Aerody question for Nick, Shaun or any genius..

[Ascend Charlie]

OK, we all know about phase lag and how a change of lift conditions (more relative airflow, less induced flow etc) to one part of a disc will manifest itself approximately 90 degrees later. Evidenced by flapback, and inflow roll.

However, I am not convinced by Recirculation.

Supposedly, hovering close to a cliff, wall or hangar will recirculate the airflow through the disc closest to the obstacle, and the loss of lift there will cause the helicopter to drift towards that obstacle.

Eh?? If there is less lift at the nearest section of the blade, the loss of lift will manifest itself about 90 degrees later, and the aircraft will move PARALLEL to the obstacle.

Please shoot holes in this if you can, it is a bit of a puzzle. I have spent plenty of time hovering near obstacles and have never felt any tendency to drift towards it.

[[email protected]]

I have spent plenty of time hovering near obstacles and have never felt any tendency to drift towards it. Me too - I suspect this is one that Nick might debunk as an urban myth.

[Sloppy Link]

I seem to recall it was something to do with increased induced flow. Not FI/QHI so no doubt someone of that exalted community will remind me in a suitably patronising manner that I am wrong. Bitter? Moi? I only bonged my minima by 20' for chrissake but I'm over it now.

[Flyting]

I had to pick up cargo from a tightly, high walled, securety pad, which was never a problem in the 350....but when I landed there with the B205, it was a bitch purely from recirculation. Never heard or felt it by hovering by a cliff face as the air wouldn't recirculate much. Needs to have the ground and a wall/cliff next to it.

[NickLappos]

The main question is "What effect does a wall have when imposed next to a rotor?" If we picture the rotor as producing a downward stream we then would like to know what the wall does to that stream, and what that means for rotor behavior and pilot workload.
All the following is conjecture, I have no relevant experience or test data to support it, but like a good Grimm's Fairey Tale, here goes: My gut feel is that the wall will behave (if sufficiently close) like an end plate, restricting the outflow, breaking up the recirculation at that azimuth and helping the rotor become somewhat more efficient for that brief moment when it passes the wall. This would produce some extra lift for the power, and also flap the blade upward later in its rotation. For a US rotor rotating CCW fro above, this would seem to want to make a right side wall cause a slight pitch up.
I also believe this effect would be small, easily disturbed by wind or wall roughness, and probably only induced if the rotor were operated within a few feet of the wall (perhaps 5% blade span - 1 foot for a 20 foot rotor- probably far, far closer than safety would dictate).

When we introduce the ground at the bottom of the wall, the wal will restrict the outflow, and probably confuse the recirculation. My guess the effect would be very unsteady, and not benificial at all, costing hover power and controol workload. When compounded with wind, this wall/ground effect would be problematic. Since it disturbs the rotor wash, I believe the effect would be worse for higher disk loading rotors (bigger helicopters).

I am charging my little model helicopter, and will try some wall hovering in a few minutes and report back.

[ShyTorque]

Welcome back, Nick.

I was always taught that anything that might cause recirculation reduced rotor efficiency, but whether a wall can do this is open to debate.

Just for fun, on a similar subject, when you have charged your little toy and solved this problem, if it's an indoor helicopter, please next try hovering it just under a flat room ceiling (I mean within an inch or so) and let us know the result! :E

I've tried this many times with a miniature helicopter I used to have, with repeatable results. It was infra-red controlled and had constant pitch blades, with rpm controlling rotor power. Whenever it got too close to a ceiling, the damn thing seemed to get suddenly drawn upwards then become almost "stuck" to it. I assume this is because of greatly reduced inflow giving a much more efficient rotor, similar to ground effect but much more pronounced. Having reduced rotor power almost to nothing to get it to drop away, it was difficult to stop it falling so fast that it hit the floor. Almost as if it instantly went into a vortex ring condition.

[Um... lifting...]

One suspects that the only firm conclusion that will be reached from the wall-hovering model helicopter experiments is that the lady of the house will require you to engage in paint touch-up before being permitted to play on PPRuNe any further.

[boguing]

Wall effect and ceiling effect certainly exist in my house.

My explanation is that the presence of the wall is a restriction on the air flowing into that side of the rotor disc, not only does it block the free flow of air because it's a wall, but because Reynolds painted it and air sticks to it.

The rotor disc keeps 'pulling' air in though, so any that comes from the direction of the wall has to be accelerated into and through the gap between wall and rotor, which reduces the local pressure.

Consequently the higher pressure on the opposite side blows the rotor towards the wall.

[RVDT]

Maybe it's all bollix?

The proximity to the ceiling restricts development of the tip vortices and subsequent induced drag?

Henceforth the rotor becomes less inefficient?

Just sayin'

[Frying Pan]

Umm..yes...


When I recirculate air around the room, my wife hits the roof


Is that the same?

[paco]

According to Ray Prouty, if you are hovering next to a wall, say on the left hand side, close enough to cause recirculation on that side, left to itself, the helicopter will drift to the rear, but there will also be a non-standard cyclic position that may take you by surprise if you are not quick enough to detect it.

Phil

[CYHeli]

What about an optical illusion where you are (visually) drawn to the wall and it is pilot induced?


It is very pronounced when you have wind coming over a building and you try to land on the lee side in the down draft, but that would be a different story again.