Dynamics - N-per-rev Vertical Vibration
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To: Hovering in the wings
The movie that you alluded to (there may be several) was made by attaching a 16 mm Camera to the head of an S-51 which was mounted on a special whirl stand that allowed the introduction of both cyclic and collective inputs. This movie had a major impact in two areas. When many helicopter pilots saw the movie they elected to never fly helicopters again and even more important it caused a major modification in the thinking of Sikorsky engineers. Up until that time they had believed that due to profile drag and relative wind speeds the blades lagged on the advancing side and led on the retreating side but the movie proved the leading and lagging to be opposite of conventional thinking at that time.
As I had mentioned in a previous post the blade was moving in a sinusoidal wave that traveled from the tip to the root much like the tethered string example that you had made in your post.
The movie that you alluded to (there may be several) was made by attaching a 16 mm Camera to the head of an S-51 which was mounted on a special whirl stand that allowed the introduction of both cyclic and collective inputs. This movie had a major impact in two areas. When many helicopter pilots saw the movie they elected to never fly helicopters again and even more important it caused a major modification in the thinking of Sikorsky engineers. Up until that time they had believed that due to profile drag and relative wind speeds the blades lagged on the advancing side and led on the retreating side but the movie proved the leading and lagging to be opposite of conventional thinking at that time.
As I had mentioned in a previous post the blade was moving in a sinusoidal wave that traveled from the tip to the root much like the tethered string example that you had made in your post.
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Dave,
I have tried for many years to get a straight answer from both Helicopter nad Vibe analysis manufacturers into the CAUSE of n per revs. Never received a straight answer yet.
My best answer has come from reading "Practical Helicopter Aerodynamics" by R.W. Prouty. Chap 4 discusses blade rigidity and the N per rev problem. Other chapters cover vibrations and design. It provides answers for N per Rev with IAS. I think the paper by Stephen Jacklin provides some answers for the harmonics of N per in flight regimes at lower IAS.
If you haven't already read it, would recommend it.
Hope it helps in your Quest....
Back to Hovering
I have tried for many years to get a straight answer from both Helicopter nad Vibe analysis manufacturers into the CAUSE of n per revs. Never received a straight answer yet.
My best answer has come from reading "Practical Helicopter Aerodynamics" by R.W. Prouty. Chap 4 discusses blade rigidity and the N per rev problem. Other chapters cover vibrations and design. It provides answers for N per Rev with IAS. I think the paper by Stephen Jacklin provides some answers for the harmonics of N per in flight regimes at lower IAS.
If you haven't already read it, would recommend it.
Hope it helps in your Quest....
Back to Hovering
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Hovering in the wings,
I feel bad that you feel that you "Never received a straight answer yet" regarding Dave's question about n/rev vibration. Where I come from, a straight answer is one that is true, so I infer from your gripe that you feel nobody has told you the truth yet.
I think you have had two folks ernestly try to describe what occurs when a small number of blades attempt to average out the lift of the machine, while each individually vibrating, circling up-wind near Mach 1 and down-wind near stall. If I can find a better reference for the issue, I will post it. Until then, I suggest that you might look inward when seeking to blame someone for your inability to grasp technical subjects that are beyond your grasp.
I feel bad that you feel that you "Never received a straight answer yet" regarding Dave's question about n/rev vibration. Where I come from, a straight answer is one that is true, so I infer from your gripe that you feel nobody has told you the truth yet.
I think you have had two folks ernestly try to describe what occurs when a small number of blades attempt to average out the lift of the machine, while each individually vibrating, circling up-wind near Mach 1 and down-wind near stall. If I can find a better reference for the issue, I will post it. Until then, I suggest that you might look inward when seeking to blame someone for your inability to grasp technical subjects that are beyond your grasp.
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Hovering in the Wings
Thanks for the leads. Unfortunately, Prouty's "Practical Helicopter Aerodynamic" is out of print. I will look into the paper by Stephen Jacklin.
The first couple of paragraphs of Nick's posting, combined with other information that he has previously provided, go a long way toward describing some of the sources of nP vibration.
It appears that the primary source is due to the asymmetrical airflow during forward flight and this might (nothing is impossible <img src="eek.gif" border="0"> ) be an intrinsic feature. The angle of attack of the blades' elements will be different at different locations on the disk. Increasing a blade's angle of attack causes the coefficient of lift and the coefficient of drag to increase as well. Since any increases in these two coefficients are not quite equal, both the lift and the drag can not be made equal on the advancing side and the retreating side. If the lift is equalized between the advancing and retreating sides, then the difference in drag will result in a nP vibration.
The web page link, which Nick gave, lists some secondary sources such as retreating blade stall, advancing tip compression and vortices from previous blades. I suspect that these are temporal and can be eliminated or greatly minimized.
The fuselage, stabilizers, and the tail rotor (for those who must put up with such a handicap <img src="smile.gif" border="0"> ) are, no doubt, additional sources.
Perhaps an electronic, or a mechanical means beyond the swashplate, will or are advancing the control of the rotor above 1P.
________
The above may have many errors and is certainly incomplete. Corrections and expansion, please.
[ 17 January 2002: Message edited by: Dave Jackson ]</p>
Thanks for the leads. Unfortunately, Prouty's "Practical Helicopter Aerodynamic" is out of print. I will look into the paper by Stephen Jacklin.
The first couple of paragraphs of Nick's posting, combined with other information that he has previously provided, go a long way toward describing some of the sources of nP vibration.
It appears that the primary source is due to the asymmetrical airflow during forward flight and this might (nothing is impossible <img src="eek.gif" border="0"> ) be an intrinsic feature. The angle of attack of the blades' elements will be different at different locations on the disk. Increasing a blade's angle of attack causes the coefficient of lift and the coefficient of drag to increase as well. Since any increases in these two coefficients are not quite equal, both the lift and the drag can not be made equal on the advancing side and the retreating side. If the lift is equalized between the advancing and retreating sides, then the difference in drag will result in a nP vibration.
The web page link, which Nick gave, lists some secondary sources such as retreating blade stall, advancing tip compression and vortices from previous blades. I suspect that these are temporal and can be eliminated or greatly minimized.
The fuselage, stabilizers, and the tail rotor (for those who must put up with such a handicap <img src="smile.gif" border="0"> ) are, no doubt, additional sources.
Perhaps an electronic, or a mechanical means beyond the swashplate, will or are advancing the control of the rotor above 1P.
________
The above may have many errors and is certainly incomplete. Corrections and expansion, please.
[ 17 January 2002: Message edited by: Dave Jackson ]</p>
Thanks for the great topic Dave, and the user friendly explanation Nick (or is it "Nickii" <img src="wink.gif" border="0"> ?). Always wondered about the "Sikorsky shuffle". I thought it was to do with main rotor vorticey interacting with the tail rotor at that speed rather than the extra Nper vibration that most aircraft feel going through translational. The UH-60 certainly got that extra Nper feel, but it "shuffled" in yaw as well - or is my memory suspect?
Hovering in the wings: You have used my favourite word: irregardless. You said: "Either way, irrigardlesss of method of excitation".
Irrigardless. Does that mean:
"without regard to"
"without without regard to"
"without regardless to", or
"with regard to" (due to resolving the double negative)?
Sorry, at my grammatical level I cannot point ANY fingers, but I couldn't resist my favourite word.
Lu: I saw the Chopper film too, and it was mounted on a UH-1H in flight. No chopper pilot I know quit flying because of it, but it did give us a great story to impress the bar fraus with!!
Hovering in the wings: You have used my favourite word: irregardless. You said: "Either way, irrigardlesss of method of excitation".
Irrigardless. Does that mean:
"without regard to"
"without without regard to"
"without regardless to", or
"with regard to" (due to resolving the double negative)?
Sorry, at my grammatical level I cannot point ANY fingers, but I couldn't resist my favourite word.
Lu: I saw the Chopper film too, and it was mounted on a UH-1H in flight. No chopper pilot I know quit flying because of it, but it did give us a great story to impress the bar fraus with!!
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Dave,
Sorry to hear Prouty's book is out of publication. If you are interested, email me a fax number and I'll fax a couple of relevant pages to you which I think may help shed light....
Nick,
It would be great if we could someday get together and discuss vibe issues in a bar (with a whiteboard) over a few beers where all the problems of the world are solved <img src="wink.gif" border="0"> . Until then I guess I will have to learn to live with my "inability to grasp technical subjects that are beyond (my) grasp"....
Been fun,
Back to Hovering
Sorry to hear Prouty's book is out of publication. If you are interested, email me a fax number and I'll fax a couple of relevant pages to you which I think may help shed light....
Nick,
It would be great if we could someday get together and discuss vibe issues in a bar (with a whiteboard) over a few beers where all the problems of the world are solved <img src="wink.gif" border="0"> . Until then I guess I will have to learn to live with my "inability to grasp technical subjects that are beyond (my) grasp"....
Been fun,
Back to Hovering
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Hovering in...
I'll take you up on that beer, I think n/rev becomes crystal clear by about the 4th, as does my grasp of particle physics....
Dave,
try searching for Ray Prouty's book at the used book web site:
<a href="http://www.abe.com" target="_blank">http://www.abe.com</a>
I also recommend Stepniewski and Key "Rotary Wing Aerodynamics" Dover press.
I'll take you up on that beer, I think n/rev becomes crystal clear by about the 4th, as does my grasp of particle physics....
Dave,
try searching for Ray Prouty's book at the used book web site:
<a href="http://www.abe.com" target="_blank">http://www.abe.com</a>
I also recommend Stepniewski and Key "Rotary Wing Aerodynamics" Dover press.
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Nick
Thanks for mentioning the used book site. Will look into it, and also look deeper into 'Rotary Wing Aerodynamics'.
Coincidentally, I ordered Prouty's new book a couple of weeks ago. Its called 'Military Helicopter Design'. As you know, he has a very clear way of writing so it should make for interesting reading.
[ 18 January 2002: Message edited by: Dave Jackson ]
Correction to previous post.
The book is not new. It was originally published in 1989, with corrections in 1998 reprint, but it looks very interesting.
[ 18 January 2002: Message edited by: Dave Jackson ]</p>
Thanks for mentioning the used book site. Will look into it, and also look deeper into 'Rotary Wing Aerodynamics'.
Coincidentally, I ordered Prouty's new book a couple of weeks ago. Its called 'Military Helicopter Design'. As you know, he has a very clear way of writing so it should make for interesting reading.
[ 18 January 2002: Message edited by: Dave Jackson ]
Correction to previous post.
The book is not new. It was originally published in 1989, with corrections in 1998 reprint, but it looks very interesting.
[ 18 January 2002: Message edited by: Dave Jackson ]</p>
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Dave,
The Stepniewski book has a good first section that describes why 1/rev is the flapping resonance frequency (as varied slightly by damping and hinge offset) and also describes the ways that the n/rev root shears are affected by forward flight. Gave me new insight, as did this thread (always does!).
The Stepniewski book has a good first section that describes why 1/rev is the flapping resonance frequency (as varied slightly by damping and hinge offset) and also describes the ways that the n/rev root shears are affected by forward flight. Gave me new insight, as did this thread (always does!).
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helmet fire,
Early H-3's had some "tail shake" due to rotorhead wash going into the tail, solved with a carefully shape main rotor pylon collar (the horse collar). In my first days in the Pilot's Office, I had been told this was the "shuffle" and that it was not intrinsic to any other Sikorskys. I have always thought the "Sikorsky Shuffle" was more of a conveniently aliterative name than an actual condition.
Early H-3's had some "tail shake" due to rotorhead wash going into the tail, solved with a carefully shape main rotor pylon collar (the horse collar). In my first days in the Pilot's Office, I had been told this was the "shuffle" and that it was not intrinsic to any other Sikorskys. I have always thought the "Sikorsky Shuffle" was more of a conveniently aliterative name than an actual condition.
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The discussion to-date has address the probable sources of nP vibration during high-speed flight, but little has been said about this vibration during translation.
Could this vibration be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?
Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?
[ 19 January 2002: Message edited by: Dave Jackson ]</p>
Could this vibration be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?
Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?
[ 19 January 2002: Message edited by: Dave Jackson ]</p>
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Dave Jackson asked (Nick added paragraph letter headings):
A) Could this vibration (vibration during translation) be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?
B) Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?
. .Nick sez:. .A) I believe that translational vibration is mostly due to aerodynamic duisturbence of the blade by the previous blade wake, and is especially a problem on some helos during approach when the descent allows more BVI (blade vortex interference). Ever fly a BO-105? The panel is shock mounted to prevent atomic dissassembly of the gages, I think.
OTOH, I do believe that stabilizer vibration can affect n/rev anywhere in the envelope, as at least one Sikorsky model has n/rev that is strongly influenced by main wake on the tail at mid cruise speed.
B) I don't believe this has much affect, Dave. I have flown many machines for vibration tuning where we adjust the CG (and therefore the steady flapping angle) with no appreciable change in the n/rev. In fact, flapping is highest at forward CG, and for many Sikorsky models, forward CG is the smoothest.
A) Could this vibration (vibration during translation) be (partially) due to the horizontal stabilizer and its change of location in respect to the rotor's downwash?
B) Could it also be that the forward tilt of the rotor, has resulted in the axis of the tip path plane to not be concentric with the axis of the mast and this results in a oscillating dynamic load, since the hub acts as a knuckle joint, not a constant-velocity joint?
. .Nick sez:. .A) I believe that translational vibration is mostly due to aerodynamic duisturbence of the blade by the previous blade wake, and is especially a problem on some helos during approach when the descent allows more BVI (blade vortex interference). Ever fly a BO-105? The panel is shock mounted to prevent atomic dissassembly of the gages, I think.
OTOH, I do believe that stabilizer vibration can affect n/rev anywhere in the envelope, as at least one Sikorsky model has n/rev that is strongly influenced by main wake on the tail at mid cruise speed.
B) I don't believe this has much affect, Dave. I have flown many machines for vibration tuning where we adjust the CG (and therefore the steady flapping angle) with no appreciable change in the n/rev. In fact, flapping is highest at forward CG, and for many Sikorsky models, forward CG is the smoothest.
Dave,
The horizontal and vertical stabs will certainly have an effect on n/rev in many aircraft.
On the Bell 412 for example, horizontal stab rigging will affect the 4-per felt in the cabin.
Similarly, if the 4-per on the 407 is noticeable at 60kts, moving very slightly out of trim with right pedal will considerably reduce it.
The horizontal and vertical stabs will certainly have an effect on n/rev in many aircraft.
On the Bell 412 for example, horizontal stab rigging will affect the 4-per felt in the cabin.
Similarly, if the 4-per on the 407 is noticeable at 60kts, moving very slightly out of trim with right pedal will considerably reduce it.
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Nick
Thanks for mentioning the BVI (blade vortex interference) and the fact that it is particularly noticeable on some helicopters when descending during an approach.
This enhances the concern regarding BVI in helicopters with twin rotors, such as the coaxial configuration, where the lower rotor never gets out from under the vortices of the upper rotor.
Your point B) is also noted.. .________
CTD
Prouty appears to confirm what you are saying. He mentions that the "Designers at Bell generally placed horizontal stabilizers forward on the tail boom so that they are in the wake during hover and never have an awkward transition going from outside the wake to inside it."
Thanks for mentioning the BVI (blade vortex interference) and the fact that it is particularly noticeable on some helicopters when descending during an approach.
This enhances the concern regarding BVI in helicopters with twin rotors, such as the coaxial configuration, where the lower rotor never gets out from under the vortices of the upper rotor.
Your point B) is also noted.. .________
CTD
Prouty appears to confirm what you are saying. He mentions that the "Designers at Bell generally placed horizontal stabilizers forward on the tail boom so that they are in the wake during hover and never have an awkward transition going from outside the wake to inside it."
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Dave
There is a very good round up on this whole topic in the latest edition of AHS Vertiflite Vol 47 No.4 page 40 titled Dynamics 2000-2001 by Cynthia Callahan of Boeing. None of the topics are in any great depth but it could point you to some useful contacts as it gives a good idea as to who is researching what in the fields of Individual Blade Control /Active Twist Rotors / Smart airframe response etc.
If you don’t have a copy AHS will ask you $20US for one tel (703) 684 6777 or email [email protected]
Happy hunting
Wunper <img src="wink.gif" border="0"> . . <img src="cool.gif" border="0">
There is a very good round up on this whole topic in the latest edition of AHS Vertiflite Vol 47 No.4 page 40 titled Dynamics 2000-2001 by Cynthia Callahan of Boeing. None of the topics are in any great depth but it could point you to some useful contacts as it gives a good idea as to who is researching what in the fields of Individual Blade Control /Active Twist Rotors / Smart airframe response etc.
If you don’t have a copy AHS will ask you $20US for one tel (703) 684 6777 or email [email protected]
Happy hunting
Wunper <img src="wink.gif" border="0"> . . <img src="cool.gif" border="0">
