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Edit: I'm beginning to think this is more suited to the physics stack exchange, but I'll keep this here for a while...

Being bored mindless, surfing the interwebz, I came across this video, where a Sukhoi SU-57 is performing aerobatics:

Youtube: Су-57 с новым звуком пилотаж на форуме Армия 2020 Кубинка

At timestamp 1:45 the plane makes a pass, emitting a distinct screech with prominent components at about 1000, 2500 and 3600Hz.

This screech may very well be from external "add on" equipment such as radar reflectors (see this Q&A), as I kind of combine such noise to some kind of "fault" in the aerodynamics, and therefore I think during design phase sources of such disturbances in airflow would be eliminated from the airframe.

As first hand observations from real life, two examples: sailplanes and the B757, close encounter of which I have found to be memorable: the sailplanes for being very quiet, emitting a slight whisper with maybe a dash of a faint whistle at a high speed fly-by, and the B757s making a very distinct and sharp hizz on short finals (B757 is categorized as "heavy", not due to its size or mass, but for the nasty wake turbulence it makes).

So: can one draw conclusions about planes aerodynamic efficiency based on acoustic observations, and if so, to what extent?

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    $\begingroup$ Any aerodynamic noise will be drowned out by engine noise. $\endgroup$ – StephenS Dec 18 '20 at 22:30
  • $\begingroup$ Ummm... no. I have very different experiences. Yours may differ, of course. $\endgroup$ – Jpe61 Dec 19 '20 at 13:02
  • $\begingroup$ What is your experience hearing a B757 in flight with both engines shut down? Even at idle, they produce a staggering amount of noise. $\endgroup$ – StephenS Dec 19 '20 at 16:15
  • $\begingroup$ To answer your question, none. But: many a times have I observed a plethora of jets, large and small from tens to tens of thousands of feet away. I'm no stranger to seats of them either, and behold, my ears can separate noises from engine and, say, fully extended flaps; a fighter jet performing a full g pull makes distinctly different sound than one travelling at same speed and configuration at 1g. Now: I'm not going to argue about this with you no more. To back off a little: I can't actually hear nothing but the engine noise of a jet at cruising altitude if I'm not on the plane myself 🙃 $\endgroup$ – Jpe61 Dec 19 '20 at 18:44
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Invariably, aerodynamic efficiency improves with newer designs and so it was from 747-200/300 (not glass cockpit) to 747-400 (winglets + increased span + glass cockpit) to B777 to B787, from A300 to A350. Industry wide pilots were never disappointed by this and proud to be flying the latest lowest drag 'slippery' airplane.

But using noise as an indication of aerodynamic efficiency, could have peculiar factors come into play. The ultra quiet and ultra efficient B787 is so 'clean' or 'slippery' that it requires great anticipation and confidence in technique to fly both the 'speed control' and the CDA Continuous Descent Approach as published at some very major airports.

Many a time, it cannot be flown without the use of speed brakes, the use of which, is (was?) anathema for many pilots.

The normal expectation is that as long as the speed is above the selected or target speed with speed brakes deployed to aid in reducing the speed, and especially during descent/ the engines would be at flight-idle. Contrary to this, a design feature of the autoflight system pushes the thrust levers up so that they are not at idle but producing thrust when the pilot is doing her best to make the airplane decelerate! So, earlier generation airplanes would have thrust at idle at these points, but the 787 does not.

Even under these circumstances it probably has a smaller noise footprint but an inability to decelerate as per the ATC published speed control targets, could for eg result in gear being lowered earlier than the published preference and this all adds to the sound. The Gear extension point is actually being monitored for variations at such airports.

Despite the above information, theoretically, aerodynamic efficiency could certainly be gauged on the basis of noise provided you were to compare like to like - the comparison would have to be between aircraft similar category, all the airplanes would have to be doing the same maneuver in equivalent configuration eg.

  • passing 500ft on final approach, max landing weight, landing config with gear down, on speed, and stabilised on approach

OR

  • level fly by in clean config, height 400ft, 270kts speed.

And there's no accounting for what factors, apart from aerodynamics and engines maybe discovered to be contributing to the noise level.

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  • $\begingroup$ Thank you for your answer, I have not had the pleasure of riding 787, but A350 is quite quiet too, and slippery as well. Regarding approach procedures, they are not set in stone, and CDAs will be adjusted as the planes "evolve". And I agree, feels criminal CDAs are flown with speedbrakes open, partially defeats the purpose, but I think it is economically and -logically better the CDA cuts fuel consumption of the less efficient planes rather than the more efficient ones. More gets saved that way. $\endgroup$ – Jpe61 Dec 19 '20 at 18:55
  • $\begingroup$ I've modded the answer a bit, thank you. $\endgroup$ – skipper44 Dec 19 '20 at 20:20

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