Iran hostage rescue helicopter limitations

Brief — the motivation for tilt-rotor research. Helicopters too noisy; tip-speed approaches Mach 1.

The Defense Department got interested in a different type of helicopter after the Iranians took over the American Embassy in 1979. They tried to rescue the hostages and the helicopters weren't successful. Helicopters tend to announce their arrival — they're noisy, inherent with the blade whipping around. If you're moving forward, the tip of the advancing blade approaches the speed of sound and gets very noisy; the receding tip is not. If you tilt the rotor and turn it into a rotor for an airplane, the tip speed never gets close to the speed of sound, and they can be much quieter.