Re: Aircraft speed and weight at cruise.

Date:         24 Oct 97 04:33:14 
From:         John Weiss <John.R.Weiss@Boeing.*NOSPAM*.com>
Organization: Boeing Commercial Airplane Group
References:   1
Followups:    1
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jf mezei wrote:
> -A wing generates a lift value which is proportional to the air speed.
> -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 ?

The lift produced by the wing is proportional to both airspeed and angle
of attack.

> 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 ?) ????

The pilot (or autopilot) trims the aircraft to maintain level flight.
If the engine thrust is not reduced, the AOA will decrease and the
airspeed will increase.  If the AOA is not reduced, the thrust and
airspeed are reduced.

> 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 ?

The tradeoff is made in fuel burn.  To make the schedule, the cruise
airspeed may be somewhat faster or slower than the optimum cruise speed
for the given conditions.

At any gross weight, there is a wide range of airspeeds over which the
airplane _can_ fly, and a narrower range over which it is _usually_
flown for performance and economy reasons.

> Would a fully loaded plane not travel faster then a same plane but
> lightly loaded ?

No.  It would require a higher AOA (with higher drag) to push it at the
same airspeed.
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