Posted: Sat Jan 19, 2013 6:22 pm Post subject: airflow recirculation
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!
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? _________________ CPL(H), FI(H), H269, R22, B206
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
It seem to be flying by " no pun intended" but all good stuff!
How's tricks with you? has that fog eventually lifted?
Joined: Aug 23, 2005 Posts: 266 Location: On a course.... golf course
Posted: Mon Jan 21, 2013 5:31 am Post subject:
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.
Joined: May 14, 2005 Posts: 90 Location: Wycombe Air Park, Calgary, Dubai
Posted: Sun Aug 18, 2013 2:22 pm Post subject:
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.
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