Brooks (brooksmoses) wrote,

In-flight science and data aggregation

As I previously discovered, Virgin America has wifi on all of their flights to and from Boston. This leads to some interesting things, such as using FlyteComm to track the flight that one's currently on. Of course, that is somewhat redundant, since the in-flight entertainment system contains a map with live position updates.

However, they don't quite have the same data. The airplane's system gives the current airspeed (519 MPH), whereas FlyteComm reports the current groundspeed (449 MPH). From this and elementary subtraction, we can deduce that we've got a 70MPH headwind, and from that it's pretty clear why it's a 6 hour 18 minute flight from Boston to SFO, whereas it was only 5 hours and 35 minutes the other way.

Also, the airplane's system gives us the current outside temperatute (-85F). That gives us another interesting bit of information; look up the speed of sound at that temperature, which turns out to be 647 MPH; thus, we're travelling right at Mach 0.8 -- which is pretty typical for a commercial airliner, as I recall, and right at the edge between "subsonic" and "transonic".

On the flight out here, the sun was just right so that I could see the shadow of the shock wave that forms over the top of the wing. According to Bernoulli's principle, since the air above the wing is at lower pressure (as is obvious from the fact that pressure differential across the wing is holding up the airplane), the air going over the wing must be going faster than the general airspeed of the plane. Empirically, it's going enough faster to be going Mach 1.0 -- the speed of sound -- when the airplane is going Mach 0.8, and so that is the critical Mach number of the wing. (There are some nice diagrams at that link.) That's actually why this is the optimum speed of the aircraft; at subsonic speeds, going faster is more efficient because you get more lift from the same weight of wing -- or, alternately, the same lift from a smaller wing -- but when the shock wave is formed, it starts decreasing the efficiency of the wing. Thus, the airplane is at its most efficient when flying at a speed such that the shock wave over the wing is just barely present.

A different thing to do with the temperature is compare it to the Standard Atmosphere. The airplane's system gives the current altitude as 36319 feet, and the standard temperature at that altitude is 217K. The actual temperature of -85F is 208K, so this is 9K colder than standard -- which is not surprising; it's at least that much different on the ground.

Interestingly, in the time it took me to write this post, in which we've travelled from 60% across Colorado -- just West of Denver -- to about 85% across it, the outside temperature has warmed up to -76F, and we've sped up to 526 MPH. That's still about the same Mach number; actually slightly lower.

A completely unrelated note is that a 6 hour and 18 minute flight can be really rather boring. Which is another reason for this post.

Also, Virgin America gives their planes interesting names. ("Unicorn Chaser", chosen by Boing Boing, should be good for a spit-take by anyone who recognizes the reference.) This one is "Dark Horse".
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