Page 190 - Understanding Flight

CH07_Anderson 7/25/01 9:00 AM Page 177

Airplane Performance 177

luck would have it, the airspeed indicator on the instrument The first around-the-world
panel does it for the pilot, and many of the important air- airplane flight occurred from
speeds are marked on it. April 6 to September 28, 1924,
The airspeed indicator does not measure the true airspeed and started and ended in
of the airplane. It is really measuring the difference in Seattle.
pressure produced by the air striking the end of the Pitot tube
and the static pressure, as discussed in Chapter 1 (see Figure 1.10). A
Pitot tube can be seen on a wing of every small airplane. There are
several on large jets, mounted near the nose, and they can often be
seen as one boards the airplane. The Pitot tube is calibrated so that
the indicated airspeed and the true airspeed are the same under
standard conditions at sea level. As the airplane flies higher, there is
less air striking the Pitot tube and thus less total pressure for the
same speed. So as the density decreases the indicated airspeed
decreases. Since the airspeed indicator is calibrated at sea-level
standard conditions, the indicated airspeed is lower than the true
airspeed at a higher altitude. The airplane is actually going faster
than indicated. For the pilot to determine the true airspeed in flight,
the indicated airspeed must be corrected for air density, which is a
function of altitude and temperature.
One might think that having to make these calculations in flight is
a nuisance and that the pilot would like the airspeed indicator to read
the true airspeed. But it is the fact that the indicated airspeed is not
the true airspeed that makes the pilot’s life easier. All the critical
airspeeds of normal flight are indicated airspeeds. Therefore, even
though the speed changes with altitude for such important
procedures as climb, glide, stall, etc., the indicated airspeeds of these
maneuvers remain the same. When a pilot who is used to
landing at a sea-level airport makes a landing at a high- All the critical airspeeds of
altitude airport, the indicated approach speed is the same. normal flight are indicated
But the ground is going by much faster. So, when a maneuver airspeeds.
requiring a specific airspeed is performed, the pilot flies the
indicated airspeed.
We now have the tools to discuss the performance of the airplane.
So we will do just that, starting with takeoff and ending with the
landing.