Zero-lift drag coefficient

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. 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} imes 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: As noted earlier, C D , 0 = C D − C D , i {displaystyle C_{D,0}=C_{D}-C_{D,i}} .