some early helis used longer blades than the new ones. The long blades load the engine and prevent the rotor from achieving its most efficient rpm. It has become relatively common to trim 1 inch off some older heli's blades. (careful... gotta rebalance the rotor)

The glow engines on .30 size helis need to turn 10,000 to 14,000 rpm to be in thier peak power band. If you get a tachometer, and measure the rotor speed, its just a matter of knowng the gear ratio to know the engine rpm. You can find the gear ratio by... marking the clutch bell with a "Sharpie" or other felt pen, count how many times that goes around for one rotation of the main rotor. (Turn the rotor backward... or you might have to turn the clutch bell... auto-rotation system disengagement can drive you nuts trying to turn the rotor never making the clutch move.)