In aerodynamics, the zero-lift drag coefficient C D , 0 {\displaystyle C_{D,0}} is a dimensionless parameter which relates an aircraft's zero-lift drag force to its size, speed, and flying altitude. Mathematically, zero-lift drag coefficient is defined as C D , 0 = C D − C D , i {\displaystyle C_{D,0}=C_{D}-C_{D,i}} , where C D {\displaystyle C_{D}} is the total drag coefficient for a given power, speed, and altitude, and C D , i {\displaystyle C_{D,i}} is the lift-induced drag coefficient at the same conditions. Thus, zero-lift drag coefficient is reflective of parasitic drag which makes it very useful in understanding how "clean" or streamlined an aircraft's aerodynamics are. For example, a Sopwith Camel biplane of World 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 C D , 0 {\displaystyle C_{D,0}} value of 0.0161 for the streamlined P-51 Mustang of World War II which compares very favorably even with the best modern aircraft. The drag at zero-lift can be more easily conceptualized as the drag area ( f {\displaystyle f} ) which is simply the product of zero-lift drag coefficient and aircraft's wing area ( C D , 0 × S {\displaystyle C_{D,0}\times S} where S {\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. In another comparison with the Camel, a very large but streamlined aircraft such as the Lockheed 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 the cube root of the ratio of power to drag area, that is:
V m a x ∝ p o w e r / f 3 {\displaystyle V_{max}\ \propto \ {\sqrt[{3}]{power/f}}} .
Estimating zero-lift drag Source: As noted earlier, C D , 0 = C D − C D , i {\displaystyle C_{D,0}=C_{D}-C_{D,i}} . The total drag coefficient can be estimated as:
C D = 550 η P 1 2 ρ 0 [ σ S ( 1.47 V ) 3 ] {\displaystyle C_{D}={\frac {550\eta P}{{\frac {1}{2}}\rho _{0}[\sigma S(1.47V)^{3}]}}} , where η {\displaystyle \eta } is the propulsive efficiency, P is engine power in horsepower, ρ 0 {\displaystyle \rho _{0}} sea-level air density in slugs/cubic foot, σ {\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 ρ 0 {\displaystyle \rho _{0}} , the equation is simplified to:
C D = 1.456 × 10 5 ( η P σ S V 3 ) {\displaystyle C_{D}=1.456\times 10^{5}({\frac {\eta P}{\sigma SV^{3}}})} . The induced drag coefficient can be estimated as:
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