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(2) Pressure-differenoe theory.-Figure 5 shows the motion of the particles of an air stream passing a flat plate held at right angles to the flow. The air is deflected from its course some distance in front of the plate and has a complex eddying motion in rear o.f it. In front of the plate the air is under an increased pressure, while behind the plate there is an area of reduced pressure. The drag can be con› sidered as due to the difference between the pressures in front of and behind the plate. 

8. Shape coefficients.-a. The two systems in common use for ex-. pressing air resistance are the engineering and the absolute. 

(1) Under the engineering system the formula is- 

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where Rv=air resistance due to pressure difference. 

A = cross sectional area normal to the air stream in square feet. 

V =velocity of motion in miles per hour. 

K IJ)=an empirically determined constant depending on the shape of the solid and the mass density of the air. 

In lighter than air practice the letter "K," minus subscript, is used to denote K IJ) when the mass density of the air is standard (0.00237 pound per cubic foot, which is the •value when the pressure is 29.92 inches and the temperature is 60° F.). 

(2} The absolute system, adopted by the National Advisory Com› mittee for Aeronautics, uses the formula: 

where p= mass density of the air. 

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v= velocity of motion in feet per second. 

K v=an empirical shape coefficient. 

2 ’;’ .is the dynamic pressure per unit of area or the velocity head of the air stream. This formula has more definite physical interpreta› tion than the engineering formula from both the momentum and pressure-difference theories. Before studying aerodynamic data, the system which is being used should always be determined. 

b. Some of the first practical tests made to determine the effect of shape upon the resistance offered the motion of solids through the air were .conducted by Eiffel. Since then studies have been conducted by various investigators until at present the store of in› formation on this subject is quite elaborate. Figures 5 to 15 give the action of the air on various shapes together with the values of K. 

