Lift-to-drag ratio ($L/D$)

The lift-to-drag ratio $L/D$ compares the lift supporting an aircraft with the drag opposing its motion. For $L>0$ and $D>0$, a larger ratio means less drag for a given lift. If both coefficients use the same dynamic pressure $q$ and reference wing area $S$, then $L/D=C_L/C_D$. Ratios using mismatched reference areas do not have this interpretation.

A parabolic drag polar provides a useful worked model:Here $C_{D0}$ is the zero-lift drag coefficient. In a simple finite-wing model, $k=1/(\pi eAR)$ relates lift-induced drag to wing aspect ratio and a span efficiency factor.^[ref] A fitted whole-aircraft $k$ can also incorporate additional effects; it need not be determined by geometry alone.

The maximum lift-to-drag ratio follows by differentiation:At this point the two modeled contributions to drag coefficient are equal. With the stipulated $C_{D0}=0.020$, $e=0.8$ and $AR=8$, $k\simeq 0.049736$, $C_L^{\ast }\simeq 0.6341$ and $(L/D{)}_{\max }\simeq 15.85$. The optimum is usable only if the actual wing can attain that lift coefficient in the regime where the model is valid; aerodynamic stall can intervene.

For steady unpowered descent in still air, the force balance gives $\tan \gamma =D/L$, where $\gamma$ is the glide angle. Thus the stipulated optimum corresponds to $\gamma \simeq 3.61{}^{\circ }$ and about $15.85$ units of horizontal travel per unit of height lost.^[ref] Wind and changing flight conditions alter travel over the ground. An airfoil section's ratio also should not be substituted for that of an entire aircraft, which includes other sources of drag.

The maximum lift-to-drag ratio is the largest attainable ratio of positive lift to drag within stated operating conditions. It depends on both the drag polar and the range of lift coefficients the vehicle can actually attain.

For the positive parabolic drag polar $C_D=C_{D0}+k{C}_L^2$, a short proof uses the arithmetic-geometric mean inequality:Consequently
provided that coefficient is attainable. Equality requires equal modeled zero-lift and lift-dependent drag contributions. This bound supplies a global argument, rather than merely locating a stationary point by differentiation.

A lift-constrained maximum lift-to-drag ratio can be lower. With $C_{D0}=0.020$, $k=0.050$ and allowed $0<C_L\le 0.50$, the unconstrained optimum $C_L^{\ast }\simeq 0.6325$ is excluded. Since the ratio increases up to that point, the constrained optimum is at $C_L=0.50$, giving $L/D=15.3846$, below the unconstrained $15.8114$. A stall-limited interval should therefore be applied before claiming the fitted optimum is reachable.

Parameter bounds on maximum lift-to-drag ratio follow from monotonicity. If $C_{D0}\in [c_{-},c_{+}]$ and $k\in [k_{-},k_{+}]$ are positive intervals, then the unconstrained maximum lies in
This interval arithmetic bound is conservative when the fitted parameters are correlated and not all interval combinations are jointly possible. It quantifies uncertainty in the assumed polar; it does not include failure of the polar model itself.

Parameter bounds on maximum lift-to-drag ratio describe how an allowed set of drag polar parameters limits the optimum predicted by that model. For the positive parabolic drag polar, write $c=C_{D0}>0$ andThis is the unconstrained model optimum; an allowable range of lift coefficients can reduce the attainable value.

The derivatives $\partial F/\partial c=-F/(2c)$ and $\partial F/\partial k=-F/(2k)$ show that $F$ decreases with either parameter. Consequently, positive bounds $c\in [c_{-},c_{+}]$ and $k\in [k_{-},k_{+}]$ implyThis interval arithmetic argument is global over the rectangle, without a small-error approximation.

An original example uses $c\in [0.018,0.022]$ and $k\in [0.045,0.055]$. The predicted optimum ratio lies between about 14.374 and 17.568. Its optimizing lift coefficient lies in the different intervalor approximately $[0.5721,0.6992]$. A range of performance values and a range of operating points answer different questions.

A rectangular enclosure can be conservative. For a separate joint-parameter example, suppose fitting constrains $k=0.001/c$ exactly over a positive interval of $c$. Then the product $ck$ is constant and every allowed pair gives $F=15.8114$, even though each parameter varies. Combining independent marginal extremes would include pairs excluded by that relation.

For small random parameter errors, first-order propagation instead givesThe covariance term is needed when parameters are correlated, consistent with the primary metrological propagation law.^[ref] A standard uncertainty is not automatically a hard interval bound. Neither calculation includes failure of the assumed polar outside its fitted regime, and neither by itself identifies one operating point that is optimal for every parameter pair.

A lift-constrained maximum lift-to-drag ratio is the best ratio available within an allowed range of lift coefficients, rather than over every coefficient admitted algebraically by a drag polar. It is a constrained optimization problem whose feasible set must be specified before claiming an attainable optimum.

For the parabolic drag polar with positive $c=C_{D0}$ and $k$, defineIts derivative isThe ratio increases up to $x_{\ast }=\sqrt{c/k}$ and decreases afterward. The exact constrained solution is thereforeThis proof handles an interior optimum and either active endpoint; simply inserting an upper limit into the unconstrained formula does not.

An original example takes $c=0.020$ and $k=0.050$, giving $x_{\ast }\simeq 0.6325$ and unconstrained ratio 15.8114. If the allowed interval is $[0.20,0.45]$, the maximum occurs at 0.45 and is about 14.9378. If the allowed interval is instead $[0.90,1.20]$, it occurs at the lower endpoint 0.90 and is about 14.8760. A lower bound can be active as well as an upper bound.

In steady level flight, a speed interval can induce a lift-coefficient interval. With fixed gravitational weight $W$, mass density $\rho$ and wing area $S$,The endpoints reverse because the function decreases with speed. Additional limits, including the usable range before aerodynamic stall, require intersecting feasible intervals.

The simple finite-wing contribution motivates a quadratic polar, but changing configuration, Mach number or Reynolds number can change its fitted parameters.^[ref] Restricting the mathematical interval does not validate the polar across that interval. Wind and ground-distance objectives also define a different optimization from the aerodynamic ratio considered here.