Couple of blade construction questions.
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Sorry to revive this old thread again, but I have a new couple of questions:
1. In most fast helicopters it seems to me that the rotor disc area seems to be erring on the smaller size. Now I know Mr. Lappos mentioned that a bigger rotor will stall quicker since it turns slower - is this a factor on the retreating blade that limits top speed? What I'm getting at - does a fast helicopter HAVE to have a smaller rotor disc, or is it just so that they happen to be mounted on choppers with strong engines (S-76, A109, MD500 etc) that can really pull well?
2. A high wing loading on an airplane normally means that the ride will be smoother and the initial turn quicker (but sustained turns slower). Does the same apply to rotor discs and their loading? High disc loading = smoother ride?
Thank you. Your answers are always very good and informative.
1. In most fast helicopters it seems to me that the rotor disc area seems to be erring on the smaller size. Now I know Mr. Lappos mentioned that a bigger rotor will stall quicker since it turns slower - is this a factor on the retreating blade that limits top speed? What I'm getting at - does a fast helicopter HAVE to have a smaller rotor disc, or is it just so that they happen to be mounted on choppers with strong engines (S-76, A109, MD500 etc) that can really pull well?
2. A high wing loading on an airplane normally means that the ride will be smoother and the initial turn quicker (but sustained turns slower). Does the same apply to rotor discs and their loading? High disc loading = smoother ride?
Thank you. Your answers are always very good and informative.
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1. Retreating blade stall begins at the tip of the retreating blade due to two factors - downflap and reduced blade velocity. If you shorten the blade, you shorten downflap and also reduce blade velocity causing a corresponding large reduction in lift so you must increase rotational velocity of the blade to compensate. Of course, you will eventually get to a point where you've increased rotational velocity so much that you run into compressibility (supersonic flow) on the advancing blade. The record breaking Westland Lynx walked a fine line between retreating blade stall and advancing blade compressibility, and that's why it's the realm of test pilots and engineers. Screw up your temperature or pressure calculations just a little bit and you've lost your rotor system. 
2. I'd say definitely, yes. High wing (or disc) loading = smoother ride. I actually had some first time helo passengers mention this just today. They were expecting the helicopter to be bumpy and the smoothness surprised them. The exception, of course, is at high speed because a heavy helo gets into retreating blade stall earlier, and therefore the vibration levels for a heavy helo are higher at speeds close to Vne than those for a light helo.
2. I'd say definitely, yes. High wing (or disc) loading = smoother ride. I actually had some first time helo passengers mention this just today. They were expecting the helicopter to be bumpy and the smoothness surprised them. The exception, of course, is at high speed because a heavy helo gets into retreating blade stall earlier, and therefore the vibration levels for a heavy helo are higher at speeds close to Vne than those for a light helo.
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Thanks.
Let's say I was mad enough to want to design my own helicopter. Let's also assume I'm otherwise sane and responsive and need more answers:
1. As a tail rotor flies at speed, it presumably encounters the same effects as a main rotor - that the retreating blade has less bite. This gets continually worse the faster you go. But, at speed, a tail rotor also becomes less important as the weather-waning fin and structure of the helicopter now provides most of the anti-torque. My question is: do I need to construct the tail rotor with the same complex lead/lag and flapping hinges as the main rotor, or can I just ignore the fact that the retreating blade will stall and design it rigidly?
2. Is good autorotational qualities mainly achieved by having a low disc loading, or a heavy rotor? The two are not the same.
3. I'm a little bit confused about the autorotational aerodynamics over a blade. I've always assumed that the angle of attack of the inner part of the blade that drives the autorotation had to be negative to sustain blade rpm. And since the blade is twisted more at the root, that part could have negative AoA, whilst the rest of the blade had positive AoA. I've been told that's not the case - a rotor will autorotate even if this is not the case or all parts of the blade have positive AoA (like in a Autogyro). Why I'm asking is simple - I want to know if a variable chord blade (a blade that's wider at the root than the tip), but with NO blade twist will autorotate.
4. As a blade spins its centrifugal force will want to pull the blade outwards. At the same time the weight of the helicopter + the g's you pull will want to bend the blade upwards. These are forces one has to bear in mind. But, common sense also suggests that the faster the blade spins and the more centrifugal force it encounters, the less it will want to bend. Or to put it another way, you trade bend forces for centrifugal forces at speed. At what point or speed does this start to affect the calculations and is there some kind of rule of thumb here? I'm assuming that a certain speed, one could all but ignore the bending forces and just construct a blade to take the (very high) centrifugal forces, no?
Sincerely,
your local noodle sharpener.
Let's say I was mad enough to want to design my own helicopter. Let's also assume I'm otherwise sane and responsive and need more answers:
1. As a tail rotor flies at speed, it presumably encounters the same effects as a main rotor - that the retreating blade has less bite. This gets continually worse the faster you go. But, at speed, a tail rotor also becomes less important as the weather-waning fin and structure of the helicopter now provides most of the anti-torque. My question is: do I need to construct the tail rotor with the same complex lead/lag and flapping hinges as the main rotor, or can I just ignore the fact that the retreating blade will stall and design it rigidly?
2. Is good autorotational qualities mainly achieved by having a low disc loading, or a heavy rotor? The two are not the same.
3. I'm a little bit confused about the autorotational aerodynamics over a blade. I've always assumed that the angle of attack of the inner part of the blade that drives the autorotation had to be negative to sustain blade rpm. And since the blade is twisted more at the root, that part could have negative AoA, whilst the rest of the blade had positive AoA. I've been told that's not the case - a rotor will autorotate even if this is not the case or all parts of the blade have positive AoA (like in a Autogyro). Why I'm asking is simple - I want to know if a variable chord blade (a blade that's wider at the root than the tip), but with NO blade twist will autorotate.
4. As a blade spins its centrifugal force will want to pull the blade outwards. At the same time the weight of the helicopter + the g's you pull will want to bend the blade upwards. These are forces one has to bear in mind. But, common sense also suggests that the faster the blade spins and the more centrifugal force it encounters, the less it will want to bend. Or to put it another way, you trade bend forces for centrifugal forces at speed. At what point or speed does this start to affect the calculations and is there some kind of rule of thumb here? I'm assuming that a certain speed, one could all but ignore the bending forces and just construct a blade to take the (very high) centrifugal forces, no?
Sincerely,
your local noodle sharpener.
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Adam,
1. Allowing TR to teeter gives the lightest design. There is no design reason for rigid tail rotors - fenestrons normally use cyclic trim to keep TR in plane.
2. Low disk loading allows low auto rates. High disk inertia supplies energy to gain forward speed from hover and/or flare. Both are desirable features.
3. Any blade will autorotate as long as the pitch is set low enough to allow no net torque across span. A tapered blade will be more efficient in hover, but will suffer in forward flight.
4. Rotor coning is carefully considered in any helicopter design. A compliant root is desirable to allow a range of operational g-loadings.
1. Allowing TR to teeter gives the lightest design. There is no design reason for rigid tail rotors - fenestrons normally use cyclic trim to keep TR in plane.
2. Low disk loading allows low auto rates. High disk inertia supplies energy to gain forward speed from hover and/or flare. Both are desirable features.
3. Any blade will autorotate as long as the pitch is set low enough to allow no net torque across span. A tapered blade will be more efficient in hover, but will suffer in forward flight.
4. Rotor coning is carefully considered in any helicopter design. A compliant root is desirable to allow a range of operational g-loadings.
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Solidity Ratio (Sigma) is the actual blade planform area divided by the disk area. It is a means of non-dimentionalising various rotor performance coefficients. The assumption is that the hub area is trivial.
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But blade solidity can be achieved in two ways, no?
Either have a few fat chord blades, or many narrower ones. Surely there must be differences in how they perform and fly even if they have exactly the same blade area?
Either have a few fat chord blades, or many narrower ones. Surely there must be differences in how they perform and fly even if they have exactly the same blade area?
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Adam,
Prouty has a chapter entitled 'Tail Rotor Design' in his book 'Helicopter Aerodynamics'. Some considerations are; noise, power consumption, and aspect ratio.
Perhaps, some of the answers to the tail-rotor blade count are the same as those for the main-rotor blade count currently being discussed on the thread Number of rotor blades?
Dave
Prouty has a chapter entitled 'Tail Rotor Design' in his book 'Helicopter Aerodynamics'. Some considerations are; noise, power consumption, and aspect ratio.
Perhaps, some of the answers to the tail-rotor blade count are the same as those for the main-rotor blade count currently being discussed on the thread Number of rotor blades?
Dave
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Thanks, I've ordered Helicopter Performance, Stability and Control from Prouty now. Hopefully that incorporates most of the info from the older discontinued books.
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OK, one last question.
On an articulated head, the blade is allowed to hinge in both lead/lag and up/down. Lead/lag is all fine to me, but the up down thing I have some trouble understanding.
Isn't it correct to assume that as the helicopter gains lift and the blades cone upwards, the blade must reach its up hinge stop. If not, the blade would just continue to fold upwards producing no lift and leaving the heli on the ground.
Now if this is the case - then what possibility does the blade have to flap upwards in the hinge if it's already at the up stop? None, I would assume. And if that's the case, that means that the blade itself absorbs that up-flap. Which makes it a rigid rotor blade in flight, by all definitions. So what's the point of having that hinge if it has no effect in flight and isn't allowed to hinge?
This ancient rotor blade in flight footage which you've all seen, seems to describe the above. From the age of the film, I'm assuming this is articulated head (rigid rotors are after all a pretty new invention). But look at thhe blade closest to the hub - it doesn't move at all. The rest of the blade and the tip is doing all the bending.
YouTube - Slow motion video of a helicopter rotor blade
Don't get it. Please explain.
On an articulated head, the blade is allowed to hinge in both lead/lag and up/down. Lead/lag is all fine to me, but the up down thing I have some trouble understanding.
Isn't it correct to assume that as the helicopter gains lift and the blades cone upwards, the blade must reach its up hinge stop. If not, the blade would just continue to fold upwards producing no lift and leaving the heli on the ground.
Now if this is the case - then what possibility does the blade have to flap upwards in the hinge if it's already at the up stop? None, I would assume. And if that's the case, that means that the blade itself absorbs that up-flap. Which makes it a rigid rotor blade in flight, by all definitions. So what's the point of having that hinge if it has no effect in flight and isn't allowed to hinge?
This ancient rotor blade in flight footage which you've all seen, seems to describe the above. From the age of the film, I'm assuming this is articulated head (rigid rotors are after all a pretty new invention). But look at thhe blade closest to the hub - it doesn't move at all. The rest of the blade and the tip is doing all the bending.
YouTube - Slow motion video of a helicopter rotor blade
Don't get it. Please explain.
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Adam,
CF pulls the blades out. Even with no hinge stop they would not go straight up. If you consider lift and CF as roughly perpendicular, the coning angle will be defined by adding the CF and lift vectors together. At flat pitch there is no coning angle, at high power there is significant coning angle. CF is much bigger than lift so the angle never gets crazy, i.e. straight up.
Juan de la Cierva's contribution to the helicopter was the flapping hinge. On the advancing side in forward flight, there is greater airspeed which would result in greater lift, and on the retreating side the opposite is true. As the advancing blade flaps up, the relative wind changes such that there is less AOA on the blade, and less lift. On the retreating side the blade is flapping down, which due to the change in relative wind produces more AOA and more lift. This tends to equalize the dis-symmetry of lift. However, as the blades flap up and down, they are trying to conserve their rotational momentum. Spin yourself in your office chair, pull you legs & arms in, you speed up. The lead/lag hinges allow the blade to accelerate and decelerate without causing huge bending moments at the root.
-- IFMU
CF pulls the blades out. Even with no hinge stop they would not go straight up. If you consider lift and CF as roughly perpendicular, the coning angle will be defined by adding the CF and lift vectors together. At flat pitch there is no coning angle, at high power there is significant coning angle. CF is much bigger than lift so the angle never gets crazy, i.e. straight up.
Juan de la Cierva's contribution to the helicopter was the flapping hinge. On the advancing side in forward flight, there is greater airspeed which would result in greater lift, and on the retreating side the opposite is true. As the advancing blade flaps up, the relative wind changes such that there is less AOA on the blade, and less lift. On the retreating side the blade is flapping down, which due to the change in relative wind produces more AOA and more lift. This tends to equalize the dis-symmetry of lift. However, as the blades flap up and down, they are trying to conserve their rotational momentum. Spin yourself in your office chair, pull you legs & arms in, you speed up. The lead/lag hinges allow the blade to accelerate and decelerate without causing huge bending moments at the root.
-- IFMU
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Wonderful thread!
Folks!
This is fantastic reading. When I started my PPL(H) 1993 one of the first things I did was to obtain a subscription to Rotor & Wing, and from reading some re-runs of Prouty articles I decided that I just HAD to get "Practical Helicopter Aerodynamics". That was great reading and a really good starting point for some basic understanding of what actually transpires above your head during flight.
I have always had a need to understand things and would instantly repspond to , to take a fictive axample, a driving teachers statement "You need to press the clutch before putting this thing in gear" with a solid "WHY?" Prouty gave me that, and I must add that Nick Lappos´ answers keep my gray cells in motion.
Thank you all! (and keep this thread going so that everyone with a stick in each hand gets a better understaning of "things on the roof")
All the best,
PF
This is fantastic reading. When I started my PPL(H) 1993 one of the first things I did was to obtain a subscription to Rotor & Wing, and from reading some re-runs of Prouty articles I decided that I just HAD to get "Practical Helicopter Aerodynamics". That was great reading and a really good starting point for some basic understanding of what actually transpires above your head during flight.
I have always had a need to understand things and would instantly repspond to , to take a fictive axample, a driving teachers statement "You need to press the clutch before putting this thing in gear" with a solid "WHY?" Prouty gave me that, and I must add that Nick Lappos´ answers keep my gray cells in motion.
Thank you all! (and keep this thread going so that everyone with a stick in each hand gets a better understaning of "things on the roof")
All the best,
PF
