Lift-to-Drag Ratio

Lift divided by drag is the closest thing aeronautics has to a single figure of merit. It says how many newtons of weight an aircraft carries for every newton of thrust it needs to hold, and it sets the glide ratio, the range and, through the power curve, most of what the aircraft can do.

Because lift and drag share the same dynamic pressure and the same reference area, those cancel out and $L/D = C_L/C_D$ exactly. That has a consequence worth sitting with: $L/D$ is a property of the SHAPE and the ATTITUDE, not of the size, the weight, the altitude or the airspeed. A scale model and the full-size aircraft have the same best $L/D$. A loaded sailplane and an empty one glide the same DISTANCE from the same height — the heavier one simply flies faster and gets there sooner, which is why competition pilots carry water ballast on strong days and dump it before landing.

The numbers are worth knowing because they place any new aircraft immediately. A Cessna 172 is around 9. A modern airliner in cruise reaches 17 to 20. A high-performance sailplane exceeds 50, and the best open-class machines pass 60. The Wright Flyer managed about 8.3. An albatross is roughly 20, and the flying squirrel — which is genuinely gliding, not merely falling — is close to 2.

Every aircraft has one angle of attack at which $L/D$ is best, and since angle of attack maps onto indicated airspeed for a given weight and configuration, that is the number printed as the best-glide speed. Fly faster and the parasite drag has grown; fly slower and the induced drag has grown; either way the ratio falls. On the low-speed side, the fall is steep and it arrives together with the region of reversed command, so an engine failure flown too slowly is worse than one flown too fast in both distance and controllability.

The one place $L/D$ misleads is on range for a jet, which maximises range not at best $L/D$ but somewhat faster, at the speed that maximises $V \times L/D$ — because a jet's fuel flow tracks thrust rather than power. A propeller aircraft, whose fuel flow tracks power, does maximise range at best $L/D$. Same airframe, same polar, two different answers, and the difference is entirely in what the engine charges for.