On fixed wing aircraft, the wing is the principal contributor to the lift produced by the airplane. While producing the lift, wing induces an angle of attack on the stream around it. The induced angle is positive ahead of the wing and is called upwash. Behind the wing, the induced angle is negative and is called downwash.
The magnitude of this downwash is dependent upon the relative location of the horizontal tail to the wing and the lift generated from the wing. There are many ways in which the downwash can be calculated and added into the aerodynamics but one of the simplest is to use the approach defined by ‘Roskam’. This calculates a downwash gradient based upon the wing geometry and the horizontal and vertical separation of the Neutral Points.
Airplane Design Part VI by Dr Jan Roskam provides a series of equations that can be used to calculate the downwash gradient of the tail based upon the wing geometry and separation of the Wing and Tail Neutral Points. These can be added into j2 Builder to produce a parameterised downwash gradient (de_da).
With the downwash gradient found, the next step is to convert that into a downwash value for the Horizontal Tail.
By default, the angle of attach used in the downwash gradient is the free stream value. However, downwash is a function of the lift not the angle of attack. This is achieved by creating an alpha equivalent (alpha eq) value that is a function of the wing clean lift curve slope and the total lift on the wing. Thus at zero lift, not zero alpha, there is no downwash.
The resulting downwash angle can then be applied to the Empennage by adjusting the angle of attack through twist.