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Propeller acts as brake?

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Propeller acts as brake?

Old 07-29-2008, 08:03 AM
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Default RE: Propeller acts as brake?

However, nearly all model helicopters have symetrical blades for the same reason that most aerobatic airplanes have symetrical wings, so they can fly upside down just as well as they can right side up and inverted autorotations are a pretty standard stunt for the RC heli guys.
Yes but the blades still create lift with angle of attack, the pilot adjusts the pitch to maximize the lift. Our props are maximized to create thrust. If you have an adjustable pitched prop then puting it in maximum foward pitch would be the least drag, medium a little more drag, stopped prop even more drag, and reverse pitch may have more drag.
Old 08-05-2008, 06:46 AM
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Default RE: Propeller acts as brake?

I think you should investigate this in 2 different way. A freewheeling prop and a prop that is attached to an engine.

The aoutorotation is not freewheeling. The engine is trying to force it to turn slower but the air is trying to freewheel the blades. Thats why there is drag. If the blades were not attached to the engine and they could freewheel completely free then the slowing effect of the aoutorotation would not work. (I also think that heli blades have alot to do with kinetic energy stored in them during autorotation but thats not the subject)

I think same works with the airplanes. Props can freewheel easier on an electric motor (if esc was set not to break) than IC ones because of the compression. Free flight props are the easiest to free wheel.

Freeflight props are huge. If you force them to stop during the glide thats huge friction. They often test glide them without the props attached. And so they let them freewheel to reduce the drag during the glide. Lets say we force the prop of a freeflight plane to rotate at 10 rpm. Thats a rotating prop but it will again produce drag.

To me (this is my not so educated guess) the more we force a prop not to freewheel the more drag we get and forcing it to come to a complete stop would be the worst case, producing the most drag.
Old 08-05-2008, 03:39 PM
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Default RE: Propeller acts as brake?


ORIGINAL: encan

I think you should investigate this in 2 different way. A freewheeling prop and a prop that is attached to an engine.

The aoutorotation is not freewheeling. The engine is trying to force it to turn slower but the air is trying to freewheel the blades. Thats why there is drag. If the blades were not attached to the engine and they could freewheel completely free then the slowing effect of the aoutorotation would not work. (I also think that heli blades have alot to do with kinetic energy stored in them during autorotation but thats not the subject)

I think same works with the airplanes. Props can freewheel easier on an electric motor (if esc was set not to break) than IC ones because of the compression. Free flight props are the easiest to free wheel.

Freeflight props are huge. If you force them to stop during the glide thats huge friction. They often test glide them without the props attached. And so they let them freewheel to reduce the drag during the glide. Lets say we force the prop of a freeflight plane to rotate at 10 rpm. Thats a rotating prop but it will again produce drag.

To me (this is my not so educated guess) the more we force a prop not to freewheel the more drag we get and forcing it to come to a complete stop would be the worst case, producing the most drag.
Model heli's have a one way bearing that seperates the main shaft and main gear and allows the engine to slow to idle while the rotor is free to rotate at any speed. Pratice them every time out on my Raptor. Just in case. You never know when the engine might quit.

Just wanted to set the record straight on the rotor freewheeling in auto's. IT IS FREEWHEELING.
Old 08-05-2008, 09:44 PM
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Default RE: Propeller acts as brake?

Yes, a heli's rotor is freewheeling during autorotation, but there is a retarding force on the rotation. It's the air resistance of the blade tips. If the rotor has a 36 inch diameter and it is rotating 1800 rpm, those rotor tips are going through the air at over 190 mph and at that speed, air resistance becomes extreme, even with the blade lined up with the airflow.

The least braking of all with power out would be an adjustable pitch prop with the pitch adjusted to infinity, i.e. "feathered".

A very coarse pitched prop will windmill very slowly and if allowed to freewheel, will have only slightly more drag than a feathered prop.

When you have something like a 16X4 prop at 60 mph (88 ft/sec) that prop will windmill three revolutions per ft or 88 X 3 revolutions per second (264 rps or 15,840 rpm). This results in a blade tip speed of 1,105 ft per second and the air resistance at that speed is extreme, much, much higher than the drag that blade would experience braked to a stop and only experienceing the drag of going 60 mph with the blade broadside to the airflow.

There is some pitch to diameter ratio where there is a crossover point, course pitch props=less drag freewheeling, fine pitch props= less drag braked.

BTW, airplanes can also come down in autorotation, we call it a "flat spin".
Old 08-06-2008, 01:43 AM
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Default RE: Propeller acts as brake?

OK, i never had a heli so i shouldnt have talk about autorotation on helis. Sorry... The oneway bearing makes sense.


B.L.E i understand all the things you say but i wanna ask a question then. Why does a prop is windmill? I think because of the friction. Doesnt the friction gets lower as the prop spins faster in moving air and isnt it almost zero when the prop is pushing the air at same speed with the speed of the plane? I mean wouldnt there be more friction between the prop and the air if the prop was not spinning? If we had a plane moving forward 15 MPH and the prop was only rotating just enough to push the air stream with 15 MPH then what friction forces apply to this prop?

If i get you right (sorry my native is not english) the drag you mention about the 16x4 prop, is just trying to keep this prop slow. And therefore has a similar effect of a compressing engine. But a stopped prop directly creates a drag to slow down the plane. How can we compare these two?

Old 08-06-2008, 06:49 AM
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Default RE: Propeller acts as brake?

In that 16X4 windmilling at 15,840 rpm, that's theoretical, the actual rpm will likely be lower, notice the tip speed of 1100 ft per second. That's nearly as fast as a standard velocity .22 short bullet goes. That's only about 50 fps slower than the speed of sound. Trust me, it takes a lot of power to move a prop tip that fast through air even if the blade is lined up with the airflow.

Anybody out there got an electric helicopter and a wattmeter? How about measuring the zero pitch power consumption of your rotor blades at full rpm and compare that to the power consumption at the same head rpm without any blades on the head. I think it would be interesting and revealing to see just how much power is used just to spin the blades through the air without making any thrust.
Old 08-06-2008, 07:25 AM
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Default RE: Propeller acts as brake?

Lets say in a wind tunnel, 50 MPH wind is going through. And you run en electric motor with a prop pushing wind 50 MPH of air(if it was a flying model then the air speed would be 0 -which is not posibble-) would that require more amps than running the motor unloaded?

Is this same thing as tunring a prop in steady air with blades at 0 incedence like the test you wanted to see?
Old 08-06-2008, 12:29 PM
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Default RE: Propeller acts as brake?

What we need is for someone to put a 14X6 prop on a bearing supported shaft that can be locked and unlocked easily. Place said assembly in a wind tunnel and measure the resistance both free spinning and stopped. Anyone here have access to a wind tunnel? If so, will you do the experiment for the forum members?
Old 08-06-2008, 02:03 PM
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Default RE: Propeller acts as brake?

I always thought the original question was regarding a prop on an engine running at idle versus a stopped engine. A free-wheeling prop wont answer that question.
Old 08-06-2008, 06:28 PM
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Default RE: Propeller acts as brake?

You're right. New deal on the wind tunnel test folks. It needs to be with an idling engine and then with the engine off.

I was thinking of the comparison with the heli rotor in auto and that's where the freewheeling idea came from. My BAD.
Old 08-06-2008, 08:26 PM
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Default RE: Propeller acts as brake?

Does it have to be an engine? Would a motor with a brake be OK?
Old 08-07-2008, 06:59 AM
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Default RE: Propeller acts as brake?


ORIGINAL: Jezmo

You're right. New deal on the wind tunnel test folks. It needs to be with an idling engine and then with the engine off.

I was thinking of the comparison with the heli rotor in auto and that's where the freewheeling idea came from. My BAD.
My prediction is that as the engine, or motor idles slower and slower, the braking will increase until a point is reached where the blades are at such a high angle of attack to the airflow that they stall. At that point, the propeller will "let go" of the air and the windmilling torque will dramatically go down and the braking force will dramatically go down as well.

On some full scale airplanes with props pitched for high speed, you actually hear the engine suddenly slow down as the airspeed increases to a certain point because the blades are suddenly not in a partial stall.
Old 08-07-2008, 11:53 AM
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Default RE: Propeller acts as brake?

My prediction is that as the engine, or motor idles slower and slower, the braking will increase until a point is reached where the blades are at such a high angle of attack to the airflow that they stall. At that point, the propeller will "let go" of the air and the windmilling torque will dramatically go down and the braking force will dramatically go down as well.
Exactly.
Old 08-13-2008, 04:30 PM
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Default RE: Propeller acts as brake?

Easy, just epoxy a prop to a dowel rod. Make a wooden frame consisting of a base and 2 upright supports for the rod. Holes are drilled in the frame to accept the rod. The other end of the rod has a bearing epoxied onto it. Leave a few inches of play so that the shaft assembly can slide forward and back.

On the front mount, drive a smooth framing nail within the arc of the propeller so that it can not turn. Now, open the sunroof on your car and stick this thing out on the roof. The prop can not turn because of the framing nail, yet you can slide the shaft forward a bit while holding the bearing. Measure how much force it takes to move it slightly. Now, push further forward until it clears the nail. It will begin to rotate. Again measure the force it takes to keep it in a forward position.

Be advised that this will very likely disintegrate and disfigure, dismember, and/or blind you. Happy experimenting.

BTW: I predict a spinning prop has less brake effect. It just seems like common sense to me, however I know there are scientific facts that defy logic..
Old 08-13-2008, 05:26 PM
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Default RE: Propeller acts as brake?


ORIGINAL: kriegsmacht

Easy, just epoxy a prop to a dowel rod. Make a wooden frame consisting of a base and 2 upright supports for the rod. Holes are drilled in the frame to accept the rod. The other end of the rod has a bearing epoxied onto it. Leave a few inches of play so that the shaft assembly can slide forward and back.

On the front mount, drive a smooth framing nail within the arc of the propeller so that it can not turn. Now, open the sunroof on your car and stick this thing out on the roof. The prop can not turn because of the framing nail, yet you can slide the shaft forward a bit while holding the bearing. Measure how much force it takes to move it slightly. Now, push further forward until it clears the nail. It will begin to rotate. Again measure the force it takes to keep it in a forward position.

Be advised that this will very likely disintegrate and disfigure, dismember, and/or blind you. Happy experimenting.

BTW: I predict a spinning prop has less brake effect. It just seems like common sense to me, however I know there are scientific facts that defy logic..
That would test a free-wheeling prop, not the originally posted idling engine, where the rpm is a constant and not just getting rotated by the air coming at it. So this test would not be valid in this case. It is already established that the question is not about a free-wheeling prop.
Old 08-13-2008, 08:40 PM
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Default RE: Propeller acts as brake?

Ah.. Ok I scanned instead of read carefully. Bad habit.

In that case, I think if you eleminate the variables you are left with just angle of attack on a propeller blade, and the resulting drag along the axis of the plane's movement. A way to recreate that could be to take a prop and blow on it with an air compressor nozzle, turning it to various orientations and AOA's up to and including full "stall".

I make no claims that this post contains original ideas, or makes sense.

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