Zero-lift drag coefficient
In aerodynamics, the zero-lift drag coefficient 
  
    
      
        
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    {\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 
  
    
      
        
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        =
        
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    {\displaystyle C_{D,0}=C_{D}-C_{D,i}}
  , where 
  
    
      
        
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    {\displaystyle C_{D}}
   is the total drag coefficient for a given power, speed, and altitude, and 
  
    
      
        
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    {\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 
  
    
      
        
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    {\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.