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Aviation History
1948
1948 - 2238.PDF
786 FLIGHT DECEMBER 30TH, 1948 — i-i -TJLufBSWgrrMtff^' f What the Airscrew Designer is Doing to Meet of SANS-TAIL, SANS-WINDMILL : Ten more of the original batch of 15 Northrop XB-35 airscrew-driven bombers ore being converted to jetted long-range scouts, with a maximum re- connaissance range of 7,500 miles. Thirty more RB-49 Jet-wings are being built under licence by Convair at Fort Worth. WINDMILLS OR JETS? WHILE the jet enthusiasts are happily engaged inprophesying .the early demise of the airscrew, theindustry is emphatically stating that it intends to remain in business. Further, to judge by the promises ofthe airscrew designer, his section of the industry shows indications of trespassing well into the domain of the jet.In a recent S.A.E. paper discussing the propulsion of future long-range transports, L. H. Enos, of the Curtiss-WrightPropeller Division presents a plausible case not only for the survival of the airscrew, but for its extension as far up thespeed scale as 550 m.p.h. Which is about 100 m.p.h. better than the Whittlers are prepared to admit. Efficiency with Safety- According to its proponents, the industry definitely aims to furnish airscrews that: (1) give the highest possible efficiency in pounds of thrust per shaft h.p. per pound of airscrew weight, and (2) operate safely and without restrictions for long periods of time ; in other words, aerodynamic efficiency combined with structural integrity. Efficiencies as high as 85 per cent are claimed for thin- bladed airscrews at cruising speeds around 400 m.p.h., with a loss of only 5 per cent (i.e., 80 per cent efficiency) when operating at a high speed of 550 m.p.h. These efficiencies are based upon full-scale flight tests of single-rotation props using a thrust meter and torque meter. Assuming, however, that some of the cbming breed will be turbo- propped transports operating at 30,000-35,oooft, dual- rotation props are almost certainly a necessity in order to hold the blade tip Mach limitation below 0.9. If this is so, the quoted efficiencies probably lean a bit on the optimistic side. As for the structural side of the partnership, with its bearing on safety, it is freely admitted that vibration problems have, become increasingly difficult during recent years, particularly in the case of piston-engined aircraft. The turbo-prop installation, however, uses a very small r.p.m. range and is not subject to the firing torque fluctua- tions of the piston engine, hence the stress problem so far is easier to handle. To which the jet proponents point out that the combined weight of airscrew and reduction gearing will approach the weight of the engine itself and, moreover,'will open up a whole new field of complex problems concerned with gears and control systems. C. L. Johnson,Lockheed's chief research engineer, estimates that existing airscrew problems will be increased three-to five-fold ifprojected into the realm of new transports with higher - speeds, wing loadings and powers. Braking Drag Intensively developed over the past five years or so, theadoption of the reverse-pitch airscrew has added yet one more gadget to the increasing complexity of modernairscrew-driven aircraft, whether piston-engined or turbo- propped. For cutting down the landing run, especially onan iced runway where wheel braking might be treacherous, and for ground manoeuvring such as turning and backingup, the reverse-pitch airscrew has become an absolute necessity on large civil airliners and military aircraft.Whether one looks upon this as a token of the march of mechanical progress (from the simple to the sublime, shallwe say?) or as an unnecessary evil, depends of course upon one's personal predilection for airscrews or jets during thenext decade. Sidestepping for the moment whether such gadgetry isgoing to remain long enough on the scene to pay for its expensive birth, board and keep, we agree that thetechnique of windmill locomotion on the ground when a large multi-engined aircraft is backing up can be quite adiverting spectacle. We recently watched a squad of Boeing B-50S toeing the tarmac by milling back and forthon their throttles, and the sight brought home to us the associated problem of providing adequate rearward viewfrom the cockpit. The B-50, due to its flush nose-type windshielding, calls for a lookout member of the crewstationed aft for signalling purposes—a most intriguing sight which reminded us of a railway brakesman hangingon the back of a shunting loco or wagon during marshalling operations in a freight yard, Airscrew pitch-reversal on the ground has now beenlogically extended into the flight regime, the idea being to provide braking drag for increasing the rate of descent at B 24
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