if you were over 7K feet it was worth doing, but is a tricky maneiver.