Ditching a helicopter: (incl pictures)
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Phewww!!! Getting a bit off track there guy's! only just found this one but what fun it is.
I dont want to take away anything from anybodies wonderful arguements but one thing is in my mind getting back to the original question.
Sod all this gyrosopic precession bull***t, but if you had to ditch would you really give a toss what was going on with the rotor head / disc / fuselage. Id be out of the bloody thing like a shot and certainly wouldn't hang around to find out who's theories or experiences were correct.
HeliEng - How many times you been ditched and how is yor floatation equipment!! :o
I dont want to take away anything from anybodies wonderful arguements but one thing is in my mind getting back to the original question.
Sod all this gyrosopic precession bull***t, but if you had to ditch would you really give a toss what was going on with the rotor head / disc / fuselage. Id be out of the bloody thing like a shot and certainly wouldn't hang around to find out who's theories or experiences were correct.
HeliEng - How many times you been ditched and how is yor floatation equipment!! :o
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Dave, Heedm, Nick, Lu
Great work once again throwing all the ideas up on the table and letting them battle themselves out.
Being what I would call "slow" I look for the simple proofs to validate an idea or to help me along in the understanding of a concept.
On a bell head asking whether it's a predominantly gyroscopic effect or an aerodynamic one would simply require spinning the aircraft up sans blades. Do the control inputs still have the same efect on the tips of the grips?
The building blocks of understanding that are happening here really are good.
Cheers from the high desert
Great work once again throwing all the ideas up on the table and letting them battle themselves out.
Being what I would call "slow" I look for the simple proofs to validate an idea or to help me along in the understanding of a concept.
On a bell head asking whether it's a predominantly gyroscopic effect or an aerodynamic one would simply require spinning the aircraft up sans blades. Do the control inputs still have the same efect on the tips of the grips?
The building blocks of understanding that are happening here really are good.
Cheers from the high desert
Iconoclast
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To: Tgrendl
“On a bell head asking whether it's a predominantly gyroscopic effect or an aerodynamic one would simply require spinning the aircraft up sans blades. Do the control inputs still have the same effect on the tips of the grips”?
All things being normal there would be no effect, as the grips would not deflect. The same thing would be true for any other helicopter. There is no aerodynamic effect because there are no airfoils. Because there is no aerodynamic effect there is no gyroscopic precession.
Gyroscopic precession is a function of a force being applied to one side of the disc or an imbalance of forces across the disc. In this case the force is a resultant of the differences in aerodynamic lift across the disc. Although the advancing blade has its’ rotational velocity plus the relative wind it has less pitch than the retreating blade. Whatever pitch is taken from the advancing blade it is added to the retreating blade thus it generates more lift even though the relative wind is moving in the same direction. This differential of forces (greater lift) will be applied to the retreating side of the disc and the imbalance of forces causes the disc to raise 90-degrees after the application of the force. This in turn causes the entire disc to dip down over the nose and rise over the tail. Some individuals will look at this as a pure aerodynamic function, which causes the blades to fly to the commanded position, and gyroscopic precession be damned. I will totally believe that theory if someone can convince me that there is a law or rule or theory in aerodynamics that explains why this phenomenon takes place in 90-degrees as opposed to 35-degrees or 125-degrees.
In the case of retreating blade stall the force imbalance is greater on the right (advancing) side and the imbalance is much greater by several orders of magnitude so as to cause the disc to flap / blow back with minimal warning.
To: Dave Jackson
You have a marvelous capacity to evade certain discussions (read argument) by using a lot of engineering platitudes.
[ 21 November 2001: Message edited by: Lu Zuckerman ]
“On a bell head asking whether it's a predominantly gyroscopic effect or an aerodynamic one would simply require spinning the aircraft up sans blades. Do the control inputs still have the same effect on the tips of the grips”?
All things being normal there would be no effect, as the grips would not deflect. The same thing would be true for any other helicopter. There is no aerodynamic effect because there are no airfoils. Because there is no aerodynamic effect there is no gyroscopic precession.
Gyroscopic precession is a function of a force being applied to one side of the disc or an imbalance of forces across the disc. In this case the force is a resultant of the differences in aerodynamic lift across the disc. Although the advancing blade has its’ rotational velocity plus the relative wind it has less pitch than the retreating blade. Whatever pitch is taken from the advancing blade it is added to the retreating blade thus it generates more lift even though the relative wind is moving in the same direction. This differential of forces (greater lift) will be applied to the retreating side of the disc and the imbalance of forces causes the disc to raise 90-degrees after the application of the force. This in turn causes the entire disc to dip down over the nose and rise over the tail. Some individuals will look at this as a pure aerodynamic function, which causes the blades to fly to the commanded position, and gyroscopic precession be damned. I will totally believe that theory if someone can convince me that there is a law or rule or theory in aerodynamics that explains why this phenomenon takes place in 90-degrees as opposed to 35-degrees or 125-degrees.
In the case of retreating blade stall the force imbalance is greater on the right (advancing) side and the imbalance is much greater by several orders of magnitude so as to cause the disc to flap / blow back with minimal warning.
To: Dave Jackson
You have a marvelous capacity to evade certain discussions (read argument) by using a lot of engineering platitudes.
[ 21 November 2001: Message edited by: Lu Zuckerman ]
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With apologies to connavar, and probably many others.
_________
tgrendl
[(aerodynamic) vertical thrust] = [(dynamic) impressed angular momentum]
Without the blades, there is no means of applying the above to the rotor.
This is the story;
The aerodynamicist will collect a large number of variable, such as; pitch, angle of attack, RRPM, forward velocity, air density, chord, airfoil profile centrifugal force etc., etc. etc. He will then calculate [the height that the blade will fly to], at the azimuths of interest.
The dynamicist will simply come along and steal from the aerodynamicist [the blades vertical thrust at all azimuths], which is a byproduct of the aerodynamicist's hard work. He will then blend this thrust into the rotor's rotational momentum, which he also stole from the aerodynamicist, and then calculate [the height that the blade will be forced to] at the azimuths of interest.
[ 22 November 2001: Message edited by: Dave Jackson ]
_________
tgrendl
[(aerodynamic) vertical thrust] = [(dynamic) impressed angular momentum]
Without the blades, there is no means of applying the above to the rotor.
This is the story;
The aerodynamicist will collect a large number of variable, such as; pitch, angle of attack, RRPM, forward velocity, air density, chord, airfoil profile centrifugal force etc., etc. etc. He will then calculate [the height that the blade will fly to], at the azimuths of interest.
The dynamicist will simply come along and steal from the aerodynamicist [the blades vertical thrust at all azimuths], which is a byproduct of the aerodynamicist's hard work. He will then blend this thrust into the rotor's rotational momentum, which he also stole from the aerodynamicist, and then calculate [the height that the blade will be forced to] at the azimuths of interest.
[ 22 November 2001: Message edited by: Dave Jackson ]
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Lu said
>"You have a marvelous capacity to evade certain discussions (read argument) by using a lot of engineering platitudes."<
You can't have it both ways!!!!!!
Either;
Accept an overly simplified explanation ~ by blind faith.
~or~
Understand the mathematical proof ~ by reading and comprehending.
Your choice.
[ 22 November 2001: Message edited by: Dave Jackson ]
>"You have a marvelous capacity to evade certain discussions (read argument) by using a lot of engineering platitudes."<
You can't have it both ways!!!!!!
Either;
Accept an overly simplified explanation ~ by blind faith.
~or~
Understand the mathematical proof ~ by reading and comprehending.
Your choice.
[ 22 November 2001: Message edited by: Dave Jackson ]
Iconoclast
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To: Dave Jackson
It is very easy to prove aerodynamic precession by using a Bell analogy indicating that in 180-degrees of rotation the rotor has aligned itself with the swash plate. Now, try to justify that theory by addressing a multi blade rotor system where the rotor never aligns itself with the swashplate.
In the Bell analogy the pitch horn leads the blade by 90-degrees and you can justify the theory since when the swashplate is tipped down over the longitudinal centerline the advancing blade is disposed over the lateral centerline which is 90-degrees ahead of the longitudinal centerline. On the Robinson when the swashplate is tipped down over the longitudinal centerline with forward pitch the advancing blade is not over the lateral centerline but it is advanced 18-degrees ahead of the lateral centerline when it has maximum forward pitch input. Now, address this using aerodynamic precession.
As you said, you can’t have it both ways.
It is very easy to prove aerodynamic precession by using a Bell analogy indicating that in 180-degrees of rotation the rotor has aligned itself with the swash plate. Now, try to justify that theory by addressing a multi blade rotor system where the rotor never aligns itself with the swashplate.
In the Bell analogy the pitch horn leads the blade by 90-degrees and you can justify the theory since when the swashplate is tipped down over the longitudinal centerline the advancing blade is disposed over the lateral centerline which is 90-degrees ahead of the longitudinal centerline. On the Robinson when the swashplate is tipped down over the longitudinal centerline with forward pitch the advancing blade is not over the lateral centerline but it is advanced 18-degrees ahead of the lateral centerline when it has maximum forward pitch input. Now, address this using aerodynamic precession.
As you said, you can’t have it both ways.
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Lu posted 21 November 2001 19:39
>"Some individuals will look at this as a pure aerodynamic function...... I will totally believe that theory if someone can convince me "<
Lu posted 21 November 2001 23:19
>"It is very easy to prove aerodynamic precession"<
[ 21 November 2001: Message edited by: Dave Jackson ]
>"Some individuals will look at this as a pure aerodynamic function...... I will totally believe that theory if someone can convince me "<
Lu posted 21 November 2001 23:19
>"It is very easy to prove aerodynamic precession"<
[ 21 November 2001: Message edited by: Dave Jackson ]
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To: Dave Jackson
>"Some individuals will look at this as a pure aerodynamic function...... I will totally believe that theory if someone can convince me "<
This is what I said:
Some individuals will look at this as a pure aerodynamic function, which causes the blades to fly to the commanded position, and gyroscopic precession be damned. I will totally believe that theory if someone can convince me that there is a law or rule or theory in aerodynamics that explains why this phenomenon takes place in 90-degrees as opposed to 35-degrees or 125-degrees.
>"It is very easy to prove aerodynamic precession"<
This is what I said:
It is very easy to prove aerodynamic precession by using a Bell analogy indicating that in 180-degrees of rotation the rotor has aligned itself with the swash plate. Now, try to justify that theory by addressing a multi blade rotor system where the rotor never aligns itself with the swashplate.
No more evasion. Please address my comments in detail.
>"Some individuals will look at this as a pure aerodynamic function...... I will totally believe that theory if someone can convince me "<
This is what I said:
Some individuals will look at this as a pure aerodynamic function, which causes the blades to fly to the commanded position, and gyroscopic precession be damned. I will totally believe that theory if someone can convince me that there is a law or rule or theory in aerodynamics that explains why this phenomenon takes place in 90-degrees as opposed to 35-degrees or 125-degrees.
>"It is very easy to prove aerodynamic precession"<
This is what I said:
It is very easy to prove aerodynamic precession by using a Bell analogy indicating that in 180-degrees of rotation the rotor has aligned itself with the swash plate. Now, try to justify that theory by addressing a multi blade rotor system where the rotor never aligns itself with the swashplate.
No more evasion. Please address my comments in detail.
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Dave said, The aerodynamicist will collect a large number of variable, such as; pitch, angle of attack, RRPM, forward velocity, air density, chord, airfoil profile centrifugal force etc., etc. etc. He will then calculate [the height that the blade will fly to], at the azimuths of interest.
Thank you. We agree. Since NONE of our discussions mentioned pitch, angle of attack, forward velocity, air density, chord, or airfoil profile then obviously we weren't discussing this from an aerodynamics point of view.
Aerodynamics is "The dynamics of bodies moving relative to gases, especially the interaction of moving objects with the atmosphere." (The American Heritage® Dictionary of the English Language, Fourth Edition)
We weren't discussing anything about aerodynamics. What we were talking about were the effects of forces that we all took for granted, forces that are generated aerodynamically.
Dave said, "Why talk about "impressed angular momentum' when one can eliminate the middlemen, go right to the source and just say 'thrust'?" Because the two are different. You add thrust to angular momentum the same way you add oranges to anecdotes...you can't. Besides, thrust is an intermediary. The thrust produced by the blade is mostly used to hold the helicopter off the ground. A small part of the thrust is used to create a moment on the blade which imparts angular momentum to that blade, which is summed with the angular momentum the blade already has, .... are you getting it now?
Dave said, "There is only one force that is of interest. It is the only one that is variable and it is aerodynamic. The "original angular momentum" is an uninteresting constant. Or at least the RRPM better be a constant or there are serious problems."
You may find it uninteresting, but just like high school geometry, you can't do without it. The mass of planets doesn't change considerably, but if NASA ignored that uninteresting constant, those rockets just wouldn't get anywhere.
Also, there is not only one force that is variable. Drag is variable. Weight is variable (remember weight is a vector, it's direction changes as the attitude of the blade and/or helicopter changes). Depending on how you choose to label all the interactions the rotors have with their environment, there can be many more changing forces. Of course, it doesn't matter how you label them, they all have to be considered no matter how much they interest you.
Yes, we do agree on most of this stuff. You still seem to hold aerodynamic precession as a very special concept. I'm not sure exactly what your theory is, and as Lu was wondering, how it explains what the apparent lag is. A more general theory from a dynamics point of view (keep in mind that aerodynamics is a subset of dynamics) does explain that apparent lag, and it doesn't need to know the air density to explain it.
_______________________
Lu said, "I will totally believe that theory if someone can convince me that there is a law or rule or theory in aerodynamics that explains why this phenomenon takes place in 90-degrees as opposed to 35-degrees or 125-degrees."
Read that long post on the Gyroscopic Precession thread that I keep mentioning. The reason it would happen is written there. BTW, the reason for it is neither aerodynamic nor gyroscopic.
_______________________
Connavar you're right. Don't think about this kind of stuff when you're ditching. Right now, I doubt if any of us involved in this conversation are ditching, so I guess that's okay with you.
I ditched once. We got out like a shot, never got wet, and the helicopter slowly sank. We sat back, thought about it, and salvaged a helicopter worth at least $20 million dollars. I guess thinking does pay off. Hey, my employer still hasn't paid us for that salvage.
_________________
Thomas coupling if you're having problems with the big words I'll be happy to translate for you.
You also asked us to lighten up. Okay.....a three legged dog walks into a bar and says, "I'm lookin' for the man who shot my paw."
Matthew.
[ 22 November 2001: Message edited by: heedm ]
[ 22 November 2001: Message edited by: heedm ]
Thank you. We agree. Since NONE of our discussions mentioned pitch, angle of attack, forward velocity, air density, chord, or airfoil profile then obviously we weren't discussing this from an aerodynamics point of view.
Aerodynamics is "The dynamics of bodies moving relative to gases, especially the interaction of moving objects with the atmosphere." (The American Heritage® Dictionary of the English Language, Fourth Edition)
We weren't discussing anything about aerodynamics. What we were talking about were the effects of forces that we all took for granted, forces that are generated aerodynamically.
Dave said, "Why talk about "impressed angular momentum' when one can eliminate the middlemen, go right to the source and just say 'thrust'?" Because the two are different. You add thrust to angular momentum the same way you add oranges to anecdotes...you can't. Besides, thrust is an intermediary. The thrust produced by the blade is mostly used to hold the helicopter off the ground. A small part of the thrust is used to create a moment on the blade which imparts angular momentum to that blade, which is summed with the angular momentum the blade already has, .... are you getting it now?
Dave said, "There is only one force that is of interest. It is the only one that is variable and it is aerodynamic. The "original angular momentum" is an uninteresting constant. Or at least the RRPM better be a constant or there are serious problems."
You may find it uninteresting, but just like high school geometry, you can't do without it. The mass of planets doesn't change considerably, but if NASA ignored that uninteresting constant, those rockets just wouldn't get anywhere.
Also, there is not only one force that is variable. Drag is variable. Weight is variable (remember weight is a vector, it's direction changes as the attitude of the blade and/or helicopter changes). Depending on how you choose to label all the interactions the rotors have with their environment, there can be many more changing forces. Of course, it doesn't matter how you label them, they all have to be considered no matter how much they interest you.
Yes, we do agree on most of this stuff. You still seem to hold aerodynamic precession as a very special concept. I'm not sure exactly what your theory is, and as Lu was wondering, how it explains what the apparent lag is. A more general theory from a dynamics point of view (keep in mind that aerodynamics is a subset of dynamics) does explain that apparent lag, and it doesn't need to know the air density to explain it.
_______________________
Lu said, "I will totally believe that theory if someone can convince me that there is a law or rule or theory in aerodynamics that explains why this phenomenon takes place in 90-degrees as opposed to 35-degrees or 125-degrees."
Read that long post on the Gyroscopic Precession thread that I keep mentioning. The reason it would happen is written there. BTW, the reason for it is neither aerodynamic nor gyroscopic.
_______________________
Connavar you're right. Don't think about this kind of stuff when you're ditching. Right now, I doubt if any of us involved in this conversation are ditching, so I guess that's okay with you.
I ditched once. We got out like a shot, never got wet, and the helicopter slowly sank. We sat back, thought about it, and salvaged a helicopter worth at least $20 million dollars. I guess thinking does pay off. Hey, my employer still hasn't paid us for that salvage.
_________________
Thomas coupling if you're having problems with the big words I'll be happy to translate for you.
You also asked us to lighten up. Okay.....a three legged dog walks into a bar and says, "I'm lookin' for the man who shot my paw."
Matthew.
[ 22 November 2001: Message edited by: heedm ]
[ 22 November 2001: Message edited by: heedm ]
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Hi guys,just felt a need to throw in a couple of comments.I've never worked on two bladed heads so most of my experience comes from 3,4 and 5 bladed heads. The comments i've been reading about pitch horn offset seem to miss the point as i understand it which is that the pitch on the blade will reach it maximum deviation 90degs before the effect is felt.so the pitch horn offset can be ignored because it is the position of the blade centre line that is important as the response is at 90degs to this.
Somebody mentioned the control spider on a lynx (sorry forgot who) and said that with system the sinusoidal nature of the swashplate can be overcome, this however is not the case.The end result of the spider is exactly the same as a swashplate. Another point made about the lynx head is that it is rigid and will suffer the effects of procession more so than fully articulated head,although the head is semi-rigid the articulation is achieved in the flex of the titanium sleeves and central star,so the blades still flap,lead and lag the same as any other helicopter and so shouldn't feel procession more or less than any other helicopter.
If i've misunderstood any of the above comments i'm sure somebody will put me right
Somebody mentioned the control spider on a lynx (sorry forgot who) and said that with system the sinusoidal nature of the swashplate can be overcome, this however is not the case.The end result of the spider is exactly the same as a swashplate. Another point made about the lynx head is that it is rigid and will suffer the effects of procession more so than fully articulated head,although the head is semi-rigid the articulation is achieved in the flex of the titanium sleeves and central star,so the blades still flap,lead and lag the same as any other helicopter and so shouldn't feel procession more or less than any other helicopter.
If i've misunderstood any of the above comments i'm sure somebody will put me right
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I put forth this input just to confuse the issue....whilst doing conversion training for new CH-47 Chinook pilots at Fort Sill, Oklahoma....a USAF crew flying their H-3 Sikorsky made a water landing and made mock of the fact that Chinooks could not do that. Much to their chagrin....we splashed down next to them and cruised circles around them.
Point being....unless you have fixed floats or a boat hull, that is not a recommended maneuver. Emergency floats are notably unreliable and should be considered just that....emergency use only.
If used in salt water...which usually connotes the presence of waves...the quicker one exists the aircraft the better...and in light of the current issue of underwater breathing apparatus to Navy/Coast Guard aircrews....I think there is a message there!
Also...one commercial operator I have worked for asked for us to ensure a "float failure" after the crew and passengers were out of the aircraft should we land in salt water. Pilots were encouraged to wear a knife for just such eventualities. Seems insurance value of the aircraft was enhanced if it was submerged permanently.
Point being....unless you have fixed floats or a boat hull, that is not a recommended maneuver. Emergency floats are notably unreliable and should be considered just that....emergency use only.
If used in salt water...which usually connotes the presence of waves...the quicker one exists the aircraft the better...and in light of the current issue of underwater breathing apparatus to Navy/Coast Guard aircrews....I think there is a message there!
Also...one commercial operator I have worked for asked for us to ensure a "float failure" after the crew and passengers were out of the aircraft should we land in salt water. Pilots were encouraged to wear a knife for just such eventualities. Seems insurance value of the aircraft was enhanced if it was submerged permanently.
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Gentlemen, especially believers in precession. A point to ponder for you.
The Autogyro (yes I’ve flown those as well). The control of the rotor in simple autogyros is direct movement of the teetering hub; this input is in phase with the disk movement.
Lets look at the forces applied when commanding the disk to tilt forward.
1. An upward force is applied at the BACK of the disk (6 o’clock).
2. A tiny movement of the disk causes the advancing blade to experience an instantaneous reduction in effective pitch angle reducing the angle of attack. The disk begins to move, does it:
A. Tilt up on the side of the advancing blade (3 o’clock [for example]) following the laws of precession?
Or
B. Tilt forward.
Given that we already know that the controls are rigged in phase we know that the effects of precession are not noticeable to the pilot and the disk tilts forward.
Now imagine if you used a swashplate instead. Where would you want to reduce the pitch if you wanted the disk to tilt forward (teetering, undamped head)?
GA
PS. With over 1500 helicopter deck landings I can assure you that the disk follows the helicopter, which follows the deck, at normal rotor speeds no matter what the pitch or roll. Why? Because if it didn’t it would experience a change in its’ attitude relative to the control orbit which would return the disk to its’original position as though the pilot had commanded the disk to level itself with respect to the aircraft. However gust of wind, or turbulence, especially with a vertical component alter the induced velocity and disturb the disk, at low rotor rpm the blade sail can be quite significant and tip strikes on the deck are not unheard of.
PPS Heedm – In response to an earlier post of yours, the Robinson you saw hovering tilted to one side was experiencing an effect called “Tail Rotor Roll”, more or less all conventional single rotor helicopters experience this, it has nothing to do with gyroscopes.
The Autogyro (yes I’ve flown those as well). The control of the rotor in simple autogyros is direct movement of the teetering hub; this input is in phase with the disk movement.
Lets look at the forces applied when commanding the disk to tilt forward.
1. An upward force is applied at the BACK of the disk (6 o’clock).
2. A tiny movement of the disk causes the advancing blade to experience an instantaneous reduction in effective pitch angle reducing the angle of attack. The disk begins to move, does it:
A. Tilt up on the side of the advancing blade (3 o’clock [for example]) following the laws of precession?
Or
B. Tilt forward.
Given that we already know that the controls are rigged in phase we know that the effects of precession are not noticeable to the pilot and the disk tilts forward.
Now imagine if you used a swashplate instead. Where would you want to reduce the pitch if you wanted the disk to tilt forward (teetering, undamped head)?
GA
PS. With over 1500 helicopter deck landings I can assure you that the disk follows the helicopter, which follows the deck, at normal rotor speeds no matter what the pitch or roll. Why? Because if it didn’t it would experience a change in its’ attitude relative to the control orbit which would return the disk to its’original position as though the pilot had commanded the disk to level itself with respect to the aircraft. However gust of wind, or turbulence, especially with a vertical component alter the induced velocity and disturb the disk, at low rotor rpm the blade sail can be quite significant and tip strikes on the deck are not unheard of.
PPS Heedm – In response to an earlier post of yours, the Robinson you saw hovering tilted to one side was experiencing an effect called “Tail Rotor Roll”, more or less all conventional single rotor helicopters experience this, it has nothing to do with gyroscopes.
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Glad to see we’ve got back too the original question.
Interesting comment about “ensuring” float failure! My view is that given time pop out floats will deflate anyway leading to the airframe capsizing.
Anybody who has done Helicopter Underwater Egress (Escape) Training will know that a float failure is assumed hence the emphasis on evacuation of the helicopter whilst inverted underwater.
As an aside one of my friends was doing his HUET revalidation yesterday. Somehow he managed to split his head open whilst evacuating upside down. He is now flying today with a dispensation for not having revalidated his HUET and a second dispensation for flying with a pre existing condition. He looks quite comical. Anybody who sees a rather large captain landing on a tropical platform with what looks like a turban on his head will know who I’m talking about.
A suggestion to any pilots out there who don’t undergo HUET training as part of their job is, I would highly recommend attending a course. I saw an advert recently in a flying magazine offering such courses to private pilots. It may have been Flyer Magazine and through Biggin Hill Helicopters for less than GBP100 but I’m not too sure. Not only does such a course teach you how to evacuate a helicopter upside down underwater but it also teaches preparedness for such an incident and gives you the confidence and respect needed to handle life threatening incidents like this. These courses will be held around any major offshore flying base. In the UK, Great Yarmouth, Aberdeen, Fleetwood and Hull all have facilities to do this training. Money well invested.
Interesting comment about “ensuring” float failure! My view is that given time pop out floats will deflate anyway leading to the airframe capsizing.
Anybody who has done Helicopter Underwater Egress (Escape) Training will know that a float failure is assumed hence the emphasis on evacuation of the helicopter whilst inverted underwater.
As an aside one of my friends was doing his HUET revalidation yesterday. Somehow he managed to split his head open whilst evacuating upside down. He is now flying today with a dispensation for not having revalidated his HUET and a second dispensation for flying with a pre existing condition. He looks quite comical. Anybody who sees a rather large captain landing on a tropical platform with what looks like a turban on his head will know who I’m talking about.
A suggestion to any pilots out there who don’t undergo HUET training as part of their job is, I would highly recommend attending a course. I saw an advert recently in a flying magazine offering such courses to private pilots. It may have been Flyer Magazine and through Biggin Hill Helicopters for less than GBP100 but I’m not too sure. Not only does such a course teach you how to evacuate a helicopter upside down underwater but it also teaches preparedness for such an incident and gives you the confidence and respect needed to handle life threatening incidents like this. These courses will be held around any major offshore flying base. In the UK, Great Yarmouth, Aberdeen, Fleetwood and Hull all have facilities to do this training. Money well invested.
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I thought I'd toss in my 2cents about gyroscopic precession and helicopter swashplate rigging angles. Kirilian laid out the factors that describe how a rotor does not obey the gyroscopic precession rules (because it is not a gyroscope in any real way). The swashplate rigging angle is affected by many factors, and is never really 90 degrees in any helicopter!
Before all the mechanics jump up and describe (accurately) that their helicopter is rigged exactly at 90 degrees (forward stick makes the swashplate tilt exactly to the right), let me clarify one point:
The rigging of the helicopter is the offset put in to phase the cyclic exactly to match the motion of the aircraft. The test of perfection is that the pilot moves the cyclic forward and the nose goes down, with no roll at all (and vice-versa). Well, that never really is achieved! We compromise in two important ways:
1) Speed changes the rigging angle (we call it Gamma) so that there is a big change between hover and Vne. As I recall, high speed needs less Gamma, hover needs to most. The difference for the S-76 was about 10 degrees, for the Comanche it is even more! We usually compromise at a Vcruise setting and let it go. The S-76 flew with 3 different sets of mixing bellcranks, and we picked the "best" set to match our opinions. For Comanche, with the Fly By Wire, we tune the Gamma to match each flight condition.
2) There is a rotor tilt for short term, medium term and long term that is different. The short term is seen in big stick inputs done fast, and washes out quickly. This one is usually under-mixed, since it is seen only in abrupt, unusual maneuvers (unless you are dogfighting in a Comanche). The BO-105 folks know this one when they pull load factor and there is a big need to retrim roll.
The meduim term slow response is there as the "constant" term and is the one we address with mechanical mixing in most helos. This is the one that sets Gamma. You find this if you ask for 5 degrees per second roll rate, and look to see if any pitch retrim is needed.
The longest term is the roll and pitch retrim needed for changes in airspeed, which we do not set up mechanically, we let the auto-pilot or pilot worry about this. This one sets the S-76 and Robinson stick somewhat to the right as speed increases, about 1" or so from hover to cruise. Don't be confused by this one, at any point in the flight, if the pilot pushes the stick forward, the nose goes down with almost no roll, even though the stick is moving with airspeed in a slightly rightward direction.
Let me forwarn LU, I will answer no questions he asks on this subject, as he has read all this before and has NEVER understood a word of it. If any other ppruners have a question or disagreement, fire away!
Before all the mechanics jump up and describe (accurately) that their helicopter is rigged exactly at 90 degrees (forward stick makes the swashplate tilt exactly to the right), let me clarify one point:
The rigging of the helicopter is the offset put in to phase the cyclic exactly to match the motion of the aircraft. The test of perfection is that the pilot moves the cyclic forward and the nose goes down, with no roll at all (and vice-versa). Well, that never really is achieved! We compromise in two important ways:
1) Speed changes the rigging angle (we call it Gamma) so that there is a big change between hover and Vne. As I recall, high speed needs less Gamma, hover needs to most. The difference for the S-76 was about 10 degrees, for the Comanche it is even more! We usually compromise at a Vcruise setting and let it go. The S-76 flew with 3 different sets of mixing bellcranks, and we picked the "best" set to match our opinions. For Comanche, with the Fly By Wire, we tune the Gamma to match each flight condition.
2) There is a rotor tilt for short term, medium term and long term that is different. The short term is seen in big stick inputs done fast, and washes out quickly. This one is usually under-mixed, since it is seen only in abrupt, unusual maneuvers (unless you are dogfighting in a Comanche). The BO-105 folks know this one when they pull load factor and there is a big need to retrim roll.
The meduim term slow response is there as the "constant" term and is the one we address with mechanical mixing in most helos. This is the one that sets Gamma. You find this if you ask for 5 degrees per second roll rate, and look to see if any pitch retrim is needed.
The longest term is the roll and pitch retrim needed for changes in airspeed, which we do not set up mechanically, we let the auto-pilot or pilot worry about this. This one sets the S-76 and Robinson stick somewhat to the right as speed increases, about 1" or so from hover to cruise. Don't be confused by this one, at any point in the flight, if the pilot pushes the stick forward, the nose goes down with almost no roll, even though the stick is moving with airspeed in a slightly rightward direction.
Let me forwarn LU, I will answer no questions he asks on this subject, as he has read all this before and has NEVER understood a word of it. If any other ppruners have a question or disagreement, fire away!
Hi heliport, will think of something suitable....have been participating on our own (police/HEMS)forum which we are trying to get off the ground (identical to the PPrune infrastructure). Won't give the link unless someone wants to join...
Hang in there HP
back to sleeeeeep, now
Hang in there HP
back to sleeeeeep, now
Iconoclast
Join Date: Sep 2000
Location: The home of Dudley Dooright-Where the lead dog is the only one that gets a change of scenery.
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To: Everyone else but Nick because he won’t answer
Nick sez:
“Before all the mechanics jump up and describe (accurately) that their helicopter is rigged exactly at 90 degrees (forward stick makes the swashplate tilt exactly to the right), let me clarify one point”:
Response:
I know of no helicopter that when the cyclic is displaced forward the swashplate tilts to the right.
Regarding mechanics that accurately describe their respective helicopters as having a phase angle or rigging angle of 90-degrees. Sikorsky mechanics other than S-76 mechanics can make that claim Hughes and Apache mechanics can make that claim Aerospatial mechanics can make that claim, as can many others.
Nick sez:
The rigging of the helicopter is the offset put in to phase the cyclic exactly to match the motion of the aircraft. The test of perfection is that the pilot moves the cyclic forward and the nose goes down, with no roll at all (and vice-versa). Well, that never really is achieved! We compromise in two important ways:
Response:
You use the term rigging and rigging angle interchangeably. Rigging angle (your usage) can change. (See below) but the helicopters rigging is constant once it is set. If it changes you are in deep trouble.
Nick sez:
1) Speed changes the rigging angle (we call it Gamma) so that there is a big change between hover and Vne. As I recall, high speed needs less Gamma, hover needs to most. The difference for the S-76 was about 10 degrees, for the Comanche it is even more! We usually compromise at a Vcruise setting and let it go. The S-76 flew with 3 different sets of mixing bellcranks, and we picked the "best" set to match our opinions. For Comanche, with the Fly by Wire, we tune the Gamma to match each flight condition.
Response:
You use the term “rigging angle”, as something that is dynamic and it may be an accurate choice of words. However those of us that are not used to the term or the term Gamma will visualize the rigging of the helicopter flight controls as being dynamic. I do not mean dynamic in that it moves in response to pilot input I mean that they might think that the controls are dynamic in respect to speed changes. The pilot will adjust his controls to compensate for speed changes or in compensation for external forces acting on the rotor system. I can accept that.
Regarding the S-76 flying with three sets of bellcranks I think you should clarify the reason for doing so. From what I understand, there was a goof-up on the gearbox casting and the servo attach fittings were not in the proper place as compared to other Sikorsky designs. The basic Sikorsky design places the Fore and aft servo 45-degrees ahead of the longitudinal axis and the left lateral servo 45-degrees after the longitudinal axis and the right lateral servo 180-degrees out from the left lateral servo. All Sikorsky helicopters up to the S-76 had their servos mounted as described above. Please do not include the Blackhawk. The three different mixing units were developed to obtain the optimum response from a given pilot input. In concert with the servo mounting relative to the two axes on other Sikorsky helicopters the pitch horns led the blades by 45-degrees. This meant that the blade over the right side of the helicopter would be at the lowest pitch and the blade over the left side would be at maximum pitch. The two blades in-between (assuming a four-blade system) would be at the collective pitch setting when the pilot pushed forward cyclic. As a result, the disc would dip down over the nose. I choose to believe it is gyroscopic precession and others like you say it is aerodynamic precession. It works either way.
I have a few other question s but they won’t be answered so why ask.
Nick sez:
“Before all the mechanics jump up and describe (accurately) that their helicopter is rigged exactly at 90 degrees (forward stick makes the swashplate tilt exactly to the right), let me clarify one point”:
Response:
I know of no helicopter that when the cyclic is displaced forward the swashplate tilts to the right.
Regarding mechanics that accurately describe their respective helicopters as having a phase angle or rigging angle of 90-degrees. Sikorsky mechanics other than S-76 mechanics can make that claim Hughes and Apache mechanics can make that claim Aerospatial mechanics can make that claim, as can many others.
Nick sez:
The rigging of the helicopter is the offset put in to phase the cyclic exactly to match the motion of the aircraft. The test of perfection is that the pilot moves the cyclic forward and the nose goes down, with no roll at all (and vice-versa). Well, that never really is achieved! We compromise in two important ways:
Response:
You use the term rigging and rigging angle interchangeably. Rigging angle (your usage) can change. (See below) but the helicopters rigging is constant once it is set. If it changes you are in deep trouble.
Nick sez:
1) Speed changes the rigging angle (we call it Gamma) so that there is a big change between hover and Vne. As I recall, high speed needs less Gamma, hover needs to most. The difference for the S-76 was about 10 degrees, for the Comanche it is even more! We usually compromise at a Vcruise setting and let it go. The S-76 flew with 3 different sets of mixing bellcranks, and we picked the "best" set to match our opinions. For Comanche, with the Fly by Wire, we tune the Gamma to match each flight condition.
Response:
You use the term “rigging angle”, as something that is dynamic and it may be an accurate choice of words. However those of us that are not used to the term or the term Gamma will visualize the rigging of the helicopter flight controls as being dynamic. I do not mean dynamic in that it moves in response to pilot input I mean that they might think that the controls are dynamic in respect to speed changes. The pilot will adjust his controls to compensate for speed changes or in compensation for external forces acting on the rotor system. I can accept that.
Regarding the S-76 flying with three sets of bellcranks I think you should clarify the reason for doing so. From what I understand, there was a goof-up on the gearbox casting and the servo attach fittings were not in the proper place as compared to other Sikorsky designs. The basic Sikorsky design places the Fore and aft servo 45-degrees ahead of the longitudinal axis and the left lateral servo 45-degrees after the longitudinal axis and the right lateral servo 180-degrees out from the left lateral servo. All Sikorsky helicopters up to the S-76 had their servos mounted as described above. Please do not include the Blackhawk. The three different mixing units were developed to obtain the optimum response from a given pilot input. In concert with the servo mounting relative to the two axes on other Sikorsky helicopters the pitch horns led the blades by 45-degrees. This meant that the blade over the right side of the helicopter would be at the lowest pitch and the blade over the left side would be at maximum pitch. The two blades in-between (assuming a four-blade system) would be at the collective pitch setting when the pilot pushed forward cyclic. As a result, the disc would dip down over the nose. I choose to believe it is gyroscopic precession and others like you say it is aerodynamic precession. It works either way.
I have a few other question s but they won’t be answered so why ask.
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Heedm
>>""Why talk about 'impressed angular momentum' when one can eliminate the middlemen, go right to the source and just say 'thrust'?"< Because the two are different."<.
True, but force (or torque) changes the state of rotational inertia and rotational inertia is a component of angular momentum.
>"Also, there is not only one force that is variable. Drag is variable. Weight is variable."<
True again, but for conceptual simplicity, it is easier to consider them as being constants.
>"You still seem to hold aerodynamic precession as a very special concept. I'm not sure exactly what your theory is, and ..... how it explains what the apparent lag is."<
Phase lag is one of the reasons why I like this hypothetical aerodynamic precession. For me, it is easier to aerodynamically envision the relative phasing between the control plane and the tip path plane, when considering a blade that is flying to position in less than 180-degrees.
If phase lag is to be discussed, others might prefer that it be on a new thread.
I agree with all you say 100%. Well; not to go overboard [as in ditching], lets say 99.9%.
Whatever. This discussion is certainly thought provoking and educational, for me at least.
_____________
In a hypothetical world, one could have a teetering rotor, with absolute rigidity between the blades (to resist coning), and no mass.
There is no 'original angular momentum' in this hypothetical world since the rotor has no mass. If the engine stops, the rotor will immediately stop revolving, since there is nothing to oppose the drag.
In this hypothetical aerodynamic world, it will still be possible to fly the rotor disk to position, whereas it will be impossible to move it to position by gyroscopic forces.
Now; if helicopter designers can continue to reduce the mass of helicopters far enough, gyroscopic precession will take its last flight - out the window.
_____________
Nick
Thank you ~ again.
Your explanation of Gamma is very timely. I have been reading and rereading documents on Gamma in the Sikorsky ABC. As you know from flying the craft, it had an in-flight variable Gamma from less than 20-degrees to greater than 60-degrees. The graphs show the results of different Gamma at different forward speeds. It has been difficult to understand, but interesting.
Your current description of Gamma and forward speed is making things easier. Thanks.
Now back to rereading your posting.
>>""Why talk about 'impressed angular momentum' when one can eliminate the middlemen, go right to the source and just say 'thrust'?"< Because the two are different."<.
True, but force (or torque) changes the state of rotational inertia and rotational inertia is a component of angular momentum.
>"Also, there is not only one force that is variable. Drag is variable. Weight is variable."<
True again, but for conceptual simplicity, it is easier to consider them as being constants.
>"You still seem to hold aerodynamic precession as a very special concept. I'm not sure exactly what your theory is, and ..... how it explains what the apparent lag is."<
Phase lag is one of the reasons why I like this hypothetical aerodynamic precession. For me, it is easier to aerodynamically envision the relative phasing between the control plane and the tip path plane, when considering a blade that is flying to position in less than 180-degrees.
If phase lag is to be discussed, others might prefer that it be on a new thread.
I agree with all you say 100%. Well; not to go overboard [as in ditching], lets say 99.9%.
Whatever. This discussion is certainly thought provoking and educational, for me at least.
_____________
In a hypothetical world, one could have a teetering rotor, with absolute rigidity between the blades (to resist coning), and no mass.
There is no 'original angular momentum' in this hypothetical world since the rotor has no mass. If the engine stops, the rotor will immediately stop revolving, since there is nothing to oppose the drag.
In this hypothetical aerodynamic world, it will still be possible to fly the rotor disk to position, whereas it will be impossible to move it to position by gyroscopic forces.
Now; if helicopter designers can continue to reduce the mass of helicopters far enough, gyroscopic precession will take its last flight - out the window.
_____________
Nick
Thank you ~ again.
Your explanation of Gamma is very timely. I have been reading and rereading documents on Gamma in the Sikorsky ABC. As you know from flying the craft, it had an in-flight variable Gamma from less than 20-degrees to greater than 60-degrees. The graphs show the results of different Gamma at different forward speeds. It has been difficult to understand, but interesting.
Your current description of Gamma and forward speed is making things easier. Thanks.
Now back to rereading your posting.
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I'll respond to everyone in one post.
Nick, thank you for that practical discussion. While I enjoy getting into the theory, I don't lose sight of the fact that the pilot makes the finesse corrections as the flight parameters change. I don't mind going head-to-head with you on physics but for cockpit knowledge, I bow to your training and experience.
You said, "Kirilian laid out the factors that describe how a rotor does not obey the gyroscopic precession rules...." Was that the Lock's number post, or did I miss something?
I agree that many factors will affect this angle, but no matter how many factors there are, basic physics still applies. If the rotor flapping hinge is coincident with the axis of rotation, then an angle of precisely 90 degrees is measured between a maximum force and maximum displacement. If the hinge is offset, then the angle will reduce.
Jed A1, I do a two day HUET and EBS course every 3-5 years. I've always been comfortable in the water, but I learnt on day one that I would have not escaped an upsidedown ditching without the training and confidence that that course gave me. You're right to recommend it.
Grey Area, first of all, I was kidding about that left turning Robinson. Part of a long discussion.
Secondly, the forward movement of the stick in the autogyro with a teetering head doesn't directly put an upwards force on the disk at 6 o'clock. It's a teetering head, so when a blade is at 6 o'clock, the hinge is normal to the force. Consider what would happen with the blade at 6 o'clock but the rotors not turning. What I believe does happen is the forward movement of the stick causes less pitch at the 3 o'clock and more pitch at the 9 o'clock. The process from here follows the apparent 90 degree lag discussions.
Kyrilian what is the gamma that is in Lock's formula? The way gamma has been used here is that it is the phase angle between blade pitch and blade displacement (don't know the precise definition of that angle).
Matthew.
[ 23 November 2001: Message edited by: heedm ]
Nick, thank you for that practical discussion. While I enjoy getting into the theory, I don't lose sight of the fact that the pilot makes the finesse corrections as the flight parameters change. I don't mind going head-to-head with you on physics but for cockpit knowledge, I bow to your training and experience.
You said, "Kirilian laid out the factors that describe how a rotor does not obey the gyroscopic precession rules...." Was that the Lock's number post, or did I miss something?
I agree that many factors will affect this angle, but no matter how many factors there are, basic physics still applies. If the rotor flapping hinge is coincident with the axis of rotation, then an angle of precisely 90 degrees is measured between a maximum force and maximum displacement. If the hinge is offset, then the angle will reduce.
Jed A1, I do a two day HUET and EBS course every 3-5 years. I've always been comfortable in the water, but I learnt on day one that I would have not escaped an upsidedown ditching without the training and confidence that that course gave me. You're right to recommend it.
Grey Area, first of all, I was kidding about that left turning Robinson. Part of a long discussion.
Secondly, the forward movement of the stick in the autogyro with a teetering head doesn't directly put an upwards force on the disk at 6 o'clock. It's a teetering head, so when a blade is at 6 o'clock, the hinge is normal to the force. Consider what would happen with the blade at 6 o'clock but the rotors not turning. What I believe does happen is the forward movement of the stick causes less pitch at the 3 o'clock and more pitch at the 9 o'clock. The process from here follows the apparent 90 degree lag discussions.
Kyrilian what is the gamma that is in Lock's formula? The way gamma has been used here is that it is the phase angle between blade pitch and blade displacement (don't know the precise definition of that angle).
Matthew.
[ 23 November 2001: Message edited by: heedm ]