Lycoming XR-7755, 1943

8 Jan 1943 – History of Lycoming's Large Engines and Their Potential Use in the Convair B-36

Starting in 1932, Lycoming began work on the XO-1230 (aka the X-1), a liquid-cooled 180° V-12 for the U.S. Army Air Corps (USAAC) based on a USACC cylinder design. With a 5.25" bore, 4.75" stroke and 1,233.91 in³ displacement, both Lycoming and the USSAC hoped this engine would produce at least one horsepower per cubic inch and weigh less than one pound per horsepower. Development proceeded slowly for a number of reasons, chief among which were the technical challenges and that the project was perpetually underfunded because the USAAC as a whole was underfunded. The XO-1230 ultimately produced 1,200 hp at 3,400 rpm on test and flew in a Vultee XA-19A, but was not reliable at that power setting and attracted no interest among aircraft manufacturers. The XO-1230 project was abandoned late in 1938.
Around 1938, Lycoming tried to salvage the O-1230 design by gearing two of them together to form the XH-2470 (aka the X-2), a liquid-cooled vertically-opposed H-24 displacing 2467.83 in³ with a 2,300 hp at 3,300 rpm takeoff rating. The U.S. Navy selected this engine to power the Curtiss XF14C-1 and partially funded its development. The USAAC also selected and partially funded the engine to power the Vultee XP-54 after P&WA abandoned its X-1800, which had been the original engine selection. The XH-2470 flew a few times in the XP-54, but was unreliable
At its own expense, Lycoming next developed the BX (aka X-4? Don't ask what happened to the X-3!) engine, an upsized version of the XH-2470. With its 5.75" bore, 5.25" stroke and 3,271.87 in³ displacement it had a 3,150 hp takeoff rating. This was the first indication of Lycoming trying to provide engines for the Consolidated Vultee Aircraft Corporation (CVAC) XB-36 heavy bomber. Again, it was not selected and was consequently never produced.
The USAAC realized that in order to power the intercontinental bombers it envisioned it needed to incentivize engine manufacturers to design and build very powerful engines with extremely low specific fuel consumption (sfc). To this end, it issued Request for Data R40-D on 6 Mar 1940. This solicited designs of 4,000 to 5,500 hp aircraft engines with guaranteed fuel specifics at 40%, 50%, 60%, and 70% takeoff power. Studebaker and Lycoming submitted proposals that saw further action. The Lycoming solution, the XH-4940 (aka X-5) was essentially a doubled-up XH-2470 displacing 4,935.64 in³.
Lycoming then decided it preferred a liquid-cooled 36-cylinder engine consisting of three 60° V-12s with three crankshafts geared together (aka X-6). Initially, with a 6.125" bore, 6.250" stroke and 6,629.56 in³ displacement it had a 5,000 hp at 2,900 rpm takeoff rating, a 4,000 hp at 2,650 rpm normal rating and weighed about 4,900 lb.
Ultimately, Lycoming settled on the XR-7755 (aka X-7), a liquid-cooled four-row 36-cylinder radial with a 6.375" bore, 7.750" stroke, 7,756.33 in³ displacement and 8.5:1 compression ratio resulting in a 5,000 hp at 2,600 rpm takeoff rating and 4,000 hp at 2,300 rpm normal rating that weighed about 6,000 lb.

Sometime in late 1942 the Army Air Corps asked CVAC to compare the Lycoming BX to the P&WA Wasp Major, which had been selected to power its XB-36 bomber. On 8 Feb 1943, CVAC released its report, "Study – Lycoming BX Engine", which included drawings, an installation narrative, and performance and weight comparisons.

The engine, attached to the rear spar, drove a pusher propeller located aft of the wing trailing edge. Two General Electric type BM turbosuperchargers per engine were located side-by-side in the lower portion of the nacelle. Cooling air to the coolant radiator, oil coolers and intercoolers was supplied from leading edge ducts and exhausted through an annular opening around the propeller spinner. Flaps controlled the cooling air flow control for each cooling unit by varying its exit area. A main air duct leading from the wing leading edge divided into three just ahead of the rear spar; the center duct supplied the coolant radiator and the outer ducts supplied the oil coolers on either side of the coolant radiator. The two intercoolers, located outboard of the oil coolers, had separate leading edge entrances on either side of the main duct entrance.

Ground cooling was achieved via augmenters into which a portion of the exhaust from the aft three cylinders of the upper two cylinder banks was discharged into the radiator and oil cooler ducts near the duct exits. These exhaust jets entrained cooling air, augmenting its flow through the cooling system. Exhaust for the augmenters came from an auxiliary exhaust manifold connected by a valve to the upper-cylinder exhaust manifolds. This auxiliary manifold was only used for ground operations as the valve retracted flush against with the main manifold and closed the auxiliary outlet during normal operation. Cooling for the engine exhaust shrouds was provided by air led from the engine air inlet ahead of the turbocharger and discharged into the intercooler ducts aft of the intercoolers near their exhaust duct exits. For ground operation, air from one of the two turbochargers was bypassed to the exhaust shrouds. Backflow to the normal inlet was prevented by a valve, which for normal operation closed the bypass duct. The engine compartment was ventilated directly from a duct located between the two turbocharger air intake ducts. This air dumped into the engine compartment and exhausted through ports in the radiator and oil cooler ducts just aft of the augmenter jets. Ventilation for ground operation was provided by jets in the radiator and oil cooler ducts.

Dual General Electric type BM turbochargers provide greater economy for cruising operation. At high altitudes and low powers where the turbocharger efficiency begins to drop, provision is made for bypassing one turbocharger, thereby doubling the exhaust gas flow through the other turbocharger and maintaining good turbocharger efficiency.

Lycoming BX and P&WA Wasp Major Installation Comparison (dated 8 Jan 1943)

Lycoming BX Engine Installation	P&WA Wasp Major Engine Installation	BX and Wasp Major Nacelle Outlines	BX Inboard, Center and Outboard Nacelle Outlines

XB-36 Study with Six Liquid-Cooled Turbosupercharged Lycoming BX versus P&WA Wasp Major Engines
The better BX engine cruising performance is largely due to the lower sfc (0.37 versus 0.40 lb/hp/hr). Takeoff is also improved due to the BX engine's higher horsepower (3,150 hp versus 3,000 hp).
Metric	Condition 1 BX Lower SFC Increases Range	Condition 2 BX Lower SFC Increases Bomb Load for 10,000 Mile Range	Condition 3 BX Lower SFC Reduces Fuel and Gross Weight Required for 10,000 Mile Range with 10,000 lb Bombs
	BX	Wasp Major	BX	Wasp Major	BX	Wasp Major
Gross Weight (lb)	265,000	265,000	265,000	265,000	251,500	265,000
Maximum Range (mi)	10,750	10,000	10,000	10,000	10,000	10,000
Bomb Load (Dropped 1/2 Range)	20–500 lb	20–500 lb	35–500 lb	20–500 lb	20–500 lb	20–500 lb
High Speed at 30,000 ft (mph)	366*	369	366*	369	369	369
Service Ceiling at 0.3 Range (ft)	40,800	40,000	40,500	40,000	41,300	40,000
Takeoff Over 50 ft Obstacle (ft)	4,750	5,000	4,750	,5000	4,350	5,000
* The lower BX engine installation drag is estimated to increase top speed by 2 mph; however, using the available gear ratios and favoring cruising gives a propeller tip speed loss that reduces the top speed by 5 mph, thereby giving 366 mph for the BX engine installation.

XB-36 Range versus Bomb Load, 265,000 lb Gross Weight

(Optimum Altitude Cruise with Bombs Dropped at Half Range)

Estimated Weight Comparisons for the XB-36
(Six Lycoming BX Engines versus Six P&WA Wasp Major Engines)
Item	Lycoming BX	P&WA Wasp Major
Engines (6 with two-speed reduction gear, including 15 lb/eng for residual oil)	19,890	20,636
Nacelles
Engine Mounts (6)	1,820	1,520
Flexible Mounts	420	804
Cowling	1,550	1.786
Miscellaneous	86	86
Total	(3,876)	(4,196)
Engine Accessories
Turbochargers (12)	3,000	3,000
Turbocharger Supports	89	89
Intercoolers (12)	851	851
Intercooler Supports	48	48
Oil Coolers (12)	800	642
Oil Cooler Supports	35	30
Exhaust System (excluding heat exchangers)	1,750	1,575
Air Ducts	1,170	932
Fuel Pumps (6)	24	24
Miscellaneous	76	76
Total	(7,843)	(7,267)
Power Plant Controls
Turbocharger Controls	20	20
Turbocharger Regulators	45	45
Regulator Supports	15	15
Throttle and Mixture	149	149
Cooling Air Exit Flaps	250	119
Power Plant Controls in Fuselage	120	120
Propeller	150	150
Miscellaneous	84	84
Total	(833)	(702)
Propellers (6)
Total	(6,210)	(6,210)
Systems
Starting
Gear Boxes and Shafts	150	150
Portable Starter and Shaft	50	50
Oil	261	261
Fuel (excludes protection)	737	737
Power Plant Instruments	113	113
Cooling
Fans	–	1,602
Coolant (70% ethylene glycol, 30 gal/eng at 9 lb/gal)	1,620	–
Coolant Radiators (6)	1,650	–
Piping, Expansion Tank, Radiator Supports	450	–
Total	(4,290)	(1,602)
Total Power Plant	44,253	41,964
Notes: 1) Other items in weight empty are estimated to be the same for both engine types 2) Increase in weight empty with BX engine = 2,289 lb. 3) Decrease in fuel to maintain constant gross weight (oil is 4% fuel by volume) = 2,289 / 6.3 = 363 gal

1 Jun 1943. Materiel Command (hereinafter MatCmd) civilian employees J. Glen Blackwood and Opie Chenoweth briefed Lycoming representatives W.K. Cooper, Samuel K. Hoffman, P.G. Garlent and E.A. Johnson on an engine development program in which Lycoming might participate. It was a long-range bomber engine with a 5,000 hp takeoff rating, very low cruise specific fuel consumption at 25 to 80% normal power, two-speed dual-rotation reduction gear, fuel injection, 36" height if practical and Grade 130 fuel usage. No constraints were placed on design, cylinder arrangement, number or size. No altitude rating was required although an exhaust turbosupercharger could be used to provide sea-level power at altitude or achieve takeoff power. MatCmd suggested that Lycoming's program might include calibration and 50-hr endurance tests of several different-sized cylinders, twin-cylinder endurance test, design studies and calculations for the full-up engine, durability tests on the two-speed reduction gear, fuel injection, coolant pump, ignition system, accessory section, oil pump and similar items, an option of furnishing a few engines. Additional items could then cover 50-hr and 150-hr tests. Chenoweth expressed a preference for a fixed-price contract. MatCmd and Lycoming at that time referred to this proposed engine as X-5. This was essentially a twinned XH-2470 with 48 cylinders.[8 Jun 1943 Memorandum Report ENG-57-503-888. Conference between Lycoming and Materiel Command Representatives on a Proposed Engine Development. RG342 RD3024 Engines - Lycoming - General 1933-45 (XR-7755)]

18 Jun 1943. Lycoming had abandoned the X-5 concept in favor of the X-6 (described below). A conference at the Materiel Center included Collins and Hoffman from Lycoming, and Col Gillespie, Chenoweth and Blackwood from MatCmd. A later meeting included Collins and Hoffman from Lycoming, and Lt. A. Dernbach (Propeller Lab), Mr. Dunham (Aircraft Lab) and Blackwood. Lycoming proposed to pick a cylinder size and concentrate on its development, making the best possible engine with that size rather than experimenting with different sizes prior to engine development. Lycoming's first choice was a 6.125" x 6.250" bore and stroke, with which it hoped to save time by using existing piston forgings. This choice resulted in a multi-cylinder engine with the following characteristics:

Takeoff Rating = 5,000 at 2,900 rpm
Normal Rating = 4,000 hp at 2,650 rpm
Bore, Stroke, Displacement, Compression Ratio = 6.125", 6.250", 6,629.56 in³, 8.0:1
Length x Width x Height = 110" x 62.5" x 62.5"
Basic Weight = 4,900 lb
Basic Specific Weight = ~ 1 lb/hp
Specific Weight with Two-Speed Dual-Rotation Reduction Gear = ~1.2 lb/hp
Cylinder Arrangement = Three 60° V-12s (36 cylinders) with three crankshafts. Cylinders were in blocks of three.
Supercharger, Carburetor or Fuel Injection, Grade 130 fuel
SAE No. 70 single-rotation propeller; SAE No 60-60 dual-rotation propeller

MatCmd suggested that this engine might weigh more than an engine using fewer crankshafts. Hoffmen said that studies made of radial designs revealed that the master rod, crankpins and knuckle pins become so large that there is no weight savings. While MatCmd considered the X-6 engine arrangement satisfactory, Chenoweth opined that the relatively small displacement might result in sacrificing sfc or power. Chenoweth suggested to err on the side of too much displacement in order to safeguard future development and limit bmep to 190 psi.

Hoffman agreed to study the 6.500" x 6.500" cylinder size as a means to reduce bmep and increase sfc. MatCmd suggested eliminating the gear-driven supercharger since it reduced sfc due to the power required to drive it; the turbochargers could be used instead. Hoffman was reluctant to pursue such a radical design. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

Cylinder Bore x Stroke (inches)	Displacement (in³)	bmep at 2,900 rpm (psi)
6.125 x 6.250	6,629.56	206
6.375 x 6.375	7,325.42	186
6.500 x 6.500	7,664.84	176

28 Aug 1943. Letter Contract W 33-038 ac-564 (11169) for single-cylinder experimental engines applicable to Lycoming X-6 development was signed.

18 Nov 1943. Prescott visited Lycoming and came away with the following observations:

Lycoming was now working with a 6.375" bore and 6.750" stroke for the X-6; this was the same as the eventual X-7 with its four rows of nine cylinder configuration.
The first single had a 6.375" bore and 5.500" stroke due to appropriating the same crankshaft that had been used on the BX engine.
A single-cylinder engine was to be ready to test by 15 Jan 1944.
Torsional vibration orders that were potentially concerning were 1 1/2, 2 1/2, 3 1/2 and 4 1/2.
Lycoming was to propose a firing order and 3-crankshaft phasing that would eliminate one vibration period and use dynamic vibration absorbers to address the remaining torsional vibration orders.
A single 6-cylinder block design was to be used throughout.

[Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

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