Perhaps my logic is off on this, but I figured I'd ask anyway. Now, airspeed is not very accurate (i.e. if I'm correct, it's not an absolute speed, like groundspeed). So why is it used? It would seem that you could get a much more accurate reading from an inertia-based speedometer/accelerometer.

Well, that is what all the newer planes display. The readout in the ADI is INS or IRS derived, but the airplane wings only care about airspeed, so in the newer planes the pitot input is run through the computer so unlike the little Cessna or Cub the airspeed displayed is pretty accurate. It is corrected for temperature, compressability and altitude. Even in the old 707 the KFIS system made corrections and would display true airspeed although not as well as the more modern planes.

Lou

Interesting... thanks for the info.  :)

Now here's something else I was wondering: If a plane is travelling at oh, .83 or .84 Mach, and it's being pushed along by a strong tailwind, then why does it not feel the effects of reaching transonic speeds?

boeing247 asked: If a plane is travelling at oh, .83 or .84 Mach, and it's being pushed along by a strong tailwind, then why does it not feel the effects of reaching transonic speeds?


Try this example:

You are in a row boat and you are able to row at three knots.

The river is moving at two knots.

As you leave shore and row up stream you are only able to make one knot RELATIVE to the shore, but your speed on the water is three knots. If you turn and go with the river (downstream) you can still only do three knots in the water, but your speed relative to shore will be five knots.

Same with a plane. As you taxi out for takeoff you read the ground speed on the GPS or INS readout. If you are taxing downwind your ground speed will remain the same since you are still on the ground and the tires have friction. Now, turn into the wind for takeoff. If the plane is a Cub for this example, the takeoff speed is around 40 knots. If the wind is calm, the takeoff speed is 40 knots. If the wind is blowing at 20 knots the takeoff speed is still 40 knots, but the ground roll is very different.

The reason pilots want to takeoff and land into the wind is to have a shorter ground run.

You are on final, and the conditions above exist. The wind is blowing at 20 knots. If you land your Cub into the wind the ground speed will be around 20 knots, but the airspeed will still be 40 knots. This will result in a shorter ground roll. If you were to try and land downwind with the same wind, your airspeed would still be 40 knots, but your ground speed would be 60 knots (40 + 20 ). Same at cruise, the airspeed is M .84 and you have a 150 knot tail wind. The airspeed is still M .84, but the ground speed is 150 knots faster.

This is why we use airspeed when flying the plane, because the plane only knows how fast the air is moving over the wing.

Lou

Remember Bram, that if wind is travelling at 10kts, your plane is going 10kts aerodynamically, although you're staionary in relation to teh ground

Okay, that makes sense. I figured that it was something like that.

Did I post my name somewhere earlier?  :-?