Resonance in helicopter testing is important, because it prevents this from happening in the air:
and from teh side:
The edited version of the Popular Science article by Michael Moyer explains what is happening:
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 not 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.
Shaking things like bridges, skyscrapers or helicopters, at their resonant frequencies, 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 the 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, that bird’s goose is cooked.
Thanks to P. Reid for forwarding this!
and from teh side:
The edited version of the Popular Science article by Michael Moyer explains what is happening:
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 not 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.
Shaking things like bridges, skyscrapers or helicopters, at their resonant frequencies, 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 the 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, that bird’s goose is cooked.
Thanks to P. Reid for forwarding this!
1 comment:
I'm sure you do this just to provoke me. The article you quote is more or less correct, but I think it over simplifies some in unnecessary ways.
Not everything resonates. Some things are designed not to resonate (legs under ancient record turntables, e.g.). Some things have multiple resonances.
The article is close on about resonance: for various reasons different systems have resonances, frequencies at which they can be made to oscillate / vibrate at very large amplitudes with very little dissipation by suitable external forces. The mechanical problem is the lack of dissipation, which is what allows the big amplitude, and it's the big amplitude that can rip things apart, generally through mechanical fatigue (think bending a wire back and forth to make it snap).
It's not always destructive; resonance is sometimes desirable, e.g,, if you were making a glass harmonica from wine glasses you'd want a good, clear resonance from good crystal.)
Of the many tests that hardware about to be launched into space goes through is a vibration test: the device is shook at frequencies all across the spectrum to look for resonances--frequently spotted when things fly off the equipment. It's a very scary test to put things through.
One of the most famous examples of destructive resonance was the collapse of the Takoma Narrows bridge, in 1940. I still find this video rather harrowing. Watch for Prof. Farquharson -- he was known to absolutely all physics grads in my day thannk to a famous tape loop about--wait for it!--resonance phenomena. The narration said: "Note how Prof. Farquharson walks along a node of the oscillations." Physicists--always droll.
Modern skyscrapers, btw, are built to sway but not to resonate. They avoid resonance with inertial mass dampers that are built into the tops of the buildings. Some of the designs are quite extraordinary.
Post a Comment