Windmilling vs Stationary Prop Drag

desertdog71

desertdog71

Girthy Member
OK, guys have fun with this one.

WHY? Does a windmilling prop produce more drag than a stationary prop. You cannot bring the feather argument into this. We are talking specifically about fixed pitch props only. We all know that it is a fact that a windmilling prop has more drag but I need a good explanation why that is.
 
I'm sure Seagull or TG will be along with the mathematical reason, but to put it simply, think about what the prop is turning while it is windmilling. The engine (pistons, crankshaft, pumps, etc) is no longer producing power to move those parts, the prop is extracting energy from the air and moving those parts, thus drag.
 
Dugie8 said:
I'm sure Seagull or TG will be along with the mathematical reason, but to put it simply, think about what the prop is turning while it is windmilling. The engine (pistons, crankshaft, pumps, etc) is no longer producing power to move those parts, the prop is extracting energy from the air and moving those parts, thus drag.
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That was my reasoning also. However this has become a huge debate amongst instructors and students at White Air. So I am looking for more input.
 
desertdog71 said:
That was my reasoning also. However this has become a huge debate amongst instructors and students at White Air. So I am looking for more input.
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What else could it be? You stated not feathered (fixed pitch) so I guess you could take into account the surface are of the prop blades, parasite drag, induced drag and such (the thing is a wing). All things being equal I would be curious to see just how much drag all that would be, without the prop driving the engine.
 
The follow up to that part is:

Then why wouldn't the prop just stop instead of working so hard to crank the engine?
 
desertdog71 said:
The follow up to that part is:

Then why wouldn't the prop just stop instead of working so hard to crank the engine?
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Because you have enough force (airflow) to overcome the friction and "drag" within the engine. Just like pulling a boulder across a level surface, once you overcome the weight X the coefficient of friction you the boulder will move until that force becomes less than (W x CoF).

Just like with a twin when the accumulator pops the blades out of feather, the airflow produces enough force to overcome all the drag within the engine and turn the prop.
 
I used to race a 36 foot sailboat and it didnt have a foldable prop. If you pop'd it into neutral (unlocked it) underway you'd instantly feel the boat decelerate as the prop spun up.

As for your question, try comparing the drag of a 3ft diameter prop and a 3ft diameter solid disk. Something tells me that the windmilling prop will have even more drag than the disk, as it extracts energy from the entire area, where with the disk the air will just move around it.

This makes sence to me in theory (it is true in other applications, like river turbines or windmills), but i dont know its true on a twin because the prop might not be allowed to spin fast enough with the drag of the engine to be a really efficient windmill.
 
desertdog71 said:
I need a good explanation why that is.
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In short: the prop has a negative AOA and produces negative thrust. Why doesn't it stop? It's reached an equilibrium position.

Think about this:

In normal powered flight, the AOA seen by the blade is a combination of the aircraft's relative wind (RW) and the rotation of the prop. When the aircraft isn't moving, the AOA is solely due to the rotation of the blade and whatever blade angle was built into propeller. As it starts to move forward under its own power, the AOA of the blade starts to decrease, because the aircraft RW combines with the wind produced by prop rotation. A fixed pitch prop will rotate faster due to a reduction in AOA and hence drag.

Because the blade is an airfoil, there is drag opposite to the direction of rotation. The drag is proportional to the blade's AOA, just like the aircraft's main wing. The propeller requires torque to overcome this drag and it’s the function of the engine to supply this torque.

When the engine cuts off, the drag opposing the rotation of the propeller starts to slow it down, along with the internal friction of the engine, so its contribution to the blade AOA decreases, and the RW of the aircraft starts to play a larger role. As long as there is drag opposing the rotation, the propeller will continue to slow. Eventually, the AOA on the blade reaches zero. However, since the airfoil is cambered, it's still generating lift and drag, plus it's still dragging the crankshaft along with it, so the rotation continues to slow and the AOA moves to the other side of the propeller.

The AOA is now negative and at some point, there will be zero lift generated. However, the propeller is still moving at a certain RPM and is turning the engine over, so there is still drag. The propeller continues to slow. Now as the prop slows, the negative AOA increases instead of decreases, so there is a growing force that provides negative thrust and opposes the slowing tendency of the propeller. Eventually, the thrust provided by the blade in the direction of rotation will just equal the drag of the propeller and the engine and equilibrium is established.

Let me know if that's not coherent. Difficult without pictures.
 
A windmilling prop acts as a "flat plate" out in the open airstream. Because it is rotating at a high rpm it is here ,there, there, and everywere in between in a very short matter of time. Its all about the flat plate drag.

Next
 
Champcar said:
A windmilling prop acts as a "flat plate" out in the open airstream. Because it is rotating at a high rpm it is here ,there, there, and everywere in between in a very short matter of time. Its all about the flat plate drag.
Next
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The empty space between the prop blades also travels around real fast, so there's practically no prop there at all. Therefore, there should be almost no drag. :)
 
tgrayson said:
The empty space between the prop blades also travels around real fast, so there's practically no prop there at all. Therefore, there should be almost no drag. :)
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TG

I've gone back and forth over this. On the Dash8 we rarely (in over a 1000 hours only once) used reverse on landing. We would only pull the power levers back into "disc". All this did was go into ground beta which was basically zero pitch on the prop (no forward or "reverse" thrust). You could feel the airplane come to a real quick stop. I know we weren't getting any reverse thrust out of them since it pulled no torque when you did it.

Is there a difference between "discing" with power on or off. For turbo props (especially free turbine ones) I would think not???
 
Dugie8 said:
TG

I've gone back and forth over this. On the Dash8 we rarely (in over a 1000 hours only once) used reverse on landing. We would only pull the power levers back into "disc". All this did was go into ground beta which was basically zero pitch on the prop (no forward or "reverse" thrust). You could feel the airplane come to a real quick stop. I know we weren't getting any reverse thrust out of them since it pulled no torque when you did it.

Is there a difference between "discing" with power on or off. For turbo props (especially free turbine ones) I would think not???
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Interesting questions. Be advised that I have almost no hands-on experience with turboprops.

<<All this did was go into ground beta which was basically zero pitch on the prop (no forward or "reverse" thrust). >>

With a forward motion of the aircraft and a true zero blade pitch, the blades *must* be seeing a negative AOA, hence producing reverse thrust.

<<I know we weren't getting any reverse thrust out of them since it pulled no torque when you did it.>>

How is torque measured exactly?

<<Is there a difference between "discing" with power on or off. For turbo props (especially free turbine ones) I would think not???[/quote]>>

This is the question that stumped me a bit, so I had to turn to an Excel spreadsheet to run some numbers. On one hand, adding power tends to reduce the negative AOA, reducing thrust, but it increases the velocity of the air seen by the prop (q pressure), increasing thrust. Which effect dominates?

I made some arbitrary assumptions in my calculations, aircraft velocity = 100 knots, prop diameter = 10 feet, blade at zero pitch angle.

Here are some representative numbers:
Code: 
  RPM          Tip velocity (knots)         Prop AOA (degrees)    Thrust Factor
   500                 185                               33                  1
  1000                 326                               18                1.7
  1500                 476                               12                2.5
  2000                 628                                9                3.2
  2500                 782                                7                4.0
So it appears that keeping the RPM high will always result in more reverse thrust. Also note that at low RPM, the propeller blades are stalled, resulting in a loss of thrust, so my “1” for thrust factor is a poor choice. However, the trend is what we’re interested in.

Now, a true beta mode would increase the thrust at all non-stalled AOA’s, but the blades would also be stalled at a larger range of low RPM, increasing the benefit of more power.

I’m curious as to how this jibes with turbo prop operations. My King Air manual says when using beta, keep the condition levers in high idle.
 
The Dash8's prop governor worked a little different than most. From idle to about 20 degrees PLA the prop rpm (pitch was allowed to fluctuate a bit) was governed by the engine ECU (electronic contol unit) basically all it did was use fuel flow to keep the prop RPM where you wanted it (I think ground range was 800 or 600 rpm?? with the PLA below a point for quiet taxi). There was a prohibited range too for vibration reasons.

Now, torque was measure on this engine by reading the displacement of sensors on the power turbine shaft. Think of two tubes, one insed the other as the prop was put under load one tube would "twist" while the other remained undeformed. Sensors lined the inside and outside of the shafts and would pick up the amount of "twist", and convert that to torque as a percentage on the gauge.

IIRC, on the ground torque was anywhere from 10 to 15 and on landing going into "disc" produced no more than 15%.

I don't have the manuals anymore, but I do remember that the difference between disc and actual reverse was a fairly large number of degrees and getting the props into reverse required the power levers to be pulled past a certain point, otherwise the ECU and the Prop gove would keep them at a "positive" angle (the whole beta range in flight protection thing).

My real "heartburn" with the whole disc idea is, as you said, there is just as much air between the blades two and the drag you feel comes more from the air acting on the props to turn the engine. But in this case there is nothing to turn other than the reduction gear box and the power turbine, and that turns fairly easy (you could stand there and hold the prop stationary while the engine was started, ATRs have a break on the prop shaft so they can use the engine like an APU without the prop out there spinning, almos the same engine as the Dash). So without the internal drag from engine components, this discing effect must be from the props spinning at 1200 rpm??
 
Dugie8 said:
rop was put under load one tube would "twist" while the other remained undeformed. Sensors lined the inside and outside of the shafts and would pick up the amount of "twist", and convert that to torque as a percentage on the gauge.
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Where I was going with this is that the device might not be setup to detect the torque provided by the airflow, as opposed to the engine.

<<My real "heartburn" with the whole disc idea is, as you said, there is just as much air between the blades two >>

That post was a joke to show that his reasoning was flawed. :)

<<the drag you feel comes more from the air acting on the props to turn the engine. >>

On a more basic level, the reverse thrust is entirely dependent on the AOA and velocity experienced by the propeller blade. However, it's the internal drag that determines what that AOA and RPM will be.

Remember in a piston engined aircraft, at low pitch, you may have a 12 degree positive setting at the low pitch stop. When you kill an engine, the AOA must move from a positive to a negative AOA as the prop slows. How negative will depend on how much internal friction it has. However, the resulting negative thrust will be a function of the blade AOA (negative) and RPM.

In the turboprop, you cheated. You deflected the blade angle instantly to 0, whereas the piston has to travel from 12 to 0 before it can start going negative. So your discing reproduced essentially a 12 degree AOA change from the very start.
 
Dugie8 said:
So without the internal drag from engine components, this discing effect must be from the props spinning at 1200 rpm??
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Dugie:

The analysis I went though showed me something I didn’t expect. The drag of a windmilling prop doesn’t appear to be due to the drag of the engine. This is shown not only by your experience in the Dash 8, but also by the table that I generated. The greatest amounts of reverse thrust are generated by high RPM. However, the greater the drag of the engine, the lower the resulting RPM. Hence the drag of the engine *reduces* the amount of reverse thrust available.

Why would this be? It’s not reverse thrust per se that is pulling the engine through its revolutions, it’s the torque produced by that thrust which does this. The propeller will end up producing whatever thrust is necessary so that its associated torque matches the torque required by the engine at a particular RPM. The torque produced by the prop (in the direction of rotation) is greater at low RPM, because the lift vector of the propeller blade at high AOA is more in the direction of rotation than it is at lower AOA Like this:

windmilling.png


Because a free turbine requires less torque, the propeller will stay at a higher RPM than a piston engine aircraft, producing more reverse thrust.

How about a reality check? Do you think perhaps this is demonstrated by a helicopter autorotation, when you disconnect the rotor blade from the drive shaft? I wonder if in a piston engined aircraft a *slight* addition of power might increase drag, rather than decrease it.
 
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