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HeliTorque :: View topic - airflow recirculation
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HeliTorque Forum Index » Ground School

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airflow recirculation
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paddywak
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PostPosted: Sat Jan 19, 2013 6:22 pm    Post subject: airflow recirculation Reply with quote

I have been back in the books again in order to help me write my own notes on P.O.F

Looking at explanations on recirculation when hovering in close proximity to buildings, I have found conflicting views.
I had always been under the impression that any effect on the rotor would take place 90 degrees further on in the P.O.R.

Take for example a helicopter hovering in a no wind condition with an anti-clockwise rotor system facing North. In front of the helicopter is a building less than 2/3 the rotor diameter.

If the recirculation is causing an increased induced flow at the 12 o-clock position, then the effect would be a loss of lift 90 degrees further on in P.O.R. More lift to pilots 3 o-clock and less lift to the pilots 9 o-clock.
If the disc then tilts to the left then we would move sideways to the left on an East - West axis, or would we?

Some say we would be sucked in towards the building!

Any views / comments on this...?
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GregNZ
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PostPosted: Sat Jan 19, 2013 10:41 pm    Post subject: Reply with quote

Hey mate,

I know where you got that explanation from and I queried it as well. Every other book I've read and everyone I've asked say it's wrong and you will indeed get dragged TOWARDS the building, not 90deg to it.

How's the course going?
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paddywak
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PostPosted: Sun Jan 20, 2013 10:41 am    Post subject: Reply with quote

Hello buddy, im about 20 hours in and quite intense. I don't seem to have enough hours in the day. 3 sorties a day then catching up on ground school stuff until midnight most nights. I've even given up alcohol this month Shocked

It seem to be flying by " no pun intended" but all good stuff!
How's tricks with you? has that fog eventually lifted?
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Ascend_Charlie
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PostPosted: Mon Jan 21, 2013 5:31 am    Post subject: Reply with quote

This question raises its ugly head regularly, and every time the answer comes up as "Err.... yeah, the aircraft should move parallel to the obstruction, not towards it...??"

I don't have the answer. Try Shawn Coyle's "Cyclic and Collective" for a possible solution.

Has anybody ever heard of an aircraft being "sucked in"?

I have hovered very close to cliff faces, landed next to open and closed hangar doors and other solid obstacles, and never felt that I was being drawn in any particular direction by recirculation. Maybe it's one of those great furphies like LTE.
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flip2
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PostPosted: Mon Jan 21, 2013 12:09 pm    Post subject: Reply with quote

Thank goodness you didn't mention the word "gyroscope" Tied Up then you'd really open a can of worms Wink

I don't know the conclusive answer to your question, but I suspect when people claim to get "sucked in" to a nearby obstruction it could have something to do with pilot fixation on said obstruction.

Like Ascend_Charlie, I've never really noticed it... perhaps any aerodynamic effects are so slight that we automatically compensate for it?
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paddywak
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PostPosted: Fri Feb 01, 2013 5:09 pm    Post subject: Reply with quote

"Gyroscope".....ha,ha , there ive done it now!

Cheers guys, Ive never felt the effect either Laughing
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paco
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PostPosted: Sun Aug 18, 2013 2:22 pm    Post subject: Reply with quote

When hovering next to a building or other obstruction, the downwash at that point goes through the rotor disk multiple times, thereby increasing the induced flow and by definition reducing the angle of attack. As the other side of the disc hasn't changed its characteristics, there is more lift on that side which pushes you towards whatever it is you are hovering next to.

If you were talking about individual blades, the phase lag might come into it, but we are talking about the rotor disc as a whole.

Phil
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Ascend_Charlie
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PostPosted: Thu Aug 22, 2013 10:51 am    Post subject: Reply with quote

Phil, the same thing can be said about inflow roll and also about flapback.

it seems that this phenomenon of "more lift here than over there, move it 90 or so degrees in the direction of rotation of the disc, then let it roll" only applies in SOME situations.

The bigger question to be asked is :

"Has it ever REALLY happened?" Anyone? Anyone? Bueller? Fry?
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Ascend_Charlie
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PostPosted: Thu Aug 22, 2013 10:51 am    Post subject: Reply with quote

Phil, the same thing can be said about inflow roll and also about flapback.

it seems that this phenomenon of "more lift here than over there, move it 90 or so degrees in the direction of rotation of the disc, then let it roll" only applies in SOME situations.

The bigger question to be asked is :

"Has it ever REALLY happened?" Anyone? Anyone? Bueller? Fry?
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paco
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PostPosted: Thu Aug 22, 2013 9:33 pm    Post subject: Reply with quote

Well, it must have happened to someone otherwise we wouldn't be teaching how to avoid it Smile

Phil
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