Overview

The Gipsy Twelve was designed for air cooling via ducts that introduced cooling air from the rear, an important departure from previous aircraft engine practice. The ducts delivered air at controlled pressure and flow rate from wing leading edge openings that did not materially affect lift and drag. de Havilland bragged that it had lower cooling drag than any other engine in production. The cooling air was forced at pressure to galleries outside the two six-cylinder banks through ducts that followed an easy sweep and were unobstructed by auxiliary units. Air flowed over the cylinder fins to the space between the banks and exhausted downward and aft. In high speed aircraft the cooling air flow had to limited to the cooling requirements; the Gipsy Twelve did this via an air exit gill located on the engine nacelle underside.

Maj Frank B.Halford, F.R.Ae.S., M.S.A.E., whose record as a successful aero engine designer began in 1916, designed the first Gipsy engine in 1927. The original Gipsy One 100-120 hp unit was successful from the start, the prototype establishing a 1927 world speed record for light aircraft. Progressively exploring the qualities of small frontal area, reliability, durability, installation simplicity, and accessibility, Halford's designs had proceeded in logical stage-by-stage efficiency and reliability advances.

General Description

Under the British Air Ministry nomenclature system, the name Gipsy King was applied to Gipsy Twelve engine equipped with auxiliaries to Air Ministry specifications. The Gipsy Twelve and Gipsy King I were thus identical except for auxiliaries and minor external features,. The following data applied to both except when stated otherwise.

The Gipsy Twelve power unit and its cowling could be quickly removed or installed. Magnetos, constant-speed governor units and vacuum pumps were in exceptionally convenient positions on the engine front. The starter was accessibly mounted on the top cover. The generator, with flexible drive, was housed conveniently on the side. Oil filters and relief valves access was unobstructed. The carburetter was fixed below the engine with all adjustments on one side and beneath. The fuel pumps, when supplied, could be fitted with remote priming control, making it unnecessary to open the cowling. The Breeze ignition harness was neatly arranged. The only auxiliaries mounted on the engine rear face were those that did not require attention in service, such as the carburetter jacket pressure relief valve, the tachometer drive and flexible generator drive. The hydraulic pump for undercarriage and flap operation, when supplied, was mounted vertically on the rear face with connections all located just below the top cowling.

A useful auxiliary equipment choice was available. At the engine front either a vacuum pump (for navigation instruments) or an oil pump (for automatic pilot and for hydraulic jacks to control cooling gill) could be fitted. On the rear a generator drive only, a generator drive and high-power hydraulic pump, a hydraulic pump only, or an air compressor only (for wheel brakes etc.), could be fitted. Alternative gearboxes were supplied to provide the different shaft speeds required for individual auxiliaries.

Cooling Ducts

Automatic boost controls and four-position mixture controls were built into the carburetter; the four positions were automatic weak, automatic rich, extra rich for starting, and slow-running cut-off. Warm engine oil circulated in a jacket around the supercharger inlet volute above the carburetter and through the two hollow carburetter butterfly valves. These provisions in conjunction with hot and cold induction air-intake control eliminated ice formation.

Cooling System Details. As the cooling ducts differed from the standard practice, airframe engine installation designers were required to follow the general layout of Drawing S.K.8230. Air entered through openings Al and A2 in the wing leading edge and was led through fixed ducts to galleries B1 and B2, one on the outside of each cylinder bank. These air-tight galleries were designed to be removable for sparking plug access and were joined to the fixed ducts so that engine movement was accommodated. After passing between the cylinder and head cooling fins, the air escaped at C; de Havilland recommended that the outlet opening area be controlled to meet variable airspeed and engine cooling load conditions. During climbs the relative areas gave a 5 inHg pressure drop minimum.

Gipsy Twelve Engine Data

Performance

International Rating = 405/420 bhp at 2,400 rpm and 7,500 ft, zero boost (2,286 m)
Maximum Rating = 410/425 bhp at 2,450 rpm and 7,750 ft, zero boost (2,362 m)
Maximum Takeoff Rating = 505/525 bhp at 2,600 rpm and Sea Level

Performance Curves

Fuel Consumption

Climbing, 319 bhp, 2,400 rpm, full throttle, sea level, normal rich = 31.5 gph
Cruising, 345 bhp, 2,200 rpm, automatic rich = 0.63 pt/bhp/hr
Maximum Level Flight, 425 bhp, 2,450 rpm, sea level, automatic rich = 34.5 gph
Oil Consumption at Normal rpm = 6 – 14 pt/hr

 

Oil Temperature Limits

Minimum for Takeoff and Ground Running = 25°C
Maximum for Continuous Cruising = 70°C
Peak for Climbing = 80°C

Cylinder Temperature Limits

Maximum Climbing = 230°C
Maximum Cruising = 210°C
Emergency Maximum (5 min) = 240°

Oil Pressure Limits

Normal Oil Pressure = 60 psi main; 15 psi auxiliary
Emergency Minimum (5 min) = 40 psi main; 10 psi auxiliary

Valve Timing: IO = 32.75° BTC; IC = 82.5 ABC; EO = 79.25° BTC; EC = 36.25°ABC
Valve Clearances: Inlet = 0.005"; Exhaust = 0.005"

Engine Component Details

Top Cover with Booster Coil and Starter, Crankshaft, Airscrew Shaft, Supercharger Drive Shaft, Crankcase, Supercharger Assembly	Supercharger Views, Cylinders, Connecting Rods, Pistons, Heads, Air and Hydraulic Pumps, Generator Drive

The cylinder head was an aluminium alloy casting held to the cylinder barrel by four high-strength steel studs screwed at their upper ends to the crankcase. The joint between the head and the cylinder was made by a copper-asbestos washer that fitted into a recess in the cylinder head. Flanged bronze valve guides were fitted for one inlet and one exhaust valve. The high-expansion steel valve seats were shrunk and peened into the cylinder head. Two 14 mm sparking plugs fitted to each cylinder head side provided dual ignition. Liberal fin area on each cylinder head provided ensured adequate cooling.

The cylinder was a carbon steel forging machined externally to form cooling fins and ground internally. Special attention to wall thickness and fin depth avoided distortion and promoted even cooling. The cylinders were specially treated for corrosion protection. The cylinder barrel was spigoted into the cylinder head using an intervening copper-asbestos washer. The other barrel end was fitted into the crankcase by means of a flange on the barrel; an oil-tight joint was formed by compressing a dermatine (nitrile butadiene rubber) ring between the flange radius and the crankcase bore chamfered edge.

The slipper-type piston was machined from an upset aluminium alloy forging designed so that the crown stress was taken directly to the gudgeon pin, which floated in both the piston and connection rod the small end. It was retained by an external circlips and washers at each end. Three rings were fitted to each piston; the two rings closest to the piston crowns were compression rings and the ring closest to the crankshaft was a scraper type that removed oil from the cylinder wall and deflected it through a series of small drilled holes to piston interior and back to the crankcase.

The H-section connecting rods were machined all over from 65-ton nickel chrome steel forgings. The master rod was forked at its big end to receive the plain connecting rod. Both rods were bronze-bushed at their small ends. The forked rod big end was fitted with a steel-backed lead-bronze split bearing that ran directly on the crankshaft. This bearing was made with an outside surface of white metal at its centre to provide a bearing for the plain rod. Oil holes were drilled through the bearing to lubricate the plain rod big end.

The crankshaft was a nickel-chrome-steel forging machined all over; it rotated in eight main bearings. Journals and crankpins were hollow, capped at both ends and drilled to supply pressure lubrication for the big end bearings. The crankshaft front was machined to form driving dogs for the vacuum pump and airscrew governor, tapered and keyed to receive the airscrew reduction gear, and machined to carry the camshafts and auxiliaries driving gear.

The airscrew shaft was a nickel chrome steel forging, machined all over, and provided at its front end with splines on which the airscrew rested. Towards the rear the large reduction gear was spigoted on a flange and bolted into place. The airscrew shaft front end ran in a combined radial and thrust bearing fitted in the front cover. The shaft was hollow and internally splined at its rear to take the supercharger driving shaft coupling. The deep-section Elektron crankcase casting had seven massive cross webs that carried the main bearing low down in the crankcase, thereby forming an extremely stiff box section. In addition the case was reinforced on each side by integrally cast webbing and additional strength was given by two transverse bolts at the centre main bearing. The main bearing caps were held in place with studs and were a push fit in between the boss webs that were machined accordingly. At the front crankcase end the cross web carried both the second main crankshaft bearing and also a bronze-backed white metal airscrew shaft bearing pressed into the web. The cross webs were also bored at either side to take the two camshaft bearings. The crankcase front had facings to carry the two magnetos while the crankcase rear was machined to accept the supercharger casing. The Elektron top cover had an integrally cast housing at its rear that carried the starter gear and electric starter. The booster coil was also mounted on this cover. Inside, on the cover's left, a passage was cast from front to rear that served as the main oil gallery.

Two steel camshafts each with twelve integral cams, ran in seven plain bearings housed in the lower crankcase on either side. The five intermediate journals for each shaft were supported by bearings in the crankcase cross webs. The bearing diameters were large enough to enable the camshafts to be withdrawn from the front. The front bearing was magnesium alloy and the rear bearings duralumin; they were held in position by dowel bolts and lock washers that passed through the crankcase. The camshaft bearings were lubricated from the hollow camshafts in which oil was maintained at a pressure of 20 psi. The camshaft spur driving gear was vernier-keyed to the extreme front end and secured by a nut and tab washer. Each cam operated a sliding tappet that lifted the valve by means of a D.T.D. 130 push rod and rocker mechanism; springs closed the valves. The tappet end was square at the cam end to prevent rotation and was fitted with a ball end to engage the push rod. The tappet reciprocated in a flanged guide, housed and bolted in the crankcase. The push rod had a cup at its upper and a ball at its lower end. The steel rocker, which had a phosphor bronze bush, pivoted about its offset centre on a hardened steel spindle and was held in a stamped steel bracket. At the push rod end the rocker was tapped to receive a hardened steel screw with a cup end and locknut, which was used to adjust valve clearance. A hardened steel pad was riveted into the rocker's other end. A telescopic cover enclosed the push rod and seated outwardly under the action of an enclosed central spring against a Dermatine ring in the tappet guide flange at the crankcase and a similar ring in a recess on the cylinder head top side. The valve gear was completely enclosed by cast Elektron covers held in position on the cylinder head underside by a special nut.

The valves were of special alloy steel (exhaust, D.T.D. 49A, and inlet S. 62). The exhaust valve face was stellited, a process that had been found essential when running engines on fuels containing tetraethyl lead. Both exhaust and inlet valve stem ends were stellited to withstand wear. Double concentric valve springs were fitted between the valve guide flange and the valve stem collar. The collar was held in position on the valve stem by split taper collets.

The timing gears and all auxiliary drives, except for the supercharger, were driven from a gear mounted directly behind the reduction gear at the crankshaft front end. This crankshaft area was subject to the least torsional vibration and proved the smoothest drive for accessories. From the crankshaft gear the drive passed through double idler gears situated on either crankshaft side to the two camshafts. Bevel gears integral with the front camshaft gears drove the two magnetos. The rear of the left B-bank camshaft was splined externally and drove the Dowty hydraulic pump, generator and metering pump, all at its rear end, via a horizontal coupling shaft . It also drove the oil pump through bevel gears and a vertical shaft. Machined integrally with the vertical oil pump drive shaft were worm gears that drove the dual revolution indicator and the fuel pump.

The supercharger was bolted to the crankcase rear end and was driven through a long drive shaft splined at its front end into a coupling in the airscrew shaft rear, and at its rear end into another coupling that drove the supercharger impeller through a layshaft. The D.T.D. 130 impeller forging was machined all over and was dynamically balanced to enable it to run in ball bearings at a maximum speed of approximately 20,000 rpm. The impeller drew the mixture from an S.U. carburettor fitted low on the supercharger casing right side, and compressed it to a pressure controlled by the cam on the automatic boost control to a maximum of 32 psi during takeoff conditions. On leaving the impeller the mixture's kinetic energy was converted into pressure by diffuser blades that passed the mixture into the delivery volute casing. From the delivery volute the mixture passed into a large main manifold between the cylinders banks. Four three-branch manifolds carried the mixture to the cylinder head inlet ports. Directly above the carburetter the mixture passed through a jacketed casing section with engine scavenge oil constantly passing. Hot oil was also fed to the two hollow butterfly valves in the carburettor itself so that with the heater jacket, all possibility of ice formation was obviated.

Lubrication Diagram

Lubrication. The oil pumps and filters formed a detachable unit bolted to the supercharger rear underside. A gear type pump in the oil sump drew oil from a separate tank via a gauze filter situated in the sumps left side and delivered it under pressure to an Auto Klean filter that removed the finest foreign matter particles. The oil flow then passed through the auxiliary filter, which was bolted to the oil sump, and on to the engine. A relief valve used shims to set the main oil pressure to 60 psi. After passing through the pressure filter the oil divided into two streams. The main stream flowed upwards to the top cover and along a cast in gallery connected by drillings to the crankshaft main bearings, into the crankshaft through the hollow journals and to the crankpin big end journals; this stream also lubricated the airscrew shaft bearing, the rear main bearing, the camshaft idler spindles, and reduction gear oil jet. The second stream passed through a balanced piston mechanism that automatically reduced the pressure to approximately 20 psi. The oil at this reduced pressure lubricated the hollow camshafts, the magneto drives, and the accessory drives. The pistons, cams and tappets were lubricated by the continual mist in the crankcase caused by the oil splash from the main bearings, big ends and camshafts. A drilling in the right camshaft rear bearing connected with a hole in the supercharger carrier case and lubricated the supercharger layshaft. Holes in the layshaft gear assisted in maintaining an oil mist that lubricated the gears.

Oil drawn from the crankcase by the reduced pressure in the supercharger casing lubricated the front impeller bearing. A Tecalemit metering pump at the engine rear delivered oil to the rear impeller bearing. Every important bearing was pressure lubricated, but the oil flow was not excessive and a reasonable lubricant working temperature was maintained. Airscrew governor oil was fed at engine pressure from the top cover to a drilling in the crankcase left top facing that connected with the front main bearing oil hole. Oil flowed to the drive housing and on to the airscrew governor, which increased the pressure to about 180 psi. The oil was then fed to the rear airscrew shaft bearing and along the hollow shaft to the airscrew.

Oil that collected in the crankcase and supercharger casing base was scavenged by two gear type pumps through two scavenge filters also contained in the oil sump, and delivered through an external pipe to the hollow butterfly valve spindles in the carburetter, and then to the oil jacket on the supercharger casing. The oil tank return pipe was connected to a union on the oil jacket cover. To avoid excessive pressure in the butterfly valve spindles, a bypass valve in the supercharger casing delivered return oil directly to the jacket. A central screw held the valve gear cover in place and the joint was sealed by a Hallite C-section seal. Attached to each of these covers was a vent stand pipe to vent excess oil and fumes. A plug was provided for filling and also determines the level of the oil in the cover. Rocker movement splashed oil over all rocker gear moving parts.

The two oil scavenge filters, one oil pressure filter and one oil suction filter were contained in the same Elektron casting that formed the main oil sump and carried the oil pumps. A long auxiliary felt pressure filter with a relief valve was bolted onto two facings on the oil sump. All filters were accessible for cleaning by unscrewing their hexagonal caps. The main pressure filter was of the Auto Klean type. Dismantling for cleaning was only necessary after every 250 hrs flying. The filter unit was removed by taking off the four nuts and split pins. The tommy bar, however, was to be turned frequently in order to clear the filter.

Engine Controls

Gipsy Twelve engine controls were arranged for pulley operation. The outer pulley, mounted at the engine rear, operated the magneto and governor controls; the centre pulley actuated the throttle control; the inner pulley controlled the four-position mixture lever. All the pulley levers were provided with screw stops by which the correct control travel could be adjusted. The inner and centre pulley levers were connected by control rods to bellcranks and then to the carburettor throttle and mixture control levers. The outer pulley carrying the main spindle was connected to the rear centre control shaft, which was then connected to the main centre control shaft by a flexible coupling. Similarly the front end was connected to the control casing rear. By means of a suitable cam and gear contained in this control casing the magneto and airscrew governor controls were synchronized.

The controls could also be arranged with levers for rod operation, in lieu of pulleys. A lever with a ball end was attached to the control shaft rear end of the for connection to the aircraft control. Throttle and mixture controls were directly operated.