I was wondering: how can a propeller plane, single engine or otherwise, be flying along relatively silently and then start making noise, often very loud--and then turn it off just as quickly? Are they descending/ascending? It seems like the engine must be straining. Thank you.
It is unusual to change prop speed quickly and only briefly, so I would suggest two possible explanations:
- On the ground: The pilot tests the propeller pitch control. This he/she does by revving up the engine while the propeller is at a low pitch setting. Then the pitch is increased, which increases the aerodynamic forces on the prop and, consequently, the noise it radiates out. Since this test takes only a few seconds, the result is just what you described (but not while flying overhead).
- In the air: Propellers don't radiate noise equally in all directions. This is especially pronounced on the T-6 "Texan", where the noise intensity increases dramatically when you stand directly abeam of the aircraft. The plot below is taken from this report on propeller noise. Probably what you hear is caused by the variation in noise (and dominant frequencies) while the plane flies overhead.
Directivity plot of propeller noise. The details depend a lot on the specific aircraft and propeller type.
Most of the noise of a propeller plane comes from the propeller tips. And an easy way to change the noise is to change the tip-speed, either suddenly up, suddenly down, or gently.
People on the ground notice the change in the noise of an airplane, so if you need to change the propeller speed, it's best to do this gently so there's no sudden 'whoosh' of the tips.