How calculate propeller efficiency if I know propeller trust?

  • #1
How calculate propeller efficiency if I know propeller trust ant motor power and motor efficienty?
 

Answers and Replies

  • #2
10
0
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
 
  • #3
2,985
15
You cant covert thrust to watts. Thrust is not power.
 
  • #4
convert thrust to watts (or any standardized unit of energy)...
How conwert thrust to watts?
 
  • #5
2,985
15
How conwert thrust to watts?
Again, no.

Side: It's spelled convert.
 
  • #6
10
0
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
Gold Member
2,572
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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.
 
  • #8
2,017
85
You need a speed to convert thrust to a power.
 
  • #9
FredGarvin
Science Advisor
5,066
9
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:

[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
10
0
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
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.
 
  • #11
10
0
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
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.
 
  • #12
russ_watters
Mentor
19,951
6,441
No, sorry, that's wrong too. A plane with no time isn't getting anywhere, even if it has speed!
 
  • #13
FredGarvin
Science Advisor
5,066
9
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.
 
  • #14
10
0
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
19,951
6,441
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.
 
  • #16
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
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?
 
  • #17
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 [Broken]

Then air created by propeller push plane to opposite direction than fly plane...
 
Last edited by a moderator:
  • #18
2,985
15
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)
 
  • #19
2,985
15
Be and not bad calculate propeller efficienty with following effect...:


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

 
Last edited by a moderator:
  • #20
I think how get propeller efficienty in practic is need that run two cars same weight. One powered wheels, second powered propeller. And what it runs in flat track without stops 30 km/h (kilometers per hour), who long 1000 kilometers. And look how many gasoline one car waste and second. If one car waste 70 liters gasoline and seconds (propeller powered) 100 liters. Then eficienty propeller is:

1/70 = 0,014285714285714285714285714285714

1/100 = 0,01

0,01 / 0,014285714285714285714285714285714 = 0,700000000000000000000000000035

0,700000000000000000000000000035 * 100 = 70 %
 
  • #21
2,017
85
It's early and my brain hurts from a night of drinking.

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
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)

Because for me more simple use meters than feets or inches.
 
  • #23
1. Why on earth do you think at 30km/h a prop would give better fuel efficiency than driveshafts and wheels?
I not think that. Car with proppeller waste more fuel whan car driven with wheels.
 
  • #24
2,017
85
I not think that. Car with proppeller waste more fuel whan car driven with wheels.
Fair enough, I read it wrong (told you, my brain wasnt up to speed). The method you specified is still crap though.

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
It's early and my brain hurts from a night of drinking.

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.
100 l for car and 70 l for prop.
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.
Aero car (propeller powered car):
34yewt5rhtgde.jpg

34yhethbf.jpg


Code:
Why do you think this is calculating the propellor efficiency?
How other?
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?
Car powered propeller must be like normal car.
This car good:
3254yewthy46.jpg


This car powered propeller not good for experiment, because is not like normal car:
e4w56w4t5rhdr.jpg
 

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