Re: Aircraft speed and weight at cruise.

Date:         24 Oct 97 04:33:15 
From:         capndavey@aol.com (Capndavey)
Organization: AOL http://www.aol.com
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>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.
>
>-The force of the lift should always equal the weight of the aircraft
> otherwise the plane would either climb or drop.

Close.  Lift ALWAYS equals weight, or the airplane will ACCELERATE
vertically, until it stabilizes at a new constant rate. In an elevator
you feel the extra "lift" only when you start going up, not the whole way.
Same thing going down.  Once you are no longer ACCELERATING, lift equals
weight. Most students, and even some more advanced pilots have a hard time
with that one.

>-The weight of an aircraft changes as fuel is exhausted during cruise.
>
>QUESTION:
>
>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 ?

Assume ANY flight: the answer is no. See above.

>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 ?) ????

If your car suddenly got lighter (say, your mother-in-law fell out) it
would begin to go faster unless you let off the gas a bit. As the fuel
burns down, the airplane is lighter; it needs less lift, meaning less
drag;  it will then go faster, or will climb,  unless power is reduced,
or pitch lowered, or some of each. This is so gradual, mind you, that you
don't see it happening. You control the flight path of the airplane, the
wing makes the amount of lift it needs to. Example, if you pick up a
bucket, you don't have to "adjust" the amount of pull on the handle. The
pull on the handle is enough to hold up the bucket. If you ACCELERATE
the bucket, it's a different story.

>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 time stays the same (pretty much). The amount of fuel needed is what
varies.

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

The lighter airplane would go faster for the same power setting, because
it needs less lift, and the wing is therefore making less drag.

My examples may be a bit lame, and it's a complex subject. I hope this
helps even  a little.

Dave Simpson
dsimpson@dbsis.com