= Glider
{disambiguate=aircraft}
{wiki}
A <glider (aircraft)> is an <aircraft> designed for sustained flight without continuous engine <thrust>. Its <aircraft wings> obtain aerodynamic <lift> from motion relative to the air. It can descend steadily while its <speed> remains constant: loss of <gravitational potential energy>, rather than a continuous reduction of <kinetic energy>, can supply the work against <drag>.<Four Forces on an Airplane by NASA Glenn Research Center>[Gliders and the distinction between propulsion and aerodynamic support]{ref}
In still air and a steady unpowered glide, the mechanical power balance is
$$
DV=mgw,
$$
where $V$ is <airspeed> and $w>0$ is downward speed. This follows from $D=W\sin\gamma$ and $w=V\sin\gamma$ in <glide angle>. Motion of the surrounding air changes the ground-relative energy account; rising air can offset a glider's descent relative to that air.
Two performance objectives should be distinguished. The <minimum sink speed> minimizes the downward rate, while the <best glide speed> maximizes horizontal distance per unit height lost in still air. They need not be the same speed.
For an original comparison, stipulate two steady operating points: $V_1=20\,\mathrm{m/s}$ with $w_1=0.6\,\mathrm{m/s}$, and $V_2=30\,\mathrm{m/s}$ with $w_2=0.7\,\mathrm{m/s}$. Their horizontal-distance ratios are
$$
\frac{\sqrt{V_1^2-w_1^2}}{w_1}\approx33.3183,\qquad
\frac{\sqrt{V_2^2-w_2^2}}{w_2}\approx42.8455.
$$
Among these two stipulated points, the slower one stays aloft longer for a given height loss, but the faster one covers more horizontal distance. From $1000\,\mathrm m$, the corresponding times are about $1666.7$ and $1428.6\,\mathrm s$. These invented points illustrate the competing objectives; they are not a measured performance curve.
A simple motivating experiment tracks a small unpowered model's position and height through a nearly steady portion of flight, then compares the vertical and horizontal motion. NASA identifies paper and lightweight model gliders as accessible examples.<Glide Angle by NASA Glenn Research Center>[Opening description and Flight Path]{ref} Repeating with controlled geometry and launch conditions distinguishes the resulting motion from a claim based solely on a large <wing aspect ratio>.
Back to article page
