Oct 12, 2012, 09:29 AM Registered User Joined Oct 2004 2,488 Posts Why should there be any thrust if the propulsor can't accelerate air to a speed higher than that of the plane?
Oct 12, 2012, 11:42 AM
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Quote:
 Originally Posted by Brandano Why should there be any thrust if the propulsor can't accelerate air to a speed higher than that of the plane?
Tell me what is pushing your plane along at "a speed" (against the resistance of aerodynamic drag) if there is no thrust.

Not only that, if your propulsor is not producing any thrust it is not doing any work so it won't need any power input. Why do you need "wide open throttle" to achieve maximum speed? Where is the power going?
Last edited by TangoKilo; Oct 12, 2012 at 12:12 PM.
 Oct 12, 2012, 12:58 PM B for Bruce The 'Wack, BC, Canada Joined Oct 2002 10,094 Posts Into countering the aerodynamic drag.
Oct 12, 2012, 01:25 PM
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Quote:
 Originally Posted by TangoKilo Tell me what is pushing your plane along at "a speed" (against the resistance of aerodynamic drag) if there is no thrust. Not only that, if your propulsor is not producing any thrust it is not doing any work so it won't need any power input. Why do you need "wide open throttle" to achieve maximum speed? Where is the power going?
If the propeller is providing zero available thrust, then the work from thrust available is W = T_A * V_infinity = 0. However, the work from the motor is the product of the angular velocity of the shaft and the torque. In steady state, the torque on the shaft from the motor is equal and opposite to the torque on the shaft from the propeller, which will be nonzero.
Oct 12, 2012, 03:40 PM
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Quote:
 Originally Posted by TangoKilo Tell me what is pushing your plane along at "a speed" (against the resistance of aerodynamic drag) if there is no thrust. Not only that, if your propulsor is not producing any thrust it is not doing any work so it won't need any power input. Why do you need "wide open throttle" to achieve maximum speed? Where is the power going?
A slight dive? If the efflux velocity is matching that of the free airstream the engine is just wasting energy as heat. A plane could never exceed the speed of the efflux from its propeller or jet engines in balanced level flight, which is one reason why hypersonic engines are exotic affairs or rockets. This does not mean that a plane can't go faster than the speed a prop can accelerate air on a stand. The prop adds speed to the airstream, so what you measure on a stand is not what you will have on the plane.
Oct 12, 2012, 07:36 PM
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Quote:
 Originally Posted by DPATE If the propeller is providing zero available thrust, then the work from thrust available is W = T_A * V_infinity = 0. However, the work from the motor is the product of the angular velocity of the shaft and the torque. In steady state, the torque on the shaft from the motor is equal and opposite to the torque on the shaft from the propeller, which will be nonzero.
If the propeller is doing no work, there is no resistance to rotation (torque).

Angular velocity times nothing equals nothing.

Torque required from motor - none. Power required - none.
Oct 12, 2012, 07:46 PM
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Quote:
 Originally Posted by Brandano A slight dive? If the efflux velocity is matching that of the free airstream the engine is just wasting energy as heat. A plane could never exceed the speed of the efflux from its propeller or jet engines in balanced level flight, which is one reason why hypersonic engines are exotic affairs or rockets. This does not mean that a plane can't go faster than the speed a prop can accelerate air on a stand. The prop adds speed to the airstream, so what you measure on a stand is not what you will have on the plane.
"A plane could never exceed the speed of the efflux from its propeller..." but "This does not mean that a plane can't go faster than the speed a prop can accelerate air on a stand."

It is twenty five minutes past one here, I've had a good dinner and a few glasses of cognac. I can't be bothered to work out what you mean. Good night.
Oct 12, 2012, 08:14 PM
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Quote:
 Originally Posted by TangoKilo If the propeller is doing no work, there is no resistance to rotation (torque). Angular velocity times nothing equals nothing. Torque required from motor - none. Power required - none.
The there will be profile drag on the propeller whether it is producing thrust or not. Of course, this drag produces a torque on the shaft.
Oct 13, 2012, 05:25 AM
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Quote:
 Originally Posted by DPATE The there will be profile drag on the propeller whether it is producing thrust or not. Of course, this drag produces a torque on the shaft.
OK, let me put it another way.

If the propeller is doing no work in producing thrust, there is very little resistance to rotation (torque).

Angular velocity times very little equals an insignificant amount.

Torque required from motor - insignificant amount.

Power required - insignificant amount.

Battery life at maximum speed - about three years.

(Last sentence was a joke by the way, before you correct me)
 Oct 13, 2012, 07:01 AM Registered User Joined Oct 2004 2,488 Posts Surface drag is quadratically related to speed. The same is true for friction drag in the bearings of the motor. Energy is dissipated as heat in both cases. You can do no work and waste a lot of energy all the same.
 Oct 13, 2012, 07:26 AM Claus at Area 52 copenhagen Denmark Joined May 2002 2,511 Posts stumax did a little program that did the job 70 mm exhaust 1.8 pounds = 45.6 m/sec
Oct 13, 2012, 10:27 AM
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Quote:
 Originally Posted by TangoKilo If the propeller is doing no work in producing thrust, there is very little resistance to rotation (torque).
The propeller of a 3D plane in a hover is supplying zero power to the airplane ( power = thrust times velocity). In a hover, the propeller experiences significant resistance to rotation. The input power (torque times RPM) is likely quite high.
Oct 13, 2012, 12:45 PM
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Quote:
 Originally Posted by ShoeDLG The propeller of a 3D plane in a hover is supplying zero power to the airplane ( power = thrust times velocity). In a hover, the propeller experiences significant resistance to rotation. The input power (torque times RPM) is likely quite high.
Those are opposite situations. In hover, the induced power is quite high (relatively large amount of acceleration of the air). When the speed of a ducted fan aircraft reaches the efflux velocity (if that could happen), the induced power is zero but the parasite power (from drag of the aircraft) and profile power (from drag of the propeller blades) are quite high.

In the first situation, the total power required by the motor is just the induced power. In the second situation, there is no contribution from induced power.
Last edited by Montag DP; Oct 13, 2012 at 01:12 PM.
 Oct 13, 2012, 02:26 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 673 Posts Montag, I wasn't comparing the induced to the profile power, but instead comparing the power delivered by the propeller to a hovering airplane to the power required to turn the propeller.
 Oct 14, 2012, 06:38 PM Registered User Joined Oct 2012 45 Posts Gentlemen, this is becoming a bit silly. What on earth is “parasite power” or “profile power”? It doesn’t matter if you are hovering or flying straight and level. In the first case the thrust is countering weight, in the second the thrust is countering drag. If the drag, (in the second hypothetical case) were to be the same as the weight (in the first hypothetical case) the same amount of thrust would be required. Thrust is the equal and opposite reaction to a moving mass of air relative to a frame of reference, so the velocity term must be the velocity of that moving air relative to the same frame of reference. It makes no difference if the model is moving (has momentum) or not as long as the same frame of reference is used. Just because the hovering model is not moving does not mean that no power is being produced. If a motor is drawing a number of Watts of electrical power there must be a number of Watts of output power (depending on the efficiency) whether the body, to which the motor is attached, is moving or not. It is a mass of air which is moving and that mass of air and its velocity are factors of the output power. Similarly, the propulsion system of a model moving at maximum speed is also producing thrust. The thrust is that required to equal the drag at that speed and is a factor of the output power. Output power is half mass flow times velocity squared. There is electrical power going in and there is mechanical power (in the form of moving air) going out. The efficiency of a propulsion unit is the percentage of power going in which is converted to useful power output. If, for example, the efficiency of a propulsion unit is 75%, the useful power output is 75% of the electrical power input. Profile drag, bearing friction, and wasted heat are only a part of the 25% losses, they will only consume a small part of the power input. The power required to turn the propeller is part of the efficiency equation, the power transferred to the air by the turning propeller is another part of the same equation. Incidently, shaft power is torque times angular velocity, not RPM.