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Aviation History
1943
1943 - 1577.PDF
JUNE -17111, 1943 FLIGHT 635 Post-war Transport Aircraft Part IN of Dr. Edward P. Warner's Wilbur Wright Memorial Lecture (Continued from page 617, June 10th issue) NO one who has to deal with problems of regulation,either of airworthiness or of the conditions of air-craft operation, can fail to be deeply sensible of his responsibility. Mistaken regulation can put a drag on development, or it can warp it into unsound and unprofit- able directions. The United Kingdom has entrusted the formulation and application of airworthiness regulation to a semi-public body, of which most of the members are chosen from the industry. In the United States the responsibility remains with an agency oi Government; but we both have the same problems. On the one hand we are constrained by the desire to include margins of safety that will leave no possibility of avoidable hazard—on th>3 other, by the desire to impose no burdens on operation, and no economic handicaps on aircraft design, which do not genuinely correspond to the interest of safety. The processes of regulation tend to grow more and more elabor- ate as aeronautical science and the art of aircraft design accumulate new refinements. It grows progressively more difficult, yet at the same time progressively more impor- tant, to keep regulation abreafst of technical develop- ment, so that there may be no regulatory time-lag in the practical application of the products of research and invention. Engine Failure The underlying principle of the present American regulations, in so far as they impose minimum limits of performance, is that it should be possible for a trans- port aircraft, under the actual condition of its operation, to sustain a failure of any one engine at any instant of the aircraft's flight without thereby creating any serious hazard ; and that the standard requirements should include enough margins of performance to give reasonable insur- ance that the operation would still be safe in spite of high temperature, turbulence, or other sub-standard atmo- spheric conditions, sub-standard condition of the air- craft, or failure to maintain the optimum flight path and air speed with the rigour which could be expected ot a test pilot putting the type through its official trials. The American airworthiness regulations now require that every new type of aircraft which is to be certificated for transport use shall undergo, in addition to suitable tests for stability and control, enough landing and take-off trials to make it possible to compute the landing and take-off paths with reasonable accuracy for any weight within the probable operating range of the aircraft, and for any altitude above sea-level. The landing distance is measured in still air from a height of 50ft. above the ground to the point of bringing the aircraft to rest; and at the instant of passing the 5ott. altitude the speed must be at least 30 per cent, in excess ot the stalling speed. The flaps or their equivalent must be so set that the rate of climb with take-off power will be at least 0.07 times the square of the stalling speed in miles per hour. The landing distance having been thus determined for a suitable range of weights and altitudes, the requirement is imposed upon transport operators making use of the air- craft that it shall be so loaded for any particular flight that the landing distance as shown by the results, of the official- trials will not exceed 60 per cent, ot the actual length of runway available at the field where the landing is to be made. The conditions with respect to take-off are substantially more complicated. The fundamental assumption in that case is that the take-off run is started on full power; that one engine stops at the most critical instant of the take- off ; and that it must be possible, whatever the instant at which the engine failure occurs, either to continue the flight with safety or to bring the aircraft to rest withinthe confines of the field. The aircraft must leave the ground at a speed at least 20 per cent, in excess of thepower-off stalling speed, and at least 10 per cent, in excess of that at which it can be kept under full directionalcontrol. Ike use of an aircraft in transport operation must then meet the requirement that no take-off may be made with a load greater than will permit the aircraft, in following the demonstrated path for the case of critical engine failure, to reach a height of at least 50ft. by the time it reaches the edge of the field, and thereafter to clear all obstruc- tions by at least 50ft. vertically or 300ft. horizontally until it has attained an altitude completely clear ot the sur- rounding terrain. It is further required that the rate of climb with any one engine inoperative and its airscrew windmilling, and with the other engines operating at take-off power, shall be at least 0.035 times the square of the stalling speed of the aircraft with the flaps set y.s in the.take-off tests. Since an aircraft may take off and land under very favourable conditions and yet have to pass over high and rugged terrain en route, the regulations include a further stipulation that the rate of climb with one engine stopped, the propeller feathered, and the remaining engines operat- ing at the maximum rate of power for continuous output, must be at least 0.02 times the square of the stalling spec-d at an altitude 1,oooft. above the highest terrain that has to be passed in the course of the flight. To meet these conditions imposes a new set of restric- tions on the designer's choice of fundamental design ratios. In order that an aircraft may be introduced into post-war transport service in* the United States, if the present re- quirements remain unchanged, it will have to have a rate of climb of o.O35V^an with one engine inoperative and its airscrew windmilling. In the post-war state of the art as I now foresee it, that will be approximately equivalent to a requirement that the product ol the power loading and the square root of the span loading should not exceed 21 in the twin-engine aircraft, or 30 in a four-engine one. Span Loading—Take-off and Cruising The restrictions imposed on span loading by the take-offrequirements may now be compared with the conclusions on span loading that were previously derived from con-siderations of cruising efficiency alone. The comparison can be made through the tabulation: — Take-offPower Loading. 6 8 10 12 15 Probable Cruising Speed at 10,oooft.(Assuming W/S not in Excess of 40). 300 280 250 228 212 185 Optimum Span Loadingfor Cruising. 7.2 5d 5-3 4-7 4-3 3-4 Maximum Allowable Span Loading for Single-Engine Climb at Take-off in a Twin-Engine Aircraft (Assuming Aspect Ratio 12). 17.6 12.2 6.8 4.4 3-i 2.0 (The smaller of the two values of span loading, ib underlined in each case.) • With take-off power loadings of more than about ir, theability to meet the regulatory requirements for climb with one engine inoperative becomes critical in determining thespan loading in twin-engine aircraft. For a twin-engine aircraft with an aspect ratio of 12, theproduct of take-off power loading and the square'root oi
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