How calculate propeller efficiency if I know propeller trust?

  • #26
2,017
85
Your calculation is crap. The above post is similarly pointless to furthering your argument.

Fred showed you how to calculate prop efficiency.

/Thread.
 
  • #27
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, thats not mean, that it not have power. I think, hat possible convert trust to watts.
 
  • #28
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?
 
  • #29
2,017
85
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
 
  • #30
... 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!
 
  • #31
2,017
85
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 dont get why you have done those 'calculations' as they are meaningless (mostly becuase loss coefficients are rubbsih and pulled out of thin air) and dont 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.
 
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  • #32
Is there some sort of language barrier here? Where are you from vissarion?
I from Lithuania. I not wery good understand english language.
 
  • #33
2,017
85
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).
 
  • #34
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.
 
  • #35
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85
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.
 
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  • #36
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.
Then I say motor output power, I to output power include all lost power. I not think about transmision. I think that motor shaft be directly placed to propeller.
 
  • #37
2,017
85
Then you are assuming no transmission losses.

This means that when you say motor power, this is the same as when other people say shaft power.

Which is ok, so long as you say it. Otherwise it is confusing.
 
  • #38
Then you are assuming no transmission losses.

This means that when you say motor power, this is the same as when other people say shaft power.

Which is ok, so long as you say it. Otherwise it is confusing.
I assuming that no reduction.


This means that when I say motor output power, this is the same as when other people say shaft power.
 
  • #39
2,017
85
I assuming that no reduction.


This means that when I say motor output power, this is the same as when other people say shaft power.
Yes. :)
 
  • #40
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Hi everybody!

I am new here and found this thread when I seaked for propeller efficiency at Google. Since many years I have been fascinated of the propeller and more than 20 years ago I got the "crazy idéa" of making a pedal bike with a propeller. (I have still not made it) I have read a lot about it and found out some things.

First of all: it's obvious that something driving in an elastic or unfirm ground cannot be that efficient as when driving at firm ground ,like wheel drive. (But it had been just that cool with pedalling a propeller!)
Taking one example: when rowing a boat you easy notice it's more efficient when you are on shallow water and can press the oars against the water bed than against the water.

Also the size of the propeller related to the speed is important. For example: putting a small one horse power AC airplane engine with propeller to a bicycle would be a bad solution. Though one horse power produced by a cyclist will give a common wheel drived racing bike a speed of around 60km/h this small AC plane propeller would barely give enough thrust to move the bike with the cyclist forward at all. Yes, it would but it would go very slowly. The propeller area is way to small for this purpose.

But: if the cyclist would pedalling a much larger propeller with, for example 1,5m size, and maybe 4 blades, this would give the bike much higher speed with only ,5 horsepower.

The efficiency depends of several factors, and one basic factor (I think the most important) is the blade (wing)area. A propeller with 4 blades and 2 meter diameter would give twice the static thrust than a 2 bladed with 1 meter size with same power. This is when stationary. That because the speed of the propeller would be halvened when the area is 8 times larger.
That would likely not work well to the AC airplane however, because the propeller would rotate too slowly for the purpose.

If I understand it right according to the earlier discussion and posts, an efficiency of a propeller of 50% compared to a wheel would mean that the actual propeller makes the demand of twice the power to reach (hold) the same speed. If the efficiency is 70% it would mean that the actual propeller would need around 1,4 times the power to reach the same speed.

But it's very important to take in consider the size of the propeller for the actual situation and condition. A propeller with 1m diameter would with human power allow climbing only very modest uphill, while pedalling a helicopter rotor could make it possible to climb slalom slopes. (hypothetically of course when it's practically impossible, just counting of the blade area)

Apologize for my somewhat lacking english, maybe I don't always find the correct words. Welcome with replies.

Regards, Patric, Sweden
 
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  • #41
Then propeller diameter is for example 0.457 m and pich is 0.254 m (18x10 inches) and propeller rotation speed is for example 4000 rpm then propeller max eficienty be at speed (propeller eficienty max be then sliding propeller is 20-30 %):

4000/60 s = 66.666666666666666666666666666667 revolution per second.

66.666666666666666666666666666667 * 0.254 = 16.933333333333333333333333333333 meters per second.

16.933333333333333333333333333333 * 3.6 = 60.96 km/h

60.96 * 0.75 = 45.72 km/h.

So max propeller efficienty of propeller 18x10 inch with 4000 rpm be at speed 45.72 km/h.

If propeller is 12x6 inch (305 cm diameter and pitch 152 cm) and rpm is 4000, then propeller max efficienty be at speed:

66.67 rps * 0.152 = 10.13384 m/s

10.13384 * 3.6 = 36.481824 km/h

36.481824 * 0.75 = 27.361368 km/h.

If propeller is 12x6 inch (305 cm diameter and pitch 152 cm) and rpm is 6000, then propeller max efficienty be at speed:

6000/60 = 100 revolution per second

100 * 0.152 = 15.2 m/s

15.2 m/s * 3.6 = 54.72 km/h

54.72 km/h * 0.75 = 41.04 km/h

If you not ride on that speed, then propeller eficienty is max ( maybe 60-80 %), then propeller efficienty can be 10 precent if you ride about, not 41.04 km/h, but 7 km/h.

Propeller efficienty posible maybe known use ventilator and blow to propeller. If we know ventiliator motor output power, ventilator efficienty and know power what generates generator on that is propeller and we know generator efficienty, then we can calculate propeller efficienty.

I read what some propellers max efficienty is 0.9.

Thats propellers max efficienty is 0.9:

http://www.aerosila.ru/index.php?actions=main_content&id=17

http://www.aerosila.ru/index.php?actions=main_content&id=18
 
  • #42
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Thanks for your reply vissarion.eu!

Interesting things you share, though it may be a bit over my physical knowledge to count out these details. But according to what you say and the values you show the efficiency of a propeller is very depending of the right size and pitch for the intended speed.

About the efficiency, did I got it right that this is the same thing as the percentual thrust compared to wheel drive at the actual speed (and assuming wheel drive had 100% efficiency)?

Regards, Patric
 
  • #43
Thanks for your reply vissarion.eu!

Interesting things you share, though it may be a bit over my physical knowledge to count out these details. But according to what you say and the values you show the efficiency of a propeller is very depending of the right size and pitch for the intended speed.
For slow (10-30 km/h) moving cars, bicycles driven with propeller beter, of corse, larger propeler, than smaller propeller and beter more blades propeller, than less blades propeller, but with higher rpm if you want bigest eficienty (for example for bicycle driven with propeller if we want speed 20 km/h need about 0.6 meter propeller with 4 blades and 2000 rpm propeller rotating speed; need about 5 kg thrust to pull bicycle, who weight with rider about 80 Kg).
About the efficiency, did I got it right that this is the same thing as the percentual thrust compared to wheel drive at the actual speed (and assuming wheel drive had 100% efficiency)?
I, how is in reallity, do not known.
 
  • #44
graph1.jpg


http://www.electric-bikes.com/betterbikes/index.html [Broken]

Jet when fly not have friction with ground, but it has wings, so wings frictring to air.
Bicycle driven with propeller not have wing, who fricting to air, but have wheels, who fricting to ground and ball-bearings (and not front wheel bicycle have free-wheel, what about 3 times make biger friction, about 3-4 times faster stop back wheel, than front wheel, when you rotate its with same speed).

I think, that showen in image is not correct (how you think?), because then possible be ride with about 20 W motor on 8 km/h speed.

Must be like this, I think:
http://img202.imageshack.us/img202/6835/resistance.png [Broken]
 
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  • #45
I calculate what need 1000 rpm propeller 0.5 meter pitch propeller and diameter propeller we can choose 2-1.7 meter with two blades or 1.7-1.5 with 4 blades for bicycle to run 22.5 km/h.

1000/60 = 16.666666666666666666666666666667 r/s

0.5 * 16.666666666666666666666666666667 = 8.3333333333333333333333333333333 m/s

8.3333333333333333333333333333333 * 3.6 = 30 km/h

30 * 0.75 = 22.5 km/h
 
  • #46
I be bulded propeller driving bicycle with motor who use about 500-800 W power, bettery weight be 5.3 kg, motor weight be about 300 g, propeller be 18x10 inches APC. My wheight be about 55 kg. Motor rpm be about 6000-3000 and I ride on it without pedaling on flat about 10-15 km/h.
 
  • #47
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For slow (10-30 km/h) moving cars, bicycles driven with propeller beter, of corse, larger propeler, than smaller propeller and beter more blades propeller, than less blades propeller, but with higher rpm if you want bigest eficienty (for example for bicycle driven with propeller if we want speed 20 km/h need about 0.6 meter propeller with 4 blades and 2000 rpm propeller rotating speed; need about 5 kg thrust to pull bicycle, who weight with rider about 80 Kg).
Pedalled drived airplanes have a 2-bladed propeller of around 3m diameter, so I understand that the most efficient is to have a large propeller which is rotating not much faster than the vehicle. About the air and rolling resistance I think I read that a racing bike has not much more rolling resistance than 5N and I think the air resistance may be around 10N in 30km/h. I therefore think 5kg(50N) is the total resistance in 50+km/h.

Here is a guy who actually made a propeller trike:
http://www.treehugger.com/files/2007/06/the-diy-propeller-trike.php
 
  • #48
Pedalled drived airplanes have a 2-bladed propeller of around 3m diameter, so I understand that the most efficient is to have a large propeller which is rotating not much faster than the vehicle.
Bigest propeller have two advantages (good things):
1 good thing. Slow rotating, lees friction brearing.
2 good thing. It blow less to himself, like in this image:http://img21.imageshack.us/img21/5701/orast.png [Broken]

And is other good thing. Then pitch is, for example, 10 inches and diameter propeller is 40 inches, then that propeller very not efficien. Best proportion is then pitch and diameter is about 1:1, for example best proportion is 10x10 inches, 1x1 meter, 2x2 meters. Midle propeller angle of atack must be about 45 degree.

Big propeller only not good things is that it must be wery thin profile (for pedal, propeller driven bicycle, who speed is about 20 km/h), so it easy breake.

And, maybe, difficult make transmision to rotate for exampla 0.5 meter propeller with pedal. Much easy, maybe, make for 3 meter diameter propeller transmision and beter be efficient transmision.

Small propeller( for example 0.5x0.5 meter), who efficienty max is on 20 km/h and we need that speed, for esample, not make enought thrus. So need make smaller pich, for example 0.5 diameter meter and pitch 0.1 meter and make biger rpm, but how I said that propeller proportion is not efficient. Or can use 0.5 meter diameter and 0.5 pitch meter propeller with highest rpm, but then it and now be not efficient, because max efficienty it be for example on 50 km/h, on 20 km/h it eficienty be, let say, about 30 precent (on 50 km/h be about 70 precent), so it makes enough thrust to ride on 20 km/h, but eficienty propeller be only 30 precent, so using bigest propeller, who diameter can be 3 meters and pitch can be 3 meters. And it make enough thrust to ride or fly on 20 km/h and it max efficienty is on 20 km/h.

For example in this video


propeller efficienty max is on about 30-40 miles per hour. But it not make enought thrust to pull bicycle in that speed so it ride on about 10-20 km/h, but if human little bedal it ride on 35 mph how say author that video:

"It will run 35 mph - just need to pedal a little."
 
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