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

- Thread starter vissarion.eu
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- #1

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

- #2

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- #3

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

- #4

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How conwert thrust to watts?convert thrust to watts (or any standardized unit of energy)...

- #5

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- #6

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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.

- #7

Mech_Engineer

Science Advisor

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Incorrect, thrust is expressed as a unit of force (e.g. newtons, pounds force, etc.), not as a unit of 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.

- #8

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You need a speed to convert thrust to a power.

- #9

FredGarvin

Science Advisor

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Please point us to ANY reference that lists thrust as a measure of 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.

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

[tex]\eta_p = \frac{T*u}{P_{shaft}}[/tex]

where:

[tex]\eta_p[/tex] = propeller efficiency

[tex]T[/tex] = thrust

[tex]u[/tex] = aircraft speed

[tex]P_{shaft}[/tex] = shaft horsepower provided by the engine to the propeller

- #10

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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.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:

[tex]\eta_p = \frac{T*u}{P_{shaft}}[/tex]

where:

[tex]\eta_p[/tex] = propeller efficiency

[tex]T[/tex] = thrust

[tex]u[/tex] = aircraft speed

[tex]P_{shaft}[/tex] = shaft horsepower provided by the engine to the propeller

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.

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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.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:

[tex]\eta_p = \frac{T*u}{P_{shaft}}[/tex]

where:

[tex]\eta_p[/tex] = propeller efficiency

[tex]T[/tex] = thrust

[tex]u[/tex] = aircraft speed

[tex]P_{shaft}[/tex] = shaft horsepower provided by the engine to the propeller

- #12

russ_watters

Mentor

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

- #13

FredGarvin

Science Advisor

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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.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.

- #14

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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]

- #15

russ_watters

Mentor

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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.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.....

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

- #16

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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: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:

[tex]\eta_p = \frac{T*u}{P_{shaft}}[/tex]

where:

[tex]\eta_p[/tex] = propeller efficiency

[tex]T[/tex] = thrust

[tex]u[/tex] = aircraft speed

[tex]P_{shaft}[/tex] = shaft horsepower provided by the engine to the propeller

(2 kg * 30 m/s) / 1 HP = 60 procents?

- #17

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Be and not bad calculate propeller efficienty with following effect...: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.

http://img21.imageshack.us/img21/5701/orast.png [Broken]

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

Last edited by a moderator:

- #18

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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 p

- #19

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For reference, I have found it much better to google images rather than trying to draw them myself because you can usually find ones of higer quality. See:Be and not bad calculate propeller efficienty with following effect...:

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

Last edited by a moderator:

- #20

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1/70 = 0,014285714285714285714285714285714

1/100 = 0,01

0,01 / 0,014285714285714285714285714285714 = 0,700000000000000000000000000035

0,700000000000000000000000000035 * 100 = 70 %

- #21

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Even in my lowered state of mental agility I can tell something about the above just doesnt seem quite right.

1. Why on earth do you think at 30km/h a prop would give better fuel efficiency than driveshafts and wheels?

Run it with numbers the bother way around.

100 l for car and 70 l for prop.

Using the same equations you have, just flipped.

You get 142% efficiency. Wow.

2. Why do you think this is calculating the propellor efficiency?

3. What do you think the propellor efficiency acutally is, as you seem to be using a different definition to every textbook ive ever read on the subject?

Look at what variables you have included in your 'calculation'.

Vehicle speed working axially to the prop.

Power output, from getting fom A to B to measure fuel useage.

Trust required to remain at set speed of 30km/h.

Compare the above to the equation that we know that works for prop efficiecnt that you dismissed.

- #22

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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)

Because for me more simple use meters than feets or inches.

- #23

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I not think that. Car with proppeller waste more fuel whan car driven with wheels.1. Why on earth do you think at 30km/h a prop would give better fuel efficiency than driveshafts and wheels?

- #24

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Fair enough, I read it wrong (told you, my brain wasnt up to speed). The method you specified is still crap though.I not think that. Car with proppeller waste more fuel whan car driven with wheels.

I think there is little more to say on this, multiple textbooks disagree with you. Others have explained prop efficiency sufficiently well. Either you are dismissing it because you are stubborn or becuase you have a non standard definition of propellor efficiency.

- #25

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Even in my lowered state of mental agility I can tell something about the above just doesnt seem quite right.

1. Why on earth do you think at 30km/h a prop would give better fuel efficiency than driveshafts and wheels?

Run it with numbers the bother way around.

100 l for car and 70 l for prop.

Using the same equations you have, just flipped.

You get 142% efficiency. Wow.

2. Why do you think this is calculating the propellor efficiency?

3. What do you think the propellor efficiency acutally is, as you seem to be using a different definition to every textbook ive ever read on the subject?

Look at what variables you have included in your 'calculation'.

Vehicle speed working axially to the prop.

Power output, from getting fom A to B to measure fuel useage.

Trust required to remain at set speed of 30km/h.

Compare the above to the equation that we know that works for prop efficiecnt that you dismissed.

You not good me understand. I not say 100 l for car and 70 l for prop. I say 100 l for propeller powered car and 70 l for normal car.100 l for car and 70 l for prop.

Aero car (propeller powered car):

Code:

`Why do you think this is calculating the propellor efficiency?`

Car powered propeller must be like normal car.3. What do you think the propellor efficiency acutally is, as you seem to be using a different definition to every textbook ive ever read on the subject?

This car good:

This car powered propeller not good for experiment, because is not like normal car:

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