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Aircraft 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" these values.
One of the most important considerations in flight is the balance of forces
maintained between thrust, drag, lift, and weight.

Balance of Forces

An aircraft in flight retains energy in two forms; kinetic energy and potential
energy. Kinetic energy is related to the speed of the airplane, while potential
energy is related to the altitude above the ground. The two types of energy
can be exchanged with one another. For example when a ball is thrown
vertically into the air, it exchanges the kinetic energy (velocity imparted by the
thrower), for potential energy as the ball reaches zero speed at peak altitude.

When an airplane is in stabilized, level flight at a constant speed, the power

has been adjusted by the pilot so that the thrust is exactly equal to the drag. If
the pilot advances the throttle to obtain full power from the engine, the thrust
will exceed the drag and the airplane will begin to accelerate. The difference
in thrust between the thrust required for level flight and the maximum available
from the engine is referred to as "excess thrust". When the airplane finally
reaches a speed where the maximum thrust from the engine just balances the
drag, the "excess thrust" will be zero, and the airplane will stabilize at its
maximum speed.

Notice that this "excess thrust" can be used either to accelerate the airplane to
a higher speed (increase the kinetic energy) or to enter a climb at a constant
speed (increase the potential energy), or some combination of the two.

Excess Thrust Energy Exchange

There are energy exchange equations which can be used to relate the rate of
change of speed (or acceleration) to the rate of change of altitude (or rate of
climb). (These equations are introduced later.) In this way, level flight
accelerations (accels.) at maximum power can be used to measure the
"excess thrust" over the entire speed range of the airplane at one altitude.
This "excess thrust" can then be used to calculate the maximum rate of climb
capability for an aircraft.

Takeoff

The takeoff is a critical maneuver in any airplane. The airplane will usually be
carrying a payload (passengers, cargo, weapons) and often a full load of fuel.
The resulting heavy weight means that a high speed must be reached before
the wings can generate sufficient lift, thus a long distance must be traveled on
the runway before lift-off. After lift-off, the heavy weight will result in a
relatively slow acceleration to the speed for best angle of climb.

After lining the aircraft up on the runway, the pilot applies the brakes
(accomplished by applying pressure to the top of the rudder pedals - each
pedal controls its respective wheel). The throttles are then advanced to
military power (100% RPM). As the engines wind up, the engines and
instruments are given a "last minute" check. (Pilots do a lot of "checks" to
ensure that everything is going OK. After all, if something were to happen, you
can't just pull off to the side of the road!) When everything is ready, the brakes
are released and the airplane accelerates down the runway. At a pre-
determined speed, the pilot pulls back on the stick to pitch the airplane
upward about five degrees. Although the nose wheel is off the ground, the
main gear remains on the runway because there is not yet enough airflow
over the wings to create sufficient lift to raise the aircraft. After a little while,
the airplane reaches the speed (90 knots) at which its wings produce lift
slightly greater than its weight and it takes off.

While the airplane climbs away from the runway the pilot must raise the
landing gear (this decreases the drag) and the flaps, then let it accelerate to
the desired climb speed. Once this speed is reached, it is maintained by
raising the nose slightly and "trimming" off all control stick pressures.
Straight and Level Flight

If an airplane maintains a given altitude, airspeed, and heading, it is said to be

in "straight and level flight." This condition is achieved and maintained by
equalizing all opposing forces. Lift must equal weight so the airplane does not
climb or descend. Thrust must equal drag so the airplane does not speed up
or slow down. The wings are kept level so the airplane does not turn. Any
imbalance will result in a change in altitude or airspeed. It is the pilot's
responsibility to prevent or correct for such an imbalance.

Proper trim is essential for maintaining this balance. If the pilot, by being "out
of trim," is forced to maintain a given amount of stick pressure, the arm
holding the stick will eventually tire. But in the short term the pilot must very
precisely hold that pressure -- any change will result in a change in attitude. If
the airplane is properly trimmed, the correct stick position is held
automatically, and no pressure need be exerted.

Obviously, an airplane cannot remain indefinitely in this ideal condition. Due to

mission, airspace, and fuel requirements, the pilot must change the airspeed,
altitude, and heading from time to time.

Speed

Speeding up and slowing down is not simply a matter of changing the throttle
setting (changing the force produced by the engines). Airspeed can also be
changed by changing the drag. Many aircraft are equipped with a
"speedbrake" for this purpose -- a large metal plate that can be extended out
into the windstream, increasing parasite drag and slowing the airplane.
As an airplane speeds up or slows down, the amount of air passing over the
wing follows suit. For instance, to maintain a constant altitude as the airspeed
is decreasing, the pilot must compensate for this decreased airflow by
changing the AOA (pulling back on the stick) to equalize the amount of lift to
the weight of the airplane. All this works nicely until stall speed is reached,
when an increase in AOA is met with a decrease in lift, and the airplane, its
weight not completely countered by lift, begins to dramatically lose altitude.
Conversely, an increase in airspeed must be met with a decrease in the AOA
(moving the stick forward) to maintain a constant altitude. As airspeed
increases or decreases, trim must be changed as well.

Mach number is the most influential parameter in the determination of range

for most jet-powered aircraft. The most efficient cruise conditions occur at a
high altitude and at a speed which is just below the start of the transonic drag
rise. The drag (and thus the thrust required to maintain constant Mach
number) will change as the weight of the airplane changes. The angle of
attack (and thus the drag) of an airplane will become slightly lower as fuel is
used since the airplane is becoming lighter and less lift is required to hold it
up.

Climb

Climbs and descents are accomplished by using power setting respectively

higher or lower than that required for level flight. When an airplane is in level
flight, just reducing the power begins descent. Instead of pulling back on the
stick to maintain altitude as the airspeed slows, the pilot keeps the stick
neutral or pushes it forward slightly to establish a descent. Gravity will provide
the force lost by the reduction in power. Likewise, increased power results in a
climb.

Airspeed can be controlled in a climb or descent without changing the throttle

setting. By pulling back on the stick and increasing the climb rate or by
decreasing the descent rate, the airspeed can be decreased. Likewise,
lowering the nose by pushing forward on the stick will effectively increase the
airspeed. In most climbs and descents, this is the way airspeed is maintained.
A constant throttle setting is used and the pilot changes pitch in small
increments to control airspeed.

If the pilot were to fly a climb such that the airplane was at the best-climb
speed as it passed through each altitude, it would be achieving the best
possible rate of climb for the entire climb. This is known as the "best-climb
schedule" and is identified by the dotted line.
Flying the best-climb schedule will allow the airplane to reach any desired
altitude in the minimum amount of time. This is a very important parameter for
an interceptor attempting to engage an incoming enemy aircraft. For an
aircraft that is equipped with an afterburner, two best climb schedules are
determined; one for a Maximum Power climb (afterburner operating) and one
for a Military Power climb (engine at maximum RPM but afterburner not
operating). The Max Power climb will result in the shortest time but will use a
lot of fuel and thus will be more useful if the enemy aircraft is quite close. The
Mil. Power climb will take longer but will allow the interceptor to cruise some
distance away from home base to make the intercept.

For cargo or passenger aircraft the power setting for best climb is usually the
maximum continuous power allowed for the engines. By flying the best-climb
schedule the airplane will reach it's cruise altitude in the most efficient
manner, that is, with the largest quantity of fuel remaining for cruise.

Range

One of the most critical characteristics of an airplane is its range capability,

that is, the distance that it can fly before running out of fuel. Range is also one
of the most difficult features to predict before flight since it is affected by many
aspects of the airplane/engine combination. Some of the things that influence
range are very subtle, such as poor seals on cooling doors or small pockets of
disturbed air around the engine inlets.
Turns

The aerodynamics of a turn widely misunderstood, since many people think

that the airplane is "steered" by the stick or the rudder pedals (probably the
result of thinking of the airplane as a sort of "flying car.") A turn is actually the
result of a change in the direction of the lift vector produced by the wings.

A pilot turns an airplane by using the ailerons and coordinated rudder to roll to
a desired bank angle. As soon as there is bank, the force produced by the
wings (lift) is no longer straight up, opposing the weight. It is now "tilted" from
vertical so that part of it is pulling the airplane in the direction of the bank. It is
this part of the lift vector that causes the turn. Once the pilot has established
the desired bank angle, the rudder and the aileron are neutralized so that the
bank remains constant.

When part of the lift vector is used for turning the airplane, there is less lift in
the vertical opposing weight. If the pilot were to establish a bank angle without
increasing the total amount of lift being produced, the lift opposing the weight
would decrease, and the resulting imbalance would cause in a descent. The
pilot compensates by pulling back on the stick (increasing the AOA and
therefore lift). By increasing the total lift, the lift opposing the weight can
balance out the weight and control level flight. This increase in total lift also
increases lift in the turn direction and results in a faster turn.
As the bank angle increases, the amount of pull required to maintain level
flight increases rapidly. It is not possible to maintain level flight beyond a given
bank angle because the wings cannot produce enough lift. An attempt to fly
beyond this point will result in either a stall or a descent.

Physiologically speaking, the most important part of a turn is the necessity to

pull "Gs". As the back pressure is increased to maintain level flight, the
increased force is felt as an increase in "G" level. In a 30 degree bank, 1.2 G
is required to maintain level flight. The G level increases rapidly with an
increase in bank; at 60 degrees, it goes to 2.0 G, and it takes 9.0 G to fly a
level 84 degree bank turn. As long as there is enough airspeed, the G level
can be increased in any bank angle by pulling back on the stick.

Finishing the turn, a simple matter of leveling the wings by using the ailerons
and coordinated rudder, takes time; the airplane continues turning until the
wings are level, so the roll-out must be started a little prior to reaching the
desired heading. Back-stick pressure must also be released as bank
decreases or the aircraft will climb.

Maneuverability
Airplanes are not limited to being a relatively fast means of getting
somewhere. Long ago thrill-seeking pilots discovered that aircraft have the
potential for providing loads of fun while getting nowhere fast. Aerobatics are
an essential skill for fighter pilots; and the training that it gives to pilots in
position orientation and judgment is considered so vital that a great deal of
time is spent teaching these maneuvers. 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.

Maneuverability

Aileron Roll
Aileron Roll The aileron roll is simply a 360 degree roll accomplished by
putting in and maintaining coordinated aileron pressure. The maneuver is
started slightly nose high because, as the airplane rolls, its lift vector is no
longer countering its weight, so the nose of the airplane drops significantly
during the maneuver. Back stick pressure is maintained throughout so that
even when upside down, positive seat pressure (about 1 G) will be felt. As the
airplane approaches wings-level at the end of the maneuver, aileron pressure
is removed and the roll stops.

Loop

Loop A loop is simply a 360 degree change in pitch. Because the airplane will
climb several thousand feet during the maneuver, it is started at a relatively
high airspeed and power setting (if these are too low, the airspeed will decay
excessively in the climb and the maneuver will have to be discontinued.) The
pilot, once satisfied with the airspeed and throttle setting, will pull back on the
stick until about three Gs are felt. The nose of the airplane will go up and a
steadily increasing climb will be established. As the maneuver continues,
positive G is maintained by continuing to pull. The airplane continues to
increase its pitch until it has pitched through a full circle. When the world is
right-side-up again, the pilot releases the back stick pressure and returns the
aircraft to level flight.

MISTAKES

Any time you place yourself in a several thousand pound machine and force it
to travel through the air at high speeds and altitudes, there is going to be
some risk. Many think that the primary risk in flying is mechanical failure or
weather. Contrary to this belief, most airplanes (even those made of cloth and
wood) that crash do so as a result of pilot error --frequently from attempting to
fly too slow!

Stall

The stall is the initial result of letting the airspeed decay below what is
required for the wings to produce sufficient lift. With insufficient lift to
counteract aircraft weight, the airplane is not being "held up" by the wings any
more and it accelerates toward the ground. At low altitude, the stall can be
immediately disastrous but with enough altitude below, the pilot can take
action to recover.

Recovery from the stall is accomplished by correcting the condition that led to
it. Since the stall is caused by attempting to fly at too high an AOA, the pilot
must immediately reduce the AOA by moving the stick forward. At the same
time, the throttle is advanced to full power to rapidly increase the airspeed
needed for a return to level flight or climb.

Aircraft are almost always designed to give some warning prior to a stall. In
very large aircraft, special sensors detect the impending stall and physically
shake the control stick. Cessna uses a buzzer located in the wing root for its
light aircraft. High-performance aircraft have a horizontal stabilizer placed so
that, as a stall is approached, the turbulent air coming off the top of the wing
hits the horizontal stabilizer and shakes the flight controls. In extreme
conditions, the whole airplane will shake. These warnings are difficult to
ignore; they give the pilot sufficient time to act to prevent the stall.

Spin

If a stall is maintained and yaw is somehow induced, a spin can result. Spins
can be recognized by high descent and roll rates, and a flight path that is
straight down. Clearly, this is a situation to be entered with some forethought.
Harder to recover from than a stall, and much more dangerous in terms of
altitude loss, the spin is an extremely complex maneuver and beyond the
scope of this text. The good news is that if you do not stall, you cannot spin.

Landing

"All good things must come to an end," and most flights end with a landing.
The relative difficulty of this maneuver is often expressed by a student pilot
after the first solo flight: "The first thought that came to mind after I took off
was `Oh boy, now I've gotta land this thing!'"

After lining the airplane up with the runway and configuring it properly (landing
gear, proper flap setting, speedbrake out), the pilot uses the throttle setting to
maintain the proper airspeed (100 knots) and uses the elevators and ailerons
to keep the airplane headed for the runway. The airplane is set up in a shallow
descent (about three degrees) aimed at the near end of the runway. If this part
of the landing, the "final approach" is flown correctly, it will look like the jet is
headed for a collision with the approach end of the runway.

As the airplane closes in on the approach end, the pilot begins to ease the
stick back to level off the airplane several feet above the runway and slows to
landing speed by reducing the power to idle. As the airplane levels off just
above the ground in idle power, it will lose speed rapidly because there is little
or no thrust to counter the drag. The pilot continues to move the stick back to
increase the AOA and keep the airplane flying for just a little while longer. In a
well-flown landing, the airplane will settle to the ground just before the stall
AOA is reached.

Now a land-based vehicle, the airplane is controlled with the brakes and
slowed to taxi speed.

The Axis System

A good understanding of the basic axis system used to describe aircraft

motion is necessary to appreciate flight data. 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.
 Translational Axes Rotational Axes

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.)

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This page last modified: August 15, 2003