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
is the total drag coefficient for a given power, speed, and altitude, and
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.
value of 0.0161 for the streamlined P-51 Mustang of World War II[1] which compares very favorably even with the best modern aircraft.
) which is simply the product of zero-lift drag coefficient and aircraft's wing area (
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 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: As noted earlier,
is the propulsive efficiency, P is engine power in horsepower,
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.
, the equation is simplified to: The induced drag coefficient can be estimated as: where
is the lift coefficient, AR is the aspect ratio, and