Compute aerodynamic drag from air density, airspeed, reference area, and drag coefficient, and get the lift-to-drag ratio — the single most important efficiency figure of merit for an aircraft. The calculator also reports the corresponding lift and the dynamic pressure that drives both forces.
Drag is the aerodynamic force opposing an aircraft's motion through the air; overcoming it is the entire job of the engine in cruise. The lift-to-drag ratio L/D measures how much lift you get for each unit of drag, and it sets the glide angle, the cruise efficiency, and ultimately the range of an aircraft. This calculator evaluates both from the standard drag equation and the drag and lift coefficients.
D = ½ · ρ · V² · S · C_D, exactly parallel to the lift equation L = ½ρV²S·C_L.
The two share the same dynamic pressure q = ½ρV² and reference area S, differing only in their coefficient. Dividing one by the other cancels q and S, giving the clean result L/D = C_L / C_D — the aerodynamic efficiency depends only on the ratio of the coefficients, not on speed or altitude directly.
Total drag is the sum of two parts. Parasite (zero-lift) drag comes from skin friction and pressure (form) drag and grows with V²; it dominates at high speed. Induced drag is the unavoidable penalty of producing lift — the energy left in the wingtip vortices — and it falls with V² at fixed weight, dominating at low speed. The drag coefficient is often modelled as C_D = C_D0 + C_L²/(π·e·AR), where C_D0 is the parasite term and the second term is the induced drag with aspect ratio AR and span efficiency e.
Plotting C_D against C_L traces the drag polar, the signature curve of an aircraft's aerodynamics. Its parabolic shape comes directly from the C_L² induced-drag term. The point on the polar where a line from the origin is tangent gives the maximum L/D; the corresponding C_L is the best-glide / best-range lift coefficient. The polar lets a designer read off the drag for any flight condition at a glance.
In a power-off glide, the ratio L/D equals the glide ratio: an aircraft at L/D = 15 travels 15 m forward for every 1 m of altitude lost. For powered flight, range scales with L/D (jets) or with the related C_L^½/C_D (propellers) through the Breguet range equation. Maximizing L/D — through high aspect ratio, clean surfaces, and laminar flow — is therefore central to fuel economy and endurance.
It varies widely with design. A modern airliner cruises near L/D ≈ 17–20, a high-performance sailplane reaches 40–60, a light general-aviation aircraft is around 10–15, and a blunt re-entry capsule may be below 1. Higher aspect ratio and lower parasite drag push the maximum L/D up.
Parasite drag is everything not associated with producing lift — skin friction, form drag, and interference — and it increases with the square of speed. Induced drag is the cost of generating lift via trailing vortices and decreases with the square of speed at fixed weight. Total drag is minimized (and L/D maximized) where the two are equal.
Best L/D occurs at a specific lift coefficient (where induced drag equals parasite drag), not a fixed speed. Solve for the speed that produces that C_L at your weight and altitude: V = sqrt(2W/(ρ·S·C_L)). Heavier weight or thinner air raises that best-L/D speed, which is why glide and best-range speeds increase with altitude and load.
The maximum achievable L/D is a property of the aerodynamic shape and does not depend on weight — but the speed at which you achieve it does. A heavier aircraft must fly faster to reach the best-L/D lift coefficient, so it glides the same distance from a given height but does so more quickly.
In incompressible flow C_D rises at low speed because the lift coefficient (and thus induced drag) is large. At high subsonic speed, compressibility adds wave drag once shock waves form near the drag-divergence Mach number, and C_D climbs sharply. So the constant-C_D assumption here is a snapshot for one flight condition; use a drag polar or Mach analysis across the envelope.