Inaerodynamics, thezero-lift drag coefficient${\displaystyle C_{D,0}}$ is a dimensionless parameter which relates an aircraft's zero-liftdragforce to its size, speed, and flying altitude.

Mathematically, zero-liftdrag coefficient is defined as${\displaystyle C_{D,0}=C_D-C_{D,i}}$, where${\displaystyle C_D}$ is the total drag coefficient for a given power, speed, and altitude, and${\displaystyle C_{D,i}}$ is thelift-induced drag coefficient at the same conditions. Thus, zero-lift drag coefficient is reflective ofparasitic drag which makes it very useful in understanding how "clean" or streamlined an aircraft's aerodynamics are. For example, aSopwith Camel biplane ofWorld War I which had many wires and bracing struts as well as fixed landing gear, had a zero-lift drag coefficient of approximately 0.0378. Compare a${\displaystyle C_{D,0}}$ value of 0.0161 for the streamlinedP-51 Mustang ofWorld War II^[1] which compares very favorably even with the best modern aircraft.

The drag at zero-lift can be more easily conceptualized as thedrag area (${\displaystyle f}$) which is simply the product of zero-lift drag coefficient and aircraft's wing area (${\displaystyle C_{D,0}\times S}$ where${\displaystyle S}$ is the wing area). Parasitic drag experienced by an aircraft with a given drag area is approximately equal to the drag of a flat square disk with the same area which is held perpendicular to the direction of flight. The Sopwith Camel has a drag area of 8.73 sq ft (0.811 m²), compared to 3.80 sq ft (0.353 m²) for the P-51 Mustang. Both aircraft have a similar wing area, again reflecting the Mustang's superior aerodynamics in spite of much larger size.^[1] In another comparison with the Camel, a very large but streamlined aircraft such as theLockheed Constellation has a considerably smaller zero-lift drag coefficient (0.0211 vs. 0.0378) in spite of having a much larger drag area (34.82 ft² vs. 8.73 ft²).

Furthermore, an aircraft's maximum speed is proportional to thecube root of the ratio of power to drag area, that is:

${\displaystyle V_{max}\propto \sqrt[3]{power/f}}$.^[1]

As noted earlier,${\displaystyle C_{D,0}=C_D-C_{D,i}}$.

The total drag coefficient can be estimated as:

${\displaystyle C_D=\frac{550\eta P}{\frac{1}{2}{\rho }_0[\sigma S(1.47V{)}^3]}}$,

where${\displaystyle \eta }$ is thepropulsive efficiency, P is engine power inhorsepower,${\displaystyle {\rho }_0}$ sea-level air density inslugs/cubic foot,${\displaystyle \sigma }$ is the atmospheric density ratio for an altitude other than sea level, S is the aircraft's wing area in square feet, and V is the aircraft's speed in miles per hour. Substituting 0.002378 for${\displaystyle {\rho }_0}$, the equation is simplified to:

${\displaystyle C_D=1.456\times {10}^5(\frac{\eta P}{\sigma S{V}^3})}$.

The induced drag coefficient can be estimated as:

${\displaystyle C_{D,i}=\frac{C_L^2}{\pi A\text{R}ϵ}}$,

where${\displaystyle C_L}$ is thelift coefficient,AR is theaspect ratio, and${\displaystyle ϵ}$ is the aircraft'sefficiency factor.

Substituting for${\displaystyle C_L}$ gives:

${\displaystyle C_{D,i}=\frac{4.822\times {10}^4}{A\text{R}ϵ{\sigma }^2{V}^4}(W/S{)}^2}$,

where W/S is thewing loading in lb/ft².

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• Drag (physics)

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