FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1960
1960 - 0588.PDF
588 GNOME . . . of fuel density. This signal is used for operating an associatedtop-speed governor for the gas generator and also as a control signal for the hydraulic actuator that sets the incidence of thecompressor inlet guide vanes and first three rows of stator blades. The pressure rise across the function-generator is fed to theactuator control and used, together with an intake air temperature signal, to establish the desired angular position of the blades fora particular compressor operating condition in terms of its equiva- lent r.p.m. (i.e., blade relative Mach number). The actuating fluidfor the control jack is supplied by a bleed of high-pressure fuel from downstream of the pump. This is returned to the fuel systemat the inlet to the low pressure centrifugal filter. The flow control unit ensures that the fuel flow passed to theengine through the throttle is a function only of throttle position and altitude, the change in flow being that required to achievea sensibly constant gas-generator speed for any given throttle setting. It performs this duty by balancing the pressure dropacross the throttle against an ambient pressure capsule and using any out-of-balance to vary the flow through the servo valve thatcontrols the pump-stroke setting actuator—this change modifying pump output and hence throttle pressure drop in the sense neededto restore the balance. The variation in pressure drop with ambient pressure is that required to give the desired flow com-pensation with altitude. The fuel injectors are of Simplex design, there being eight oneach of two manifolds operating in parallel. The flow divider upstream of the manifolds arranges that at low flows, fuel is fedto only eight of the injectors, thus ensuring that adequate atomiza- tion of the fuel occurs during the starting cycle. A component fitted ahead of the Lucas pump which is of par-ticular interest is the centrifugal filter. This has been designed and developed by Dynamic Filters Inc, and is an engine-drivenun;t capable of exceptionally effective filtration of the fuel with respect to both solid and liquid foreign matter. The filter containsa rotating cylindrical member incorporating inner and outer annular cavities through which the fuel is passed in series.While passing through the cavities the fuel experiences high radial acceleration forces, equivalent at engine rated r.p.m. to 344g inrhe inner cavity and 688g in the outer cavity. The contaminent centrifuged out by these forces passes through louvred shells inthe inner wall of either cavity, to be retained in separate dirt compartments. With the Gnome it is the object of the control system (diagramC) that the pilot can set the required rotor speed by means of the speed-select lever and from then onwards can virtually forget theengine, and concentrate solely on his flying. However, to provide for the contingency of an electrical supply or speed signal failure,the pilot is enabled very rapidly to assume direct manual control of the throttle. This is achieved by means of an emergencychange-over toggle, operation of which may be facilitated by its positioning on one of the flight control levers. The emergencytoggle is mechanically linked to the throttle actuator and when depressed engages a clutch within the assembly to give directmechanical control of the throttle. This action can be completed virtually instantaneously following realization of an electricalfailure, and a device can be incorporated in the linkage from the twist grip to centralize the movement of the grip irrespective ofthe throttle setting at the moment of clutch engagement. (Should an electrical failure in fact occur the throttle motor automatically FLIGHT, 29 April l-^o locks in the position of failure, and a pilot-warning light is at. Provision is also made against the possibility of a runawayoccurring following a transmission failure between the power turbine and the rotor. Driven off the power turbine shaft by aworm and skew gear is a Lucas overspeed-trip governor incor- porating a rotating, eccentric circular collar pivoted to and heldconcentric with the drive shaft by the action of a spring, "jSx a preset degree of overspeed of the power turbine the centrifugalforce on the collar overcomes the resistance of the spring, and the collar moves out instantaneously to an eccentric position. Thistrips a lever arm, releasing a spring-loaded plunger to close the shut-off cock in the fuel controller via direct mechanical linkageThe pilot can re-set the trip governor by closing the shut-off cockj and re-light the engine. Although the standard Gnome power unit does not include agearbox, reduction boxes have been developed by de Havilland as optional extras. These provide a range of step-down ratios withvarious distances between input and output centres. A typical box gives a ratio of 3.26:1, with both directions of rotation available.The layout of this box comprises a simple train of double helical gearing with idler—for opposite rotation a small second idler isadded. The drive from the power turbine shaft into the input shaft of the box is achieved via a simple polygon coupling. This iscarried on the power turbine shaft and permits easy attachment of the gearbox to the engine. The box is bolted at its input face tothe rear mounting face of the engine in the recess of the exhaust duct. In this instance, therefore, engine mounting pads are pro-vided either side of the gearbox main casting. A special feature of the gearboxes is the inclusion of a deHavilland hydraulic torquemeter. This makes use of the tendency for the individual gears comprising a double helical pair, to moveaxially in opposite directions when transmitting torque. For the purpose of the torquemeter, the relationship between the gearhelix angles in the train is such that the low speed, output pair tend to move apart under load. This movement is restrained fromoccurring by carrying the gears on separate, co-axial shafts with (at the forward end) a piston on the inner shaft carrying the reargear, and an encasing cylinder on the outer shaft carrying the forward gear. Slight differential movement of the shaft allows oilunder pressure to pass through radial drillings in the outer shaft, across an annular port in the inner shaft and raise the hydraulicpressure in the space enclosed by the piston and cylinder. The resulting axial pressure forces act to oppose the shaft movement,and equilibrium is achieved by permitting a small metered bleed of oil to pass via a drilling to the underside of the piston. Becauseboth the piston and cylinder are rotating, an additional source of hydraulic pressure results from the centrifuging of the oil, anda sufficient radial depth of oil is contained in a reservoir under the piston to equalize these pressure forces either side of the piston.The oil eventually escapes down between the diaphragms of the Manchester type coupling at the forward end of the shaftsand spills through holes at its periphery. A second row of radial drillings in the outer shaft, downstreamof the annular port, enables a tapping of the "equilibrium" hydraulic pressure—proportional to the torque being transmitted—to provide a signal to the torquemeter gauge in the cockpit. In addition, an electric pressure switch in this oil line is arranged toopen at a pre-set value of hydraulic pressure equivalent to the maximum safe torque limit for a particular installation. Thetemporary opening of the torque limiting switch, which is wired into the throttle actuator control circuit, ensures that the fuel-flowis held steady until power-turbine speed rises or torque falls. GREMLINS IN ANTARCTICA A COPY of a telegram sent by the leader of the 1960 BelgianAntarctic ExDedition to the Centre National de Recherches Polaires in Brussels has been forwarded to Flight by the Directorof the Centre, Cmdt F. E. Bastin of the Belgian Air Force, with the comment that it expresses "in a somewhat funny way thetroubles experienced when one attemms to fly in these barren lands." The telegram from the leader, Maj Derom, is as follows: — From the "Base Roi Baudouin," 1st April 1960 "Long and arduous hours of gremlinological investigation have beenrewarded by the discovery of some new species of gremlins stop are now convinced that it was a false belief that microbes, gremlins andother pests could not live in Polar regions. About their origin in Antarctica, we suppose, in fact we are now certain, that they havebeen brought over by the first mechanical bird landing on the shelf. The rapid evolution towards the new species described hereunder isprobably an effect of the well-known psychological state called 'antarc- sollicitism,' a certain laisser-aller by which mating cannot be kept in thesuitable clan. Gremlins totally unheard of before. The following species have been very active this year demanding tremendous scientific andphysical efforts by our air and ground crews. "Primo: Snowreapers At the slightest breeze, they revel in fillingthe tail of Otter and Cessna with fresh snow. A powerful stream of hot air could only melt the snow but up to now we have been unable to kill this sort of creature who, seen under the microscope, seemsto be wearing little asbestos anoraks. "Secundo: Skisuckers In great concentration around parking places.They are of an off-white colour, have 23 paws in the form of half spheres of magdebourg and a tail of the ice anchor type. As soon as anaircraft comes to a standstill they emerge from the snow and, making a half-roll to the left or to the right, put their paws on the tefloncovered skis and anchor rheir tails down in the continent, making it impossible for the aircraft ro advance any farther. The only way to getrid of these suckers is the use of hammer and snowspades. "Tertio: Peskimo Polaris Like the European type suffers most prob-ably from a cold bottom but instead of pinching the spark of the plugs they steal the BTUs of the janitrol heater, leaving none for pilot andcrew. This lack of heat has been compensated by wearing more items of very expensive clothing and by the supply of fair amount of variousfirewaters. This is very detrimental to the finances of the National Center of Polar Research. Careful investigations by our specialists havebeen unable to locate the hiding-place of this most pestilential airweed. "P.S. We also had some trouble, but in a minor way, with otherspecies. The hochalkers, erasing carefully die distributing horizon and delicately whitewashing the blooming lot in a 360° non-dimensiona!haze. The sastru diggers, another kind of bastard, whose fretting ot the snow surface may turning a landing into a haphazard affair andreduce that lovely immense Antarctic aerodrome into a mere Austere landing strip."
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
