Langley helicopter supersonic blade tip experiment

Cited to explain why conventional helicopters are speed-limited around 200 mph — once the advancing blade tip approaches Mach 1, vibrations and instabilities will rip the blades off. NASA built and flew an experimental helicopter with a jet engine on the back to break the barrier deliberately; it's now an outdoor exhibit at the NASA Hampton facility.

Down in Hampton, Virginia, there's a NASA facility where they do test flights, and they have a helicopter out there. NASA once flew a helicopter that had more than Mach 1 blades. Typical helicopters might be limited to 200 miles an hour — they can't go 250 miles an hour because you'll rip off the blades from the vibrations of trying to break the sound barrier. You get a sonic boom and all kinds of vibrations.

NASA Langley fitted rocket motors to a helicopter to push forward speed to 240 mph, driving advancing blade tips supersonic to study the instability.

It's the blade speed, the tip blade speed, and specifically the advancing tip blade speed. When that goes supersonic, you run into all kinds of stability problems. At Langley, Virginia, at the materials lab right outside, they have a helicopter that they once put some rocket motors behind, and they actually flew it with the rocket motors pushing it forward to like 240 miles an hour. The blade tips had gone supersonic on the advancing blade tip. So maybe it's only 200 miles an hour going forward for the fuselage, but the advancing blade tip is going a lot faster than that.

NASA Langley pushed a test helicopter past sonic tip speed for a few seconds. The general lesson: helicopters are noise-and-vibration-limited at ~200 mph because the advancing rotor tip approaches Mach 1.

It was a V-22 Osprey that was the first aircraft in when they captured Osama bin Laden. Why? Because they're quiet. Helicopters are noisy. Airplane mode is nice and quiet. They can go three hundred miles range. Anybody know why helicopters can't go faster? This thing can go three hundred miles an hour, whereas helicopters can't go more than about two hundred miles an hour. [Tom gestures with rotor blades.] If you're going in that direction, the rotor blades are going around like this. There's no problem for the rotor blade that's retarding, but the one that's going forward is sweeping around, and when it reaches sonic velocity, all of a sudden you get vibrations and noise. You can't exceed the speed of sound on a helicopter blade without having tremendous energy. NASA did it — if you go to Langley you'll see a test helicopter where they actually did go faster with the tip speeds above the speed of sound, but only for a few seconds.