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 downwash on to the tailplane.
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 downwash 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.
In jet-driven aircraft the tail plane is often set very high
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,
