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If a plane flies with a jet stream, its speed relative to the ground is the sum of the speed of the jet stream and the speed of the plane relative to the air in the jet stream. If the speeds that are added together are subsonic, can the resulting speed of the plane relative to the ground become supersonic? If it can, does the plane create a sonic boom on the ground?

Aspro
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    Consider the converse case, a supersonic jet flying into a headwind. Would you expect the sonic boom to go away? – Ralph J Oct 22 '20 at 15:15
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    The speed of sound, supersonic flight, and the sonic boom are irrelevant to ground speed. The only thing that counts is the air speed and atmospheric conditions. – Dean F. Oct 22 '20 at 16:26
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    752 Knots = 865 MPH = 1,393 KPH https://groundspeedrecords.com/wof-top-3/?aircraft-manufacturer=Boeing&wpvaircraftmodel=b747-400&term_id=432 – Mike Sowsun Oct 22 '20 at 16:30
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    "Supersonic speed relative to the ground" does not make sense. The speed of sound is always relative to the surrounding fluid (air). Air molecules do not care how the earth below is moving – 60levelchange Oct 23 '20 at 11:12

1 Answers1

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Yes, the ground speed of a subsonic aircraft could be larger than the speed of sound. As an example, during the record time for a subsonic Atlantic crossing a Boeing 747 reached speeds of up to 1327 km/h:

Experts are hailing a British Airways flight as the fastest subsonic New York to London journey.

The Boeing 747-436 reached speeds of 825 mph (1,327 km/h) as it rode a jet stream accelerated by Storm Ciara.

(BBC)

The speed of sound at sea level is usually around 1235 km/h. The speed of sound is however temperature dependent and will be lower at high altitudes, therefore the ground speed in this case was higher than the speed of sound, regardless of the altitude.

This does not make the aircraft supersonic. A supersonic flight implies that the TAS (True AirSpeed) is higher than the local speed of sound. This was never the case for this 747 flight.

If it can, does the plane create a sonic boom on the ground?

No, a sonic boom can only be created locally by an aircraft flying faster than the local speed of sound. It is unrelated to the ground speed. Consider the following illustration of a sonic boom:

Sonic Boom
(image source: Wikipedia)

You can see a line of sound waves currently hitting the person standing on the ground. This is the sonic boom you would hear.

I modified this image for subsonic flight:

Subsonic Sound

You can see that the circles never intersect and can therefore never form a line of sound waves. If the aircraft is in a tailwind, the sound waves would be emitted in the same way, they would just move faster across the ground as a whole, but there is no relative difference between them (and the aircraft). Therefore, there is still no sonic boom even when the aircraft's ground speed is faster then the speed of sound.

Note however that the wind speed would change with altitude. The jet stream does not extend all the way to the ground. This would distort the circles when they get closer to the ground, but it cannot create a sonic boom.

Bianfable
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    "No, a sonic boom would be created locally" - Isn't that meant to read "NOT" ? – MikeB Oct 23 '20 at 09:03
  • Is there any danger of the plane "falling out of" the jet stream, or encountering a patch of air which is moving slower which would (at the least) creating severe turbulence and at worst cause the plane to unintentionally go supersonic (at least for a brief time)? – Michael Oct 23 '20 at 16:50
  • If the plane deliberately made a sharp turn (or dive or climb) then it could exit the jetstream. – Ross Presser Oct 23 '20 at 18:09
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    @Michael Not really. The edges of the jet stream are relatively sharp compared to normal changes in wind speeds. They are however not real discontinuities. There is still some smooth transition into slower wind speeds. Even if there would be a sharp change in wind speed or direction (called windshear), the doppler radar would pick it up and warn the pilots. – Bianfable Oct 23 '20 at 18:25
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    What happens when the sound waves from the plane overtake the sound waves emitted earlier, if the plane is moving faster than the sound waves due to a tailwind? Does it just sound like it's flying backwards, as you hear the noise moving away from you instead of closer? – nick012000 Oct 24 '20 at 05:40
  • @nick012000 That cannot happen. The sound waves are also travelling in the air, which is moving w.r.t. the ground, just like the plane. The whole picture of plane and emitted sound waves does not change with the wind. – Bianfable Oct 24 '20 at 10:12
  • @Bianfable What about when the plane is traveling through a layer of air that ends above the ground, though? Imagine a situation where the air from ground level to 10000 feet is stationary, and then at 10000 feet, a layer of air moving mach .4 starts, with a small laminar flow transition region between them. If a plane is flying at mach .8 at 20000 feet, then it would be moving at mach 1.2 relative to a person observing it on the ground, and it'll overtake the sound of its own motion once that sound leave the laminar flow zone and enters the stationary air, right? – nick012000 Oct 24 '20 at 10:38
  • @nick012000 Yes, the plane at 20,000 ft could overtake its own soundwaves at lower altitudes, but so what? It will not overtake the sound waves at 20,000 ft since they will move with the wind. It does not really matter that the sound waves below the aircraft are smoothly slowed down in the transition region. This cannot create a sonic boom. – Bianfable Oct 24 '20 at 10:52
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    @Bianfable I didn't say it would, but the person on the ground would hear it moving away from them because it overtakes its sound waves, right? – nick012000 Oct 24 '20 at 11:05