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How to Calculate Downwash

When an aircraft goes to lift, the force creates an opposite reaction of air flowing downward. This is referred to as downwash. Lift is dependent on downwash. When the air forced downward is greater than the weight of the aircraft, it will then start to rise. As the vehicle moves upward, the air pushes down at the trailing tip of the wing. To calculate downwash, you need the angle of the wing, the induced drag and the lift coefficient.

Instructions

- 1
  Calculate the lift coefficient. The final wing lift coefficient is know as Cl. It will be equal to the stream lift coefficient (Clo) divided by 1 plus the (Clo) divided by pi (3.14159) times the aspect ratio (AR). To find AR, you square the wingspan (s) and divide by the wing area (A). The final equation will look like this: Cl = Clo / (1 + Clo /[pi * AR]). The downwash angle is the ratio between lift coefficient and aspect ratio.
- 2
  Calculate the induced drag. The induced drag coefficient is the ratio of the square of the lift coefficient to the aspect ratio, times some constant, which would be the exact number of the lift force at that given time. Induced drag starts out as small value when lift is low, and increases rapidly when lift rises. It will look like this: CDI (induced drag) = Cl^2/Pi*AR
- 3
  Calculate the downwash force (F). F is equal to the mass of the air (m) times its new downward velocity (VE, or velocity times epsilon) per unit time. The mass of air is given by its volume times its density (p), and the volume in unit time is V times the "swept" area A. This culminates in the equation F = pAV (VE). This force has an effect on both the lift and drag coefficients, making downwash a fluid formula, directly linked to the amount of force exerted on the aircraft at any given time pushing it upward.

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