Future rotorcraft control systems
Nick Said:
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.
Nick,
Let's not go planning the funeral for hydraulic systems just yet. Tough to beat hydraulic power for its combination of load generating capability AND bandwidth. For a given electric system, bandwidth goes down with load. As the load or bandwidth requirements go up, the electric servo gets bigger. Seems the tradeoff goes to electrics for smaller aircraft to me.
But, I do agree the future is electric. Whether it is electric-hydraulic or just electric, time will tell.
-- IFMU
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.
Nick,
Let's not go planning the funeral for hydraulic systems just yet. Tough to beat hydraulic power for its combination of load generating capability AND bandwidth. For a given electric system, bandwidth goes down with load. As the load or bandwidth requirements go up, the electric servo gets bigger. Seems the tradeoff goes to electrics for smaller aircraft to me.
But, I do agree the future is electric. Whether it is electric-hydraulic or just electric, time will tell.
-- IFMU
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Graviman,
Where did this statement come from?
The tri-teetering hub is an attempt to give greater control authority to very light rotorcraft. It is not intended for high-speed flight.
This Lockheed article from the first Issue of Stu Field's new magazine Experimental Helo might be of interest.
Dave
For a start you will always get upwash (fountain effect) in the circle (rotor planform) of zero velocity
Just in case it is reverse velocity utilization that has lead you to the tri-teetering hub ...
This Lockheed article from the first Issue of Stu Field's new magazine Experimental Helo might be of interest.
Dave
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Dave,
"Where did this statement come from?"
I made it up!
Seriously though it is common sense. Stepniewski describes potential theory in his "Rotary-Wing Aerodynamics", which is the origin of CFD. With CFD, as with so many C.A.E. activities, common sense goes a long way to getting the right answer. The computer just tells you if you were right. 
If you think about a rotor operating in forward velocity, the ideal is to have a positive pressure across the entire underside of the disk. Around the perimeter of the circular reverse velocity region there will be no flow across the aerofoil. There is therefore no way that the aerofoil can generate downwash, or the pressure gradient associated with it. The practical upshot is that the air will leak from below to above. If you consider his chapter on vortex theory you see that this means that two opposed vortices will spill off either side of the zero velocity region. Ergo you will get an effect similar to the fountain effect that Prouty nicely describes in one of his books (Heli aerodynamics, i think) - where the air leakage at the hub actually forms a miniature tornado from the airflow towards top (beanies help).
Perhaps, though, i should have been more specific with that statement. The conventional will suffer the worst, but any twin rotor will lessen the effect. Either coaxial or intermeshing have a gap between opposing rotors, so will allow the effect to occur with reverse velocity utilisation (leading to further blade slap). An interleaver will suffer less, and indeed i did wonder if this was the reason behind your recent migration to interleavers. My reason against use of RVU with interleavers is that you still need a large blade AOA in forward flight to keep a good downwash, at both root and tip, which will be away from optimum. There will be forward speed where the zero aerofoil velocity regions will coincide, but this is over the fuselage so no big deal.
Basically if twin rotor hubs are rigid, feather the reatreating blade and design the hub for the rotor lift inbalance. As i say intermeshing looks good here. Your project, your choice...
----
"...tri-teetering hub is an attempt to give greater control authority to very light rotorcraft."
Right, i wasn't sure though. Must admit to still wondering what advantages this head would give over say the Sikorsky/Schweizer 300 articulating lead (rigid with lead-lag). This is generally percieved as having superior flight characteristics to the R22. The tri-teetering is a sort of half-way design.
Gyro stabilised Schweizer 300 - mmmmmmmmmmmmmmm...
----
From the (rather interesting) site:
"Prouty further states, the "hinge-less" design suffers from the characterization that "they all shook". Lockheed added a fourth blade and controlled the shaking to a more pilot acceptable level."
I don't recall any provision being made for lead-lag on the 186/XH51 rotor heads. This would go some way to explaining the vibration. Indeed the original CL475 had a 2-blade rigid rotor!
Mart
[Edit:typos]
"Where did this statement come from?"
I made it up!
Seriously though it is common sense. Stepniewski describes potential theory in his "Rotary-Wing Aerodynamics", which is the origin of CFD. With CFD, as with so many C.A.E. activities, common sense goes a long way to getting the right answer. The computer just tells you if you were right. If you think about a rotor operating in forward velocity, the ideal is to have a positive pressure across the entire underside of the disk. Around the perimeter of the circular reverse velocity region there will be no flow across the aerofoil. There is therefore no way that the aerofoil can generate downwash, or the pressure gradient associated with it. The practical upshot is that the air will leak from below to above. If you consider his chapter on vortex theory you see that this means that two opposed vortices will spill off either side of the zero velocity region. Ergo you will get an effect similar to the fountain effect that Prouty nicely describes in one of his books (Heli aerodynamics, i think) - where the air leakage at the hub actually forms a miniature tornado from the airflow towards top (beanies help).
Perhaps, though, i should have been more specific with that statement. The conventional will suffer the worst, but any twin rotor will lessen the effect. Either coaxial or intermeshing have a gap between opposing rotors, so will allow the effect to occur with reverse velocity utilisation (leading to further blade slap). An interleaver will suffer less, and indeed i did wonder if this was the reason behind your recent migration to interleavers. My reason against use of RVU with interleavers is that you still need a large blade AOA in forward flight to keep a good downwash, at both root and tip, which will be away from optimum. There will be forward speed where the zero aerofoil velocity regions will coincide, but this is over the fuselage so no big deal.
Basically if twin rotor hubs are rigid, feather the reatreating blade and design the hub for the rotor lift inbalance. As i say intermeshing looks good here. Your project, your choice...
----
"...tri-teetering hub is an attempt to give greater control authority to very light rotorcraft."
Right, i wasn't sure though. Must admit to still wondering what advantages this head would give over say the Sikorsky/Schweizer 300 articulating lead (rigid with lead-lag). This is generally percieved as having superior flight characteristics to the R22. The tri-teetering is a sort of half-way design.
Gyro stabilised Schweizer 300 - mmmmmmmmmmmmmmm...
----
From the (rather interesting) site:
"Prouty further states, the "hinge-less" design suffers from the characterization that "they all shook". Lockheed added a fourth blade and controlled the shaking to a more pilot acceptable level."
I don't recall any provision being made for lead-lag on the 186/XH51 rotor heads. This would go some way to explaining the vibration. Indeed the original CL475 had a 2-blade rigid rotor!
Mart
[Edit:typos]
Last edited by Graviman; 6th Jun 2005 at 20:17.
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Mart,
Vortex theory is really neat and interesting, but it is too complex a theory to use at this level of evaluation, particularly when trying to apply it to twin main-rotors. Blade element theory and common logic will do.
I strongly believe, until proven wrong, that the Intermeshing configuration is the best configuration for small rotorcraft, where speed and agility are important.
I think, until proven wrong, that the Interleaving configuration may be the best for large transport rotorcraft, where lift and speed are important but high maneuverability is not.
It might offer two advantages. One, that its undersling may eliminated the need for lead/lag hinges. And two, that its large flapping hinge offset will allow for a smaller obliquity when used in an Intermeshing configuration.
Reguarding the 3-blade and 4-blade Lockheed, the Sikorsky S-69 (XH-59) ABC coaxial had quite rigid 3-blade rotors. It was intended to fly at 250 kts, however it only got up to 220 kts due to the high vibrations. This would appear to be the reason that the sketch of their new X2 coaxial series shows 4-blade rotors.
Dave
Vortex theory is really neat and interesting, but it is too complex a theory to use at this level of evaluation, particularly when trying to apply it to twin main-rotors. Blade element theory and common logic will do.
the reason behind your recent migration to interleavers.
I think, until proven wrong, that the Interleaving configuration may be the best for large transport rotorcraft, where lift and speed are important but high maneuverability is not.
Must admit to still wondering what advantages this head would give over say the Sikorsky/Schweizer 300 articulating lead (rigid with lead-lag).
Reguarding the 3-blade and 4-blade Lockheed, the Sikorsky S-69 (XH-59) ABC coaxial had quite rigid 3-blade rotors. It was intended to fly at 250 kts, however it only got up to 220 kts due to the high vibrations. This would appear to be the reason that the sketch of their new X2 coaxial series shows 4-blade rotors.
Dave
Last edited by Dave_Jackson; 6th Jun 2005 at 23:50.
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Hi Dave,
"Blade element theory and common logic will do."
Agreed. These days you would go straight from blade element calcs to CFD. Building a ground rig will be a good way to avoid some of those ScArY licencing costs though.
"I strongly believe ... that the Intermeshing configuration is the best configuration for small rotorcraft..."
I strongly believe that you are right!
"...the Interleaving configuration may be the best for large transport rotorcraft ..."
Agreed. The additional payload justifies the additional drivetrain complexity. I get puzzled sometimes since the questions are not in the context of a specific project, so i am not always sure where the discussion is going.
"(Tri-teetering hub) ... undersling may eliminated the need for lead/lag hinges."
OK. Need to be proven/optimised on a ground rig, for perfect blade hub stiffness match.
"...large flapping hinge offset will allow for a smaller obliquity ..."
This is heading towards a rigid hub though. I can't help but wonder if a hysteric elastomer lead-lag bush would work as well. Again, though, it just needs proving out on a ground rig.
"... S-69 (XH-59) ABC coaxial had quite rigid 3-blade rotors ... high vibrations. ... X2 coaxial series shows 4-blade rotors."
Again, i wonder about the use of elastomeric lead-lag bushes to avoid coriolis induced vibration. Don't forget 4 bladed rotor will introduce (potentially avoidable) profile drag, over 3 bladed rotors.
Mart
[Edit: Ammendment]
"Blade element theory and common logic will do."
Agreed. These days you would go straight from blade element calcs to CFD. Building a ground rig will be a good way to avoid some of those ScArY licencing costs though.
"I strongly believe ... that the Intermeshing configuration is the best configuration for small rotorcraft..."
I strongly believe that you are right!
"...the Interleaving configuration may be the best for large transport rotorcraft ..."
Agreed. The additional payload justifies the additional drivetrain complexity. I get puzzled sometimes since the questions are not in the context of a specific project, so i am not always sure where the discussion is going.
"(Tri-teetering hub) ... undersling may eliminated the need for lead/lag hinges."
OK. Need to be proven/optimised on a ground rig, for perfect blade hub stiffness match.
"...large flapping hinge offset will allow for a smaller obliquity ..."
This is heading towards a rigid hub though. I can't help but wonder if a hysteric elastomer lead-lag bush would work as well. Again, though, it just needs proving out on a ground rig.
"... S-69 (XH-59) ABC coaxial had quite rigid 3-blade rotors ... high vibrations. ... X2 coaxial series shows 4-blade rotors."
Again, i wonder about the use of elastomeric lead-lag bushes to avoid coriolis induced vibration. Don't forget 4 bladed rotor will introduce (potentially avoidable) profile drag, over 3 bladed rotors.
Mart
[Edit: Ammendment]
Last edited by Graviman; 10th Jun 2005 at 15:12.
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One last area: Collective control.
The theme of this thread is: to design a cost effective control system for a light helicopter, reducing pilot workload, and thus time required to master that machine. The contention is that a system based on the Lockheed CL475 stability gyro, being entirely mechanical with no active components, is well suited to the task. For helicopters already requiring power assist, an electic servo system with gyro control is well suited (along with the additional cost/complexity involved).
On area not yet covered is collective control for auto-rotation. The only method i can think of is the addition of an electric servo to correct collective position for RRPM errors. Since engine normally supplies required power, and is auto throttled in an R22, this system would normally not detect a need for collective correction. If activated the pilot would easilly be able to overpower the servo, in the same way that he can overpower the throttle. Light and horn could be part of the same system. The pilot would detect a mild resistance to flaring, which would only serve to remind him/her that the RRPM was reducing.
Does anyone feel that these conclusions would result in a machine that was easier to fly, and therefore less likely to suffer incident?
Mart
On area not yet covered is collective control for auto-rotation. The only method i can think of is the addition of an electric servo to correct collective position for RRPM errors. Since engine normally supplies required power, and is auto throttled in an R22, this system would normally not detect a need for collective correction. If activated the pilot would easilly be able to overpower the servo, in the same way that he can overpower the throttle. Light and horn could be part of the same system. The pilot would detect a mild resistance to flaring, which would only serve to remind him/her that the RRPM was reducing.
Does anyone feel that these conclusions would result in a machine that was easier to fly, and therefore less likely to suffer incident?
Mart
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Graviman,
I have been following this post for awhile now and I am very excited to hear some of this stuff from engineering folks like you. I'm not an engineer by any means but I have had some ideas in my head for some time that are very similar to yours. This being the case I"ll share my ideas and let everyone fire away.
I have often wondered why we connect the collective directly to the rotor control system and then have to engineer a way to make the engine maintain RRPM. My idea, however crazy it may be, is to connect the collective to the engine power control only and use a flyweight driven governor in the rotor system to automatically maintain RRPM. The flyweights would be allowed to swing outward with increased RRPM and when they do they would be linked to the pitch horns and could increase blade pitch. With this system if you lost engine power the collective would automatically be lowered due to the flyweights retracting (under action of a spring) from reduced RPM. This could be a handy feature in a non forgiving aircraft like the R22 where the reaction time needed is very fast. The problem with this design would be in autorotation where we are expecting the RRPM to decay but we still want to raise the pitch in the blades. This could be remedied by a simple magnetic brake assy. which would lock the collective to the pitch change mechanism. I have this all worked out in my pee brain but explaining it in text is a bit tough.
Now that we have a set of flyweights near the head controlling the RRPM we could possibly incorporate your gyro stabilization set up. If the weights are there and already connected to the pitch horns for rotor control you are halfyway there.
This whole thing is similar to the governors in fuel control units on turbine engines. You can even anticipate a droop in RPM by changing the tension of the spring in the system which we could also do in our new rotor system. I think this set up would have some good qualities in that it would almost be impossible to get behind the power curve as the pitch of the blades would only increase as fast as the engine power. Lowering pitch in case of an engine failure would be an automatic event and not one the pilot needs to perform conciously. Couple all this with Gravimans gyro cyclic control and you could have a pretty safe small helicopter. I agree that once the aircraft grows in size that electric stabilization equipment may be better suited as you would need some type of power assist anyway be it electric pneumatic hydraulic or whatever.
Ok so now is your chance to tell the grease ball mechanic to go back to his tools and leave the engineering to the pros! Hope I didn't get too far off topic. I just thought it was interesting that smart folks were coming up with ideas not far from my hair brained schemes!
Max
I have been following this post for awhile now and I am very excited to hear some of this stuff from engineering folks like you. I'm not an engineer by any means but I have had some ideas in my head for some time that are very similar to yours. This being the case I"ll share my ideas and let everyone fire away.
I have often wondered why we connect the collective directly to the rotor control system and then have to engineer a way to make the engine maintain RRPM. My idea, however crazy it may be, is to connect the collective to the engine power control only and use a flyweight driven governor in the rotor system to automatically maintain RRPM. The flyweights would be allowed to swing outward with increased RRPM and when they do they would be linked to the pitch horns and could increase blade pitch. With this system if you lost engine power the collective would automatically be lowered due to the flyweights retracting (under action of a spring) from reduced RPM. This could be a handy feature in a non forgiving aircraft like the R22 where the reaction time needed is very fast. The problem with this design would be in autorotation where we are expecting the RRPM to decay but we still want to raise the pitch in the blades. This could be remedied by a simple magnetic brake assy. which would lock the collective to the pitch change mechanism. I have this all worked out in my pee brain but explaining it in text is a bit tough.
Now that we have a set of flyweights near the head controlling the RRPM we could possibly incorporate your gyro stabilization set up. If the weights are there and already connected to the pitch horns for rotor control you are halfyway there.
This whole thing is similar to the governors in fuel control units on turbine engines. You can even anticipate a droop in RPM by changing the tension of the spring in the system which we could also do in our new rotor system. I think this set up would have some good qualities in that it would almost be impossible to get behind the power curve as the pitch of the blades would only increase as fast as the engine power. Lowering pitch in case of an engine failure would be an automatic event and not one the pilot needs to perform conciously. Couple all this with Gravimans gyro cyclic control and you could have a pretty safe small helicopter. I agree that once the aircraft grows in size that electric stabilization equipment may be better suited as you would need some type of power assist anyway be it electric pneumatic hydraulic or whatever.
Ok so now is your chance to tell the grease ball mechanic to go back to his tools and leave the engineering to the pros! Hope I didn't get too far off topic. I just thought it was interesting that smart folks were coming up with ideas not far from my hair brained schemes!
Max
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Graviman & maxtork;
It sounds like you guys are talking about a Rotor Governor.
The following may be informative;
It sounds like you guys are talking about a Rotor Governor.
The following may be informative;
- A search on this subject will reveal one or more previous PPRuNe threads.
- The web page Control - Flight - Governor
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MaxTork,
I've been thinking about your system, and i think it may have merit. It might be worth sketching out some of your ideas, so Dave can post them on his site. It would struggle with some of the inverted flight threads on this forum, but we are only talking R22. It would need careful packaging, but may well be possible to incorperate it into the gyro system i am proposing.
"The flyweights would be allowed to swing outward with increased RRPM and when they do they would be linked to the pitch horns and could increase blade pitch."
The system "gain factor" needs to be very high. Since RRPM is constant in theory flyweights would be at same swing. This means that autorotation may still be lower than nominal RRPM, albeit marginal. Ideally you want the RRPM to be controlled by error correction, which implies active.
"With this system if you lost engine power the collective would automatically be lowered due to the flyweights retracting (under action of a spring) from reduced RPM."
True enough. I think you need a robust mechanism for flaring though, which must feel natural to a pilot already potentially stressed. I can see problems triggering the magnetic clutch.
I suspect in the real world this system may be an ideal candidate for electric control though (like throttle already is). Probably the pilot needs direct control over collective, with a system to assist the pilot make the right choices. Gas turbine power curve lagging could be cured with the system i'm proposing resisting pilot input (although he can over power it). The pilot would soon learn to anticipate the response, but would not feel he was only taking a vote in collective control.
Actually gas turbine lag is a good justification for a hybrid helicopter, using electric motors as suggested by Dave J. Motors would be directly coupled to gearbox, and would make up RRPM errors left by turbine response. If powerful enough variable RRPM strategies allow much higher figure of merit across rotor required thrust range.
Mart
[Edit: speelin]
I've been thinking about your system, and i think it may have merit. It might be worth sketching out some of your ideas, so Dave can post them on his site. It would struggle with some of the inverted flight threads on this forum, but we are only talking R22. It would need careful packaging, but may well be possible to incorperate it into the gyro system i am proposing.
"The flyweights would be allowed to swing outward with increased RRPM and when they do they would be linked to the pitch horns and could increase blade pitch."
The system "gain factor" needs to be very high. Since RRPM is constant in theory flyweights would be at same swing. This means that autorotation may still be lower than nominal RRPM, albeit marginal. Ideally you want the RRPM to be controlled by error correction, which implies active.
"With this system if you lost engine power the collective would automatically be lowered due to the flyweights retracting (under action of a spring) from reduced RPM."
True enough. I think you need a robust mechanism for flaring though, which must feel natural to a pilot already potentially stressed. I can see problems triggering the magnetic clutch.
I suspect in the real world this system may be an ideal candidate for electric control though (like throttle already is). Probably the pilot needs direct control over collective, with a system to assist the pilot make the right choices. Gas turbine power curve lagging could be cured with the system i'm proposing resisting pilot input (although he can over power it). The pilot would soon learn to anticipate the response, but would not feel he was only taking a vote in collective control.
Actually gas turbine lag is a good justification for a hybrid helicopter, using electric motors as suggested by Dave J. Motors would be directly coupled to gearbox, and would make up RRPM errors left by turbine response. If powerful enough variable RRPM strategies allow much higher figure of merit across rotor required thrust range.
Mart
[Edit: speelin]
Last edited by Graviman; 15th Jun 2005 at 03:39.
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Graviman,
Glad to hear that I wasn't totally out in left field. I agree that packaging would be a bit of a chore but I think it could be done.
As for the gain factor I think this could be overcome much as we do in an engine governor. By anticipating the droop in rpm we can compensate for static droop conditions. By simply changing the spring tension which is balancing the centrifugal force acting on the weightsyou can change the rpm. This same mechanism could be made part of the mag brake for autorotations. Lock the spring in it's place and now you are acting directly on the pitch change mechanism for the flare.
I don't think the pilot workload would be a problem either. Pilots already need to react quickly to lower the collective during an engine failure. If we take this action out of the loop he can simply enter his auto and set up for the landing and on the way down after he is set he can energize the brake. On the way down seems a less stressful time to have to react then imediately after an engine failure...although I stand by to be corrected from our pilot types who may be reading.
I will take your advice and make some sketches of what I have in mind. I'd be happy to send it to Mr Jackson if he is at all interested. Although I believe he already has this idea in his head as well considering his page on "rotor governors" which is precisely what I am talking about.
Max
Glad to hear that I wasn't totally out in left field. I agree that packaging would be a bit of a chore but I think it could be done.
As for the gain factor I think this could be overcome much as we do in an engine governor. By anticipating the droop in rpm we can compensate for static droop conditions. By simply changing the spring tension which is balancing the centrifugal force acting on the weightsyou can change the rpm. This same mechanism could be made part of the mag brake for autorotations. Lock the spring in it's place and now you are acting directly on the pitch change mechanism for the flare.
I don't think the pilot workload would be a problem either. Pilots already need to react quickly to lower the collective during an engine failure. If we take this action out of the loop he can simply enter his auto and set up for the landing and on the way down after he is set he can energize the brake. On the way down seems a less stressful time to have to react then imediately after an engine failure...although I stand by to be corrected from our pilot types who may be reading.
I will take your advice and make some sketches of what I have in mind. I'd be happy to send it to Mr Jackson if he is at all interested. Although I believe he already has this idea in his head as well considering his page on "rotor governors" which is precisely what I am talking about.
Max
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Max,
Howabout positioning the brake trigger so that it trips once the collective has been lowered to the autorotation position? Heli will fly normally, other than turbine lag limiting. At engine failure collective becomes ineffective, while blades set pitch for auto, until pilot lowers it to begin auto descent. Flare is as normal, but no need for any additional check (which may well be forgotten in the heat of the moment). This basically gives a machine with a virtually non-existant H-V curve for safe flight (although pilot must still flare).
I would have to see sketches of the mag brake overide to comment further, but see what you are getting at. If it could be packaged with the Lockheed gyro, then this would be a neat intallation. Horn and light would still trigger when engine response fell below limits, but maybe horn can go out when collective lowered.
Out of interest do multi-spool turbine engines suffer from lag as badly as single spool? In auto engines the smaller twin turbo is effectively an intermediate pressure turbine, designed to respond to transients until larger (low pressure) turbo can spool up/down.
Mart
Note: I was planning to discuss tail rotor control strategies next(or general yaw control for counter-rotating helis). My accidental threadjacking of the thread below shows me what potential problems exist for such a system...
http://www.pprune.org/forums/showthr...5&pagenumber=2
Howabout positioning the brake trigger so that it trips once the collective has been lowered to the autorotation position? Heli will fly normally, other than turbine lag limiting. At engine failure collective becomes ineffective, while blades set pitch for auto, until pilot lowers it to begin auto descent. Flare is as normal, but no need for any additional check (which may well be forgotten in the heat of the moment). This basically gives a machine with a virtually non-existant H-V curve for safe flight (although pilot must still flare).
I would have to see sketches of the mag brake overide to comment further, but see what you are getting at. If it could be packaged with the Lockheed gyro, then this would be a neat intallation. Horn and light would still trigger when engine response fell below limits, but maybe horn can go out when collective lowered.
Out of interest do multi-spool turbine engines suffer from lag as badly as single spool? In auto engines the smaller twin turbo is effectively an intermediate pressure turbine, designed to respond to transients until larger (low pressure) turbo can spool up/down.
Mart
Note: I was planning to discuss tail rotor control strategies next(or general yaw control for counter-rotating helis). My accidental threadjacking of the thread below shows me what potential problems exist for such a system...
http://www.pprune.org/forums/showthr...5&pagenumber=2
Last edited by Graviman; 18th Jun 2005 at 16:06.
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Graviman,
Well I finnaly got around to doing some drawings. Although very crued they do give an example of what I am talking about. Unfortunately I don't know how to post a power point slide nor do I have a place to host it. If you are interested I would be happy to email it to you. After seeing it on paper I got excited again and started thinking about making a small spinning model to see how it works in 3D.
As for your question about twin spool engines I would imagine that two spools would be better but it would also add considerable complexity and therefore cost to the engine. In a small helicopter engine I don't think it would be warranted as modern turbines can accelerate from min to max power in about 1 second if you didn't have to worry about the torque limitations on the drivetrain. I believe with a rotor governor you could simply pull collective as you wish while watching the torque gauge and the RRPM will be maintained all by it's self. But again I'm just a grease monkey so I stand to be corrected.
Max
Well I finnaly got around to doing some drawings. Although very crued they do give an example of what I am talking about. Unfortunately I don't know how to post a power point slide nor do I have a place to host it. If you are interested I would be happy to email it to you. After seeing it on paper I got excited again and started thinking about making a small spinning model to see how it works in 3D.
As for your question about twin spool engines I would imagine that two spools would be better but it would also add considerable complexity and therefore cost to the engine. In a small helicopter engine I don't think it would be warranted as modern turbines can accelerate from min to max power in about 1 second if you didn't have to worry about the torque limitations on the drivetrain. I believe with a rotor governor you could simply pull collective as you wish while watching the torque gauge and the RRPM will be maintained all by it's self. But again I'm just a grease monkey so I stand to be corrected.
Max
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By all means send me drawings, Max. It would be useful to keep discussion in this thread though. I prefer CAD to models, but it is my approach to getting stuff done.
What is the cause of torque delay in a turbine? I presume it is mostly to do with stators reacting to pilot input. Does falling behind the power curve present real problems to novice pilots? Is this the reason behind your concept, or are you most concerned about safe autorotation entry?
Mart
What is the cause of torque delay in a turbine? I presume it is mostly to do with stators reacting to pilot input. Does falling behind the power curve present real problems to novice pilots? Is this the reason behind your concept, or are you most concerned about safe autorotation entry?
Mart
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Graviman,
I emailed you the simple drawing I made. Anyone else who may be interested feel free to ask as I would be happy to share it.
I would love to make a CAD model in 3D as I agree that would be the easiest way to see how things work but I work with CAD programs about as well as I sing and I can't carry a note in a bucket!
Now on to something I know at least a little about. To answer your question about turbine torque lag. There are actually two types of turbines used in helicopters today. The free shaft turbine and the direct shaft turbine. Most current types use the free shaft variety. This type can be described as a jet engine blowing on a pinwheel. We have basically a jet engine with a second turbine mounted close behind. This second turbine is not attached in anyway rotatonally to the jet engine rotating assembly but only to the out put shaft of the engine. When the pilot increases load on the rotor system by pulling up on the collective we automatically increase fuel flow to the engine which creates more hot gasses to impact the second (or free) turbine. A by product of this is that the jet engine (known as N1 or NG) which has no load on it other than that of the compressor drag increases in speed. Normally this happens very quickly and becomes almost transparent to the pilot. However we do have two limits to consider.
First is the fact that as we increase the load and subsequently fuel flow we create a great amount of hot gas to impact our free turbine which changes this energy to a twisting force we know as torque. If we arent careful the torque may be greater than the loads the rest of the drivetrain can handle resulting in broken parts. So one cause for this "lag" is the simple fact that we need to avoid overtorquing the drivetrain. In real life the engine is quite happy to produce more torque than the drivetrain will handle and in a very short time. So, you could pull up hard on the collective and the engine would quickly adapt to the load and you would see the torque gauge run right past the redline without much lag at all.
The other limiting factor is compressor surge. During this power increase phase we need to meter our fuel carefully so as not to build the fire in the engine to fast. If we do the rate of compression becomes less than the rate of combustion and the fire starts to squirt out the front of the engine and not the rear. This is what is known as compressor surge. So we can say that if we had a ful flow of 10 gallons per hour and we wanted to increase to 50 gallons per hour we couldn't dump all that extra fuel in at one point or we could have a surge. We can increase the fuel flow over what we have and as the engine accellerates and we have more air being compressed we cna continually add more fuel. This situation does cause a slight lag in power output in a turbine engine however with the efficient new compressor designs this can be minimized to a point where it is almost a non issue.
My thoughts on the rotor governor are that it would help with the complexity of the fuel control system of the engine as now it would not need to have a governor system built in. It would help with entry into autorotation as the reaction time needed would be relaxed greatly. There may be a slight decrease in pilot workload when considering "getting behind the power curve" but this is usually an issue with piston engine helicopters more than turbines. So for your R22 example with the old Lycoming mill I would say it would be great as you have no torque limit to concern yourself with. The pilot could pull all the collective he/she wanted without fear of getting ahead of the engine. In a turbine ship the bennefits are much less since you would always need to fly the torque gauge as the engine could overpower the drivetrain.
Once again my diarreah of the mouth has spread to my typing fingers. I hope my explanations helped at least someone and I didn't teach everyone stuff they already knew. Now on to find Mr Jackson!
Max
I emailed you the simple drawing I made. Anyone else who may be interested feel free to ask as I would be happy to share it.
I would love to make a CAD model in 3D as I agree that would be the easiest way to see how things work but I work with CAD programs about as well as I sing and I can't carry a note in a bucket!
Now on to something I know at least a little about. To answer your question about turbine torque lag. There are actually two types of turbines used in helicopters today. The free shaft turbine and the direct shaft turbine. Most current types use the free shaft variety. This type can be described as a jet engine blowing on a pinwheel. We have basically a jet engine with a second turbine mounted close behind. This second turbine is not attached in anyway rotatonally to the jet engine rotating assembly but only to the out put shaft of the engine. When the pilot increases load on the rotor system by pulling up on the collective we automatically increase fuel flow to the engine which creates more hot gasses to impact the second (or free) turbine. A by product of this is that the jet engine (known as N1 or NG) which has no load on it other than that of the compressor drag increases in speed. Normally this happens very quickly and becomes almost transparent to the pilot. However we do have two limits to consider.
First is the fact that as we increase the load and subsequently fuel flow we create a great amount of hot gas to impact our free turbine which changes this energy to a twisting force we know as torque. If we arent careful the torque may be greater than the loads the rest of the drivetrain can handle resulting in broken parts. So one cause for this "lag" is the simple fact that we need to avoid overtorquing the drivetrain. In real life the engine is quite happy to produce more torque than the drivetrain will handle and in a very short time. So, you could pull up hard on the collective and the engine would quickly adapt to the load and you would see the torque gauge run right past the redline without much lag at all.
The other limiting factor is compressor surge. During this power increase phase we need to meter our fuel carefully so as not to build the fire in the engine to fast. If we do the rate of compression becomes less than the rate of combustion and the fire starts to squirt out the front of the engine and not the rear. This is what is known as compressor surge. So we can say that if we had a ful flow of 10 gallons per hour and we wanted to increase to 50 gallons per hour we couldn't dump all that extra fuel in at one point or we could have a surge. We can increase the fuel flow over what we have and as the engine accellerates and we have more air being compressed we cna continually add more fuel. This situation does cause a slight lag in power output in a turbine engine however with the efficient new compressor designs this can be minimized to a point where it is almost a non issue.
My thoughts on the rotor governor are that it would help with the complexity of the fuel control system of the engine as now it would not need to have a governor system built in. It would help with entry into autorotation as the reaction time needed would be relaxed greatly. There may be a slight decrease in pilot workload when considering "getting behind the power curve" but this is usually an issue with piston engine helicopters more than turbines. So for your R22 example with the old Lycoming mill I would say it would be great as you have no torque limit to concern yourself with. The pilot could pull all the collective he/she wanted without fear of getting ahead of the engine. In a turbine ship the bennefits are much less since you would always need to fly the torque gauge as the engine could overpower the drivetrain.
Once again my diarreah of the mouth has spread to my typing fingers. I hope my explanations helped at least someone and I didn't teach everyone stuff they already knew. Now on to find Mr Jackson!
Max
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Max,
I owes you an apology. I forwarded the .ppt file to my works address, since i don't have powerpoint, but was so engrossed in the problem i am investigating i totally forgot. Dumb engineers, eh!
Can you send a jpeg or similar?
"Most current types (turbine helis) use the free shaft variety."
Of course - i'm thinking direct shaft . I even have a nice sample of an A250 spool up with rotor turbine in tow as my computer startup, so should have realised.
"I hope my explanations helped at least someone and I didn't teach everyone stuff they already knew."
Nice write-up, Max. Don't mind relearning stuff in context, since it helps me make connections about design techniques. Now i realise we are primarily talking free power turbines i'm right with you.
When piston helis get below the power curve, are we talking naturally aspirated or turbo? I can see problems with torque converters, EGR, and turbos (in fact frequently do
), but don't see why (say) an R22 would suffer lag - seemed quick enough for my poor piloting anyhow .
Mart
[Edit: spialeeng]
I owes you an apology. I forwarded the .ppt file to my works address, since i don't have powerpoint, but was so engrossed in the problem i am investigating i totally forgot. Dumb engineers, eh!
Can you send a jpeg or similar?"Most current types (turbine helis) use the free shaft variety."
Of course - i'm thinking direct shaft
. I even have a nice sample of an A250 spool up with rotor turbine in tow as my computer startup, so should have realised."I hope my explanations helped at least someone and I didn't teach everyone stuff they already knew."
Nice write-up, Max. Don't mind relearning stuff in context, since it helps me make connections about design techniques. Now i realise we are primarily talking free power turbines i'm right with you.
When piston helis get below the power curve, are we talking naturally aspirated or turbo? I can see problems with torque converters, EGR, and turbos (in fact frequently do
), but don't see why (say) an R22 would suffer lag - seemed quick enough for my poor piloting anyhow .Mart
[Edit: spialeeng]
Last edited by Graviman; 25th Jun 2005 at 10:26.
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MaxTork,
Must admit it took me a while to see the subtlety of this, but i see what you are getting at. Basically you are fitting in additional weights into the Lockheed gyros to force RRPM control. Very neat, since this obviates the need to find methods of getting collective input into the rotor head.
I was thinking along the lines of a govenenor fitted as part of the collective control. This would clearly still need mixers in the pitch links so that collective and cyclic would have authority over the blade pitch. I think i like your system better...
You mentioned about varying the mast head spring to keep RRPM exactly constant with varying collective pitch, how would this work? Also, how does the pilot override the mechanism for collective flare? I still think that putting the collective down should trigger this, maybe with the servo i mentioned for continued hands off collective control.
A model is the best way to test the dynamics of this system. I just suggest the cad as a good (but not necessary) way to get there. Definately this concept would need a ground rig to prove it out, maybe based on the Sikorsky/Schweizer S300 head. Machined wood is ideal for this...
BTW anyone else getting the feeling that the Lockheed gyro system deserves a reappearance in light private helis - best wishes and Godspeed Lu...
Mart
[Edit: 'cos i types faster than i thinks ]
Must admit it took me a while to see the subtlety of this, but i see what you are getting at. Basically you are fitting in additional weights into the Lockheed gyros to force RRPM control. Very neat, since this obviates the need to find methods of getting collective input into the rotor head.
I was thinking along the lines of a govenenor fitted as part of the collective control. This would clearly still need mixers in the pitch links so that collective and cyclic would have authority over the blade pitch. I think i like your system better...
You mentioned about varying the mast head spring to keep RRPM exactly constant with varying collective pitch, how would this work? Also, how does the pilot override the mechanism for collective flare? I still think that putting the collective down should trigger this, maybe with the servo i mentioned for continued hands off collective control.
A model is the best way to test the dynamics of this system. I just suggest the cad as a good (but not necessary) way to get there. Definately this concept would need a ground rig to prove it out, maybe based on the Sikorsky/Schweizer S300 head. Machined wood is ideal for this...
BTW anyone else getting the feeling that the Lockheed gyro system deserves a reappearance in light private helis - best wishes and Godspeed Lu...
Mart
[Edit: 'cos i types faster than i thinks
]
Last edited by Graviman; 25th Jun 2005 at 11:09.
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Graviman,
I think there are a number of ways to make this set up work. We could use the swashplate as usual for cyclic and collective pitch control. Or we could use the swashplate and pitch links for cyclic control only and use the hub spring tension for collective. I'm not sure at this point which would be easier. Either way the weight assembly that is controlling the pitch is balanced between the two sets of springs, so if we change either one we upset the balance and get the required pitch response.
If we think of the hub springs as being pinched between the gyroscopic weight assembly on one side and a floating plate within the mast on the other side. If we move the plate up and down we can change the tension of the spring and therefore the balance which sets the pitch. So in the flare we can lower the floating plate and lower the tension of the springs which would allow high pitch anlges with low rotor RPM.
I hope my ramblings make a bit of sense. I can maybe try to work out some animations of some sort to explain it all better. I really think this whole thing could work and be a bennefit.
Max
I think there are a number of ways to make this set up work. We could use the swashplate as usual for cyclic and collective pitch control. Or we could use the swashplate and pitch links for cyclic control only and use the hub spring tension for collective. I'm not sure at this point which would be easier. Either way the weight assembly that is controlling the pitch is balanced between the two sets of springs, so if we change either one we upset the balance and get the required pitch response.
If we think of the hub springs as being pinched between the gyroscopic weight assembly on one side and a floating plate within the mast on the other side. If we move the plate up and down we can change the tension of the spring and therefore the balance which sets the pitch. So in the flare we can lower the floating plate and lower the tension of the springs which would allow high pitch anlges with low rotor RPM.
I hope my ramblings make a bit of sense. I can maybe try to work out some animations of some sort to explain it all better. I really think this whole thing could work and be a bennefit.
Max
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This gets more interesting...
"...ramblings make a bit of sense ... this whole thing could work and be a bennefit."
Right with you MaxTorq, but some concerns to be addressed still.
A better approach would be to cone the existing Lockheed gyro, with one set of weights (or offset pivot). RRPM would be slightly dependant on heli g loading, even though counter spring will be stong. This can be solved by a shaft centre counter weight, being careful not to upset the original gyro aumentation. Also the spring will need to be soft, and pre-stretched, to give the required sensitivity. Even so high pitch will require slightly higher RRPM, or worse autorotation requires slightly lower RRPM.
This last problem really bothers me. The ideal is a mechanism that keeps govenor in exactly the same position (ie fixed RRPM), over a very large range of pitch angles. In steam engines the govenor didn't need to move very much. I can't see any obvious way of solving this without an electric motor, which could well be part of the original "pilot assist" system i was considering (since it would also gently fight too fast collective input for turbines).
A purely mechanical system is always preferable, for safety and reliability reasons. I am willing to accept a hybrid approach for collective, but am not yet convinced that a complex sensor-electric system can outperform simple gyro augmentation of cyclic - Lu's account was very positive, before blade divergence. If pilot over-ride is really required (should the pilot actually be there?) then a motor could "trim" the collective input - probably better to let pilot have direct control, but let system "suggest" corrections...
Another thought is pilot induced oscillation. In hover if a pilot pulled more collective, to correct mild sink, an over-ride system might inadvertantly encourage more input to overcome "that wretched control system" . A correcting force in the collective would simply encourage the pilot to "slow down" his/her responses. A pilot in a panick is likely to fight the control system more, which may make matters much worse - worse case a failing engine with reduced power near to ground...
Mart
Right with you MaxTorq, but some concerns to be addressed still.
A better approach would be to cone the existing Lockheed gyro, with one set of weights (or offset pivot). RRPM would be slightly dependant on heli g loading, even though counter spring will be stong. This can be solved by a shaft centre counter weight, being careful not to upset the original gyro aumentation. Also the spring will need to be soft, and pre-stretched, to give the required sensitivity. Even so high pitch will require slightly higher RRPM, or worse autorotation requires slightly lower RRPM.
This last problem really bothers me. The ideal is a mechanism that keeps govenor in exactly the same position (ie fixed RRPM), over a very large range of pitch angles. In steam engines the govenor didn't need to move very much. I can't see any obvious way of solving this without an electric motor, which could well be part of the original "pilot assist" system i was considering (since it would also gently fight too fast collective input for turbines).
A purely mechanical system is always preferable, for safety and reliability reasons. I am willing to accept a hybrid approach for collective, but am not yet convinced that a complex sensor-electric system can outperform simple gyro augmentation of cyclic - Lu's account was very positive, before blade divergence. If pilot over-ride is really required (should the pilot actually be there?) then a motor could "trim" the collective input - probably better to let pilot have direct control, but let system "suggest" corrections...
Another thought is pilot induced oscillation. In hover if a pilot pulled more collective, to correct mild sink, an over-ride system might inadvertantly encourage more input to overcome "that wretched control system"
. A correcting force in the collective would simply encourage the pilot to "slow down" his/her responses. A pilot in a panick is likely to fight the control system more, which may make matters much worse - worse case a failing engine with reduced power near to ground...Mart
Last edited by Graviman; 27th Jun 2005 at 21:27.
> A purely mechanical system is always preferable, for safety and > reliability reasons
Whoops! I should go yank out my fuel injection and electronic ignition out of my car, slap in a carb and distributor. I've been following the system, and I have to wonder if 1) we are fixing a problem that really exists and 2) if you really want an easy to fly helicopter, why not fly by wire?
I've been following this thread with interest, but I think the challenges of implementing it, much less certifying it would overcome the advantages of a 'purely mechanical system.'
Good luck!
-IFMU
Whoops! I should go yank out my fuel injection and electronic ignition out of my car, slap in a carb and distributor. I've been following the system, and I have to wonder if 1) we are fixing a problem that really exists and 2) if you really want an easy to fly helicopter, why not fly by wire?
I've been following this thread with interest, but I think the challenges of implementing it, much less certifying it would overcome the advantages of a 'purely mechanical system.'
Good luck!
-IFMU