Nick doesn't like my resistance to his take on ground effect, but I still feel it is relevant:
In Ground effect - the air approaching the aeroplane (which is close to the surface) is forced up over the aircraft and has little or no effect...and it is also forced between the fuselage and the surface. The latter causes the air pressure to build which induces a ground effect...similar to the Ekranoplane in the picture. This 'effect' is utilised by the designer to assist the motion of the vehicle over the surface.
In addition there is Ground cushion. This develops when the downwash from rotors impacts the surface and bounces outwards and inwards. The inwards vectors induce addtional air pressure, or a 'bubble' which to some extent, supports the airframe, reduces the induced airflow and increases lift which allows the operator to reduce the collective to maintain position.
It also causes the static tube to register a downward indication in the VSI.
There are 2 effects consequently, each being sourced differently.
WRT VRS Vs SWP:
My take for what it's worth:
SWP: you arrive into the hover at your listed weight and respective approach speed and these two factors are too much for the remaining power margin you have left in the collective. Hence you run out of arresting power to stop the momentum of the arriving vehicle. I liken it to driving too fast and the brakes being too ineffective to stop you.
VRS: Is disc related and is to do with placing the vehicle in an aerodynamically unstable situation w.r.t.: RoD / Speed and disc angle.
Once again, my observations suggest that the North Americans tend to confuse the two, whereas the Europenas gravitate more towards VRS and its effects and barely talk about SWP.
two cnets worth
