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-   -   How calculate propeller efficiency if I know propeller trust? (http://www.physicsforums.com/showthread.php?t=346361)

 vissarion.eu Oct17-09 03:19 AM

How calculate propeller efficiency if I know propeller trust?

How calculate propeller efficiency if I know propeller trust ant motor power and motor efficienty?

 twasnow Oct18-09 07:40 PM

Re: How calculate propeller efficiency if I know propeller trust?

convert thrust to watts (or any standardized unit of energy), motor output power is motor input power times efficiency, convert to the same standardized unit, Then thrust power divided by motor output power, and you have your propeller efficiency

 Cyrus Oct18-09 10:05 PM

Re: How calculate propeller efficiency if I know propeller trust?

You cant covert thrust to watts. Thrust is not power.

 vissarion.eu Oct19-09 03:10 AM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by twasnow (Post 2399483) convert thrust to watts (or any standardized unit of energy)...
How conwert thrust to watts?

 Cyrus Oct19-09 07:47 AM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by vissarion.eu (Post 2400004) How conwert thrust to watts?
Again, no.

Side: It's spelled convert.

 twasnow Oct19-09 02:57 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by Cyrus (Post 2399700) You cant covert thrust to watts. Thrust is not power.

Thrust is power and it is expressed as either Jules / Second, horsepower, Foot-pounds / Second, or of course Watts.

all of these are easily convertible.

 Mech_Engineer Oct19-09 03:41 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by twasnow (Post 2400727) Thrust is power and it is expressed as either Jules / Second, horsepower, Foot-pounds / Second, or of course Watts. all of these are easily convertible.
Incorrect, thrust is expressed as a unit of force (e.g. newtons, pounds force, etc.), not as a unit of power.

 xxChrisxx Oct19-09 03:52 PM

Re: How calculate propeller efficiency if I know propeller trust?

You need a speed to convert thrust to a power.

 FredGarvin Oct19-09 03:54 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by twasnow (Post 2400727) Thrust is power and it is expressed as either Jules / Second, horsepower, Foot-pounds / Second, or of course Watts. all of these are easily convertible.
Please point us to ANY reference that lists thrust as a measure of power.

To answer the OP you need to know the aircraft speed to calculate the efficiency:

$$\eta_p = \frac{T*u}{P_{shaft}}$$

where:
$$\eta_p$$ = propeller efficiency
$$T$$ = thrust
$$u$$ = aircraft speed
$$P_{shaft}$$ = shaft horsepower provided by the engine to the propeller

 twasnow Oct19-09 09:38 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by FredGarvin (Post 2400794) Please point us to ANY reference that lists thrust as a measure of power. To answer the OP you need to know the aircraft speed to calculate the efficiency: $$\eta_p = \frac{T*u}{P_{shaft}}$$ where: $$\eta_p$$ = propeller efficiency $$T$$ = thrust $$u$$ = aircraft speed $$P_{shaft}$$ = shaft horsepower provided by the engine to the propeller
Sorry, I have mis-spoke, While thrust is a force, without any motion there is no work being performed, if there is no work there is 0 efficiency so long as you are putting in ANY power.

So to correct, I assumed that by "I have thrust of the propeller" he meant, I have the thrust power, and it was expressed as such, other wise to calculate the efficiency we also need a speed giving us Nm/s a.k.a. Watts. Seeing as he is from Europe, has broken English, and didn't provide an air speed or say he was given that variable, I think my first assumption is probably correct.

None the less I shouldn't have repeated his mistake and called it "thrust" instead of "thrust power"

If it was expressed as a force than Great, it better have a speed along with it... well or a crazy slew of other variables so that a theoretical air speed can be calculated, fortunately due to the fact that we have "power in" we would not need to calculate the theoretical max power assuming no drag.

vissarion.eu we really need more info. what are the variables (at least as units) that you were provided.

 twasnow Oct19-09 09:48 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by FredGarvin (Post 2400794) Please point us to ANY reference that lists thrust as a measure of power. To answer the OP you need to know the aircraft speed to calculate the efficiency: $$\eta_p = \frac{T*u}{P_{shaft}}$$ where: $$\eta_p$$ = propeller efficiency $$T$$ = thrust $$u$$ = aircraft speed $$P_{shaft}$$ = shaft horsepower provided by the engine to the propeller
Oh and one other thing Mr. perfect, your calculation there provides the efficiency of a plane getting from point A to point B, Not the propeller efficiency.

 russ_watters Oct19-09 10:09 PM

Re: How calculate propeller efficiency if I know propeller trust?

No, sorry, that's wrong too. A plane with no time isn't getting anywhere, even if it has speed!

 FredGarvin Oct19-09 10:14 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by twasnow (Post 2401237) Oh and one other thing Mr. perfect, your calculation there provides the efficiency of a plane getting from point A to point B, Not the propeller efficiency.
Care to try again? I have 4 or so propulsion references on my desk right now plus a handful of places on line I can cite for the definition of propulsive/propeller efficiency. You are flat out wrong on all accounts.

 twasnow Oct19-09 10:21 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by FredGarvin (Post 2401282) Care to try again? I have 4 or so propulsion references on my desk right now plus a handful of places on line I can cite for the definition of propulsive/propeller efficiency. You are flat out wrong on all accounts.

How is your equation not about plane efficiency it is using aircraft speed

what if the propeller he is talking about is in a fan, or say a pump.....[edited]

 russ_watters Oct19-09 10:24 PM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by twasnow (Post 2401296) How is your equation not about plane efficiency it is using aircraft speed what if the propeller he is talking about is in a fan, or say a pump.....
Because you don't use the word "thrust" to describe what a propeller is doing when it is powering a desk fan. "Thrust" means propelling an object.

Anyway, it only takes a relatively slight modification of that equation to change it to describe a desk fan.

 vissarion.eu Oct20-09 01:24 AM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by FredGarvin (Post 2400794) Please point us to ANY reference that lists thrust as a measure of power. To answer the OP you need to know the aircraft speed to calculate the efficiency: $$\eta_p = \frac{T*u}{P_{shaft}}$$ where: $$\eta_p$$ = propeller efficiency $$T$$ = thrust $$u$$ = aircraft speed $$P_{shaft}$$ = shaft horsepower provided by the engine to the propeller
One horsepower is 745,69987158227022 W. Then, if motor is output power is 745,69987158227022 W, aircraft speed = 30 meters per second, trust is, then its fly what speed, 2 kilograms, propeller efficiency is:
(2 kg * 30 m/s) / 1 HP = 60 procents?

 vissarion.eu Oct20-09 01:40 AM

Re: How calculate propeller efficiency if I know propeller trust?

Quote:
 Quote by twasnow (Post 2401237) Oh and one other thing Mr. perfect, your calculation there provides the efficiency of a plane getting from point A to point B, Not the propeller efficiency.
Be and not bad calculate propeller efficienty with following effect...:
http://img21.imageshack.us/img21/5701/orast.png

Then air created by propeller push plane to opposite direction than fly plane...

 Cyrus Oct20-09 01:57 AM

Re: How calculate propeller efficiency if I know propeller trust?

You have way, way, too many significant digits.

1 HP = 745.7W

Also, why did you use metric units, and then throw in HP? The denominator should be in watts.

Side: It's spelled percent. (No o, or s)

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