Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thedoc on 17/04/2015 17:50:01
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Seb asked the Naked Scientists:
I heard lately that a Boeing went near the sound speed (barrier) because of a jet stream from behind. So If a plane has a 300 mph wind from behind, does that plane needs to go faster to create the sonic boom? That would mean to go 600 mph above the sound barrier speed?
What do you think?
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The"sound barrier" is related to air speed, i.e. the rate at which a plane travels through the immediately surrounding air. Whilst a conventional airliner could probably exceed Mach 1 in a dive without significant damage, they aren't designed to be efficient above an indicated air speed of about Mach 0.9 - 0.95.
Matters are further complicated by the fact that true air speed (the speed at which molecules pass the aircraft) is much greater than indicated air speed (the rate at which molecules impact the aircraft) at high altitude, because there are fewer molecules per cubic meter.
Anyway, give a 300 mph tailwind, a 600 mph TAS equates to 900 mph over the ground, so the plane would indeed be travelling over the surface faster than sound, but travelling through the air at less than Mach1 so would not generate a sonic compression boom.
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Many years ago, on an international flight (probably a 747?), I was watching the flight statistics, and I was bemused to see that our ground speed exceeded the speed of sound, for exactly the reasons you indicate.
But the plane itself was not breaking the sound barrier or creating a sonic boom.