From: email@example.com (Mark Drela) Organization: Massachusetts Institute of Technology Date: 29 Jul 96 02:29:31 References: 1 2 3 4 5
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In article <airliners.1996.1303@ohare.Chicago.COM>, firstname.lastname@example.org (C. Marin Faure) writes: |> In article <airliners.1996.1258@ohare.Chicago.COM>, email@example.com (Keith |> Barr) wrote: |> |> > In article <firstname.lastname@example.org>, |> > C. Marin Faure <email@example.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"