Part 11 (1/2)
In order that this nose-heavy tendency should not exist when the thrust is working and descent not required, the centre of thrust is placed a little below the centre of drift or resistance, and thus tends to pull up the nose of the aeroplane.
The distance the centre of thrust is placed below the centre of drift should be such as to produce a force equal and opposite to that due to the C.G. being forward of the C.L. (see ill.u.s.tration above).
LOOPING AND UPSIDE-DOWN FLYING.--If a loop is desired, it is best to throttle the engine down at point A. The C.G. being forward of the C.P., then causes the aeroplane to nose down, and a.s.sists the pilot in making a reasonably small loop along the course C and in securing a quick recovery. If the engine is not throttled down, then the aeroplane may be expected to follow the course D, which results in a longer nose dive than in the case of the course C.
[Ill.u.s.tration: Position A. Path B. Path C. Path D.]
A steady, gentle movement of the elevator is necessary. A jerky movement may change the direction of motion so suddenly as to produce dangerous air stresses upon the surfaces, in which case there is a possibility of collapse.
If an upside-down flight is desired, the engine may, or may not, be throttled down at point A. If not throttled down, then the elevator must be operated to secure a course approximately in the direction B. If it is throttled down, then the course must be one of a steeper angle than B, or there will be danger of stalling.
[Footnote 16: ”In effect” because, although there may be actually the greatest proportion of keel-surface in front of the vertical axis, such surface may be much nearer to the axis than is the keel-surface towards the tail. The latter may then be actually less than the surface in front, but, being farther from the axis, it has a greater leverage, and consequently is greater in effect than the surface in front.]
[Footnote 17: The reason the C.P. of an inclined surface is forward of the centre of the surface is because the front of the surface does most of the work, as explained on p. 62.]
CHAPTER III
RIGGING
In order to rig an aeroplane intelligently, and to maintain it in an efficient and safe condition, it is necessary to possess a knowledge of the stresses it is called upon to endure, and the strains likely to appear.
STRESS is the load or burden a body is called upon to bear. It is usually expressed by the result found by dividing the load by the number of superficial square inches contained in the cross-sectional area of the body.
[Ill.u.s.tration: Cross Sectional area]
Thus, if, for instance, the object ill.u.s.trated above contains 4 square inches of cross-sectional area, and the total load it is called upon to endure is 10 tons, the stress would be expressed as 2-1/2 tons.
STRAIN is the deformation produced by stress.
THE FACTOR OF SAFETY is usually expressed by the result found by dividing the stress at which it is known the body will collapse by the maximum stress it will be called upon to endure. For instance, if a control wire be called upon to endure a maximum stress of 2 cwts., and the known stress at which it will collapse is 10 cwts., the factor of safety is then 5.
COMPRESSION.--The simple stress of compression tends to produce a crus.h.i.+ng strain. Example: the interplane and fuselage struts.
TENSION.--The simple stress of tension tends to produce the strain of elongation. Example: all the wires.
BENDING.--The compound stress of bending is a combination of compression and tension.
[Ill.u.s.tration]
The above sketch ill.u.s.trates a straight piece of wood of which the top, centre, and bottom lines are of equal length. We will now imagine it bent to form a circle, thus:
[Ill.u.s.tration]
The centre line is still the same length as before being bent; but the top line, being farther from the centre of the circle, is now longer than the centre line. That can be due only to the strain of elongation produced by the stress of tension. The wood between the centre line and the top line is then in tension; and the farther from the centre, the greater the strain, and consequently the greater the tension.
The bottom line, being nearest to the centre of the circle, is now shorter than the centre line. That can be due only to the strain of crus.h.i.+ng produced by the stress of compression. The wood between the centre and bottom lines is then in compression; and the nearer the centre of the circle, the greater the strain, and consequently the greater the compression.
It then follows that there is neither tension nor compression, _i.e._, no stress, at the centre line, and that the wood immediately surrounding it is under considerably less stress than the wood farther away. This being so, the wood in the centre may be hollowed out without unduly weakening struts and spars. In this way 25 to 33 per cent. is saved in the weight of wood in an aeroplane.