Extendible Main Rotors
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A single wing on one side of the helicopter would produce another (roll) problem in the hover from downwash.
I think we have a good compromise with the present designs, which have evolved over a number of years into a good old engineering and aerodynamic compromise. I remain still to be convinced by the tilt-rotor concept, which is too specialised and so inherently relatively inflexible. I think a better way forward would be to resurrect the Gyrodyne/Rotordyne concept.
I think we have a good compromise with the present designs, which have evolved over a number of years into a good old engineering and aerodynamic compromise. I remain still to be convinced by the tilt-rotor concept, which is too specialised and so inherently relatively inflexible. I think a better way forward would be to resurrect the Gyrodyne/Rotordyne concept.
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I've seen parts of the Rotordyne at heli museum, ShyTorque, it an interesting concept. The problem was noise from the tip jets, and the poor efficiency would have limited hover time. Rotor gearbox weights are not such a problem for a heli this size now.
The asym-wing concept would only work with a high effective hinge offset and advanced SAS or FBW. These are both concepts which are fundamental to the coaxial high speed heli, so are feasible. In essence i am suggesting, without backing from calculation, that the empty weight of the asym-wing may be lower than a coaxial with strong blades. This also removes the complexity of the second rotor.
The asym-wing concept would only work with a high effective hinge offset and advanced SAS or FBW. These are both concepts which are fundamental to the coaxial high speed heli, so are feasible. In essence i am suggesting, without backing from calculation, that the empty weight of the asym-wing may be lower than a coaxial with strong blades. This also removes the complexity of the second rotor.
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Update
Hello Riff Raff, Graviman:
There are two concepts floating around here.
1. Extendible/Rectractible Rotor Blades.
2. Extendible/Retractible aerofoil Chord
After reading the threads of comments it seemed that an extendible/retractible chord would be "theoretically" more useful than an extendible/retractable rotor. Boiling down the idea "why do this" is simple "in theory land":
To offer further control of ascent/decent characteristics. How much it offloads or loads up torque, Nr, ... we don't know. Yes, you're right Riff Raff, in theory it all looks pretty, and yes in reality the complex mechanisms to get to the "pretty" end up making it "ugly". As Joe Sutter said in his book-when he was finished with the entire 747 design he stood back, looked at the damn thing like a piece of art and said yes that looks right. I know you're probably shaking your heads on that but I do believe in simple statements and simple solutions like that. Example: does the A-380 look right to you?
Graviman I'll send you over the first ideation sketch of the extendible rotor in good old .dxf From there you should be able to see what I mean by simple solutions. Same goes for extendible chords. I beleive there's always an answer. Every time I board a 747 I do so still not believing that this thing can fly. Yet she flies gracefully and safely. Our brains are more than capable of figuring this stuff out.
There are two concepts floating around here.
1. Extendible/Rectractible Rotor Blades.
2. Extendible/Retractible aerofoil Chord
After reading the threads of comments it seemed that an extendible/retractible chord would be "theoretically" more useful than an extendible/retractable rotor. Boiling down the idea "why do this" is simple "in theory land":
To offer further control of ascent/decent characteristics. How much it offloads or loads up torque, Nr, ... we don't know. Yes, you're right Riff Raff, in theory it all looks pretty, and yes in reality the complex mechanisms to get to the "pretty" end up making it "ugly". As Joe Sutter said in his book-when he was finished with the entire 747 design he stood back, looked at the damn thing like a piece of art and said yes that looks right. I know you're probably shaking your heads on that but I do believe in simple statements and simple solutions like that. Example: does the A-380 look right to you?
Graviman I'll send you over the first ideation sketch of the extendible rotor in good old .dxf From there you should be able to see what I mean by simple solutions. Same goes for extendible chords. I beleive there's always an answer. Every time I board a 747 I do so still not believing that this thing can fly. Yet she flies gracefully and safely. Our brains are more than capable of figuring this stuff out.
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Those who think that extending the blades somehow just increase the torque don't understand rotors very well. It goes without saying that the increase in rotor diameter also incurs a change in the rotor rpm to adjust tip speed appropriately. Usually, that means holding tip speed constant and reducing rpm. Torque goes up linearly with reduced rpm at fixed power, but lift goes up by the square root of the disk area. Therefore, lift increases markedly against power at fixed torque. Big rotors always lift more than little ones. Otherwise, helos would have rotors that fit into vest pockets.
If not for the mechanical clap-trap needed, the idea is a sound one. The first thing a designer does is size the rotor diameter for the engines and payload needed, and the ability to do so "on the fly" is (except for the mechanism) virtually all gravy.
If not for the mechanical clap-trap needed, the idea is a sound one. The first thing a designer does is size the rotor diameter for the engines and payload needed, and the ability to do so "on the fly" is (except for the mechanism) virtually all gravy.
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Nick, i was refering to the system ability to respond to transients. Angular momentum is mwr^2 or mvr, so that increasing radius momentarily drops the aerofoil speed. It requires a torque increase just until tip speed is recovered. My concern is that this introduces lead/lag modes, if the rotor radius is being continously adjusted. For steady state, or long time constant inputs, your analysis is spot on.
I'm actually slightly confused here. Isn't the rotor normally sized as large as practical for a given machine, to keep disk loading down? Why would a designer want to reduce the rotor diam for a given machine on the fly? The only reason i can think of is for reduced landing area, say for ship ops.
Henshaw, i though the variable camber design was for fixed chord? This way you allow the "clean rotor" to run near Ct/sigma of 0.125, but with the option of deploying flaps quickly for increasing lift. Rotor figure of merit efficiency would be reduced in the manouvre, but this is a temporary state. I was trying to think of a relatively simple mechanism which would improve the rotor efficiency without sacrificing the manouvre performance. Increasing chord to maintain constant Ct/sigma is a good future development, but basics first...
I'm actually slightly confused here. Isn't the rotor normally sized as large as practical for a given machine, to keep disk loading down? Why would a designer want to reduce the rotor diam for a given machine on the fly? The only reason i can think of is for reduced landing area, say for ship ops.
Henshaw, i though the variable camber design was for fixed chord? This way you allow the "clean rotor" to run near Ct/sigma of 0.125, but with the option of deploying flaps quickly for increasing lift. Rotor figure of merit efficiency would be reduced in the manouvre, but this is a temporary state. I was trying to think of a relatively simple mechanism which would improve the rotor efficiency without sacrificing the manouvre performance. Increasing chord to maintain constant Ct/sigma is a good future development, but basics first...
Last edited by Graviman; 23rd Sep 2007 at 13:44. Reason: Clarification only.
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Today's helicopters require the rotor to product the lift and the forward propulsion.
It would appear that the inclusion of a propulsive device, such as a propeller, will reduce the thrust that is demanded from the rotor, for a given forward velocity. This lower thrust from the rotor will mean a reduction of the high pitch angle on the retreating blades. In turn, this will result in a retreating tip that is further from stall and a retreating root that is producing less negative lift.
All of the above should result in a slightly fast forward speed for a given power. Perhaps, the Lynx speed record could be considered as an example of this.
In addition, since less of the 'given power' is being consumed by the main rotor it means that less power is consumed by the tail rotor or the drag of the rudder to counter the rotor's torque. This was previously look at by Sikorsky when they placed a swingable tail rotor on an existing craft.
Makes one think that a union of the helicopter and the gyrocopter may be in the future.
Makes one think that a union of a symmetrical twin rotor helicopter and the gyrocopter may be in the future.
It would appear that the inclusion of a propulsive device, such as a propeller, will reduce the thrust that is demanded from the rotor, for a given forward velocity. This lower thrust from the rotor will mean a reduction of the high pitch angle on the retreating blades. In turn, this will result in a retreating tip that is further from stall and a retreating root that is producing less negative lift.
All of the above should result in a slightly fast forward speed for a given power. Perhaps, the Lynx speed record could be considered as an example of this.
In addition, since less of the 'given power' is being consumed by the main rotor it means that less power is consumed by the tail rotor or the drag of the rudder to counter the rotor's torque. This was previously look at by Sikorsky when they placed a swingable tail rotor on an existing craft.
Makes one think that a union of the helicopter and the gyrocopter may be in the future.
Makes one think that a union of a symmetrical twin rotor helicopter and the gyrocopter may be in the future.
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Dave, i've always supported your high speed intermeshing concept. Kaman apparently offered an intermeshing solution at the same time as Seasprite, but the Navy chose conventional. This may have held back counterrotating helicopter development. I suspect the reason was more to do with head clearance on deck than performance. Unfortunately development of X2 seems to have ground to a halt, which is a great shame.
The modified S-61 was part of the competition that lead to the AAFSS, for which Cheyenne was designed. The competitors were all compounded machines with Bell getting 274kts and Lockheed getting 263kts. This gives an idea of maximum achievable speed with a pusher prop compounded helicopter. I am suggesting an adaptation to compounding which would alleviate the retreating blade stall, while minimising hover download and parasitic mass.
The asym-wing is very much a compromise, with the objective being the minimum solution for the specific problem of retreating blade stall. A movable tail rotor may well be part of that solution. The single wing would produce anti-torque at speed, so tail rotor could become a pusher. Part of me recoils from the concept, but i'm sure it would be viable. Basically it has the lowest parts count.
The modified S-61 was part of the competition that lead to the AAFSS, for which Cheyenne was designed. The competitors were all compounded machines with Bell getting 274kts and Lockheed getting 263kts. This gives an idea of maximum achievable speed with a pusher prop compounded helicopter. I am suggesting an adaptation to compounding which would alleviate the retreating blade stall, while minimising hover download and parasitic mass.
The asym-wing is very much a compromise, with the objective being the minimum solution for the specific problem of retreating blade stall. A movable tail rotor may well be part of that solution. The single wing would produce anti-torque at speed, so tail rotor could become a pusher. Part of me recoils from the concept, but i'm sure it would be viable. Basically it has the lowest parts count.
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at the risk of being a big bully picking on little old Sikorsky ....
Mart; you mention;
Perhaps there are problems developing appropriate airfoils for high-speed flight. In addition, the two rotors and the propeller are all in very close aerodynamic proximity to each other.
Recall that the nighttime promotional picture of the prototype showed a fuselage etcetera but no rotors or propeller. One would think that Engineering would develop and test the complexities first.
Perhaps this quote from the AHS Rotary Wing History thread says much.
"Unfortunately development of X2 seems to have ground to a halt, which is a great shame."
Recall that the nighttime promotional picture of the prototype showed a fuselage etcetera but no rotors or propeller. One would think that Engineering would develop and test the complexities first.
Perhaps this quote from the AHS Rotary Wing History thread says much.
"Perhaps his [Igor] greatest accomplishment was in his skills as a showman in selling the helicopter to the military, ...."
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Dave, unfortunately i suspect there may be some internal politics going on. I have quietly wondered whether there has been an incident which damaged the machine - i sincerely hope not. A project of this nature is not going to be cheap, and will tie up many engineers during it's development. I imagine much of the groundwork has already been done on the ABC.
Getting everything stiff enough, while keeping mass low, will be a headache. The one problem i am aware of is flexure in the extended driveshaft to the top rotor. Sikorsky may have been overly ambitious on rotor clearance/flexure.
This is what got me thinking about the asym-wing concept as a quick and dirty method to achieve the X2 intended performance. The Comanche parts bin could also be further used, particularly that 15% hinge offset rotor. The final design may not be a pure helicopter, but so what as long as the machine delivers 250+kts...
Getting everything stiff enough, while keeping mass low, will be a headache. The one problem i am aware of is flexure in the extended driveshaft to the top rotor. Sikorsky may have been overly ambitious on rotor clearance/flexure.
This is what got me thinking about the asym-wing concept as a quick and dirty method to achieve the X2 intended performance. The Comanche parts bin could also be further used, particularly that 15% hinge offset rotor. The final design may not be a pure helicopter, but so what as long as the machine delivers 250+kts...
Last edited by Graviman; 23rd Sep 2007 at 09:14.
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Just a quick check on Sky website reveals nothing new on X2. There is one sound heavy lifter concept though:
http://www.sikorsky.com/file/popup/0,9604,2190,00.pdf
Designing 2 4-blade rotors to do the work of one 8-blade makes sense from the weight point of view. I guess it is smaller machines which would suffer cost and weight penalties. Of course tip servo vibration damping might allow the return to 3-blade designs, then again i imagine the each rotor 3P vibration was out of phase due to 60 degree azimuth seperation. I've never read about S-69 suffereing vibration issues.
If i had to guess: Sky is waiting for tiltrotor verdict before making a pitch...
http://www.sikorsky.com/file/popup/0,9604,2190,00.pdf
Designing 2 4-blade rotors to do the work of one 8-blade makes sense from the weight point of view. I guess it is smaller machines which would suffer cost and weight penalties. Of course tip servo vibration damping might allow the return to 3-blade designs, then again i imagine the each rotor 3P vibration was out of phase due to 60 degree azimuth seperation. I've never read about S-69 suffereing vibration issues.
If i had to guess: Sky is waiting for tiltrotor verdict before making a pitch...
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If i had to guess: Sky is waiting for tiltrotor verdict before making a pitch...
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Gents,
As far as I know, the X2 is humming along, preparing for its next step. Funding has been slow, I hear, because the funds are purely internal R&D funds, what is left over after profit, and therefore subject to the natural market forces.
Several good friends, experts in their field are working daily on the aircraft.
As far as I know, the X2 is humming along, preparing for its next step. Funding has been slow, I hear, because the funds are purely internal R&D funds, what is left over after profit, and therefore subject to the natural market forces.
Several good friends, experts in their field are working daily on the aircraft.
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That is good to hear, Nick. So no news really is good news then in this case. My concern was that the project may have lost momentum.
Good luck to the X2 team. Take your time, and get it right. When it comes many jaws will drop...
Good luck to the X2 team. Take your time, and get it right. When it comes many jaws will drop...
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Just an un-thought thought.
As Nick has mentioned, the objective of extendable blades is to improve the hover/cruse efficiency of the tilt-rotor (or tilt-wing) craft by having the disk area greater during hover than during cruse.
Perhaps the proposed V-44 quad-rotor , or a modified version of it, could provide this. During hover this craft would be supported by four rotor disks. During cruise each front and rear rotor would rotate so that their propellers shared a common axis of rotation. If the gap between the front and rear props was minimal they would act as coaxial propellers, as used by high speed aircraft.
In other words the total effective disk area in cruise is half of the total effective disk area during hover.
Dave
Perhaps the proposed V-44 quad-rotor , or a modified version of it, could provide this. During hover this craft would be supported by four rotor disks. During cruise each front and rear rotor would rotate so that their propellers shared a common axis of rotation. If the gap between the front and rear props was minimal they would act as coaxial propellers, as used by high speed aircraft.
In other words the total effective disk area in cruise is half of the total effective disk area during hover.
Dave
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Dave, that looks like a bandaid on a bandaid. I think i'll patiently wait for X2, if for no other reason than parts count!
The Monk, your observation is spot on and is my main objection to the extendible rotor concept. As you let the blades out you are extracting energy from the rotor system (ie it is pulling you). This means the kinetic energy and thus speed of the aerofoil must go down. As you pull the blades back in you are inserting energy into the rotor system (ie you pull it). This means the kinetic energy and thus speed of the aerofoil must go up.
This is why i say that for each adjustment the lift initially goes the wrong way. Any attempt to force the lift to remain constant for Nick's induced power saving forces lots of lead-lag energy into the system. The best you can hope for is a trim system with a 10+ second response time.
Much better to do something with the blade camber...
The Monk, your observation is spot on and is my main objection to the extendible rotor concept. As you let the blades out you are extracting energy from the rotor system (ie it is pulling you). This means the kinetic energy and thus speed of the aerofoil must go down. As you pull the blades back in you are inserting energy into the rotor system (ie you pull it). This means the kinetic energy and thus speed of the aerofoil must go up.
This is why i say that for each adjustment the lift initially goes the wrong way. Any attempt to force the lift to remain constant for Nick's induced power saving forces lots of lead-lag energy into the system. The best you can hope for is a trim system with a 10+ second response time.
Much better to do something with the blade camber...
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Constant Chord
Graviman,
That is correct. The concept I'm working on is for constant chord that extends (like flaps but not flaps) thus limiting the variables to offer as much control as needed. Again, I see variable length and variable chord as two different animals with different reasons for being.
Henshaw
That is correct. The concept I'm working on is for constant chord that extends (like flaps but not flaps) thus limiting the variables to offer as much control as needed. Again, I see variable length and variable chord as two different animals with different reasons for being.
Henshaw
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Here is another related idea, which might be of interest.
It is by Anton Flettner in 1955.
Air vehicle with Rotary Wing ~ US Patent 2,881,989.
Dave
It is by Anton Flettner in 1955.
Air vehicle with Rotary Wing ~ US Patent 2,881,989.
Dave
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This one actually flew, Dave:
http://airandspace.si.edu/research/a...ft/herrick.htm
The problem with a modern stop rotor is the need to stop the rotor without the tips breaking off at high KIAS. Active vibration supressing tips at the very least are required. These also need to handle potentially divergent forces when the rotor is at 180' azimuth...
http://airandspace.si.edu/research/a...ft/herrick.htm
The problem with a modern stop rotor is the need to stop the rotor without the tips breaking off at high KIAS. Active vibration supressing tips at the very least are required. These also need to handle potentially divergent forces when the rotor is at 180' azimuth...
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Mart,
The 'Stopped Rotor' is an interesting but very difficult concept. I do not believe that Flettner is considering this concept. His objective is to reduce the downwash of the rotor that is 'wasted' on the wings during hover and slow speed flight.
Last year's patent by Northrop Grumman ~ US 7,137,589 is a variant of Flettner's.
Dave
The 'Stopped Rotor' is an interesting but very difficult concept. I do not believe that Flettner is considering this concept. His objective is to reduce the downwash of the rotor that is 'wasted' on the wings during hover and slow speed flight.
Last year's patent by Northrop Grumman ~ US 7,137,589 is a variant of Flettner's.
Dave