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Understanding constant speed props

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Understanding constant speed props

Old 23rd Jul 2014, 08:32
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Understanding constant speed props

I'm doing some reading to about constant speed props, prior to using one. One article 'Pelicans Perch: Those Marvelous Props' is frequently recommended so I'm starting there. It raises a question which I'm hoping someone can answer.

The article starts by taking about the different props used to optimise different performance (take off, climb, cruise etc). One statement says: "Some props used counterweights to balance aerodynamic forces so that if the prop were under a load, the blades would flatten out automatically, increasing RPM and power." I understand why the RPM would increase but I don't understand why the power would increase. Applying my school boy physics, I thought power would be a function of the pitch and the RPM. So, if the RPM increases but the pitch flattens out, the net power would still be the same. Where am I going wrong? And why does a flatter blade work any better for the take off than it does, say, for a climb or cruise? I seem to be missing some part if the jigsaw here.
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Old 23rd Jul 2014, 09:07
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Power = RPM x Torque

If Torque remains constant then Power is proportional to RPM.
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Old 23rd Jul 2014, 09:22
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In the absence if other changes (such as more fuel to provide more energy for the engine), what happens to torque as RPM increases? I'm guessing it decreases, and hence brings me back to my question. Anything else seems like the physics equivalent of a free lunch. Not disagreeing (because doubtless your formula is correct) but still puzzled.
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Old 23rd Jul 2014, 10:49
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I suppose you could approach this as an angle of attack issue.

Consider a wing. With the aircraft flying level, the wing is providing just sufficient lift to counteract gravity. Increase the angle of attack a little and the aircraft will climb slowly. Increase the angle some more and the aircraft will climb faster. Increase the angle of attack beyond a certain point and the wing will stop producing meaningful lift.

With a prop in still air, the angle of attack is the pitch angle. As the aircraft speeds up, the prop experiences a decreasing angle of attack due to the velocity of the air approaching the "front" of the prop. The angle of attack can be maintained by increasing the pitch a little. As the airspeed increases, the pitch angle needs to be increased to maintain a constant angle of attack. Hence we coarsen the prop as airspeed increases.

You can change the prop angle of attack by varying the pitch or by speeding it up/slowing it down or speeding up/slowing down the aircraft.

The "take-off" prop is fixed to have a (relatively fine) pitch suitable for accelerating from zero to climb speed. A "climbing" prop has a coarser pitch that is optimised for the speed in climb.

The optimum pitch is the one for a given airspeed that transmits the maximum available engine power to the air. The maximum available engine power varies with the throttle position.

Clear as mud?
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Old 23rd Jul 2014, 11:05
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Nobody has mentioned that an engine's power output is non-linear with respect to RPM...Therefore,A CS propcan alter pitch to allow the engine to stay on the peak of it's power/ torque curve.

If you like, it's the Aero equivalent of a car's CVT automatic gearbox...there really isn't room on the average SEP to put a multi-speed gearbox on the front of the engine...and changing gear might prove a challenge to many pilots
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Old 23rd Jul 2014, 11:16
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In the specific question asked, the RPM increases. That means more air into the engine, and more fuel from the carb or fuel injection. That is one way to explain more power. If everything is unchanged, including RPM and fuel flow I don't think power will change. No free lunch!
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Old 23rd Jul 2014, 11:44
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higher rpm means you can burn more fuel in a given time, hence more power.
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Old 23rd Jul 2014, 15:39
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Thanks everyone, especially Worrab. The angle of attack explanation was the one I needed, and now things fall into place. I had overlooked the effect of the vectors in play once there is airspeed.
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Old 24th Jul 2014, 09:28
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I CDUK... You could look up some stats on Lycoming engines, and find out that in aviation use they are practically rev limited for safety purposes, and to ensure a long life. The Red-Line is set at 2800 rpm.

If you were to install one in a car, then you could most likely rev it to 4000+rpm where the power would peak to a maximum. So in aviation use you are in the situation where the power always increases as the revs rise to the arbitrary Red-line figure.

A fixed bladed prop needs power proportional to the square of its speed, because as revs rise... More air is passed backwards. That air is moved back quicker. (~ M.V ). The intersection of the power available (straight) line, and power required (curved) line, will give the rpm for that particular engine/prop configuration.

A constant speed prop is just mechanically governed to keep the engine speed constant. In a loop for instance... When climbing upwards it sets a low pitch, and when diving it sets the coarse pitch.

Last edited by phiggsbroadband; 24th Jul 2014 at 15:27. Reason: get the formula right Phi...
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Old 29th Jul 2014, 16:12
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One statement says: "Some props used counterweights to balance aerodynamic forces so that if the prop were under a load, the blades would flatten out automatically, increasing RPM and power."
What I think the article is referring to, with this sentence, is some (experimental) props that change pitch automatically when the centrifugal forces and/or induced drag forces change. So the pitch change does not come from some clever mechanism in the hub, but the blades themselves change shape under various loads. The hub is then essentially one from a fixed pitch prop, as it does not need to do anything other than to hold the blades rigidly to the prop shaft.

AFAIK these props are not (widely) used anymore. Most, if not all of the C/S props today have a mechanism in the hub which rotates the blades and thus changes the pitch of the blades. The shape of the blade in such an arrangement is not changing at all.
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