Making Sense of Best Glide and Glide Ratio

[201Steve]

On 2/16/2025 at 6:45 PM, MikeOH said:

 My biggest fear is going for an airport close to the edge of the glide ring vs. accepting a for-certain makeable off-field site.

This. 
 

So many people have died within a quarter mile of an airport trying to save the airplane. 
 

if I have an engine failure, I care nothing about the airplane. Whatever consequence comes, I’m fine with it so long as I’m sitting at my dinner table that night. 

[TheBearFlies]

I may have gone through that discussion too fast and missed something. Best glide speed per the POH based on weight. That speed will in theory give you minimal altitude loss for distance, traveled. Glide Ratio= distance traveled for altitude lost. Pitch for best glide don’t exceed critical AOA, target a landing spot, run through engine fail procedures.

[Max Clark]

6 hours ago, 201Steve said:

So many people have died within a quarter mile of an airport trying to save the airplane. 
 

if I have an engine failure, I care nothing about the airplane. Whatever consequence comes, I’m fine with it so long as I’m sitting at my dinner table that night. 

I ended up adding buffer to my glide ratio for the rings display. Figured it would help remove temptation if something happened. 

[Max Clark]

Vbg:

3368 lbs 91.5 KIAS

3200 lbs 89 KIAS

2900 lbs 84.5 KIAS

2600 lbs 80 KIAS

FF performance profile has 89 KIAS and 11.5:1 Glide

G3x was set for 85

GTN was set for 89

POH Emergency procedures spell out 85 KIAS in multiple places - this is the number I’m going to use and set everything to

The Hartzel 3 blade prop drops “approximately 7%” on glide ratio - but trying to figure out what the starting number is based on that image…

[jlunseth]

My POH also has a graphical display of Best Glide and Maximum Glide Distance. The Best Glide table shows speeds as low as 76 at 2300 pounds and a high of 87 at 2900 pounds. I just use 81 IAS because it closely approximates the typical load when I am flying alone. Two things to remember. The most common fatal mistake in an engine out is to sit there dumbfounded, nose high during takeoff. You need to be spring loaded to drop the nose every takeoff to avoid a stall. Second, during practice for my commercial and working on circling a spot on the runway for an engine out landing, I found that best glide would sometimes trigger a stall alert. Makes sense - stall at a 60 degree bank angle is 90 which is higher than best glide, and on top of that, when flying with an engine out there is no prop draft over the wing helping the wing not to stall. So if I tried too hard to make my descent shallow I could get into a stall situation. Stall trumps Best Glide Speed so drop the nose as necessary even if that means breaking best glide.

There have been alot of discussions in the Forum about prop effect. From what I recall, the Best Glide chart assumes a windmilling prop. It is possible to stop the prop, it requires a quick pitch up and then dropping the nose to avoid a stall, and it increases glide distance. Don't recall the number. I have put this in my memory bank if I am ever in the flight levels and needing as much glide range as possible, but I have never practiced it and don't think I would try it at lower altitudes because of the stall risk.

Edited February 25 by jlunseth

[PT20J]

I’ve heard lots of discussion about which is the lowest drag: windmilling vs stopped prop. According to this graph from Aerodynamics for Naval Aviators, it depends on blade angle. This agrees with data from an old NACA report I looked up once. 

What is apparent (and easily verifiable if you try it) is that pulling the prop control all the way back greatly improves glide. In my J, the rpm is in the yellow arc at best glide with the engine at idle and the prop set for 2500 rpm. The vibration reminds me to pull the prop control back. 

[Ragsf15e]

24 minutes ago, PT20J said:

I’ve heard lots of discussion about which is the lowest drag: windmilling vs stopped prop. According to this graph from Aerodynamics for Naval Aviators, it depends on blade angle. This agrees with data from an old NACA report I looked up once. 

What is apparent (and easily verifiable if you try it) is that pulling the prop control all the way back greatly improves glide. In my J, the rpm is in the yellow arc at best glide with the engine at idle and the prop set for 2500 rpm. The vibration reminds me to pull the prop control back. 

Pulling it (prop rpm) back is so effective that it almost feels like accelerating forward.  The first few times I tried it, I didn’t really believe it (I had a 3 blade prop so maybe even more effective?).  
When practicing, I wasn’t completely comfortable with having the engine set like that for long, but I didn’t have a reason for that.  It just turns so slow (and sounds different) that it’s different than you’re use to.  It always ran back up just fine.

[Hank]

Surely your POHs have something like this from my little Owners Manual. There are speeds and glide ratios for both windmilling and stopped props; unsurprisingly, stopped props create less drag, with improved glide characteristics. What surprised me just now is the slightly lower Vbg for stopped prop.

It's right before the Performance Tables, same section 

 

[PT20J]

1 hour ago, Hank said:

What surprised me just now is the slightly lower Vbg for stopped prop.

 

That surprises me, also. Best glide occurs at the max L/D speed. In fact, the glide ratio is simply the ratio of lift to drag. Since lift = weight, it's also the minimum total drag speed. Total drag is comprised of induced drag (which decreases with airspeed) and parasitic drag (which increases with airspeed.) The minimum total drag occurs when each drag component is contributing half of the total. Since stopping the prop reduces parasitic drag, it should cause the minimum drag to shift to a higher airspeed, but the chart shows lower.

[Andy95W]

[A64Pilot]

9 hours ago, PT20J said:

I’ve heard lots of discussion about which is the lowest drag: windmilling vs stopped prop. According to this graph from Aerodynamics for Naval Aviators, it depends on blade angle. This agrees with data from an old NACA report I looked up once. 

What is apparent (and easily verifiable if you try it) is that pulling the prop control all the way back greatly improves glide. In my J, the rpm is in the yellow arc at best glide with the engine at idle and the prop set for 2500 rpm. The vibration reminds me to pull the prop control back. 

That is an easy one to answer without any chart.

Spinning a dead engine over takes a huge amount of energy, so where does this energy come from?

Easy, the only place it can, from the potential energy of the aircraft, robbing all that potential energy and maintaining kinetic energy reduces glide a LOT.

But here’s the thing, go out and stop the prop, I had to pull mine all the way until just before stall, going that slow she was dropping pretty fast, then dropping the nose to get back to best glide ate a lot of altitude so I’m convinced unless your really high that you will actually have a poorer glide ratio stopping the prop.

Funny because I can stop my C-140 prop easily, it must be because the constant speed prop is flatter at low RPM?

[A64Pilot]

10 hours ago, jlunseth said:

My POH also has a graphical display of Best Glide and Maximum Glide Distance. The Best Glide table shows speeds as low as 76 at 2300 pounds and a high of 87 at 2900 pounds. I just use 81 IAS because it closely approximates the typical load when I am flying alone. Two things to remember. The most common fatal mistake in an engine out is to sit there dumbfounded, nose high during takeoff. You need to be spring loaded to drop the nose every takeoff to avoid a stall. Second, during practice for my commercial and working on circling a spot on the runway for an engine out landing, I found that best glide would sometimes trigger a stall alert. Makes sense - stall at a 60 degree bank angle is 90 which is higher than best glide, and on top of that, when flying with an engine out there is no prop draft over the wing helping the wing not to stall. So if I tried too hard to make my descent shallow I could get into a stall situation. Stall trumps Best Glide Speed so drop the nose as necessary even if that means breaking best glide.

Heres my 2c, never climb at vx, except to transition directly to vy, and vy should be avoided as too slow, why?

Nobody I don’t care how slick you are if the engine quits at vy your going to find yourself either in or very close to stall, at that point your have to do the thing your brain is screaming not to, to point the nose at the ground to break a stall, but you don’t have the altitude for that so at best you crash flat in a “mush” close to stall with high rate of descent. Vx to stall is only 6 kts with flaps up.

Why do I say Vy is too slow too? because a 60 degree angle of bank takes from memory a min of 1.4 times stall speed, and any normal human will lose some speed if one quits, you might want some maneuverability if it does quit and that means as a min 1.4 times stall. That’s somewhere around 88 kts, you do climb flaps up right? so use flaps up stall speed, my J I think at max gross thats 63 kts

88 kts is all over Vy, so you need to be a little faster if you accept that your not Super Pilot and you will freeze for a very few seconds before you accept that it has quit.

So if your climbing at mid 90’s kts and it quits you may have a decent chance of maneuvering to make either the impossible turn or that field off your left or fight shoulder after losing five kts or so before you get your act together.

Personally I use 100 kts, I like even numbers and a little extra fudge on my cake. Oh and the engine doesn’t get hot as fast too.

 

Edited February 26 by A64Pilot

[MikeOH]

42 minutes ago, A64Pilot said:

...Vx to stall is only 6 kts with flaps up.

Say what???

I can't speak to a "J", but Vx is 94 mph and clean stall is 68 mph for my "F".  That's a lot more than 6 kts!

[Hank]

1 hour ago, A64Pilot said:

Vx to stall is only 6 kts with flaps up.

. . .

Why do I say Vy is too slow too? because a 60 degree angle of bank takes from memory a min of 1.4 times stall speed, and any normal human will lose some speed if one quits, you might want some maneuverability if it does quit and that means as a min 1.4 times stall.

What kind of Mooney do you fly where Vx is only Stall + 6??? 

For my C, Vx = 80 mph; Vy = 100 mph - Altitude. 

Here are my stall speeds:

So yes, Vy in a climb is near stall speed in a 60° bank while holding altitude, which won't happen after engine failure.

I've had my engine cut out in cruise when the tank ran dry, with no burble. I was fat, dumb and happy, watching the clock and fuel pressure. It suddenly got pretty quiet, my wife jumped up and the plane rapidly pitched down to maintain IAS.

This should also happen in climb, if you've got the trim set and aren't pulling back on the yoke. Don't you trim yoke forces away in all phases of flight, or do you hold the yoke back for a 10-15 minute climb? (Oh, my aching arms!)

Anyway, with a dead engine and turning to your landing spot, are you really going to a) hold level altitude, and b) bank 60 freaking degrees? I hope to fly with a dead engine the same way I do with an operating engine, except at 100-105 mph and descending; all turns near standard rate but certainly below 30° bank. Down low, the landing spot better be out the windshield before any turns start!

So realistically, my stall speed will be in the low 70s in all turns and I'm climbing 95+ IAS before transitioning to 105 mph.

How am I going to stall and die by climbing at Vy??? Even if it takes me several seconds to lower the nose to 105 if the engine craps out? Regardless, it would be "exciting" if it happened down low, say < 1000 agl.

[PT20J]

According to my 1994 M20J POH, clean, power off, gross weight, level flight stall speed is 62 KIAS and Vx at sea level is 66 KIAS.

Best angle of climb is the speed where there is the maximum difference between thrust available and thrust required. The bigger the engine, the lower Vx. The F-15 supposedly has a thrust/weight ratio of 1.17:1, so it's Vx is zero.

[MikeOH]

35 minutes ago, PT20J said:

According to my 1994 M20J POH, clean, power off, gross weight, level flight stall speed is 62 KIAS and Vx at sea level is 66 KIAS.

Best angle of climb is the speed where there is the maximum difference between thrust available and thrust required. The bigger the engine, the lower Vx. The F-15 supposedly has a thrust/weight ratio of 1.17:1, so it's Vx is zero.

I'm having trouble understanding the enormous difference between the F and J.  What's the horsepower on your J?

[PT20J]

19 minutes ago, MikeOH said:

I'm having trouble understanding the enormous difference between the F and J.  What's the horsepower on your J?

200. Thrust required = drag, so lower drag will also reduce Vx. 

94 mph seems way too high for Vx. Especially when Hank’s C is 80.

[MikeOH]

1 minute ago, PT20J said:

200. Thrust required = drag, so lower drag will also reduce Vx. 

94 mph seems way too high for Vx. Especially when Hank’s C is 80.

200 HP, same as my F.

 

[PT20J]

31 minutes ago, MikeOH said:

200 HP, same as my F.

 

Vx should increase with altitude and Vy should decrease with altitude and they converge to the same speed at the absolute altitude. The chart shows Vy decreasing with altitude but doesn’t show Vx increasing. Looks like Mooney just picked one number and stuck with it.

 

 

[A64Pilot]

5 hours ago, PT20J said:

Vx should increase with altitude and Vy should decrease with altitude and they converge to the same speed at the absolute altitude. The chart shows Vy decreasing with altitude but doesn’t show Vx increasing. Looks like Mooney just picked one number and stuck with it.

 

 

Some people like to have very precise numbers, but I can tell you that in truth that “close is good enough” Actually when you go out and fly an instrumented airplane you won’t get good solid repeatable numbers even though your certain CG and weight are identical. Then if you do a representable sample of aircraft like you should but the FAA will not allow, your scatter factor is even larger.

CG in particular for glide ratio affects the numbers more than you might affect. 

I think Mooney was smart for picking a number and sticking with it, because we need one number to remember in case of need we obviously can’t pull out a chart and determine an exact number and I doubt many can fly a precise exact speed anyway, not without concentrating on just that one parameter. I can’t anyway.

Over 20 years ago when the D model Apache came out one of its pages on the MFD was a performance page that constantly calculated all performance numbers and updated them for density altitude, gross weight etc. I’d suspect modern airliners do the same? Do any of the glass cockpits in our aircraft do that yet? When you were flying right at the edge it was nice to have.

[Max Clark]

21 hours ago, jlunseth said:

From what I recall, the Best Glide chart assumes a windmilling prop.

That's what's detailed on the POH Glide chart.

[Max Clark]

11 hours ago, A64Pilot said:

Heres my 2c, never climb at vx, except to transition directly to vy, and vy should be avoided as too slow, why?

That's an interesting thought exercise: in engine failure do you want less pitch up/more airspeed, or do you want more altitude?

I much prefer cruise climb in general, but understand the teaching of Vy to 1,000' AGL especially after training in S22T where CAPS is unavailable below 500' AGL.

[A64Pilot]

20 minutes ago, Max Clark said:

That's an interesting thought exercise: in engine failure do you want less pitch up/more airspeed, or do you want more altitude?

I much prefer cruise climb in general, but understand the teaching of Vy to 1,000' AGL especially after training in S22T where CAPS is unavailable below 500' AGL.

If you get to fly heavy as in very performance limited aircraft for awhile you learn that maneuvering especially is an exercise in energy management. Altitude and airspeed are interchangeable. Bob Hoover had it down in his Rockwell flight demos, he even did them in the single engine 114 which wasn’t a good performing airplane.

Having said that my preference is airspeed, reason is if an engine quits, first thing that happens is the prop governor trying to maintain RPM flattens the prop to min pitch, that’s a big speed brake, and or instinct is to maintain altitude, the two rapidly reduce airspeed. Go out and try it, just pull power in a hard climb at altitude and see how rapidly speed decays. The best decision Sully made in my opinion was to not try the impossible turn, the FAA tried to hammer him on it, “proving” it was possible in the Sim, but by the time he diagnosed the problem, then went through the mental process of accepting it he probably couldn’t have made it. That was in my opinion the one good thing in the movie, it showed that well. If Sully couldn’t react instantly, well I doubt I will either.

In the belief that nobody, myself included will react instantly and will find themselves close or into a stall if climbing at Vx I like to carry a little extra speed hoping that by the time I recognize the problem and react I’ll find myself close to best glide speed, which i’d nothing else gives me enough speed for a flare to reduce vertical speed to close to zero before the forced landing, if your close to stall, there is no flare and you will hit hard.

Besides it the only way I can keep cyl head temps down, Cyl head temps are what limit my climb gradient, I’m talking above the first 1000 ft or so, but once they get higher than I like they won’t come down until cruise, but climbing faster keeps them from ever getting up to begin with. I’m a firm believer that Cyl head temps directly correspond the cylinder stress.

Now ALL this is opinion of course and well we are all allowed our opinions, but I like a little extra airspeed up to final of course, on short final I fly slower than recommended to prevent float as I’m often flying into and out of short grass fields. A little extra speed gives me time to think.

 

Edited February 26 by A64Pilot

[PT20J]

Deakin’s take on climb speeds.

https://www.advancedpilot.com/articles.php?action=article&articleid=1842

Airspeed and altitude are both forms of energy — airspeed being kinetic and altitude being potential — and are thus interchangeable (neglecting losses due to drag). But, conversion requires acceleration which has a time element. If you are close to the ground and descending, you don’t have a lot of time. 

[Pinecone]

14 hours ago, A64Pilot said:

Heres my 2c, never climb at vx,

Although there is the consideration that some aircraft climb so flat that they cannot get back to the runway, even from higher altitudes.  So a Vx climb might be better in some cases.

Reference the AOPA Impossible Turn video where the Bonanza could not get back because it was too far away.

EDIT--------------------

After reading the Deakin article above, what he says makes a lot of sense.