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Aviation History
1948
1948 - 2239.PDF
DECEMBER 30TH, 1948 FLIGHT 787 eoooi-I 7OOO- EMERGENCY DESCENT (DOUGLAS C-S4) Rate of Descent vs. Indicated Air Speed. moderate air speeds. This permits a multi-engined civil or military aircraft to be quickly and safely brought to the ground in the event of emergency ;.t high altitude, while in the case of combat aircraft it also provides a safe and evasive technique for obtaining rapid changes of speed and altitude. Such, at any rate, is the thought of Curtiss- Wright airscrew designers who have been conducting these aerial experiments on a Douglas C-54 transport, and to whom we are indebted for the accompanying data. This figure clearly shows the air- brake effectiveness of reversing the four airscrews, compared with the more conventional drag methods of landing gear and wing flap exten- sion ; moreover, the airscrews are designed to cater for this condition, whereas lowering of the flaps is re- stricted by specific structural design criteria to moderate speeds only. The high rate of descent of 5,000- 7,000ft/min under complete control at relatively low air speeds is a very desirable feature. For example, in the event of failure of the cabin pressurizing system when flying at high altitude, it might make all the difference between survival and disaster, since it is unlikely that oxygen masks could be distributed in time to save all the passengers. (All the same, speaking personally, we dislike the thought of catching up with our stomach while going down the lift at over a mile a minute!) On recent civil transports where the reverse pitch control is de- signed for ground operation only, inadvertent reversal of the pro- peller in flight is controlled by a microswitch in- stalled in the reversing circuit and actuated by the landing gear torque-scissors. This switch controls a solenoid-operated throttle stop, which prevents airscrew reversal when the aircraft is NOT resting on its landing gear. When the latter is compressed under load, the throttle can be pulled through what is normally the closed position, continued movement increasing the power for reverse thrust. Apparently, for reversal in flight, some form of over-ride device is provided to release the throttle stop, or else the landing gear connection is eliminated altogether. Whatever method is adopted it is more likely that the gadgetry will increase rather than decrease. Anti-icing Along the rest of the airframe, the problem, of providing adequate anti-icing protection for the airscrew has received a great deal of study by the airscrew manufacturers. The first generally used method of de-icing consisted of slinger rings at the hub, feeding alcohol directly on to the blades. This scheme had severe limitations and was followed by the development of grooved rubber shoes along the leading edge of the blatie, which resulted in some improvement, but was still not entirely adequate. The type now in common use incorporates an electrically heated external shoe, which provides a much improved anti-icing element, in place of the earlier de-icing methods. (The distinction, of course, between anti-icing and de-icing is the difference between prevention and cure.) • Just as thermal anti-icing has superseded the rubber shoe- on the wing and tail surfaces, so may the airscrew be expected to parallel them. The use of hot air for furnishing the anti-icing requirements of airscrews has been under test for several years and will go into service test on military aircraft next year. This method leaves the blade surface free of any irregularities and is a further improvement over the present external rubber shoe, although the latter has B 25 • EMERGENCY DESCENT"(DOUGLAS C-54) LANDING GEAR & WING FLAPS DOWN (FORWARD PITCH) LANDING GEAR & WING FLAPS UP (FORWARD PITCH 120 130 14O 15O 16O 17O INDICATED AIR SPEED (MPH) only a very small effect on the efficiency at cruising speeds, so small indeed as to be practically negligible. The other approach to the problem is to use an electric- ally heated internal shoe, which also improves the blade surface This scheme is employed on the Curtiss electric four-bladed airscrews of the Boeing Stratocruiser. It consists of electric resistance heating elements connected in series and running lengthwise down the blade, each opposite pair of blades being heated in unison. Airscrew heating is conducted through a rotary timer with eight fixed contacts and a wiping con- tact, which channels an 8-volt D.C. flow into each pair of blades in turn. Both the hot air and the electric resistance methods should decrease the repair and overhaul work now required on the external shoe type. Seeing is Believing ! So much, then, for the efforts of the airscrew designer to keep in step with airframe development. Within its appropriate sphere the modern airscrew is a highly efficient thrust mechanism and it is hardly likely to go out of fashion alto- gether. Oddly enough, even Jack Northrop has recently gone on record in favour of the turbo-prop engine for a long-range transport cruising at 500 m.p.h.—on the score of fuel economy, of course. In one sense, perhaps, this design philosophy is not so odd when viewed in relation to the big Northrop 10,000 h.p. Turbodyne engine now under active develop- ment. Nonethless, the case history of the XB-35 all-wing bomber; which has been plagued with an unending series of airscrew troubles ever since it first flew in June, 1946, is not a very healthy record in favour of the modern high-powered airscrew. (Incidentally, one suspects that the asymmetric wing flow into the pusher prop does not lessen the vibration headaches of the unfortunate airscrew designer.) It was not, in fact, until two of the original order for 15 were converted into YB-49 jets that this unique aircraft began to show its real capabilities. A certain touch of irony has been injected into this particular case history by the announcement that the U.S.A.F. has now ordered the conversion of the remaining piston-engined XB-35S into jets. All, that is, with the exception of three guinea-pigs which, apparently, will continue the difficult struggle to evolve a satisfactory dual- rotation pusher prop installation—possibly in combination with a piston-turbo compound or a turbo-prop engine. By the time this has been successfully accomplished, the production Jet-Wing—whether RB-49 long-range scout or B-49 long-range bomber—should be mounting more power- ful turbo-jets and climbing farther up the speed and range ladder. Right now, a visitor to Southern California can walk by the Northrop plant and actually see this windmill- jet conversion process taking place al fresco in the sun—for all the world, as well as the engineer, to see. And seeing is the most convincing form of believing. A PLASTIC PROTECTION NEW raw material for protecting metallic articles andcomponents against damage and corrosion has been pro- duced by Clear Glass Products, Ltd., of Southampton, knownas " Avigel " plastic peel, this substance is easy to apply and remove and affords protection for any metal against oxidation,acids or chemical attack, or sea water. The substance can be melted in a suitable container at about 150 deg C, andparts to be protected are dipped in the fluid at about 170 deg C. A slit enables the coating to be peeled off and, in theinterests of economy, returned to the melting pot.
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