Pratt and Whitney R1830 (Twin Wasp)
The R1830 is a 14 cylinder, two row piston engine (seven cylinders per row) rated at 1350 hp, with a capacity of 1830 cubic inches (hence the designation R, for radial, 1830).
When America entered World War One (nineteen months before it ended) American-built French Hispano-Suiza engines were manufactured by Wright-Martin, Frederick B. Rentschler was in charge of this production. After the war had ended the Wright-Martin board set aside $3 million dollars to create an aviation company and Rentschler was asked to set up the new company. As well as producing, refining and improving the Hispano-Suiza engines being produced under licence, Rentschler called for new designs. To this end he employed George Mead (engineer), Andrew Van Dean Wilgoss (designer), Charles Marks (production engineer) and John Borrup (machine shop superintendent). Donald Brown who had worked for Wright-Martin during the war also joined the team. This nucleus of men formed Wright Aeronautical.
The American Navy as early as 1921 had been seeking a 200 hp air cooled radial and the development of this engine (a nine cylinder unit) was in the hands of a Charles Lawrence. Lawrence sought a merger with Wright. After Wright had purchased the Lawrence company, Rentschler assigned Mead and Willgoos to revise the engine design. After further work by Sam Heron and E.T.Jones the famous J-5, better known as the Whirlwind, emerged. It was this engine which powered the Spirit of St Louis on Lindbergh’s historic flight from New York to Paris in May 1927. After the success of the J-5, Rentschler became at odds with his board of directors, none of whom were willing to finance further experimental and development engineering. This disagreement lead to Rentschler leaving Wright.
Rentschler set up an aero engine company from scratch, receiving backing from a machine-tool manufacturer, Pratt and Whitney. He sought out some of the members of the design team he had put together at Wright, other members of the design team asked to join him. The group that designed the J-5 now undertook the design of a 400 hp air cooled radial, weighing less than 650 pounds that they had learnt (from Chance Vought, an old friend of Rentschler) the navy would require. The design was from scratch, there was no commitment to existing designs, jigs or fixtures. In 1920 Mead had designed an engine in which he had reversed the usual procedure of that time by splitting the crankshaft and employing a solid master rod. Mead and Willgoss decided to follow this practice in the new engine and to make the crankcase forged and split through the plane of the cylinders so that the same forging could be used for each half of the crankcase. This led to a strong and light crankcase where the load would be spread uniformly between the front and rear main bearings. In July the team were joined by Earle.A Ryder. Ryder and Willgoss set to work on the arrangements for the valves, cylinder head finning, rocker boxes and push rods. Willgoss designed the rocker box so that it was an integral part of the cylinder casting and Ryder designed a telescoping cover for the push rods. Mead devised a rotary induction system which made possible the use of only one carburettor.
Mead and Willgoss worked out a system of cylinder barrel finning that would result in thinner and more numerous fins which gave greater cooling characteristics. Willgoss designed the accessories arrangement at the rear which made servicing of accessories possible without dismantling the engine. As the prototype neared completion the engine adopted the name ‘Wasp’.
The first run was made at 380 hp, the second at 410 hp and the third at 425 hp. In the next eight weeks the engine underwent continuous tsting. It swept through the Navy qualification 50 hour test, the final reading (the last of 221 made) showing 415 hp at 1890 rpm. The engine weighed just under 650 pounds. The ‘Wasp’ took to the air on the 5th May 1926, it was an outstanding success.
In the early 1930’s Wright stopped development on all its engines except the R-1820 Cyclone. By concentrating all its research facilities into the this one engine the Wright development engineers were able to achieve 700 hp with reliability. The Cyclone was chosen to power the DC3 and B-17.
Pratt and Whitney had started work on their first production two row configuration (the R-1830 basically one Wasp bolted behind another Wasp, hence the name Twin Wasp) in 1929. The success achieved by Wright’s with the Cyclone caused Luke Hobbs, Pratt and Whitney’s engineering manager, to follow suit and concentrate development of the R-1830 equipped with single-stage and two-stage supercharging. At the early stages of World War II the R-1830 developed more power, was more rugged and had a greater service life than the Cyclone.
In the summer of 1936 the R-1830 had attained a take-off rating of 1200 hp, but was about to undergo its only major research problem. On a radial engine, number one piston is attached to the crankshaft by a special rod (master rod) that is much stronger and larger than all the other connecting rods, the other rods being attached to journals machined on the master rod. The loads applied to all seven pistons are hence transmitted to the master rod bearing, this bearing has to withstand enormous loads/forces. Earlier research into the master rod bearing resulted in a bearing which had a thin layer of lead on a thin layer of silver which itself was upon a thin layer of copper, the copper adhering to a steel backing. This bearing had given outstanding performance. Testing carried out in mid 1936 showed the thin layer of lead within the bearing material to be suffering what appeared to be an eroding action. After a time it became apparent that corrosion rather than erosion was the problem. The problem was one of making the lead impervious to acid corrosion. Adding the smallest quantities of indium to the lead produced the answer. A manufacturing method was evolved which would produce the bearing in such shape and accuracy that it was ‘pre-fitted’ to the master rod. With the approach of war, Pratt and Whitney made its discovery available to other American engine manufacturers as well as British aero engine manufacturers. As engine powers increased the new master rod bearing withstood all and every load applied to it.
In 1939 Pratt and Whitney matched its R-1830 against the liquid cooled Allison V-1710 in an appraisal organised by the War Department. Despite winning the trial the R-1830 was not chosen. A derivative of the Twin Wasp known as the Double Wasp (an 18 cylinder, two rows of nine, 2500 hp, 2800 cubic inches capacity radial) powered many American fighters (Corsair, Thunderbolt). The Twin Wasp and Double Wasp proved to be equally as successful.
In 1938 France placed a $2 million order for R-1830’s, in 1939 it instigated a further $83 million order for these engines. This meant that the monthly output of 375 Twin Wasps from the factory at East Hartford had to be increased to a monthly output of 1420, a 379% increase. To this end the work force was increased by 3000 and the plant was expanded to its maximum workable limit. When France collapsed Britain took over the French contract. When America decided to increase its aircraft output, to accomodate the Double Wasp the facilities at East Hartford could not cope. The American automotive industry became involved. Buick, and then later Buick together with Chevrolet took over production and assembly of the Twin Wasp. East Hartford dealt with production of the Double Wasp. 173,618 Twin Wasps were made, a production number never to be exceeded by any other aero engine ever manufactured.
The constructional layout of the Pratt and Whitney R-1830 (Twin Wasp) consists of five major sections: front section (which contains the propeller reduction gear), crankcase, cylinders, supercharger section and rear section. The crankcase and cylinders are often referred to as the ‘power section’.
Number one cylinder is the top cylinder on the rear row. The cylinder numbering on the front row (engine viewed from front), starting slightly to the left of top and going ACW, is 2, 4, 6, 8, 10, 12, 14. The rear row cylinders numbering, again viewed from the front, starting at top and going ACW, is 1, 3, 5, 7, 9, 11, 13.
Firing order is 1:10:5:14:9:4:13:8:3:12:7:2:11:6
Most B-24’s used either the R-1830-43, 43A, 65 or 65A units. Most 43/43A units were fitted with the Bendix-Stromberg injection type carburettor whilst most 65/65A units were fitted with the Chandler-Evans (CECO) direct metering carburettor. The 43/65 units were usually fitted with Scintilla magnetos whereas the 43A/65A units normally had Bosch magnetos. The 43A and 65A units were built under licence by Buick and Chevrolet.
All power plants were geared 16:9 and rated at 1200 hp (at sea level). On B-24’s thrust was provided by a Hamilton Standard three bladed propeller of 11 feet 7 inches diameter.
Each engine was attached to its mount by eight flexible mounts. The nacelles were designed to permit engine and mount to be removed as a unit, concept of ‘power egg’. The distinctive oval engine cowls of all later Liberators resulted from placing air scoops on each side of the engine. The RH aperture, viewed from the front, passed air to the intercooler and turbo whilst the LH provided air for the supercharger and the oil cooler as well as allowing small amounts of cooling air to pass over the generator. Each scoop had a small diversion to cool one of the twin magnetos fitted.
Both magnetos fitted to the back of the rear section - right magneto fired the front set of plugs, left the rear set. Each cylinder contained a front and a rear sparking plug.
To replace a magneto, if it was known that one magneto was timed perfectly, the correctly timed magneto was as a guide to replacing the other magneto.
If both magnetos were to be replaced it was recommended that one magneto be timed to the engine and the second magneto be synchronised with the first.
Timing marks placed on the propeller thrust plate and slinger ring used as propeller shaft rotation would differ from crankshaft rotation due to the reduction gearing.
The drive shaft of the magneto contained a fourteen lobe cam. Assuming that one magneto was known to be in perfect time, the propeller was rotated ACW until the cam lobe, on the correctly timed magneto was in the cam dwell preceeding firing to No 1 cylinder. A .001 inch (one thousandths of an inch) feeler gauge was placed between the ignition points. The propeller shaft was then rotated very slowly ACW until the feeler strip became loose (i.e. the points were beginning to open). At this stage a mark was placed on the slinger ring and aligned with a mark placed on the thrust plate. With these marks aligned the same procedure was carried out to the untimed magneto. If any rotational adjustment was necessary it was carried out by rotating the untimed magneto body on its mounting studs.
When a magneto had been removed without establishing aligning marks prior to removal, a stiff piece of wire was inserted through the front spark-plug hole of No 1 cylinder and the propeller rotated ACW until TDC compression (both valves closed) was established. The wire was held firmly against the piston crown and a file mark was scratch on the piece of wire two inches above the edge of the spark-plug hole, at the same time a mark was placed on the de-icer slinger ring. The propeller shaft was then rotated CW until the mark on the wire lined up with the edge of the spark-plug hole. A mark was placed on the thrust plate that was perfectly square with the mark previously placed on the slinger ring.
The propeller was then rotated ACW until the mark on the wire again aligned with the edge of the spark-plug hole. Another mark was made on the thrust plate that again was perfectly square with the mark previously placed on the slinger ring.
Using a flexible tape located around the edge of the thrust plate a third mark, which was exactly central to the other two marks located on the thrust plate, was placed on the thrust plate. A final mark, to establish the required 25 degree advance, was located one and five thirty seconds of an inch (1 5/32) to the left of the central mark. The magneto was then fitted with this advance mark being aligned with the original mark placed on the de-icer slinger ring.
The pressure type downdraught carburettor had twin venturis, each with a throttle butterfly. Located in the air intake above the venturi was a sealed (nitrogen and inert oil) metallic bellows which was secured to a contoured needle valve. The bellows would expand/contract with changes in air density due to variation in altitude, temperature and supercharging. The needle valve controlled the flow of air to the impact air chamber in the fuel regulating unit.
The fuel regulator contained two air (impact/venturi) and two fuel chambers (unmetered/metered) each separated by a diaphragm. All four diaphragms were connected to a fuel poppet valve. One of the air chambers measured impact pressure (air pressure prior to entering the venturi) whilst the other air chamber measured venturi pressure. With the engine running the venturi pressure was always below impact pressure. The greater the air speed at the venturi (due to an increase in throttle opening commanded by the pilot) the greater the pressure drop with a resultant pressure difference (referred to as air metering force) between impact pressure and venturi pressure. The air metering force opened the poppet valve which admitted fuel under pressure from the fuel pump to the unmetered fuel chamber. Fuel pump pressure was also fed to a fuel control unit.
Within the fuel control unit was a valve disc plate whose position was controlled by the mixture control lever. The lever had four positions; idle cut-off, automatic lean, automatic rich and full rich. Control lever movement (by the pilot) rotated the disc plate, the position of which permitted or restricted fuel flow from the power enrichment, automatic lean and automatic rich jets. The power enrichment and automatic lean jets reduced the fuel pump pressure to a pressure known as metered fuel pressure.
In idle cut-off all three jets were closed off thus there was no fuel flow from the carburettor. In automatic lean only the automatic lean jet supplied metered fuel whilst in automatic rich both the automatic lean and automatic rich jets supplied metered fuel. In full rich the power enrichment jet as well as the automatic rich jet and automatic lean jet supplied metered fuel to the cylinders.
Metered fuel pressure existed in the second fuel chamber of the fuel regulator. The difference in pressure between the two fuel chambers (one at unmetered fuel pressure the other at metered fuel pressure) was known as fuel metering force. Whenever the engine was running the air metering force would be balanced by the fuel metering force.
If there was a change in the air metering force there would be an automatic change in the fuel metering force hence an increase/decrease in the weight of air delivered to the engine would create a corresponding increase/decrease in the required weight of fuel delivered.
An adaptor, located below the throttle plate and venturi, was responsible for discharging the metered fuel into the air stream.
The charge air, taken in via one of the four intakes (two on either side of the oval cowling) was passed directly to the centrifugal compressor. The compressor had two turbines coupled to a common shaft. One turbine was driven by the pressure of the exhaust gases. The other turbine compressed the inducted air which raised the pressure above atmospheric. There was an oil supply to the turbine shaft. A waste gate (controlled by the pilots supercharger control via a pressure regulator), when opened, allowed the exhaust gases to escape hence the speed of the turbines was reduced with a consequent reduction in compression to the inducted air. Air from the intake side of the intercooler was passed through a cooling duct to help reduce the considerable temperature within the turbine unit. A cooling cap, visable from underneath the wing, also helped to reduce the internal temperature within the unit.
When air is compressed its temperature is raised and as a result its density (mass) is reduced. To achieve maximum mass the compressed air that left the turbine unit passed through an intercooler, the cooling medium being ram air from another of the four intakes fitted to the side of the engine cowling. The amount of cooling air being allowed through the intercooler was controlled by two shutters, the positions of which was altered by an electric motor switched from a control operated by the pilots.
From the intercooler the charge air was passed to the carburettor where the quantity of fuel and air, as determined by the throttle control in the cockpit, was passed to the engine driven supercharger where a further pressure increase took place, to both the fuel and the air. The compressed fuel/air was then passed via the induction system to the cylinders. A boost gauge tapping was taken from the induction system after the air had been through its two compression phases, the second after the mix with 100 octane fuel.
General procedures for dismantling the R-1830:
Thrust bearing nut released. Propeller shaft runout checked (should not exceed 0.005 in). Ignition conduit, leads and magnetos removed, together with breather tube (rear crankcase) and No 8 cylinder rockerbox.
Front section: Thrust bearing assembly, oil pump and reduction gearing (16:9) removed.
Cylinders: Intake pipes, sump, rockerbox covers (from rear row and then front row) removed. Valve adjusting screw locknuts loosened. Pushrod cover nuts unfasten. In turn each cylinder turned to TDC compression (both valves closed) and each rocker depressed (using tool) to allow removal of pushrods and covers. External oil tubes removed. Each cylinder removed with piston positioned at TDC. Master rod in each row (one master rod located in No 12 other in No 5) left to last. If more than two adjacent studs had failed or more than two adjacent nuts had come loose during engine operational use the cylinder was scrapped and all the studs on that particular cylinder mounting pad were replaced.
The piston pin and piston would be taken off from their connecting rod immediately the relevant cylinder has been removed. Piston rings taken off their piston. Ignition manifold and inter-cylinder deflectors removed. Rockers drifted out. Valve springs compressed, valve stem locks and valves removed.
Power section: Remove reduction gear coupling, support plate assembly and front scavange pump intermediate drive gear. Loosen cam retaining nut, remove crankcase front section, remove cam retaining nut, turn cam one complete revolution (to push tappet rollers away from cam lobes) and remove cam, camshaft front bearing, valve tappets, rollers and pins. Remove oil tubes and crankshaft front plug. Undo four master rod bolts, ensure master rod is in its full forward position and using a slide hammer remove master rod from the crankshaft.
Remove nuts securing crankcase section to supercharger section, using suitable hoist lift power section away from supercharger section. Remove crankshaft front bearing inner race, lift out cam reduction gear and spacer.
Rear crankcase section: Remove nuts from crankcase bolts, remove bolts and lift off crankcase. Remove nuts securing rear cam bearing to crankcase, push out valve tappets as far as possible and use puller to remove rear cam bearing. Remove crankcase rear breather baffle, oil tappet guide screws, roller pins and rollers. Withdraw valve tappet. Remove oil drain tubes. Remove bolts securing crankshaft rear gear to crankshaft and then remove gear, spacer and rear bearing roller retainer from the crankshaft. Lift off the rear bearing outer race and roller assembly. Remove and dismantle rear master rod in the same way that the front master rod was dismantled. Remove crankcase centre section, crankshaft centre bearing and outer race retainer then remove the outer race from the liner in crankcase centre section. Remove nuts, bolts and screws securing two retaining shoes to the crankshaft.
Use a slide hammer to remove rear shoes then remove front shoes and crankshaft centre bearing. Use a hydraulic puller to remove rear bearing inner race. Remove front and rear crankpin plugs. Remove the allen plugs from the top, bottom and rear of the crankshaft centre cheek and from the top of the crankshaft rear cheek.
Rear section: Install a holder on the forward side of the supercharger, remove the previously loosened starter jaw nut and pull off the starter jaw. Remove all covers from the rear case together with the nuts holding the magneto adaptors to the rear section and then pull off the magneto adaptors and drive gears. Remove the magneto drive pinion, tachometer drive, auxiliary drive gear and auxiliary intermediate drive gear. Remove rear oil pump and seperate the rear, centre and front sections of the pump. Undo the nuts securing the rear section to the intermediate rear case and seperate the two casings ensure that the engine generator drive gear and intermediate drive gear do not fall out. Pull off the plug spacer and remove the pressure relief valve, by-pass valve, rear section breather tube, rear breather body, generator drive gear oil seal and vacuum pump drive.
Intermediate rear section: Tab back the lockwasher and undo the nut from the forward end of the accessory drive shaft. Remove the shaft adaptor with a puller and withdraw the accessory drive gear and shaft from the intermediate rear section. Remove the generator drive and generator intermediate drive gears from the casing. Remove both impeller intermediate drive gears. Undo the impeller shaft locknut. Remove the impeller shaft front and rear cover plates. Drift the impeller shaft from the supercharger and intermediate rear case. Remove the front and rear bearings and seals. Remove nuts from diffuser support bolts, nuts securing intermediate rear case to supercharger case and lift off the intermediate rear case. Remove impeller. Remove fuel drain valve connection and valve. Finally undo the bolts securing the oil pressure tube and bracket assembly to the supercharger case.
Prior to assembly, which is basically the reverse procedure of dismantling, all components would undergo a rigourous inspection. To mention but two; linkrods would be permitted a 0.010 inch mis-alignment (the measurement being taken at each end of the piston pin) and the crankshaft would be allowed a 0.002 inch runout at its centre bearing. The two master-rods and their bearings would receive special inspection. The Pratt and Whitney overhaul manual allocates 65 pages to the inspection techniques appropriate to each component and 91 pages to repair techniques, again appropriate to each and every engine component. The manual also contains a section relating to lockwires.
When ever a component required to be ‘locked’, a wire was passed through the ‘top’ bolt, then wound prior to passing through the ‘bottom’ bolt, where it was again wound. Three thicknesses (0.025/0.040 and 0.0521) of wire are used throughout the engine.