Help! I've done something stupidly obvious to my helos...

...but I can't, for the life of me, figure out what.
I changed the default flight to one where the plane was sat in a parking space with its engine off, but all electrical/fuel switches on, just awaiting a mouse-click on the starter button. Now, when I create a flight with one of my add-on helicopters (Medco Huey or PaveHawk) they sit there with their rotors turning at almost 100%, but as soon as I pull up on the collective (ie. joystick throttle) the rpm's fall away, the helo doesn't do more than lurch, and it just sits there at about 50% rpm until the collective is bottomed again.
I know I must have left something switched off, but I've scanned every pixel of both helicopters' control panels in both 2D and VC views for a switch I've missed, but I can't find one. I've tried Ctrl-E to do an auto engine start but to no avail.
What have I missed? I just know it's going to be something painfully obvious...
I changed the default flight to one where the plane was sat in a parking space with its engine off, but all electrical/fuel switches on, just awaiting a mouse-click on the starter button. Now, when I create a flight with one of my add-on helicopters (Medco Huey or PaveHawk) they sit there with their rotors turning at almost 100%, but as soon as I pull up on the collective (ie. joystick throttle) the rpm's fall away, the helo doesn't do more than lurch, and it just sits there at about 50% rpm until the collective is bottomed again.
I know I must have left something switched off, but I've scanned every pixel of both helicopters' control panels in both 2D and VC views for a switch I've missed, but I can't find one. I've tried Ctrl-E to do an auto engine start but to no avail.
What have I missed? I just know it's going to be something painfully obvious...