Helicopter Rotor Tip Speed RPM Calculator

Rotor tip speed: v = 2·π·r·(RPM/60)

A helicopter's main rotor lives in a fairly narrow band, somewhere between 250 and 500 RPM depending on rotor diameter. The bigger the disc, the lower the RPM, and that trade keeps the blade tips well under the speed of sound. The Bell 206 main rotor, for example, cruises at roughly 395 RPM. Tail rotors run far faster, in the 2,000 to 3,000 RPM range, simply because their radius is so small. What you really watch is the tip Mach number, usually held around Mach 0.5 to 0.7. Push the tips closer to Mach 1 and you start paying for it with compressibility drag, retreating-blade stall and a sonic boom that wrecks aerodynamic efficiency and shakes the aircraft hard. As a worked example, 400 RPM on a 5.5 m radius gives a tip speed of 230 m/s, or Mach 0.68 at sea level.

Applications

You'll see this come up in rotor blade design, in calibrating the NR-100 governor (which holds NR, the rotor RPM, at 100% through FADEC), in maintenance work and ground runs, in vibration analysis at 1/rev and n/rev, in autorotation engineering, and in noise certification studies.

FAQ

Why never reach Mach 1 at the tip? Once a sonic shock forms on the advancing blade, you get drag divergence, heavy vibration, lost lift and structural fatigue. Supersonic tip speeds just aren't acceptable, whether you judge them aerodynamically or by the noise they make.

Why does the tail rotor spin so much faster? Its radius is small, often under 1 m, so reaching a comparable tip speed and producing enough anti-torque thrust takes a much higher RPM.

What is NR-100? NR is the rotor RPM written as a percentage of the nominal value, so NR-100 means the governor is keeping the rotor at exactly 100% of its design speed.