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[Nickbat]

Was that...


A) Me listening to the weather forecast?

B) Me warning the wife about what's to come later?

Incredibly strong jet stream. I saw a tip about this on a weather forum.

A Boeing 747 normally has a max cruising speed up to Mach 0.88 (583kts). I just noticed a 747-800 (Polar Air) flying east at 636kt (731mph)!!! 

http://www.flightradar24.com/PAC956

 

I suppose that raises an interesting engineering question. I presume that an aircraft can exceed its max speed if it pushed by a tail wind since the air resistance is so much lower that it does not stress the airframe. Thus, in theory at least, a 747 could break the sound barrier if it has a strong enough tail wind? 

nope .. as the speed of sound is also affected by the speed of air movement .. exceeding the speed of sound means exceeding the RELATIVE speed of sound in your locale .. or the "air speed" .. the mach ratio remains the same regardless of windspeed,   so mach .65 in still air is 430 knots ground speed .. in 100 knot headwind its 330 knots ground speed, in a 100 knot tailwind its 530 knots ground speed ... the "airspeed" or mach no remains constant ...at 430 knots / .65

Thanks for the explanation.

[Rods2]

Was that...


A) Me listening to the weather forecast?

B) Me warning the wife about what's to come later?

Incredibly strong jet stream. I saw a tip about this on a weather forum.

A Boeing 747 normally has a max cruising speed up to Mach 0.88 (583kts). I just noticed a 747-800 (Polar Air) flying east at 636kt (731mph)!!! 

http://www.flightradar24.com/PAC956

 

I suppose that raises an interesting engineering question. I presume that an aircraft can exceed its max speed if it pushed by a tail wind since the air resistance is so much lower that it does not stress the airframe. Thus, in theory at least, a 747 could break the sound barrier if it has a strong enough tail wind? 

nope .. as the speed of sound is also affected by the speed of air movement .. exceeding the speed of sound means exceeding the RELATIVE speed of sound in your locale .. or the "air speed" .. the mach ratio remains the same regardless of windspeed,   so mach .65 in still air is 430 knots ground speed .. in 100 knot headwind its 330 knots ground speed, in a 100 knot tailwind its 530 knots ground speed ... the "airspeed" or mach no remains constant ...at 430 knots / .65

Thanks for the explanation.

Mach speed also varies with air temperature, pressure and humidity, with temperature variation making the biggest difference. This means that the speed of Mach one varies. The reason it is used is that the airflow effects over an object will be the same for any particular Mach number. At 30,000 feet it is about 0.89 of that at sea level.

Ground speed and indicated air speed are very different where indicated airspeed is a relative airflow speed measured with a pitot tube. Ground speed is not only dependent on wind speed but also aircraft attitude as it will decrease when you are climbing or diving.