Lift-to-Drag Ratio
Lift-to-drag ratio (L/D) is the aerodynamic lift a wing or aircraft generates divided by its drag — a direct measure of efficiency. It equals the glide ratio for unpowered flight and improves with higher aspect ratio, ranging from about 10:1 for a herring gull to 60:1 for the best sailplanes.
The lift-to-drag ratio (L/D) is the lift generated by an aerofoil or aircraft divided by its aerodynamic drag. It is the key measure of aerodynamic efficiency for a given flight condition and varies with airspeed, peaking at one particular speed rather than at the speed of minimum drag. A central property is that maximum L/D is independent of the aircraft's weight, wing area, or wing loading. For an unpowered glide at constant speed, L/D numerically equals the glide ratio — forward distance covered per unit of altitude lost. Total drag combines induced drag (dominant at low speed) and form or parasitic drag (dominant at high speed), so a plot of L/D against speed forms a characteristic U-shape. A higher aspect ratio (longer, narrower wings) raises L/D mainly by cutting induced drag, which is why endurance and soaring aircraft use slender wings. Representative values: a herring gull about 10:1 and a wandering albatross about 20:1; recreational sailplanes around 30:1 and the best up to 60:1; airliners like the Boeing 747 about 17.7:1 and the Airbus A340 about 19:1; the record-setting Virgin Atlantic GlobalFlyer about 37:1. The practical importance is energetic: higher L/D directly reduces the power or fuel needed for flight — broadly, doubling L/D halves the energy required to cover a given distance — which is why it dominates the design of long-endurance and solar aircraft.