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 travelled 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.