Re: A340 speed

From:         drela@athena.mit.edu (Mark Drela)
Organization: Massachusetts Institute of Technology
Date:         29 Jul 96 02:29:31 
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In article <airliners.1996.1303@ohare.Chicago.COM>, faurecm@halcyon.com (C. Marin Faure) writes:
|> 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.
|> >
|>
|> 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.

Don't be so hard on yourself.
Induced drag only depends on lift L and dynamic pressure q ,
as can be seen from the usual lifting-line result:

                            2
                  (L / span)
   D_induced  =  ------------
                   pi  q  e

In level flight, where L is fixed, induced drag clearly varies
as  1 / q .  Bringing the angle of attack into the argument just
complicates things unnecessarily, IMHO.


  Mark Drela                          First Law of Aviation:
  MIT Aero & Astro          "Takeoff is optional, landing is compulsory"