Re: Aircraft speed and weight at cruise.

Date:         24 Oct 97 04:33:14 
From:         "Edward Lee" <>
References:   1
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jf mezei <"[non-spam]jfmezei"> wrote in article <airliners.1997.2480@ohare.Chicago.COM> ...
> Sorry if this is very basic:
> Correct me if assumptions as wrong:
> -A wing generates a lift value which is proportional to the air speed.

Lift is proportional to square value of the airspeed.

> -The force of the lift should always equal the weight of the aircraft
>  otherwise the plane would either climb or drop.
> -The weight of an aircraft changes as fuel is exhausted during cruise.
> Assuming a LAX-SYD flight with no winds. Towards the end of the flights
> when the plane is much lighter, doesn't the lift provided by the wings
> exceed by quite a bit the lighter weight of the aircraft ?
> Is this change so trivial that it is not an issue, or do pilots take
> this into consideration (reducing speed to reduce lift or what ?) ????

You can either be flying at a lower speed or climb higher to an altitude
where air density is less, since lift is also proportional to air density.
If you are flying at the same speed, the pitch angle of the airplane will
reduce, the lift coefficiency will also be reduced, thence the lift will
stay the same to maintain altitude.

> If speed is adjusted to match the lift with the weight of the aircraft,
> how does this affect airline schedules  where cargo loads may influence
> the time it takes for the airctaft to get to destination ?
> Would a fully loaded plane not travel faster then a same plane but
> lightly loaded ?

Reducing airspeed will affect the trip time, but will not be so significant
.... Reducing speed to 0.82 mach (speed of sound at that altitude) compare
to flying at 0.86 mach means 10 minutes more flight time for a ten hour
flight. Since airplane will climb higher as the weight reduced, same
mach number of flying at higher altitude will reduce true airspeed of the
airplane, because the speed of sound is normally lower at higher altitude
due to temperature is lower at higher altitude and the speed of sound is
proportional to temperature. Chances are lighter airplane flying at the
same mach number may not be faster than heavier airplane flying at
same mach number, but definitely will cost less fuel to fly the trip.

=========== Gravity, it's the law! ===========
Mr. Dar-Ping Lee    B747 Pilot, China Airlines
Base: Taipei, Taiwan ROC [TPE/RCTP]     Email:,