The Aeroplane Speaks [36]
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.
Thus, if, for instance, the object illustrated 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.
[cwts. = centerweights = 100 pound units as in cent & century. Interestinly enough, this word only exists today in abbreviation form, probably of centreweights, but the dictionary entries, even from a hundred years ago do not list this as a word, but do list c. or C. as the previous popular abbreviation as in Roman Numerals] The word listed is "hundredweight. Michael S. Hart, 1997]
COMPRESSION.--The simple stress of compression tends to produce a crushing 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.
The above sketch illustrates 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:
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 crushing 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.
The strength of wood is in its fibres, which should, as far as possible, run without break from one end of a strut or spar to the other end. A point to remember is that the outside fibres, being farthest removed from the centre line, are doing by far the greatest work.
SHEAR STRESS IS such that, when material collapses under it, one part slides over the other. Example: all the locking pins.
Some of the bolts are also in shear or ``sideways'' stress, owing to lugs under their heads and from which wires are taken. Such a wire, exerting a sideways pull upon a bolt, tries to break it in such a way as to make one piece of the bolt slide over the other piece.
TORSION.--This is a twisting stress compounded of compression, tension, and shear stresses. Example: the propeller shaft.
NATURE OF WOOD UNDER STRESS.--Wood, for its weight, takes the stress of compression far better than any other stress. For instance: a walking-stick of less than 1 lb. in weight will, if kept perfectly straight, probably stand up to a compression stress of
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.
Thus, if, for instance, the object illustrated 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.
[cwts. = centerweights = 100 pound units as in cent & century. Interestinly enough, this word only exists today in abbreviation form, probably of centreweights, but the dictionary entries, even from a hundred years ago do not list this as a word, but do list c. or C. as the previous popular abbreviation as in Roman Numerals] The word listed is "hundredweight. Michael S. Hart, 1997]
COMPRESSION.--The simple stress of compression tends to produce a crushing 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.
The above sketch illustrates 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:
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 crushing 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.
The strength of wood is in its fibres, which should, as far as possible, run without break from one end of a strut or spar to the other end. A point to remember is that the outside fibres, being farthest removed from the centre line, are doing by far the greatest work.
SHEAR STRESS IS such that, when material collapses under it, one part slides over the other. Example: all the locking pins.
Some of the bolts are also in shear or ``sideways'' stress, owing to lugs under their heads and from which wires are taken. Such a wire, exerting a sideways pull upon a bolt, tries to break it in such a way as to make one piece of the bolt slide over the other piece.
TORSION.--This is a twisting stress compounded of compression, tension, and shear stresses. Example: the propeller shaft.
NATURE OF WOOD UNDER STRESS.--Wood, for its weight, takes the stress of compression far better than any other stress. For instance: a walking-stick of less than 1 lb. in weight will, if kept perfectly straight, probably stand up to a compression stress of