Propeller windmilling, the truth

Salty said:
So, I need to read the linked docs still, been traveling, but here’s where I lose it. If turning the engine is what causes the drag, then why is it so hard to stop the engine from turning? I stalled the plane and still couldn’t stop the windmilling in a skycatcher with the engine not running. If it’s causing so much drag, why won’t it stop spinning?
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Airspeed does not have to go to zero for the plane to stall, and the forward motion of the plane is still driving the propeller. The propeller will continue to turn until the dead engine's resistance to being turned exceeds the tendency of the airspeed to cause it to turn.

In the forced landing I referred to, the propeller did not stop turning until I started the turn to exit the runway, which was far below stall speed. (A larger engine might have stopped sooner.)
 
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Salty said:
So, I need to read the linked docs still, been traveling, but here’s where I lose it. If turning the engine is what causes the drag, then why is it so hard to stop the engine from turning? I stalled the plane and still couldn’t stop the windmilling in a skycatcher with the engine not running. If it’s causing so much drag, why won’t it stop spinning?
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Next time, kill the engine yank, back abruptly on the yoke/stick and try to get that airspeed as close to 0 as you can. It might just stop then. Of course then you gotta hope your starter works to get it going again. :eek:
 
Salty said:
So, I need to read the linked docs still, been traveling, but here’s where I lose it. If turning the engine is what causes the drag, then why is it so hard to stop the engine from turning? I stalled the plane and still couldn’t stop the windmilling in a skycatcher with the engine not running. If it’s causing so much drag, why won’t it stop spinning?
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Because it's constantly getting more energy from the airstream. Think of it this way:
  • gravity is pulling the plane straight downwards
  • the wings are redirecting that energy mostly into forward movement via lift (without a running engine, it has to sacrifice some altitude to capture that energy)
  • as the plane moves forward through the air, the windmilling prop converts some of the relative wind to drag, which steepens your descent path (the wings have to sacrifice even more altitude to get enough energy to keep the airplane flying)
 
32 knots is not a whole lot of energy spinning the prop, yet it kept turning the engine. Intuitively, that tells me the engine isn’t taking much energy to spin it. I’m not arguing, just explaining why I have such a hard time understanding it.
 
Salty said:
32 knots is not a whole lot of energy spinning the prop, yet it kept turning the engine. Intuitively, that tells me the engine isn’t taking much energy to spin it. I’m not arguing, just explaining why I have such a hard time understanding it.
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The prop is efficient at converting rotation to movement through the air, so it's also (unfortunately) efficient at converting movement through the air to rotation. Wind turbines have much bigger blades, of course, but one of them can capture enough energy in a 5–10 knot wind to power a city block (or more), and a 5-knot wind can move a 2,000 ton wooden sailing ship. And even at the stall, the wings are producing enough lift to hold up your 1,000–3,000 lb ++ plane (you don't suddenly drop straight down like a stone).

If windmilling engines didn't create a lot of drag, twin pilots wouldn't waste so much time practicing feathering the dead engine.
 
Rockymountain said:
65 Knots? That is barely relevant to most planes that people are flying where best glide speeds are much higher than that. Drag is exponential, not linear with wind speed.
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My Vg is only 65 mph, so don't get so uppity. It is only the 3rd most popular airplane ever produced - C150, behind two others with 65 kt Vg the C172 and PA28.
 
I don't believe all this bollocks about a windmilling propeller being equivalent to a disk of the same size.

Look! here's an online tool to calculate the drag of things: https://www.omnicalculator.com/physics/drag-equation

Putting in a 75" diameter disk moving at 70kts at around 2000ft standard atmosphere gets you a drag force of around 450lbf. That's close to the maximum thrust of a 172 (which would be 500-600lbf according to random people on the internet, but sounds plausible). Is allowing the prop to windmill the equivalent of running a prop in the other direction at about 75% power? Of course not.
 
flyingron said:
Nonsense. Drag is not exponential with the wind speed. It is proportional to the square of the speed. It is more than linear however.
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Is there a subtlety I’m missing in your statement? Something that is “proportional to the square” sure sounds exponential to me.
 
Ah, posting too early again. You’re using the pure mathematical meaning of exponential, in which the exponent is the variable, where the exponent here is fixed. Most non mathematic obsessed people consider any curve that is sharper than linear and continues to grow sharper, to be exponential, like y = x^2.
 
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I think I might have some insight about the confusion of a windmilling propeller being like a disk of the same size. When analyzing the operation of propellers it is common to use "actuator disk theory". Here the propeller is modeled as a disk, but the disk is not solid. The disk imparts forces on the wind stream and can add or extract energy (propeller vs windmill). A solid disk has a drag coefficient of 1.28. Wind turbines on the other hand, which are trying to extract as much energy as possible, have a drag coefficient of about 0.75.

My guess is that after a game of telephone actuator disk theory turned into a wind milling prop == flat disk of the same size.
 
Well, converting an irrelgular object into a "flat plate" or flat disk is common aerodynamics. I think someone just got over zeealous in assuming the size was the same as the prop "disk". Again, it's clearly wrong because it's clear that you can make quite a difference in drag just by changing the pitch of a variable pitch prop, even without feathering it.
 
steingar said:
A rather silly discussion. Not much you can do about it on a fixed prop. Here's a good question for everyone. If you're trying to stop the prop to extend your glide you likely haven't got a workable engine. My question is without oil pressure can you feather a complex prop?
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You will have oil pressure to turn the blades to stop the prop position as the plane then needs to be flown very close to stall speed AFTER reaching said position. IOW FIRST you change the prop pitch and THEN you slow the plane down.
 
Snowmass said:
You will have oil pressure to turn the blades to stop the prop position as the plane then needs to be flown very close to stall speed AFTER reaching said position. IOW FIRST you change the prop pitch and THEN you slow the plane down.
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I think you might have it wrong here. Most sources advise to slow to best glide speed.
 
steingar said:
I think you might have it wrong here. Most sources advise to slow to best glide speed.
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I don't follow. POH best glide speed assumes prop windmilling with prop blades aligned as much as possible into the relative wind. However; to stop the prop I had to slow to almost stall. Once the prop stopped you can increase glide speed somewhat, hopefully, to a new best glide speed. If anybody REALLY wants the answer then test; don't argue. Of course the purpose of many posts is to just argue endlessly.
 
Snowmass said:
I don't follow. POH best glide speed assumes prop windmilling with prop blades aligned as much as possible into the relative wind. However; to stop the prop I had to slow to almost stall. Once the prop stopped you can increase glide speed somewhat, hopefully, to a new best glide speed. If anybody REALLY wants the answer then test; don't argue. Of course the purpose of many posts is to just argue endlessly.
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Unfortunately, credible testing is expensive, time-consuming, and requires a fair degree of skill (both stick-and-rudder and statistical), so when we go up to test in our planes, the results aren't very reliable (and will often be whatever the person set out to find).

Fortunately, NACA report 464 tested just this question many years ago — see my earlier post. https://www.pilotsofamerica.com/com...dmilling-the-truth.132499/page-2#post-3132908

In their series of experiments, at 100 mph a stopped prop with 17° pitch produced only 6% less drag than a windmilling one turning the engine, and since the drag from the prop would be only a tiny percentage of the total drag acting on an aircraft, it seems unlikely that stopping the prop would make much of a difference in glide range, old pilots' tales notwithstanding.
 
Salty said:
So, I need to read the linked docs still, been traveling, but here’s where I lose it. If turning the engine is what causes the drag, then why is it so hard to stop the engine from turning? I stalled the plane and still couldn’t stop the windmilling in a skycatcher with the engine not running. If it’s causing so much drag, why won’t it stop spinning?
Click to expand...

David Megginson said:
...In their series of experiments, at 100 mph a stopped prop with 17° pitch produced only 6% less drag than a windmilling one turning the engine, and since the drag from the prop would be only a tiny percentage of the total drag acting on an aircraft, it seems unlikely that stopping the prop would make much of a difference in glide range, old pilots' tales notwithstanding.
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So it appears that the answer to Salty's question above is that turning the engine is only causing about 6% of the drag from the prop.
 
David Megginson said:
Unfortunately, credible testing is expensive, time-consuming, and requires a fair degree of skill (both stick-and-rudder and statistical), so when we go up to test in our planes, the results aren't very reliable (and will often be whatever the person set out to find).

Fortunately, NACA report 464 tested just this question many years ago — see my earlier post. https://www.pilotsofamerica.com/com...dmilling-the-truth.132499/page-2#post-3132908

In their series of experiments, at 100 mph a stopped prop with 17° pitch produced only 6% less drag than a windmilling one turning the engine, and since the drag from the prop would be only a tiny percentage of the total drag acting on an aircraft, it seems unlikely that stopping the prop would make much of a difference in glide range, old pilots' tales notwithstanding.
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All flying might be considered expensive but testing is no more expensive that just flying around. Testing does require thinking and some effort but is not that difficult. You can measure the the true glide ratio for YOUR plane by climbing to a safe altitude in very calm air (early morning over water?) then noting glide CAS and loss of altitude vs. time converting to TAS and doing simple arithmetic. Also stop the prop and the do the same. Then you will know could even be a hero by reporting back with what you found.

Or just argue and argue and prove nothing which is the popular choice.
 
Snowmass said:
All flying might be considered expensive but testing is no more expensive that just flying around. Testing does require thinking and some effort but is not that difficult. You can measure the the true glide ratio for YOUR plane by climbing to a safe altitude in very calm air (early morning over water?) then noting glide CAS and loss of altitude vs. time converting to TAS and doing simple arithmetic. Also stop the prop and the do the same. Then you will know could even be a hero by reporting back with what you found.
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You go do that with your airplane. I'm not shutting down my engine in flight unless I have to. What if it decides not to start back up?

Snowmass said:
Or just argue and argue and prove nothing which is the popular choice.
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Put your money where your mouth is, or just be the pot calling the kettle black.
 
steingar said:
You go do that with your airplane. I'm not shutting down my engine in flight unless I have to. What if it decides not to start back up?


Put your money where your mouth is, or just be the pot calling the kettle black.
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It sounds like he already did:
Snowmass said:
I don't follow. POH best glide speed assumes prop windmilling with prop blades aligned as much as possible into the relative wind. However; to stop the prop I had to slow to almost stall. Once the prop stopped you can increase glide speed somewhat, hopefully, to a new best glide speed. If anybody REALLY wants the answer then test; don't argue. Of course the purpose of many posts is to just argue endlessly.
Click to expand...
 
steingar said:
You go do that with your airplane. I'm not shutting down my engine in flight unless I have to. What if it decides not to start back up?


Put your money where your mouth is, or just be the pot calling the kettle black.
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I’ve done it too. Why would a healthy engine not start back up again? It’s really not all that risky. I did it at 3000 feet right above a runway though.
 
Snowmass said:
All flying might be considered expensive but testing is no more expensive that just flying around. Testing does require thinking and some effort but is not that difficult. You can measure the the true glide ratio for YOUR plane by climbing to a safe altitude in very calm air (early morning over water?) then noting glide CAS and loss of altitude vs. time converting to TAS and doing simple arithmetic. Also stop the prop and the do the same. Then you will know could even be a hero by reporting back with what you found.
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The pros will develop a test plan in advance that requires multiple flights on different days, hire a test pilot to fly them, add monitoring instrumentation to ensure that even the test pilot isn't deviating too far from the specs, then repeat the test on multiple days under different conditions before they're confident coming to a conclusion.

An individual pilot (especially an amateur like me) will go up once, do a quick ad-hoc comparison flight, and present the results with 10x the confidence the pros have in presenting theirs. You'll note how almost everyone who buys a speed mod for their airplane finds that it makes it fly faster and lowers the stall speed and shortens the takeoff distance and cuts their fuel burn by 20%. It's fun to do those kinds of tests — no disagreement there — and they'll help you get more familiar with your plane, but their value for the flying public at large is (unfortunately) limited.
Or just argue and argue and prove nothing which is the popular choice.
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Or option 3, post a link to an actual scientific research report, like I did earlier. There may well have been other scientific studies about stopped, feathered, and windmilling props since NACA did theirs in the 1930s, and they might contradict the earlier report, so people are welcome to find and post them to continue the discussion.
 
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Palmpilot said:
So it appears that the answer to Salty's question above is that turning the engine is only causing about 6% of the drag from the prop.
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Not exactly, because they also tested a free-turning prop against one that was turning the engine. According to that study, a free-spinning prop (unclutched from the engine, which we can't do in our planes) produced only about 60% as much drag. It's difficult to be draw overly-precise conclusions, because the windmilling will also cause the prop to turn more slowly (thus reducing its own induced drag), but it's fair to conclude that about 40% of the drag comes from the work of turning the engine, and the rest comes from the prop itself.
 
Salty said:
I’ve done it too. Why would a healthy engine not start back up again? It’s really not all that risky. I did it at 3000 feet right above a runway though.
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My perfectly healthy engine didn't start one day after a fuel stop because the starter failed. I can think of a few others.

I'm just wondering what the comments would look like in the Aviation Mishaps section if there was an incident where the cause was the pilot shutdown the engine just to see how the plane would fly with the engine off and then couldn't get it restarted and misjudged the glide to the runway.

And if there's no procedure in the POH on starting an engine in flight, would that mean it's not supposed to be shutdown during flight in the first place? And if that's the case, and an incident occurred, what FAR would you have violated?
 
donjohnston said:
My perfectly healthy engine didn't start one day after a fuel stop because the starter failed. I can think of a few others.

I'm just wondering what the comments would look like in the Aviation Mishaps section if there was an incident where the cause was the pilot shutdown the engine just to see how the plane would fly with the engine off and then couldn't get it restarted and misjudged the glide to the runway.

And if there's no procedure in the POH on starting an engine in flight, would that mean it's not supposed to be shutdown during flight in the first place? And if that's the case, and an incident occurred, what FAR would you have violated?
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it wasn't perfectly healthy then was it? ;)
 
David Megginson said:
Not exactly, because they also tested a free-turning prop against one that was turning the engine. According to that study, a free-spinning prop (unclutched from the engine, which we can't do in our planes) produced only about 60% as much drag. It's difficult to be draw overly-precise conclusions, because the windmilling will also cause the prop to turn more slowly (thus reducing its own induced drag), but it's fair to conclude that about 40% of the drag comes from the work of turning the engine, and the rest comes from the prop itself.
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I stand corrected.
 
donjohnston said:
...And if there's no procedure in the POH on starting an engine in flight, would that mean it's not supposed to be shutdown during flight in the first place? And if that's the case, and an incident occurred, what FAR would you have violated?
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If you do it in a careless or reckless manner so as to endanger the life or property of another, then 91.13 comes into play.

There is a regulation that prohibits violating limitations. I just checked a random POH, and while there is no minimum RPM listed in the limitations section, there is a minimum oil pressure listed.
 
Salty said:
it wasn't perfectly healthy then was it? ;)
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The "engine" was perfectly healthy. It was the starter that failed. The engine was running fine and would have continued running until it ran out of fuel... had I not shut it down.
 
donjohnston said:
The "engine" was perfectly healthy. It was the starter that failed. The engine was running fine and would have continued running until it ran out of fuel... had I not shut it down.
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My rule for flying is the same as the one I taught my kids about life: don't go looking for trouble, because it can find you on its own.

Shutting down a healthy engine in flight sounds to me like looking for trouble (even if it turns out fine 99% of the time).
 
So now motor glider pilots are violating FAR's? Sheesh.

I guess I'm just a reckless nut job having logged dozens of flights without an engine.
 
David Megginson said:
My rule for flying is the same as the one I taught my kids about life: don't go looking for trouble, because it can find you on its own.

Shutting down a healthy engine in flight sounds to me like looking for trouble (even if it turns out fine 99% of the time).
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If you are actually that risk adverse, then I recommend that you never leave the ground in an airplane.
 
Salty said:
If you are actually that risk adverse, then I recommend that you never leave the ground in an airplane.
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You could say the same thing about, say, flying VFR into IMC, as if the only two options were "zero risk" or "unlimited risk". Most of us draw a line somewhere between the two extremes.

I take calculated risks in aviation when they are legal, manageable, and necessary to get where I'm going (e.g. flying single-pilot IFR, or flying at night), but never pointless ones (e.g. buzzing someone's house, flying under a bridge, or shutting down an engine in flight). I know trouble might find me, but that doesn't mean I have to go out looking for it.
 
David Megginson said:
You could say the same thing about, say, flying VFR into IMC. The only two options are "zero risk" or "unlimited risk". Most of us draw a line somewhere. I take calculated risks in aviation when they are legal and bring a benefit (e.g. flying single-pilot IFR, or flying at night), but never pointless ones (e.g. buzzing someone's house, flying under a bridge, or shutting down an engine in flight). There are enough necessary risks in aviation without bringing superfluous ones into it. I know trouble might find me, but that doesn't mean I have to go out looking for it.
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I suspect you are ignoring far greater risks than those you mention. Do you fly at night, over water, over trees, over mountains or hills, do you switch fuel tanks in flight, are you ever outside glide range of a runway? If you're honest, you'll say yes to at least one of those, and probably more that I didn't list, that are just as great a risk or more, than shutting down an engine in flight, over a runway at 3000 feet AGL.
 
Salty said:
So now motor glider pilots are violating FAR's? Sheesh.

I guess I'm just a reckless nut job having logged dozens of flights without an engine.
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You do you know there is a fundamental difference between a motor GLIDER and a single engine airplane, right?

And who said shutting down the engine is a FAR violation?
 
donjohnston said:
You do you know there is a fundamental difference between a motor GLIDER and a single engine airplane, right?

And who said shutting down the engine is a FAR violation?
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Read the thread. It was suggested.

Tell me, just what is the fundamental difference between a motor glider and an aircraft above a runway at 3000 AGL with its engine off?
 
donjohnston said:
One has a primary design to operate without power. That's why it has the word "GLIDER" in its name. :rolleyes:
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So? There is no practical difference in the scenario I described.
 
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