I am very much a beginner but I'm curious about this. When you invert, why doesn't the copter immediately slam into the ground? Gravity and the blades would be pulling down. It doesn't happen on the FMS trainer.

Because of negative pitch

which is...? Sorry I'm a heli noob

Pitch is the angle of attack for the blades. If you have positive pitch, you go up, negative pitch, you go down. If you roll into inverted flight, the controls are reversed. negative pitch makes you go up, and positive pitch to go down. If you are not careful, and roll into inverted flight and do not adjust yourself at the controls, you will simply fly straight into the ground. Hope this helps some.

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\ positive / negative and the blades are spinning left. when u have positive pitch u get air blowing down and when u have negative u get air blowing up

Got all of that but HOW does the tx know that the copter rolled over? Or how does the tx know to change the pitch from positive to negative. I obviously haven't tried this with a copter but the simulator just lets you barrel roll all day. I'm not changing anything on the tx.

You flick a swich on the tx and it reverses the controlls needed to be reversed, I have the tx that comes with the honey bee cp2 and it's switch is on the top left side

Actually most people no longer use an invert switch. The switch is useless when you are flipping over repeatedly and such. We use a combination of throttle and pitch curves programmed into the radio so that you have full throttle at the top and bottom of the stick and postive pitch at the top of the stick and negative pitch at the bottom. Basically when you roll inverted you just pull the throttle stick back to what you would usually associate with idle. There are multiple curves programmed into a heli radio generally and the 1 for inverted flight is usally called Idleup 1 or Stunt 1 with normal mode allowing only upright flight.

FWIW, most airplane radios do have the software to create the required throttle and pitch curves for helis. Some radios have it all though such as the 7, 8 and 9 channel radios from JR and Futaba, and some of the newer 6 channel radios so they can fly planes and helis.

Now we use something called "Idle Up"

In position 1 on the switch, the throttle and collective are linked together.
Think of the throttle stick as a clock. With the stick at the 6'o clock position, the motor is at a dead stop, and there is 0 pitch in the blades.
As you move the throttle up towards 12 'o clock, head speed increases and positive pitch increases.

In position 2 (commonly referred to as idle up) The throttle is set at a specific RPM, and the stick controlls the collective.
At the 6'oclock position, full headspeed(throttle) is applied and full negative pitch is applied.
At mid stick position, hovering headspeed(throttle) is applied and 0 or "flat" pitch is applied.
At 12' o clock position, full headspeed(throttle) is applied and full positive pitch is applied.

Positive pitch = air blowing down.
Negative pitch = air blowing up.

So to get the heli inverted.

The pilot pulls it up into a hover by moving the throttle stick towards 12' o clock until the heli lifts off. Once he gets into a stable hover, then he flips the "idle up" switch.
He bumps the throttle towards 12' o clock to make the heli "leap" upwards a little, while applying forward cyclic. As the heli starts to nose over into inverted flight, the pilot moves the throttle stick towards 6'o clock to apply negative pitch. Once he comes back over to right side up flight, the pilot moves the throttle stick back towards the 12' o clock position to return to positive pitch.