Military aeroplanes; an explanatory consideration of their characteristics ... by Grover Cleveland Loening

Author:Grover Cleveland Loening
Language: eng
Format: epub
Publisher: W.S. Best printing co., 1916

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STRESS ANALYSIS FOR BIPLANE TRUSS

The loiids at the connecting points U, U', U", called panel points, are indicated on the diagram, and are due to the air load on the wings. The heavy line wires are the "flying wires." taking the stresses due to these loads; and the dashed lines in the truss diagram are the 'landing wires." taking the weight of wings on landing.

In the graphical stress method, each panel point is considered in order, and for each one a closed triangle or polygon of forces is drawn. The force polygons must all close, since the point is in equilibrium.

First, a "sense" of rotation for the diagram is chosen and indicated by the arrow as clockwise, and a .scale to which to lay off the forces is chosen. Then, on the truss diagram, the regions between forces are lettered A, B. C. etc., the forces considered being only the forces carr>nng the truss load. That is why the compression in L" U" is not considered, since it is carried to U" and from there over the truss. Taking the first point U". we have the force between A and B. called ab. = 205 lbs. total, and the force of compres.sion in U"U', called be and a third force, the tension in the wire ca. Thus, there are three forces at this point. The magnitude of one is known and the direction of the other two. so that a force triangle, as given on the stress diagram abc. may be drawn, such that ab =- 205 lbs. to scale, be. is parallel to U' U', and ac is parallel to U"L'. Their point of intersection establishes the closing point of the triangle, thus determining ac and be in lbs., merely by reading their lengths to the same .scale to which ab was laid off.

Panel point L' is now taken, the forces being taken in the same order going around the point clockwise. First, we have ac. already solved and then cd. the strut compression, the direction of which we know, so we draw cd thru c, parallel to U'L'. To obtain the rest of the polygon it is now necessary to consider ea. acting upwards at L'. which is laid off on the vertical, and then to close the polygon the other force line de. may be drawn thru e. parallel to L'L. the point d being located by the intersection of the lines of action of the two unknown forces thru e and c. Thus, with d found, cd and de are readily read to scale.

A similar process is employed for the rest of the tru.ss.

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A moment's thought on the manner in which the air force on the wings Hfts the rest of the machine, will lead to the simple conception that an aeroplane is virtually a swing bridge, turned upside down, with a uniform static load of the simplest kind, equal in average intensity to 1800/335 = 5.

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