planemad_bk: any movement, like twirling your keys around your finger, produces vibrations. The complications produced by having an engine + gearbox + prop all vibrating
and generating more vibrations in the aircraft to which they are attached ... the designers have to try to get the vibrations to balance each other for, if they interact they can multiply the vibration and this leads to uncomfortable moments.
Sometimes, the vibration from the engine will resonate with another item and
that leads to the kind of doubled-up vibration that is destructive. During design and testing, they can find and eliminate most of these. If, however, there is a vibration only present in a narrow bandwidth of revs, then it may be best to just avoid it, providing of course that it is not in an optimal place on the torque curve!
Many motor vehicles have a similar problem and they are tuned to be stable at all legal speeds but it is not unusual for a car to have a sweet-spot when the vibrations are minimal and the speed is maximal, particularly as the car gets older and components are worn in unpredictable ways. Then, when you find that sweet-spot, all you have to do is explain this to the traffic police.
