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Aviation History
1961
1961 - 0240.PDF
240 FLIGHT, 24 February 1961 SYSTEM SURVEY. delivered in less than two months' time, will have duplicated auto-stabilizers and conventional gyro instruments. The object is to be able to operate in the existing relatively high ceilings, but inreduced slant visibility. The pilot would continue to control attitude largely by visual reference to the ground, but would navi-gate by means of radio aids, notably duplicated Decca Navigator. It seems to be generally agreed that the helicopter working onshort routes into city centres cannot afford for economic reasons to become directly involved in fixed-wing traffic patterns. Thispre-supposes the use of discfete routes, which will probably not be effectively marked by existing radio beacons. The normal VHFnavaids are in any case unlikely to provide adequate coverage at low altitudes over built-up areas. An area-coverage aid strongly sug-gests itself—unless Doppkr used on short sectors, or MF beacons, can provide acceptable accuracy. The ability of Doppler to operateaccurately at low level over built-up areas is not yet proved. There is also a likelihood that the large helicopters will carry a"conventional" aid, such as ILS, to enable them to use fixed-wing airports as a diversion. There seems to be no acceptable alterna-tive to VHF radio as a communications medium. Over southern and central England VHF coverage at low altitude is very good,but in the New York area, for instance, several repeater stations have been set up to ensure coverage at the heights at which thehelicopters in the experimental all-weather programme fly. The equipment mentioned above is conventional, and the con-siderations governing its installation are those of weight and safety. Yet, with such adjuncts as cabin-address and intercom, itcould account for an addition of something like 1,0001b in weight and the choice would therefore have to be shrewdly balanced witheconomy. It is likely that the large helicopters will operate over sectordistances of up to 200 miles, involving about one-and-a-half hours' flying. With an additional allowance of up to lhr fordiversion or other contingencies, a total flight-time or trip length of some 2hr to 2ihr would have to be reckoned. For this length oftime it would be necessary to provide the pilot with some form of assistance beyond plain autostabilization. There are two alterna-tives—either a flight director instrumentation specially designed for helicopters, or an autopilot Both can achieve the same result,the director retaining the pilot as the servo system, while the autopilot leaves the human pilot free to perform other tasks. Themore the autopilot is designed to do, the more necessary does a monitoring device become. With autopilots designed to makeautomatic approaches down to speeds below the minimum-power level, an independent director instrument for monitoring purposeswill almost certainly be a firm requirement. The"mission profile" diagrams (Figs 2 to 5) show the prob-able stages of helicopter IFR development, from the present special VFR limits to the ultimate virtually blind-flying stage. SpecialVFR (Fig 2) implies limited use of navaids, because map-reading is fairly easy, especially at present cruising speeds of about 80kt.In a 500ft ceiling and half-mile visibility, attitude control is always possible by visual reference to the ground and none of thepresent helicopters carries even autostabilization. Severe limita- tions apply, particularly in adjusting routes to obtain obstacleclearance and, in single-engined helicopters, following routes which provide continuous forced-landing facilities in the form ofwater or open ground. Autostabilization and better conventional flight instruments willallow operations in rather poorer visibility, but there will be virtually no improvement in take-off and landing conditions,except by the use of an area-coverage navaid allowing slowing- down and initial preparations for landing to be completed whilethe heliport is still out of sight. The first extension—the ability to cruise in or above cloud, butwith the same minima for landing or take-off—is illustrated in Fig 3. It implies the ability to fly the aircraft for prolongedperiods on instruments and to navigate without sight of the ground. "VFR on top" is then possible. Autostabilization and atleast an efficient director system for cruising speeds become a necessity. As soon as the helicopter operates in or above cloud,twin-engined safety is also a must, because allowance for forced- landing cannot be effectively made once the ground is out of sight. The advantage of en route IFR capability is a much greaterfreedom in choosing routes and the ability, for example, to fly airways during all or part of inter-city flights. High ground or tallbuildings no longer provide en route hazards, although they must still be allowed for in the climb and descent phases. An additionalrequirement is, however, effective icing protection; and the weight of this must count as a penalty of en route IFR capability. One complication of flying fixed-wing airways on the longerinter-city routes will occur in joining and leaving airways. The helicooter could leave airways at the edge of the terminal controlzone for a city and make its way on special VFR lanes at low altitudes, but this might prove lengthy and difficult in places such A Chicago Helicopter Airways S-58 flying over typical American city- centre scenery. Transport helicopters must be able to operate beside, rather than over, such terrain 500ft CLOUD BASE SPECIAL VFR ROUTE ADJUSTMENT FOR OBSTACLES AND ENGINE FAILURE Fig 2. Flight profile of present special VFR type of operations IFR CRUISE AND NAVIGATION ROUTE FREEDOM 500ft CLOUD BASE Fig 3. Flight profile for the first stage of IFR operation, involving en route IFR flight as London, where the main approach lanes to the heliport area onthe Thames lie close to the most frequently used ILS approach to London Heathrow. Fig 4 illustrates the probable configuration of an IFR approachin ceiling and visibility corresponding approximately to existing fixed-wing minima—ceiling between 100ft and 200ft and visibilityof about i mile. The end of the navigation or en route leg would be followed by a descent, in cloud at normal cruising speed, to aninitial approach height. The helicopter would then have to slow down to its final approach speed of about 30kt and lock-on to thefinal-approach aid, descending on a 15° glide-slope, still in cloud. In the worst conditions, the time and the visual range available onemerging from the cloud-base would probably not be sufficient to guarantee a successful transition to hover by visual reference alone,and some automatic slowing-down and levelling-off mode would be required in the director or autopilot system. Visual contactwith high-intensity lighting would certainly be established during the final slowing-down stage and the final hover and landing couldbe made visually. The lowest speed attained during the automatic slowing-down manoeuvre would be governed by the minimum leveiengine-out speed of the helicopter (in the case of the Rotodyne this speed is about 26kt). In the lowest visibility conditions someautomatic means of overshooting would be necessary. In this type of operation take-off minima would depend on therapidity with which the helicopter could safely make the transition
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