Future rotorcraft control systems
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Future rotorcraft control systems
Having recently had a go at R22 flying, i realise that the biggest impediment to wider helicopter appeal is the cost 
. The hardest skill to master is the hover, so why burn good cash trying to learn a skill that is (from an engineering perspective) a design defect. Some of you high hour folks will disagree, since it's so long since you hovered toddled that you just see it as part of the machine quirks - most will have progressed on to fully augmented machines anyway, so won't see the point of this thread.
I was always impressed with the story of the Lockheed CL475, where a fixed wing pilot jumped in and flew the heli with NO additional training! Although unable to find any e-refs on the mechanics system, i have found a write up on the AH56A which (after development of the 186) the CL475 led to:
http://www.internetage.com/cartercopters/pics9.htm
This is quite a read, but the basics are: The pilot flies the gyro, the gyro flies the heli. This is fundamentally different from the Bell system, in that the gyro has full control of the heli and the pilot is not fighting the gyro. It is similar in design to the Hiller system, but is fully mechanical, without aerodynamic interaction outside of the blades.
Although mechanically simple, the dynamics take more understanding. Basically the gyro is connected to the pitch links and physically flies the rotor cyclic input (ie conventional rotor dynamics apply). The pilot is connected to the gyro via a swash plate, but with a 90 degree lead due to gyro precession. Since the gyro wants to "stay put", any uncommanded input (eg gust) that moves heli results in a stabilising rotor cyclic input. The practical upshot is the gyro will stay in the same relative orientation in the helicopter (or otherway round). When the pilot puts in a commanded cyclic input, the gyro moves (precesses) in the way the pilot wants the heli to go, and the heli just follows the gyro.
The clever bit, which always confuses folks, is the system details. Since the gyro is always at the same relative position to the heli, there are springs in the control links. These do nothing more than allow a reasonable movement, for the desired rate of pitch or roll. The really subtle bit is the flap pitch couple in the blade, which allows the rotor system to aply force into the gyro. This just produces an effective rotor dihedral (similar to flapback), only in this case the entire heli will gently pitch/roll in opposition to any movement. This just means the pilot has to maintain stick displacement for a given longitudinal (or lateral) velocity - just like a fixed wing.
I read/skimmed with interest similar trends developing in RC helis:
http://www.w3mh.co.uk/articles/html/csm9-11.htm
The main benefit for full size machines would be hands-off stability, without ANY reduction in controlability. Basically it is bringing full flight control systems to private helis. This opens up even more applications to helis, and reduces accident rates - no reverting to basic reflexes due to say IMC disorientation.
So how 'bout it? Is it time for the Lockheed system to make a return, but this time to private helis?
Mart
. The hardest skill to master is the hover, so why burn good cash trying to learn a skill that is (from an engineering perspective) a design defect. Some of you high hour folks will disagree, since it's so long since you hovered toddled that you just see it as part of the machine quirks - most will have progressed on to fully augmented machines anyway, so won't see the point of this thread.I was always impressed with the story of the Lockheed CL475, where a fixed wing pilot jumped in and flew the heli with NO additional training! Although unable to find any e-refs on the mechanics system, i have found a write up on the AH56A which (after development of the 186) the CL475 led to:
http://www.internetage.com/cartercopters/pics9.htm
This is quite a read, but the basics are: The pilot flies the gyro, the gyro flies the heli. This is fundamentally different from the Bell system, in that the gyro has full control of the heli and the pilot is not fighting the gyro. It is similar in design to the Hiller system, but is fully mechanical, without aerodynamic interaction outside of the blades.
Although mechanically simple, the dynamics take more understanding. Basically the gyro is connected to the pitch links and physically flies the rotor cyclic input (ie conventional rotor dynamics apply). The pilot is connected to the gyro via a swash plate, but with a 90 degree lead due to gyro precession. Since the gyro wants to "stay put", any uncommanded input (eg gust) that moves heli results in a stabilising rotor cyclic input. The practical upshot is the gyro will stay in the same relative orientation in the helicopter (or otherway round). When the pilot puts in a commanded cyclic input, the gyro moves (precesses) in the way the pilot wants the heli to go, and the heli just follows the gyro.
The clever bit, which always confuses folks, is the system details. Since the gyro is always at the same relative position to the heli, there are springs in the control links. These do nothing more than allow a reasonable movement, for the desired rate of pitch or roll. The really subtle bit is the flap pitch couple in the blade, which allows the rotor system to aply force into the gyro. This just produces an effective rotor dihedral (similar to flapback), only in this case the entire heli will gently pitch/roll in opposition to any movement. This just means the pilot has to maintain stick displacement for a given longitudinal (or lateral) velocity - just like a fixed wing.
I read/skimmed with interest similar trends developing in RC helis:
http://www.w3mh.co.uk/articles/html/csm9-11.htm
The main benefit for full size machines would be hands-off stability, without ANY reduction in controlability. Basically it is bringing full flight control systems to private helis. This opens up even more applications to helis, and reduces accident rates - no reverting to basic reflexes due to say IMC disorientation.
So how 'bout it? Is it time for the Lockheed system to make a return, but this time to private helis?
Mart
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The Lockheed design concept.
To: Graviman
The reason a fixed wing pilot can with minimal helicopter skills fly a Lockheed helicopter is because the helicopter flies like a fixed wing aircraft. The pilot displaces the cyclic, which in turn applies a nutating force to the gyro (via springs) and the gyro responds 90-degrees later in the direction of rotation. When the gyro is displaced it applies a force via the pitch links to the blades causing them to respond albeit not at 90-degrees. This was a major design fault with the AH-56 and before the fix could be applied the program was cancelled. Once the blades had been displaced by the cyclic input the pilot would return the cyclic to its’ neutral position very much like a fixed wing aircraft. By returning the cyclic to the neutral position the nutating force was removed and the gyro would remain in the commanded position and the blades would follow. On a conventional helicopter the pilot has to maintain the cyclic in the commanded position in order to maintain the control input.
The couple between the rotorblades and the rotorhead was so strong that the cyclic could not be displaced when on the ground. Squat switches had to be unloaded which would remove the mechanical locks from the cyclic control system. Without the squat switches the helicopter could be tipped over with cyclic input.
The reason a fixed wing pilot can with minimal helicopter skills fly a Lockheed helicopter is because the helicopter flies like a fixed wing aircraft. The pilot displaces the cyclic, which in turn applies a nutating force to the gyro (via springs) and the gyro responds 90-degrees later in the direction of rotation. When the gyro is displaced it applies a force via the pitch links to the blades causing them to respond albeit not at 90-degrees. This was a major design fault with the AH-56 and before the fix could be applied the program was cancelled. Once the blades had been displaced by the cyclic input the pilot would return the cyclic to its’ neutral position very much like a fixed wing aircraft. By returning the cyclic to the neutral position the nutating force was removed and the gyro would remain in the commanded position and the blades would follow. On a conventional helicopter the pilot has to maintain the cyclic in the commanded position in order to maintain the control input.
The couple between the rotorblades and the rotorhead was so strong that the cyclic could not be displaced when on the ground. Squat switches had to be unloaded which would remove the mechanical locks from the cyclic control system. Without the squat switches the helicopter could be tipped over with cyclic input.
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Are you saying the Lockheed system deserves to see the light of day again, or should it be consigned to the history books? Most folks seem to feel that way about the Bell bars and Hiller paddles.
"On a conventional helicopter the pilot has to maintain the cyclic in the commanded position in order to maintain the control input."
I thought that the spanwise offset of the pitch horn from articulating hinge was to allow blade lift to cause a nutating "feedback" force to the gyro. This meant that the gyro would gently respond to a continued rotor input, from say forward flight. Unless the pilot provided a constant input the effect was like fixed wing longitudinal and lateral dihedral.
"The couple between the rotorblades and the rotorhead was so strong that the cyclic could not be displaced when on the ground."
Yeah this could be a problem, but the cyclic should be central on a conventioanl during run-up and shut-down anyway. Maybe a cyclic lock or limiter in place of the normal frictions. It does prove that the machine could "fly" hands off though...
Mart
"On a conventional helicopter the pilot has to maintain the cyclic in the commanded position in order to maintain the control input."
I thought that the spanwise offset of the pitch horn from articulating hinge was to allow blade lift to cause a nutating "feedback" force to the gyro. This meant that the gyro would gently respond to a continued rotor input, from say forward flight. Unless the pilot provided a constant input the effect was like fixed wing longitudinal and lateral dihedral.
"The couple between the rotorblades and the rotorhead was so strong that the cyclic could not be displaced when on the ground."
Yeah this could be a problem, but the cyclic should be central on a conventioanl during run-up and shut-down anyway. Maybe a cyclic lock or limiter in place of the normal frictions. It does prove that the machine could "fly" hands off though...
Mart
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Lockheed design revisited.
To: Graviman
There is no articulating hinge on the AH-56 rotorhead as the head is rigid. The blade is attached to the rotorhead by a series of hinges, which allow pitch change. It could be said that there is an apparent flapping hinge on the rigid rotorhead and this flapping point would change with aerodynamic loading. If the blade did in fact flap any movement of the blade in relation to the gyro would result in a pitch change which would provide a restoring force to the blade returning it to its’ commanded position.
The gyro being rigid in space would provide a resistance to movement caused by blade feedback and being rigid would cause the blade pitch to change and not cause the gyro to move as a result of any feedback.
I thought that the spanwise offset of the pitch horn from articulating hinge was to allow blade lift to cause a nutating "feedback" force to the gyro. This meant that the gyro would gently respond to a continued rotor input, from say forward flight. Unless the pilot provided a constant input the effect was like fixed wing longitudinal and lateral dihedral.
The gyro being rigid in space would provide a resistance to movement caused by blade feedback and being rigid would cause the blade pitch to change and not cause the gyro to move as a result of any feedback.
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The "shotgun" question approach:
"If the blade did in fact flap any movement of the blade in relation to the gyro would result in a pitch change which would provide a restoring force to the blade returning it to its’ commanded position."
Ah OK. Thanks for clearing me up on this, Lu. I had read the Speciality Press book on the AH56, but got this point wrong. So the rotor really did just do exactly what it was told and nothing more. Must admit, as i typed my original post i was wondering how much lateral cyclic that "feedback" would require for forward flight.
Was there any part of the system which provided for hands off speed stability? I'm also curious as to whether the system had any way of providing sideslip stability? Basically would the system also fly forwards and sideways hands-off?
Do you have any views about whether this system should be reconsidered? I am always astonished by the pure skill (and technical knowledge) of heli pilots, but equally alarmed when i read about accidents that this flight control system may have avoided...
BTW how are things with you at the moment?
Mart
Ah OK. Thanks for clearing me up on this, Lu. I had read the Speciality Press book on the AH56, but got this point wrong. So the rotor really did just do exactly what it was told and nothing more. Must admit, as i typed my original post i was wondering how much lateral cyclic that "feedback" would require for forward flight.
Was there any part of the system which provided for hands off speed stability? I'm also curious as to whether the system had any way of providing sideslip stability? Basically would the system also fly forwards and sideways hands-off?
Do you have any views about whether this system should be reconsidered? I am always astonished by the pure skill (and technical knowledge) of heli pilots, but equally alarmed when i read about accidents that this flight control system may have avoided...
BTW how are things with you at the moment?
Mart
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graviman,
You started out so well on this thread, noting how RC aircraft are stabilized, and now you are sliding off into the never never land of blade flapping as some kind of stability solution. Heaven help us from the designers who think more and varied hinges and complex configurations will solve the helo's handling problems, when simple stabilizers can be bought on computer cards for a buck!
The only true solution to stability and control for future helicopters is in electronic systems that are cheap and powerful, not in screwy mechanisms that seem to be stable, but are really just complex.
Stay with the electronic theme, it is the winner.
Also, the Lockheed Cheyenne was a loser from the start, but nobody remembers its flaws, they just remember the marketing hype that was spread about it when it was being sold. The program was cancelled for technical reasons that had no solution. That stabilizr bar did not work very well, in fact it needed a good old fashioned electronic stability system anyway. The stab bar went unstable at high speed, and caused at least two accidents, one that killed the crew. I'll bet you didn't see that on any marketing web site!
You started out so well on this thread, noting how RC aircraft are stabilized, and now you are sliding off into the never never land of blade flapping as some kind of stability solution. Heaven help us from the designers who think more and varied hinges and complex configurations will solve the helo's handling problems, when simple stabilizers can be bought on computer cards for a buck!
The only true solution to stability and control for future helicopters is in electronic systems that are cheap and powerful, not in screwy mechanisms that seem to be stable, but are really just complex.
Stay with the electronic theme, it is the winner.
Also, the Lockheed Cheyenne was a loser from the start, but nobody remembers its flaws, they just remember the marketing hype that was spread about it when it was being sold. The program was cancelled for technical reasons that had no solution. That stabilizr bar did not work very well, in fact it needed a good old fashioned electronic stability system anyway. The stab bar went unstable at high speed, and caused at least two accidents, one that killed the crew. I'll bet you didn't see that on any marketing web site!
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Nick
Did you tell us that future helos will be designed and built unstabilized but pilotable because of the computer take over ??
If so, why electric command on helos are not the rule even on light ?
Jean
Did you tell us that future helos will be designed and built unstabilized but pilotable because of the computer take over ??
If so, why electric command on helos are not the rule even on light ?
Jean
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drakkar,
You ask the right question. The entre world has slipped past the use of mechanical controls, opting for simple mechanisms, and complex computer code to house the stability.
Examples:
The fuel injection of virtually every car is fly-by-wire, where the sensors know the rpm, temperature, altitude, exhaust, throttle setting (which has no mechanical connection to the fuel valve!) and others. The computations "know" when you back off throttle, when it rains, when you are in the mountains, and they give you ideal control and high economy.
Airport people-movers have entirely automatic controls to run the trains, open the doors, make the announcements, and even shut themselves down for maintenance. No people are involved with the routine operation while thousands of people use them for mass transport.
The latest Boeing and Airbus aircraft have all their handling made up inside their computers, the control surfaces are placed where they make the most sense structurally and drag-wise, and not where they lend the best natural stability.
The latest fighter configurations have no "natural" stability, they derive it all from their computers, and they are unflyable without the computers that quell their bad characteristics.
Those who chase good handling by adding hinges and mechanisms are like those villians in the Jules Verne novels, who use monsterous steam-powered blimps to destroy the world. This is thinking anachronistically (look that one up, its a doozey!)
BTW, light helicopters already use electronics instead of screwy mechanisms, if by "light" we mean a few ounces. Their electronics do all the stabilization, and do it very well. Expect someone soon to introduce this as the new means to control a light helo, it is coming. The investment right now is too big, but only because of certification. The electronic control system for a helicopter could cost 10% of the cost to machine the control rods of today's light helos, if the controls were made in large enough quantities. The controls for a modern computer-controlled elevator (one axis of the 6 axies a helo needs) cost about $10 to build.
Regarding what we can expect with computer controls, I wrote an article or two on the Comanche controls, I will try to dig them up and post them. A non-pilot could lift Comanche to a hover, move precisely in any direction, and land again without any training. It didn't just fly like an airplane, it flew like a dream.
You ask the right question. The entre world has slipped past the use of mechanical controls, opting for simple mechanisms, and complex computer code to house the stability.
Examples:
The fuel injection of virtually every car is fly-by-wire, where the sensors know the rpm, temperature, altitude, exhaust, throttle setting (which has no mechanical connection to the fuel valve!) and others. The computations "know" when you back off throttle, when it rains, when you are in the mountains, and they give you ideal control and high economy.
Airport people-movers have entirely automatic controls to run the trains, open the doors, make the announcements, and even shut themselves down for maintenance. No people are involved with the routine operation while thousands of people use them for mass transport.
The latest Boeing and Airbus aircraft have all their handling made up inside their computers, the control surfaces are placed where they make the most sense structurally and drag-wise, and not where they lend the best natural stability.
The latest fighter configurations have no "natural" stability, they derive it all from their computers, and they are unflyable without the computers that quell their bad characteristics.
Those who chase good handling by adding hinges and mechanisms are like those villians in the Jules Verne novels, who use monsterous steam-powered blimps to destroy the world. This is thinking anachronistically (look that one up, its a doozey!)
BTW, light helicopters already use electronics instead of screwy mechanisms, if by "light" we mean a few ounces. Their electronics do all the stabilization, and do it very well. Expect someone soon to introduce this as the new means to control a light helo, it is coming. The investment right now is too big, but only because of certification. The electronic control system for a helicopter could cost 10% of the cost to machine the control rods of today's light helos, if the controls were made in large enough quantities. The controls for a modern computer-controlled elevator (one axis of the 6 axies a helo needs) cost about $10 to build.
Regarding what we can expect with computer controls, I wrote an article or two on the Comanche controls, I will try to dig them up and post them. A non-pilot could lift Comanche to a hover, move precisely in any direction, and land again without any training. It didn't just fly like an airplane, it flew like a dream.
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Hi Nick,
"...now you are sliding off into the never never land of blade flapping as some kind of stability solution."
Well not really, since i have accepted that my original "understanding" about flap pitch coupling was in error. The gyro just provided an attitude feedback reference.
"The only true solution to stability and control for future helicopters is in electronic systems that are cheap and powerful"
But rely on complicated power hydraulics to implement their control strategies. The biggest single problem area on our "simple" prototype truck is the hydraulics. I'll stick to well thought out mechanisms anyday - especially since a quick visual tells you if it is all there and working.
"Stay with the electronic theme, it is the winner."
As an electronics engineer i agree, as a mechanical engineer i disagree (i should stop studying sometime
). For a smaller R22 type machine the only way would be to introduce (say) a electric power ram, along with associated power electronics and generator. Is it doable? Yes. Is it simple? No.
"Lockheed Cheyenne ... stabilizr bar did not work very well, in fact it needed a good old fashioned electronic stability system anyway."
Well, i'm really more interested in the CL475 light heli, but there is very little documented evidence of how this system worked or performed. It had no electrical or hydraulic complexities to worry about in FMECA design studies though.
"The (AH56) stab bar went unstable at high speed, and caused at least two accidents, one that killed the crew. I'll bet you didn't see that on any marketing web site!"
This is all well documented, but nothing like as well as being involved in that nature of project. I got the impression that McNamara's Defence Procurement Policy and the shortcuts it lead to were as much an issue. Lu could probably fill in the details of the nature of the actual failures, but i gather that disk loading beyond the concept specification was partly to blame. Loss of crew on any program is clearly unacceptable, however, and must be avoided at all cost.
Mart
"...now you are sliding off into the never never land of blade flapping as some kind of stability solution."
Well not really, since i have accepted that my original "understanding" about flap pitch coupling was in error. The gyro just provided an attitude feedback reference.
"The only true solution to stability and control for future helicopters is in electronic systems that are cheap and powerful"
But rely on complicated power hydraulics to implement their control strategies. The biggest single problem area on our "simple" prototype truck is the hydraulics. I'll stick to well thought out mechanisms anyday - especially since a quick visual tells you if it is all there and working.
"Stay with the electronic theme, it is the winner."
As an electronics engineer i agree, as a mechanical engineer i disagree (i should stop studying sometime
). For a smaller R22 type machine the only way would be to introduce (say) a electric power ram, along with associated power electronics and generator. Is it doable? Yes. Is it simple? No."Lockheed Cheyenne ... stabilizr bar did not work very well, in fact it needed a good old fashioned electronic stability system anyway."
Well, i'm really more interested in the CL475 light heli, but there is very little documented evidence of how this system worked or performed. It had no electrical or hydraulic complexities to worry about in FMECA design studies though.
"The (AH56) stab bar went unstable at high speed, and caused at least two accidents, one that killed the crew. I'll bet you didn't see that on any marketing web site!"
This is all well documented, but nothing like as well as being involved in that nature of project. I got the impression that McNamara's Defence Procurement Policy and the shortcuts it lead to were as much an issue. Lu could probably fill in the details of the nature of the actual failures, but i gather that disk loading beyond the concept specification was partly to blame. Loss of crew on any program is clearly unacceptable, however, and must be avoided at all cost.
Mart
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If you look at the evolution of FADEC, you'll see the future in helo flight controls. Hydromechanical monsters were simplified with electronics. Then electronics were used to do the pilots work on the manual controls, allowing the pilot to overide as required. Eventually the manual controls were dropped and the FADEC is the only thing that can control fuel systems.
I think the scenario that drakkar mentioned is really just limited by what level of over-ride the pilot is comfortable with. Once we believe that FBW is more reliable than mechanical (augmented or not) then the manufacturers will be able to offer unstable aircraft that the computer can fly.
I'm looking forward to that day.
Matthew.
I think the scenario that drakkar mentioned is really just limited by what level of over-ride the pilot is comfortable with. Once we believe that FBW is more reliable than mechanical (augmented or not) then the manufacturers will be able to offer unstable aircraft that the computer can fly.
I'm looking forward to that day.
Matthew.
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Graviman,
Nick is obviously correct. Forget the 'loosy goosy' things. Future rotorcraft control will be electric and electronic. However, the electron should not be used to mask aerodynamic problems. It should be used to effect aerodynamic solutions.
Why can't linear induction actuators be used in place of hydraulic ones? Double or triple redundancy will provide a very high safety factor. There are also the advancements being made in piezoelectrics.
Some predictions for the not to distant future;
~ Rotors will be powered by electric motors.
~ The use of an electric disk-motor will eliminate the need for gears.
~ Dual main-rotors will have a separate motor for each rotor and the inter-phasing of these rotors will be maintained electrically by encoders.
Heck, all of this could be done in an R/C today.
Then light-activated actuators for fly-by-light.
Dave
Nick is obviously correct. Forget the 'loosy goosy' things. Future rotorcraft control will be electric and electronic. However, the electron should not be used to mask aerodynamic problems. It should be used to effect aerodynamic solutions.
Why can't linear induction actuators be used in place of hydraulic ones? Double or triple redundancy will provide a very high safety factor. There are also the advancements being made in piezoelectrics.
Some predictions for the not to distant future;
~ Rotors will be powered by electric motors.
~ The use of an electric disk-motor will eliminate the need for gears.
~ Dual main-rotors will have a separate motor for each rotor and the inter-phasing of these rotors will be maintained electrically by encoders.
Heck, all of this could be done in an R/C today.
Then light-activated actuators for fly-by-light.
Dave
Last edited by Dave_Jackson; 4th Jun 2005 at 19:35.
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Lockheed design philosophy.
To: Graviman
What Nick stated about electronic stability control is correct. However what he stated about the gyro becoming unstable is not. It was the design of the blades that caused the problems. As a matter of history the US Army kept adding equipment to the AH-56 to the point that Lockheed wanted to extend the blades to increase lift. The Army would not allow any change to the planform of the AH-56 because it would impact the support and transportation of the helicopter.
Lockheed aerodynamics engineers (read Ray Prouty) decided to redesign the blades to increase the lift without changing blade length. They accomplished this by adapting a radical design approach where at each blade station the blade had a different shape and camber. Unlike a conventional design the aerodynamic centers and centers of pressure were different at each station making the blade unstable. This instability manifested itself in blade divergence. This blade divergence was unpredictable and varied with airspeed, disc loading, and other factors.
On two occasions the divergence manifested itself in such a way that the blade struck the cockpit while the helicopter was in flight killing the pilot. This particular helicopter was equipped with a downward firing ejection seat taken from a B-47 however it was mounted in the gunner’s position and the pilot was sitting in the rear seat. It is problematic if the pilot could have ejected as the divergence was so fast and the helicopter was flying close to the waters of the Pacific. The second case of blade divergence took place in a large wind tunnel at Ames Research Center causing severe damage to both the helicopter and the wind tunnel.
The problem of divergence was turned over to Parker Bertea the makers of the AH-56 hydraulic system. I do not know if they investigated the use of electronics but the decided to go with an Electro-mechanical feedback loop. The system consisted of flap detectors and the resultant signal was transmitted via mechanical linkage the ran through the mast and the resultant signal was fed to a black box that monitored blade movement in relation to the signal being supplied to the servo. If there was a variance between the detected signal and the servo input the black box would alter the servo input to compensate for the divergence. A system very similar to this design is used on the Lynx, which has a 15-degree rigging offset. When the pilot pushes cyclic the electronic detectors measure any difference between pilot input and blade response and a signal is sent to the servos to counter the effect of the offset.
The Parker Bertea design worked perfectly according to Lockheed and Army test pilots but it was rife with single point catastrophic failures. By this time the program was cancelled.
What Nick stated about electronic stability control is correct. However what he stated about the gyro becoming unstable is not. It was the design of the blades that caused the problems. As a matter of history the US Army kept adding equipment to the AH-56 to the point that Lockheed wanted to extend the blades to increase lift. The Army would not allow any change to the planform of the AH-56 because it would impact the support and transportation of the helicopter.
Lockheed aerodynamics engineers (read Ray Prouty) decided to redesign the blades to increase the lift without changing blade length. They accomplished this by adapting a radical design approach where at each blade station the blade had a different shape and camber. Unlike a conventional design the aerodynamic centers and centers of pressure were different at each station making the blade unstable. This instability manifested itself in blade divergence. This blade divergence was unpredictable and varied with airspeed, disc loading, and other factors.
On two occasions the divergence manifested itself in such a way that the blade struck the cockpit while the helicopter was in flight killing the pilot. This particular helicopter was equipped with a downward firing ejection seat taken from a B-47 however it was mounted in the gunner’s position and the pilot was sitting in the rear seat. It is problematic if the pilot could have ejected as the divergence was so fast and the helicopter was flying close to the waters of the Pacific. The second case of blade divergence took place in a large wind tunnel at Ames Research Center causing severe damage to both the helicopter and the wind tunnel.
The problem of divergence was turned over to Parker Bertea the makers of the AH-56 hydraulic system. I do not know if they investigated the use of electronics but the decided to go with an Electro-mechanical feedback loop. The system consisted of flap detectors and the resultant signal was transmitted via mechanical linkage the ran through the mast and the resultant signal was fed to a black box that monitored blade movement in relation to the signal being supplied to the servo. If there was a variance between the detected signal and the servo input the black box would alter the servo input to compensate for the divergence. A system very similar to this design is used on the Lynx, which has a 15-degree rigging offset. When the pilot pushes cyclic the electronic detectors measure any difference between pilot input and blade response and a signal is sent to the servos to counter the effect of the offset.
The Parker Bertea design worked perfectly according to Lockheed and Army test pilots but it was rife with single point catastrophic failures. By this time the program was cancelled.
There are two elements to this discussion, controllability and stability. Entry level helicopters are generally very controllable and exhibit docile flying qualities while not being very stable in the classic sense. These machines use rotor inertia, stabilizer bars or a combination of the two to achieve the pleasant flying qualities we have grown accustom to. By contrast, larger more complex machines use electronic solutions for stability as Nick mentioned to achieve true stability. As an example the S-92 is inertially stabilized in a hover. It uses rate gyros, accelerometers or an inertial reference system to neutralize any tendency to drift that would be common to an un-augmented machine.
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Wow - good stuff!
Lu,
Thanks for the info - you really know your stuff on this. My understanding then is that the basic mechanical system is very reliable (Blade divergence is a potential problem on any project). Do you know much about the history of the CL475 or 186? Did the 186 progress to hydraulic or was this a Cheyenne feature?
----
Matthew,
"Once we believe that FBW is more reliable than mechanical (augmented or not) then the manufacturers will be able to offer unstable aircraft that the computer can fly."
I accept this, and even recently went for a job interview to a supplier for aerospace electrohydraulic and electromechanical systems (what's left of the auto industry has taken on e-steering in a big way). However, if there was a simple mechanical solution for light helis why overly complicate it? On large machines that require servo assistance i can see the point, but an R22?!?
BTW my car does not have power assist, and at 160'000 miles of hard driving i don't want it - more to go wrong...
----
"Why can't linear induction actuators be used in place of hydraulic ones?"
No reason at all. I even like some of the systems on offer. The point is for an R22 size machine why introduce the complexity of power assist? A gyro as part of the swash plate is simple and effective. Find me a fault with that system and i'll drop it...
"~ Rotors will be powered by electric motors."
Yes. I'm even working on a fuel cell concept that may offer that dream, but i'm first designing a diesel engine as a control and a fall back research option (got to know your stuff to get funding).
"~ The use of an electric disk-motor will eliminate the need for gears."
No. Electric motors have a maximum torque/mass figure driven by the 1 tesla flux in the mag circuit. Gears are good in that they allow the motor to spin at high RPM to maximise power/mass. Disc motors reduce mass/torque (less mag circuit iron) but would still benefit from high rpm.
"~ Dual main-rotors will have a separate motor for each rotor and the inter-phasing of these rotors will be maintained electrically by encoders."
Highly unlikely. For intermeshing/interleaving the potential modes of failure for this would be catastrophic, this would fast be rejected by any sane design team. The safest means of autorotative (which already implies one mode of failure) synchronisation is the humble gear. SBS tandem would fly with desynchronised rotors, but i can guess how Nick would feel about having to watch 2 NR guages while looking for a soft spot.
Still, solid state will come...
Mart
[edit:typos]
Thanks for the info - you really know your stuff on this. My understanding then is that the basic mechanical system is very reliable (Blade divergence is a potential problem on any project). Do you know much about the history of the CL475 or 186? Did the 186 progress to hydraulic or was this a Cheyenne feature?
----
Matthew,
"Once we believe that FBW is more reliable than mechanical (augmented or not) then the manufacturers will be able to offer unstable aircraft that the computer can fly."
I accept this, and even recently went for a job interview to a supplier for aerospace electrohydraulic and electromechanical systems (what's left of the auto industry has taken on e-steering in a big way). However, if there was a simple mechanical solution for light helis why overly complicate it? On large machines that require servo assistance i can see the point, but an R22?!?
BTW my car does not have power assist, and at 160'000 miles of hard driving i don't want it - more to go wrong...
----
"Why can't linear induction actuators be used in place of hydraulic ones?"
No reason at all. I even like some of the systems on offer. The point is for an R22 size machine why introduce the complexity of power assist? A gyro as part of the swash plate is simple and effective. Find me a fault with that system and i'll drop it...
"~ Rotors will be powered by electric motors."
Yes. I'm even working on a fuel cell concept that may offer that dream, but i'm first designing a diesel engine as a control and a fall back research option (got to know your stuff to get funding).
"~ The use of an electric disk-motor will eliminate the need for gears."
No. Electric motors have a maximum torque/mass figure driven by the 1 tesla flux in the mag circuit. Gears are good in that they allow the motor to spin at high RPM to maximise power/mass. Disc motors reduce mass/torque (less mag circuit iron) but would still benefit from high rpm.
"~ Dual main-rotors will have a separate motor for each rotor and the inter-phasing of these rotors will be maintained electrically by encoders."
Highly unlikely. For intermeshing/interleaving the potential modes of failure for this would be catastrophic, this would fast be rejected by any sane design team. The safest means of autorotative (which already implies one mode of failure) synchronisation is the humble gear. SBS tandem would fly with desynchronised rotors, but i can guess how Nick would feel about having to watch 2 NR guages while looking for a soft spot.
Still, solid state will come...
Mart
[edit:typos]
Last edited by Graviman; 4th Jun 2005 at 21:36.
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Graviman,
If this is getting too far off topic, please say so.
Your knowledge of electricity is far above mine, so I get to question you.
Mass/Torque:
An electric motor that is extremely fast and small has an advantage. However, the helicopter rotor is a large disk with insignificant thrust comes from its center. Why could not a reasonably large diameter, linear induction disk motor be an integral part of a special rotor hub, which is shaped like a very large Frisbee? It might have thousands of poles (with 50% or 33% of them activate at one time) and air bearings to maintain the gap. The weight will be kept down by the preceding integration, composite construction and the elimination of the gearbox.
To double the motor's speed on coaxial helicopters, the motor could be located between the rotors, with the 'armature' connected to one rotor and the 'stator' to the other rotor.
Potential Intermeshing failure:
What if triple redundancy was applied to all facets of the drive. In other words, separate batteries, motors, and controls etc. Electrical systems are very reliable, but, if one system went down, the other two would probable still provide slow level flight.
The large electrical force and its large moment arm should be able to maintain rotor inter-phasing, considering that CNC milling machines can inter-phased multiple motors to within a 10th to 100th of a degree.
Just a demented idea.
Dave
If this is getting too far off topic, please say so.
Your knowledge of electricity is far above mine, so I get to question you.
Mass/Torque:
An electric motor that is extremely fast and small has an advantage. However, the helicopter rotor is a large disk with insignificant thrust comes from its center. Why could not a reasonably large diameter, linear induction disk motor be an integral part of a special rotor hub, which is shaped like a very large Frisbee? It might have thousands of poles (with 50% or 33% of them activate at one time) and air bearings to maintain the gap. The weight will be kept down by the preceding integration, composite construction and the elimination of the gearbox.
To double the motor's speed on coaxial helicopters, the motor could be located between the rotors, with the 'armature' connected to one rotor and the 'stator' to the other rotor.
Potential Intermeshing failure:
What if triple redundancy was applied to all facets of the drive. In other words, separate batteries, motors, and controls etc. Electrical systems are very reliable, but, if one system went down, the other two would probable still provide slow level flight.
The large electrical force and its large moment arm should be able to maintain rotor inter-phasing, considering that CNC milling machines can inter-phased multiple motors to within a 10th to 100th of a degree.
Just a demented idea.
Dave
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Hi Dave,
Not worried about topic, as long as thread is constructive.
"Why could not a reasonably large diameter, linear induction disk motor be an integral part of a special rotor hub, which is shaped like a very large Frisbee?"
Why not, but aerospace motors and generators run at 80'000 rpm regardless of shape. Any less and you are just introducing unecessary weight.
"It might have thousands of poles (with 50% or 33% of them activate at one time) and air bearings to maintain the gap."
Why not. Sounds unecessarily complex to me though and doesnt get around the torque/mass limitation. You are just packing more copper into the 1 Tesla mag flux.
"The weight will be kept down by the preceding integration, composite construction and the elimination of the gearbox."
Nope. You still need an iron return circuit, which is the cause of the 1 Tesla limit. Careful design can just about get up to 2 Teslas. Nano carbon fibre promises room temp superconductivity, but you won't be able to buy 'em for quite a while.
"To double the motor's speed on coaxial helicopters, the motor could be located between the rotors, with the 'armature' connected to one rotor and the 'stator' to the other rotor."
Sounds draggy. Why not just use an epicyclic reduction gearset (lowest mass for given torque) and run the motor at 80'000 rpm, or whatever is centrifugally achievable? Why not use a gas turbine...
----
"What if triple redundancy was applied to all facets of the drive."
Then, as long as pilot (or more likely operator) has common sense to ground aircraft on single failure, the crew only crash and die during a triple failure. What if a +ve lightning strike over the north sea wipes out all of the redundancy?
"The large electrical force and its large moment arm should be able to maintain rotor inter-phasing..."
Agreed, but so should gears. My concern comes from the fact that the best motor will likely be a brushless DC requiring power electronics, unless you want all the wear and reliability problems of a brush. The power electronics, although reliable, do go wrong and never when you want them too. You are talking about aerospace motors which will have to run at high rpm and are going to be expensive. When was the last time you drove a hybrid car? I drive a diesel.
----
"Just a demented idea."
My concern comes from the fact that the idea comes from a false premise (assuming you are trying to avoid driveshafts in SBS tandems). Tail rotors in themselves are not bad things, but counter-rotating rotors offer better potential for aircraft flexibility.
The main problem with ANY rotorcraft is the retreating portion of the rotor. In a conventional heli the retreating portion of the rotor limits top speed. In a counter-rotating heli the retreating portion can still introduce unecessary power loss, through distorted downwash distribution. Intermeshers will suffer vortex spill blade slap, and interleavers will still have to operate retreating tip at higher than ideal AOA. Either work at highest aerodynamic efficiency with outboard advancing.
Why not just accept the loss and just use root/tip control to perfectly feather the retreating blade? If you do this the Stepniewski rotation intermesher wins hands down, and ANY other configuration is just unecessary complexity.
Unicopter, with a tandem instructor seat, is a good design concept - why not just stick with it?
Mart
[Edit: typos]
Not worried about topic, as long as thread is constructive.
"Why could not a reasonably large diameter, linear induction disk motor be an integral part of a special rotor hub, which is shaped like a very large Frisbee?"
Why not, but aerospace motors and generators run at 80'000 rpm regardless of shape. Any less and you are just introducing unecessary weight.
"It might have thousands of poles (with 50% or 33% of them activate at one time) and air bearings to maintain the gap."
Why not. Sounds unecessarily complex to me though and doesnt get around the torque/mass limitation. You are just packing more copper into the 1 Tesla mag flux.
"The weight will be kept down by the preceding integration, composite construction and the elimination of the gearbox."
Nope. You still need an iron return circuit, which is the cause of the 1 Tesla limit. Careful design can just about get up to 2 Teslas. Nano carbon fibre promises room temp superconductivity, but you won't be able to buy 'em for quite a while.
"To double the motor's speed on coaxial helicopters, the motor could be located between the rotors, with the 'armature' connected to one rotor and the 'stator' to the other rotor."
Sounds draggy. Why not just use an epicyclic reduction gearset (lowest mass for given torque) and run the motor at 80'000 rpm, or whatever is centrifugally achievable? Why not use a gas turbine...
----
"What if triple redundancy was applied to all facets of the drive."
Then, as long as pilot (or more likely operator) has common sense to ground aircraft on single failure, the crew only crash and die during a triple failure. What if a +ve lightning strike over the north sea wipes out all of the redundancy?
"The large electrical force and its large moment arm should be able to maintain rotor inter-phasing..."
Agreed, but so should gears. My concern comes from the fact that the best motor will likely be a brushless DC requiring power electronics, unless you want all the wear and reliability problems of a brush. The power electronics, although reliable, do go wrong and never when you want them too. You are talking about aerospace motors which will have to run at high rpm and are going to be expensive. When was the last time you drove a hybrid car? I drive a diesel.
----
"Just a demented idea."
My concern comes from the fact that the idea comes from a false premise (assuming you are trying to avoid driveshafts in SBS tandems). Tail rotors in themselves are not bad things, but counter-rotating rotors offer better potential for aircraft flexibility.
The main problem with ANY rotorcraft is the retreating portion of the rotor. In a conventional heli the retreating portion of the rotor limits top speed. In a counter-rotating heli the retreating portion can still introduce unecessary power loss, through distorted downwash distribution. Intermeshers will suffer vortex spill blade slap, and interleavers will still have to operate retreating tip at higher than ideal AOA. Either work at highest aerodynamic efficiency with outboard advancing.
Why not just accept the loss and just use root/tip control to perfectly feather the retreating blade? If you do this the Stepniewski rotation intermesher wins hands down, and ANY other configuration is just unecessary complexity.
Unicopter, with a tandem instructor seat, is a good design concept - why not just stick with it?
Mart
[Edit: typos]
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CL-475
To: Graviman
When I worked on the Cheyenne I walked through a door leading to a warehouse that was adjacent to our offices. Hanging from the rafters was a Lockheed CL-475. It differed from the version with the gyro bar (similar to the Bell stabilizer bar) in that the gyro was a circular ring that was mounted around the rotorhead. I do not know if this was a variant of the CL-475 or a modified version. In any case I couldn’t get any information on it because there were so few knowledgeable Lockheed types in our group.
I saw the 186 in several variants flying on test flights but I was unable to get close to them. One major point was that this helicopter was so stable and flew so well that the Lockheed designers decided to scale up the dynamic system in designing the Cheyenne. Needless to say this did not work
Do you know much about the history of the CL475 or 186? Did the 186 progress to hydraulic or was this a Cheyenne feature?
I saw the 186 in several variants flying on test flights but I was unable to get close to them. One major point was that this helicopter was so stable and flew so well that the Lockheed designers decided to scale up the dynamic system in designing the Cheyenne. Needless to say this did not work
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Hydraulics are not necessary at all, graviman. The "power assist" could very well be electric, such electric jackscrew actuators are in normal use today on many aircraft, and they have the force to be used wherever pilot muscle power alone can be used to control an aircraft. these cost about $1500 apiece, so all four axies of control should be less than $10,000 cost including the computation. Simple inertial sensors are also quite cheap, when they are useful for stability only (Flight Quality vs Nav Quality Inertial Sensors). The entire cost for the auto fly could be 12000 today, at very low quantities, at high quantities, the cost could be down to $2000 or so.
Unmentioned by the stabilizer bar advocates is that the bar specifically ignores pilot input unless the input is held long enough. This means that mechanical stability systems almost always reduce plot controllability and control response as necessary costs of their use.
As I said before, I see no reason to make complex mechanical systems (stabilizer bars, dual rotors, specially designed flapping hinges, etc) to achieve ease of control and high stability.
Unmentioned by the stabilizer bar advocates is that the bar specifically ignores pilot input unless the input is held long enough. This means that mechanical stability systems almost always reduce plot controllability and control response as necessary costs of their use.
As I said before, I see no reason to make complex mechanical systems (stabilizer bars, dual rotors, specially designed flapping hinges, etc) to achieve ease of control and high stability.
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Nick,
All the point you make have validity, with one exception. At the risk of belaboring a point, I do not believe that a 'rotorcraft' can achieve significantly higher forward speeds without having some configuration of twin counterrotating main-rotors.
I think you will agree that high forward velocities necessitate a reduction in the rpm of the rotor(s) so that compression at the advancing tip does not become a problem. This high forward velocity combined with the slower turning rotor(s) puts a significant portion of the retreating side of the rotor into reverse velocity.
Mart suggests simply feathering the retreating blades, however, logic, and others, suggest that this reverse velocity should be utilized by putting the blades on the retreating side into a negative pitch.
Two years ago Sikorsky proposed that high forward velocities could be achieved with their single rotor Reverse Velocity Rotorcraft. However, the forgoing linked web page gives a strong and valid reason why I believe that the single rotor proposition is fatally flawed.
IMHO, Sikorsky's recent return to the twin-rotor configuration reaffirms the necessity for the lateral symmetry, which is provided by twin main-rotors through the complete range of forward speeds.
Dave
All the point you make have validity, with one exception. At the risk of belaboring a point, I do not believe that a 'rotorcraft' can achieve significantly higher forward speeds without having some configuration of twin counterrotating main-rotors.
I think you will agree that high forward velocities necessitate a reduction in the rpm of the rotor(s) so that compression at the advancing tip does not become a problem. This high forward velocity combined with the slower turning rotor(s) puts a significant portion of the retreating side of the rotor into reverse velocity.
Mart suggests simply feathering the retreating blades, however, logic, and others, suggest that this reverse velocity should be utilized by putting the blades on the retreating side into a negative pitch.
Two years ago Sikorsky proposed that high forward velocities could be achieved with their single rotor Reverse Velocity Rotorcraft. However, the forgoing linked web page gives a strong and valid reason why I believe that the single rotor proposition is fatally flawed.
IMHO, Sikorsky's recent return to the twin-rotor configuration reaffirms the necessity for the lateral symmetry, which is provided by twin main-rotors through the complete range of forward speeds.
Dave
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Lu,
"... Lockheed CL-475 ... the gyro was a circular ring that was mounted around the rotorhead."
This is the system i am really interested in, since it was basically just a gyro augmenting a rigid rotor in a light helicopter. Mechanically very simple, with no need for power assist.
"... 186 ... was so stable and flew so well that the Lockheed designers decided to scale up the dynamic system ..."
Shows system works on a medium size machine. A modern implementation of this would clearly be electronic with laser gyros and electric linear actuators.
----
Nick,
"The "power assist" could very well be electric ... the cost could be down to $2000 ..."
Agreed for a medium heli, but why introduce more active systems on a light heli trainer?
"... stabilizer bar ... specifically ignores pilot input unless the input is held long enough. This means that mechanical stability systems almost always reduce plot controllability and control response as necessary costs of their use."
Ah, you're getting confused with the Bell system here. The Bell-bars were designed to supply attitude inertia into the teetering rotorhead, to "low pass" pilot input. IMHO it is a bad system and should not be considered. The Lockheed system did not affect rotor attitude inertia, but just put an attitude reference into the system - as Lu highlights, control was exceptional.
"As I said before, I see no reason to make complex mechanical systems (stabilizer bars, dual rotors, specially designed flapping hinges, etc) to achieve ease of control and high stability."
Agreed, but if we take (say) a Sikorsky/Schweizer 300, then i am just talking about a single gyro in (say) the swash plate mech. That $2000 would be overkill on this modification, and (provided a CL475 type mech) the control would be exceptional.
Due (only) to limited finances i have only had an introduction to the R22. Most of the flying was suprisingly similar to gliding, once you retrain your feet (no collective but there is an airbrake lever). Mastering the hover was clearly going to ingest to much cash for my tastes, hence this thread...
----
Dave,
"... feathering the retreating blades, however, logic, and others, suggest that this reverse velocity should be utilized by putting the blades on the retreating side into a negative pitch."
I'm not against this, but am convinced that you can never get close to the optimum downwash distribution using this technique. This goes for conventionals, coaxials, intermeshers and interleavers. For a start you will always get upwash (fountain effect) in the circle (rotor planform) of zero velocity. The alternative is the V22...
Just in case it is reverse velocity utilisation that has lead you to the tri-teetering hub, check out Widgeon's post (especially the links to the pics) in the thread:
http://www.pprune.org/forums/showthr...83#post1923883
Mart
[Edit:ammendments]
"... Lockheed CL-475 ... the gyro was a circular ring that was mounted around the rotorhead."
This is the system i am really interested in, since it was basically just a gyro augmenting a rigid rotor in a light helicopter. Mechanically very simple, with no need for power assist.
"... 186 ... was so stable and flew so well that the Lockheed designers decided to scale up the dynamic system ..."
Shows system works on a medium size machine. A modern implementation of this would clearly be electronic with laser gyros and electric linear actuators.
----
Nick,
"The "power assist" could very well be electric ... the cost could be down to $2000 ..."
Agreed for a medium heli, but why introduce more active systems on a light heli trainer?
"... stabilizer bar ... specifically ignores pilot input unless the input is held long enough. This means that mechanical stability systems almost always reduce plot controllability and control response as necessary costs of their use."
Ah, you're getting confused with the Bell system here. The Bell-bars were designed to supply attitude inertia into the teetering rotorhead, to "low pass" pilot input. IMHO it is a bad system and should not be considered. The Lockheed system did not affect rotor attitude inertia, but just put an attitude reference into the system - as Lu highlights, control was exceptional.
"As I said before, I see no reason to make complex mechanical systems (stabilizer bars, dual rotors, specially designed flapping hinges, etc) to achieve ease of control and high stability."
Agreed, but if we take (say) a Sikorsky/Schweizer 300, then i am just talking about a single gyro in (say) the swash plate mech. That $2000 would be overkill on this modification, and (provided a CL475 type mech) the control would be exceptional.
Due (only) to limited finances i have only had an introduction to the R22. Most of the flying was suprisingly similar to gliding, once you retrain your feet (no collective but there is an airbrake lever). Mastering the hover was clearly going to ingest to much cash for my tastes, hence this thread...
----
Dave,
"... feathering the retreating blades, however, logic, and others, suggest that this reverse velocity should be utilized by putting the blades on the retreating side into a negative pitch."
I'm not against this, but am convinced that you can never get close to the optimum downwash distribution using this technique. This goes for conventionals, coaxials, intermeshers and interleavers. For a start you will always get upwash (fountain effect) in the circle (rotor planform) of zero velocity. The alternative is the V22...
Just in case it is reverse velocity utilisation that has lead you to the tri-teetering hub, check out Widgeon's post (especially the links to the pics) in the thread:
http://www.pprune.org/forums/showthr...83#post1923883
Mart
[Edit:ammendments]
Last edited by Graviman; 5th Jun 2005 at 14:36.