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?
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5$\begingroup$ Consider the converse case, a supersonic jet flying into a headwind. Would you expect the sonic boom to go away? $\endgroup$– Ralph J ♦Oct 22, 2020 at 15:15
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7$\begingroup$ 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. $\endgroup$– Dean F.Oct 22, 2020 at 16:26
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1$\begingroup$ 752 Knots = 865 MPH = 1,393 KPH groundspeedrecords.com/wof-top-3/… $\endgroup$– Mike SowsunOct 22, 2020 at 16:30
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3$\begingroup$ "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 $\endgroup$– 60levelchangeOct 23, 2020 at 11:12
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1 Answer
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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:
(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:
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.
answered Oct 22, 2020 at 13:59
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$\begingroup$ "No, a sonic boom would be created locally" - Isn't that meant to read "NOT" ? $\endgroup$– MikeBOct 23, 2020 at 9:03
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$\begingroup$ 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)? $\endgroup$– MichaelOct 23, 2020 at 16:50
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$\begingroup$ If the plane deliberately made a sharp turn (or dive or climb) then it could exit the jetstream. $\endgroup$ Oct 23, 2020 at 18:09
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2$\begingroup$ @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. $\endgroup$ Oct 23, 2020 at 18:25
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1$\begingroup$ 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? $\endgroup$ Oct 24, 2020 at 5:40