Piezoelectric energy harvesters can draw energy from the mechanical loads and the vibrations in the fuselage/wings/blades of aircraft.
Ok, think about this: in a perfect airplane, there would be no vibration. Every erg of energy being used would go into the propulsion. Any vibrations are, by definition, wasted energy.
Think about a specific example. You know the vibrations in a turboprop plane that make it sort of "buzz" inside the fuselage? That's caused by air off the props that hits the fuselage. That air is being pushed by the engine, yet it's not pushing the aircraft forward. That is the definition of wasted energy.
Ok, so real aircraft waste some energy, big deal. But how much? Obviously, the thing is flying, right? So it can't be that much. I mean, if it was 50%, that would be 2000 hp of wind smacking into the fuse - you'd need a parachute, not ear protection! So we're talking about what, 1%? 3%
So that's why we don't use piezos to recuperate. You're trying to collect this tiny bit of power. Even at 100% efficiency, and I'd say 10% is more likely, we're still talking about a tiny fraction of the power you need to do anything.
In fact, the only use of piezos in this role is the exact opposite. The Q400 (and others I assume) power a set of piezos on the fuselage that provide the opposite vibration to the air off the props, and thereby lower cabin noise.