Category Archives: Aircraft Engines

Packard X-2775 front

Packard X-2775 24-Cylinder Aircraft Engine

By William Pearce

In late 1926, Lt. Alford Joseph Williams approached the Packard Motor Car Company (Packard) regarding a high-power engine for a special aircraft project. Williams was an officer in the United States Navy and believed that air racing contributed directly to the development of front-line fighter aircraft. The United States had won the Schneider Trophy two out of the last three races, and another win would mean permanent retention of the trophy for the US. However, the US government was no longer interested in supporting a Schneider team.

Packard X-2775 front

The original Packard X-2775 (1A-2775) was a direct-drive engine installed in the Kirkham-Williams Racer. A housing extended the propeller shaft to better streamline the engine. Two mounting pads were integral with the crankcase, and a third was part of the timing gear cover at the rear of the engine. Note the vertical intake in the center of the upper Vee.

Williams was assembling a group of investors to fund the design and construction of a private racer to participate in the Schneider contest. In addition, the US Navy was willing to indirectly support the efforts of a private entry. With the Navy willing to cover the development of the engine, Packard agreed to build a powerful engine for Williams’ Schneider racer. On 9 February 1927, the US government officially announced that it would not be sending a team to compete in the 1927 Schneider race, held in Venice, Italy. On 24 March 1927, it was announced that a private group of patriotic sportsmen had formed the Mercury Flying Corporation (MFC) to build a racer for the Schneider Trophy contest that would be piloted by Williams. The aircraft was built by the Kirkham Products Corporation and was known as the Kirkham-Williams Racer.

Packard had started the initial design work on the engine shortly after agreeing to its construction, even though a contract had not been issued. Once the Navy had the funds, Contract No. 3224 was issued to cover the engine’s cost. To speed development of the powerful engine, Packard combined components of two proven V-1500 engines to create a new 24-cylinder engine. The new engine was designated the Packard 1A-2775, but it was also commonly referred to by its Navy designation of X-2775.

Packard X-2775 case drive rod crank

The X-2775’s hexagonal, barrel-type crankcase, timing gear drive and housing, connecting rods, and crankshaft. Note the walls inside of the crankcase, and the crankshaft’s large cheeks that acted as main journals.

The Packard X-2775 was designed by Lionel Melville Woolson. The engine was arranged in an X configuration, with four banks of six cylinders. The upper and lower banks retained the 60-degree bank angle of the V-1500. This left 120-degree bank angles on the sides of the engine. As many V-1500 components were used as possible, including pistons, the basic valve gear, and the induction system. At the front of the X-2775, the propeller shaft ran in an extended housing and was coupled directly to the crankshaft, without any gear reduction. The extended housing allowed for a more streamlined engine installation.

A single-piece, cast aluminum, hexagonal, barrel-type crankcase was used. Two engine mounting pads were provided on each side of the crankcase, and a third pad was incorporated into the side of the timing gear housing, which mounted to the rear of the engine. The crankcase was designed to support landing gear or floats connected to the forwardmost engine mounting pad. Seven integrally cast partitions were provided inside the crankcase. The partitions were hollow at their center and were used to support the crankshaft. The seven single-piece main bearings were made of Babbitt-lined steel rings, shrunk into the crankcase’s partitions, and retained by screws from the outer side of the flanged partition. The partitions had a series of holes around their periphery that allowed for the internal flow of oil as well as enabled assembly of the engine’s connecting rods.

Packard X-2775 manifold and valve spring

Upper image is the valve port arrangement that was integral with the valve and camshaft housing. The drawing includes the ports to circulate hot exhaust gases around the intake manifold to ensure fuel vaporization. The lower image is the unique valve spring arrangement designed by Lionel Woolson. Helically-twisted guides (left) held the seven small springs (center) to make the complete spring set (right).

The crankshaft was positioned about 1.5 in (38 mm) above the crankcase’s centerline and had six crankpins. The crankshaft’s cheeks acted as main journals. The cheeks were perfectly circular and were 7.75 in (197 mm) in diameter. This design increased the main bearing surface area to support the engine’s power but kept the crankshaft the same overall length as the crankshaft used on the V-1500 engine. A longer crankshaft would result in a longer and heavier engine, as well as necessitating the design and manufacture of new valve housings and camshafts. At 161 lb (73 kg), the crankshaft was around twice the weight of the crankshaft used in the V-1500 engine. The X-2775’s crankshaft was inserted through the center of the crankcase for assembly.

Each connecting rod assembly was made up of a master rod and three articulated rods. The end cap, with its two bosses for the articulating rods, was attached to the master rod by four studs. The articulated rods had forked ends that connected to the blade bosses on the master rod. The forked end of each articulated rod was tapped and secured to the master rod by a threaded rod pin. Once assembled, two bolts passed through the connecting rod assembly to further secure its two halves and also secured the pins of the articulated rods. To accommodate the crankshaft being approximately 1.5 in (38 mm) above center in the crankcase, the lower articulated rods were 1.5 in (38 mm) longer than the other rods. When the engine was viewed from the rear, the master rods were attached to pistons in the upper left cylinder bank.

Packard X-2775 section

Sectional view of the X-2775 engine. The engine mount is depicted on the left, and the landing gear or float mount is on the right. Note the spark plug position. The revised engine had provisions for four spark plugs—two on each side of the cylinder.

Individual steel cylinders of welded construction with welded-on steel water jackets were mounted to the crankcase via 10 studs. The cylinder’s combustion chamber had machined valve ports and was welded to the top of the cylinder barrel. Five studs protruded above each cylinder’s combustion chamber and were used to secure the cast aluminum valve and camshaft housing. Each bank of six cylinders had a single valve and camshaft housing.

Each cylinder had two intake and two exhaust valves. The valves were arranged so that one intake and one exhaust valve were on the Vee side of the cylinder, and the pairing was duplicated on the other side of the cylinder. The valve and camshaft housing collected the exhaust gases from two adjacent cylinders and expelled it out one of three exhaust ports. The valve and camshaft housing also had an integral intake manifold that fed three cylinders. The valves for each cylinder bank were actuated by a single overhead camshaft driven by an inclined shaft at the rear of the engine. The two inclined shafts for each Vee engine section were driven by a vertical shaft geared to the crankshaft. The lower vertical shaft was extended to drive one fuel, one water, and four oil pumps. The shafts were enclosed in the timing gear housing that mounted to the back of the engine. The valve covers of the lower cylinders also formed sumps for engine oil collection. Oil was circulated through various passageways in addition to the hollow crankshaft and hollow camshaft. The exhaust valve had a hollow stem for oil cooling.

The valve springs were designed by Woolson and were a unique design. Rather than the valve stem passing through the center of one or two valve springs, a set of seven smaller springs encircled the valve stem. Each of the seven springs was mounted on a guide, and the set was contained in a special retainer. The seven spring guides were given a slight helical twist. The special valve spring set distributed the spring load evenly around the valve stem, reduced the likelihood of a valve failure due to a spring breaking, prevented valve springs from setting, and also rotated the valve during engine operation. The valve rotation was one revolution for about every 40 revolutions of the crankshaft.

Packard X-2775 front and back

Front and rear views of the original X-2775 illustrate that the engine was narrow but rather tall. The ring around the propeller shaft was a fixed attachment point for the engine cowling.

Each cylinder’s combustion chamber had a flat roof with a spark plug on each side of the cylinder. The spark plugs were fired by a battery-powered ignition system via four distributors driven at the rear of the engine. Two distributors were positioned behind each 60-degree cylinder bank Vee. In each cylinder, one spark plug was fired by an upper distributor, and one spark plug was fired by a lower distributor. Separate induction systems were positioned in the upper and lower cylinder Vees. Each system consisted of a central inlet that branched into a forward and rear section. Each section had a carburetor and fed six cylinders. This gave the engine a total of four carburetors—two in each upper and lower vee. Control rods linked the carburetors to the distributors so that ignition timing was altered with throttle position. A port in the valve and camshaft housing fed exhaust gases through a jacket surrounding the manifold to which the carburetor mounted. The exhaust gases heated the intake manifold to better vaporize the incoming fuel charge.

Packard’s V-1500 engine had a 5.375 in (137 mm) bore and a 5.5 in (140 mm) stroke. The X-2775 had the same 5.375 in (137 mm) bore, but the stroke was shortened to 5.0 in (127 mm). However, the three articulated connecting rods had a slightly longer stroke of 5.125 in (130 mm). Each of the six cylinders served by a master rod had a displacement of 113.5 cu in (1.86 L), and each of the 18 cylinders served by an articulated rod had a displacement of 116.3 cu in (1.91 L). The total displacement for the engine was 2,774 cu in (45.5 L). The X-2775 produced a maximum of 1,250 hp (932 kW) at 2,780 rpm and was rated for 1,200 hp (895 kW) at 2,600 rpm. At 2,000 rpm, the engine had an output of 800 hp (597 kW). The X-2775 was 77.5 in (1.97 m) long, 28.3 in wide (.72 m), and 45.2 in (1.15 m) tall. The weight of the initial X-2775 was 1,402 lb (636 kg).

Packard X-2775 no 2 supercharged

The second X-2775 incorporated a Roots-type supercharger driven from the propeller shaft. Difficulty was encountered with fuel metering since the carburetors were positioned on the pressure side of the supercharger. The supercharged engine was never installed in an aircraft.

The X-2775 engine was completed in June 1927 and subsequently passed an acceptance test, which involved the engine running continuously at full throttle for one hour. Williams was involved with testing the X-2775 at Packard to gain experience with its operation. The engine was then shipped out for installation in the Kirkham-Williams Racer, which was finished in late July. The racer and the X-2775 made their first flight on 25 August. Despite achieving speeds around 270 mph (435 km/h), the racer had issues that could not be resolved in time for the Schneider Trophy contest, scheduled to start on 23 September. The Kirkham-Williams Racer was subsequently converted to a land plane, and Williams flew the aircraft over a 3 km (1.9 mi) course unofficially timed at 322.42 mph (518.88 km/h) on 6 November 1927. However, that speed was with the wind, and Williams later stated that the true speed was around 287 mph (462 km/h). Higher speeds had been anticipated. The aircraft was then shipped to the Navy Aircraft Factory (NAF) at Philadelphia, Pennsylvania.

Around late June 1927, rumors indicated that the Schneider competition would be faster than the Kirkham-Williams Racer. As a result, the Navy added a second X-2775 engine to its existing contract with Packard. The second engine incorporated a supercharger for increased power output. In the span of 10 weeks, Packard had designed, constructed, and tested the new engine. The second X-2775 engine was, again, direct drive. However, the propeller shaft also drove a Roots-style supercharger with three rotors (impellers). A central rotor was coaxial with the propeller shaft, and it interacted with an upper and lower rotor that supplied forced induction to the respective upper and lower cylinder banks. For the upper Vee, air was brought in the right side of the supercharger housing and exited the left side, flowing into a manifold routed between the upper cylinder banks. For the lower Vee, the flow was reversed—entering the left side of the supercharger and exiting the right. The supercharged X-2775 weighed around 1,635 lb (742 kg).

Because of the very tight development schedule, the rotors were given generous clearances. This reduced the amount of boost the supercharger generated to only 3.78 psi (.26 bar), which increased the X-2775’s output to 1,300 hp (696 kW) at 2,700 rpm. Tighter rotor tolerances would yield 4.72 psi (.33 bar) of boost and 1,500 hp (1,119 kW) at 2,700 rpm. However, it is not known if improved rotors were ever built. Although completed around August 1927, the supercharged engine was never installed in the Kirkham-Williams Racer.

Packard X-2775 NASM left

The first X-2775 engine was reworked with a propeller gear reduction, new cylinders, new valve housings, and a new induction system. This engine was installed in the Williams Mercury Racer. (NASM image)

The Navy felt that adding a propeller gear reduction to the engine would be more beneficial than the supercharger. To this end, the unsupercharged engine was removed from the Kirkham-Williams Racer as the aircraft was disassembled in the NAF around early 1928. The engine was returned to Packard for modifications. A new aircraft, the Williams Mercury Racer, was to be built, and the first X-2775 engine with the new gear reduction and other modifications would power the machine.

A planetary (epicyclic) gear reduction was built by the Allison Engineering Company in Indianapolis, Indiana. This gear reduction mounted to the front of the engine and turned the propeller at .677 crankshaft speed. Other modifications to the X-2775 included using cylinders and valve housings from an inverted 3A-1500 (the latest V-1500) engine and revising the induction and ignition systems.

The new cylinders increased the engine’s compression (most likely to 7.0 to 1) and had provisions for two spark plugs on both sides of the cylinder. Still, only two spark plugs were used, with one on each side of the cylinder. The new induction was a ram-air system with inlets right behind the propeller. The air flowed into a manifold located deep in the cylinder bank’s Vee. Two groups of two carburetors were mounted to the manifold. Each carburetor distributed the air/fuel mixture to a short manifold that fed three cylinders. The revised ignition system used two magnetos and did away with battery power. The magnetos were mounted to the rear of the engine and driven from the main timing gear. The improved X-2775 was occasionally referred to as the 2A-2775, but it mostly retained the same 1A-2775 Packard designation of its original configuration. The geared X-2775 produced 1,300 hp (969 kW) at 2,700 rpm and weighed around 1,513 lb (686 kg). The gear reduction added about 3 in (76 mm) to the engine, resulting in an overall length of 80.5 in (2.04 m). The width was unchanged at 28.3 in (.72 m), but the revised induction system reduced the engine height slightly to 43.25 in (1.10 m).

Packard X-2775 NASM front

The revised X-2775 took advantage of ram-air induction. Intakes directly behind the Williams Mercury Racer’s spinner fed air into manifolds at the base of the cylinder Vees. Note the spark plugs on both sides of the cylinders. (NASM image)

The updated X-2775 engine was installed in the Williams Mercury Racer in July 1929. In early August, flight testing was attempted on Chesapeake Bay near the Naval Academy in Annapolis, Maryland. While the aircraft was recorded at 106 mph (171 km/h) on the water, it could not lift off. The Williams Mercury Racer was known to be overweight, and there were questions about its float design. The trouble with the racer caused it to be withdrawn from the Schneider Trophy contest, scheduled to start on 6 September in Calshot, England. Later, it was found that the Williams Mercury Racer was some 880 lb (399 kg), or 21%, overweight. Some additional work was done on the aircraft, but no further attempts at flight were made.

Of the original X-2775, Woolson stated that the engine ran for some 30 hours, and at no time was mechanical trouble experienced or any adjustments made. Williams made some comments about the X-2775 losing power, but he otherwise seemed satisfied with the engine and did not report any specific issues. Assistant Secretary of the Navy for Aeronautics David S. Ingalls did not make any negative comments about the engine, but he said Commander Ralph Downs Weyerbacher of the NAF felt that the engine was not satisfactory. However, the basis for Weyerbacher’s opinion has not been found.

There were essentially no X-2775 test engines. Only two engines were made, and the second engine was never installed in any aircraft. The very first X-2775 built was installed in the Kirkham-Williams Racer, and the majority of the issues encounter seemed to come from the aircraft, and not the engine. This scenario repeated itself two years later with the Williams Mercury Racer. The X-2775 did not have any issues propelling the updated racer at over 100 mph (161 km/h) on the surface of the water, but it was the aircraft that was overweight and unable to take flight. If the engine were significantly flawed, it would not have survived its time in the Kirkham-Williams Racer, have been subsequently modified, and then installed in the Williams Mercury Racer. This same engine, Serial No. 1, was preserved and is in storage at the Smithsonian National Air and Space Museum.

Packard offered to build additional X-2775 engines for anyone willing to spend $35,000, but no orders were placed. In the late 1930s, Packard investigated building an updated X-2775 as the 2A-2775. The 2A-2775 was listed as a supercharged engine that produced 1,900 hp (1,417 kW) at 2,800 rpm and weighed 1,722 lb (781 kg). Some sources indicate the engine was built, although no pictures or test data have been found.

Packard X-2775 NASM top

Top view of the X-2775 illustrates the two sets of two carburetors, with each carburetor attached to a manifold for three cylinders. The intake manifold can be seen running under the carburetors. (NASM image)

“The Packard X 24-Cylinder 1500-Hp. Water-Cooled Aircraft Engine” by L. M. Woolson S.A.E. Transactions 1928 Part II. (1928)
“Internal Combustion Engine” US patent 1,889,583 by Lionel M, Woolson (granted 29 November 1932)
“Valve-Operating Mechanism” US patent 1,695,726 by Lionel M, Woolson (granted 18 December 1928)
“Lieut. Alford J. Williams, Jr.—Fast Pursuit and Bombing Planes” Hearings Before a Subcommittee of the Committee on Naval Affairs, United States Senate, Seventy-first Congress, second session, on S. Res. 235 (8, 9, and 10 April 1930)
“Packard “X” Type Aircraft Engine is Largest in World” Automotive Industries (8 October 1927)
Master Motor Builders by Robert J. Neal (2000)
Packards at Speed by Robert J. Neal (1995)
Jane’s All the World’s Aircraft 1929 by C. G. Gray (1929)

Kirkham-Williams Racer no cowl

Kirkham-Williams Seaplane Racer (1927)

By William Pearce

Lt. Alford Joseph Williams was an officer in the United States Navy and a major proponent of aviation. Williams believed that air racing contributed directly to the development of front-line fighter aircraft. In 1923, Williams won the Pulitzer Trophy race and later established a new 3 km (1.9 mi) absolute speed record at 266.59 mph (429.04 km/h). In 1925, Williams finished second in the Pulitzer race, but his main disappointment was not being selected as a race pilot for the Schneider Trophy team. Williams was also not selected for the 1926 Schneider team. That year was a particularly bad showing from the United States despite its advantage of hosting the Schneider contest.

Kirkham-Williams Racer front

The Kirkham-Williams Racer was built to compete in the 1927 Schneider Trophy contest and to capture the world speed record. Note how the large Packard X-24 engine dictated the shape of the aircraft.

Williams could see that racing was not a priority for the US military and decided to take matters into his own hands. In late 1926, Williams sought the support of investors to build a private venture Schneider racer. Since the US had won the Schneider Trophy two out of the last three races, another win would mean permanent retention of the trophy. Williams received further support from various departments in the US Navy, and the Packard Motor Car Company (Packard) was willing to design a new engine provided the Navy paid for it. On 9 February 1927, the US government officially announced that it would not be sending a team to compete in the 1927 Schneider race, held in Venice, Italy. The plans that Williams, the Navy, and Packard had implemented moved forward, and a syndicate to fund the private entry racer was announced on 24 March 1927. The Mercury Flying Corporation (MFC) was formed by patriotic sportsmen for the purpose of building the racer to compete in the 1927 Schneider Trophy contest, with Williams as the pilot.

Although the US government was not directly supporting MFC’s efforts, the US Navy was willing to lend indirect support by transporting the racer to Italy and providing a Packard X-2775 engine for the project. The X-2775 (Packard model 1A-2775) was a 1,200 hp (895 kW), water-cooled, X-24 engine that had been under development by Packard since 1926. The engine was a result of the talks initiated by Williams for a power plant intended specifically for a race aircraft. Ultimately, the engine was covered under a Navy contract. The X-2775 was one of the most powerful engines available at the time.

Kirkham-Williams Racer wing radiator

The racer had some 690 sq ft (64.1 sq m) of surface radiators covering its wings. Fluid flowed from a distributor line at the wing’s leading edge, through the tubes, and into a collector line at the wing’s trailing edge. Tests later indicated that the protruding radiator tubes doubled the drag of the wings.

Williams had decided that the racer should be designed along the same lines as previous Schneider racers built by the Curtiss Aeroplane and Motor Company (Curtiss). MFC contracted the Kirkham Products Corporation (Kirkham) to design and construct the racer. Kirkham’s founder was engineer and former Curtiss employee Charles K. Kirkham, and a number of other former Curtiss employees worked for the company, such as Harry Booth and Arthur Thurston. Booth and Thurston had been closely involved with the racers built at Curtiss. The aircraft was named the Kirkham-Williams Racer, but it was also referred to as the Kirkham-Packard Racer, Kirkham X, and Mercury X.

The Kirkham-Williams Racer was constructed in Kirkham’s faciality in Garden City, on Long Island, New York. The biplane aircraft consisted of a wooden fuselage built around the 24-cylinder Packard engine. The engine mount, firewall, and cowling were made of metal. The upper and lower surfaces of the wooden wings were covered with longitudinal brass tubes to act as surface radiators for cooling the engine’s water and oil. The specially-drawn tubes had an inverted T cross section and protruded about .344 in (8.73 mm) above the wing, creating a corrugated surface. The tubes were .25 in (6.35 mm) wide at their base and .009 in (.23 mm) thick. Around 12,000 ft (3,658 m) of tubing was used, and the oil cooler was positioned on the outer panel of the lower right wing. The water or oil flowed from the wing’s leading edge to a collector at the trailing edge. The aircraft’s twin floats were also made from wood and housed the racer’s main fuel tanks. The floats were attached by steel supports that were covered with streamlined aluminum fairings. The forward float supports were mounted directly to special pads on the engine. The cockpit was positioned behind the upper wing, and a headrest was faired back along the top of the fuselage into the vertical stabilizer. A framed windscreen protected the pilot. A small ventral fin extended below the aircraft’s tail.

Kirkham-Williams Racer starter

The Packard X-2775 engine barely fit into the racer. The engine cowling mounted to arched supports running from the cylinder banks to a ring around the propeller shaft. The Hucks-style starter, powered by four electric motors, is connected to the propeller hub. Note that the forward float strut is mounted to the engine’s crankcase.

The Kirkham-Williams Racer had an overall length of 26 ft 9 in (8.15 m). The fuselage was 22 ft 9 in (6.93 m) long, and the floats were 21 ft 3 in long (6.48 m). The upper wing had a span of 29 ft 10 in (9.09 m), and the lower wing’s span was 24 ft 3 in (7.39 m). The racer was 10 ft 9 in (3.28 m) tall and weighed 4,000 lb (1,814 kg) empty and 4,600 lb (2,087 kg) fully loaded. The aircraft carried 60 gallons (227 L) of fuel, 35 gallons (132 L) of water, and 15 gallons (57 L) of oil. The direct-drive Packard engine turned a two-blade, ground-adjustable, metal propeller that was 8 ft 6 in (2.59 m) in diameter and built by Hamilton Standard. A Hucks-style starter driven by four electric motors engaged the propeller hub to start the engine. Carburetor air intakes were positioned in the upper and lower engine Vees and were basically flush with the cowling’s surface.

Packard was involved with the aircraft’s construction, and Williams was involved with the engine’s development. The Kirkham-Williams Racer was finished in mid-July 1927 and transported later that month to Manhassest Bay, on the north side of Long Island. Weather delayed the first tests until 31 July. Taxi tests revealed that the float design was flawed and caused a large amount of spray to cover the aircraft and cockpit. The spray resulted in damage to the propeller during a high-speed taxi test. In addition, the aircraft was around 450 lb (204 kg) overweight.

Kirkham-Williams Racer launch

Lt. Al Williams prepares the racer for a test on Manhassest Bay. The cockpit was designed around Williams, and he was the only one to taxi or fly the aircraft. Note the support running between the vertical and horizontal stabilizers.

With the Schneider race just over a month away, little time was left to properly test the aircraft and transport it halfway around the world. Williams requested a postponement of the Schneider race for one month, but the British contingent declined the request. To make matters worse, Williams had been very optimistic about the aircraft’s test schedule and repeatedly promised an attempt on the world speed record. Issues with the Kirkham-Williams Racer resulted in a continual push-back of Williams’ proposed speed flights.

With a repaired propeller and new floats, the Kirkham-Williams Racer was ready for additional tests on 16 August. An oil leak and air in the water-cooling system caused Williams to cancel the day’s activities before any real testing had been done. On 17 August, high-speed taxi tests were finally sufficiently completed. Williams announced that the Kirkham-Williams Racer’s first flight would be the following day, but unfavorable weather caused that date to be pushed back. The racer’s first flight was on 25 August, and it should be noted that this was the first flight for the X-2775 engine as well. Some sources state that Williams made two speed runs at an estimated 250 mph (402 km/h). However, Williams stated that no speed runs were attempted on the first flight. While 250 mph (402 km/h) is an impressive speed for the time, it was most likely an estimation made by observers and not achieved over a set course. The second flight that day was cut short because of engine cooling issues caused by air in the cooling system.

Kirkham-Williams Racer runup

Williams is in the cockpit running up the X-2775 engine. The registration X-648 has been applied to the tail. The fuselage was painted blue, with the wings, floats, and rudder painted gold. Note the rather imperfect finish of the fuselage, just before the tail.

Unfavorable weather resulted in more delays, and it was not until 29 August that Williams was able to take the Kirkham-Williams Racer up for another flight during a brief break between two storm fronts. Williams made a high-speed run, and the racer was unofficially timed at 275 mph (443 km/h). Later, Williams would say the speed was probably around 269 mph (433 km/h), but he and others felt the aircraft was capable of 290 mph (467 km/h). Weather again caused delays, and three takeoff attempts on 3 September had to be aborted on account of pleasure boats straying into the aircraft’s path and causing wakes.

On 4 September, a good, extended flight was made, after which Williams reported the aircraft was nose-heavy and became increasingly destabilized at speeds above 200 mph. The issue was with the orientation of the floats. Modifications were made, and the aircraft flew again on 6 September. Williams reported improved handling, but some issues remained. The Navy had held the cruiser USS Trenton at the Brooklyn Navy Yard with the intention of transporting the Kirkham-Williams Racer to Italy in time for the Schneider contest, which was to start on 23 September. However, Williams cancelled any attempts to make the Schneider race on 9 September, citing the nose-heaviness and also float vibrations.

Kirkham-Williams Racer no cowl

Williams stands on the float, with work going on presumably to clear air from the cooling system, which was a reoccurring issue. The copper radiators covered almost all of the wing’s surface area. Note that the interplane struts protruded slightly above the wings.

During the time period above, it was felt that the Kirkham-Williams Racer may not have been competitive, and Packard was asked to build a more powerful engine. In the span of 10 weeks, Packard designed, constructed, and tested a supercharged X-2775 engine. The Roots-type supercharger was installed on the front of the engine and driven from the propeller shaft. Liberal tolerances were used because of the lack of time, and the supercharger generated only 3.78 psi (.26 bar) of boost. The supercharged engine produced 1,300 hp (696 kW), which was only a slight power increase. The engine was not installed, because the minor gain in power was offset by the added weight and complexity of the supercharger system.

With the Schneider race out of reach, the Kirkham-Williams Racer was converted to a landplane with the intent to set a new world speed record. The floats were removed, and two main wheels attached to streamlined struts were installed under the engine. A tail skid replaced the small fin under the aircraft’s rudder. In addition, the X-2775 engine was fitted with a new cowling and spinner that gave the aircraft a more streamlined nose.

Kirkham-Williams Racer landplane front

Williams reported making four emergency landings in the racer at Mitchel Field, but the causes of the forced landings have not been found. The aircraft was fitted with the same direct-drive X-2775 engine as the seaplane. The intake of the upper Vee engine section can just be seen above the cowling.

The modifications to the Kirkham-Williams Racer were completed by late October 1927, and the aircraft was taken to Mitchel Field on Long Island, New York. Williams’ initial tests found the plane heavy with a landing speed of around 100 mph (161 km/h). Williams felt Mitchel Field was not an ideal place for experimental work with the aircraft, but the MFC did not have funds to seek a better location. Williams ended up making four forced landings at Mitchel Field in the Kirkham-Williams Racer.

On 6 November, Williams flew the aircraft over a 3 km (1.9 mi) course and was unofficially timed at 322.42 mph (518.88 km/h). This speed was significantly faster than the then-current records, which were 278.37 mph (447.99 km/h) set by Florentin Bonnet on 11 November 1924 for landplanes, and an absolute record of 297.70 mph (479.10 km/h) set by Mario de Bernardi on 4 November 1927. Some were skeptical of Williams’ speed, especially since it was achieved in only one direction and with the wind reportedly blowing at 40 mph (64 km/h). Williams announced that an official attempt on the record would soon be made, but no further flights of the Kirkham-Williams Racer were recorded. Later, Williams stated that the racer’s still-air speed on the 6 November 1927 run was around 287 mph (462 km/h), which was much lower than anticipated.

Williams had the aircraft disassembled and shipped to the Naval Aircraft Factory (NAF) in Philadelphia, Pennsylvania to further evaluate ways to improve the racer’s speed. A section of the wing was removed and tested by the National Advisory Committee for Aeronautics in their wind tunnel at Langley Field, Virginia. The test results indicated that the corrugated surface radiators decreased lift, doubled drag, and slowed the aircraft by some 20 mph (32 km/h). While at the NAF, the disassembled Kirkham-Williams Racer was used as the basis for Williams’ 1929 high-speed aircraft—the Williams Mercury Racer.

Kirkham-Williams Racer landplane

In landplane form, the Kirkham-Williams Racer had a more streamlined nose and an added tailskid. The machine looked every bit a racer and was one of the fastest aircraft in the world, even at only 287 mph.

Schneider Trophy Seaplanes and Flying Boats by Ralph Pegram (2012)
Schneider Trophy Racers by Robert Hirsch (1993)
Master Motor Builders by Robert J. Neal (2000)
Racing Planes and Air Races Volume II 1924–1931 by Reed Kinert (1967)
Full Scale Investigation of the Drag of a Wing Radiator by Fred E. Weick (September 1929)
“Lieut. Williams’ Racing Seaplane” by George F. McLaughlin, Aero Digest (September 1927)
“Lieut. Alford J. Williams, Jr.—Fast Pursuit and Bombing Planes” Hearings Before a Subcommittee of the Committee on Naval Affairs, United States Senate, Seventy-first Congress, second session, on S. Res. 235 (8, 9, and 10 April 1930)

Daimler-Benz DB 604

Daimler-Benz DB 604 X-24 Aircraft Engine

By William Pearce

In July 1939, the RLM (Reichsluftfahrtministerium, or Germany Air Ministry) issued specifications for a new medium bomber capable of high-speeds. Originally known as Kampfflugzeug B (Warplane B), the aircraft proposal was eventually renamed Bomber B. The Bomber B specification requested an aircraft that could carry a 2,000 kg (4,410 lb) bomb load 3,600 km (2,237 mi) and have a top speed of 600 km/h (373 mph). To power the Bomber B aircraft, the RLM requested engine designs from BMW, Junkers, and Daimler-Benz. The respective companies responded with the BMW 802, the Junkers Jumo 222, and the Daimler-Benz DB 604.

Daimler-Benz DB 604

The Daimler-Benz DB 604 was designed in 1939 to power the next generation of German fast bombers under the Bomber B program. However, the engine was not selected for production.

The DB 604 was an all-new, liquid-cooled, 24-cylinder engine. Four banks of six cylinders were arranged in an “X” configuration with each cylinder bank spaced at 90 degrees. The X-24 engine consisted of a two-piece aluminum alloy crankcase split horizontally at its center. The engine’s single crankshaft had six crankpins that were spaced at 0 degrees, 120 degrees, 240 degrees, 240 degrees, 120 degrees, and 0 degrees. This arrangement resulted in cylinders firing evenly at every 30 degrees of crankshaft rotation. Attached to each crankpin was a master connecting rod that accommodated three articulated connecting rods. A gear reduction at the front of the engine turned the propeller at .334 crankshaft speed. A supercharger mounted to the rear of the engine had an upper and a lower outlet. Each outlet was connected to two intake manifolds that ran along the inner Vee side of the cylinder banks.

The DB 604’s fuel system was located in the upper and lower Vees of the engine and consisted of fuel injection pumps and individual fuel injectors for each cylinder. Each cylinder had two intake and two exhaust valves, all of which were actuated by a single overhead camshaft. The camshaft for each cylinder bank was driven via a vertical shaft from the rear of the engine. The exhaust ports were positioned in the left and right Vees, as were the two spark plugs per cylinder. The spark plugs were fired by two magnetos positioned in the left and right Vees and mounted to the propeller gear reduction housing.

Daimler-Benz DB 604 side

The DB 604 was a rather compact design. A magneto can be seen at the front of the engine between the exhaust ports of the upper and lower cylinder banks. Note the supercharger at the rear of the engine. (Evžen Všetečka image via

The DB 604 had a 5.31 in (135 mm) bore and stroke and displaced 2,830 cu in (46.4 L). The engine had a 7.0 to 1 compression ratio and weighed 2,381 lb (1,080 kg). The DB 604 prototype was first run in late 1939. The first engine produced 2,313 hp (1,725 kW) at 3,200 rpm. This engine may have had a single-speed supercharger. The DB 604 A and DB 604 B engines were produced quickly after the first prototype. These engines had a two-stage supercharger that provided 6.17 psi (.43 bar) of boost. The difference between A and B versions was the rotation of the engine’s crankshaft. The DB 604 A/B had a maximum output at 3,200 rpm of 2,660 hp (1,984 kW) at sea level and 2,410 hp (1,797 kW) at 20,600 ft (6,279 m). The engine’s maximum continuous output was 2,270 hp (1,693 kW) at sea level and 2,120 hp (1,581 kW) at 21,000 ft (6,401 m), both figures at 3,000 rpm. Maximum cruise power was at 2,800 rpm, with the engine producing 1,830 hp (1,365 kW) at sea level and 1,860 hp (1,387 kW) at 20,000 ft (6,096 m). The DB 604 was flight tested in a Junkers Ju 52 trimotor transport, but it is not clear which version of the engine was tested. At least five DB 604 engines were made.

The Bomber B proposals that moved forward as prototypes were the Dornier Do 317, Focke-Wulf Fw 191, and Junkers Ju 288. Despite the DB 604 showing some promise, the RLM chose the Jumo 222, and work on the DB 604 was stopped in September 1942. No records have been found that detail the DB 604’s reliability, and many other X-24 aircraft engine designs were prone to failure. The sole surviving Daimler-Benz DB 604 engine is on display at the Flugausstellung L.+ P. Junior museum in Hermeskeil, Germany.

Daimler-Benz DB 604 right

Some of the fuel injection equipment is just visible in the engine’s upper Vee. The sole surviving DB 604 engine is on display at the Flugausstellung L.+ P. Junior museum in Hermeskeil, Germany. (Evžen Všetečka image via

Ultimately, the Ju 288 was selected as the winner of the Bomber B program. Delays with the 2,500 hp (1,964 kW) Jumo 222 led to it being substituted with the 2,700 hp (2,013 kW) Daimler-Benz DB 606, and that engine was later replaced by the 2,950 hp (2,200 kW) DB 610. The DB 606 consisted of two DB 601 inverted V-12 engines coupled side-by-side, while the DB 610 was the same arrangement but with two DB 605 engines. The Ju 288 aircraft and the Jumo 222 engine never entered large-scale production.

An enlarged version of the DB 604 was contemplated, with the engine’s bore increased .2 in (5 mm) to 5.51 in (140 mm). This gave the engine a displacement of 3,044 cu in (49.9 L). The larger 90-degree, X-24 engine was very similar to the DB 604 but incorporated a three-speed, three-stage supercharger. The engine was forecasted to produce 3,450 hp (2,575 kW) at 36,089 ft (11,000 m). Development of the larger engine did not progress beyond the initial design phase.

Daimler-Benz DB 604 left

Despite a number of X-24 aircraft engines being made, none truly were produced beyond the prototype phase, and the DB 604 was no exception. Note that the two intake manifolds between the upper (and lower) cylinder banks were connected at the front of the engine to equalize pressure. (Evžen Všetečka image via

Flugmotoren und Strahltriebwerke by Kyrill von Gersdorff, et. al. (2007)
German Aero-Engine Development A.I.2.(g) Report No. 2360 by G. E. R. Proctor (22 June 1945)
Luftwaffe: Secret Bombers of the Third Reich by Dan Sharp (2016)
Jane’s All the World’s Aircraft 1945–46 by Leonard Bridgman (1946)

Lycoming XR-7755-3

Lycoming XR-7755 36-Cylinder Aircraft Engine

By William Pearce

Since 1933, the Lycoming Division of the Aviation Manufacturing Corporation had worked to create a high-power engine for the United States Armed Forces. Its first attempt was the 1,200 hp (895 kW), 12-cylinder O-1230, which was outclassed by the time it first flew in 1940. Lycoming’s second attempt was the 2,300 hp (1,715 kW), 24-cylinder XH-2470. The engine had shown some promise, but its performance was eclipsed by other engines when the XH-2470 was first flown in 1943. Lycoming set out to design an engine that was more powerful than any other and that would meet the power needs of future large aircraft.

Lycoming XR-7755-3

The Lycoming XR-7755 was the most powerful aircraft engine in the world when it was built. The XR-7755 was the culmination of Lycoming’s experience with radial and liquid-cooled engines. Conceived in 1943, such a large engine was not needed by the time it first ran in 1946.

In mid-1943, Lycoming engaged in talks with personnel from the US Army Air Force (AAF) at Wright Field, Ohio. Different sources list the involvement of the Air Materiel Command, Air Technical Service Command, and the Power Plant Lab. By December 1943, the engine concept had been solidified as a very large displacement, high-compression, liquid-cooled engine designed for optimum fuel economy and intended to power the next generation of very large aircraft. Lycoming’s experimental engine was designated XR-7755 and given the “Materiel, Experimental” code MX-434.

Clarence Wiegman headed the Lycoming XR-7755 design team. The engine consisted of nine banks of four inline cylinders positioned radially with 40-degrees of separation around a forged steel crankcase. This formed a 36-cylinder inline radial engine. The crankcase was made up of five sections, each split vertically through the cylinders. The crankcase sections were secured together by nine bolts that extended the length of the case. The individual steel cylinders each had their own water jacket. Each bank of four cylinders shared a common cast aluminum cylinder head. Each four-cylinder bank was secured to the crankcase by 16 long studs that passed through the cylinder head.

Lycoming XR-7755-3 stand

The worker gives some perspective to the XR-7755’s large size. However, the engine’s three-ton (2.74 t) weight is hard to imagine. The engine’s two magnetos and four distributors are visible on the front of the cylinder banks.

Each cylinder had one intake and one exhaust valve. Both valves were sodium cooled, with a hollow stem for the intake valve and a hollow stem and head for the exhaust valve. The valves for each bank of cylinders were actuated by a single overhead camshaft, driven via a vertical shaft at the front of the engine. Each camshaft had two sets of lobes for different valve timing—one lobe set was optimized for power and the other set for economic cruise. The camshafts shifted axially to engage the desired set of lobes. When the camshaft was shifted, the spark plug timing was automatically changed. Ignition was provided by two magnetos and four distributors. Each unit was camshaft-driven and mounted to the front of a separate cylinder bank. The spark plug leads passed through the valve covers and to the spark plugs, which were positioned in opposite corners of each cylinder.

Lycoming XR-7755-1 test stand

The XR-7755-1 on the test stand with its single propeller shaft. With each of the 36-cylinders displacing 215 cu in (3.5 L), witnessing the XR-7755 run was most likely a very memorable event. Note the robust upper engine support.

The crankshaft had four crankpins, each spaced at 180 degrees. The crankshaft was made up of five sections and assembled through the four one-piece master connecting rods. The crankshaft sections were joined at the rear of the crankpin via face splines and secured by four bolts. Five roller bearings supported the crankshaft in the crankcase.

At the rear of the engine was a single-stage, single-speed supercharger. The supercharger’s impeller was 14.4 in (366 mm) in diameter and spun at six times crankshaft speed. The supercharger fed air to nine intake manifolds, each mating with the right side of a cylinder bank. Fuel was provided to the cylinder via either a carburetor or fuel injection. Individual exhaust stacks were attached to the left side of each cylinder. Provisions were also made to incorporate two turbosuperchargers.

Although a single rotation engine was tested, the engine accommodated contra-rotating propellers using SAE #60L-80 spline shafts. The inner shaft rotated counterclockwise, and the outer shaft rotated clockwise. A two-speed propeller gear reduction was hydraulically shifted and used planetary gears. A .2460 reduction was available for high engine speeds, and a .3536 reduction was used for cruise operations with low engine rpm.

Lycoming XR-7755-1 Maxwell and Cervinsky

Lycoming workers Red Maxwell (left) and Paul Cervinsky (right) pose next to the completed XR-7755-1. It appears Maxwell is ready for the big engine to be stuffed in an airframe to see what it will do. Note the ring on the nose case and around the propeller shaft. No other image found has that ring.

The XR-7755 had a 6.375 in (162 mm) bore and a 6.75 in (171 mm) stroke. The engine displaced 7,756 cu in (127.1 L) and had a compression ratio of 8.5 to 1. The XR-7755 produced 5,000 hp (3,728 kW) at 2,600 rpm (.2460 propeller gear) for takeoff, 4,000 hp (2,983 kW) at 2,300 rpm (.2460 propeller gear) for normal operation, and 3,000 hp (2,237 kW) at 2,100 rpm (.3536 propeller gear) for cruise power. Specific fuel consumption at normal cruise power was .43 lb/hp/hr (262 g/kW/hr), but the rate dropped to around .38 lb/hp/hr (231 g/kW/hr) at low cruise power of around 1,500 hp (1,119 kW). The engine was 61.0 in (1.55 m) in diameter, 66.25 in (1.68 m) tall, and 121.35 in (3.08 m) long. The XR-7755 weighed 6,050 lb (2,744 kg).

Lycoming XR-7755 ad Dec 1946

Lycoming ad from December 1946 featuring the XR-7755. If the engine was not going to go into production, Lycoming might as well get some press out of it. One can only wonder how those responsible for marketing imagined the huge, liquid-cooled engine would factor into the decision-making process of a person buying a small, air-cooled engine.

The XR-7755 was first run in July 1946. At the time, some 10,000 hours of single-cylinder testing had been completed. The Lycoming factory was located near a residential area. Reportedly, a nearby grocery store’s canned goods would vibrate off the shelves as the XR-7755 underwent high-power tests. A good neighbor, Lycoming went to the store and installed strips on the shelf edges to keep the cans from falling. At takeoff power, the XR-7755’s fuel consumption was 580 gallons (2,196 L) per hour, or 20.62 fl oz (.61 L) per second. The engine’s coolant pump flowed 750 gpm (2,839 l/m) to dissipate 95,600 BTUs (23,956 kcal) per minute. The flow rate was enough to fill a 55 gallon (208 L) drum every 4.4 seconds. The oil pump circulated 71 gpm (269 l/m) at 100 psi (6.89 bar). Lycoming had an optimistic opinion of the engine and believed that an output of 7,000 hp (5,220 kW) was possible.

Most sources indicate that two XR-7755 engines were built: an XR-7755-1 with a single rotation propeller shaft and an XR-7755-3 with a contra-rotating propeller shaft. Both of these engines used carburetors. There is some indication, including the recollections of those who had family members involved with the project, that a third engine was built: an XR-7755-5 with fuel injection. Reportedly, the -1 underwent a 50-hour test run, but the results are not known. The -3 was delivered to the AAF in 1946, but it is unlikely this engine underwent much testing. It is not clear what happened to the -5, if it was completed. By the time the XR-7755 had run, the concept of an aircraft larger than the Convair B-36 Peacemaker had fallen out of favor, as had the idea of modifying the B-36 with larger piston engines. Rather, jets would be used to improve performance of the aircraft. There was no application for the XR-7755 in a post-war world with the performance of jet aircraft quickly being realized. The XR-7755 never flew.

Lycoming XR-7755 AAF Fair Oct 1945

The XR-7755 on display at the Army Air Forces Fair held at Wright Field, Ohio in October 1945. Note what appears to be a mockup of the contra-rotating propeller shafts.

One curious anomaly in the XR-7755’s story is an appearance of the engine at the Army Air Forces Fair held at Wright Field, Ohio in October 1945. This predates the engine’s run date and its supposed delivery to the AAF. However, the engine appears to have a mockup of its contra-rotating propeller shafts installed. It would seem that the engine is not complete and was shipped the 460 miles (740 km) from Lycoming’s factory in Williamsport, Pennsylvania to Dayton, Ohio to be displayed with other unusual treasures from the war. Presumably, the engine was returned to Lycoming after the show and was subsequently completed and tested in 1946.

The sole XR-7755-3 has been preserved by the Smithsonian National Air and Space Museum and is on display in the Steven F. Udvar-Hazy Center in Chantilly, Virginia. Many consider the XR-7755 the largest aircraft engine ever built. However, the Soviet IAM M-44 (8,107 cu in / 132.8 L) of 1933 and Yakovlev M-501 (8,760 cu in / 143.6 L) of 1952 were larger engines. At the time it first ran, the XR-7755 was the world’s most powerful reciprocating aircraft engine.

Lycoming XR-7755-3 NASM

The restored XR-7755-3 on display in the Steven F. Udvar-Hazy Center of the Smithsonian National Air and Space Museum. The bottom of the engine is on the left, marked by the drain tube from the gear reduction housing and the sump built into the valve cover. Note the two spark plug leads for each cylinder passing through opposite sides of the valve covers. (Sanjay Acharya image via Wikimedia Commons)

“5,000-Hp. Lycoming Revealed” by J. H. Carpenter, Aviation (December 1946)
“The Evolution of Reciprocating Engines at Lycoming” by A. E. Light, AIAA: Evolution of Aircraft/Aerospace Structures and Materials Symposium (24–25 April 1985)
Aircraft Engines of the World 1948 by Paul Wilkinson (1948)
The History of North American Small Gas Turbine Aircraft Engines by Richard A. Leyes II and William A. Fleming (1999)
Studebaker’s XH-9350 and Their Other Aircraft Engines by William Pearce (to be published)

Lycoming XH-2470 side

Lycoming XH-2470 24-Cylinder Aircraft Engine

By William Pearce

The Lycoming Division of the Aviation Manufacturing Corporation was located in Williamsport (Lycoming County), Pennsylvania. The company had started producing aircraft engines in the late 1920s. In 1937, Lycoming became aware that its most powerful engine to date, the 12-cylinder O-1230, would not produce the power needed for future frontline military aircraft. Development of the O-1230 engine started in 1933, but the anticipated power needs of state-of-the-art aircraft were beyond what the 1,200 hp (895 kW) engine could provide. Lycoming moved quickly to apply knowledge gained from the O-1230 to a new aircraft engine.

Lycoming XH-2470 side

The design of the Lycoming XH-2470 started with the concept of mounting two O-1230 engines to a common crankcase. Note that the propeller shaft is raised above the centerline of the engine.

Lycoming started the design of its new engine in 1938, and detailed design work commenced in mid-1939. The 24-cylinder engine had an H-configuration and consisted of components from two O-1230 engines combined with a new crankcase. The new two-piece aluminum crankcase was split horizontally and accommodated a left and right crankshaft. Each crankshaft served two banks of six cylinders, with one bank above the engine and the other bank below. Fork-and-blade connecting rods were used, with the forked rods serving the lower cylinders. The H-24 engine was designated XH-2470 and given the “Materiel, Experimental” code MX-211. The US Army Air Corps (AAC) initially felt that the engine was too small, but the US Navy supported the design. The Navy ordered a single prototype engine on 11 December 1939, and the AAC started to show some interest in the engine in 1940.

The Lycoming XH-2470 utilized individual cylinders that consisted of a steel barrel screwed and shrunk into an aluminum head. The liquid-cooled cylinder was surrounded by a steel water jacket. The aluminum head had a hemispherical combustion chamber with one intake valve and one sodium-cooled exhaust valve. A cam box was mounted to the top of each cylinder bank, and each cam box contained a single camshaft that was shaft-driven from the rear of the engine.

Lycoming XH-2740 top

Top view of the XH-2470 shows the intake manifold positioned between the cylinder banks. The narrow engine could be installed horizontally (on its side) in an aircraft’s wing. Bell and Northrop pursued this installation for project aircraft, but the designs were not built.

A downdraft carburetor fed fuel into the supercharger’s 12 in (305 mm) diameter impeller mounted to the rear of the engine. Lycoming had experimented with direct fuel injection on test cylinders and planned to have fuel injection available for the XH-2470, but it is unlikely that any complete engines ever used fuel injection. The XH-2470-1, -3, and -7 engines had a single-speed, single-stage supercharger that was driven at 6.142 times crankshaft speed. The XH-2470-5 had a two-speed supercharger that was driven at 6.06 and 7.88 times crankshaft speed. Intake manifolds ran between the upper and lower cylinder banks. Depending on the installation, exhaust gases were either expelled from the outer side of the cylinders via individual stacks or collected in a manifold common to each cylinder bank. Provisions were made for the engine to accommodate a turbosupercharger.

The XH-2470-1, -3, and -5 were available with a single-rotation propeller shaft using a SAE #60 spline shaft. The -1 and -3 had a .38 gear reduction. The -5 was listed as having a two-speed reduction, but the ratios have not been found. The XH-2470-7 had contra-rotating propeller shafts and a two-speed gear reduction with speeds of .433 and .321. The contra-rotating shafts were SAE #40-60 splines, with the inner shaft rotating counterclockwise and the outer shaft rotating clockwise. The gear reduction for all engines was achieved through spur gears, and the propeller shaft was positioned above the engine’s centerline. The engine could be installed in either a vertical or horizontal position.

Lycoming XH-2470-2 drawing

The XH-2470-2 and -4 were engines intended for the Navy. The -2 was similar to the AAF’s engine with a single propeller shaft. The -4 had contra-rotating propeller shafts and was similar to the -7.

The H-2470 had a 5.25 in (133 mm) bore and a 4.75 in (121 mm) stroke. The engine’s total displacement was 2,468 cu in (40.4 L), and it had a 6.5 to 1 compression ratio. The H-2470 produced 2,300 hp (1,715 kW) at 3,300 rpm at 1,500 ft (457 m) for takeoff, 2,000 hp (1,491 kW) at 3,100 rpm at 3,500 ft (1,067 kW) for normal operation, and 1,300 hp (969 kW) at 2,400 rpm at 15,000 ft (4,572 m) for cruise operation. In addition, the two-speed supercharged engine could achieve 1,900 hp (1,417 kW) at 3,300 rpm at 15,000 ft (4,572 m) for emergency power and 1,750 hp (1,305 kW) at 3,100 rpm at 15,000 ft (4,572 m) for normal operation. The XH-2470 had a 3,720-rpm overspeed limit for diving operations. The single-rotation engines were 89.9 in (2.28 m) long, 30.5 in (.77 m) wide, 50.3 in (1.28 m) tall, and weighed 2,430 lb (1,102 kg). The contra-rotating XH-2470-7 was approximately 114 in (2.90 m) long and weighed 2,600 lb (1,179 kg).

Before the XH-2470 had even run, Lycoming proposed a variant of the engine to satisfy the AAC’s Request for Data R40-D, which was issued on 6 March 1940. R40-D sought the design of a 4,000 to 5,500 hp (2,983 to 4,101 kW) aircraft engine for use in long-range bombers. Lycoming proposed coupling two H-2470 engines together, creating a 48-cylinder XH-4940. The XH-4940 would produce 4,800 hp (3,579 kW) at 3,100 rpm up to 8,500 ft (2,591 m) with the aid of a single-speed, single-stage supercharger. The engine had a projected maximum speed of 3,400 rpm and would weigh 6,200 lb (2,812 kg). The AAC’s R40-D ended up going nowhere, and the request was cancelled in mid-1940.

Lycoming XH-2470 test stand

An XH-2470 mounted on a test stand with a tractor propeller. Installed in the XP-54 as a pusher; the blades on the XH-2470 had their angle reversed. Note the individual exhaust stacks.

The XH-2470 was first run in July 1940. The engine was proposed for the Curtiss XF14C Naval fighter and the Vultee XP-54 Swoose Goose AAC fighter. The XP-54’s original power plant was the Pratt & Whitney X-1800 (XH-2240 / XH-2600), but development of this engine stopped in October 1940. It was the cancellation of the X-1800 that led to the AAC’s interest in the XH-2470, and the AAC ordered 25 (later increased to 50) engines in October 1940. The AAC was also interested in potentially using the XH-2470 to power the Lockheed XP-58 Chain Lightning. Bell and Northrop also expressed interest in the engine for future projects.

The XH-2470 completed a Navy acceptance test in April 1941. At the time, the XF14C and XP-54 prototypes were in the detailed design stage. However, the Army Air Force (AAF—the AAC had changed its name in June 1941) continued to alter the XP-54 requirements throughout 1941. Added to the Vultee project were Turbosuperchargers, a pressurized cockpit, and the option of the Wright R-2160 Tornado engine. It was not until 1942 when R-2160 development was seriously behind schedule that the engine was dropped from the XP-54 and a more focused installation of the XH-2470 was presented. An XH-2470-7 engine with contra-rotating propellers was intended for the XP-54, but a single rotation engine was substituted because of delays with the contra-rotating gearbox. The AAF specified two Wright TSBB turbosuperchargers for the first XP-54 prototype and a single, experimental General Electric (GE) XCM turbosupercharger for the second aircraft. Reportedly, the Navy ordered 100 XH-2470 engines in May 1942. However, this may have been a total of 100 engines on order with 50 going to the AAF and 50 to the Navy.

The first XP-54 (41-1210) made its first flight on 15 January 1943, taking off from Muroc (now Edwards) Air Force Base, California. With the exception of the first flight, the XH-2470 engine installed in the XP-54 turned a 12 ft 2 in (3.71 m) Hamilton Standard propeller. Although the aircraft handled well, its development had suffered through constant changes in design and intended role. The aircraft underperformed, and the XH-2470 engine had some issues, such as oil foaming at high RPM or at altitudes above 20,000 ft (6,096 m).

Lycoming XH-2470 Vultee XP-54

The Vultee XP-54 was a very large aircraft. Even so, the installation of the XH-2470 appears to be quite cramped. Note the large exhaust manifold linking the engine to the turbosupercharger, which was positioned behind the cockpit.

The first XP-54 was flown to Wright Field, Ohio on 28 October 1943. After the next flight, a close inspection of the XH-2470 revealed some minor issues as well as damage to the supercharger impeller. The engine was removed and sent to Lycoming for repairs. The cost to fix the engine was more than the AAF was willing to pay, which showed the AAF’s lack of interest in the XH-2470 program. The first XP-54 was removed from flight status and used as a source of spare parts for the second XP-54 aircraft. The first XP-54 had completed 86 flights and accumulated 63.2 hours of flight time.

To make matters worse, the Navy cancelled its XH-2470 order in December 1943, deciding to power the XF14C with a turbosupercharged Wright R-3350 instead. Factors that influenced this change were the Navy’s long-standing preference for air-cooled engines, a shift of the XF14C’s role to that of a high-altitude fighter, issues with the XH-2470’s developmental progress, and doubts that the engine would be ready in time to see combat during World War II. At the same time, Lycoming had moved on to another aircraft engine project, the 36-cylinder XR-7755. Lycoming had invested over $1,000,000 of its own money into the XH-2470 engine.

Lycoming XH-2470 Vultee XP-54 top

The two exhaust outlets from the turbosupercharger protrude quite visibly behind the cockpit. The panel behind the exhaust was stainless steel, and hot exhaust burned the paint off the cowling on early flights. The upper cowling was later replaced with an unpainted stainless steel unit, and the rudders were painted around the same time. (Aerospace Legacy Foundation Archive image)

The second XP-54 prototype (42-108994, but incorrectly painted as 41-1211) made its first flight on 24 May 1944, taking off from Vultee Field. After at least three flights, the GE XCM turbosupercharger and the XH-2470 were removed from the aircraft. Some incompatibility between the turbosupercharger and engine had caused damage to both units. A new engine and turbosupercharger were installed, and the XP-54 flew again in December 1944. The second XP-54 made at least 10 flights, the last ending with an engine failure on 2 April 1945. The airframe had accumulated 10.7 hours of flight time.

At least one XH-2470 engine has been preserved. An XH-2470-7 is in storage at the Smithsonian National Air and Space Museum. The engine, which was never installed in any aircraft, has contra-rotating propellers and a two-speed gear reduction. The Smithsonian also lists an XH-2470-1 engine from the XP-54 in its inventory. However, no further evidence of this engine’s existence has been found.

The preserved XH-2470-7 is in storage at the Smithsonian National Air and Space Museum. Although the engine was never installed in any aircraft, at least it may be displayed one day. (NASM image)

The preserved XH-2470-7 is in storage at the Smithsonian National Air and Space Museum. Although the engine was never installed in any aircraft, at least it may be displayed one day. (NASM image)

Aircraft Engines of the World 1947 by Paul Wilkinson (1947)
American Secret Pusher Fighters of World War II by Gerald H. Balzer (2008)
Development of Aircraft Engines and Fuels by Robert Schlaifer and S. D. Heron (1950)
U.S. Experimental & Prototype Aircraft Projects Fighters 1939–1945 by Bill Norton (2008)
Experimental & Prototype U.S. Air Force Jet Fighters by Dennis R. Jenkins and Tony R. Landis (2008)
The History of North American Small Gas Turbine Aircraft Engines by Richard A. Leyes II and William A. Fleming (1999)
Studebaker’s XH-9350 and Their Other Aircraft Engines by William Pearce (to be published)
Preliminary Model Specification for Engine Aircraft Model XH-2470-4 for Opposite Rotating Propellers by Aviation Manufacturing Corporation Lycoming Division (18 April 1940)
“The Evolution of Reciprocating Engines at Lycoming” by A. E. Light, AIAA: Evolution of Aircraft/Aerospace Structures and Materials Symposium (24–25 April 1985)