Re: Aircraft speed and weight at cruise.

Date:         16 Nov 97 19:58:46 
From:         "Ricky" <rwai@aol.com>
References:   1 2
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Don Stokes wrote in message ...
>In article <airliners.1997.2480@ohare.Chicago.COM>,
>jf mezei  <"[non-spam]jfmezei"@videotron.ca> wrote:
>>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.
>
>This assumption is wrong, or at least incomplete.  It should contain the
>qualification "at a given angle of attack".  This is important because...
>
>>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 angle of attack is adjusted as the plane lightens.  When heavy,
>the plane flies a bit more nose-up than when light; by dropping the nose,
>less lift is generated.  This is (part of) the "trim" of the aircraft.

The statement "A wing generates a lift value which is proportional to the
air speed" is completely right.  since:

    L=1/2*pho*V^2*S*CL_alpha * alpha

        L is lift, pho is density, V is speed, S is area, CL_alpha is
        lift curve slope, and alpha is angle of attack

Therefore,
    L is proportional to air speed is a completely right statement
    L is also proportional to density, wing area, and lift curve slope

When you said the "Assumption is wrong", you are completely
wrong.  "or at least incomplete", then your statement also incomplete,
since lift also relate to density, and area of the wing.

Ricky Wai, Aerospace Engineer