The Radial Engine

The radial engine is an internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel. This configuration was very commonly used in aircraft engines before being superseded by turbo-shaft and turbojet engines. It is a reciprocating engine.

The cylinders are connected to the crankshaft with a master-and-articulating-rod assembly. One cylinder has a master rod with a direct attachment to the crankshaft. The remaining cylinders pin their connecting rods attachments to rings around the edge of the master rod (see animation). Four-stroke radials almost always have an odd number of cylinders, so that a consistent every-other-piston firing order can be maintained, providing smooth running.

Debate Of Use

Click on Picture to enlarge

A Continental radial engine, 1944

The debate about the merits of the radial vs. the inline continued throughout the 1930s, with both types seeing some use. The radial was more popular largely due to its simplicity, and most navy air arms had dedicated themselves to the radial because of its improved reliability for over-water flights and better power/weight ratio for aircraft carrier takeoffs. Although inline engines offer smaller frontal area than radials, inline engines require the added weight and complexity of cooling systems and are generally more vulnerable to battle damage.

The vast majority of radial-engineaircraft designed since the 1930s were also were fitted with NACA cowlings to reduce drag and to also enhance forward thrust by virtue of its airfoil effect.

The Radial Engine Principal

+ Larger Font | - Smaller Font

Inside The Radial Engine

Master-and-articulating-rod Assembly

The master-and-articulating-rod assembly is used on X-type engines, radial-type engines, and on some V-type engines. The master rod is similar to any other connecting rod except that it is constructed to provide for the attachment of the articulated rods on the big end.

The articulated rods are fastened by knuckle pins to a flange around the master rod. Each articulated connecting rod has a bushing of nonferrous metal, usually bronze, pressed or shrunk into place to serve as a knuckle-pin bearing. The knuckle pins may be held tightly in the master-rod holes by press fit and lock plates or they may be of the full-floating type.

If the big end of the master rod is made of two pieces, the cap and the rod, the crankshaft is made of one solid piece. on the other hand, if the rod is made of one piece, then the crankshaft may be of either two-piece or three-piece construction. Regardless of the type of construction, the usual bearing surfaces must be supplied.

It should be understood that the type of connecting rod used in an engine depends largely on the cylinder arrangement. If the cylinders are arranged in a line parallel to the crankshaft, the connecting rod is similar to that used in most automobile engines. However, certain types of aircraft engines have a system of connecting rods connected to the same crankshaft bearing, called an articulating connecting-rod assembly. The main rod or master rod joins one of the pistons with the crankshaft, and the other rods, called articulating rods or link rods, connect the other pistons to this same master connecting rod.

Inside The Radial Engine

Master-and-articulating-rod Assembly

Most often attributed to the American F.D.Farwell the rotary engine may have had an earlier beginning in a compressed-air engine worked out by the Australian pioneer Lawrence Hargrave some eight or nine years prior. It is certain, however, that the French brothers Seguin brought the engine into commercial and mechanical life based on the conceptions of one brother (Laurent). It was in 1907 that his 7-cyl rotary was born - and it came to be known as the Gnome. This engine was followed by a succession of designs by many manufacturers, most of which were successful.

We are accustomed to seeing someone swing the prop to start an engine. This was not always necessary because a crank could be engaged to the gear on the rear of the thrust plate and enough rotary motion could be generated to get the engine started. When you compare this with the starting of a radial or an inline the reason why rotaries started easier can be seen. In the case of the radial or inline, it was necessary to set the inards of the engine in motion. The rotary was started by rotating the engine. The mass of the rotary added to the starting function and assisted the effort. The rotary was its own inertial starter!

The rotary engine gained quick acceptance because of its remarkable power to weight ratio. The only comparable ratios came from the brilliant mind of an American, Charles Manly. He had, in the very early years of the 1900s, achieved P/W ratios that even rotaries did not match until 1916. His 5-cyl 4-stroke static radial gave a ratio of 2.4 lb per hp dry and 4.0 with all of its plumbing attached. How remarkable was his achievement can be seen in a comparison of the Wright's engine which delivered one hp per 15 lbs and the 1912 Gnome rotary of 80hp which had a 2.625 ratio. Manly did not produce his engine commercially: the brothers Seguin did.

Rotary Engine Theory

100 hp Gnome Monosoupe (The Science Museum, London)

That the rotary engine dominated the early years of aviation is evident - although there were some very fine engines extant such as the twins of Duthiel-Chalmers and Darracq, the Antoinette by Levavasseur, and those of Fiat.

The demise of the rotary came about for several reasons. Among the most important of these was the large rotating mass of the engine which produced gyroscopic forces. These forces had their useful features - if the pilot could master them before something happened to lessen his desire to fly. It provided the Sopwith Camel with remarkable turning power. However, the engine also delivered sharp torque reversals when the ignition was cut which was tough on the engine mounts and the airframe.

Another problem encountered by rotary engine designers was met when trying to meet the demand for greater power. The size of the engine could be expanded in only two directions: make it larger in circumference, make it more than one row (deeper). The problem with the first solution was that this just made the gyroscopic forces even more unmanageable. The second way out of the problem provided much the same effect and the rear bank of cylinders were hard to cool.

There are other reasons that would have tended against the use of the rotary into more modern times and the greatest of these would be its enormous appetite for oil. The fuel was mixed with air as it was introduced through a primitive "carburetor" - usually in the tail end of the crankshaft. Via this route it made its way to the crankcase where is picked up all of the oil that was loose. When the fuel mixture was introduced to the combustion chamber it was very much a mix of fuel, air, and castor oil.

The imperfect combustion of any engine is not equaled by that of a rotary. The castor oil, being the least combustible of the two liquids, was spewed out into the atmosphere. It would be but a short time before the whole of the slipstream area of the aeroplane would be well coated with castor oil. The pilot would be soaking up oil at a fairly rapid rate as well. It is arguable that the reason for cowling the engine had as much to do with trying to control the wildly spewing oil as it was to do with the concepts of streamlining. The usual practice was to direct the oil underneath the fuselage by opening up the bottom of the cowl.

However, a cowling is not a favorite item to a rotary. The cylinders are air-cooled. As has been mentioned, the use of two banks of cylinders caused trouble enough. The cowling made the engine much hotter that it liked. The reason for the cutout in the bottom of the cowl, then, was to direct the spray of oil as well as to aid in cooling the engine. Some of the cowlings of WWI aeroplanes show evidence of extra cooling openings being cut into them by mechanics in the field.

Many people remark about the pleasantness of the odor of burnt castor oil. Out in the open where one's exposure is contrasted with other scents, it can be an enjoyable sensation. It is still nice if you are saying, "bye-bye" to the pilot before you go back to your mechanic's tasks. But to sit behind an engine that is spraying you with unburnt - as well as burnt - castor oil is quite another matter after a few hours. The oil is known for its purgative qualities. It would be impossible to expose oneself to such an atmosphere and not experience certain difficulties.

Rotary Engines and Specifications:

Model: 80 Hp
Le Rhône 110 Hp
Le Rhône
(Type J) Oberrursel
U.R. II

Date: Circa 1916 Circa 1916 Circa 1916

Cylinders: 9 9 9

Configuration: Rotary,
Air cooled Rotary,
Air cooled Rotary,
Air cooled

Horsepower: 80 hp
(59 kw) 113 bhp
(84 kw) 110 hp
(82 kw)

R.P.M.: 1,200 rpm 1,200 rpm NA

Bore & Stroke: 4.1 in x 5.5 in
105mm x 140mm 4.4 in x 6.7 in
112mm x 170mm 4.9 in x 5.9 in
124mm x 150mm

Displacement: NA 920 in³ 995 in³/16.3 L

Weight: NA 330 lbs (149 kg)

It is the need for cooling that is part of the reason that pilots 'blipped' their engines. One could not use a throttle on them because they had such great need of motion to keep them cool. That they were allowed to stop to descend is true but the combustion had ceased during that time. (Of course, starting them up again could be an exciting experience. If they were not too loaded with the explosive fuel mixture - they might do just that: explode. If badly loaded in one or two cylinders, the rough running could cause considerable concern before it cleared.)

Although the cowling did cause them to overheat, It also allowed them to produce greater power as the air trapped within the cowl was easier to "stir" with the cylinders than would be a stream of high velocity air directed at the front of the engine.

They were easy to start by diving to turn the prop - which turned the engine. And they have been known to run with the most awesome damage inflicted on one or more cylinders.

There are many stories about the gyroscopic forces and their ability to turn a sorely pressed pilot out of danger. The most engaging terms used to describe the turn of a Camel was said by Dick Day: "Why, it puts both eyes on the same side of your nose!"

Rotary Engine Theory

100 hp Gnome Monosoupe (The Science Museum, London)

Rotary Engines and Specifications:

Model:	80 Hp Le Rhône	110 Hp Le Rhône (Type J)	Oberrursel U.R. II
Date:	Circa 1916	Circa 1916	Circa 1916
Cylinders:	9	9	9
Configuration:	Rotary, Air cooled	Rotary, Air cooled	Rotary, Air cooled
Horsepower:	80 hp (59 kw)	113 bhp (84 kw)	110 hp (82 kw)
R.P.M.:	1,200 rpm	1,200 rpm	NA
Bore & Stroke:	4.1 in x 5.5 in 105mm x 140mm	4.4 in x 6.7 in 112mm x 170mm	4.9 in x 5.9 in 124mm x 150mm
Displacement:	NA	920 in³	995 in³/16.3 L
Weight:	NA	330 lbs (149 kg)

COURTESY OF
The Aviation History On-Line Museum.

Multi-row Radials

Click on Picture to enlarge

The Lycoming XR-7755 was the largest reciprocating ever built. It produced 5,000 HP at 2,600 RPM.

Originally radial engines had but one row of cylinders, but as engine sizes increased it became necessary to add extra rows. Most did not exceed two rows, but the largest radial engine ever built in quantity, the Pratt & Whitney Wasp Major, was a 28-cylinder 4-row radial engine used in many large aircraft designs in the post- World War II period. The USSR also built a limited number of 'Zvezda' 42-cylinder diesel boat engines featuring 6 rows with 7 banks of cylinders, bore of 160 mm (6.3 in), stroke of 170 mm (6.7 in), and total displacement of 143.5 liters (8,756 in³). The engine produced 4,500 kw (6,000 hp) at 2,500 rpm .

The Pratt And Whitney R-4360

Click on Picture to enlarge

P&W R-4360 Radial Engine

Called the “corncob” because of it long, slim and bumpy shape, this 28 cylinder, 4 row, air-cooled radial engine was the largest radial engine to reach series production. How P & W managed to cool that last row of cylinders, with air that had already gone past three rows of hot cylinders—and make it work--was pure magic. Weight of this 4,360 cube monster was 3,600 lbs. Horsepower ratings ranged from 3,000 to 4,360

The B-36 used six of these motors. So complex was the starting drill that it took hours to get all of them running properly. Ditto for the shutdown sequence or heat would damage them severely—think of a subway train with square wheels and you get the idea. Engine analyzers were developed to keep all 168 cylinders running properly during the 10,000 mile missions these B-36s flew—a benefit we all now share. Finally, Howard Hughes’ famous “Spruce Goose” (that was made from birch plywood) used eight of these monsters.

The Lycoming XR-7755

The displacement was 7,755 cubic inches. When compared to Lycoming's largest production engine in production today which displaces 720 cubic inches, it was more than 10 times larger!

This huge engine was 10 feet long, 5 feet in diameter and weighed 6,050 pounds. It produced 5,000 HP at 2,600 RPM, and the target was 7,000. It used 580 GPH of av gas at the 5,000 HP rating.

There were nine overhead camshafts which could be shifted axially for METO power in one position and cruise at the other. Two great shafts emerged for coaxial propellers, and there was a two speed gear-change box between the crankshaft and the propeller shafts.

Development of the XR-7755 began at Lycoming in Williamsport in the summer of 1943. With the end of World War II in 1945, the military no longer had a need for an engine of this size, and development of the XR-7755 stopped at the prototype stage.

Click on Picture to enlarge
	Experimental Engineers with the XR-7755 Here is the XR-7755 in the test cell at Lycoming along with the team of experimental engineers responsible for its testing and qualification. Photo courtesy Robert Ribando. William Ribando can be seen in the white shirt and tie just to the left of the engine.

During those years, Lycoming put together a team, under the leadership of VP Engineering Clarence Wiegman, to develop this super-size engine.

The engine now resides at Silver Hill of the Smithsonian Institute.

Click on Picture to enlarge

Lycoming XR-7755 This behemoth is the largest piston aircraft engine ever built in the free world. From 7,755 cubic inches (bore of 6.375", stroke of 6.75") and 7,050 lbs it gave 5,000 horsepower under test in 1944; 7,000 horsepower was the development target. Nine liquid cooled inline four cylinder engines about a common crankshaft. Two contra-rotating prop shafts. The camshafts each consisted of two sets of lobes. One set of lobes for takeoff, the other for economy cruise. The camshafts were shifted axially to switch lobe sets. 580 gallons of fuel per hour at takeoff power, BSFC of 0.43 at cruise. Paul McBride of Lycoming says it was for the B-36 but political pressures caused the B-36 to be fitted with the Pratt & Whitney R-4360 instead. Awesome. A survivor still exists at the Smithsonian's Garber facility. See it if you can.

Lycoming XR-7755
Specifications:
Date:	1943
Displacement:	7,755 cu. in. (127 liters)
Cylinders:	36
Bore and Stroke::	6.4 in. (162 mm) x 6.8 in. (171 mm)
Weight:	6,050 lbs. (2,744 kg)
Performance:
Horsepower:	5,000 hp (3,728 kw) at 44 in Hg.MP.
RPM:	2,600
Configuration:
4-cycle, 4 row radial, liquid cooled, 2 speed geared dual rotation propeller drive with turbo-supercharger.

Modern Radials

Click on Picture to enlarge

Pratt & Whitney R-1340 radial engine mounted in Sikorsky H-19 helicopter

At least three companies build radials today. Vedeneyev engines produces the M-14P model, 360 HP radial used on Yakovlevs, and Sukhoi Su-26 and Su-29 aerobatic aircraft. The M-14P has also found great favor among builders of experimental aircraft, such as the Culp's Special, and Culp's Sopwith Pup , Pitts S12 "Monster" and the Murphy "Moose". 110 HP 7-cylinder and 150 HP 9-cylinder engines are available from Australia's Rotec Engineering. Miniature radial engines for model airplanes use are also available from OS and Saito of Japan and Technopower. The Saito firm is known for making three different sizes of three-cylinder radials, and a five cylinder example.