Re: Airplane drag

[John Denker <jsd@MONMOUTH.COM>]

At 02:31 PM 9/12/99 -0600, Jim Green wrote:
> If I understand the site correctly, the drag _does_ pass through a minimum
> as airspeed increases

Yes, under the usual reasonable assumptions.  That's a well-known fact.

> -- and a so called  "induced drag" _does_ continue to
> decrease with increasing airspeed.

Yes, UtURA.  Also well-known.

> Now if I could just figure out what "induced drag" is and what causes it,
> maybe I could teach my kid a little physics. (:-)

Check out
  http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_induced_drag
and more generally
  http://www.monmouth.com/~jsd/how/htm/airfoils.html#sec_vortices

> But it looks like "induced drag" is one of those inventions engineers
> (including chemists) invent when they don't understand how to do it
> correctly.

The scorn and condescension is not helpful.  There are lots of people who
know how to do it correctly.  And some of them know how to introduce
approximations and simplifications when talking to non-experts.

Part of the reason people are fascinated by airplanes is that they are so
different from other things.  Throwing a model airplane is simple, but it
is radically different from throwing a rock.

Parts of the story are complicated, but the most important parts of the
story are very simple.  But remember, even the simple parts of the story
are fundamentally different from the usual baby-physics that people are
taught.