Zerolift drag coefficient

In aerodynamics, the zerolift drag coefficient C_{D,0} is a dimensionless parameter which relates an aircraft's zerolift drag force to its size, speed, and flying altitude.
Mathematically, zerolift drag coefficient is defined as C_{D,0} = C_{D} − C_{D,i}, where C_{D} is the total drag coefficient for a given power, speed, and altitude, and C_{D,i} is the liftinduced drag coefficient at the same conditions. Thus, zerolift 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, Sopwith Camel biplane of World War I festooned with wires, bracing struts, and fixed landing gear, had a zerolift drag coefficient of approximately 0.0378, compared to 0.0161 for the streamlined P51 Mustang of World War II^{[1]} which compares very favorably even with the best modern aircraft.
The zerolift drag coefficient can be more easily conceptualized as the drag area (f) which is simply the product of zerolift drag coefficient and aircraft's wing area ( where 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 m^{2}), compared to 3.80 sq ft (0.353 m^{2}) for the P51. 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 the Lockheed Constellation has a considerably smaller zerolift 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:
 ^{[1]}.
Estimating zerolift drag^{[1]}
As noted earlier, C_{D,0} = C_{D} − C_{D,i}.
The total drag coefficient can be estimated as:
 ,
where η is the propulsive efficiency, P is engine power in horsepower, ρ_{0} sealevel air density in slugs/cubic foot, σ 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}, the equation is simplified to:
 .
The induced drag coefficient can be estimated as:
 ,
where C_{L} is the lift coefficient, A is the aspect ratio, and is the aircraft's efficiency factor.
Substituting for C_{L} gives:
 ,
where W/S is the wing loading in lb/ft².
References
 ^ ^{a} ^{b} ^{c} ^{d} Loftin, LK, Jr.. "Quest for performance: The evolution of modern aircraft. NASA SP468". http://www.hq.nasa.gov/pao/History/SP468/cover.htm. Retrieved 20060422.
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