PhysicsRider

Miniature Aircraft makes a tachometer specifically for rotor blades... You can check their speed on the ground or in the air while flying... Only thing is you need another person to check the head speed while you fly, or vice versa... Here is a blurb from their site regarding the tach:

0524 - X-Cell Optical Heli Tachometer


Designed by and for R/C Heli enthusiasts to provide the ultimate in safety and accuracy while measuring main rotor speed of any heli at any time, from lift-off to landing, regardless of altitude, air speed or attitude.

Package Contents: 1 each

You can get it for $125 from Ricks RC: www.ronlund.com for more info from the manufacture, visit: http://www.x-cellrchelicopters.com/Store/index.asp