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Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

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3

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Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

As a general rule, I'm told GA planes generate 4 lbs of thrust per horsepower.

So if you use a 4 bladed prop, does it generate anything close to 8 lbs of thrust per hp? In other words, say for an ultralight, where you're only flying 60 mph, and prop drag isn't an issue, can you use an engine with half the horsepower, or some other fraction thereof?

Will that shorten my takeoff roll? Conceptually, takeoff roll is just the distance to accelerate from zero to my minimum takeoff speed.

Since F=ma, then a=F/m, so if I want to accelerate twice as fast, I need twice the force, or thrust ( probably more than twice the thrust, to overcome rolling drag, etc.)

Is this correct, at least conceptually?

aerodynamics

share|improve this question

asked 10 hours ago

FredFred

19244 bronze badges

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• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Related: aviation.stackexchange.com/questions/37720/…
  $endgroup$
  – AEhere
  10 hours ago
  

  
  
  
  

add a comment | 

3

$begingroup$

Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

As a general rule, I'm told GA planes generate 4 lbs of thrust per horsepower.

So if you use a 4 bladed prop, does it generate anything close to 8 lbs of thrust per hp? In other words, say for an ultralight, where you're only flying 60 mph, and prop drag isn't an issue, can you use an engine with half the horsepower, or some other fraction thereof?

Will that shorten my takeoff roll? Conceptually, takeoff roll is just the distance to accelerate from zero to my minimum takeoff speed.

Since F=ma, then a=F/m, so if I want to accelerate twice as fast, I need twice the force, or thrust ( probably more than twice the thrust, to overcome rolling drag, etc.)

Is this correct, at least conceptually?

aerodynamics

share|improve this question

asked 10 hours ago

FredFred

19244 bronze badges

$endgroup$

• 
  
  
  

  
  2
  

  
  
  
  
  
  

  
  $begingroup$
  Related: aviation.stackexchange.com/questions/37720/…
  $endgroup$
  – AEhere
  10 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

Does a 4 bladed prop have almost twice the thrust of a 2 bladed prop?

As a general rule, I'm told GA planes generate 4 lbs of thrust per horsepower.

So if you use a 4 bladed prop, does it generate anything close to 8 lbs of thrust per hp? In other words, say for an ultralight, where you're only flying 60 mph, and prop drag isn't an issue, can you use an engine with half the horsepower, or some other fraction thereof?

Will that shorten my takeoff roll? Conceptually, takeoff roll is just the distance to accelerate from zero to my minimum takeoff speed.

Since F=ma, then a=F/m, so if I want to accelerate twice as fast, I need twice the force, or thrust ( probably more than twice the thrust, to overcome rolling drag, etc.)

Is this correct, at least conceptually?

aerodynamics

share|improve this question

asked 10 hours ago

FredFred

19244 bronze badges

$endgroup$

share|improve this question

asked 10 hours ago

FredFred

19244 bronze badges

share|improve this question

asked 10 hours ago

FredFred

19244 bronze badges

asked 10 hours ago

FredFred

19244 bronze badges

asked 10 hours ago

FredFred

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2 Answers
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7

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At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive.

Due to blade interference, it will also generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.

share|improve this answer

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I guess so in theory. That assumes the prop was optimized in the first place, but in the GA world, I see ground clearance limiting prop diameter in many cases. Case in point, many float pilots put on a larger prop once they are on floats, my soaring club wanted better climb performance from their tow plane, so they bolted on a 2nd prop, offset 90deg to the first one. Lazair did the same thing, all without increasing horsepower. I'd say there is probably at least a 20-30+% improvement, or else no one would bother doing it. Of course cruising speed would suffer.
  $endgroup$
  – Fred
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  This is probably the case of a prop optimized for cruise at full course pitch where you need full power. I'm thinking they couldn't get their prop pitch fine enough and there was extra power to be used.
  $endgroup$
  – Fred
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @Fred If you're diameter limited, then optimization may be achieved at more than 2 blades. This is why large airliners can have 40 blades in their fan.
  $endgroup$
  – Therac
  6 hours ago
  

  
  
  
  

add a comment | 

3

$begingroup$

This is not conceptually correct, for the following reason: Doubling the number of blades on your prop will not double the thrust unless your engine is powerful enough to drive the prop at the same speed as the 2-bladed case.

For small planes with fixed-pitch props, a prop and engine combination is chosen for a given aircraft so that when the engine is running at its maximum RPM setting and full throttle, the prop is absorbing the full power output of the engine. If one then adds some more blades to the prop, the engine will be loaded down too heavily for it to run at its maximum power setting and the power output of the engine will go down and so will the thrust generated by the prop.

share|improve this answer

answered 7 hours ago

niels nielsenniels nielsen

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7

$begingroup$

At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive.

Due to blade interference, it will also generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.

share|improve this answer

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I guess so in theory. That assumes the prop was optimized in the first place, but in the GA world, I see ground clearance limiting prop diameter in many cases. Case in point, many float pilots put on a larger prop once they are on floats, my soaring club wanted better climb performance from their tow plane, so they bolted on a 2nd prop, offset 90deg to the first one. Lazair did the same thing, all without increasing horsepower. I'd say there is probably at least a 20-30+% improvement, or else no one would bother doing it. Of course cruising speed would suffer.
  $endgroup$
  – Fred
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  This is probably the case of a prop optimized for cruise at full course pitch where you need full power. I'm thinking they couldn't get their prop pitch fine enough and there was extra power to be used.
  $endgroup$
  – Fred
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @Fred If you're diameter limited, then optimization may be achieved at more than 2 blades. This is why large airliners can have 40 blades in their fan.
  $endgroup$
  – Therac
  6 hours ago
  

  
  
  
  

add a comment | 

7

$begingroup$

At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive.

Due to blade interference, it will also generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.

share|improve this answer

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I guess so in theory. That assumes the prop was optimized in the first place, but in the GA world, I see ground clearance limiting prop diameter in many cases. Case in point, many float pilots put on a larger prop once they are on floats, my soaring club wanted better climb performance from their tow plane, so they bolted on a 2nd prop, offset 90deg to the first one. Lazair did the same thing, all without increasing horsepower. I'd say there is probably at least a 20-30+% improvement, or else no one would bother doing it. Of course cruising speed would suffer.
  $endgroup$
  – Fred
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  This is probably the case of a prop optimized for cruise at full course pitch where you need full power. I'm thinking they couldn't get their prop pitch fine enough and there was extra power to be used.
  $endgroup$
  – Fred
  7 hours ago
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @Fred If you're diameter limited, then optimization may be achieved at more than 2 blades. This is why large airliners can have 40 blades in their fan.
  $endgroup$
  – Therac
  6 hours ago
  

  
  
  
  

add a comment | 

$begingroup$

At the same size and rpm, a 4-bladed prop will require twice the horsepower to drive.

Due to blade interference, it will also generate less than twice the thrust. So lbs thrust per hp will generally slightly decrease, as long as the propeller is in its optimal tip airspeed band.

share|improve this answer

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

$endgroup$

share|improve this answer

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

answered 10 hours ago

TheracTherac

9,5472828 silver badges4040 bronze badges

3

$begingroup$

This is not conceptually correct, for the following reason: Doubling the number of blades on your prop will not double the thrust unless your engine is powerful enough to drive the prop at the same speed as the 2-bladed case.

For small planes with fixed-pitch props, a prop and engine combination is chosen for a given aircraft so that when the engine is running at its maximum RPM setting and full throttle, the prop is absorbing the full power output of the engine. If one then adds some more blades to the prop, the engine will be loaded down too heavily for it to run at its maximum power setting and the power output of the engine will go down and so will the thrust generated by the prop.

share|improve this answer

answered 7 hours ago

niels nielsenniels nielsen

3,06611 gold badge55 silver badges1616 bronze badges

$endgroup$

add a comment | 

3

$begingroup$

This is not conceptually correct, for the following reason: Doubling the number of blades on your prop will not double the thrust unless your engine is powerful enough to drive the prop at the same speed as the 2-bladed case.

For small planes with fixed-pitch props, a prop and engine combination is chosen for a given aircraft so that when the engine is running at its maximum RPM setting and full throttle, the prop is absorbing the full power output of the engine. If one then adds some more blades to the prop, the engine will be loaded down too heavily for it to run at its maximum power setting and the power output of the engine will go down and so will the thrust generated by the prop.

share|improve this answer

answered 7 hours ago

niels nielsenniels nielsen

3,06611 gold badge55 silver badges1616 bronze badges

$endgroup$

add a comment | 

$begingroup$

This is not conceptually correct, for the following reason: Doubling the number of blades on your prop will not double the thrust unless your engine is powerful enough to drive the prop at the same speed as the 2-bladed case.

For small planes with fixed-pitch props, a prop and engine combination is chosen for a given aircraft so that when the engine is running at its maximum RPM setting and full throttle, the prop is absorbing the full power output of the engine. If one then adds some more blades to the prop, the engine will be loaded down too heavily for it to run at its maximum power setting and the power output of the engine will go down and so will the thrust generated by the prop.

share|improve this answer

answered 7 hours ago

niels nielsenniels nielsen

3,06611 gold badge55 silver badges1616 bronze badges

$endgroup$

share|improve this answer

answered 7 hours ago

niels nielsenniels nielsen

3,06611 gold badge55 silver badges1616 bronze badges

answered 7 hours ago

niels nielsenniels nielsen

3,06611 gold badge55 silver badges1616 bronze badges

answered 7 hours ago

niels nielsenniels nielsen

3,06611 gold badge55 silver badges1616 bronze badges