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Engine failure - glide tests


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Following up on my recent dual mag failure that resulted in an off-field landing just shy of an airport in my M20J. Luck was with me that day to find a smooth field and no damage to the plane. If only there was a way I could have extended the glide to make it just a bit further to the runway. Turns out there is a way but it seems to be a secret. To my knowledge I’ve never received training for it and don’t recall ever seeing it mentioned in a POH.

Simply pulling the prop control back to course pitch on a windmilling propeller extends the glide a considerable amount. I set about collecting some data today.

Flight test in calm air and light winds. Starting at 7000MSL, idle cut-off, windmilling propeller, 90 KIAS, 10nm glide.

First test, prop control full forward in flat pitch. Descent rate of 750fpm, over the airport at 1500MSL. That’s 550ft/nm.

Second test, same starting location and parameters except prop control pulled full back to course pitch. Descent rate of 600fpm, over the airport at 2600MSL. That’s 440ft/nm.

If my math is correct, that’s a 25 percent better glide. Wish I learned this previously but if there’s ever another engine out pulling the prop should be an automatic reach.

Fly safe.

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2 hours ago, ArtVandelay said:

For those of us with EFBs, that’s a glide ratio of 13.8.

My only question is if there’s a problem with lack of oil pressure, will the prop still feather?

No, it won’t increase pitch, it takes pressure to overcome the spring.

Also of course it won’t help in the event the engine isn’t windmilling.

‘It will help in the most common reason for engine failure though, fuel starvation.

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1 hour ago, BravoWhiskey said:

Same on my POH.  Prop full forward for all Emergency procedures.  For the CFIs out there, pulling the prop back is a maneuver for commercial isn’t it?  

Didn’t use to be, I was never taught it, but it’s something you learn talking to other people.

Most Commercial training is I think geared for you to move onto bigger aircraft, turbines and they feather of course, the glide ratio increase from feathering is so great that most crop dusters that dead stick don’t make the intended landing area, they end up in the trees past the field they were making for.

‘My opinion is you need to go out and practice things like this, just cruise at lowest power possible for a couple of minutes to cool the engine off, then idle cutoff, just bringing it to idle isn’t really a dead engine. At altitude of course with forced landing area planned and easy to make, uncontrolled airfield is good.

‘Next step is stopped prop, which is very easy on my C-140, and you can’t pull a fixed prop anyway, but my Mooney has to be in a stall to stop the prop and the altitude lost in slowing to stall and stall recovery I’m sure more than negates any glide ratio gain from a stopped prop. So good idea in the 140, bad idea in the Mooney, so things change from aircraft to aircraft.

Flew a 172 once that was so down on Compression that no matter how deep the stall was, the prop just would not stop, airplane flew fine though and seemed to make good power.

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I was taught this in my Commercial ASEL training.

You can feel the difference in drag.  Just set up best glide, trim, then pull the prop back.  We did it at idle power, not shutting down the engine.

So emergency procedure for engine might be prop full forward initially, but if you can't get it started, then prop full low RPM for best glide.  

And remember, if you need some drag, you can push the prop control forward again.

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7 hours ago, ArtVandelay said:

For those of us with EFBs, that’s a glide ratio of 13.8.

My only question is if there’s a problem with lack of oil pressure, will the prop still feather?

On a feathering prop the lack of pressure is what makes it feather.    Most of us don't have those, though.    If a windmilling prop is turning fast enough for the prop governor to generate sufficient pressure to drive the prop to course pitch, it can do that, but the course pitch will reduce the rpm, so I don't know how well it really works in practice.    Testing it with the engine running at idle won't reveal that, either, I suspect.

 

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In the flat pitch glide at 90KIAS (idle cut-off) my engine was windmilling at 1400RPM.

In full pitch glide the engine dropped to 700RPM.  I was a little concerned, especially getting closer to the ground, that the engine might stop.  Lack of oil pressure would then place the prop back into flat pitch.  It's my recent understanding, however, that a dead stick is best for glide, even at flat pitch.  I wasn't up for trying to collect that data.

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Yes, supposedly a stopped prop is the lowest drag.  And the drag difference from flat to coarse pitch is minimal.

But you have to get very slow to stop the prop.  And if you go to fast, it will start windmilling again.

In your case, even if the prop stopped, you still have the starter. :)

 

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7 hours ago, EricJ said:

On a feathering prop the lack of pressure is what makes it feather.    Most of us don't have those, though.    If a windmilling prop is turning fast enough for the prop governor to generate sufficient pressure to drive the prop to course pitch, it can do that, but the course pitch will reduce the rpm, so I don't know how well it really works in practice.    Testing it with the engine running at idle won't reveal that, either, I suspect.

 

We have enough oil pressure at idle speed to operate the prop governor, of course the governor is itself an oil pump, boosting pressure to operate the pump, so it should, but I’ve not tried it.

Be wary of doing it at idle engine running, the pitch could be high enough to even add thrust, and with the engine idling, the fuel burned probably at least off sets the energy that’s normally stolen from the aircraft to spin that dead motor, so in other words it’s pretty sure your glide engine running will be much better than actual dead stick.

I’ve stopped the prop on my Mooney, it was much harder to stop than my 140, and much easier to get it spinning again, from I assume a much flatter prop in the Mooney at min pitch.

Go try yourself, but I came away with the opinion that stopping the prop on my J just isn’t a viable technique.

I say give the prop pull a try if you lose the engine, worst thing that could happen is that it won’t work, which means nothing will change

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15 minutes ago, GeeBee said:

Actually you have to get going fairly fast to windmill after a stop. On my PA-18 you had to go well into the yellow arc. Constant speed prop may be less, but also many times higher compression ratio engine.

 

I’ve got a video doing it in my 140 with a C-85 engine, which is very low compression, and like you said I had to dive into the yellow arc to get it spinning, but my J model with its high compression engine didn’t require nearly as much speed, I could see you getting it spinning again accidentally.

My theory is at min pitch, the Mooney’s prop is much flatter, and as an old Helicopter test pilot, I can assure you flat pitch is the way to get things spinning, if you lose an engine on a helicopter, you had better be dropping the collective to reduce pitch in a hurry.

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Loss of oil pressure makes a feathering prop go toward feather and a non-feathering prop go toward flat pitch. This is kind of academic because it is the pressure from the governor that counts, so this only tells you what happens if the governor fails in a particular way. If the engine oil pressure gets low enough, the prop will stop when the engine seizes.

It takes energy to turn the engine, and the faster it turns, the more energy it takes. When the engine quits, the only energy source is potential energy (altitude). So, clearly the faster you turn the engine, the more it will affect your glide.

Whether the lowest drag comes with a stopped prop or a windmilling prop in high pitch depends on the compression ratio, number of cylinders, engine friction, high pitch stop angle and prop blade area. The windmilling prop has to extract energy from the airstream to turn the engine. However, a stopped prop presents significant flat plate area to the airstream which causes more drag than a windmilling prop. So, the correct answer is: it depends. The NACA did a study on this as I recall, but I don't have the report number handy.

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20 hours ago, Skyland said:

Following up on my recent dual mag failure that resulted in an off-field landing just shy of an airport in my M20J. Luck was with me that day to find a smooth field and no damage to the plane. If only there was a way I could have extended the glide to make it just a bit further to the runway. Turns out there is a way but it seems to be a secret. To my knowledge I’ve never received training for it and don’t recall ever seeing it mentioned in a POH.

Simply pulling the prop control back to course pitch on a windmilling propeller extends the glide a considerable amount. I set about collecting some data today.

Flight test in calm air and light winds. Starting at 7000MSL, idle cut-off, windmilling propeller, 90 KIAS, 10nm glide.

First test, prop control full forward in flat pitch. Descent rate of 750fpm, over the airport at 1500MSL. That’s 550ft/nm.

Second test, same starting location and parameters except prop control pulled full back to course pitch. Descent rate of 600fpm, over the airport at 2600MSL. That’s 440ft/nm.

If my math is correct, that’s a 25 percent better glide. Wish I learned this previously but if there’s ever another engine out pulling the prop should be an automatic reach.

Fly safe.


Somebody did some extensive testing with his Mooney trying to find the same info a few years ago…

He wrote about his experiments with a ton of detail….

Now to remember Chuck’s screen name…  Find @cnoe…   :)

 

When you have altitude… there is a lot than can be done to extend the glide distance… (weather and glide rings can be really helpful)

Vbg… is weight dependent… (Long Body POH)

Stopped prop is reported best for Mooneys… keeping stopped can be a challenge… (resource?)

Prop pulled way back helps… pushed way forwards (2700 rpm… you can feel the downshift / braking action)

Doing testing with the engine running… provides plenty of power… it won’t be the same with the Engine actually off… (slightly mis adjusted idle causes long landings…)

Engine friendly idle… is a lot of power…. (1krpm for splash oil and heat on the ground )

 

+1 For Nate Jaros’ book on the subject… get an E-copy from Amazon for a handful of dollars… very CB friendly! And our Mooneys do better than his Brand B device…   :)

 

Best regards,

-a-

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1 hour ago, PT20J said:

The windmilling prop has to extract energy from the airstream to turn the engine. 

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Is the energy being extracted from the airstream, or from the kinetic and potential energy in the aircraft?

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6 minutes ago, MikeOH said:

Is the energy being extracted from the airstream, or from the kinetic and potential energy in the aircraft?

At that point…..

It’s nearly all potential energy from there…. (Once set up for best glide, the level of kinetic energy is mostly constant all the way to the ground….)

Conserve as much energy as you can…

The rest is all drag…

 

Where we can gain some energy… use a tail wind.   :)

 

Mooney sailors have learned to pick up some good ground speed…

Best regards,

-a-

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8 minutes ago, MikeOH said:

Is the energy being extracted from the airstream, or from the kinetic and potential energy in the aircraft?

Depends on your frame if reference. The propeller doesn’t “know” if it is moving through the air, or the air is moving past it (as in a wind tunnel).

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Just now, PT20J said:

Depends on your frame if reference. The propeller doesn’t “know” if it is moving through the air, or the air is moving past it (as in a wind tunnel).

But, it is NOT in a wind tunnel. The aircraft is moving IN the air mass; I maintain you are NOT extracting any energy from the airstream.  The energy is coming from the kinetic and potential energy of the aircraft.  Pedantic, sure.  But, it is NOT a frame of reference issue.

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1 hour ago, MikeOH said:

But, it is NOT in a wind tunnel. The aircraft is moving IN the air mass; I maintain you are NOT extracting any energy from the airstream.  The energy is coming from the kinetic and potential energy of the aircraft.  Pedantic, sure.  But, it is NOT a frame of reference issue.

Certainly you are correct that for an airplane in flight with an inoperative engine, the energy source is the kinetic and potential energy of the airplane.

However, in the paragraph cited, I was only discussing the propeller drag independent of whether it is attached to an airplane or in a wind tunnel. In the NACA reports, the tests were all done in a wind tunnel, and that’s what was in my mind as I wrote the paragraph.

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I did a practice engine out landing at an airport starting at 3k. Before i was always annoyed with the gear horn beeping as i practiced but this time i just pulled the mixture to idle cutoff and this was way better. No beeping as my throttle was above idle but didn’t matter as the engine wasn’t producing any power so more realistic of what my glide ratio would be. Once i setup my 87 knots indicated i pulled the prop control back and it truly felt like i released the emergency brake on a car. The vvi went from 750 to 400. Next time I’ll try to stop the prop to see if i can and if the vvi drops even more. As i circled around and came up to finale i realized i was too high so extended the gear and even though that helped i saw i was still going to land halfway down the runway so I instinctively pushed the prop control back in and it took a few seconds but the rpms rose up and i could feel the drag come back and i landed in the first third of the runway. As i flared i pushed the mixture back in too to get the engine running again. Now the i interesting  part is that i normally to help slow in the pattern have my RPM’s set at 2200 and if I’m 100 or so kias or faster when i push the prop control in, i will hear and feel the prop speed up. But if i slow to say 80 kias when i push the prop control in i get no change in RPM as the prop is already at the fine pitch stops at that speed. All that to say if you pull the prop control back past where it would normally command 2200 it will slow to an rpm much lower when you have an engine not producing any power at a slow speed. I don’t know how slow you would have to get the engine turning to lose oil pressure in the prop governor but it’s obviously lower than my max course commanded pitch setting at 80 kias can do. Again it would be interesting to be at max course pitch and slow up maybe to stall speed to see if the pitch would stay or would the governor finally lose enough oil pressure as the engine was coming to a stop and have the prop go back to fine pitch?  Maybe next time i goto shutdown the engine I’ll pull the prop to max course pitch before mixture cutoff and once the engine stops go out and measure the pitch to see if it’s different or the same as when i shutdown with the pitch in fine position. 

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3 hours ago, Will.iam said:

I did a practice engine out landing at an airport starting at 3k. Before i was always annoyed with the gear horn beeping as i practiced but this time i just pulled the mixture to idle cutoff and this was way better. No beeping as my throttle was above idle but didn’t matter as the engine wasn’t producing any power so more realistic of what my glide ratio would be. Once i setup my 87 knots indicated i pulled the prop control back and it truly felt like i released the emergency brake on a car. The vvi went from 750 to 400. Next time I’ll try to stop the prop to see if i can and if the vvi drops even more. As i circled around and came up to finale i realized i was too high so extended the gear and even though that helped i saw i was still going to land halfway down the runway so I instinctively pushed the prop control back in and it took a few seconds but the rpms rose up and i could feel the drag come back and i landed in the first third of the runway. As i flared i pushed the mixture back in too to get the engine running again. Now the i interesting  part is that i normally to help slow in the pattern have my RPM’s set at 2200 and if I’m 100 or so kias or faster when i push the prop control in, i will hear and feel the prop speed up. But if i slow to say 80 kias when i push the prop control in i get no change in RPM as the prop is already at the fine pitch stops at that speed. All that to say if you pull the prop control back past where it would normally command 2200 it will slow to an rpm much lower when you have an engine not producing any power at a slow speed. I don’t know how slow you would have to get the engine turning to lose oil pressure in the prop governor but it’s obviously lower than my max course commanded pitch setting at 80 kias can do. Again it would be interesting to be at max course pitch and slow up maybe to stall speed to see if the pitch would stay or would the governor finally lose enough oil pressure as the engine was coming to a stop and have the prop go back to fine pitch?  Maybe next time i goto shutdown the engine I’ll pull the prop to max course pitch before mixture cutoff and once the engine stops go out and measure the pitch to see if it’s different or the same as when i shutdown with the pitch in fine position. 

I wonder what the effect of opening the throttle fully would have on glide performance with the engine shut down?  

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