Effects of downwash

In many types of aircraft the air which strikes the tail plane has already passed over the main planes, and the trailing vortices from these will cause a down- wash on to the tail plane (Fig. 5.6, overleaf). The angle of this downwash may be at least half the angle of attack on the main planes, so that if the main planes strike the airflow at 4°, the air which strikes the tail plane will be descending at an angle of 2°, so that if the tail plane were given a riggers’ angle of incidence of 2°, it would strike the airflow head-on and, if symmetrical, would provide no force upwards or downwards. Again, the angle of down – wash will, of course, change with the angle of attack of the main planes, and it is for this reason that the angle at which the tail plane should be set is one of the difficult problems confronting the designer.

As we shall discover later, its setting also affects the stability of the aero­plane, and further difficulties arise from the fact that in a propeller-driven aircraft the tail plane is usually in the slipstream, which is a rotating mass of air and will therefore strike the two sides of the tail plane at different angles.

Main plane.

Riggers’ angle of incidence 4° Angle of attack 4°

Fig 5.6 Effect of downwash on the tail plane

In jet-driven aircraft the tail plane is often set very high (Fig. 5F), to keep it clear of the hot jets, and this in turn may cause trouble since it may be shielded by the main planes at large angles of attack, resulting in what is called a deep stall and general instability, hence the low tail position illustrated in Fig. 5G.