How calculate propeller efficiency if I know propeller trust?

[vissarion.eu]

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

 

[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

 

[Cyrus]

You can't covert thrust to watts. Thrust is not power.

 

[vissarion.eu]

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

How conwert thrust to watts?

 

[Cyrus]

How conwert thrust to watts?

Again, no.

Side: It's spelled convert.

 

[twasnow]

You can't 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]

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]

You need a speed to convert thrust to a power.

 

[FredGarvin]

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]

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]

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]

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

 

[FredGarvin]

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]

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]

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]

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]

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]

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)

 

[Cyrus]

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:

​

 

[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 %

 

[xxChrisxx]

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 doesn't 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 I've 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.

 

[vissarion.eu]

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.

 

[vissarion.eu]

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.

 

[xxChrisxx]

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.

 

[vissarion.eu]

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 doesn't 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 I've 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):

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 I've ever read on the subject?

Car powered propeller must be like normal car.
This car good:

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

 

[xxChrisxx]

Your calculation is crap. The above post is similarly pointless to furthering your argument.

Fred showed you how to calculate prop efficiency.

/Thread.

 

[vissarion.eu]

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.

If human pushing car and car not going, then human use 0 watts? No! It pushing in some power, it wasting energy, it do work. It power about 200 watts. Same and with propeller. If it pushing, but car not going, that's not mean, that it not have power. I think, hat possible convert trust to watts.

 

[vissarion.eu]

Your calculation is crap. The above post is similarly pointless to furthering your argument.

Fred showed you how to calculate prop efficiency.

/Thread.

There you me put not enought details how calculate. I not understand.
For example car powered propeller have trust 100 kg.
It speed 30 km/h.
Motor output power 20000 watts.
What propeller eficienty?

 

[xxChrisxx]

if you are using the SI units.

Thrust in N
Speed in m/s
Power in Watts


You then put these numbers into the equation fred gave in post 9.

100kgf = 981N Thrust
30 kph = 8.33 m/s

Shaft HP = Rated motor output * mechanical efficiecny (assumre 90%)
For a car this value could be much much lower.

Power(shaft) = 18000 W

Efficiency = 981*8.33/18000
= .45

 

[vissarion.eu]

... Shaft HP = Rated motor output * mechanical efficiecny (assumre 90%)
For a car this value could be much much lower.

Power(shaft) = 18000 W

I say motor output power. This mean not motor power, but output power, power, that motors gives. So power is not 18000 W, but 20000 W. Little mistake you make.

Efficiency plane without air resistance (or with air resistance?) = 981*8.33/20000 = 0,4085865. Efficienty car = 0,4085865 * 0.7 (friction to ground) = 0,28601055.

Efficiency plane with air resistance = 0,4085865 * 0,9 (for example if 0,4085865 is without air resistance) = 0,36772785.

Thanks!

 

[xxChrisxx]

Motor horsepower is motor output.

SHAFT horsepower is including losses AFTER the motor but BEFORE the propeller.

We use shaft horsepower NOT motor horsepower becuase the only loss between the shaft and the final output IS the propeller. Giving... *TADAAAAA* propeller efficiency!There is no mistake, you just don't appear to understand why shaft horsepower is used and not motor.
If you used motor output you'd be finding Prop efficiency AND Transmission efficiency in one term.

Of course if you meant the power going to the prop (which most people refer to as shaft power) as motor power, then you would be right to use 20 kW.I don't get why you have done those 'calculations' as they are meaningless (mostly becuase loss coefficients are rubbsih and pulled out of thin air) and don't calulcate propeller efficiency.Is there some sort of language barrier here? Where are you from vissarion?

EDIT: I guess I should explain more throughly.
Engine power output varies at the point that you measure it. The following power outputs would be measures highest to loest.Indicated Horsepower
Power found theoretically, taking into account the thermal and volumatric efficiencies on the engine.

Brake Horsepower
Power found by measuring the output at the crankshaft (in a piston engine). Takes into account the mechanical efficiency of the engine.
In cars this would be the bhp and is found by an engine dyno.

Shaft Horsepower
Power found after the transmission. This takes into account transmission efficiency. This is the available power for application.

Output/Effective Horsepower
This is the power that the prop is using to acutally do useful work. Includes ALL effiiencies.
This power can be measured or calculated.
In cars this would be the wheelhp and can be measured by a rolling road.
If anyone on the thread can see a glaring omission or problem with the above descriptions, let me know.

 

[vissarion.eu]

Is there some sort of language barrier here? Where are you from vissarion?

I from Lithuania. I not wery good understand english language.

 

[xxChrisxx]

Ah I see, well your english is certainly better than my lithuanian. :)

In that case you may be using different words to me. What do you mean by 'motor output power'? Where was the power value taken from? (Compare to list above).

 

[vissarion.eu]

What do you mean by 'motor output power'? Where was the power value taken from? (Compare to list above).

Motor output power is power in watts for example, that makes shaft motor. For example motor input power 100 W (10 V, 10 A), motor output power 70 W. This mean, that motor eficienty is 70 %. I don't known how exactly measure motor output power. Input power value taken from volts * ampers (V * A) in electric motor.

 

[xxChrisxx]

If the value you have for the motor is taken from a book it is likely to be Power at crankshaft. Brake Horsepower.

Between motor and propellor is a transmission.

If assuming no loss from transmission.
Shaft Horsepower = Motor Horsepower.

However, transmission will always have loss in real cases. This is why I used:
20000 W *.9 = 18000 W
Example Case.

Engine is 50% thermal efficient
Engine is 90% mechanical efficienct.
Transmission is 90% efficienct.

Input fuel = 100000W
Motor output = input* efficiency = 50000 W (theoretical/indicated output)
Real Motor output = motor output*mechanical efficiency = 45000 W (brake output)
Shaft Power Available = real output* transmission efficiency = 40500 W (shaft output)

Before power gets to propeller 59.5% of power is lost.
This is why we use Shaft power.