Re: A340 speed

From:         faurecm@halcyon.com (C. Marin Faure)
Organization: Northwest Nexus Inc.
Date:         16 Jul 96 13:51:49 
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In article <airliners.1996.1258@ohare.Chicago.COM>, barr@netcom.com (Keith
Barr) wrote:

> In article <airliners.1996.1243@ohare.chicago.com>,
> C. Marin Faure <faurecm@halcyon.com> wrote:
> >The problem is drag, which as you know, goes up as the speed goes up.
>
> This is only partially correct.  Below L/D max, drag increases as you
> slow down due to the increase in induced drag.
>
> >When the cruise speed is pushed up, this extra parasite drag, plus the
> >increased induced drag you're going to get anyway...
>
> This is incorrect.  As you go faster, parasite drag increases, but since
> you are lowering your required angle of attack, induced drag decreases.
>
> Total drag (parasite + induced) is more or less U-shaped when plotted on
> a chart with airspeed on the X-axis, and Drag on the Y-Axis.  The minima
> is at your best Lift to Drag ratio, which is also your best glide speed.

I guess there's a good reason why I'm not an engineer.  I always thought
induced drag was the product of producing lift, and not necessarily in
direct relationaship to angle of attack.  I've always thought you can
increase lift two ways: by increasing the wing's angle of attack, or by
moving the wing through the air faster.  In either case, the induced drag
would go up if it's a product of producing lift.  But if induced drag is
solely related to angle of attack, then I can see how increasing speed
which allows you to develop the necessary lift at a lower AOA would not
result in an increase in induced drag.

In any event, the A-340 apparently picked up more drag than expected due
to the outboard enigines and pylons, which requires a lower cruise speed
on those flights where both very long range and a higher payload are
required.

C. Marin Faure
   author, Flying a Floatplane