Introduction to Flight Testing
Flight testing is the process of gathering information (or data) which will
accurately describe the capabilities of a particular type of airplane, and which
can be used to accurately predict and optimize the use of all airplanes of that
same type in future missions. Flight testing of research airplanes constitutes
the gathering of data in regions of the flight environment where little past information
has been obtained. This information is then used to design future airplanes which
can operate safely in this new environment.
The test maneuvers which are used to obtain this data are described in detail
in sections that follow this introduction. There are some common elements and
special terminology in these maneuver descriptions which will be introduced now
to avoid duplication.
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| Translational Axes |
Rotational Axes |
The Axis System
A good understanding of the basic axis system used to describe aircraft motion
is necessary to fully appreciate flight test data, since all measurements are
referenced to this axis system.
Aircraft translational motion is described in terms of motion in three different
directions, each direction being perpendicular to the other two (orthogonal).
Motion in the X direction is forward and aft velocity. The Y direction produces
sideways motion to the left and right, and up and down motion is in the Z direction.
The rotational motion of an aircraft can be described as rotation about the
same three axes; pitch rotation (nose up or nose down) is about the y axes, lateral
or roll rotation (one wing up or down) is about the x axis, and yaw rotation (nose
right or left) is about the z axis.
There are several slightly different versions of the basic axis system just
described. They differ primarily in the exact placement of the zero reference
lines, but are generally similar in their directions. (For example, the body-axis
system uses the fuselage center line as the x axis, while a wind-axis system uses
the direction that the aircraft is moving through the air as the x axis.)
Performance
Performance generally refers to the motion of the airplane along its flight
path, fore and aft, up or down, right or left. The term "Performance" also refers
to how fast, how slow, how high and how far. It may also refer, in general sense,
to the ability of an airplane to successfully accomplish the different aspects
of its mission. Included are such items as minimum and maximum speed, maximum
altitude, maximum rate of climb, maximum range and speed for maximum range, rate
of fuel consumption, takeoff and landing distance, weight of potential payload,
etc. There are specific maneuvers which are used to measure and quantify
these characteristics for each airplane. In many cases, flight testing takes place
in a competitive environment to select the best airplane for accomplishing a particular
mission. Since all of these performance measurements are strongly affected by
differences in the weather conditions (that is, temperature, pressure, humidity,
winds), there are some very specific and complex mathematical processes which
are used to "standardize" the test results. The "standardization" process corrects
each of the test-day measurements to an artificially created standard-day condition.
In this way valid comparisons can be made between airplanes that were tested on
different days or at different atmospheric locations.
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| Aircraft Performance |
Stability and Unstability |
Stability
Stability refers in a general sense to the rotational motion of an airplane
about its axes; roll, pitch, and yaw. "Stability" is defined as the tendency of
an object to return to an initial rest or trim condition when it is disturbed.
A marble placed in the bottom of a shallow trough is said to be stable to that
trim condition.
Oscillatory
When disturbed it will tend to return to that resting place at the bottom of
the trough. It may overshoot and oscillate back and forth, but it will continue
to seek the lowest point in the trough. Static stability is a measure of the strength
of that returning tendency.
A trough with steep sides provides a higher level of static stability for the
marble than one with shallow sides. The motion that results from a statically
stable condition is called "oscillatory". If we set the marble on a flat table,
there is no tendency for it to return to any trim point .
This condition is called "neutral stability". The motion resulting from a condition
of neutral stability is called "non-oscillatory". If we turn the trough upside
down, we can balance the marble at one point on the top of the trough, but when
disturbed it will tend to move away from the balance point at an increasing rate.
This is an example of an unstable condition, or "static instability". The motion
that results from static instability is called "divergence".
Damping
Damping is resistance to motion. Damping only exists when actual movement is
occurring. For an airplane it is usually characterized as being proportional to
the rate of movement, or velocity. (Note that "velocity" can be either translational
- speed, or rotational - revolutions). Damping is usually related to some form
of friction. If we line the trough referred to in the stability example with a
towel, the marble will still seek the lowest point in the trough (still statically
stable), but the towel has increased the friction between the marble and the smooth
sides. The marble will not move as rapidly and will not oscillate back and forth
as much as in the previous example. The towel has added damping to the motion
of the marble.
The marble rolling back and forth in a trough is a demonstration of certain
laws of physics. Engineers have applied the magic of mathematics to improve their
understanding of many of these laws of physics. The mathematical equations which
describe the motions of an airplane in flight, (or a marble in a trough) are called
"differential equations" and they are based on an advanced mathematical concept
called Calculus. By applying a marvelously simple mathematical trick, called the
Laplace Transform, engineers can identify specific mathematical terms within the
equations of motion which cause certain characteristics to occur in the observed
motions.
Once identified, these terms can be manipulated by altering the shape or location
of various aircraft components (changing the size of the tail, for example). In
this way the aircraft designer can produce an airplane that will have the desired
levels of stability and damping.
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| Damping |
Maneuverability |
Maneuverability
Maneuverability is defined as the ability to change the speed and flight direction
of an airplane. A highly maneuverable airplane, such as a fighter, has a capability
to accelerate or slow down very quickly, and also to turn sharply. Quick turns
with short turn radii place high loads on the wings as well as the pilot. These
loads are referred to as "g forces" and the ability to "pull g's" is considered
one measure of maneuverability. One g is the force acting on the airplane in level
flight imposed by the gravitational pull of the earth. Five g in a maneuver exerts
5 times the gravitational force of the earth.
Flight Test Instrumentation
Gathering Data
The purpose for flight testing is to gather data about the flight characteristics
of an airplane and its subsystems for subsequent analysis on the ground. This
data gathering process starts with sensors or transducers which have been mounted
throughout the airplane. Transducers are devices which convert mechanical measurements
into electrical signals. Different kinds of transducers are used to measure control
positions, pressures, temperatures or loads.
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| Sensors (Transducers) |
Wiring To Signal Conditioning Unit |
Recorder & Telemetry |
The electrical signal from each transducer is routed through special instrumentation
wiring to a central location in the airplane where it is connected to signal conditioning
equipment. The signal conditioner "conditions" each transducer signal to a common
format and organizes all of the signals for efficient recording. Many different
terms are used to describe the various phases or processes that are included in
this "conditioning", such as multiplexing, commutating, sub-commutating, digitizing,
analog-to-digital converting, time-code generating, pulse-code-modulating, etc.
The resulting organized stream of data is then transferred to an on-board tape
recorder and also to a telemetry transmitter. The tape recorder records the data
on magnetic tape in much the same way that music is recorded on a tape cassette.
The telemetry transmitter transmits the same data stream from the airplane to
a ground station on a selected radio frequency, in much the same way as a commercial
radio station broadcasts music to our homes.
The ground station receives the data stream and also records all of the data
on another, ground based tape recorder. The ground station also converts portions
of the data stream into electrical signals that can be displayed on indicators
or strip charts in the ground control room. In this way engineers on the ground
can monitor flight activities and can assist the pilot in the safe conduct of
the flight.
If an aircraft is expected to remain within easy range of the ground station
for all of it's flights, it may not be equipped with an on-board recorder. This
decision reduces complexity and saves weight. The data is transmitted from the
airplane to the ground and the ground recorders are the only source of data for
post flight analysis.
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| Ground Station and Control Room |
Ground Station and Control Room Without On-Board
Recorder |
Sensors
Nose Boom
Nose boom installations are fairly standard on test aircraft. The nose boom
allows critical measurements of both pressure and the flow angles to be measured
well in front of the fuselage where the measurements are not influenced by the
shape of the aircraft.
Pitot-Static System
The pitot-static system is the basic measurement method for determining speed
and altitude. It includes two pressure measurements. Total pressure (or pitot
pressure) represents the pressure being applied to the front of the airplane as
it moves through the air. It is measured by a using a pressure transducer to measure
the pressure inside a forwarding-facing tube at the front of the nose boom. Static
pressure represents the undisturbed pressure of the atmosphere at the altitude
that the airplane is flying. It is measured by side-facing tubes or holes on the
top and bottom of the nose boom. The static pressure measurement can be related
directly, through a mathematical formula, to the altitude that the airplane is
flying. The difference between the pitot and static pressure can be related (through
another mathematical formula) to the speed of the airplane through the air.
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| Nose Boom Pressure Sensors |
Nose Boom Angle Sensors |
Angle of Attack and Sideslip Vanes
Immediately behind the total and static pressure tubes on the nose boom are
two vanes (very much like miniature weather vanes) that pivot freely on posts
extending vertically and horizontally from the nose boom. A transducer measures
the position of these vanes relative to the fuselage centerline. The resulting
angles are called angle of attack and angle of sideslip. Both are key measurements
for determining the stability of an airplane.
Gyros and Accelerometers
Miniature gyroscopes (gyros) measure the rate of rotation about the three axes
mentioned earlier (pitch rate, roll rate, and yaw rate). Accelerometers measure
the linear acceleration along the same three axes (fore and aft - X, sideways
- Y, and up and down - Z). The three accelerometers and three gyros are usually
very carefully aligned and mounted near the aircraft's center of gravity, often
on the some mounting platform.
Strain Gages
Loads are measured by mounting strain gages on the structural parts to be monitored.
These sensors are very small wires which are bonded to the structure. When the
structure is under load there will be a slight expansion or contraction of the
part due to the load. This minute change in dimension is sensed by the strain
gage which produces an electrical signal in much the same manner as the other
transducers.
Author: Robert G. Hoey
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Introduction
