Loads on Tail Plane:
Where the four main forces can be satisfactorily balanced in themselves, the duty of the tail plane is merely to act as a “stand-by.” Therefore it will be set at such an angle of attack that it carries no load in normal flight; at high speeds it must carry a down load (Fig 7.3), because at high speed the main aerofoils will be at a small angle of attack, the centre of pressure will move backwards, the wing pitching moment about the centre of gravity will be nose-down, and so the tail must be held down to counteract this tendency; correspondingly, at low speeds – i.e. at large angles of attack of the main planes – the tail plane must carry an upward load (Fig 7.3).
Since the tail plane is equally likely to have to carry an upward or a downward load, it is usually of symmetrical camber, and therefore provides no lift when the angle at which it strikes the airflow is 00. But when the four main forces cannot be satisfactorily balanced in themselves, the tail plane may be called upon to provide a more or less permanent balancing force either upwards or downwards. It is then called a lifting tail. To provide the lift it may be cambered in the same way -as an ordinary aerofoil, or, as the force required is often a downward one, it may even be shaped like an inverted aerofoil.
Effect of Downwash:
There is one very important point to be remembered in connection with the angles at which tail planes are set: in many types of aircraft the air which strikes the tail plane has already passed over the main planes, which cause a downwash on to the tail plane (Fig 7.3).
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 40, the air which strikes the tail plane will be descending at an angle of 20, so that if the tail plane were given a riggers’ angle of incidence of 20, 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.