Balanced flight
From Wikipedia, the free encyclopedia
Balanced Flight is the state an aircraft is in when it is traveling at a constant heading, with a constant airspeed, and at a constant altitude. In other words, the aircraft is not accelerating. This occurs because all of the four main forces that act on an aircraft in flight are in perfect balance, and they cancel each other out.
The four main forces that act on an aircraft in flight are thrust, drag, lift, and gravity.
Thrust: Thrust is the force that accelerates the aircraft forward. It is a result of any mass of air (or other gasses such as in a rocket) that is accelerated backwards in an engine. This can be caused in a number of ways such as by a rotating propeller or by the igniting of an air/fuel mixture in a gas turbine engine.
Drag: Drag is the force that accelerates the aircraft backwards, and it counteracts (is opposite of) thrust. It is the result of the resistance (inertia) of the air molecules which are displaced as the aircraft moves through them. Drag varies with different aircraft shapes and with the positions of movable surfaces such as flaps and control surfaces (ailerons, elevators, and rudders).
Lift: Lift is the force that accelerates the aircraft upwards, keeping it in the air. It is caused by the difference in air pressure on the upper and lower surfaces of the wing. The airflow impacts the wing at an angle (angle of attack or AOA). This compresses the air underneath the wing, resulting in higher airpressure, and it lowers the airpressure above the wing, as the air has to curve downward before exerting pressure on the wing. To optimize lift further, many airfoils are designed with the top of the wing bulging outward slightly, forcing the upper stream of air to flow faster, which results in lower pressure (Bernoulli's principle).
Gravity: Gravity is the force of attraction between any two masses. Therefore, gravity pulls the aircraft straight down, and counteracts lift. On Earth gravity is, for the most part, constant. However, it does decrease very slightly with altitude.
When an aircraft is not in balanced flight, the thrust/drag pair of forces is out of balance, the lift/gravity pair of forces is out of balance, or both pairs are out of balance. When either thrust or drag is greater than its counterpart, the aircraft gains or loses airspeed. When either lift or gravity is greater than its counterpart, the aircraft gains or loses altitude. However, it should be noted that the above explanation is a very simplified one. In reality the horizontal, vertical, and lateral components of all these forces vary as the aircraft rotates around its three axes (roll, pitch, and yaw) resulting in a complex, net behavior of the aircraft.
