CHAPTER VI.
THE QUESTION OF SUSTENTATION
THE CENTRE OF PRESSURE.
§ 1. Passing on now to the study of an aeroplane actually in the air, there are two forces acting on it, the upward lift due to the air (i.e. to the movement of the aeroplane supposed to be continually advancing on to fresh, undisturbed virgin air), and the force due to the weight acting vertically downwards. We can consider the resultant of all the upward sustaining forces as acting at a single point—that point is called the "Centre of Pressure."
Suppose A B a vertical section of a flat aerofoil, inclined at a small angle a to the horizon C, the point of application of the resultant upward 'lift,' D the point through which the weight acts vertically downwards. Omitting for the moment the action of propulsion, if these two forces balance there will be equilibrium; but to do this they must pass through the same point, but as the angle of inclination varies, so does the centre of pressure, and some means must be employed whereby if C and D coincide at a certain angle the aeroplane will come back to the correct angle of balance if the latter be altered.
In a model the means must be automatic. Automatic stability depends for its action upon the movement of the centre of pressure when the angle of incidence varies. When the angle of incidence increases the centre of pressure moves backwards towards the rear of the aerofoil, and vice versa.
Let us take the case when steady flight is in progress and C and D are coincident, suppose the velocity of the wind suddenly to increase—increased lifting effect is at once the result, and the fore part of the machine rises, i.e. the angle of incidence increases and the centre of pressure moves back to some point in the rear of C D. The weight is now clearly trying to pull the nose of the aeroplane down, and the "lift" tending to raise the tail. The result being an alteration of the angle of incidence, or angle of attack as it is called, until it resumes its original position of equilibrium. A drop in the wind causes exactly an opposite effect.
Fig. 42.
§ 2. The danger lies in "oscillations" being set up in the line of flight due to changes in the position of the centre of pressure. Hence the device of an elevator or horizontal tail for the purpose of damping out such oscillations.