The T/O speed is weight dependent (because the amount of lift needed is weight dependent).
I think you're confusing groundspeed with airspeed.
It's airspeed that is important ie the speed of the a/c related to the airmass in which it located and is shown on the airspeed indicator.
Groundspeed is the airspeed plus or minus any tail- or headwind effect. Note that groundspeed is irrelevent WRT the aerodynamics ie flying the thing.
You don't calculate the required speed from the h/wind &/or crosswind. You extract the required speed from the a/c's runway performance charts then wait for that airsspeed to be reached.
Using a standard atmosphere (ie the pressure/temp/density for which the airspeed indicator is calibrated) and ignoring instrument inaccuracies etc then in nil wind Airspeed = Groundspeed.
If (say) 60 kts is the t/o speed then in the above conditions Airspeed will need to equal 60kts. Groundspeed would also happen to equal 60kts but that irrelevent as far as flying is concerned.
Lined up on the runway with a 20 kt headwind then as far as the a/c's wings are concerned it's already moving at 20 kts even though the wheels aren't rolling. When the a/c eventually accelerates along the runway to 60 kts airspeed & gets airborne the groundspeed would happen to be 40 kts.
If there was a 60kt headwind then the a/c would already be doing 60kts (as far as the wings are concerned) and could get airborne immediately ie without accelerating down the runway. The g/s in this case would be zero.
If there was a tailwind during the takeoff roll eg 20kts then the a/c would have to accelerate to this groundspeed for the wings to consider themselves at a zero speed. It would then have to gain another 60kts for the wings to be at 60 kts airspeed. In doing so, the a/c will have accelerated to 80 kts groundspeed. It's possible for a tyre speed limitation to come into effect here since the wheels will be rolling along at the groundspeed and not whatever the airspeed is.
This works similarly for landing.
The crosswind component is largely irrelevent *except for* mainly some handling/controllability/sideload on the undercarriage issues. This imposes a limit to how much x/wind component is acceptable. There are skills that you will be taught to deal with these issues.
It's the headwind/tailwind component that is important re the take off and landing distance.
Considering the constantly varying strength & direction of the wind then approximations are accurate enough to derive the h/wind, t/wind & x/wind components.
Last edited by Tinstaafl; 5th Feb 2003 at 01:10.