AIRPLANE designing is the only branch of structural engineering making any pretense of strength calculation in which such calculations are based upon a direct empirical assumption or specification of the total forces acting upon specific parts of the structure and, as a rule, having little or no relation to one another. Whether our load factors are sound throughout, no one can be certain. Although the United States Department of Commerce and the British authorities are each satisfied with its own load factors, neither has given an adequate reason for believing its practice to be right. Whereas the American and the British specifications have suggested until recently, and the British regulations still indicate, that weight of the airplane alone determines the maximum loads to which the wing structure is subjected, the author asserts positively that this is not true and that the gross weight may not even be a primary factor.
The putting of specifications on a basis of rational analysis is advocated, and the first requirement is said to be to go back of the loads themselves to the conditions under which they arise, calculating the loads upon any part of the structure from a given condition of flight.
Four conditions of flight or maneuvers are set forth and discussed, formulas for calculation of wing load factors being given and charts presented. Factors of safety are discussed and true factors of safety suggested for different flight conditions.
Conclusions drawn from the investigation are that (a), except for airplanes of less than 6000-lb. weight or those having balanced elevator controls, the weight of the airplane is or should be a minor element in determining load factors; (b) power loading ranks first in importance for big airplanes; (c) wing loading ranks second in large planes, but on machines of less than 4000-lb. gross weight, wing loading, weight and size and shape of the tail surfaces should be preeminent.