72 | SEPTEMBER 2019 FLYINGMAG.COMT
he airspeed indicator may be the
oldest and most fundamental of
the f light instruments, but it is also
the one least suited to its job, which is
primarily not to tell us how fast we are
going but rather where we are in the
flight envelope. It is pleasant to know,
as we cruise along, that we are indicat-
ing 137 knots; but the airspeed indica-
tor is most important at low speeds,
close to the stall, and it is there that it
is most likely to play us false.
We use airspeed in two ways. As a
gauge of dynamic pressure—that is,
of the force exerted by moving air on
the airplane—it tells us during the
takeoff roll when to rotate. Dynamic
pressure is also the defining parame-
ter for gear and f lap operating speeds,
single-engine minimum control speed
and never-exceed speed—although
the last is actually a true airspeed
based on f lutter considerations.
Many of the characteristic speeds of
an airplane—stalling, maneuvering,
turbulence penetration, glide, best
rate or angle of climb—are proxies forcertain angles of attack. As proxies,
however, they are unreliable. They
fail to factor in what I will tempo-
rarily call, without attempting to
pronounce it aloud, the Gweight.
Gweight is the weight of the airplane
multiplied by the G loading, and at
any given moment, it is equal to the
lifting force being produced by the
wing. A 3,000-pound airplane in a
60-degree banked turn must pull 2 Gs
to maintain altitude; its Gweight is
6,000 pounds. If a 1-G gust strikes theairplane during the turn, its Gweight
rises to 9,000 pounds.
It’s difficult for us to relate airspeed
to the Gweight because airspeed is
proportional to Gweight ’s square
root. Suppose that, our 3,000-pound
airplane has a clean stalling speed of
70 kias, its best rate-of-climb speed is
100 kias, and its maneuvering speed
is 125. At the end of a f light, with only
the pilot and minimal fuel aboard, its
Gweight is only 2,000 pounds; now it
stalls at 57 kias and climbs at 82. Its
maneuvering speed is down to 100.
In a 60-degree bank at gross weight,
however, its stalling speed is 100—
throw in that gust, and it becomes 121.
In other words, depending on
circumstances, the stalling speed,
that life-preserving number, can have
almost any value.
Angle of attack, on the other hand,
is for all practical purposes directly
proportional to Gweight. All airplanes
reach maximum lift at some angle of
attack, usually 15 to 18 degrees above
the zero-lift angle; they make half thatTECHNICALITIES FLYING Opinion
A MODEST PROPOSAL
THE FIRST THING WE DO, LET’S KILL ALL THE AIRSPEED INDICATORSBy Peter GarrisonUNFORTUNATELY,
PILOTS ARE
INDOCTRINATED FROM
THE VERY START OF
THEIR TRAINING WITH
A RELIGIOUS REGARD
FOR THE AIRSPEED
INDICATOR.