= Aircraft
{wiki}
An \b[aircraft] is a vehicle capable of flight through the atmosphere. Its support can involve aerodynamic <lift> or <buoyancy>. A <fixed-wing aircraft> obtains aerodynamic <lift> from its <aircraft wings> as it moves relative to the surrounding air. An engine can supply <thrust>, but propulsion and the aerodynamic support of <gravitational weight> are distinct functions.<Four Forces on an Airplane by NASA Glenn Research Center>[Lift, Drag, Thrust and Weight]{ref}
For straight, level flight at constant <velocity>, neglect vertical <thrust> and align the remaining <thrust> with the flight direction. The <force balance> is
$$
L=W=mg,\qquad T=D.
$$
Here $L$ is <lift>, $D$ is <drag>, $T$ is <thrust> and $W$ is <gravitational weight>. For a stipulated $1000\,\mathrm{kg}$ vehicle with $g=9.81\,\mathrm{m/s^2}$ and <lift-to-drag ratio> $L/D=12$, this requires \b[$L=9810\,\mathrm N$ and $T=D=817.5\,\mathrm N$]. At an air-relative <speed> of $50\,\mathrm{m/s}$, $TV=40.875\,\mathrm{kW}$ is the useful propulsive mechanical power; an engine's input power can be larger because of losses.
An unpowered <glider (aircraft)> can instead maintain a steady descending <velocity>. If its <glide angle> below the horizontal is $\gamma$, resolving <gravitational weight> perpendicular and parallel to the flight path gives
$$
L=W\cos\gamma,\qquad D=W\sin\gamma,\qquad \tan\gamma=\frac{D}{L}.
$$
Consequently an assumed constant $L/D=12$ corresponds to $\gamma\simeq4.76\,{}^\circ$ and 12 units of horizontal travel per unit of height lost in still air.<Glide Angle by NASA Glenn Research Center>[Flight Path and Tangent Angle]{ref} The loss of <gravitational potential energy> supplies the work against <drag>; the <kinetic energy> need not be falling during that steady descent.
These balances concern translation of the vehicle. Maintaining its orientation also requires a balance of <torques> about its <centre of mass>. An <aircraft> can satisfy an instantaneous vertical <force balance> while still having an unwanted <pitching moment>.
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