V-Flyer

On my flight back from Vegas the skymap said groundspeed 744mph
how fast is that on the mach scale is that...just out of interest, and is the plane actually flying at that speed or is it the speed of the plane plus the speed of the jetstream?

edit, just seen a similar question in general, sorry would still appreciate your thoughts on this one though[:I]

See here for more info on the mach scale, though it looks slightly complicated at times[:I]

VS.

Well I can tell you that the speed of sound at sea level is 760mph. So it would appear that you were approaching transonic speeds. However, I have never known how that actually relates to air speed. In my confused mind I am thing that air speed must be faster as the arc you will fly in the air is longer than on the ground therefore if you are covering 744 ground miles in one hour I come to some unfounded conclusion that you must be travelling faster at roughly 40,000 feet. I don't even want to go into the working of Pi (sp?) and the difference in Radius between the earth's circumference and the earth's circumference plus 40,000 ft.

My brain hurts....

Paul

Just found this link that explains quite well however didn't address my thoughts on the 'Arc Theory'
http://www.aerospaceweb.org/question/in ... 0210.shtml

Paul

The difference in speed at an arc height of 36000 compared to an arc at sea level is about 37 MPH (I think!). As the speed egg was looking at was most likely from a GPS though, it should be allowing for the height and measuring the speed along an arc at the height you are at then converting it to a ground speed. So I think that egg was traveling at 744 across the ground, about 710mph along and imaginary piece of land 36,000 feet high (although as this land is not charted there may be some air accidents!) and some other speed through the moving air. Willing to be corrected though.

Of course the ground speed would be affected by which chart datum the GPS was working to. I was assuming WGS84 which arcs at sea level if I remember correctly.

I see that there are two similar threads today!!!

Let's see how much fluid mechanics I can remember from my degree - (not very much)

The Mach speed scale is a relative scale i.e. it measures the speed of an object relative to the fluid it is surrounded by. (i.e. object= EGGs plane, fluid=air). Therefore it is not really possible to convert a ground speed into a mach number as the mach number will be dependent on the speed that the surrounding fluid is travelling. For a 744 to get up to 740mph ground speed there must have been one hell of a tail wind and in actual fact the mach number was probably pretty much the same as in any other 744 flight.

On a side note there is a reason that supersonic planes are pointy and subsonic planes more rounded. I do not think I would want to be on a 744 that broke mach one!

Originally posted by oxmatt
On a side note there is a reason that supersonic planes are pointy and subsonic planes more rounded. I do not think I would want to be on a 744 that broke mach one!

Indeed, it'd be more than mach one that would find itself broken... [:w]

well it was a very bumpby ride most of the way, defo had that speed on the skymap though

EGG, not sure if you read that article I linked to but essentially if an aircraft is flying with an airspeed of 500mph plus has a tail wind of 100mph, the Ground Speed is 600mph, however the aircraft is only classed with travelling at 500mph.

With that theory in mind, if we could ever get tail wind speed up to 300mph (will never happen I know) then I guess we could look at ground speed of approaching 1000mph.

Paul

How long was your flight EGG? I once remember the return from SFO - LHR coming in at around 8 1/2 hours, with tailwinds I recall of up to and over 150mph (all from memory but it was very fast with constant high speed tail winds - in fact I recall the journey as being slightly disappointing as any London-San Fran trip I actually like to stay in the air as long as possible!).

The flight was 8 hours 54 mins.