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Aviation History
1960
1960 - 0042.PDF
42 FLIGHT, 8 January 196' ft* SYSTEM SURVEY Towards All-weather Helicopters THE BEA helicopter unit, based in the old "beehive" terminalbuilding at Gatwick airport, is responsible for some pioneering work in the development of all-weather commercial helicopteroperations. Although a Sycamore, a Bell 47G and a 47J and a Whirlwind are operated on charter work of various kinds, anotherWhirlwind is used purely for development flying. In this aircraft, G-AOCF, a Flight representative flew last week with Capt J. W.Reid, flight technical development officer of the unit, and later talked to R. A. Davis, who is in charge of the electronic laboratory.We saw the operation of the Newmark helicopter autopilot, Decca Mk 8 and Flight Log and the lighting and angle-of-approachindicator layout beside the "beehive." The Sperry Zero Reader, Marconi AD.722 ADF and the VOR receiver which have beenevaluated were not in use on this occasion. Actual blind landings, using only Decca large-scale Flight Logcharts, were achieved in a Sycamore some six years ago; and the BEA lighting pattern for night and poor-weather use, based onthe Calvert principle, was described in Flight for April 11, 1958. Since then the BEA unit has done much of the development flyingfor the Newmark autopilot and the Sperry Zero Reader has been modified for use during helicopter approaches. Additional instru-mentation is being developed including a speed-lock for the autopilot, a "barber's pole" vertical flight director for peripheralvision, and more advanced ground lighting. Principal unfulfilled requirements are a precise height indicator and a groundspeedindicator. A centimetric ILS is reported to be under development. The ADF and VOR now work effectively, interference fromrotor modulation having been overcome by modifications to the VOR receiver. The main aerial is beneath the fuselage; butbecause this leads to loss of signal in turns a gyro-operated switch selects a second aerial above the fuselage in such attitudes. During the air-test flight last week, Capt Reid demonstratedthe Newmark autopilot in its three operating modes. First he flew in the cruise setting, the autopilot taking attitude signals from theSperry gyro vertical on the co-pilot's panel and attitude being adjusted by trim knobs on the main controller. Stabilization forpitch, roll and yaw axes was separately switched on. With a switch on the control column Capt Reid then selected the stabilize mode,in which rate-gyro signals provide short-term stabilization and pilot's control demands are superimposed from sensers at the baseof the cyclic stick. The last mode is hover, in which gyro- magnetic compass heading signals maintain a selected headingduring stabilized hovering. Collective pitch is manually con- trolled in all three modes. Complete circuits were made in thestabilize setting, and the equipment worked well in bumpy air. The first approach was made visually on a 20° glide-slopedefined by the two Calvert red/white VGPI lights located beside the cross of Lorraine pattern of high- and low-intensity lighting,the let-down being started from 1,200ft. The 20" slope looked more like 45 ° from the cockpit and very good downward visibilityis necessary to keep the lights in sight. VGPI reference was lost at about 150ft, but by that time the transition was beginning. Wenext made a 15° approach on the lighting pattern, using Decca Cockpit of the Westland Whirlwind used by the BEA helicopte experimental unit for instrument-landing development. The Decci Flight Log and a mounting for VOR or ADF dial are on the coaming At extreme right are the Zero Reader controls; Decca and Newmari autopilot panels are on the central console. Brackets on the windscreer . . are for instrument-flying blinds -, Mk 8 and a large-scale chart as the only reference. Chart mark-ings showed the extended centreline and reference heights even 100ft, to which Capt Reid adhered expertly to arrive exactly over/ the pad. Visibility minima established at present are 200ft and 400yciwith Decca and 80ft and 150yd with ILS. The slender width of the pen-line on the Flight Log may represent an unacceptableerror when a touch-down must be precise. Decca approach infor- mation could be presented on an ILS-type cross-pointer, but somecomplication would result if the approach were not substantially parallel to the lattice lines. It is intended that the instrumentapproach area round a heliport should extend over a circle of only one-mile radius to avoid interference between neighbouring sites. One of the main problems in actually making these approacheslies in wind effect, both on groundspeed and on drift. When approach speeds are as low as 30kt drift may well be up to 40° andwind-shear and gradient greatly affect glide-slope performance. IAS is relatively unimportant, because the helicopter (unlike afixed-wing aircraft) is in no danger of stalling, but groundspeed is all-important in maintaining descent angle. The lighting patternmust be embedded (and therefore fixed) in the landing area and the pattern can at best cover four approach directions, so cross-winds will inevitably occur. The transition from near-autorotatiye descent to hover, which is a difficult helicopter manoeuvre even invisual flight, must be made accurately into the ground-cushion and over a limited ground distance. Some accurate height-reference isdesirable, but both the radio and sonic altimeters now available are likely to become inaccurate over built-up or accidented ground.Doppler suffers similarly. In the new turbine helicopters constant-speeding will greatlyease collective-pitch control and sufficient power will be available to hover safely at over 100ft, virtually out of ground-effect. Inthese circumstances a barometric altimeter may offer a sufficiently accurate height indication. Piloting may also be simpler if it ispossible to separate cyclic and collective control to make the former govern speed and the latter descent gradient. For helicopter use, the Zero Reader has been modified toaccept additional signals from three rate-gyros and cyclic stick- position. An IAS term will be added to assist speed control.In present 20° ILS approaches the Zero Reader is set for the approach gradient and ILS signals. BEA propose to use anintegrated director instrument display and a coupler for automatic approaches is being prepared. An automatic, programmed tran-sition mode would be provided in the autopilot so that, at about 150ft, glide-slope control would be released and the transitionmade to hover under localizer control alone. Collective-pitch control would be automatic in this phase. BEA have tried the Bendix sonic altimeter which transmitssound pulses at 3kc/s. Height indications are accurate to within 6in and maximum height is between 150ft and 300ft, but NewYork Airways have found public reaction to apparent machine- gunning created by the sound pulses to be slightly disconcerting. The "barber's pole" instrument was produced by Smiths andis designed to provide rate-of-descent instructions from combined pitch-attitude and glide-slope signals to be visible in the pilot'speripheral field of vision. It consists of a black column about lft high painted with spiral white stripes and mounted to the left ofthe central instrument group. At a demand to increase rate of descent the column rotates at an appropriate speed, producinga distinctive downward motion of the stripe pattern. Strobed rows of lights might be used, and the system might be applied tothe yaw axis, the instrument then being set horizontally. Despite notable gaps in precise height and groundspeed indica-tion, blind landing for helicopters in only relatively unobstructed sites seems to be a distinct possibility with existing equipment;and BEA have more practical experience of such operations than any other organization. The very elaborate Bell ANIP helicopterprogramme in America has not yet been conducted with such realistic requirements in mind—6° glide-slopes and unobstructedlanding areas have been used. BEA is now training more pilots to the blind-landing technique. The first operational application of this type of equipment hasbeen planned by BEA for the Rotodyne, in which 5501b of weight has been set aside for control and navigation equipment. Thiswill comprise duplicated stabilization and single autopilot, in- tegrated director instruments, ILS, duplicated VHF radio, inter-com and cabin address, navigation aids chosen from Decca, VOR or ADF and possibly a precision altimeter and some form of searchradar. The last-named may be one of the new high-definition radars. It is realized that profitable Rotodyne operation mustdepend upon very high regularity.
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