Flight Dynamics: The Swashplate and Phase-angle
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To Dave
Dave
you quote
'it now appears that the R-44 (coning) coupling is the opposite of convention'
Interesting is the word "appears", what do you imply with this word choice...
The other part of the information was that :
'this system is extremely well behaving dynamically with virtualy no overshoot and a full steady state within 1,4 revs ( =0.2 sec approx), in fact almost a plane linear amplifier'.
In plane language that means a mechanical lever.
As a control system quite good one I would say. Many hydraulic and other steering systems (including pure mechanical ones) do not behave that well.
To Chiplight
your comments are right : if well perfomed this maneuver should not increase flapping. Cran explaned that pilots may instinctively do the wrong thing namely : they might not 'steer in the wind' (cfr also for example previous comments Nick made on the LTE spin shown in the skycrane movie. Apparently you seem to find it natural to steer in the wind, but I am afraid not everyone will do this instinctively. Mind also that these blowbacks can be very brutal, and if not anticipated, correcting may be too late and if late large flapping will result (and this has nothing to see with the R22/R44 but effects in particular all theatering rotors).
To Lu
Can you give me (or help me get) the Georgia Tech report. That can help me assess the (flight path) scenario (you can pm me this)
delta3
you quote
'it now appears that the R-44 (coning) coupling is the opposite of convention'
Interesting is the word "appears", what do you imply with this word choice...
The other part of the information was that :
'this system is extremely well behaving dynamically with virtualy no overshoot and a full steady state within 1,4 revs ( =0.2 sec approx), in fact almost a plane linear amplifier'.
In plane language that means a mechanical lever.
As a control system quite good one I would say. Many hydraulic and other steering systems (including pure mechanical ones) do not behave that well.
To Chiplight
your comments are right : if well perfomed this maneuver should not increase flapping. Cran explaned that pilots may instinctively do the wrong thing namely : they might not 'steer in the wind' (cfr also for example previous comments Nick made on the LTE spin shown in the skycrane movie. Apparently you seem to find it natural to steer in the wind, but I am afraid not everyone will do this instinctively. Mind also that these blowbacks can be very brutal, and if not anticipated, correcting may be too late and if late large flapping will result (and this has nothing to see with the R22/R44 but effects in particular all theatering rotors).
To Lu
Can you give me (or help me get) the Georgia Tech report. That can help me assess the (flight path) scenario (you can pm me this)
delta3
Last edited by delta3; 29th Nov 2004 at 22:13.
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This was posted earlier today on a thread which has now disappeared.
I thought I'd post it for new members who may not know Nick's expertise and experience.
It comes from the FAA website:
I thought I'd post it for new members who may not know Nick's expertise and experience.
It comes from the FAA website:
NICK LAPPOS is the VXX Program Director for Sikorsky, responsible for the design, production, proposal and customer interface for the next generation Presidential helicopter bid.
Under his previous position as Director of S/H-92 Programs, the S-92 achieved FAA certification and was awarded the 2002 Robert J. Collier Trophy.
Nick was a Test Pilot for Sikorsky for over 25 years, and has flown over 70 different helicopter types. He was the chief R&D test pilot for over 12 years, and was Director of Test Engineering for Sikorsky in 1999 and 2000.
He flew the first flights of many experimental aircraft, including the S-76, the Shadow, Fantail and Fly-by-Wire technology demonstrators. He participated in the development of the Black Hawk, the Super Stallion, the XH-59A Advancing Blade Concept (ABC), and the RAH-66 Comanche, and has over 7,500 hours of flight time, including 950 hours of combat and 2,000 hours of Flight Test experience.
Nick graduated a Dean's List Aerospace Engineering Graduate of the Georgia Institute of Technology, and has recently been appointed to their Academy of Distinguished Engineering Alumni. He is a Fellow of the American Helicopter Society and has received their Feinberg Award as most outstanding pilot three times, once individually and twice as member of pilot teams. Nick is a Member of the Society of Experimental Test Pilots, and has received their Tenhoff Award.
He holds 16 U.S. patents on flight and engine controls and cockpit displays, and three FAI recognized helicopter world records. He has authored a number of technical papers for the AHS, SAE and has written a number of articles for Interavia, Rotor and Wing and Defense Helicopter magazines. He is a US Army Vietnam veteran and flew Cobra attack helicopters.
Under his previous position as Director of S/H-92 Programs, the S-92 achieved FAA certification and was awarded the 2002 Robert J. Collier Trophy.
Nick was a Test Pilot for Sikorsky for over 25 years, and has flown over 70 different helicopter types. He was the chief R&D test pilot for over 12 years, and was Director of Test Engineering for Sikorsky in 1999 and 2000.
He flew the first flights of many experimental aircraft, including the S-76, the Shadow, Fantail and Fly-by-Wire technology demonstrators. He participated in the development of the Black Hawk, the Super Stallion, the XH-59A Advancing Blade Concept (ABC), and the RAH-66 Comanche, and has over 7,500 hours of flight time, including 950 hours of combat and 2,000 hours of Flight Test experience.
Nick graduated a Dean's List Aerospace Engineering Graduate of the Georgia Institute of Technology, and has recently been appointed to their Academy of Distinguished Engineering Alumni. He is a Fellow of the American Helicopter Society and has received their Feinberg Award as most outstanding pilot three times, once individually and twice as member of pilot teams. Nick is a Member of the Society of Experimental Test Pilots, and has received their Tenhoff Award.
He holds 16 U.S. patents on flight and engine controls and cockpit displays, and three FAI recognized helicopter world records. He has authored a number of technical papers for the AHS, SAE and has written a number of articles for Interavia, Rotor and Wing and Defense Helicopter magazines. He is a US Army Vietnam veteran and flew Cobra attack helicopters.
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Nick,
"Just because he confused you long enough to reverse your understanding of delta three "
No, no, no. That ain't so. I was referring to a discussion between you and I, which we had on the thread Topic: Delta-3 & Phase Lag In this thread, we disagree on how the phase-lag interfaces with a delta3 teetering rotor.
At the risk of regurgitating old fodder
, I still firmly believe that a delta3 teetering rotor / phase-lag relationship cannot be equated with a rigid rotor / phase-lag relationship. They are functionally different.
If one or both of us do not fully understand the activity of teetering delta3 c/w phase-lag, is it not realistic for Lu to have his concerns?
Delta3,
"'it now appears that the R-44 (coning) coupling is the opposite of convention'"
"Interesting is the word "appears", what do you imply with this word choice..."
Burkhard Domke's picture shows that the pitch link is located between the coning hinge and the teetering hinge. From the picture and Robinson's CCW rotation, teetering will subject the blade to a negative pitch change but coning will subject the blade to a positive pitch change. This is how it appears in the picture; but what if Burkhart inadvertently put the negative in wrong-way-up. Then the pitch link would be on the trailing edge, and the delta3 activity would appear differently.
Joking aside. It is an observation of interest, intended to instigate an interesting technical discussion and expand the knowledge of the participants.
"The other part of the information was that :
'this system is extremely well behaving dynamically with virtualy no overshoot and a full steady state within 1,4 revs ( =0.2 sec approx), in fact almost a plane linear amplifier'."
May I ask where this other information can be found? Is it related to the actual helicopter or to the simulator? Thanks.
"Just because he confused you long enough to reverse your understanding of delta three "
No, no, no. That ain't so. I was referring to a discussion between you and I, which we had on the thread Topic: Delta-3 & Phase Lag In this thread, we disagree on how the phase-lag interfaces with a delta3 teetering rotor.
At the risk of regurgitating old fodder
, I still firmly believe that a delta3 teetering rotor / phase-lag relationship cannot be equated with a rigid rotor / phase-lag relationship. They are functionally different.If one or both of us do not fully understand the activity of teetering delta3 c/w phase-lag, is it not realistic for Lu to have his concerns?
Delta3,
"'it now appears that the R-44 (coning) coupling is the opposite of convention'"
"Interesting is the word "appears", what do you imply with this word choice..."
Burkhard Domke's picture shows that the pitch link is located between the coning hinge and the teetering hinge. From the picture and Robinson's CCW rotation, teetering will subject the blade to a negative pitch change but coning will subject the blade to a positive pitch change. This is how it appears in the picture; but what if Burkhart inadvertently put the negative in wrong-way-up. Then the pitch link would be on the trailing edge, and the delta3 activity would appear differently.
Joking aside. It is an observation of interest, intended to instigate an interesting technical discussion and expand the knowledge of the participants.
"The other part of the information was that :
'this system is extremely well behaving dynamically with virtualy no overshoot and a full steady state within 1,4 revs ( =0.2 sec approx), in fact almost a plane linear amplifier'."
May I ask where this other information can be found? Is it related to the actual helicopter or to the simulator? Thanks.
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Your grandmother wears combat boots. So there.
To: Anyone that might still be interested in what I have to say.
NICK SEZ: Lu Zuckerman is a dumb schmuck and doesn't know what he is talking about and If I keep on saying it soon everyone will believe me.
LU SEZ: Everyone knows I am a dumb schmuck. My brothers have been telling me that for years. Now, when are you going to answer my questions.
NICK SEZ: Lu Zuckerman is a dumb schmuck and doesn't know what he is talking about and If I keep on saying it soon everyone will believe me.
LU SEZ: Everyone knows I am a dumb schmuck. My brothers have been telling me that for years. Now, when are you going to answer my questions.
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Lawdy, here we are again.
For the benefit of young and impressionable Robinson pilots who have not shared our repeated rambles down this cul-de-sac over the aeons, a few facts:
Lu Zuckerman cannot fly and has no helicopter design experience or qualifications.
He cannot read very well, if the evidence of his repeated misquoting of sources is anything to go by, but unfortunately he can write, after a fashion.
He is impervious to reason, to fact, and in particular, to ridicule.
He is not fit to polish the boots of Nick Lappos, who aside from being one of the world's top helicopter test pilots and design experts is extremely charitable with his time in answering the legitimate questions of mere mortals in this forum.
In the beginning, when Zuckerman first posited his fantasies here, Frank Robinson agreed to come onto pprune to explain where Zuckerman had got his theories twisted around his head.
Zuckerman's strategy is to wait until he hopes the matter is forgotten and start again from the discredited beginning. Thus he is able to worry a lot of new people.
The kind of help that Mr Zuckerman sorely needs is not available on this forum.
For the benefit of young and impressionable Robinson pilots who have not shared our repeated rambles down this cul-de-sac over the aeons, a few facts:
Lu Zuckerman cannot fly and has no helicopter design experience or qualifications.
He cannot read very well, if the evidence of his repeated misquoting of sources is anything to go by, but unfortunately he can write, after a fashion.
He is impervious to reason, to fact, and in particular, to ridicule.
He is not fit to polish the boots of Nick Lappos, who aside from being one of the world's top helicopter test pilots and design experts is extremely charitable with his time in answering the legitimate questions of mere mortals in this forum.
In the beginning, when Zuckerman first posited his fantasies here, Frank Robinson agreed to come onto pprune to explain where Zuckerman had got his theories twisted around his head.
Zuckerman's strategy is to wait until he hopes the matter is forgotten and start again from the discredited beginning. Thus he is able to worry a lot of new people.
The kind of help that Mr Zuckerman sorely needs is not available on this forum.
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really ?, oops i didn't know Nicklappos is an international famous expert, really.. damn i lost any chance of getting helped...
sorry mr lappos, i didnt mean to upset you.. sorry.. want a candy ?
if only the ping pong could be instructive (humour)
Dave : i recently saw somthing on your site, a rotorhub, that looks like something i am doing ? is it pure hasard ?
cheers
sorry mr lappos, i didnt mean to upset you.. sorry.. want a candy ?
if only the ping pong could be instructive (humour)
Dave : i recently saw somthing on your site, a rotorhub, that looks like something i am doing ? is it pure hasard ?
cheers
Last edited by zeeoo; 30th Nov 2004 at 17:53.
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sideslip
Nick sez:
Question: why will the teetering rotor flap "wildly" in a sideslip?
As far as I know, the rotor doesn't care what direction its flying in. In forward flight in a R22 at close to 100 kts, the flapping is only a few degrees. There is still plenty of stick travel left with which to control the aircraft. In a full on sideslip, the airspeed won't be anywhere near as great, and the flapping will be all that much less. The only difference is that it will be lateral flapping.
The FAA says:
This does not sound like wild flapping to me. The simple realization that some lateral flapping of a couple of degrees will occur in a sideslip has been overblown to where any amount of out of trim is red flagged as a big no-no that is going to slice your tail boom off.
I don't think so.
The FAA aslo says
http://www.faa.gov/certification/air.../ASW-95-01.htm
Trouble is, low g does not increase flapping as far as I know. A teetering rotor requires a load in order to produce lift. The lift and load (weight) are a force pair. Neither one exists without the other. Remove the load(zero g) and the lift is zero. Thus there is no control since the rotor can only pull on the airframe, (it cannot produce torque thru a teeter joint.)
I would expect minimal flapping when the lift is minimal on advancing and retreating sides of the rotor.
Flapping is only a problem if the rotor speed decays (which it well might in a pushover)and then the load is reapplied. Flapping will increase as the rpm decreases.
The real issue in low g is that the tail rotor thrust will roll an R22 and application of cyclic will not stop the roll due to the loss of rotor thrust. The rotor will tilt in response to cyclic because it is still flying, but tilting the rotor is not enough, you need thrust.
No helicopter that I know except Comanche can easily withstand this enormous pedal input and retain control or rotor structural integrity, and no helicopter is required to perform that maneuver. Teetering rotors will flap wildly, and will perhaps mast bump, with disasterous results
Question: why will the teetering rotor flap "wildly" in a sideslip?
As far as I know, the rotor doesn't care what direction its flying in. In forward flight in a R22 at close to 100 kts, the flapping is only a few degrees. There is still plenty of stick travel left with which to control the aircraft. In a full on sideslip, the airspeed won't be anywhere near as great, and the flapping will be all that much less. The only difference is that it will be lateral flapping.
The FAA says:
Sideslip creates lateral flapping in excess of that encountered during normal flight. This excess flapping allows less margin for lateral cyclic maneuvering in response to a low g induced roll.
I don't think so.
The FAA aslo says
http://www.faa.gov/certification/air.../ASW-95-01.htm
Mast bumping may occur with a teetering rotor system when excessive main rotor flapping results from low g (load factor below 1.0) or abrupt control input.
I would expect minimal flapping when the lift is minimal on advancing and retreating sides of the rotor.
Flapping is only a problem if the rotor speed decays (which it well might in a pushover)and then the load is reapplied. Flapping will increase as the rpm decreases.
The real issue in low g is that the tail rotor thrust will roll an R22 and application of cyclic will not stop the roll due to the loss of rotor thrust. The rotor will tilt in response to cyclic because it is still flying, but tilting the rotor is not enough, you need thrust.
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To Chiplight
I quote you:
"Flapping , in a teetering rotor, causes the rotor tip-path plane to tilt away(flap back) from the spin axis and is the result of differing airspeeds on adv/retreating blades."
I would agree with this definition (this are the beta's in my simulator)
If the main rotor axis is not perpendicular to the required tip plane for the given flight path than flapping occurs. Flapping has two effects : the first order is it tries to "do as if the main rotor is tilted". A visualisaton aid is : think of the heli in hover and change the load and balance (aft and back), the cyclic will try to 'replicate' a fixed angle of attack in the horizontal tip path.
However inertia does not like the rotor to go away from the perpendicular so it takes extra effort to get the tip path angle, and thus different angle of attacks (remember centrifugal forces are at least 10 bigger than lift).
A well designed heli (tail wing etc) will probably have the main rotor more or less perpendicular in 'most common' (forward) flight path scenario's (that I found is the case in the R44 at least in my simulator). 50 knots lateral is not a common flight path. As I said I did not simultate this one yet exactly (perhaps spending too much time on this forum..) but it appears unlikely if I saw how much balancing (wings etc) it takes to get this sort of behaviour in forward flight (that I fooled around with a lot so far)
If you are still with me... than this means that more ore less important changes in flapping will occur (this does not mean the rotor is all over the place, just refering to the above definition). It is the changes in these directions that are important, not their steady states. Not anticipating them can/will create brutal blows (and this will create all over the place situations, because the whole heli gets involved). This is quite different from a gradual accelerated lateral flight (mind tail rotor vortices in this case see different thread)
Delta3 trying to make sense...
I quote you:
"Flapping , in a teetering rotor, causes the rotor tip-path plane to tilt away(flap back) from the spin axis and is the result of differing airspeeds on adv/retreating blades."
I would agree with this definition (this are the beta's in my simulator)
If the main rotor axis is not perpendicular to the required tip plane for the given flight path than flapping occurs. Flapping has two effects : the first order is it tries to "do as if the main rotor is tilted". A visualisaton aid is : think of the heli in hover and change the load and balance (aft and back), the cyclic will try to 'replicate' a fixed angle of attack in the horizontal tip path.
However inertia does not like the rotor to go away from the perpendicular so it takes extra effort to get the tip path angle, and thus different angle of attacks (remember centrifugal forces are at least 10 bigger than lift).
A well designed heli (tail wing etc) will probably have the main rotor more or less perpendicular in 'most common' (forward) flight path scenario's (that I found is the case in the R44 at least in my simulator). 50 knots lateral is not a common flight path. As I said I did not simultate this one yet exactly (perhaps spending too much time on this forum..) but it appears unlikely if I saw how much balancing (wings etc) it takes to get this sort of behaviour in forward flight (that I fooled around with a lot so far)
If you are still with me... than this means that more ore less important changes in flapping will occur (this does not mean the rotor is all over the place, just refering to the above definition). It is the changes in these directions that are important, not their steady states. Not anticipating them can/will create brutal blows (and this will create all over the place situations, because the whole heli gets involved). This is quite different from a gradual accelerated lateral flight (mind tail rotor vortices in this case see different thread)
Delta3 trying to make sense...
Last edited by delta3; 30th Nov 2004 at 20:21.
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Delta3,
you said
I think that is a good point,(but I don't know about the brutal part)
I'd be curious what your simulations predict for an R22 in a 30 kt sideways condition, in terms of amount of flapping. Remember, delta-3 will reduce flapping.
(This page
has flapping calculations)
Thanks
you said
It is the changes in these directions that are important, not their steady states. Not anticipating them can/will create brutal blows...This is quite different from a gradual accelerated lateral flight
I'd be curious what your simulations predict for an R22 in a 30 kt sideways condition, in terms of amount of flapping. Remember, delta-3 will reduce flapping.
(This page
has flapping calculations)
Thanks
Last edited by Chiplight; 30th Nov 2004 at 22:32.
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Which way am I flying asked the R-22 of the R-44.
In the last few posts several of you have questioned the possibility of extreme flapping due to sideslipping or flying out of trim. Several statements border on stating that these conditions would not result in flapping extremes.
Who is correct? The proponents of no flapping on this forum, or the professors at Georgia Tech. The FAA sided with Georgia Tech and issued a priority letter telling Robinson to enter cautions in the respective POHs relative to sideslipping and flying out of trim.
Now, let’s forget about whom is right or wrong but please explain the evidence of wild flapping extremes on recovered rotorheads after a loss of control incident. The blade tusks hit the droop stops with such force that they fractured. This would not occur in a zero G incident (tusk fracture) if it were simple mast bumping and resulting mast fracture. In either case there must be a significant energy exchange to result in the mast fracturing or the blade tusks fracturing and what causes this? Blade flapping. Now we must determine the cause of the flapping.
Who is correct? The proponents of no flapping on this forum, or the professors at Georgia Tech. The FAA sided with Georgia Tech and issued a priority letter telling Robinson to enter cautions in the respective POHs relative to sideslipping and flying out of trim.
Now, let’s forget about whom is right or wrong but please explain the evidence of wild flapping extremes on recovered rotorheads after a loss of control incident. The blade tusks hit the droop stops with such force that they fractured. This would not occur in a zero G incident (tusk fracture) if it were simple mast bumping and resulting mast fracture. In either case there must be a significant energy exchange to result in the mast fracturing or the blade tusks fracturing and what causes this? Blade flapping. Now we must determine the cause of the flapping.
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Chiplight,
In short, "Flapping wildly" is when the rotor bumps the mast and the head embarassingly flies off. I think the rotor removing itself helps qualify it as a wild flapping event. Perhaps you disagree. Work your words as you wish, it is fairly simple to the rest of us. Your cross checking of random phrases from various sources pulled out of context will not add to your understanding of how a teetering rotor behaves, any more than listening to Lu will help you.
Low G does not produce flapping, the pilot produces the flapping at low G when the reduced control from the main rotor allows the aircraft to start rolling upside down. In his foolish wish to stay right side up, he puts an ineffective roll cyclic command in and the rest looks to the rotor like wild flapping.
The issue is contained in the concept that teetering rotors have no control over the helicopter when there is no rotor thrust. All other types of rotor, like articulated rotors, produce control even when there is no thrust and thus allow the aircraft to stay in cyclic control. This is where the typical training handbook is quite wrong when they show the thrust being tilted by the rotor and producing control. In previous posts I explained this concept, and would be glad to repeat it, with some illustrations.
Sideslip is a special case, the teetering rotor produces excessive (wild) flapping because the pair of blades experiences strong changes in its equivilent trimmed cyclic and its relative velocity while it also experiences reduced aerodynamic damping. This leads to excessive flapping and mast contact in relatively mild sideslip maneuvers, especially if the sideslip is accompanied by strong cyclic maneuver demands.
Nick
PS I had 1,000 hours on teetering rotors and was a Standardization Instructor on them before this property was discovered. One of the Cobras in my unit in Vietnam lost its main rotor during a gun run due to these problems.
In short, "Flapping wildly" is when the rotor bumps the mast and the head embarassingly flies off. I think the rotor removing itself helps qualify it as a wild flapping event. Perhaps you disagree. Work your words as you wish, it is fairly simple to the rest of us. Your cross checking of random phrases from various sources pulled out of context will not add to your understanding of how a teetering rotor behaves, any more than listening to Lu will help you.
Low G does not produce flapping, the pilot produces the flapping at low G when the reduced control from the main rotor allows the aircraft to start rolling upside down. In his foolish wish to stay right side up, he puts an ineffective roll cyclic command in and the rest looks to the rotor like wild flapping.
The issue is contained in the concept that teetering rotors have no control over the helicopter when there is no rotor thrust. All other types of rotor, like articulated rotors, produce control even when there is no thrust and thus allow the aircraft to stay in cyclic control. This is where the typical training handbook is quite wrong when they show the thrust being tilted by the rotor and producing control. In previous posts I explained this concept, and would be glad to repeat it, with some illustrations.
Sideslip is a special case, the teetering rotor produces excessive (wild) flapping because the pair of blades experiences strong changes in its equivilent trimmed cyclic and its relative velocity while it also experiences reduced aerodynamic damping. This leads to excessive flapping and mast contact in relatively mild sideslip maneuvers, especially if the sideslip is accompanied by strong cyclic maneuver demands.
Nick
PS I had 1,000 hours on teetering rotors and was a Standardization Instructor on them before this property was discovered. One of the Cobras in my unit in Vietnam lost its main rotor during a gun run due to these problems.
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Dynamics versus steady state
Chiplight,
I looked at the mensioned equations and recognise them, but these are steady state solutions, they do not explain the transients.
Flapping, yawing, low G, steady state and transients all get being confused in this threat. I give up, this is to difficult to explain without pictures. This is exactly the reason why I am building a precise interactive simulator because it lets you see what happens, based on precise input definitions and not an undefined loose ends. It also lets you look into all variables at any moment. Even I take quite some time to understand all aspects of transients, equilibriums and steady states using the simulator. I'll leave it here and get back to you guys in a few weeks when I'll show you a precisely defined scenario of a sudden yaw.
Lu
you again quote G-Tech, I repeat my question : please help me to get this source, just throwing the name of G-Tech does not really help a lot.
Delta3
I looked at the mensioned equations and recognise them, but these are steady state solutions, they do not explain the transients.
Flapping, yawing, low G, steady state and transients all get being confused in this threat. I give up, this is to difficult to explain without pictures. This is exactly the reason why I am building a precise interactive simulator because it lets you see what happens, based on precise input definitions and not an undefined loose ends. It also lets you look into all variables at any moment. Even I take quite some time to understand all aspects of transients, equilibriums and steady states using the simulator. I'll leave it here and get back to you guys in a few weeks when I'll show you a precisely defined scenario of a sudden yaw.
Lu
you again quote G-Tech, I repeat my question : please help me to get this source, just throwing the name of G-Tech does not really help a lot.
Delta3
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Nick,
No one is disputing that low g events can lead to the rotor flying off.
I think you'll notice that I also mentioned the loss of rotor thrust as the cause of loss of control and briefly explained the reason it happens. I like to think my grasp of rotor dynamics goes beyond reading random phrases, but I'll give it to you- that was a good zinger.
Now you're stating that excessive flapping occurs in a sideslip because of "reduced aerodynamic damping" and changes in "equivalent trimmed cyclic "
If this is true, then I need you to spell it out for me, if you would. Why is damping affected by the direction of travel? And what would cyclic trim have to do with flapping? Just asking.
I am able to see delta3's point, and I guess you are saying too- that the flapping is not so much from sideways flight, but from transient stick inputs. After all, anytime the rotor is changing orientation, it is flapping by definition.
If this is the case, then it is a pilot response thing, not an inherent flaw in the teetering rotor head.
LU:
I dug around and I think I found the Georgia Tech report that you keep alluding to.
Actually it is not the report, but a summary of it contained in this NTSB special report
www.ntsb.gov/publictn/1996/SIR9603.pdf
It says there, starting on page 23, that the report was never finished due to funds running out. Also, it was a computer simulation that was never fully validated. It only proved accurate within narrow parameters. Further, they don't seem to have done much in the way of sideslip simulations, because the model wasn't fully developed.
I think you are wasting your time trying to use the Georgia tech report to bolster your arguments.
No one is disputing that low g events can lead to the rotor flying off.
I think you'll notice that I also mentioned the loss of rotor thrust as the cause of loss of control and briefly explained the reason it happens. I like to think my grasp of rotor dynamics goes beyond reading random phrases, but I'll give it to you- that was a good zinger.
Now you're stating that excessive flapping occurs in a sideslip because of "reduced aerodynamic damping" and changes in "equivalent trimmed cyclic "
If this is true, then I need you to spell it out for me, if you would. Why is damping affected by the direction of travel? And what would cyclic trim have to do with flapping? Just asking.
I am able to see delta3's point, and I guess you are saying too- that the flapping is not so much from sideways flight, but from transient stick inputs. After all, anytime the rotor is changing orientation, it is flapping by definition.
If this is the case, then it is a pilot response thing, not an inherent flaw in the teetering rotor head.
LU:
I dug around and I think I found the Georgia Tech report that you keep alluding to.
Actually it is not the report, but a summary of it contained in this NTSB special report
www.ntsb.gov/publictn/1996/SIR9603.pdf
It says there, starting on page 23, that the report was never finished due to funds running out. Also, it was a computer simulation that was never fully validated. It only proved accurate within narrow parameters. Further, they don't seem to have done much in the way of sideslip simulations, because the model wasn't fully developed.
I think you are wasting your time trying to use the Georgia tech report to bolster your arguments.
Last edited by Chiplight; 1st Dec 2004 at 01:19.
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Chiplight,
Please forgive my peevishness, I get testy when the phase of the Lu hits its peak! You are right, I skimmed your post then fired off a response, certainly too hard on you, I fear.
The sideslip in any rotorcraft induces a new freestream angle on the rotor, and thus causes the cyclic trim previously set to be significantly off. Typically, the aircraft sharply rolls away from the sideslip (positive dihedral) and usually pitches strongly although horizontal tail force changes due to sideslip can make this go either way. The retrim is significant, and when coupled with the low control power of a teetering rotor, can lead to large flapping angles. For many helicopters, the largest flapping angles are seen in low speed flight at forward CG. At this condition, the control margins can be most limited, and ripe for a sideslip to create an excessively large trim change.
The damping term is harder to explain, but the concept is that the aerodynamic flap forces on the blade are due to the changes in blade lift due to flap velocity. The response of the blade is softened by the flapping moment of inertia and the hinge offset. The aerodynamic forces in the flap direction are generally positive, where flap upward reduces the local angle of attack and thus reduces the thrust, creating a restoring force. For articulated systems, the blades see this force individually, while a teetering system experiences the flap across a pair of blades. I believe this has significantly higher inertia, and thus lower equivilent damping than a simple articulated blade. Also, the zero hinge offset teetering rotor has less natural damping as well. I have never seen it analytically shown, but typical rotor motions are much less damped than in articulated or "rigid" systems, so the blade motions are larger and require more control to stop them. As an exercise, just impose a rolling reversal on a helicopter, going from say 30 degrees bank in one direction to 30 in the other, fairly rapidly. Note the amount of cyclic required to stop the roll rate. For a Boelkow, there is almost no opposite cyclic required to stop the roll rate (high damping). For an articulated system, perhaps 1/2 inch of opposite cyclic is needed. For a teetering system, it takes a siginficant opposite cyclic to stop the roll, a measure of very weak damping. This means that in maneuvers where large maneuver rates need to be countered, teetering rotors require much more control input (more opposite flapping) to do so.
Please forgive my peevishness, I get testy when the phase of the Lu hits its peak! You are right, I skimmed your post then fired off a response, certainly too hard on you, I fear.
The sideslip in any rotorcraft induces a new freestream angle on the rotor, and thus causes the cyclic trim previously set to be significantly off. Typically, the aircraft sharply rolls away from the sideslip (positive dihedral) and usually pitches strongly although horizontal tail force changes due to sideslip can make this go either way. The retrim is significant, and when coupled with the low control power of a teetering rotor, can lead to large flapping angles. For many helicopters, the largest flapping angles are seen in low speed flight at forward CG. At this condition, the control margins can be most limited, and ripe for a sideslip to create an excessively large trim change.
The damping term is harder to explain, but the concept is that the aerodynamic flap forces on the blade are due to the changes in blade lift due to flap velocity. The response of the blade is softened by the flapping moment of inertia and the hinge offset. The aerodynamic forces in the flap direction are generally positive, where flap upward reduces the local angle of attack and thus reduces the thrust, creating a restoring force. For articulated systems, the blades see this force individually, while a teetering system experiences the flap across a pair of blades. I believe this has significantly higher inertia, and thus lower equivilent damping than a simple articulated blade. Also, the zero hinge offset teetering rotor has less natural damping as well. I have never seen it analytically shown, but typical rotor motions are much less damped than in articulated or "rigid" systems, so the blade motions are larger and require more control to stop them. As an exercise, just impose a rolling reversal on a helicopter, going from say 30 degrees bank in one direction to 30 in the other, fairly rapidly. Note the amount of cyclic required to stop the roll rate. For a Boelkow, there is almost no opposite cyclic required to stop the roll rate (high damping). For an articulated system, perhaps 1/2 inch of opposite cyclic is needed. For a teetering system, it takes a siginficant opposite cyclic to stop the roll, a measure of very weak damping. This means that in maneuvers where large maneuver rates need to be countered, teetering rotors require much more control input (more opposite flapping) to do so.
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Nick, you're forgiven. Thanks for the post. Just as I was finished editing my post, I saw yours. Now I'll have to immediately re-edit- Yikes. Phases of Lu, indeed!
Thanks for the in-depth clarification. If you are comparing teetering rotors to articulated, then I can understand more clearly what you are driving at in terms of damping or lack thereof. Good stuff.
I do think that Robinson's delta-3 must help reduce flapping over what it would be for a rotor system without such geometry, but apparently not enough.
Regards
Thanks for the in-depth clarification. If you are comparing teetering rotors to articulated, then I can understand more clearly what you are driving at in terms of damping or lack thereof. Good stuff.
I do think that Robinson's delta-3 must help reduce flapping over what it would be for a rotor system without such geometry, but apparently not enough.
Regards
Last edited by Chiplight; 1st Dec 2004 at 04:11.