Couple of blade construction questions.
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Couple of blade construction questions.
Have a couple of questions for all you aerodynamical rotorheads out there that I've always wondered about, but haven't been able to find answers to.
1. One of the reasons heli's can't go very fast is because the retreating blade will have an unnaturally high angle of attack as to push air through the disc, causing it to stall. To increase stall resistance in FW, slats are often mounted to wings. Why couldn't you mount slats to the blade that pop out when the blade retreats?
2. Most modern rotors in helicopters seem to have constant chord blades where the tip has been twisted donwards towards the tip so as to produce less lift the further away from the hub you get. But the same thing could be achieved by having the chord change (and the twist remain unchanged) - by narrowing the blade the further you go out from the hub, the less lift it would produce (this was common in earlier helicopters). A twisted blade must be so much more expensive and complicated to make, so why was the changing chord design abandoned?
3. Also, older helicopters often had wooden blade rotors, and I think a few helicopters might still do (Brantly?). But I can't recall reading or hearing anywhere that wooden blades were more susceptible to failures than metal ones. What was the main reason wooden rotor blades were abandoned?
4. Rigid rotor question. As I understand it, in a rigid rotor system, lag and hinging is 'absorbed' by the blade itself, eliminating the need for dampers and such. Now, if I just took a normal, fully articulated rotor and removed the dampers for something solid and beefed up the blades, wouldn't this become a rigid rotor? If so, why aren't all manufacturers making rigid rotor systems, since they seem to have less moving parts that can start to misbehave?
Thank you.
1. One of the reasons heli's can't go very fast is because the retreating blade will have an unnaturally high angle of attack as to push air through the disc, causing it to stall. To increase stall resistance in FW, slats are often mounted to wings. Why couldn't you mount slats to the blade that pop out when the blade retreats?
2. Most modern rotors in helicopters seem to have constant chord blades where the tip has been twisted donwards towards the tip so as to produce less lift the further away from the hub you get. But the same thing could be achieved by having the chord change (and the twist remain unchanged) - by narrowing the blade the further you go out from the hub, the less lift it would produce (this was common in earlier helicopters). A twisted blade must be so much more expensive and complicated to make, so why was the changing chord design abandoned?
3. Also, older helicopters often had wooden blade rotors, and I think a few helicopters might still do (Brantly?). But I can't recall reading or hearing anywhere that wooden blades were more susceptible to failures than metal ones. What was the main reason wooden rotor blades were abandoned?
4. Rigid rotor question. As I understand it, in a rigid rotor system, lag and hinging is 'absorbed' by the blade itself, eliminating the need for dampers and such. Now, if I just took a normal, fully articulated rotor and removed the dampers for something solid and beefed up the blades, wouldn't this become a rigid rotor? If so, why aren't all manufacturers making rigid rotor systems, since they seem to have less moving parts that can start to misbehave?
Thank you.
For 1 and 4, I think there'd be structural reasons why they wouldn't be practical.
1. Slats would have to pop in and out 5 or more times a second as the blade travelled round and would be subject to all kinds of weird forces, so just making them work without breaking would have to be a massive challenge. Then I'd imagine vibration and forces imposed on the blades themselves would create more problems than the slats solved.
4. Again just my thoughts, but I'd reckon unless blades were specifically designed to do all the flexing required for feathering, flapping & dragging as well as handling normal flight loads and vibration, they wouldn't handle it.
Also it's not just the blades, but the head components, eg AS350 starflex, that come into play for that kind of system.
It wouldn't be a matter of just beefing up the blades, they'd have to be made right to handle all the fatigue caused by repetitive flexing.
My guess is that trying to cobble some kind of rigid head system from a fully articulated one would be like creating some kind of Frankenstein's monster - probably be easier and cheaper to design the whole thing from scratch as an integrated system.
1. Slats would have to pop in and out 5 or more times a second as the blade travelled round and would be subject to all kinds of weird forces, so just making them work without breaking would have to be a massive challenge. Then I'd imagine vibration and forces imposed on the blades themselves would create more problems than the slats solved.
4. Again just my thoughts, but I'd reckon unless blades were specifically designed to do all the flexing required for feathering, flapping & dragging as well as handling normal flight loads and vibration, they wouldn't handle it.
Also it's not just the blades, but the head components, eg AS350 starflex, that come into play for that kind of system.
It wouldn't be a matter of just beefing up the blades, they'd have to be made right to handle all the fatigue caused by repetitive flexing.
My guess is that trying to cobble some kind of rigid head system from a fully articulated one would be like creating some kind of Frankenstein's monster - probably be easier and cheaper to design the whole thing from scratch as an integrated system.
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In general, just calculate the loads on a blade tip to see why fancy mechanisms don't stand a chance - the tip of a typical helo blade has between 600 and 1000 g's on it due to CF. That means a 1 oz flap is exerting about 60 pounds of whirling force on its tracks and hinges. Only for a few seconds, until it flys off, of course.
Wood is an excellent medium for low load work, but the inherent strength is very very low relative to metals. By inherent strength, I mean pounds per square inch. A wood beam must be much deeper to hold the load, so it becomes very hard to package as the loads go up, but the blade wants to be thin. Look again at that film "rotor.avi" to imagine what wood would do under the higher speed environment. Development of aluminum spar blades was one of the technological leaps for helos, allowing Vne to jump to 150 knots instead of the 100 or so prior. Blade motions and stresses at speed are not able to be handled by wood.
Here is a good discussion, note that aluminum is 10 times stronger than wood:
http://en.wikipedia.org/wiki/Tensile_strength
Some more complete strength data:
http://www.efunda.com/materials/comm...anicalStrength
Regarding swapping heads on a helo, it is not very simple. Imagine buying a set of racing tires and putting them on your Chevy Vega. Cut the wheel once and watch the front end come off. The entire helo is designed as a unit, with each connecting piece optimized for the loads it gets from its neighbor.
For a direct illustration of this, look at the mess the US Marines created when they decided to swap the 2 bladed Cobra head for a 4 bladed soft inplane rotor. They ended up redesigning the entire fuselage to chase the cracks they were getting from the higher loads of the new head. Actually, Bell was chasing these cracks, and paid handsomely for the effort. Nobody ever took out the Bell salesmen/engineers who sold this nightmare to the USMC and spanked them for their mistakes, I am sure. After all, they then turned around and sold the Yankee and Zulu, with similar development nightmares!
Wood is an excellent medium for low load work, but the inherent strength is very very low relative to metals. By inherent strength, I mean pounds per square inch. A wood beam must be much deeper to hold the load, so it becomes very hard to package as the loads go up, but the blade wants to be thin. Look again at that film "rotor.avi" to imagine what wood would do under the higher speed environment. Development of aluminum spar blades was one of the technological leaps for helos, allowing Vne to jump to 150 knots instead of the 100 or so prior. Blade motions and stresses at speed are not able to be handled by wood.
Here is a good discussion, note that aluminum is 10 times stronger than wood:
http://en.wikipedia.org/wiki/Tensile_strength
Some more complete strength data:
http://www.efunda.com/materials/comm...anicalStrength
Regarding swapping heads on a helo, it is not very simple. Imagine buying a set of racing tires and putting them on your Chevy Vega. Cut the wheel once and watch the front end come off. The entire helo is designed as a unit, with each connecting piece optimized for the loads it gets from its neighbor.
For a direct illustration of this, look at the mess the US Marines created when they decided to swap the 2 bladed Cobra head for a 4 bladed soft inplane rotor. They ended up redesigning the entire fuselage to chase the cracks they were getting from the higher loads of the new head. Actually, Bell was chasing these cracks, and paid handsomely for the effort. Nobody ever took out the Bell salesmen/engineers who sold this nightmare to the USMC and spanked them for their mistakes, I am sure. After all, they then turned around and sold the Yankee and Zulu, with similar development nightmares!
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So, whats going on with the blades here...
"Sikorsky Aircraft has submitted better than projected performance test results of its latest testing on the 4th generation rotor blade to the US Marine Corps.
The 4th Generation™ blade follows in a long chain of innovations in the area of rotor systems. It builds on the work done for Sikorsky’s state-of-the-art Growth Rotor Blade (GRB™) currently used on the UH-60M and S/H-92™ helicopter..."
... to make them so much more efficient?
"Sikorsky Aircraft has submitted better than projected performance test results of its latest testing on the 4th generation rotor blade to the US Marine Corps.
The 4th Generation™ blade follows in a long chain of innovations in the area of rotor systems. It builds on the work done for Sikorsky’s state-of-the-art Growth Rotor Blade (GRB™) currently used on the UH-60M and S/H-92™ helicopter..."
... to make them so much more efficient?
Good Questions!
1.
Cant add to the previous answers!
2.
Modern, metal sparred, rotor blades are/were cheaper to make with a constant chord as they only needed a twisting of the Spar to get the correct Wash-In/Out. In the case of Whirlwind/Wessex Blades, his was almost literally done in a big Vice-type machine that was manually twisted/tuned. The leading edges and pockets were then attached to the Spar and gaps caulked.
In the case of Carbon Fibre blades, I believe it is now easier to lay a constant chord pattern than a variable chord pattern, though I stand to be corrected. It is of course possible to lay whichever pattern you like, but time, money and difficulty all come in to play when laying large tracks of strands/mats/ribbons.
3.
They Failed! Wooden blades were fine for low weight and low rev rotors of the time but would not be able to withstand some of the more powerful strains of modern times. They were also susceptible to various warping problems and diseases in more exotic climates. Wooden rotors should not be confused with wooden airframes or wings. Wings don’t do all that twisting and bending per Rev.
4.
“For every action there is and equal and opposite reaction”, The same works for Rotors – If you make the Head rigid you must also make the surrounding structure hard enough to support the Gear (Actuators) to make those movements. and then the airframe needs to be strong enough to support the rest!
In other words; There is no sense in building a Tank if it is going to be used like a Pram!
Hope this helps
1.
Cant add to the previous answers!
2.
Modern, metal sparred, rotor blades are/were cheaper to make with a constant chord as they only needed a twisting of the Spar to get the correct Wash-In/Out. In the case of Whirlwind/Wessex Blades, his was almost literally done in a big Vice-type machine that was manually twisted/tuned. The leading edges and pockets were then attached to the Spar and gaps caulked.
In the case of Carbon Fibre blades, I believe it is now easier to lay a constant chord pattern than a variable chord pattern, though I stand to be corrected. It is of course possible to lay whichever pattern you like, but time, money and difficulty all come in to play when laying large tracks of strands/mats/ribbons.
3.
They Failed! Wooden blades were fine for low weight and low rev rotors of the time but would not be able to withstand some of the more powerful strains of modern times. They were also susceptible to various warping problems and diseases in more exotic climates. Wooden rotors should not be confused with wooden airframes or wings. Wings don’t do all that twisting and bending per Rev.
4.
“For every action there is and equal and opposite reaction”, The same works for Rotors – If you make the Head rigid you must also make the surrounding structure hard enough to support the Gear (Actuators) to make those movements. and then the airframe needs to be strong enough to support the rest!
In other words; There is no sense in building a Tank if it is going to be used like a Pram!
Hope this helps
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A further point to Q2.
IMHO.. The constant chord design lends itself better to the adjustment of trim tabs. Remember the whole idea is equal lift along the blade length (as much as possible, however it is almost impossible) but also equal lift generated from all of the blades. To do this each blade needs to be tuned as you are probably well aware, therefore in a blade with a twist, you are essentially fine tuning the twist, and in a blade which was designed to be twisted, this will create very little additional stress loading. I would think a blade designed to be flat would have problems... raising the point that if you had a variable chord, designed to be twisted.. whats the point??? I would suggest twisted blades give the designer an easier ballpark to play in to achieve equality of lift.
PPS
IMHO.. The constant chord design lends itself better to the adjustment of trim tabs. Remember the whole idea is equal lift along the blade length (as much as possible, however it is almost impossible) but also equal lift generated from all of the blades. To do this each blade needs to be tuned as you are probably well aware, therefore in a blade with a twist, you are essentially fine tuning the twist, and in a blade which was designed to be twisted, this will create very little additional stress loading. I would think a blade designed to be flat would have problems... raising the point that if you had a variable chord, designed to be twisted.. whats the point??? I would suggest twisted blades give the designer an easier ballpark to play in to achieve equality of lift.
PPS
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I have a couple of follow up questions.
1. Since load and high stress seems to be what killed off wooden blades - what if I had more blades and therefore spread the load between more of them? Then they could be thinner and more agile, no?
2. Isn't there a benefit in having more blades in that since they can share load, they can also create more lift per revolution. It must be easier to create X sq.ft of lifting area spread over 8 blades rather than over 2. And 8 thin-ish blades must make less noise than 2 huge ones.
3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
4. The constant chord design of blades seems easier to construct, as you guys pointed out. But surely, the twisting if them to create wash out must be a rather finicky thing to do. It must be very hard for two twisted blades to have the same twist at exactly the same point throughout the blade. This must create a vibrant and unruly rotor path that needs a lot of tracking. So my question is still - doesn't a variable chord rotor (that doesn't need to be twisted and matched up), create less of a headache in the end when all the work spent tracking the CC one has been factored in?
Thank you!
1. Since load and high stress seems to be what killed off wooden blades - what if I had more blades and therefore spread the load between more of them? Then they could be thinner and more agile, no?
2. Isn't there a benefit in having more blades in that since they can share load, they can also create more lift per revolution. It must be easier to create X sq.ft of lifting area spread over 8 blades rather than over 2. And 8 thin-ish blades must make less noise than 2 huge ones.
3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
4. The constant chord design of blades seems easier to construct, as you guys pointed out. But surely, the twisting if them to create wash out must be a rather finicky thing to do. It must be very hard for two twisted blades to have the same twist at exactly the same point throughout the blade. This must create a vibrant and unruly rotor path that needs a lot of tracking. So my question is still - doesn't a variable chord rotor (that doesn't need to be twisted and matched up), create less of a headache in the end when all the work spent tracking the CC one has been factored in?
Thank you!
Adam,
There is no free lunch. For one thing, as you add blades, you also add profile drag. You also add cost, because any one blade is about as hard to make as another, even if they are skinnier. So if your blade cost doesn't go down, but you have twice as many, you pay twice as much.
Large helicopters tend to have more blades, like the 53E. I suspect there is a tradeoff between disk area and solidity. If they made the disk area on a 53E such that the disk loading was like a Schweizer 300, you would need bigger aircraft carriers to land on. But, with the extra blades, you can turn more horsepower into thrust compared to if you had lesser blades.
Building blades with twist is about as hard as building them without, and keeping them true. It takes tooling and processes. So, once the tooling and processes are set up, building twisted blades is no big deal. And since we can track & balance them, that's not a big deal either.
I would also suspect there is a limit to how big you can grow a rotor, eventually the rotor would not be stiff enough to support itself when not turning. It would seem to me that getting the aeroelastics stable on a loooonger blade could be a challenge too.
I don't think it is just loads and high stress that killed wooden blades, though that is certaintly a factor. There was a high-profile FW accident before metal airliners where the wooden spar failed due to either dry rot or termites (if there are any historians out there maybe you can help me out on this statement). Metal doesn't dry rot (though it might corrode) and bugs don't eat it. And, good wood is harder to come by, it has gotten pretty expensive. If you ever build a wooden homebuilt, and have to buy spruce spars for the wing, you will pay a lot more than a comparable metal spar. The stuff may grow on trees, but slowly, and you won't be buying it at home depot.
-- IFMU
There is no free lunch. For one thing, as you add blades, you also add profile drag. You also add cost, because any one blade is about as hard to make as another, even if they are skinnier. So if your blade cost doesn't go down, but you have twice as many, you pay twice as much.
Large helicopters tend to have more blades, like the 53E. I suspect there is a tradeoff between disk area and solidity. If they made the disk area on a 53E such that the disk loading was like a Schweizer 300, you would need bigger aircraft carriers to land on. But, with the extra blades, you can turn more horsepower into thrust compared to if you had lesser blades.
Building blades with twist is about as hard as building them without, and keeping them true. It takes tooling and processes. So, once the tooling and processes are set up, building twisted blades is no big deal. And since we can track & balance them, that's not a big deal either.
I would also suspect there is a limit to how big you can grow a rotor, eventually the rotor would not be stiff enough to support itself when not turning. It would seem to me that getting the aeroelastics stable on a loooonger blade could be a challenge too.
I don't think it is just loads and high stress that killed wooden blades, though that is certaintly a factor. There was a high-profile FW accident before metal airliners where the wooden spar failed due to either dry rot or termites (if there are any historians out there maybe you can help me out on this statement). Metal doesn't dry rot (though it might corrode) and bugs don't eat it. And, good wood is harder to come by, it has gotten pretty expensive. If you ever build a wooden homebuilt, and have to buy spruce spars for the wing, you will pay a lot more than a comparable metal spar. The stuff may grow on trees, but slowly, and you won't be buying it at home depot.
-- IFMU
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Adam, here is an attempt to discuss the excellent questions you pose:
1. Since load and high stress seems to be what killed off wooden blades - what if I had more blades and therefore spread the load between more of them? Then they could be thinner and more agile, no?
Wood as a structural material has great limitations - it cannot be strong enough without being made very thick - ok for buildings, pretty bad for blades. Much of the stress is not because of the number of blades, it is because of the need to have them aerodynamically thin while they have to be skyscraper-strong. The combination makes wooden blades a marginal idea.
2. Isn't there a benefit in having more blades in that since they can share load, they can also create more lift per revolution. It must be easier to create X sq.ft of lifting area spread over 8 blades rather than over 2. And 8 thin-ish blades must make less noise than 2 huge ones.
Two issues here - the real issue is the amount of blade area for the weight of the helo. More thinner blades are in the same hole as fewer thicker ones. If you made the greater number of blades wider in chord and thicker, the wood would work, but the blades would need much more engine power to swing them around and make the necessary lift. More power than the skinnier metal or composite blades.
3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
Very true, but slower rotors stall earlier, and have a lower Vne, and lower cruise speed. In fact, that is why older helos have Vne's at about 100 knots - they have slower rotors and less blade area. Good for noise, and hover efficiency, bad for high speed.
4. The constant chord design of blades seems easier to construct, as you guys pointed out. But surely, the twisting if them to create wash out must be a rather finicky thing to do. It must be very hard for two twisted blades to have the same twist at exactly the same point throughout the blade. This must create a vibrant and unruly rotor path that needs a lot of tracking. So my question is still - doesn't a variable chord rotor (that doesn't need to be twisted and matched up), create less of a headache in the end when all the work spent tracking the CC one has been factored in?
Lots of issues raised here: Twisting a blade isn't hard, and making two or 1,000 of them exactly alike also isn't that hard, really. Variable chord could work, but the chord would be screwy - thinner at the root, thicker in the middle, and then thin at the tips. It would be harder to make, but not terribly, but the aerodynamic advantage would be slight, I think.
1. Since load and high stress seems to be what killed off wooden blades - what if I had more blades and therefore spread the load between more of them? Then they could be thinner and more agile, no?
Wood as a structural material has great limitations - it cannot be strong enough without being made very thick - ok for buildings, pretty bad for blades. Much of the stress is not because of the number of blades, it is because of the need to have them aerodynamically thin while they have to be skyscraper-strong. The combination makes wooden blades a marginal idea.
2. Isn't there a benefit in having more blades in that since they can share load, they can also create more lift per revolution. It must be easier to create X sq.ft of lifting area spread over 8 blades rather than over 2. And 8 thin-ish blades must make less noise than 2 huge ones.
Two issues here - the real issue is the amount of blade area for the weight of the helo. More thinner blades are in the same hole as fewer thicker ones. If you made the greater number of blades wider in chord and thicker, the wood would work, but the blades would need much more engine power to swing them around and make the necessary lift. More power than the skinnier metal or composite blades.
3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
Very true, but slower rotors stall earlier, and have a lower Vne, and lower cruise speed. In fact, that is why older helos have Vne's at about 100 knots - they have slower rotors and less blade area. Good for noise, and hover efficiency, bad for high speed.
4. The constant chord design of blades seems easier to construct, as you guys pointed out. But surely, the twisting if them to create wash out must be a rather finicky thing to do. It must be very hard for two twisted blades to have the same twist at exactly the same point throughout the blade. This must create a vibrant and unruly rotor path that needs a lot of tracking. So my question is still - doesn't a variable chord rotor (that doesn't need to be twisted and matched up), create less of a headache in the end when all the work spent tracking the CC one has been factored in?
Lots of issues raised here: Twisting a blade isn't hard, and making two or 1,000 of them exactly alike also isn't that hard, really. Variable chord could work, but the chord would be screwy - thinner at the root, thicker in the middle, and then thin at the tips. It would be harder to make, but not terribly, but the aerodynamic advantage would be slight, I think.
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Another problem with wooden blades - they were made by hand, by the old boys who had previously made wooden propellors, almost a generation before.
They had to be made as a set to have any chance of getting the track and balance correct. Ding one blade, change the set.
They had to be made as a set to have any chance of getting the track and balance correct. Ding one blade, change the set.
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Adam asked:
I am working on a large and slow rotor.
The Boeing Co. has a slow rotor design called Hummingbird A-160 (search google for info)
But mostly, the interest seems to be about increased speed.
If your design is slow, then wood might work OK. I think wood was abandoned because wood absorbs moisture from the air and the blade can go out of balance. Also, the blade should have chordwise balance at 25% from the Leading Edge. This requires a large steel bar in the nose of a wood blade. Why not just make the nose section out of metal? That way you have the balance and strength in one part.
The blade design and material would depend on the size and use of helicopter.
3. A big rotor diameter means that the rotor can turn slower (since the tip speed limits the design of any rotor). Slow turning rotors create much less noise. Why isn't this done more on helicopters?
The Boeing Co. has a slow rotor design called Hummingbird A-160 (search google for info)
But mostly, the interest seems to be about increased speed.
If your design is slow, then wood might work OK. I think wood was abandoned because wood absorbs moisture from the air and the blade can go out of balance. Also, the blade should have chordwise balance at 25% from the Leading Edge. This requires a large steel bar in the nose of a wood blade. Why not just make the nose section out of metal? That way you have the balance and strength in one part.
The blade design and material would depend on the size and use of helicopter.
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Good thread...
Number of blades is an interesting problem, and i suspect there is much more to it than just aerodynamics...
In theory as long as the rotor has the same solidity ratio, it produces the same lift at the same Nr for the same induced power. Since more thinner blades have approx the same wetted area the total profile drag will be the same for both rotors. Since the thinner rotor has less overall depth there is an apparent weight saving, since each thinner rotors would each carry less load. However, with stress being inverse square of section depth then the need for thicker walls makes weight ends up the same
In practice each blade will have it's own free resonant frequency, affecting the effective hinge offset. There is a need to make sure that the rotating blade frequency is not overly excited by the aerodynamic loads. There is also a need to make sure that the blade modes do not occur in phase, to transmit vibration to the hub. In newer machines this must be the case over a wider range of Nr. Finally the cost of assembling many blades to a rotor dictate that the design team use as few blades as will work.
Mart
In theory as long as the rotor has the same solidity ratio, it produces the same lift at the same Nr for the same induced power. Since more thinner blades have approx the same wetted area the total profile drag will be the same for both rotors. Since the thinner rotor has less overall depth there is an apparent weight saving, since each thinner rotors would each carry less load. However, with stress being inverse square of section depth then the need for thicker walls makes weight ends up the same
In practice each blade will have it's own free resonant frequency, affecting the effective hinge offset. There is a need to make sure that the rotating blade frequency is not overly excited by the aerodynamic loads. There is also a need to make sure that the blade modes do not occur in phase, to transmit vibration to the hub. In newer machines this must be the case over a wider range of Nr. Finally the cost of assembling many blades to a rotor dictate that the design team use as few blades as will work.
Mart
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The Reynolds number must also be considered when looking at blade count and chord width.
http://www.unicopter.com/1003.html
http://www.unicopter.com/1003.html
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Thanks - this is all very interesting. I will persist with my questions, though
1. The more blades a rotor will have, the easier it must have to stabilize itself in the case of a malfuncioning blade, I assume. Lose a blade in a 2-blade rotor and you'll shake to pieces in an instant. Lose a blade in a 10-blade rotor and you'll probably survive. Therefore more blades must be safer, no?
2. Does a multiple blade rotor (3 or upwards) of the same diameter as a 2-blade rotor produce more lift? What I'm getting at - could a hypothetical 10-blade rotor with the same diameter as a 2-blade rotor create so much more lift that I could slow the rotor RPM down to make it less noisy?
Also - thanks Mr. Lappos for answering my questions. I think I read somewhere (being quite new to this board) that you helped develop and test fly the S-76 when it came out. It's such a looker that aircraft - must be one of the most graceful helicopters out there. I do however notice that whenever one flies over me here in London, they produce a lot of noise. One can instantly recognize a S-76 approaching on it's noise alone. Is this because Sikorsky chose to go with a pretty high RPM and narrow diameter rotor for this aircraft, or is it just some acoustic anomaly?
1. The more blades a rotor will have, the easier it must have to stabilize itself in the case of a malfuncioning blade, I assume. Lose a blade in a 2-blade rotor and you'll shake to pieces in an instant. Lose a blade in a 10-blade rotor and you'll probably survive. Therefore more blades must be safer, no?
2. Does a multiple blade rotor (3 or upwards) of the same diameter as a 2-blade rotor produce more lift? What I'm getting at - could a hypothetical 10-blade rotor with the same diameter as a 2-blade rotor create so much more lift that I could slow the rotor RPM down to make it less noisy?
Also - thanks Mr. Lappos for answering my questions. I think I read somewhere (being quite new to this board) that you helped develop and test fly the S-76 when it came out. It's such a looker that aircraft - must be one of the most graceful helicopters out there. I do however notice that whenever one flies over me here in London, they produce a lot of noise. One can instantly recognize a S-76 approaching on it's noise alone. Is this because Sikorsky chose to go with a pretty high RPM and narrow diameter rotor for this aircraft, or is it just some acoustic anomaly?
Lose a blade in a 10-blade rotor and you'll probably survive
could a hypothetical 10-blade rotor with the same diameter as a 2-blade rotor create so much more lift that I could slow the rotor RPM down to make it less noisy?
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Adam, i'll do my best to help..
1. The more blades a rotor will have, the easier it must have to stabilize itself in the case of a malfuncioning blade, I assume...
Actually, i was taught by a guy who worked at Rolls in the pioneering days of gas turbines (Bill Brownhill). One of his colleagues was debating that if you spring isolate ANY rotating assy below the rotational frequency, then no out of balance vibration is transmitted. To prove his point, and develop the optimum damping system (friction was considered), he built what to me was an incredible model. The model was spun up, a button was pressed and a mass representing a blade was shed. Sure enough if critical damping was set up the model sat there, easy as you please, running out of balance!
In principle a subframe mounting system allowing low pass isolation of lateral and longitudinal vibrations, but tranferring roll pitch torques, could be developed using ball joint links, springs and dampers. In practice there would be a mass penalty, with consequential reduction in payload. For the high number multiblade solution you are proposing, all mounting points would have to be strengthened to take out of balance loads for as long as it took pilot to safely land and shutdown. It is best to design the blades not to fail.
2. Does a multiple blade rotor (3 or upwards) of the same diameter as a 2-blade rotor produce more lift?...
No, assuming the same hover power. Lift for a given power requirement is a funtion of disk loading, hence for a given weight a function of disk radius. More blades or wider chord blades will increase the solidity ratio, which is fraction of rotor occupied by blade. An increased solidity ratio will reduce each blade angle of attack for same total lift. Angle of attack is best off as high as possible for the best "figure of merit" measure of rotor system efficiency.
In practice, solidity ratio is usually chosen to give machine a good manouvering margin and high speed performance. At speed retreating blade will already be running a high AOA, so compromising hover figure of merit will allow higher speed flight before retreating blade stall.
Also - thanks Mr. Lappos for answering my questions...
Must admit i also owe a great deal to Nick for helping my understanding of rotorcraft. Theory is one thing, but in the disagreement of theory and facts it is better to respect the facts...
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Dave, agreed Reynolds number is important when optimising the blade profile for a given size, Nr, and flight regime. Detailed aerodynamic interaction of tip votices acting on following blades is also an optimisation. The point is that at the design stage there is no real reason to consider any particular number of blades, other than for reasons of rotor dynamics and rotor system cost.
Mart
1. The more blades a rotor will have, the easier it must have to stabilize itself in the case of a malfuncioning blade, I assume...
Actually, i was taught by a guy who worked at Rolls in the pioneering days of gas turbines (Bill Brownhill). One of his colleagues was debating that if you spring isolate ANY rotating assy below the rotational frequency, then no out of balance vibration is transmitted. To prove his point, and develop the optimum damping system (friction was considered), he built what to me was an incredible model. The model was spun up, a button was pressed and a mass representing a blade was shed. Sure enough if critical damping was set up the model sat there, easy as you please, running out of balance!
In principle a subframe mounting system allowing low pass isolation of lateral and longitudinal vibrations, but tranferring roll pitch torques, could be developed using ball joint links, springs and dampers. In practice there would be a mass penalty, with consequential reduction in payload. For the high number multiblade solution you are proposing, all mounting points would have to be strengthened to take out of balance loads for as long as it took pilot to safely land and shutdown. It is best to design the blades not to fail.
2. Does a multiple blade rotor (3 or upwards) of the same diameter as a 2-blade rotor produce more lift?...
No, assuming the same hover power. Lift for a given power requirement is a funtion of disk loading, hence for a given weight a function of disk radius. More blades or wider chord blades will increase the solidity ratio, which is fraction of rotor occupied by blade. An increased solidity ratio will reduce each blade angle of attack for same total lift. Angle of attack is best off as high as possible for the best "figure of merit" measure of rotor system efficiency.
In practice, solidity ratio is usually chosen to give machine a good manouvering margin and high speed performance. At speed retreating blade will already be running a high AOA, so compromising hover figure of merit will allow higher speed flight before retreating blade stall.
Also - thanks Mr. Lappos for answering my questions...
Must admit i also owe a great deal to Nick for helping my understanding of rotorcraft. Theory is one thing, but in the disagreement of theory and facts it is better to respect the facts...
----
Dave, agreed Reynolds number is important when optimising the blade profile for a given size, Nr, and flight regime. Detailed aerodynamic interaction of tip votices acting on following blades is also an optimisation. The point is that at the design stage there is no real reason to consider any particular number of blades, other than for reasons of rotor dynamics and rotor system cost.
Mart
Last edited by Graviman; 7th Apr 2007 at 08:49. Reason: Tidy up of answer 2, but no change in info.
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Thanks Graviman. But this is what I don't understand: Lift is a product of disc loading. Surely a wider blade (or one with more angle of attack) would create more downwash = lift. Hence, more blades, and the bigger they are, the more lift, no? Obviously, drag would also increase, but let's ignore that for now.
Also, more blades on a rotor system must mean that they can "slipstream" behind the other blade to some degree, and therefore have less air resistance per blade as compared to a rotor design where the blades can't "hide in the wake of the blade ahead of it", as in a 2, 3, 4, maybe even 5-blade system?
Basically, what I'm getting at - when you look at fanjet engine on an airliner, they have tremendous amounts of blades. There must be a reason for that - why don't they have just 2 fan blades if that's just as good?
Also, more blades on a rotor system must mean that they can "slipstream" behind the other blade to some degree, and therefore have less air resistance per blade as compared to a rotor design where the blades can't "hide in the wake of the blade ahead of it", as in a 2, 3, 4, maybe even 5-blade system?
Basically, what I'm getting at - when you look at fanjet engine on an airliner, they have tremendous amounts of blades. There must be a reason for that - why don't they have just 2 fan blades if that's just as good?
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The amount of blade area determines the total lift available from the rotor. A low-solidity rotor stalls earlier than a high solidity rotor, all other things being equal (the factor is called Aerodynamic blade loading. It is coefficient of thrust divided by solidity). Thus, more blades means more total lift available (more G's or more speed at 1 G).
The disk area determines the downwash, because the disk area is the size of the "pipe" through which the rotor accelerates the mass of air - the bigger the "pipe" the more mass of air acted upon, and thus the less velocity needed to be imparted on the air (momentum is mass times velocity - the less mass of air, the more the speed you need to induce to get an amount of lift). For this reason, bigger rotors make less downwash than smaller rotors - when they are both lifting the same load.
Tied to this is the fact that Power is used to turn the rotor, and the bigger the rotor, the less velocity it needs to induce on the air, so the very much less the power it needs to make the lift. Bigger rotors need less power to lift a load.
So, to lift a big load with a small engine, get a bigger rotor. To maneuver a helicopter, get more blade area.
The disk area determines the downwash, because the disk area is the size of the "pipe" through which the rotor accelerates the mass of air - the bigger the "pipe" the more mass of air acted upon, and thus the less velocity needed to be imparted on the air (momentum is mass times velocity - the less mass of air, the more the speed you need to induce to get an amount of lift). For this reason, bigger rotors make less downwash than smaller rotors - when they are both lifting the same load.
Tied to this is the fact that Power is used to turn the rotor, and the bigger the rotor, the less velocity it needs to induce on the air, so the very much less the power it needs to make the lift. Bigger rotors need less power to lift a load.
So, to lift a big load with a small engine, get a bigger rotor. To maneuver a helicopter, get more blade area.