Loss of tail rotor effectiveness is just what it sounds like. At certain orientations into the wind, the tail rotor can become less effective (because of vortex ring state) and the pilot basically has a harder time keeping the aircraft pointed in one direction. A main rotor and a tail rotor are basically the same thing, except the tail rotor is smaller and lies in a different plane. In the case of tandem rotor craft like the V-22, directional control is created by an interaction between the two rotors. If you decend too quickly and create a Vortex Ring around the rotors of a V-22, it can possibly be worse than a conventional helicopter because you don't have a tail rotor to help you recover.
So what is Vortex Ring State??? Well, it only looks like a ring in illustrations. Actually, it's a bubble of non circulating air that forms around an aircraft's rotor when its vertical rate of decent is about equal to its rotor wash velocity and there is little or no lateral movement (air circulates inside the bubble but not with the free stream air). In other words, a pocket of air forms around a helicopter when it's going straight down to fast. No circulation of air means not a whole lot of lift or control. Pilots usually think of it as flying through your own rotor wash, which is pretty close to what's happening. The same thing would happen to a propeller if an airplane flew backwards too fast, but I don't think that's much of a concern except for extreme aerobatics. A Vortex Ring isn't even the final stage. If the back flow of air becomes faster than the rotor wash, a Windmill Brake State will form. Here, all the air is forced back through the rotor the wrong way and you can actually stall out the main rotor. Once you stall the main rotor there is no recovery.
Just to recap todays lecture... A V-22 decends straight down too rapildy, it enteres a Vortex Ring state which quickly leads to a Windmill Brake. The main rotors stall and the V-22 crashes. It can all happen very fast. The best way to recover is to avoid it all together.
I hope I didn't confuse you too much. One of the hardest things I've had to learn is to translate between engineers and pilots - they don't speak the same language. A lot of this is because pilot's are usually taught the lamens explinations and don't have a real understanding of what really happens, and most enginners don't have a clue how to actually fly a plane. This leads to some very interesting design meetings. It's kinda like a Nascar team where the crew memebers have never driven a car and the drivers have never looked under the hood.
Ok, I think I've bored you enough. If you're still completely confused, there are many web sites around that talk about this subject.
James
Edited by - mahler on 01/17/2002 14:56:34
