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The Prop windmilling on RX1/Apex engines

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Pilot-34

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A free-driving rotating wing, be it a propeller, wind turbine or helicopter rotor, uses the energy of the freestream air passing its airfoils to get its drive, but due to the fact that there are three distinct regions across the span of the blade from root to tip (stalled, driving and driven), you will still have net drag (from the stalled and driven regions) as a function of the entire system being driven by said airflow. I assume this is what you were trying to verbalize? See this chart:

View attachment 99898

I've done plenty of autorotations over the years. If you lose RPM during the maneuver, you are screwed and will drop like a rock (eg "low drag scenario") unless you can get the RPM back up (higher drag, lower rate of decent, relatively speaking). Of course, you can really drive Nr way up above prescribed limits, but then you risk rotor hub failure...but we are not talking about that here.

My time flying the V-22 Osprey was a good example. You can be hauling tail at 220 knots, and when you pull power back and advance Nr to covert back to VTOL mode, the flat pitching as a result of the proprotor acceleration feels like deploying a parachute out of the back of the bird!

Read my previous reply to your traffic with the two helicopter examples. A free drive prop is simply being driven by the passing wind, and still creates drag. More than if the prop was not moving at all. Simulations and actual use prove the theory, its purely aerodynamic.
lol trying to tell me a propeller and a rotor will behave the same?


In point a fully feathered prop would NOT be the same as a fully feathered rotor.

different shapes

I think the situation between an auto rotating helicopter or Gyro And a stalled or feathered or freewheeling propeller is different.

Let me ask you this why do you think A stopped propeller would start to rotate when the brake is released?

Are you forgetting that a stopped propeller is still being driven?

On the old airliners Where the propellers designed to feather or to brake ?
 
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way_up_noth

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lol trying to tell me a propeller and a rotor will behave the same?


In point a fully feathered prop would NOT be the same as a fully feathered rotor.

different shapes

I think the situation between an auto rotating helicopter or Gyro And a stalled or feathered or freewheeling propeller is different.

Let me ask you this why do you think A stopped propeller would start to rotate when the brake is released?

Are you forgetting that a stopped propeller is still being driven?

On the old airliners Where the propellers designed to feather or to brake ?
I respect your opinions..... but a wall is a wall.... doesn’t matter if that wall is vertical or horizontal ....a spinning windmilling prop/rotor will create that wall... it’s no more complicated then that...
 

Pilot-34

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Nope
You were disregarding the differences between the shape of a propeller and the shape of a rotor blade

if you turn a rotor blade 90° to the flow of air it will have very little drag and will not rotate
If you could feather a propeller fully it would do the same

But a fixed pitch propeller becomes a different situation
It takes so much energy out of the air that it will rotate a dead engine
When it is stopped it actually has less air available to it to pull the energy out of
But a free wheeling propeller not attached to the drag of an engine will rotate at the exact speed of the lowest drag removing the least energy from the air stream.

That’s why a free wheeling windmill will start to move in the wind
that’s why a free wheeling windmill will start to move in the wind
What a free wheeling propeller spinning freely in the airstream is doing is essentially the same as rotor blades and 90° to the direction of fall.

Those are old engineers that developed feathering propellers for large transports were not stupid
The amount of force we are discussing on a larger Airliner is enough that it would have made sense to fit those propellers with a brake And not just allow them to freewheel in the breeze.

There may have been studies that prove otherwise and I would be happy to look at them but my own personal actually flying observations are what I’m going by.
 

rv7charlie

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I think I mentioned something like this earlier. Try this: mount a small prop (a decent sized prop from an RC a/c might be a good starting point) on a stick, and lock it from rotating. Drive your car at a reasonable speed with the stick held out the window, feeling (or ideally, measuring) the force on the stick. Now repeat the experiment with the prop freewheeling. Report your results.

edit: Additional points for explaining how my RV-6 can fly and maintain altitude when flying inverted. Helpful hint to the reason for the reference: airflow 'driving' the airfoil is reversed, just like a freewheeling prop.
 

trimtab

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Dragging a barn door broad side to the wind generates a fraction of the drag that dragging the barn door through the air so that lateral flow of the air occurs. The Cl (that is drag in this case) when the AOA is below the critical value is huge compared to when it is well above the critical value.
 

Pilot-34

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I think I mentioned something like this earlier. Try this: mount a small prop (a decent sized prop from an RC a/c might be a good starting point) on a stick, and lock it from rotating. Drive your car at a reasonable speed with the stick held out the window, feeling (or ideally, measuring) the force on the stick. Now repeat the experiment with the prop freewheeling. Report your results.

edit: Additional points for explaining how my RV-6 can fly and maintain altitude when flying inverted. Helpful hint to the reason for the reference: airflow 'driving' the airfoil is reversed, just like a freewheeling prop.
Good idea I think I can put this together pretty easy
Propeller With washers on each side of it a bolt through it a yardstick with bolts and washers on each side of that
Tighten everything down stopped propeller.
Tighten everything down stopped propeller.
Loosen up the propeller so it free wheels obviously free wheeling propeller
Tighten up the washers on either side of the propeller to simulate windmilling while driving an engine

that sound fair enough people?
 

rv7charlie

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Suggest using a small rod, or keep the yardstick edge to the wind (ideally, a minimum drag teardrop shape). Otherwise, the support stick will be a significant percentage of total drag and significantly corrupt the data.
 

BBerson

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If you put two identical props on each end of a stick with a pivot in the middle, the prop with the most drag will pull harder rearward on the pivot.
 

Swampyankee

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Generally, windmilling props have higher drag than stopped props, which may be important. For a crankcase-scavenged two-stroke, it’s probably important to make sure engine isn’t windmilled, as fuel flow is likely needed for lubrication, but four-strokes use engine-driven oil pumps.
 

BBerson

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The question was: do disengaged free wheeling props have more drag than a stopped prop?

From op: "there is a sprag clutch....between the engine and propeller .... so the prop spins freely when not powered...."
 
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trimtab

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A propeller in freewheel is in autorotation. The hub area is stalled, an intermediate zone is absorbing energy to rotate the blade, and the tips may or may not be producing thrust (against the direction of travel). Unlike a helicopter rotor, the blade twist changes the borders of stalled, absorbing, and propulsing blade sections quite a bit. But allowing a proper to freewheel can generate MORE drag than a propeller that is slowed down by an engine.

This fact is a consideration in the design of wind turbines. Some of the same choices that optimize efficiencies during normal operation are the same choices that autorotate well when unbridled by power extraction. On of the fail-safes to prevent a much larger mast for the wind power station in such an event (due to much higher drag forces in a runaway freewheel condition) is to design the tips to dramatically lose lift as sonic speeds are approached to avoid the autorotation induced drag (propulsive) force. These can include designs that shock at much lower speeds than other areas of the blades. This reduces wind turbine efficiency, but avoids some risk from autorotation. The usual approach is to simply not design the blade to turn fast enough for the tips to begin generating propulsion, and this lightens hubs, blades, and masts at the expense of low wind speed efficiencies.
 

BBerson

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A prop isn't a windmill. It's upside down. May or may not spin with any significant rpm. Most of the prop is likely stalled just like a stopped prop.
The 50 cent rubber band balsa models have a free wheel prop when it winds down. Why do they do that?
 
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cblink.007

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The 50 cent rubber band balsa models have a free wheel prop when it winds down. Why do they do that?
Perhaps it was designed that way because it was likely cheaper to make the parts that way. It would not rotate if the rubber band was still tensioned. I remember that as a kid when the original band snapped and had to raid the kitchen for something not as good...and the prop always was still after expending the band's energy. But would it make a difference if it was still or not in flight? Likely not by much...the Re is not all that high with an airfoil surface that small.
 

BBerson

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I think the oversize rubber band props just twist through the glide at low rpm.
Same for a dead stick gyro at 45 mph. I'll go with the actual gyro guy in post 25.... no problem with freewheeling.
 

Swampyankee

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The question was: do disengaged free wheeling props have more drag than a stopped prop?

From op: "there is a sprag clutch....between the engine and propeller .... so the prop spins freely when not powered...."
The answer is yes. A freely windmilling prop will have more than one slowed by being attached to an engine.
 

Tiger Tim

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The 50 cent rubber band balsa models have a free wheel prop when it winds down. Why do they do that?
Because they are free flight and in their case a stopped prop throws them well out of trim. A freewheeling prop also intuitively feels like it should have less drag but it’s been demonstrated that it doesn’t. There was a massive fight over this on a free flight forum I was on fifteen years ago that resulted in half the members quitting and never coming back.
 
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