= Glide angle
{title2=$\gamma$}
The <glide angle> is the inclination of a descending flight path below the horizontal. A positive $\gamma$ will denote descent here. For aerodynamic reasoning it is important to specify motion relative to the air: a steady wind can change the ground track without changing the air-relative <glide angle> at the same operating condition.
For a steady unpowered descent, resolve <gravitational weight> perpendicular and parallel to the air-relative <velocity>. With <lift> normal to that velocity and <drag> opposing it,
$$
L=W\cos\gamma,\qquad D=W\sin\gamma,\qquad
\tan\gamma=\frac DL.
$$
The still-air horizontal distance per unit height lost is therefore $L/D$. NASA's description connects this geometry with a historical observation: the Wright brothers flew parallel to a dune slope, letting the slope provide an estimate of the flight-path angle.<Glide Angle by NASA Glenn Research Center>[Flight Path and Tangent Angle, dune-slope observation and distance/height geometry]{ref}
For an original constant-velocity model, set $L/D=20$ and <airspeed> $V=25\,\mathrm{m/s}$. Then $\gamma=\arctan(1/20)\approx2.8624\,{}^{\circ}$, horizontal air-relative speed is $u=V\cos\gamma\approx24.9688\,\mathrm{m/s}$, and downward speed is $w=V\sin\gamma\approx1.24844\,\mathrm{m/s}$. In still air, a stipulated $1000\,\mathrm m$ height loss corresponds to $20\,\mathrm{km}$ horizontally.
If a constant horizontal wind component $U_w$ is positive along the direction of travel, the ground-frame horizontal speed is $u+U_w$. With no vertical wind and $u+U_w>0$,
$$
\tan\gamma_{\rm ground}=\frac{w}{u+U_w},\qquad
\frac{\text{ground distance}}{\text{height lost}}=\frac{u+U_w}{w}.
$$
A $10\,\mathrm{m/s}$ headwind makes that ratio about $11.9900$; a tailwind of the same magnitude gives $28.0100$. All three cases retain the same air-relative angle and sink rate. Their <groundspeeds> and ground-frame path angles differ.
\Image[/glide-angle-wind.png]
{title=The same air-relative glide condition gives different ground ranges in a stipulated constant headwind, still air and tailwind. Original model; unequal plotting scales mean displayed line angles are not the physical glide angles.}
This distinction separates a measured ground trajectory from an aerodynamic <lift-to-drag ratio>. Estimating the latter from height and horizontal travel requires accounting for air motion and identifying the steady portion of the trajectory.
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