Re: Stalls

From:         drinkard@bcstec.ca.boeing.com (Terrell D. Drinkard)
Organization: Boeing Commercial Airplane Group
Date:         21 Apr 94 23:18:38 
References:   1
Next article
View raw article
  or MIME structure

In article <airliners.1994.1155@ohare.chicago.com>,
Tobias Henry Lutterodt  <luterodt@phoenix.Princeton.EDU> wrote:
>
>There have been a number of incidents lately (Singapore 747-400 and others)
>where a jetliner has stalled at high speed at or near cruising altitude.
>
>I don't understand how this happens...could someone explain?

In general, a high speed stall in a commercial jet transports occurs
because the wing is put at too high an angle of attack (this is actually
the general definition of a stall).  The wing at cruise is working quite
hard, with the midspan and outboard wings near their maximum coefficients
of lift.  A small change in angle of attack can cause flow separation and
subsequent loss of lift.  This change in angle of attack can be caused by
pitching the airplane up with the elevators, or by deploying high-lift
devices (a la the MD-11 that had to land at Shemya).  The airplane can also
stall because it is moving so fast that the shockwave along the upper
surface of the wing becomes so strong that the flow separates behind it.
This is a Mach buffet.  Contributing factors are weight (which falls out as
an angle of attack) and ambient air conditions.

-- 
Terry
drinkard@bcstec.ca.boeing.com
"Anyone who thinks they can hold the company responsible for what I say has
more lawyers than sense."