Page 234 - Understanding Flight

CH08_Anderson 7/25/01 9:02 AM Page 221

Aerodynamic Testing 221

First, it must be understood that flight testing means two very differ-
ent things to commercial airplane manufacturers and to military air-
plane manufacturers. To commercial airplane manufacturers, flight
testing focuses on meeting FAA requirements. Frequently, an FAA rep-
resentative will be on board the airplane to monitor the test results.
These tests are performed to verify compliance to specific regulations.
Because of the expenses, flight tests of commercial transports are
rarely performed unless a regulation is involved.
To the military, flight tests usually mean compliance with
military specifications. This typically means verifying per-
Neil Armstrong was an X-15
formance. Because military aircraft fly close to the edge of their
(Figure 5.16) test pilot before he
operating capabilities, flight testing is used to probe the limits of
entered the astronaut corps.
the airplane. Below we discuss some of the measurements
performed in a flight test.

Flight Instrument Calibration
One of the first steps in flight testing an aircraft is to ensure the altime-
ter and the airspeed indicator are giving proper information. In most
airplanes the altimeter and airspeed indicator use pressure to deter-
mine altitude or speed. We start with a little explanation of how they
work and then discuss what is tested early in a flight test program.
An altimeter is nothing more than a simple pressure gauge or
barometer. The static pressure is measured at the static port, as
discussed in the first chapter. The static port is placed somewhere on
the surface of the airplane. Because of the airflow around the airplane,
the surfaces see a variety of static pressures that are different from the
true ambient pressure. But remember, just because the air is flowing
faster somewhere it does not mean that the static pressure is lower.
But since any place on the airplane is going to see at least some small
pressure difference from the ambient static pressure, it is important to
calibrate the altimeter.
The goal of the altimeter is to measure the pressure of the atmosphere
surrounding the airplane with as little airplane interference as possible.
Typically static ports are placed on the fuselage away from the wing,
where static pressure changes are smallest. Because the static pressure
changes with position on an airplane, the error associated with