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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.

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  • $\begingroup$ Engine noise is a very small portion of the total noise of a propeller (or rotor) aircraft. Propeller tip speed seems more likely to me to be a determining factor, especially as the propeller tips approach the transsonic region. $\endgroup$ – a CVn Feb 2 '18 at 9:54
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    $\begingroup$ When flying or when taxiing? $\endgroup$ – Koyovis Feb 2 '18 at 9:56
  • $\begingroup$ When flying overhead...I see planes that are able to turn the noise on or off--without appearing to be making dramatic movements. $\endgroup$ – Jeremy Feb 2 '18 at 12:12
  • $\begingroup$ Welcome to aviation.SE! If you're close to an airport with a lot of flight training, you may be hearing aircraft in the pattern making power changes as they take off at high/full power then reduce it again for landing. $\endgroup$ – Pondlife Feb 2 '18 at 15:49
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It is unusual to change prop speed quickly and only briefly, so I would suggest two possible explanations:

  1. 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).
  2. 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

Directivity plot of propeller noise. The details depend a lot on the specific aircraft and propeller type.

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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.

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  • $\begingroup$ Thank you for answering. I'm a bystander on the ground and have noticed this phenomenon: planes going along relatively silently, then the noise gets dramatically louder, almost like what you hear in a movie, but they don't appear from my perspective to be pulling up or diving. Do you know what causes this? Ascending or descending? $\endgroup$ – Jeremy Feb 2 '18 at 12:16
  • $\begingroup$ they are probably changing the propeller pitch. in an engine driving a "constant-speed" prop, decreasing the pitch causes the engine to rev up and on some planes this makes the prop tips go supersonic, which greatly increases the noise output of the prop at the same moment the engine noise increases in pitch. this produces a sudden WOWWWWWWW noise. This is common when the plane is on approach for landing, because the pilot wants the engine running at max RPM so full power is available if a go-around is required. $\endgroup$ – niels nielsen Feb 2 '18 at 19:46
  • $\begingroup$ Niels and RAC: thank you so much for your help. $\endgroup$ – Jeremy Feb 3 '18 at 19:39

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