One Chinook Shake Please
Everything has a beat. A rhythm. A frequency at which it likes to shake. You can rock most objects off-beat for as long and hard as you like, and nothing much will happen. But start to push and pull in time with the natural frequency (the resonant frequency) of the object in question, and it will quite literally start to fall apart. Remember the fact that an army never marches across a bridge? It is because of the resonant frequency.
The resonant frequency plays an important role when dealing with bridges, skyscrapers or helicopters. Shake these at their resonant frequency, and the back-and-forth motion spells trouble. Each push adds more and more energy to the object, energy that, if not dissipated, starts to wreak havoc. That’s what happens with our Chinook. The rotating blades begin to shake the airframe at its resonant frequency, and physics takes care of the rest: Because the blades are unable to dissipate the excess energy, the convulsions rend them from the fuselage.
Helicopters are prone to resonant effects, which is why resonance ground testing (as seen in this video) is a standard part of chopper R&D. If both blades in a twin-rotor helicopter share the same heavy vibration and the engine mounts aren’t rock-solid, the energy generated can actually make the motors start moving around the engine mounts, and the next thing you know, the bird is cooked.
See this video of a CH47 Chinook getting pooped during a resonance ground test. The video is taken from behind the chopper.
