# How calculate propeller efficiency if I know propeller trust?

1. Oct 17, 2009

### vissarion.eu

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

2. Oct 18, 2009

### twasnow

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. Oct 18, 2009

### Cyrus

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

4. Oct 19, 2009

### vissarion.eu

How conwert thrust to watts?

5. Oct 19, 2009

### Cyrus

Again, no.

Side: It's spelled convert.

6. Oct 19, 2009

### twasnow

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. Oct 19, 2009

### Mech_Engineer

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

8. Oct 19, 2009

### xxChrisxx

You need a speed to convert thrust to a power.

9. Oct 19, 2009

### FredGarvin

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

10. Oct 19, 2009

### twasnow

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. Oct 19, 2009

### twasnow

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. Oct 19, 2009

### Staff: Mentor

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

13. Oct 19, 2009

### FredGarvin

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. Oct 19, 2009

### twasnow

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. Oct 19, 2009

### Staff: Mentor

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. Oct 20, 2009

### vissarion.eu

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. Oct 20, 2009

### vissarion.eu

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

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

18. Oct 20, 2009

### Cyrus

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. Oct 20, 2009

### Cyrus

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:

20. Oct 25, 2009

### vissarion.eu

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 %