Straight and Level Flight of Aircraft
In flight an aircraft can be considered as being acted on by four forces: lift, weight, thrust, and drag. Thrust is the force generated by the engine and acts along the engine's thrust vector. Lift acts perpendicular to the vector representing the aircraft's velocity relative to the atmosphere. Drag acts parallel to the aircraft's velocity vector, but in the opposite direction because drag resists motion through the air. Weight acts through the aircraft's centre of gravity, towards the center of the Earth.
In straight and level flight, lift is approximately equal to weight. In addition, if the aircraft is not accelerating, thrust is approximately equal to drag.
In straight climbing flight, lift is less than weight. At first, this seems incorrect because if an aircraft is climbing it seems lift must exceed weight. When an aircraft is climbing at constant speed it is its thrust that enables it to climb and gain extra potential energy. Lift acts perpendicular to the vector representing the velocity of the aircraft relative to the atmosphere, so lift is unable to alter the aircraft's potential energy or kinetic energy. This can be seen by considering an aerobatic aircraft in straight vertical flight - one that is climbing straight upwards (or descending straight downwards). Vertical flight requires no lift! When flying straight upwards the aircraft can reach zero airspeed before falling earthwards - the wing is generating no lift and so does not stall. In straight, climbing flight at constant airspeed, thrust exceeds drag.
In straight descending flight, lift is less than weight. In addition, if the aircraft is not accelerating, thrust is less than drag. In turning flight, lift exceeds weight and produces a load factor greater than one, determined by the aircraft's angle of bank.
Read more about this topic: Aircraft Flight Mechanics
Famous quotes containing the words straight, level and/or flight:
“I, who travel most often for my pleasure, do not direct myself so badly. If it looks ugly on the right, I take the left; if I find myself unfit to ride my horse, I stop.... Have I left something unseen behind me? I go back; it is still on my road. I trace no fixed line, either straight or crooked.”
—Michel de Montaigne (1533–1592)
“The truth is, that common-sense, or thought as it first emerges above the level of the narrowly practical, is deeply imbued with that bad logical quality to which the epithet metaphysical is commonly applied; and nothing can clear it up but a severe course of logic.”
—Charles Sanders Peirce (1839–1914)
“Its shrill scream seems yet to linger in its throat, and the roar of the sea in its wings. There is the tyranny of Jove in its claws, and his wrath in the erectile feathers of the head and neck. It reminds me of the Argonautic expedition, and would inspire the dullest to take flight over Parnassus.”
—Henry David Thoreau (1817–1862)