Propeller windmilling, the truth

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MrManH

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MrManH
Hello everyone,

I'm back to discuss windmilling and trying to separate facts from fiction. Everyone agrees that a windmilling propeller is undesirable, but how do we explain why?

The most common analogies are:

-it's like holding a disc into the wind the size of the propeller. This is undeniably incorrect as it ignores the simple fact that no matter how fast the propeller is spinning, the spacing between the blades remains constant and air does go through.

-it's like engine braking in a car. Some of the energy that carries the vehicle forward is consumed by turning the engine. This one makes sense on a high level, but I'm not sure.

What I know to be true, based on the relationship between forward velocity and the rotational vector (RPM), is that the AOA becomes negative past a certain reduction in RPM. Therefore my explanation for the negative impact of windmilling is attributed to negative thrust. But is there more to it than just negative thrust (and the obvious form/profile drag that a propeller always exhibits)?

We probably all agree that if our propeller can't be feathered, we're better off if it's completely stopped. Depending on the engine's compression, its resistance helps slow down the propeller. Isn't that progress towards stopping the propeller and therefore reducing negative thrust?

If we go with the car braking analogy where the engine's resistance is the problem. Wouldn't a stopped engine represent infinite resistance and by that logic, be undesirable?

Let me know your thoughts!
 
MrManH said:
Hello everyone,

I'm back to discuss windmilling and trying to separate facts from fiction. Everyone agrees that a windmilling propeller is undesirable, but how do we explain why?

The most common analogies are:

-it's like holding a disc into the wind the size of the propeller. This is undeniably incorrect as it ignores the simple fact that no matter how fast the propeller is spinning, the spacing between the blades remains constant and air does go through.

-it's like engine braking in a car. Some of the energy that carries the vehicle forward is consumed by turning the engine. This one makes sense on a high level, but I'm not sure.

What I know to be true, based on the relationship between forward velocity and the rotational vector (RPM), is that the AOA becomes negative past a certain reduction in RPM. Therefore my explanation for the negative impact of windmilling is attributed to negative thrust. But is there more to it than just negative thrust (and the obvious form/profile drag that a propeller always exhibits)?

We probably all agree that if our propeller can't be feathered, we're better off if it's completely stopped. Depending on the engine's compression, its resistance helps slow down the propeller. Isn't that progress towards stopping the propeller and therefore reducing negative thrust?

If we go with the car braking analogy where the engine's resistance is the problem. Wouldn't a stopped engine represent infinite resistance and by that logic, be undesirable?

Let me know your thoughts!
Click to expand...
A lot of times, physics problems are most easily solved using an energy analysis. Here is my comparison of prop stopped and prop windmilling:

If the prop is stopped, the drag on the propeller is opposed by a force of the propeller on the air. Since the prop is moving through the air, the prop is doing work on the air, i.e., expending energy, and that energy can be calculated as the drag force times the distance that the prop moves through the air. If the forward speed of the airplane is held constant, that energy has to be supplied by the gravitational potential energy of the airplane, resulting in declining altitude.

If the prop is windmilling, you have the same energy loss occurring, but you have ADDITIONAL energy loss due to the fact that the prop is being made to turn the engine, which causes work to be done on the engine. Calculating that work is a little more complicated, but once the windmilling prop reaches a constant RPM, it appears to be torque multiplied by the total angle that the prop traverses. This energy also has to come out of the gravitational potential energy of the aircraft, thus increasing the rate at which the altitude declines.

I haven't considered possible change to the drag on the airframe, which I suppose may be affected to some degree by whether the prop is windmilling or not, but I'm assuming that to be a smaller effect.
 
Interesting discussion topic.

On a twin screw ship with controllable pitch propellers, with one engine making way and the other engine shut down, you go much faster with the dead engine free spinning like ‘windmilling’ airplane propeller than when the shaft is locked. We don’t have an option to feather the prop.

So why would an airplane propeller be opposite?
 
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Rgbeard said:
Everyone agrees with this?

are you sure?
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I've never met a pilot that thought that a windmilling unfeatherable prop was a good thing. But anyway, that's a little besides my question.
 
MrManH said:
I've never met a pilot that thought that a windmilling unfeatherable prop was a good thing. But anyway, that's a little besides my question.
Click to expand...

As a multi engine pilot, I’ve honestly never seen or cared for any debate over windmilling vs stopped in a single engine airplane.

Feathered vs non-feathered is all I ever cared about.
 
MrManH said:
I've never met a pilot that thought that a windmilling unfeatherable prop was a good thing. But anyway, that's a little besides my question.
Click to expand...

You've not met me.

If I lost engine power, the windmilling prop simply means I have a spinning engine and if I can “fix it” it’s closer to running. (I am thinking fuel starvation). Correct my fuel valve issue and I should suddenly have a running engine.
 
I'm not a physicist, so I'm sure there's a flaw in my logic that someone will point out, but if it took less energy for the prop to be stopped than spinning, wouldn't the prop stop on its own?
 
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Rgbeard said:
You've not met me.

If I lost engine power, the windmilling prop simply means I have a spinning engine and if I can “fix it” it’s closer to running. (I am thinking fuel starvation). Correct my fuel valve issue and I should suddenly have a running engine.
Click to expand...

Yep. I routinely run a tank dry prior to switching tanks when I'm on long trips. The windmilling prop helps the engine start right back up when I switch tanks and briefly engage the electric fuel pump.


Sent from my iPhone using Tapatalk Pro
 
Lindberg said:
I'm not a physicist, so I'm sure there's a flaw in my logic that someone will point out, but if it took less energy for the prop to be stopped than spinning, wouldn't the prop stop on its own?
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Yes it will stop spinning once you stop supplying energy to it. That happens when you land and the airplane comes to a stop.
 
Fearless Tower said:
Interesting discussion topic.

On a twin screw ship with controllable pitch propellers, with one engine making way and the other engine shut down, you go much faster with the dead engine free spinning like ‘windmilling’ airplane propeller than when the shaft is locked. We don’t have an option to feather the prop.

So why would an airplane propeller be opposite?
Click to expand...
I'll take a stab at this...

Two major differences-
1- The percentage of the propeller's "disc" that is occupied by the blades shows a stationary prop on a ship will cause far more drag than the thin blades of an airplane's prop.
huxDy.jpg

2- A ship has forward, reverse, and... neutral. When in neutral, the propeller can freewheel without turning the engine, and fighting each compression stroke.
Airplanes have no neutral. If the prop is turning, so is the engine.
 
I am from the dark side (helicopters) and I think a windmilling propeller would have a lot more drag than a stopped propeller

A stopped propeller with 6 inch wide blades and 72 inches long gives a 432 square inch surface causing drag

That same propeller will present a 72 inch disc when windmilling for about 4000 square inches

When a helicopter engine quits the pilot selects a flat pitch and rotor blades continue turning .... which causes so much drag (large disc) the machine descends to the ground about the speed of a descending parachute.

If the rotor was stopped the machine would drop like a brick.
 
A Martin said:
A stopped propeller with 6 inch wide blades and 72 inches long gives a 432 square inch surface causing drag

That same propeller will present a 72 inch disc when windmilling for about 4000 square inches
Click to expand...
Great analogy! I was thinking the faster the RPM, the less space between the blades (think photo-blur in a picture of a turning prop). I also remember years ago in Popular Mechanics or Science an illustration of an invention of a parachute made of a pole you hang onto and a spinning (free-wheeling) propeller overhead. Don't remember if they got anyone to test it.
 
Non Compos Mentis said:
I'll take a stab at this...

Two major differences-
1- The percentage of the propeller's "disc" that is occupied by the blades shows a stationary prop on a ship will cause far more drag than the thin blades of an airplane's prop.
View attachment 96989

2- A ship has forward, reverse, and... neutral. When in neutral, the propeller can freewheel without turning the engine, and fighting each compression stroke.
Airplanes have no neutral. If the prop is turning, so is the engine.
Click to expand...

A lot of people don’t think about this but it takes power to turn an engine, even it it isn't firing. The parasitic and friction loads can be anywhere from 5-30% of rated engine load. The largest friction load inside the engine is piston ring load. Then there is the oil pump, fuel pump, vacuum pump, valve train, charging alternator (and connected electric load), and the big killer is intake manifold vacuum! These loads combined are what slows down a car or truck when you downshift on a hill.

So if the prop is turning the dead engine and the throttle is closed the parasitic load could easily be 20-40 HP that the propeller is pulling from the air. That's a lot.
 
Capt. Geoffrey Thorpe said:
With the prop stopped, you just have the "flat plate" drag.
With it spinning, you are doing work (and creating drag) spinning the prop and engine.
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This!

The physics guys/gals will tell you that work is force X velocity. No rotation means no work (turning the engine). Yes, there will be drag as the prop is being pushed through the air.

-Skip
 
Skip Miller said:
This!
The physics guys/gals will tell you that work is force X velocity. No rotation means no work (turning the engine). Yes, there will be drag as the prop is being pushed through the air.
-Skip
Click to expand...
You are also doing work with a "flat plate" by creating much more turbulence. A windmilling prop has lower AoA, hence less drag. Therefore, instead of wasting energy creating turbulence, the windmilling prop uses the energy to turn the engine. Looking at it from an energy budget point of view is correct, but you have to factor in all energy states. A stopped prop may still be better, but the explanation does not suffice.
 
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lucius said:
You are also doing work with a flat plate by creating much more turbulence due to a much higher AoA. Why would the work to create turbulence by a stopped prop be less than the work needed to turn the engine by a windmilling prop?
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Probably because of friction in the engine, plus compressing and decompressing the air in the cylinders is not a lossless process.

Notice the amount of force you have to apply to turn the prop by hand.
 
Let’s look at this another way. For a fixed pitch prop……..

Prop spins quickly:Negative drag (Thrust). Prop spins quickly, pulls the air faster than airspeed. The more and faster air pulled, the greater the thrust.

Neutral: Reduce the propeller spinning to a speed that pulls the air at a speed matching the plane’s airspeeed and you are at the neutral point on the curve.

Drag: Reduce that even more and you continue down the curve increasing drag. As you lower the rpm, you gradually create more and more drag. At what point would slowing a prop down reverse the slope and create less drag, and then as it got even slower reverse again and start producing more drag? And what would determine such a reversal? There is not such a point or bump on that curve.

A fixed pitch prop that spins freely creates less drag than a stationary one.
 
Zeldman said:
I would bet the sudden silence really got you sweating thereby making no need for the heater..!! :lol:
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The sudden silence didn't happen until the prop stopped after the forced landing, i.e., during the landing rollout (on the runway, fortunately). Until then, the windmilling of the prop made it sound like I only had a partial power loss!
 
One test is worth a thousand opinions so if you can stop the prop try both ways is very calm air. Repeat several times to be sure.
 
The windmilling prop being a "fixed size disk" of any particular size is patently absurd and easily dispelled. Shut down (or idle) the engine and move the prop control. It will windmill at various pitches, but the drag changes dramatically.

In fact, anything over the dead stick drag, is in fact entirely analogous to your car engine braking. If the prop was just a pin wheel with nothing acting on it, the drag would be very low. The energy needed to spin that thing is due to the "pumping forces" in the cylinders. It's not just the cylinder compression or you'd be like a diesel (without a Jake brake). The energy to compress the cylinder would be about the same as the energy released by the piston being pushed back out (minus a little cylinder wall and bearing friction).
 
There was a guy up in the Chicago area. Posted on the Cessna pilot's site that he took his 182 (I think RG) and would climb up to 10,000' somewhere around Gary and glide down to around 4,000'. Restart, climb back up and do it again. Said he did half a dozen times. Oh yeah, with his dog in the plane. I'll see if I can find the post.
 
I had an instructor that would pull the mixture on you from time to time. One time we were right over FDK. "What are you going to do now, captain?" he says as it gets quiet. "Land on 23," I say. "Land on the grass runway. It will be more realistic." So, I did.
 
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?
 
Even if there were no engine attached you would have substantial drag. The helicopter example provided shows how you can auto rotate a turbine bringing a helicopter down fairly under control, and turboprops like a PT6 which have no mechanical linkage to the compressor have an amazing amount of drag if you don’t feather.

And then natures own example. If you have ever played with spiraling seeds. They fall quickly until they start rotating and then they descend slowly as they spin. So yes the engine on a recip consumes some energy, but still the most drag comes from the spinning prop. :)

On my turboprop, to simulate the performance of a feathered prop, I have to dial in 150 foot pounds of positive torque to zero out the drag of the prop. That equates to about 60 HP of wasted energy.
 
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Rockymountain said:
Even if there were no engine attached you would have substantial drag. The helicopter example provided shows how you can auto rotate a turbine bringing a helicopter down fairly under control, and turboprops like a PT6 which have no mechanical linkage to the compressor have an amazing amount of drag if you don’t feather.

And then natures own example. If you have ever played with spiraling seeds. They fall quickly until they start rotating and then they descend slowly as they spin. So yes the engine on a recip consumes some energy, but still the most drag comes from the spinning prop. :)
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The drag is almost all associated with the prop. Has nothing to do with it being a turbine.
 
MrManH said:
The most common analogies are:

-it's like holding a disc into the wind the size of the propeller. This is undeniably incorrect as it ignores the simple fact that no matter how fast the propeller is spinning, the spacing between the blades remains constant and air does go through.
Click to expand...
That's not quite the point. What they say is that it provides the same drag as a disk the size of the propeller. The drag comes not from the propeller blades themselves, but from the spinning propeller turning the crankshaft and compressing the cylinders. See your point #2.
-it's like engine braking in a car. Some of the energy that carries the vehicle forward is consumed by turning the engine. This one makes sense on a high level, but I'm not sure.
Click to expand...
That's exactly it. When your engine is firing, the combustion moves the crankshaft; when your propeller is windmilling, it's turning the crankshaft 10-20 times/second, compressing and uncompressing those cylinders. The energy to do all that has to come from somewhere, and in this case, it's from the relative wind. Energy doesn't come for free, so you pay for it in additional drag.
We probably all agree that if our propeller can't be feathered, we're better off if it's completely stopped.
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I don't fully agree. There are rare cases where I'd consider stopping the engine — e.g. I'm at high altitude over the mountains or ocean, and getting an extra couple of miles of glide is my only hope of survival — but that's a rare situation. Normally, with an engine failure in a slow piston single, you just want to set it down in the closest survivable spot, not risk adding a second emergency by flying it on the edge of the stall to stop the propeller (especially since most engine failures I've heard about happen fairly low, not at cruise altitude).
 
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