LZ,
Very intersting stuff and I'll come back to you on it separately, want to try and keep this (my present)post focussed on slopes, ground effect and recirculation with obstruction on one disc only.
All-
We did touch on some of the recirculation effects on a thread a month or so ago but I think it is well worth going into with more depth.
Thomas - You are perfectly correct on the drift toward the obstruction, it most certainly does happen (as you have demonstrated) and hopefully I can explain.
When we consider recirc. we are generally speaking about the effect at the tip area
in the first instance, in that most
(through disc) recirculation happens there. The effect will be manifested 90 degrees on and a Heli (anti clock MR) with the obstruction on its right would drift forward if uncorrected.
But this is only good for recirculation affecting the tips only - If the Heli becomes closer to the obst. more of the disc will be affected (across its diameter). If the area affected by recirc. becomes half of the disc then the blade over the tail will be low by the time it gets to the (90 deg travel)right hand position.
The cumulative effects of recirc. on the blade at the right side and now at the tail will result in drift to both the right (toward the obst.) and front.
There is a small balancing effect at work as air recirculated at the disc's FRONT has the effect of tilting the disc low on the LEFT
at the same time, so resulting drift toward
the obstacle is diminished but it is not neutralised.
But the problem is also made WORSE as stable ground effect may be present on the left, out of the area of greatest recirculation, (Alpha is increased) tilting the disc to the right again as that blade climbs.
Drifting into an obstruction is of course avoidable by keeping your distance, if only the tips are affected by recirc. on the right then only drift to the front would result (still air, uncorrected).
I prefer to keep ground effect as it pertains to slopes and recirculation as it pertains to obstruction seperate as they are different causes (although they are related by their effect on tip vortices, induced flow and Alpha) and may have simliar end results. The common factor in both is gyroscopic precession and to predict the resuting drift you must find out how much of the disc is being affected.
If only the tips experience the max. effect (in considering only one blade position) as with a slope (constantly varying ground effect) then the drift will me manifested 90 degress on from the max. position.
If the maximum effect moves 'into' or across the disc diameter (toward the blade root)then the resulting effect (disc tilt) will be manifested earlier in the cycle as gyroscopic precession will also begin its work earlier on any blade approaching.
This is why sloping ground will give a wholly different drift direction to recirculation caused by an obstacle very close to the disc.
Sloping ground will always have its max. effect at the top of the slope which will constantly strengthen and diminish through the blade's 360 dgree cycle.
It would be possible for a very steep slope to produce its max. effect on disc tilt earlier to produce drift toward the highest part of it and the logical extension of that is to think of an 'adjustable' slope becoming steeper until you end up at vertical....Which brings us to obstructions like Hangars...
Recirculation due to vertical obstruction can have a much greater effect over more of the disc area (up to a half) as seen from above.
It really only depends on when you stop calling a slope a slope at start calling it
a vertical obstruction!