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Propeller acts as brake?

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Propeller acts as brake?

Old 05-28-2008, 06:48 PM
  #26  
combatpigg
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Default RE: Propeller acts as brake?

You can't get something from nothing and a freewheeling prop is producing work as it gets pushed by the airframe. Hooked up to a motor / generator I think you will find that it will produce more power turning than the power it would consume to back feed the motor enough [with electricity] to brake the prop. I think this experiment would back up the aerodynamic reasoning.
Old 05-29-2008, 07:20 AM
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da Rock
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Default RE: Propeller acts as brake?

BTW, the pitch of the prop is very significant. Which is why the observation that FF rubber models use freewheel hardward shows one end of the spectrum, and the technical paper suggests that most model airplane props would need to be stationary for least drag. Combined, they show how important the variables are to the situation.

There is an excellent quote in the technical paper. Rules of thumb usually are only good for measuring thumbs.
Old 05-29-2008, 10:51 AM
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Tall Paul
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Default RE: Propeller acts as brake?

Piston motors generally have too much compression for a prop to freewheel, although I have a K&B 40 that will.
Electric planes need some way to halt the prop, as a turning prop adds drag by turning the motor.
The motor can act as a generator, but the amount of power it puts out is too small to be of any use.
Getting rid of the drag of the turning prop is better.
Either a folding prop, or electric braking.
Old 05-29-2008, 11:10 AM
  #29  
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Default RE: Propeller acts as brake?

da Rock

Thanks for your comments.

The djaerotech article is very interestion altho I would classify it more as assertions than scientific proof. It considers a number of possibilities none of which are exactly the question I have posed. However the general trend of it seem to support my case.

He argues mainly about energy and where it goes. It seems to me that, if you keep your engine running, you must generate some power, so where does it go if not in a faster glide.

However, I have put my exact question to them and eagerly await the answer.

david
Old 05-29-2008, 11:16 AM
  #30  
wellss
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Default RE: Propeller acts as brake?

The propeller is still absorbing that engine power, it's just lifting in the opposite direction
Old 05-29-2008, 01:00 PM
  #31  
mjfrederick
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Default RE: Propeller acts as brake?

I prefer to think of it in this way, which I find more intuitive than thinking about "negative lift" created by different angles of attack. If ((RPM * Prop Pitch)/1056) > Current airspeed, then the propeller will cause an increase in airspeed, inversely if ((RPM * Prop Pitch)/1056) < Current airspeed, then the prop will act as a "brake" because the work it is doing is less than required to maintain airspeed. Now, if the propeller is simply free-wheeling rather than the engine running at idle, the propeller will act as a break. Simple physics will tell us that you can't get more out than you put in. If the propeller is free-wheeling, the only energy input into the situation would be the force of the wind (no, I'm not forgetting gravity, but that really factors its way into the wind). Since the spinning of the propeller is not completely friction-free, there will be energy lost to heat, and the propeller will not be 100% efficient in its use of the energy. The problem most R/C pilots have is dealing with the difference between airspeed and groundspeed, when we think braking we think in terms of ground speed, but while the airplane is in the air, all we can control directly is airspeed. By the way, the 1056 is the factor required to convert distance travelled (in/min) into miles/hr. (60/(12*5280)). Now, to address davidej's question as to whether the spinning, idling prop has more drag than a stationary prop: it depends. If it is a situation where the airspeed/RPM comparison above is in equilibrium, I would say the drag is virtually equal to that of the stationary prop, but if the RPM side is less than the current airspeed the "braking" effect of the prop (for lack of a better term, drag) will be greater than that of a stationary prop because as stated in a previous post, the propeller while spinning has almost the same effect as having a flat plate mounted on the front of the airplane almost equal in diameter to the prop.
Old 05-30-2008, 08:18 PM
  #32  
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Default RE: Propeller acts as brake?


ORIGINAL: davidej

da Rock

Thanks for your comments.

The djaerotech article is very interestion altho I would classify it more as assertions than scientific proof. It considers a number of possibilities none of which are exactly the question I have posed. However the general trend of it seem to support my case.

He argues mainly about energy and where it goes. It seems to me that, if you keep your engine running, you must generate some power, so where does it go if not in a faster glide.

However, I have put my exact question to them and eagerly await the answer.

david

If the engine is running at such a speed that the prop's advance is much less than the plane's speed in the glide or dive then the prop is "flying" at a very negative angle of attack and lift is actually being generated towards the tail rather than to the front. At the same time the prop is trying to autorotate thanks to the pitch and that is speeding up the engine to where it's "coasting" at a higher RPM than the idle mixture would provide so you have engine drag slowing down the prop so it can't freewheel up to speed.

Think of the spinning prop as a terribly inefficient gyrocopter rotor that happens to be sitting in the wrong place and lifting the wrong way.

Old 05-31-2008, 11:20 AM
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Tall Paul
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Default RE: Propeller acts as brake?

One of my electrics threw the prop nut recently. With a glow motor, the prop usually takes off with it but this prop stayed on, windmilling away.
I've watched a prop from a glow motor fly about 75 feet -ahead- of the plane when it left.
Old 05-31-2008, 05:45 PM
  #34  
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Default RE: Propeller acts as brake?

When you lift your foot off the accelerator in your car it will slow down much quicker (engine braking) than if you pushed in the clutch (free wheeling). Engine braking happens until the car slows to the point where the engine speed equals the road speed it would produce. Thus if the prop could free wheel at the speed required to equal the forward speed of the plane it would not act as a brake. If it were slowed more than that by a throttled down engine, it would act as a brake. A simple explanation and it does not include losses due to all the other aerodynamics but there it is.
Old 06-01-2008, 04:55 AM
  #35  
Red B.
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Default RE: Propeller acts as brake?

ORIGINAL: davidej

Been reading this thread and saw the often-stated claim that a motor at tickover provides more drag than when deadstick.

I find this counter intuitive. It would need a change of aerodynamic regime over the prop at some point to be true.

had anyone got any real evidence to support it or a sound theoretical arguement why it should be so. I find these "you will notice when ...." statements totally lacking in any real proof.

Incidentally yachtties continually argue as to whether you shoud sail with the propeller windmilling or whether you should apply the shaft brake, so they have the same problem.

Any help would be appreciated.

David
It is a well known fact :-) amongst aeronautical engineers that under certain conditions a windmilling propeller will produce more drag than a stationary propeller. As usual things are more complicated than they appear at first glance.
I am quite certain I have some scientific articles in my archive with hard facts, but for the time I suggest you look into this thesis where there are some measurements and analysis of the drag of windmilling propellers.

[link=http://www.goshen.edu/physics/PropellerDrag/thesis.htm]Propeller Drag Thesis[/link]
Old 06-01-2008, 11:22 AM
  #36  
Tall Paul
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Default RE: Propeller acts as brake?

The big turbo-prop planes like the C-130 and P-3 have a "de-coupler" between the propellor and the gearbox to the turbine, that disconnects the propeller if the motor fails, and the propellor can't be feathered, and goes into low pitch.
A low-pitched windmilling propellor attached to the gas generator can create enough drag to overcome the control authority of the flight controls, so a Bellville spring pushes the propeller out of engagement with the shaft. The prop still windmills, making an awful shriek as the blade tips go supersonic.
Old 06-01-2008, 01:44 PM
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Default RE: Propeller acts as brake?

Back in the day Frank Zaic did some work on this topic as related to free flight models. He determined that with a low pitch/diameter ratio that it's far better to stop the prop than to let it freewheel (even if you were to use a decoupler) but at high pitch/diameter ratios where the blade angles are up around the 45 degree point that freewheeling is the better option. The range of ratio of pitch to diameter where the transition occurs is around the 1 or 1.2 pitch/diameter point. And with the optimum ratio for rubber flying being up around 1.2 to 1.4 this is why so many rubber models used freewheeling before folders became popular.

Of course a lot of power or electric flyers won't be aware of this since it's not common to use model sized props with ratios even as high as 0.8 let alone 1.2 to 1.4.
Old 06-02-2008, 09:03 AM
  #38  
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Default RE: Propeller acts as brake?

Red B

Thanks for some real data - not the usual anecdotes or unproven assertions.

However, If I read you thesis right, the propellors were windmilling almost frictionlessly. This is not the same case as I am interested in and I am not sure the results can be read across.

I am interested in a stopped prop versus one turning with the engine at idling speed. It is also slightly different to the often quoted case of a light plane with a dead engine but the prop windmilling in the airstream.

Any thoughts?

Thanks
Old 06-04-2008, 09:23 AM
  #39  
Red B.
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Default RE: Propeller acts as brake?


ORIGINAL: davidej

Red B

Thanks for some real data - not the usual anecdotes or unproven assertions.

However, If I read you thesis right, ...
It's not my thesis :-), just something I found on the Internet.

Maybe there are some research results available from the [link=http://naca.central.cranfield.ac.uk/]NACA Archives[/link] that are of interest to you.
Old 06-04-2008, 10:20 AM
  #40  
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Default RE: Propeller acts as brake?

I would say that the prop on an idling engine is exactly the same as frictionless. In any case it's been established: low pitch prop- more drag spinning; high pitch prop- more drag stationary (and 11-5 would be low pitch).
Old 06-08-2008, 02:38 PM
  #41  
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Default RE: Propeller acts as brake?

If any of you guys want a real-world, feel it in the seat of your pants demonstration, go to your local airport, and have them take you up in a light twin aircraft, and have them pull one engine back to idle. The propeller will still be turning at the normal rpm, but the engine will be producing very little power (the windmilling prop is actually feeding more hp into the engine than the engine is feeding into the prop. Next, have the instructor shut the engine down (pull the mixture control to idle-cutoff. Now the engine will still be windmilling, even though the engine is now producing zero power... you won't notice much, if any at all of a change in the drag from the windmilling propeller. Now have the instructor feather the propeller. You'll notice that the airplane will literally "leap" forward as the prop feathers because of the reduction in the drag from the windmilling propeller. It truly is an eye-opener! If you don't believe before, you certainly will after. Ask any multi-engine rated pilot...

There is a power setting used in multi-engine training for setting the "failed" engine so that it simulates having the propeller feathered, called "zero thrust"... it's the power setting where the power being fed from the engine into the propeller is approximately equal to the power that the windmilling propeller is feeding back into the engine.

.
Old 06-09-2008, 07:51 AM
  #42  
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Default RE: Propeller acts as brake?

If you want an even more dramatic real world demonstration, go up in a helicoptor and kill the engine. With a small amount of negative collective, the main rotor will windmill and let you down slowly almost as if it was a parachute. Now try the same thing with the rotor braked to a stop.
Old 06-09-2008, 09:20 AM
  #43  
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Default RE: Propeller acts as brake?

I don't care who you are, that's funny.
Old 06-09-2008, 06:32 PM
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Default RE: Propeller acts as brake?

Funny and a danged sharp way of showing how much energy a freewheeling prop/rotor can absorb when spinning or not absorbing when it is sitting still. B.L.E. that is a GREAT example.
Old 07-28-2008, 03:41 PM
  #45  
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Default RE: Propeller acts as brake?

I don't have an opinion on the main question, but I would like to note that the helicopter example proves nothing. With a helicopter in autorotation, the rotor is generating lift, but the weight of the helicopter is greater than that, so the helicopter falls. The lift, though, tries to move the helicopter toward the curved side of the blades (i.e. up). The argument about the idling prop is an argument that the drag from the prop will try to move the plane away from the curved side of the blade (i.e. backward). So the helicopter situation would be analogous only if the helicopter were upside down. I don't think you could talk a full-scale helicopter pilot into trying that experiment. Compare a plane flying straight up (or hovering) with the same plane, still vertical, and a stopped engine. The plane with the stopped engine will fall, just like the helicopter. This doesn't show that the prop was producing drag, it shows that it was producing lift (i.e., forward motion).

As for help with landings on a short field, the plane shouldn't be nose-down on final anyway, so drag is unimportant. If you want more drag, get the nose up. What the original questioner needs is a plane that can fly nose-high without stalling. Diving toward the field makes your landing run longer, because it builds up speed.
Old 07-28-2008, 03:59 PM
  #46  
Tall Paul
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Default RE: Propeller acts as brake?

"the plane shouldn't be nose-down on final anyway, so drag is unimportant. If you want more drag, get the nose up. "
.
Don't tell these guys they're doing it wrong..
Hign nose angle approaches are suitable for swept-wing jets.
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Old 07-28-2008, 04:28 PM
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da Rock
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Default RE: Propeller acts as brake?

Tall Paul,
All those pictures of full scale planes landing with full flaps are neat for sure. They all certainly are nose down. But they've also got wing AOA's that are anything but "nose down". And you will admit, won't you, that our model airplanes without flaps aren't going to be low drag or slow if we were to honk 'em in like those full scales in the pictures?

For a model without flaps to get a draggy and slow approach, we really don't want to come in nose down, do we?
Old 07-28-2008, 07:32 PM
  #48  
Tall Paul
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Default RE: Propeller acts as brake?

Of course not!
What a silly thing to say.
The model's pitch attitude has to be proper for the model's wingloading and approach speed.
High wingloading.. my Uni-Jet at 18 oz per lands with the fuselage at a much different angle (and speed) than my Easy Star.
Pontificating that -all- models -must- be drug in leads to wingip-prop-wingtip-upsicedown landings.
The individual model's characteristics determine how it approaches.
There is no -one- way to do it.
The helicopter analogy is quite correct, BTW.
An autorotation is very similar to a freewheeling propellor.
That's the reason for the clutch in the drive train.
Old 07-28-2008, 09:19 PM
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Default RE: Propeller acts as brake?


ORIGINAL: Top_Gunn

I don't have an opinion on the main question, but I would like to note that the helicopter example proves nothing. With a helicopter in autorotation, the rotor is generating lift, but the weight of the helicopter is greater than that, so the helicopter falls. The lift, though, tries to move the helicopter toward the curved side of the blades (i.e. up). The argument about the idling prop is an argument that the drag from the prop will try to move the plane away from the curved side of the blade (i.e. backward). So the helicopter situation would be analogous only if the helicopter were upside down. I don't think you could talk a full-scale helicopter pilot into trying that experiment. Compare a plane flying straight up (or hovering) with the same plane, still vertical, and a stopped engine. The plane with the stopped engine will fall, just like the helicopter. This doesn't show that the prop was producing drag, it shows that it was producing lift (i.e., forward motion).

As for help with landings on a short field, the plane shouldn't be nose-down on final anyway, so drag is unimportant. If you want more drag, get the nose up. What the original questioner needs is a plane that can fly nose-high without stalling. Diving toward the field makes your landing run longer, because it builds up speed.
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.

Even if they did have cambered blades, they could most likey do an inverted autorotation. Trainers with their flat bottom wings can fly upside down after all. The guy who trained me how to fly would fly a Gentle Lady glider inverted in slope lift and nothing has a more flat bottomed wing than a Gentle Lady.
Old 07-29-2008, 06:57 AM
  #50  
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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.

Even if they did have cambered blades, they could most likey do an inverted autorotation. Trainers with their flat bottom wings can fly upside down after all. The guy who trained me how to fly would fly a Gentle Lady glider inverted in slope lift and nothing has a more flat bottomed wing than a Gentle Lady.
Sure, but this misses the point. It's lift, not drag, that keeps a helicopter from falling in autorotation, so the fact that helicopters can autorotate doesn't "prove" that a rotating propeller generates more drag than a fixed prop.

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