Why not bigger props or fans?
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Joined: Sep 2006
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From: Los Angeles, USA
Why not bigger props or fans?
Now, I want to preface this question with the fact that I'm just a layman and amateur with a technical interest. Here goes my genuine question:
It is a well known fact within the helicopter world that the bigger the main rotor, the less power it needs to produce lift. It also makes less noise, since it turns slower.
With this in mind, how come most GA planes have such small props? This makes them noisy and producing less thrust. Why aren't they made bigger? They don't necessarily have to become heavier, since the loading per square inch/mm of material decreases proportionately. They don't become harder to balance. They do however become harder from a clearance perspective, and I suspect this is the real problem. But that seams like an easy obstacle to overcome - just mount them higher.
Taken this idea further, isn't it fair to also assume that a fanjet engine also would benefit from a bigger fan? Newer fanjets seem to be getting bigger and bigger (as consumption goes down), so something's afoot. Obviously, at some point the fan will get so big, that it would be prohibitive to encase it. Drag, and structural retaining abilities (if a blade comes off would) make it too big. And then were basically going to end up with a un-shrouded fanjet. Bit like a turboprop but with more and bigger blades. Come to think of it - hasn't this been done?
Please share your valuable ideas.
It is a well known fact within the helicopter world that the bigger the main rotor, the less power it needs to produce lift. It also makes less noise, since it turns slower.
With this in mind, how come most GA planes have such small props? This makes them noisy and producing less thrust. Why aren't they made bigger? They don't necessarily have to become heavier, since the loading per square inch/mm of material decreases proportionately. They don't become harder to balance. They do however become harder from a clearance perspective, and I suspect this is the real problem. But that seams like an easy obstacle to overcome - just mount them higher.
Taken this idea further, isn't it fair to also assume that a fanjet engine also would benefit from a bigger fan? Newer fanjets seem to be getting bigger and bigger (as consumption goes down), so something's afoot. Obviously, at some point the fan will get so big, that it would be prohibitive to encase it. Drag, and structural retaining abilities (if a blade comes off would) make it too big. And then were basically going to end up with a un-shrouded fanjet. Bit like a turboprop but with more and bigger blades. Come to think of it - hasn't this been done?
Please share your valuable ideas.
Joined: May 2005
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From: F370
Tip speed: as the prop tip becomes transonic it loses efficiency. As you have noted larger props turn more slowly, but this is mostly a design limit imposed by tip speed.
Clearance: If a prop gets very big, you start to have to design around it such as extending the gear (more weight, cabin entry considerations) or are forced into a high wing multi-engine (which is slightly less efficient)
Regarding piston engines: most props are bolted directly to the crankshaft to avoid the complications, weight and reliability issues associated with a gearbox. In this case the engine RPM will basically determine the radius of the prop, and the engine horsepower will determine the number of blades required to "absorb the power". But too many blades is not good.
Turbo-props have to have a gearbox to reduce the turbine RPM, so the prop size is mostly determined by clearance issues. RPM are generally in the range of about 1000, although it can be significantly higher and somewhat lower.
When you go to a fan design, you get rid of the problems associated with the blade tip. A larger fan can increase bypass ratio and efficiency, but it adds to weight and you need the space to mount it.
I am not qualified to discuss unducted fans or helicopters!
I hope that helps.
Clearance: If a prop gets very big, you start to have to design around it such as extending the gear (more weight, cabin entry considerations) or are forced into a high wing multi-engine (which is slightly less efficient)
Regarding piston engines: most props are bolted directly to the crankshaft to avoid the complications, weight and reliability issues associated with a gearbox. In this case the engine RPM will basically determine the radius of the prop, and the engine horsepower will determine the number of blades required to "absorb the power". But too many blades is not good.
Turbo-props have to have a gearbox to reduce the turbine RPM, so the prop size is mostly determined by clearance issues. RPM are generally in the range of about 1000, although it can be significantly higher and somewhat lower.
When you go to a fan design, you get rid of the problems associated with the blade tip. A larger fan can increase bypass ratio and efficiency, but it adds to weight and you need the space to mount it.
I am not qualified to discuss unducted fans or helicopters!
I hope that helps.
Joined: Apr 2006
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From: UK
And because the larger prop is transonic it makes a very loud noise.
Take the Stearman N2S (I think this is the correct model), the propeller is around 8foot in size and the tips go transonic, they are very loud and a lot of airfields will not allow them to land.
Take the Stearman N2S (I think this is the correct model), the propeller is around 8foot in size and the tips go transonic, they are very loud and a lot of airfields will not allow them to land.
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From: Wandering the FIR and cyberspace often at highly unsociable times
A way round the tip speed problem is to increase the solidity of the propellor. This means giving it more area, either by more blades (more complicated hub), wider blades (not so efficient) or by a contra-rotating prop behind it (more expensive).
All aviation engineering design is a compromise, usually driven by money.
All aviation engineering design is a compromise, usually driven by money.
Thread Starter
Joined: Sep 2006
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From: Los Angeles, USA
Transonic tip speeds is a non-issue - they turn slower, just like a helicopters. Like windmills - those props are huge, turn slow and make very little noise. That doesn't mean they move less air.
As long as you can design a prop where the tip speed stays subsonic, it can be any size. The bigger it gets, the slower it'll have to turn just.
As long as you can design a prop where the tip speed stays subsonic, it can be any size. The bigger it gets, the slower it'll have to turn just.
Avoid imitations
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From: Wandering the FIR and cyberspace often at highly unsociable times
Helicopter main rotors are effectively a propellor flying sideways and suffer the same problem of transonics at high cruise speeds. The advancing blade's "forward flying" airspeed is added to the blade's rotational airspeed.
Some main rotor blades have swept tips, to delay those bad high speed effects.
Trouble is, as soon as the blade "turns the corner" and becomes a "retreating" blade, the forward airspeed steals the rotational airspeed, bringing the blade close to the stall. In fact the inner sections of the blade do stall at high IAS.
These are the main limits to forward airspeed of a helicopter and one of the reasons that the tilt-rotor was designed.
Some main rotor blades have swept tips, to delay those bad high speed effects.
Trouble is, as soon as the blade "turns the corner" and becomes a "retreating" blade, the forward airspeed steals the rotational airspeed, bringing the blade close to the stall. In fact the inner sections of the blade do stall at high IAS.
These are the main limits to forward airspeed of a helicopter and one of the reasons that the tilt-rotor was designed.
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From: UK
Does that mean I CANT HAVE A 3 BLADED PROP ON MY CHEROKEE 160?
Genghis
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