Certification of Robinson Helicopters (incl post by Frank Robinson)
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Lu wrote: "This is very true if you are flying an autogyro and not a helicopter. The entire theory of flapping up of the advancing blade is based on autogyro aerodynamics. What causes flapback is the increased lift on the advancing side of the blade causing a lift differential across the disc and gyroscopic precession causes the so called flap back. It is a very mild case of retreating blade stall. Only in this case the retreating blade is not in the stall range it is just generating less lift. The pilot counters this condition by restoring the position of the disc by pushing forward cyclic or, if the swash plate remains in the original position and the pilot does not correct, the delta hinge effect will pull pitch out of the blade on one side and increase the pitch on the other side and the gyroscopic precession (if I worked this out in my head correctly) will cause the helicopter to roll left."
It is not retreating blade tip stall because the blade is not at a stalling angle of attack, it is just dissymetry of lift. The disc will not roll left, it just pitches up until the disc returns to an equilibrium state. If you have the balls to go along for the ride it will pitch up until the aircraft is flying backward, at which point it will act to pitch the nose down. A useful demonstration of aerodynamic theory.
Funny SeiesmicPilot, no mention of the Bell LTE problems, or the original mast-bumping Bells.
Good day
It is not retreating blade tip stall because the blade is not at a stalling angle of attack, it is just dissymetry of lift. The disc will not roll left, it just pitches up until the disc returns to an equilibrium state. If you have the balls to go along for the ride it will pitch up until the aircraft is flying backward, at which point it will act to pitch the nose down. A useful demonstration of aerodynamic theory.
Funny SeiesmicPilot, no mention of the Bell LTE problems, or the original mast-bumping Bells.
Good day
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To: Grisoni
Your comment about low rotor rpm being the cause of rotor divergence is because Robinson said so and, you believed it.
In none of the accident reports did it specifically state low rotor rpm was the cause. It may have implied that (Please read the newspaper article above)but in most cases it said it was unknown. Until I wrote my report about the problem with the rotorhead design and the effect of that design relative to pilotage of the helicopter. The NTSB has indicated that they plan on revisiting the reports of accidents to see if the design of the rotorhead played a part in the accidents.
Low rotor rpm will not cause rotor divergance.
When the rotor rpm drops below the centrifugal force necessary to keep the blades in their respective radial positions the blades fold up and do not contact the airframe. Read the POH.
Fuselage contact is caused by retreating blade stall and or the incorrect control input to counter the condition and or significantly high flapping angles and not low rotor rpm as you have indicated.
These high flapping angles can be caused by many control inputs and two of these inputs are mentioned in the POH. Side slip and, flying out of trim. Don't argue with me by saying that you can perform these maneuvers as I will agree with you. But, the POH and the respective AD says you can't do them or, you will cause high flapping angles and strike the fuselage.
To: Helo Teacher
Go back and reread my post. I said that it is like a mild case of retreating blade stall. I did not say it was retreating blade stall. I stated that it was dysymmetry of lift and gyroscopic precession that caused the so called flap back. If the pilot counter the condition by pushing forward cyclic the condition will stop.
I also stated that if the pilot does not counter it by pushing forward and he allows the blades to come back then the disc will no longer be parallel to the swashplate and the delta hinge effect coupled with with gyroscopic precession will cause the helicopter to roll left (assuming I got it right as I had to visualize the entire action in my mind)
As far as your comment to Seismicpilot about Bell mast bumping and subsequent rotor loss you are totally correct. At my last account there were somewhere around 53 206 and UH-1 rotor loss accidents due to incorrect recovery from zero G. As far as loss of tail rotor effectiveness it can happen to any helicopter if the conditions are right.
To: Grisoni
Nowhere in my postings have I said that the Robinson helicopters are unreliable. There have been several fatal accidents caused by the failure of control elements but these have been fixed which restores the demonstrated reliability that you indicated.
My posts have been directed at the design of the rotorhead and the rigging procedure as the cause om many of the major accidents and not the reliability of the elements involved in my report.
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The Cat
[This message has been edited by Lu Zuckerman (edited 29 November 2000).]
Your comment about low rotor rpm being the cause of rotor divergence is because Robinson said so and, you believed it.
In none of the accident reports did it specifically state low rotor rpm was the cause. It may have implied that (Please read the newspaper article above)but in most cases it said it was unknown. Until I wrote my report about the problem with the rotorhead design and the effect of that design relative to pilotage of the helicopter. The NTSB has indicated that they plan on revisiting the reports of accidents to see if the design of the rotorhead played a part in the accidents.
Low rotor rpm will not cause rotor divergance.
When the rotor rpm drops below the centrifugal force necessary to keep the blades in their respective radial positions the blades fold up and do not contact the airframe. Read the POH.
Fuselage contact is caused by retreating blade stall and or the incorrect control input to counter the condition and or significantly high flapping angles and not low rotor rpm as you have indicated.
These high flapping angles can be caused by many control inputs and two of these inputs are mentioned in the POH. Side slip and, flying out of trim. Don't argue with me by saying that you can perform these maneuvers as I will agree with you. But, the POH and the respective AD says you can't do them or, you will cause high flapping angles and strike the fuselage.
To: Helo Teacher
Go back and reread my post. I said that it is like a mild case of retreating blade stall. I did not say it was retreating blade stall. I stated that it was dysymmetry of lift and gyroscopic precession that caused the so called flap back. If the pilot counter the condition by pushing forward cyclic the condition will stop.
I also stated that if the pilot does not counter it by pushing forward and he allows the blades to come back then the disc will no longer be parallel to the swashplate and the delta hinge effect coupled with with gyroscopic precession will cause the helicopter to roll left (assuming I got it right as I had to visualize the entire action in my mind)
As far as your comment to Seismicpilot about Bell mast bumping and subsequent rotor loss you are totally correct. At my last account there were somewhere around 53 206 and UH-1 rotor loss accidents due to incorrect recovery from zero G. As far as loss of tail rotor effectiveness it can happen to any helicopter if the conditions are right.
To: Grisoni
Nowhere in my postings have I said that the Robinson helicopters are unreliable. There have been several fatal accidents caused by the failure of control elements but these have been fixed which restores the demonstrated reliability that you indicated.
My posts have been directed at the design of the rotorhead and the rigging procedure as the cause om many of the major accidents and not the reliability of the elements involved in my report.
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The Cat
[This message has been edited by Lu Zuckerman (edited 29 November 2000).]
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I have read some of the comments about the R22 helicopter printed in this forum. Most were favorable and I appreciated that. However, some were obviously misinformed, and I will comment on several of those.
SIDESLIP WITH THE R22
Concerning the caution against excessive sideslips in the R22 flight manual, this was in part due to a misunderstanding by the FAA. In the Army training film on mast bumping, it showed excessive side slipping as one cause of mast bumping. This was true for the Army Bell Cobras and Hueys, because both aircraft have high centers-of-gravity and relatively low side silhouettes due to their high-mounted powerplants and low-mounted tailcones. During a severe sideslip, the resultant fuselage drag could be below the center-of-gravity and cause the helicopter to roll out of a turn, instead of into the turn, i.e. a negative dihedral or adverse roll characteristic. Airplanes prevent this by having wings with positive dihedral.
The basic R22s and R44s have low-mounted engines, high tailcones, and aerodynamic mast fairings. Consequently, neither the basic R22 or R44 had any tendency toward adverse roll during FAA certification. However, all helicopters (including the R22 and R44) tend to have an adverse roll characteristic when they are equipped with inflated floats, because the floats move the side silhouette area down considerably. For that reason, I did not object to the caution in the R22 flight manual against extreme sideslips during forward flight.
R22 FAA TYPE CERTIFICATION
During the R22 certification, both the FAA test pilots and our own company test pilots flew the R22 through all required maneuvers and flight regimes, and it met all of the FAA regulations. No exemptions were issued for the R22 by the FAA during its certification. Also, I was not a DER (designated engineering representative) during the FAA certification of the R22. No DERs were used during its original certification. After it was certified, the FAA appointed me as a DER with limited authority, so I could approve some minor design changes which commonly occur during production of a new aircraft.
R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.
Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.
The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as “wee-wa.” This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate “wee-wa” in the R22 rotor system was calculated to be 19 degrees.
The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade’s angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees.
A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle.
Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22.
Frank Robinson
SIDESLIP WITH THE R22
Concerning the caution against excessive sideslips in the R22 flight manual, this was in part due to a misunderstanding by the FAA. In the Army training film on mast bumping, it showed excessive side slipping as one cause of mast bumping. This was true for the Army Bell Cobras and Hueys, because both aircraft have high centers-of-gravity and relatively low side silhouettes due to their high-mounted powerplants and low-mounted tailcones. During a severe sideslip, the resultant fuselage drag could be below the center-of-gravity and cause the helicopter to roll out of a turn, instead of into the turn, i.e. a negative dihedral or adverse roll characteristic. Airplanes prevent this by having wings with positive dihedral.
The basic R22s and R44s have low-mounted engines, high tailcones, and aerodynamic mast fairings. Consequently, neither the basic R22 or R44 had any tendency toward adverse roll during FAA certification. However, all helicopters (including the R22 and R44) tend to have an adverse roll characteristic when they are equipped with inflated floats, because the floats move the side silhouette area down considerably. For that reason, I did not object to the caution in the R22 flight manual against extreme sideslips during forward flight.
R22 FAA TYPE CERTIFICATION
During the R22 certification, both the FAA test pilots and our own company test pilots flew the R22 through all required maneuvers and flight regimes, and it met all of the FAA regulations. No exemptions were issued for the R22 by the FAA during its certification. Also, I was not a DER (designated engineering representative) during the FAA certification of the R22. No DERs were used during its original certification. After it was certified, the FAA appointed me as a DER with limited authority, so I could approve some minor design changes which commonly occur during production of a new aircraft.
R22 ROTOR SYSTEM
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick.
Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades. For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.
The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as “wee-wa.” This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate “wee-wa” in the R22 rotor system was calculated to be 19 degrees.
The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade’s angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees.
A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle.
Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22.
Frank Robinson
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To grice-a-roni,
actually, no. figured your types are of the gore herd.
being in oil ind'y...Bush makes more sense; unlike the robby.
maybe someday you'll get to fly a real helicopter
I've just recently taken life insurance policies out on some robby pilots; sure better odds than the lottery
heck, if Law firms can do it, why not?
actually, no. figured your types are of the gore herd.
being in oil ind'y...Bush makes more sense; unlike the robby.
maybe someday you'll get to fly a real helicopter
I've just recently taken life insurance policies out on some robby pilots; sure better odds than the lottery
heck, if Law firms can do it, why not?
Guest
Posts: n/a
To: Mr. Frank Robinson
I appreciate your input. It will take a bit of time for me to digest what you have posted. When I have it clearly in my mind I will come back with several questions and some comments about your posting.
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The Cat
I appreciate your input. It will take a bit of time for me to digest what you have posted. When I have it clearly in my mind I will come back with several questions and some comments about your posting.
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The Cat
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Seis-o-graph
Iam currently flying a real helicopter (AS 365),but I do admit that if you pay me the same $$ I would take the R22/44 any day.
There has been and still is a lot of RHC trashing going on from people that really shouldn't; it be like me Saying Hueys are dangerous because they tend to lose MR's.
Point is: If you don't like it fine, but don't Badmouth something just because.
The R22 has put out tousand of pilots in the last 15 years and the problem is not the AC but the fact that there isn't any standardisation amongst the civil flightschools.
Iam currently flying a real helicopter (AS 365),but I do admit that if you pay me the same $$ I would take the R22/44 any day.
There has been and still is a lot of RHC trashing going on from people that really shouldn't; it be like me Saying Hueys are dangerous because they tend to lose MR's.
Point is: If you don't like it fine, but don't Badmouth something just because.
The R22 has put out tousand of pilots in the last 15 years and the problem is not the AC but the fact that there isn't any standardisation amongst the civil flightschools.
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I want to keep it short today, but i have to make two statements!
To seismicpilot:
I don't know what's wrong with you, but you don't have the background knowledge of Lu. Everyone following that thread knows that i don't agree with Lu, but at least he has a point. You are just jumping on the Robinsons for whatever reason and i have to completly agree with Grisoni. A lot of the pilots in this forum, including me, fly bigger helos, but like Grisoni said, give me the same paycheck and i'm going to go back in the Robinson in a heartbeat. By the way, i instruct in the Robinson, while i'm not working on my regular job (EC135). Just for fun!!!
To Frank Robinson:
I'm not a mechanic and not a physicist either, so i don't even try to verify your statements. Like Lu, you got a point and the background...unlike seismicpilot!!!
YouWillSee
[This message has been edited by YouWillSee (edited 30 November 2000).]
To seismicpilot:
I don't know what's wrong with you, but you don't have the background knowledge of Lu. Everyone following that thread knows that i don't agree with Lu, but at least he has a point. You are just jumping on the Robinsons for whatever reason and i have to completly agree with Grisoni. A lot of the pilots in this forum, including me, fly bigger helos, but like Grisoni said, give me the same paycheck and i'm going to go back in the Robinson in a heartbeat. By the way, i instruct in the Robinson, while i'm not working on my regular job (EC135). Just for fun!!!
To Frank Robinson:
I'm not a mechanic and not a physicist either, so i don't even try to verify your statements. Like Lu, you got a point and the background...unlike seismicpilot!!!
YouWillSee
[This message has been edited by YouWillSee (edited 30 November 2000).]
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This is the message that got this thread started:
"If anyone is interested in why the Robinson R22 and R44 should never have been certificated by the FAA please contact me. Please provide your email address and your interest in the subject and I shall send you a copy of a report submitted by me to the NTSB."
Then there is this one:
Nowhere in my postings have I said that the Robinson helicopters are unreliable. There have been several fatal accidents caused by the failure of control elements but these have been fixed which restores the demonstrated reliability that you indicated.
Lu, I know before I ask this that you will massage one or both of your statements in the answer but, are these statements of yours in contradiction?
"If anyone is interested in why the Robinson R22 and R44 should never have been certificated by the FAA please contact me. Please provide your email address and your interest in the subject and I shall send you a copy of a report submitted by me to the NTSB."
Then there is this one:
Nowhere in my postings have I said that the Robinson helicopters are unreliable. There have been several fatal accidents caused by the failure of control elements but these have been fixed which restores the demonstrated reliability that you indicated.
Lu, I know before I ask this that you will massage one or both of your statements in the answer but, are these statements of yours in contradiction?
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To: FlyAny
There are two separate issues. Reliability and safety. The report reflected safety and my comment about the failed control elements reflected reliability.
The rules of reliability require you to make a prediction, based on past failure histories ,to indicate just how reliable a system is predicted to be.
Thats all it is, a prediction. The ultimate reliability that has been predicted is a performance number and not a guarantee. However, many manufacturers use the reliability to predict MTBF for a component.
The rule state that if a part fails and impacts the predicted reliability then the manufacturer can correct his error and it doesn't count against him. This is for military systems. In civil applications if a part fails it is repaired by the manufacturer if it is still on warrantee.
Once the warrantee expires the operator fixes it. If the part fails repeatedly then the FAA gets into it with the manufacturer and tells him to fix the problem. With the fix or mod the warrantee statrts all over again but only for the repairs.
Relative to safety, if what is in my report is proved correct then the FAA will step in and require a modification to the parts or the procedures.
In the case of the control element failure(s) Robinson corrected the design.
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The Cat
[This message has been edited by Lu Zuckerman (edited 30 November 2000).]
There are two separate issues. Reliability and safety. The report reflected safety and my comment about the failed control elements reflected reliability.
The rules of reliability require you to make a prediction, based on past failure histories ,to indicate just how reliable a system is predicted to be.
Thats all it is, a prediction. The ultimate reliability that has been predicted is a performance number and not a guarantee. However, many manufacturers use the reliability to predict MTBF for a component.
The rule state that if a part fails and impacts the predicted reliability then the manufacturer can correct his error and it doesn't count against him. This is for military systems. In civil applications if a part fails it is repaired by the manufacturer if it is still on warrantee.
Once the warrantee expires the operator fixes it. If the part fails repeatedly then the FAA gets into it with the manufacturer and tells him to fix the problem. With the fix or mod the warrantee statrts all over again but only for the repairs.
Relative to safety, if what is in my report is proved correct then the FAA will step in and require a modification to the parts or the procedures.
In the case of the control element failure(s) Robinson corrected the design.
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The Cat
[This message has been edited by Lu Zuckerman (edited 30 November 2000).]
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What an interesting discussion regarding the operational characteristics of the Robinson R-22 rotor hub.
A couple of points for thought;
The main reason for having a flapping/coning hinge offset is to provide faster rotor responses to cyclic inputs. This of greater importance on helicopters where the distance between the rotor disk and the CG is relatively short. It appears that on the R-22 the pitch link is inline with the flapping/coning hinge. On initial consideration, this would make sense. Mr. Frank Robinson has very clearly explained the considerations used to determine the delta-3 angle. I wonder if delta-3 angle was then used as the basis for arriving at the flapping/coning hinge offset?
It may be of interest to know that the R-22 uses 'control system geometry' for its delta-3 whereas the K-Max uses 'flap hinge geometry' for its delta-3. [ref. Helicopter Theory, page 239 ~ Wayne Johnson]. The 'flap hinge geometry' consists of changing the teetering hinge angle from 90 degrees (normal to blade span), to around 75 degrees. An operational difference between the two is that the 'flap hinge geometry' also has a lead-lad component, which is required to handle the Coriolis effect between the two interlocked rotors.
Dave J.
Project: <A HREF="http://www.SynchroLite.com" TARGET="_blank">www.SynchroLite.com</A>
A couple of points for thought;
The main reason for having a flapping/coning hinge offset is to provide faster rotor responses to cyclic inputs. This of greater importance on helicopters where the distance between the rotor disk and the CG is relatively short. It appears that on the R-22 the pitch link is inline with the flapping/coning hinge. On initial consideration, this would make sense. Mr. Frank Robinson has very clearly explained the considerations used to determine the delta-3 angle. I wonder if delta-3 angle was then used as the basis for arriving at the flapping/coning hinge offset?
It may be of interest to know that the R-22 uses 'control system geometry' for its delta-3 whereas the K-Max uses 'flap hinge geometry' for its delta-3. [ref. Helicopter Theory, page 239 ~ Wayne Johnson]. The 'flap hinge geometry' consists of changing the teetering hinge angle from 90 degrees (normal to blade span), to around 75 degrees. An operational difference between the two is that the 'flap hinge geometry' also has a lead-lad component, which is required to handle the Coriolis effect between the two interlocked rotors.
Dave J.
Project: <A HREF="http://www.SynchroLite.com" TARGET="_blank">www.SynchroLite.com</A>
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To: Dave jackson
The question you asked at the end of the first paragraph is one of many that I have posed to Frank Robinson in my response to his posting above. Hopefully I will complete the response by tomorrow morning and post it at the same time.
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The Cat
The question you asked at the end of the first paragraph is one of many that I have posed to Frank Robinson in my response to his posting above. Hopefully I will complete the response by tomorrow morning and post it at the same time.
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The Cat
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To: Mr. Frank Robinson
I welcome the opportunity to exchange Ideas with you.
The following paragraphs are keyed to the main paragraphs in you posting.
SIDESLIP WITH R22
You stated in your post that the inclusion of the restriction against sideslipping was in part due to a misunderstanding on the part of the FAA. This misunderstanding was based on the FAA’s misinterpretation of a US Army training film on mast bumping. You then went on by stating that helicopters in general have an adverse roll characteristic but not the Robinson unless it is equipped with floats
My comments regarding those statements are,
1) If the inclusion of the restrictions against sideslipping (and, possibly out of trim flight) were based on a mistake on the part of the FAA why hasn’t the Airworthiness Directive (AD 95-26-04) been modified to reflect a change in the attitude of the FAA by them admitting their mistake.
2) What does your explanation have to do with the high flapping angles and attendant loads? The restrictions were implemented to minimize the flapping angles and loads that could precipitate mast bumping and loss of the rotor or, rotor incursion.
3) You stated that you had no objection to the caution against extreme sideslip during forward flight. No where in the AD or in the respective POHs does it say anything about extreme sideslip. The POHs and the AD state,” Avoid sideslip during flight. Maintain in trim flight at all times”. Surely you must have objected to them not using the word extreme.
R22 FAA CERTIFICATION
Sooner than holding up this posting I will fill this section in when I get some information regards the certification.
R22 ROTOR SYSTEM
Your explanation places the cart in front of the horse. You stated that feathering occurs while the rotor disc is being tilted. The rotor disc is being tilted due to gyroscopic precession, which occurs due to cyclic input, which minimizes the pitch on the right blade and maximizes pitch on the left blade. Simply put, feathering causes tilt. Tilt does not cause feathering. The only feathering caused by tilt is that pitch change that results from pitch coupling. Or, the pitch coupling that results from flapping about the cone hinge and in the case of flapping about the cone hinge the pitch coupling will only occur when the pitch horn/pitch link attach point is not aligned with the cone hinge.
You further stated, “aerodynamic feathering is necessary to compensate for the difference in airspeed between the advancing blade and the retreating blades". Otherwise, you stated, the advancing blade would climb and the retreating blade would dive, and the rotor disc would tilt aft.
This would be true if you placed the helicopter on a flat bed truck and propelled the helicopter forward while maintaining the cyclic stick in the rigged neutral position. However I believe if the disc tended to flap back the pitch coupling effect would cause the disc to tip to the left. At least I think so.
NOTE: What I am about to say is more for the edification of the participants in this thread so that they can better understand my argument.
This how it happens. (All comments assume a no wind condition and we discount the effects of induced flow and transverse flow effects.
1) The pilot moves the cyclic stick forward from the rigged neutral position tilting the swashplate and changing the pitch on the left and right blades. The right blade decreases pitch and the left blade increases pitch. For the want of a better word, the blades have feathered.
2) At point 1) above the helicopter is in a hover and standing still. The feathering action described above causes an imbalance in the lift forces across the disc and this results in gyroscopic precession tilting the disc down over the nose
3) With the tilting of the disc there is a change in the thrust vector which causes the helicopter to move forward and now and only now do we have an advancing and retreating blade
4) It is true that the feathering compensates for the difference in airspeed across the disc, but it is the feathering and the resulting tilting of the disc that causes the lift differential. But, the advancing blade has decreased pitch decreasing its’ effective lift relative to the retreating blade which has increased pitch to reflect the decreased airflow due to forward flight thus equalizing the lift across the disc. As a result the helicopter is flying forward and there is no adverse roll effect.
You further stated that as the helicopter started to move forward that if the pilot did not correct the helicopter would roll to the right. What you described is the effect of transverse flow and this is common to all single rotor helicopters. Regarding the term “wee-wa”, I have been associated with helicopters since 1949 and have attended twenty helicopter factory schools and familiarization courses and I never heard the expression. That is not to say it doesn’t exist.
Elsewhere in you posting you indicated that the delta 3 angle of 18-degrees was selected as the best compromise angle to reduce the two undesirable characteristics described above which would have been present in the R22 had a 90-degree pitch link been used.
First of all, a 90-degree pitch link could never be used as to do so; you would have to eliminate the cone hinges. The fact that you have cone hinges dictates the requirement for a 72-degree pitch horn. If you had a 90-degree pitch horn the pitch coupling would be so dramatic as to render the helicopter uncontrollable.
The cone hinges were designed into the rotorhead to minimize flapping loads and to eliminate spanwise blade bending. Incorporation of these hinges dictated the offset of the rotating swashplate relative to the stationary swashplate. I can accept the fact that your designers selected the 18-degree offset and designed the rotorhead to reflect that selection. The 18-degree offset you indicated was selected to eliminate the effect of transverse flow effect.
If this is true why have so many of the Robinson pilots on this thread indicated that they roll in left cyclic to counter the effects of transverse flow?
You have stated that there is an 18-degree offset between the fixed and rotating elements of the swashplate. In the rigging procedure the blades are disposed laterally and longitudinally in respect to these two major axes just like on a Bell helicopter. The Bell has a 90-degree pitch horn so the pitch horn/pitch link are directly over the points of maximum deflection (Fore & aft and laterally). On the R22 with the blades in the same positions the pitch horn/pitch link are 18-degrees away from the points of maximum deflection. This raises several questions:
1) Because the blade must rotate the additional 18-degrees to get the pitch horn/pitch link over the points of maximum swashplate deflection the resultant gyroscopic precession would tilt the disc forward and to the left. Please explain in layman’s terms why the blades should tip down over the nose as opposed to tilting to the left when the cyclic stick is pushed forward.
2) When the blade angles are set with the blades disposed as indicated in 1) above is it possible that the pitch angles will change when the blades have traveled the additional 18-degrees on the tilted swashplate? Is it also possible that this increase or decrease in pitch angle could have an effect on blade stall under certain atmospheric conditions or at high-density altitudes? The rigging procedure requires that certain basic low collective and high cyclic angles be measured, then the collective range is checked and after that, autorotation checks are made which under most condition requires the addition of pitch. Now we add in the added pitch that results from that 18-degrees of travel on the swashplate. Is there a possibility that the rotor has too much pitch under the above stated conditions and the result is blade stall and low rotor RPM? In the rigging procedure the pitch angle is taken at the 75% span position. Have your designers ever considered what the reflected pitch would be at the root of the blade where on most helicopters the pitch readings are made. What with the negative twist of the blades and the station where the reading is taken the reflected pitch could be significantly higher. With the higher reflected pitch at the root where very little lift is created the drag can be quite significant. Possibly significant enough with the added pitch from the 18-degree offset to cause the blades to slow down.
3) In the rigging procedure the cyclic is rigged in its’ neutral position which is slightly to the right of the longitudinal center line and midway in its’ Fore & Aft travel range as dictated by the stop plate. If the movement Fore and Aft is the same, please explain how you can get different pitch angles Fore & Aft?
There are many other points I would like to raise but first I’ll give you time to digest my statements and return with your response.
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The Cat
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
I welcome the opportunity to exchange Ideas with you.
The following paragraphs are keyed to the main paragraphs in you posting.
SIDESLIP WITH R22
You stated in your post that the inclusion of the restriction against sideslipping was in part due to a misunderstanding on the part of the FAA. This misunderstanding was based on the FAA’s misinterpretation of a US Army training film on mast bumping. You then went on by stating that helicopters in general have an adverse roll characteristic but not the Robinson unless it is equipped with floats
My comments regarding those statements are,
1) If the inclusion of the restrictions against sideslipping (and, possibly out of trim flight) were based on a mistake on the part of the FAA why hasn’t the Airworthiness Directive (AD 95-26-04) been modified to reflect a change in the attitude of the FAA by them admitting their mistake.
2) What does your explanation have to do with the high flapping angles and attendant loads? The restrictions were implemented to minimize the flapping angles and loads that could precipitate mast bumping and loss of the rotor or, rotor incursion.
3) You stated that you had no objection to the caution against extreme sideslip during forward flight. No where in the AD or in the respective POHs does it say anything about extreme sideslip. The POHs and the AD state,” Avoid sideslip during flight. Maintain in trim flight at all times”. Surely you must have objected to them not using the word extreme.
R22 FAA CERTIFICATION
Sooner than holding up this posting I will fill this section in when I get some information regards the certification.
R22 ROTOR SYSTEM
Your explanation places the cart in front of the horse. You stated that feathering occurs while the rotor disc is being tilted. The rotor disc is being tilted due to gyroscopic precession, which occurs due to cyclic input, which minimizes the pitch on the right blade and maximizes pitch on the left blade. Simply put, feathering causes tilt. Tilt does not cause feathering. The only feathering caused by tilt is that pitch change that results from pitch coupling. Or, the pitch coupling that results from flapping about the cone hinge and in the case of flapping about the cone hinge the pitch coupling will only occur when the pitch horn/pitch link attach point is not aligned with the cone hinge.
You further stated, “aerodynamic feathering is necessary to compensate for the difference in airspeed between the advancing blade and the retreating blades". Otherwise, you stated, the advancing blade would climb and the retreating blade would dive, and the rotor disc would tilt aft.
This would be true if you placed the helicopter on a flat bed truck and propelled the helicopter forward while maintaining the cyclic stick in the rigged neutral position. However I believe if the disc tended to flap back the pitch coupling effect would cause the disc to tip to the left. At least I think so.
NOTE: What I am about to say is more for the edification of the participants in this thread so that they can better understand my argument.
This how it happens. (All comments assume a no wind condition and we discount the effects of induced flow and transverse flow effects.
1) The pilot moves the cyclic stick forward from the rigged neutral position tilting the swashplate and changing the pitch on the left and right blades. The right blade decreases pitch and the left blade increases pitch. For the want of a better word, the blades have feathered.
2) At point 1) above the helicopter is in a hover and standing still. The feathering action described above causes an imbalance in the lift forces across the disc and this results in gyroscopic precession tilting the disc down over the nose
3) With the tilting of the disc there is a change in the thrust vector which causes the helicopter to move forward and now and only now do we have an advancing and retreating blade
4) It is true that the feathering compensates for the difference in airspeed across the disc, but it is the feathering and the resulting tilting of the disc that causes the lift differential. But, the advancing blade has decreased pitch decreasing its’ effective lift relative to the retreating blade which has increased pitch to reflect the decreased airflow due to forward flight thus equalizing the lift across the disc. As a result the helicopter is flying forward and there is no adverse roll effect.
You further stated that as the helicopter started to move forward that if the pilot did not correct the helicopter would roll to the right. What you described is the effect of transverse flow and this is common to all single rotor helicopters. Regarding the term “wee-wa”, I have been associated with helicopters since 1949 and have attended twenty helicopter factory schools and familiarization courses and I never heard the expression. That is not to say it doesn’t exist.
Elsewhere in you posting you indicated that the delta 3 angle of 18-degrees was selected as the best compromise angle to reduce the two undesirable characteristics described above which would have been present in the R22 had a 90-degree pitch link been used.
First of all, a 90-degree pitch link could never be used as to do so; you would have to eliminate the cone hinges. The fact that you have cone hinges dictates the requirement for a 72-degree pitch horn. If you had a 90-degree pitch horn the pitch coupling would be so dramatic as to render the helicopter uncontrollable.
The cone hinges were designed into the rotorhead to minimize flapping loads and to eliminate spanwise blade bending. Incorporation of these hinges dictated the offset of the rotating swashplate relative to the stationary swashplate. I can accept the fact that your designers selected the 18-degree offset and designed the rotorhead to reflect that selection. The 18-degree offset you indicated was selected to eliminate the effect of transverse flow effect.
If this is true why have so many of the Robinson pilots on this thread indicated that they roll in left cyclic to counter the effects of transverse flow?
You have stated that there is an 18-degree offset between the fixed and rotating elements of the swashplate. In the rigging procedure the blades are disposed laterally and longitudinally in respect to these two major axes just like on a Bell helicopter. The Bell has a 90-degree pitch horn so the pitch horn/pitch link are directly over the points of maximum deflection (Fore & aft and laterally). On the R22 with the blades in the same positions the pitch horn/pitch link are 18-degrees away from the points of maximum deflection. This raises several questions:
1) Because the blade must rotate the additional 18-degrees to get the pitch horn/pitch link over the points of maximum swashplate deflection the resultant gyroscopic precession would tilt the disc forward and to the left. Please explain in layman’s terms why the blades should tip down over the nose as opposed to tilting to the left when the cyclic stick is pushed forward.
2) When the blade angles are set with the blades disposed as indicated in 1) above is it possible that the pitch angles will change when the blades have traveled the additional 18-degrees on the tilted swashplate? Is it also possible that this increase or decrease in pitch angle could have an effect on blade stall under certain atmospheric conditions or at high-density altitudes? The rigging procedure requires that certain basic low collective and high cyclic angles be measured, then the collective range is checked and after that, autorotation checks are made which under most condition requires the addition of pitch. Now we add in the added pitch that results from that 18-degrees of travel on the swashplate. Is there a possibility that the rotor has too much pitch under the above stated conditions and the result is blade stall and low rotor RPM? In the rigging procedure the pitch angle is taken at the 75% span position. Have your designers ever considered what the reflected pitch would be at the root of the blade where on most helicopters the pitch readings are made. What with the negative twist of the blades and the station where the reading is taken the reflected pitch could be significantly higher. With the higher reflected pitch at the root where very little lift is created the drag can be quite significant. Possibly significant enough with the added pitch from the 18-degree offset to cause the blades to slow down.
3) In the rigging procedure the cyclic is rigged in its’ neutral position which is slightly to the right of the longitudinal center line and midway in its’ Fore & Aft travel range as dictated by the stop plate. If the movement Fore and Aft is the same, please explain how you can get different pitch angles Fore & Aft?
There are many other points I would like to raise but first I’ll give you time to digest my statements and return with your response.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
[This message has been edited by Lu Zuckerman (edited 01 December 2000).]
Guest
Posts: n/a
Lu: In answer to Frank Robinson, you say: "There are many other points I would like to raise but first I’ll give you time to digest my statements and return with your response."
Can I suggest that you take time (as FR did)to get ALL your points together for one post:
It's unrealistic to expect FR to enter into a prolonged discussion on Prune.
It's obvious that the two of you will never agree.
Eventually, you'll have to "agree to differ" so one complete response each way should do the trick.
Just an idea?
Can I suggest that you take time (as FR did)to get ALL your points together for one post:
It's unrealistic to expect FR to enter into a prolonged discussion on Prune.
It's obvious that the two of you will never agree.
Eventually, you'll have to "agree to differ" so one complete response each way should do the trick.
Just an idea?
Guest
Posts: n/a
To: Hoverman
Your point is well taken. However the response from Frank Robinson was vague and it skirted around the topics of discussion that appeared on the rotorheads forum.
Relative to the R22 FAA CERTIFICATION I held off on responding to that as I am awaiting verification of several points made by Mr. Robinson. The additional points that I wanted to bring up at a later date involved points brought up in my report that were not discussed on the forum or, points not responded to by Mr. Robinson.
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The Cat
Your point is well taken. However the response from Frank Robinson was vague and it skirted around the topics of discussion that appeared on the rotorheads forum.
Relative to the R22 FAA CERTIFICATION I held off on responding to that as I am awaiting verification of several points made by Mr. Robinson. The additional points that I wanted to bring up at a later date involved points brought up in my report that were not discussed on the forum or, points not responded to by Mr. Robinson.
------------------
The Cat
Guest
Posts: n/a
To All:
I find it difficult to understand that with all of the negativism towards my postings and my opinions that there has been only one posting since Frank Robinson responded. I would have thought that those of you that have disagreed with me in this forum would take up the cause and tell me we told you so.
The other reason I am making this posting is to keep the thread alive in hopes that Mr. Robinson will respond to my comments about his post.
------------------
The Cat
I find it difficult to understand that with all of the negativism towards my postings and my opinions that there has been only one posting since Frank Robinson responded. I would have thought that those of you that have disagreed with me in this forum would take up the cause and tell me we told you so.
The other reason I am making this posting is to keep the thread alive in hopes that Mr. Robinson will respond to my comments about his post.
------------------
The Cat
Guest
Posts: n/a
Lu,
I suspect many of us think there isn't much else that can usefully be said, that hasn't been covered in the last eight pages.
But then, if you're aiming for this thread to beat all PPRuNe records...
------------------
Whirly
To fly is human, to hover, divine.
I suspect many of us think there isn't much else that can usefully be said, that hasn't been covered in the last eight pages.
But then, if you're aiming for this thread to beat all PPRuNe records...
------------------
Whirly
To fly is human, to hover, divine.
Guest
Posts: n/a
To : Helidrvr
In response to your request to compare the helicopter rotor system to that of an autogyro I provide the following. I know that I put much of this on the Just Helicopter forum and I may well have put it on the Rotorheads forum as well.
In order to understand what I am about to say the reader has to make a minor adjustment in how he/she views a spinning rotor system. The discussion below applies to any type of helicopter rotor system no matter if it is rigid, semi rigid or, fully articulated. Hopefully I don’t come off as being pedantic.
I would suggest that you stop thinking of individual blades flying in a circle. Instead, think of the blades as a solid disc just like the rotor on a gyroscope. And, like a gyroscope the rotor system has rigidity in space and a phase angle of 90-degrees. If you apply a force to a gyro that is on gimbals, the gyro because of precession will react 90 degrees later in the direction of rotation. The same is true on a helicopter. Cyclic input will change the pitch relationship across the disc and this will result in an imbalance of forces. If the pilot pushes the stick forward, the greater force is over the left side of the disc. This upward force (due to precession) will cause the disc to raise over the tail and drop over the nose. Aerodynamics plays a minimal part in this action. The change in disc position was caused
by the change in lift forces but the actual movement was caused by the gyroscopic forces and characteristics of the spinning disc.
On two blade rotor system the pitch horn leads the blade by 90-degrees. On this type of rotor system the swashplate tips down over the nose and the pitch horn introduces the maximum pitch input while the blades are over the lateral axis. With this 90-degree disposition of the pitch horn the phase angle will cause the blade to dip down over the nose and the helicopter flies forward.
On multi blade systems the pitch horn leads the blades by 45-degrees and the swashplate tips 45-degrees ahead of the direction of flight. 45 + 45 = 90 degree phase angle and the same thing happens. The helicopter flies forward.
Individual blades do not fly to a position, The disc is moved by gyroscopic forces. The pitch change of the individual blades simply change the balance of forces across the disc and physics does the rest.
Now, we get to autogyros. There are several types but I will address the two blade and three blade variety. On the two blade type the blade assembly (There is no individual pitch change) is tilted. This in effect decreases the pitch on one blade and increases the pitch on the opposing blade. This, like on the helicopter, creates a differential of lift across the blade assembly and this differential of forces triggers, guess what, gyroscopic precession and the disc tilts but it tilts a limited amount so that it is almost parallel with the direction of flight.
On a multi blade autogyro the rotor head is fully articulated and pitch input is exactly the same as on a helicopter with the same restrictions as on a two blade system. Limited forward tilt.
The closest thing to an autogyro that was a helicopter was the Cheyenne. In flight the helicopter was powered by a propeller in the rear and most of the lift was provided by wings. The spinning rotor provided some of the lift but it was mainly used to control the direction and attitude of the helicopter.
What it boils down to is that both the helicopter and autogyro operate under the same physical laws relative to gyroscopic precession.
[This message has been edited by Lu Zuckerman (edited 05 December 2000).]
In response to your request to compare the helicopter rotor system to that of an autogyro I provide the following. I know that I put much of this on the Just Helicopter forum and I may well have put it on the Rotorheads forum as well.
In order to understand what I am about to say the reader has to make a minor adjustment in how he/she views a spinning rotor system. The discussion below applies to any type of helicopter rotor system no matter if it is rigid, semi rigid or, fully articulated. Hopefully I don’t come off as being pedantic.
I would suggest that you stop thinking of individual blades flying in a circle. Instead, think of the blades as a solid disc just like the rotor on a gyroscope. And, like a gyroscope the rotor system has rigidity in space and a phase angle of 90-degrees. If you apply a force to a gyro that is on gimbals, the gyro because of precession will react 90 degrees later in the direction of rotation. The same is true on a helicopter. Cyclic input will change the pitch relationship across the disc and this will result in an imbalance of forces. If the pilot pushes the stick forward, the greater force is over the left side of the disc. This upward force (due to precession) will cause the disc to raise over the tail and drop over the nose. Aerodynamics plays a minimal part in this action. The change in disc position was caused
by the change in lift forces but the actual movement was caused by the gyroscopic forces and characteristics of the spinning disc.
On two blade rotor system the pitch horn leads the blade by 90-degrees. On this type of rotor system the swashplate tips down over the nose and the pitch horn introduces the maximum pitch input while the blades are over the lateral axis. With this 90-degree disposition of the pitch horn the phase angle will cause the blade to dip down over the nose and the helicopter flies forward.
On multi blade systems the pitch horn leads the blades by 45-degrees and the swashplate tips 45-degrees ahead of the direction of flight. 45 + 45 = 90 degree phase angle and the same thing happens. The helicopter flies forward.
Individual blades do not fly to a position, The disc is moved by gyroscopic forces. The pitch change of the individual blades simply change the balance of forces across the disc and physics does the rest.
Now, we get to autogyros. There are several types but I will address the two blade and three blade variety. On the two blade type the blade assembly (There is no individual pitch change) is tilted. This in effect decreases the pitch on one blade and increases the pitch on the opposing blade. This, like on the helicopter, creates a differential of lift across the blade assembly and this differential of forces triggers, guess what, gyroscopic precession and the disc tilts but it tilts a limited amount so that it is almost parallel with the direction of flight.
On a multi blade autogyro the rotor head is fully articulated and pitch input is exactly the same as on a helicopter with the same restrictions as on a two blade system. Limited forward tilt.
The closest thing to an autogyro that was a helicopter was the Cheyenne. In flight the helicopter was powered by a propeller in the rear and most of the lift was provided by wings. The spinning rotor provided some of the lift but it was mainly used to control the direction and attitude of the helicopter.
What it boils down to is that both the helicopter and autogyro operate under the same physical laws relative to gyroscopic precession.
[This message has been edited by Lu Zuckerman (edited 05 December 2000).]
