[From Bill williams UMKC 30 November 2002 12:51 PM CST]
THe reason aircraft companies publish a "stall speed" is that they are too
cheap to provide an Angle of attack meter. If they provided an AOA Then
they could correctly say the aircraft will stall at an 18 degree angle of
attack and that would be that. Actually in practice it works out well
enough, for the most part, to say that an aircraft in unaccelerated flight,
at a given weight, will stall, in the absence of turbulence, with a certain
flap setting, at a certain airspeed. But, even this doesn't accurately
define "a stall speed." With icing, the airfoil may stall at some angle of
attack quite different than a clean airfoil. You can stall an aircraft well
above the published stall speed by combining "a pull-up" relative the the
aircraft with "a rolling command" or just by an abrupt roll command alone.
If the command is generated by ailron the stall will result in a roll off
contra the direction commanded. If the roll is generated by the rudder then
it may result may be a spin in the direction of the command. If the
aircraft is improperly loaded it can stall at any speed-- in the absence of
a command. Inverted flight introduces additional complications. Plus the
wing isn't the only surface which it is possible to stall. And, stalling
the aircraft doesn't necessarily mean the aircraft is going to be
decending.But, it may be difficult to climb when the aircraft is stalled.
There are potentially all sorts of exotic possiblities. So, the
manufacture's "stall speed" includes a lot of fine print.
The point I was trying to make in calling attention to the mistake about
the explaination of the stall, was that the authors are presenting material
_as if_ they are engaging in a practical application of control theory to
actual problems encountered in aircraft operations. Their treatment of the
stall is one mistake which indicates that they are not sufficiently
acquainted with the field to do so competently. Then they return to a
discussion of the phenomena of "optical flow." The point they are
attempting to make is that a pilot might mistake a higher rate of flow of
the ground at low altitude for a higher airspeed reduce the aircraft's
airspeed and crash. On rare occassions this might take place, but I've read
a lot of accident reports and I don't recall ever seeing this listed as a
contributor to an accident.
I'm not saying _the whole_ book ought to be judged based on this section,
the mistake I saw doesn't have anything to do with control theory as such,
but it seems worthwhile to me to examine the whole book closely. And, I'm
interested in the opinions of other readers who might see things that I'd
be likely to miss because I don't have sufficient understanding of an
application to make a competent judgement.