The American Helicopter Company made a specialty of pulsejet-driven rotors. A characteristic of this type of tip-jet engine is its limit on tip speed to avoid engine blowout. The relatively low rotor speed meant large heavy blades on a small helicopter. These blades, combined with heavy tip engines, resulted in a very high rotational inertia. So much so that on landing it took a few minutes for the
rotor to decelerate enough to stop it manually. From Vertiflite, Fall 2004 (page 52).