(1) An aircraft in steady, straight‐line motion can have no resultant force or couple acting upon it. This condition is never continuously maintained in flight, and the craft proceeds in a series of oscillations or wide, corrected curves. Continuous adjustment takes place in the direction of its flight through either an inherent stability or a judicious use of the controls by the pilot, but the motion may be regarded mostly without error as steady for purposes of design. Calculations carried out on the basis of steady equilibrium have for objects the determination of optimum lay‐out; the selection of most suitable component parts; the provision of adequate and easeful control; the specification of loading for strength design; and the prediction and testing of performance. In practice, such calculations go hand in hand with others concerned with statical and dynamical stability; with accel‐erated motions; with strength and weight; and with a host of purely practical considerations. Deductions drawn from the principles discussed in this Article may not be decisive in a given case till set in proper perspective. In this connection we note, without straying from our subject matter, that many secondary factors are here neglected, whose effect the engineer has, on occasion, to take carefully into account.
Piercy, N.A.V. (1931), "Aerodynamics for Engineers: V.—Aeroplane Performance in Normal Flight—Other Steady Motions—Elements of Airscrew Theory", Aircraft Engineering and Aerospace Technology, Vol. 3 No. 2, pp. 43-46. https://doi.org/10.1108/eb029369
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