Hi
Archer,
If you are taking the R.22 intro in the USA, part of your ground will be some "awareness training" on low-"G" mast bumping and low-rotor RPM stall.
You've already gotten the explanation on what 100% Nr (rotor rpm, or RRPM) is, suffice to say that the slower the rotor is turning, the less tota lift it can produce, and the more power it takes to maintain constant lift. The R.22 Beta II runs at 104% on the tach, which is about 2650 engine RPM and 500 RRPM.
If the RRPM drops to 97%, the rotor takes more power, but the engine is also slowing (to around 2470 rpm), so it is losing ability to make power. Now 97% is no worry in the R.22, but it is where the low-rpm horn goes off. The R.22 will fly quite well at 90% RRPM, it will just have less power reserve. So 104% is better, because the rotor is more efficient, and the engine is more powerful!
On the other hand, 80% is not so good, as the power the rotor demands to provide adequate lift is now getting close to exceeding the engine's ability to provide that power. Once those curves cross, the RRPM will decay rapidly - BAD THING. This is one cause of low-rpm rotor stall, the other one being letting the RPM go too low in autorotation.
The good thing is that in the R.22, you have two tachs, a governor, a warning horn, and your own ears. The chances of an aware pilot getting into a low-RPM situation is
very small.
Low-"G" is simple. If you are in cruise flight or a cruise climb and you push the cyclic forward rapidly (as you might do with an airplane to dive), the rotor system is unloaded. Imagine an airplane which was hanging from its wings by strings - you push the plane over and the wing descends faster than the fuselage - the strings go slack.
In the R.22, what happens is the tail rotor (still producing thrust) starts to roll the airframe to the right pretty quickly, while the rotor disk does not roll! The rotor hub, which is just on a simple teetering pivot on the rotor mast, will rapidly reach the limits of its travel and start contacting the rotor mast. A couple of thumps, and the mast will bend or fracture, neither being conducive to continued flight. Again, super-easy to avoid - don't push the cyclic forward rapidly in cruise flight or from a climb.
To add a bit to the flying advice offered by others, when you are flying the helicopter, first keep your eyes outside and away from the ground, especially while hovering. Use references 50' - 100' minimum. Don't think of control
movements, just control
pressures, especially with the cyclic. The Robbie is very sensitive to the cyclic and will start to move the instant you think about moving. So if you start drifting right, just think about stopping the drift,
not moving the cyclic left - if you actually move the cyclic left, you will start going back and forth like a yo-yo as your movements will inevitably be exactly out of phase with what's required.
Instead, if you start to drift right, just put a slight bit of left
pressure on the cyclic. As soon as the drift stops, relax the pressure.
...and don't worry if you don't hover well or at all. As a CFI who has most of my hours in the Schweizer, I hopped into the R.22 after about six months out of it (with a factory IP), picked it up and almost immediately started going back and forth like a drunken boxer...
...I figured it out, but I think that IP was a bit worried for a moment!
And HAVE A BLAST! Flying a helicopter is truely something worth experiencing, as often as possible.