Hi Guys,
Way back in 2003 I flew with my Father from San Francisco to Heathrow (SFO-LHR) in a KLM MD-11, half cargo I think.
I remember looking at the flight info which read we were doing 750mph - I know that with the prevailing winds, flying from West to East is quick, but was this right? I mean, that's almost supersonic at the correct altitude.
Thanks in advance.
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Re: Ground Speed
That was probably the ground speed in Miles Per Hour. The MD11 normally would cruise at 470+/- nautical miles per hour. Depending on the Airshow was set up it could be displaying in knots or miles?
Re: Ground Speed
750mph ground speed is rare but perfectly possible.
http://www.telegraph.co.uk/news/worldne ... -time.html
Airshow will normally show in mph/kph rather than knots, as it's what the general public are used to using and therefore relate to.
http://www.telegraph.co.uk/news/worldne ... -time.html
Airshow will normally show in mph/kph rather than knots, as it's what the general public are used to using and therefore relate to.
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- Starlionblue
- Posts: 21730
- Joined:
Re: Ground Speed
The speed of sound is related only to speed through a specific medium, in this case air. It is referenced to True Air Speed, not Ground Speed.
As mentioned above, airshow is in miles and km/h. 750 mph is around 650knots. A typical jetliner cruises at 480 knots. The additional 170 knots is tail wind component. The aircraft is moving at 480 knots through the air, and the air it is traveling through is moving at 170 knots over the Earth in the same direction as the aircraft.
The speed of sound in air is 39 x square root of the temperature in Kelvin. Assuming standard temperature at 39000 feet of 216.65 Kelvin, this gives a local speed of sound of 574 knots. Thus at 480 knots True Air Speed you are doing Mach 0.83.
As mentioned above, airshow is in miles and km/h. 750 mph is around 650knots. A typical jetliner cruises at 480 knots. The additional 170 knots is tail wind component. The aircraft is moving at 480 knots through the air, and the air it is traveling through is moving at 170 knots over the Earth in the same direction as the aircraft.
The speed of sound in air is 39 x square root of the temperature in Kelvin. Assuming standard temperature at 39000 feet of 216.65 Kelvin, this gives a local speed of sound of 574 knots. Thus at 480 knots True Air Speed you are doing Mach 0.83.
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- RetiredWeasel
- Posts: 942
- Joined:
Re: Ground Speed
Starlionblue wrote:The speed of sound is related only to speed through a specific medium, in this case air. It is referenced to True Air Speed, not Ground Speed.
As mentioned above, airshow is in miles and km/h. 750 mph is around 650knots. A typical jetliner cruises at 480 knots. The additional 170 knots is tail wind component. The aircraft is moving at 480 knots through the air, and the air it is traveling through is moving at 170 knots over the Earth in the same direction as the aircraft.
The speed of sound in air is 39 x square root of the temperature in Kelvin. Assuming standard temperature at 39000 feet of 216.65 Kelvin, this gives a local speed of sound of 574 knots. Thus at 480 knots True Air Speed you are doing Mach 0.83.
Nicely explained. And since all objects on earth or in the atmosphere are rotating with the earth at 460 meters/second or about 1000 mph due to the earth's rotation we're all going "supersonic". And that doesn't count the speed of the earth's orbit. Ha.
But realistically, since the wind speed here in PHX today is about 10 mph, I'm doing about Mach .013 when walking the dog.
Re: Ground Speed
Classic post Rw, first time i've seen a Mach number quoted for the speed of walking our best friends.
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