Who needs the Concorde, anyway? On Monday, somewhere over northern Pennsylvania, a Virgin Atlantic-flown Boeing 787 Dreamliner traveling from Los Angeles to London managed to go where few commercial planes have gone before: to a speed of 801 miles per hour.
What I don't understand is why they are calling this flight "subsonic" (< speed of sound; Mach I) as the speed of sound (Mach I) at a cruising altitude of about 33,000 feet is approximately 670 miles per hour (mph), well below the reported 801 mph.
I know it has something to do with the difference between air speed and ground speed, i.e., the plane was riding a 200+ mph tail wind while in the Jet Stream, so that must be subtracted from the speed to obtain the actual air speed - or something like that.
Most passenger jets flying over the Continental US normally do so at around 500 mph; some at 600 mph. The speed is usually based on maximum fuel conservation. A trip that I took from Southern California (just east of LAX) to Philadelphia, Pennsylvania (PHL), took just under 6 hours and covered approximately 3,000 miles; speed calculation - about 500 miles per hour.
Interesting. That dang math will get you everytime. You are correct, it would be supersonic. Even the guy in the Tweet said "almost the speed of sound". Perhaps they missed the 8th or 9th grade?
You're correct about air/ground speed being the cause of the strange sounding numbers.
Ground speed is equal to airspeed + wind speed. So in this case, the ground speed was 801mph with a 200mph tail wind, making the airspeed ~600mph, less than the speed of sound.
SO, the plane doesn't get the "credit" if it's not attaining the velocities under its own propulsion? I didn't know that. Thanks @Tekno Venus !
But it's velocity is still 801 mph right?
While the plane is sitting still on the runway, it has a ground speed of 0 and an airspeed of 20 mph: (in a headwind) <---This is baffling to me. I see though the plane needs motion through air to attain lift.
You're correct about air/ground speed being the cause of the strange sounding numbers.
Ground speed is equal to airspeed + wind speed. So in this case, the ground speed was 801mph with a 200mph tail wind, making the airspeed ~600mph, less than the speed of sound.
I understand and you make sense, but the fact of the matter is that the plane is still sailing through the air at 801 mph, which is supersonic and therefore is above Mach I. Right? Or no?
A Boeing 787 flying above Mach I would start to break apart according to documentation that I've read because of the sonic booms and the increased heat on the fuselage.
Mach 1 isn't a fixed speed - it's relative to airflow, temperature, aka the "local" speed of sound. The aircraft never hit supersonic speed, even though the groundspeed was above the usual threshold.
Supersonic is the point that you break through the speed of local soundwaves, which is dependent on airflow as well. Despite the huge ground speed of the aircraft, relative to the air around it the aircraft was still flying at relatively "normal" speed - it never reached the point where it escaped the local speed of sound.
A Boeing 787 flying above Mach I would start to break apart according to documentation that I've read because of the sonic booms and the increased heat on the fuselage.
Despite the groundspeed, the Boeing was flying well within it's normal tolerances and at normal performance - no airline would ever fly outside of the incredibly safe tolerances set. The plane never got close to hitting a sonic boom or breaching the local speed of sound.
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