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What causes "blowback/flapback" when accelerating through transverse flow effect?

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What causes "blowback/flapback" when accelerating through transverse flow effect?

Old 16th Aug 2022, 15:42
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Same with doing cliff winching in an updraft in the Sea King Syd - no chance of reading any instruments at all.

However
because the inflow angle only changes at the front of the rotor disc
the inflow angle changes at the rear as well with the front seeing a bigger change than the rear, hence the dissymmetry.

Normally the vibration is associated with translational lift rather than inflow roll.

In a still air transition the inflow roll happens first, then the flapback and then, when you overcome both, the vibration of ETL.
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Old 16th Aug 2022, 17:12
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Originally Posted by [email protected] View Post
Same with doing cliff winching in an updraft in the Sea King Syd - no chance of reading any instruments at all.
The effect of which I speak is quite subtle. Seeing the instruments was not a problem.

I suspect that your vibration is likely to be caused by interaction with complex vortices, as is the vibration of translational lift.

However the inflow angle changes at the rear as well with the front seeing a bigger change than the rear, hence the dissymmetry.
I should have said that it 'progressses from the front to the rear'.

In a still air transition the inflow roll happens first, then the flapback and then, when you overcome both, the vibration of ETL.
Yup. ;-)
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Old 16th Aug 2022, 18:29
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The effect of which I speak is quite subtle. Seeing the instruments was not a problem.
Syd I realised after posting that you were referring to 'Sea King Nose' - well known to crews.

I suspect that your vibration is likely to be caused by interaction with complex vortices, as is the vibration of translational lift.
Agreed
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Old 16th Aug 2022, 18:37
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Originally Posted by [email protected] View Post
What AC said remember the roll is due to a change in inflow angle, not a massive amount but enough - the pitch is due to velocity differences between advancing and retreating sides of the disc and is V squared so much bigger
Got it, thank you!
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Old 16th Aug 2022, 18:46
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Originally Posted by [email protected] View Post
Same with doing cliff winching in an updraft in the Sea King Syd - no chance of reading any instruments at all.

However the inflow angle changes at the rear as well with the front seeing a bigger change than the rear, hence the dissymmetry.

Normally the vibration is associated with translational lift rather than inflow roll.

In a still air transition the inflow roll happens first, then the flapback and then, when you overcome both, the vibration of ETL.
I thought the vibration was due to the uneven forces across the disc, when inflow roll was at its worst. That period in slow acceleration when only part of the disc has moved out the induced flow. What would be causing a vibration at ETL?
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Old 17th Aug 2022, 00:19
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One bit that puzzles me is the "dastardly" warning that comes from hovering next to a hangar or a cliff. The warning is that the air is recirculated next to the vertical obstruction, and there is thus increased induced flow next to the hazard, less lift means the helicopter will get sucked over to the cliff or hangar and nasty things happen.

Well, if the downwash increases in that sector, the effect should be felt around 90 degrees later, which would pull the aircraft along parallel to the hazard??

And having done quite a bit of hovering next to vertical obstacle walls, I have never felt any problem with it. Any contributions?
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Old 17th Aug 2022, 09:22
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I thought the vibration was due to the uneven forces across the disc, when inflow roll was at its worst. That period in slow acceleration when only part of the disc has moved out the induced flow. What would be causing a vibration at ETL?
the rotor has to pass through the tip vortices which, in still air IGE, are spread outwards from the rotor - it is passing through those vortices that creates the vibration of ETL.

If you do an IGE transition over longish grass (an inch or two) you can see the pattern of the outflow of the rotor on the grass and as you catch up with the forward edge of that, you will experience the vibration and the onset of ETL.

AC - I'm with you on that one - heard all sort of theories about the aircraft being sucked towards the obstacle or requiring more forward or aft cyclic to prevent the aircraft moving forward or back - never felt a problem either on ops or in training moving in from a stable OGE hover to close proximity to cliffs/hangars/buildings. if there is an effect it is negligible.

Full recirculation is a different matter - had a colleague make a very firm landing in a wriggly tin fort in South Armagh - we 'fell out of the sky' as we got to the hover and used a lot of unanticipated power to cushion the landing.
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Old 17th Aug 2022, 09:39
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Originally Posted by [email protected] View Post
Full recirculation is a different matter - had a colleague make a very firm landing in a wriggly tin fort in South Armagh - we 'fell out of the sky' as we got to the hover and used a lot of unanticipated power to cushion the landing.
The dreaded “settling without enough power”……..
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Old 17th Aug 2022, 12:01
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Originally Posted by [email protected] View Post
Oh no, we'll be getting into 'wee-wah' next!
I think we did with the lag <90...

When the controls positions are recorded, the S shape that occurs is pretty to break down, the lateral change is initially from inflow roll, the hover longitudinal to ETL to high speed is from the flap back initially and then from the balance of forces with the thrust/drag couples around the beast.

Alternatively.... for a 2 blader.... the hub reaction forces are "simple"... Per Uncle Wayne


19.8 Rotorcraft Aeromechanics, Johnson (2013)

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Old 17th Aug 2022, 13:34
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Now you are using maths - so unfair

Helicopter Principles of Flight was explained by the venerable Lofty Marshall as 'an explanation of something we know happens, not mathematical proof of why it happens'
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Old 17th Aug 2022, 20:17
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Everybody knows that a helicopter is "a triumph of science and technology over common sense." Also known as "White man magic".
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Old 21st Aug 2022, 13:29
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Mast Bumper - I have found a copy of Lu's book on a dusty shelf in the office.....
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Old 21st Aug 2022, 14:29
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Originally Posted by paco View Post
Mast Bumper - I have found a copy of Lu's book on a dusty shelf in the office.....
DM sent.
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Old 22nd Aug 2022, 00:30
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May I just say that I've read every post in this thread but said nothing because I could neither add to the discussion nor explain things as clearly as some of you guys. The level of knowledge about this subject here is truly impressive. And...not to single anyone out, but Ascend Charlie should be teaching this stuff if he isn't already. His explanations are incredible in their simplicity and clarity. A lot of us old-timers know this stuff but have a hard time putting it into words and terms that a newbie might understand. If any pilot were to ever wonder why the rotor does certain things, I would point them to this thread. Nice work, men!
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Old 22nd Aug 2022, 02:34
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Thank you Mr FH1100.

Yes I used to teach this stuff from 1976 onwards, but retiring from flying and instruction leaves me with these sites. I think Crab, fdr and Shy are in a similar boat.

But if you want REAL knowledge, you have to ask Nick Lappos, Sikorsky's chief test pilot, who cruised these sites in the past. Sadly Shawn Coyle has departed the helipad, but his book Cyclic and Collective is around, with knowledge and wit combined.

And pay attention whenever John Dixson visit the site, his knowledge is the supplement to Nick's, two gentlemen who were in on some of the best testing ever.

Last edited by Ascend Charlie; 22nd Aug 2022 at 04:29.
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Old 10th Sep 2022, 22:57
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Well, not a nghtmare, but a huge flashback!
That was me a month ago, on being pointed to the AP3456 online and discovering its use of my own class notes from 1977. This stimulated me into doing something that has been in the back of my mind for ages, and I have digitised them. The only copy I have is rather poor and yellowed, but with the wonders of a modern phone it has been relatively straightforward.

I have posted these as a PDF and there is an online HTML version. Remember, these are 45 years old and completely unchanged, so do not expect too much. I welcome comments, but be gentle with an old man.
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Old 10th Sep 2022, 23:21
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Originally Posted by Ascend Charlie View Post
Thank you Mr FH1100.

Yes I used to teach this stuff from 1976 onwards, but retiring from flying and instruction leaves me with these sites. I think Crab, fdr and Shy are in a similar boat.

But if you want REAL knowledge, you have to ask Nick Lappos, Sikorsky's chief test pilot, who cruised these sites in the past. Sadly Shawn Coyle has departed the helipad, but his book Cyclic and Collective is around, with knowledge and wit combined.

And pay attention whenever John Dixson visit the site, his knowledge is the supplement to Nick's, two gentlemen who were in on some of the best testing ever.
Shawn's "little book of Autorotations" & "40 years afore the mast" are good reads. Last time we spoke he was putting together the notes on the Ray Prouty Lecture series, I read the first chapter, and it was well done, perhaps Patricia would consider letting the whole set be published. The last 10 years of his health was a tough road that he travelled with his positive attitude.
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Old 11th Sep 2022, 06:14
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On the topic of vibrations and ETL - can anyone explain WHY certain rotor systems, generally with 4 blades or more, seem to exhibit these bone shaking vibrations right around that 15-30kt airspeed range? In an Agusta it makes your vision blurry, in bigger machines like the Skycrane, it can apparently cause premature tail boom cracks.
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Old 11th Sep 2022, 11:05
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lelebebbel - my guess would be the tip vortices interfering with the following blade.

When they created the BERP blade for Lynx and subsequently EH101, the first iterations had problems with what was referred to as 'cobblestoning' in the hover - this was mostly alleviated by adding an anhedral tip to help shed the tip vortices downwards.

The more blades you have the more vortices you are producing.

You could also factor in the blade passing frequency over the cockpit and tail boom as a source of pressure pulses that could affect structural integrity if they match the resonant frequency of the airframe - I believe this is a problem on the S92 creating higher noise levels in the cockpit.
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Old 11th Sep 2022, 20:15
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The "Sikorsky shuffle."
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