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Aviation History
1961
1961 - 1401.PDF
4.C 04 0-5 0.6 07 MACH NUMBER FLIGHT, 28 September 1961 505 03 O.i 0.5 0.6 MACH NUMBER Thrust horsepower specific fuel consumption at rated thrust at an altitude of 35,000ft (left) and at sea level (Schairer). S.f.c. is defined as the fuel flow divided by thrust horsepower remaining to propel the rest of the air- craft, above that required to move the propulsion system itself single failure. Powerplants are so designed that almost anyfailure within the powerplant will not result in the powerplant destroying the aeroplane. In nearly every case, powerplants andsimilar equipment are installed in such a way that their failure during flight only slightly inconveniences the accomplishment ofthe flight and a failure is not in itself a major safety problem. The turbofan-lift type of aircraft is very attractive in those cir-cumstances where a number of people are to be carried, since it is a relatively straightforward matter to design such an aircraft sothat almost any structural or mechanical element can fail at any time and still permit safe termination or continuance of the flight.This is the only type of vertical-lift aircraft which appears to give promise of equalling the fail-safe reliability expected of fixed-wing aircraft. Therefore, it is likely that the engines will be divided into enough units to permit safe failure of any one of them. Theywill be so arranged and controlled that control will be retained, as well as adequate thrust, following the failure of any element ofthe system. Much has been said over the years about where to mountengines. It seems likely that in turbofan-lift aircraft the engines will be mounted in nacelles probably attached to the side of thebody. Possibly some of the engines will be mounted on the wing in some form of pod. Perhaps some of the units can be mounted inthe front or back ends of the body in special space provided for them. In any case, it appears that the engines will always be placedin space provided especially for them. It is most unlikely that there will be spare space in either the wing or the body for theengines. Certainly the engine installation will benefit from special- lized attention. It seems most unlikely that the engines will bemounted in the wings. Turbofan-lift aircraft will have high downwash only in a limitedarea directly under the aircraft. In those applications where there is a prepared landing spot and where noise is not too important,turbofan-lift aircraft can operate very satisfactorily. Prepared landing spots will seldom have an erosion problem with turbofanengines. Furthermore, when the landing place is specially prepared it is unlikely that long hovering times will be required. Thus, theturbofan type of direct-lift aircraft is likely to operate very suc- cessfully where there are prepared take-off and landing spots.The benefits of STOL operation of fan-lift aircraft are large and the penalties relatively small. Shrouded Propellers Small-diameter shrouded propellers are frequently proposed as a means for getting adequate vertical take- off thrust. Such devices require a lot of power and hence potentially Left, typical convertible lift-fan VTOL powerplant arrangement (Adamson and Cochran) Right, lift/weight ratios for en- gines plus fuel (Adamson and Cochran) X Ef SINE provide for high speeds, but they also implyrelatively high hovering fuel consumption. It is likely that any such arrangement can bebettered by removing the shroud ring and installing a slightly larger propeller. It ishard to believe that arrangements and clear- ance problems are so tight as to dictate ashrouded propeller rather than a conventional propeller. Tandem Tilting Arrangement Tandem tilt-ing arrangements require additional study. Tandem systems utilizing two rotors forwardand two aft are attractive from a control standpoint. Such arrangements, when pro-vided with a wing of adequate span, will have performance comparable to tilt-wings.If the propellers are relatively small and highly powered, such aircraft can be veryfast. The problem of supporting the engines in the tandem positions remains to be solved.Through tandem arrangements, it might be possible to install more than four engineswith relatively sizeable propellers and thus provide for the failure of any one engine-propeller combination without reverting to connecting mechanically the driving systems. Further attention to such aircraft is wellwarranted. Short Take-off and Landing In the area of short take-off andlanding, it is evident that much attention will be given to vectoring the lifting thrust in the directions which will give shortest possibletake-off and landings. It is probable that most current arrangements are inadequate in this matter, but it is likely that the deficienciescan be readily corrected. Of special interest in the short take-off and landing discussion isthe question of blown flaps. It is entirely possible that flaps with extensive blowing will be very attractive for the STOL mode ofoperation. Further testing of blown-flap arrangements is urgently needed to clarify their characteristics. When aircraft with a vertical-landing capability are to be flownlong ranges, such as across oceans, it is difficult to understand why the take-off should not be made in the short-take-off mode.It would appear that much attention should be given to aircraft in which short take-offs are used for flight ranges beyond about1,000 miles, but which provide for vertical take-off with full pay- load and vertical landing with full payload for short-range opera-tion. Cranes An aerial crane is a specialized type of vertical take-offaircraft where range is usually of little or no importance but where hovering endurance is exceedingly important and efficient weight-carrying capability is primary. The data available to the author indicate that lightly loaded rotors or propellers will be the basis forsuch aerial cranes. The primary attention will go into obtaining satisfactory light, efficient hovering performance. Engine-out Safety The subject of engine-out safety in VTOL aircraft, mentionedby Mr Schairer, was discussed in detail in the paper Some Con- siderations In Selecting VTOL Propulsion Systems by A. P. Adamsonand D. Cochran of the General Electric Company. The problem, the authors stated, had been approached in two ways: the liftingdevice might be interconnected; or many small lifting devices might be used. Both methods required a degree of engine over-sizing oremergency ratings if engine-out aircraft climb performance was to be retained. Interconnected systems had the advantage that rolltorques due to engine power variations which might be caused by hot gas re-ingestion would tend to be smoothed out. Lift fans would generally be interconnected by means of hot gasducts, so that the aircraft would remain balanced in the event of a single engine failure. Direct-lift jet engines or turbofan engineswould not be interconnected, and the opposing engines would be shut down to maintain balance. Lifting propellers would generallybe interconnected by shafts and gears to maintain balance in the DIVERTER VAIVE SFC=.4O LIFT TURBOFAN SFC=.65 LIFT TURBOJET t )sFC=l.3 3 6 OPERATING TtME-MINUTES
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