Lycoming XR-7755, 1947

In early September 1947, Maj Gen Carl A. Spaatz created the ad-hoc Heavy Bombardment Committee (HBC), which consisted of representatives from Headquarters, Army Air Forces, the Strategic Air Command, the Air Materiel Command and the Air University. Gen Spaatz directed the HBC to investigate methods of delivering an atomic attack on an enemy 4,350 nautical miles from continental U.S. bases and recommend actions to the USAF Aircraft and Weapons Board , which examined studies showing that high speed limited possible attacks on U.S. bombers to a 55° arc of the bombers tail cone. The A&WB, faced with the high cost of extremely long range bombers, opted for speed as its top priority. A key technology to achieving both speed and range was in-flight refueling.

During 1946 and 1947, in-flight refueling experiments were converging on a workable system. Starting with a British system in which a long hose was unreeled from a tanker and then snared by a rig on the receiving bomber, concepts progressed to a probe-and-drogue scheme, which was also less than ideal. The flying boom concept that ultimately emerged was designed by B.A. Holman, a German scientist working for the AMC, and developed by Boeing. This consisted of a telescoping aluminum tube with moveable wings at its end that could be flown up, down, and side-to-side by an enlisted crewmember. It offered many advantages over hose systems; it was free from weather interference, stable at high speeds, had better lighting and was more easily separated in an emergency. It could pass up to 500 gallons of fuel per minute versus the 100 gpm British system limit. Flight testing started on 16 Sep 1948, and thereafter all future bombers were equipped with single-point refueling receptacles. Boom-equipped KB-29s started arriving in September 1950 and by December 1951 there were 14 tanker squadrons.

In-flight refueling had eliminated the range extension problem.
[Gentry, SMSgt M.A. "Background Paper on the History of Air Refueling" (Montgomery, Alabama: Air Force Enlisted Heritage Research Institute, File 100.106. 3 May 1994).]
[Julian, Thomas A. "The Origins of Air Refueling in the United States Air Force" Technology and the Air Force: A respective Assessment. Neufeld, Jacob, George M. Watson, Jr., and David Chenoweth, eds. (Washington, DC: Air Force History and Museums Program, 1997)]

All the foregoing left Blackwood trying to plan a future for the XR-7755. He came up with a equitable formula that produced a figure of merit (FoM) for comparing turboprop and reciprocating engines: FoM_500mph = (EngineWeight / CruiseHorsepower) + (CruiseSpecificFuelConsumption * CruiseHorsepower * MissionDuration). The cruise power and fuel consumption for each engine were for a power setting that would achieve a 500 mph cruise speed in a representative heavy bomber. Smaller FoMs were better than larger ones. Blackwood's study results appear in Table 47-1, and from this it becomes clear that the XR-7755, despite all the effort that had been expended to make it fuel efficient, was not only worse than any of the turboprops, but also worse than the Pratt & Whitney R-4360! This must have been a sobering revelation. Blackwood, of course, was unaware of the specifics outlined above, but surely must have been aware of the trends. Also of note is that all of the high-output turboprops in Blackwood's study abandoned. [RG 342 RD2311. XR-7755, 503-602 Current]

Table 47-1. Lycoming XR-7755 Compared to Turboprops in Developent, circa 1947
Manufacturer	Designation	ESHP (TO)	Length (in)	Dia (in)	Weight (lb)	Specific Area (hp/ft²)	Specific Volume (hp/ft³)	Specific Weight (hp/lb)	FoM @ 500mph (lb)
									5 hrs	10 hrs	15 hrs	22.5 hrs	30 hrs
Lockheed/Menasco	L-1000S-1								2.0	3.5	4.9	7.2	9.3
Frederich Flader	XT33-FF-1	5,900	154.9	51.4	4,805	409	32	1.23	3.5	5.7	7.7	10.8	14.3
Wright	XT35-W-1	5,500	152.0	59.0	4,450	290	23	1.24	3.5	5.7	8.1	11.2	14.3
Wright	XT35-W-3	8,900	168.0	59.0	5,950	469	33	1.50	3.2	5.3	7.4	10.6	13.8
Northrop/Turbodyne	XT37-NA-1	7,500	167.0	44.2	5,050	704	51	1.49	3.3	5.5	7.6	10.8	14.1
Northrop/Turbodyne	XT37-NA-3	10,000	167.0	46.0	6,000	866	62	1.67
Allison	XT39-A-1	9,000	176.0	39.0	3,700	1,085	74	2.43	3.0	5.3	7.2	10.3	13.3
General Electric	XT41-GE-1	2,700	89.9	40.0	2,400	309	41	1.13	4.0	6.5	8.5	11.6	15.3
Wright	XT43-W-1	10,000	163.6	45.0	6,400	905	66	1.56	3.3	5.4	7.4	10.7	13.9
Pratt & WHitney	R-4360-43	4,300	102.0	54.0	3,720	270	32	1.16	3.7	5.8	8.4	11.5	14.9
Lycoming	XR-7755-1	5,000	121.4	61.0	6,050	246	24	0.83	4.7	6.5	9.3	13.0	16.8

15 Jan 1947. The AMC wrote Lycoming regarding Contract No. W 33-038 ac-564 stating that: 1) The AMC was revising its program related to this contract and found it necessary to stop the procurement of an additional engine and on the compounded XR-7755 engine version. 2) AMC was not in the position to extend the basic contract beyond what was then obligated. Contract modifications were being considered that would insure a 50-hr flight approval test IAW AAF Specification XR-28398-A was completed even at the expense of sacrificing other contract items. AMC wanted to eliminate, or hold in abeyance, items pertaining to future development so that remaining funds would go toward this test with a minimum of accompanying development. This might have required modified ratings, if necessary, since the test was to be accomplished with engines and parts then available. More information is available in the Contracts and Administrative Section. [RG 342 RD 2311 XR-7755 Contract W-33-038AC-564 Proposed Procurement & Mods]

18 Feb 1947. During a telcon from Lycoming, Ryder and Pinto (the new Army Air Forces Resident Inspector) advised Blackwood that a propeller nut wrench was needed. The reworked supercharger had arrived along with an injection pump installation drawing. Lycoming intended to install the supercharger and fuel injection pump on engine #101, which was headed back to the test stand. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

26 Feb 1947. J.H. Carpenter from Lycoming advised Blackwood that Lycoming had put the work reduction into effect and had stopped building the fourth parts set to the extent possible (i.e., no new jobs were started, but those in process were completed). Engine #101 was on the dynamometer, which was having vibration trouble; the engine was rear mounted, but its front end had to be stabilized. The carburetor was erratic up to 1,600 rpm. The dynamometer bearing had heated up, which was being investigated. The run was about four hours short of completion. Engine #102 was being built up with the new link pin retainer and was expected to be ready to run in a couple of weeks; if problems arose with the #102 run, then new parts would be installed. Reduction gear parts were still 3-4 weeks away. The 3-cylinder test engine was back in its test stand. The new link pin retainer and link pins plated with silver-lead-indium were in. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

Engine #101 Run-in Log
Power (bhp)	4,500 to 5,100	4,000 to 4,500	3,000 to 4,000	2,000 to 3,000	1,000 to 2,000	0 to 1,000
Time (hr:min)	1:15	0:47	1:52	3:00	4:27	4:46

4 Mar 1947. During a telcon from Lycoming, Ryder and Pinto reported that #101 had been run in to 2,570 rpm and 5,000 hp. Test engineers were fixing oil leaks, and when the run-in was complete a 50-hr test would begin on 5 or 8 Mar 1947. It appeared Lycoming would just get by with a 100°F induction air inlet temperature and no ram pressure requirements, a marginal situation. The hydraulic pump Blackwood had sent had a 16-tooth spline instead of the 12-tooth spline required by the drive; Blackwood advised the test engineers to proceed without the pump. After engine #101 had completed a 15.5 hr run-in, Lycoming provided a chart of time at power. The 50-hr test would begin Monday, 10 Mar 1947. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

4 Apr 1947. During a telcon from Lycoming, Carpenter apprised Blackwood that the dynamometer on engine #102 was down with front bearing trouble and had been for over a week. This had happened during run-in up to 1,900 rpm. The 3-cylinder engine's main drive shaft to auxiliary section driving fuel injector and oil pump twisted off with a torsional failure due to a bad 1 1/2 order vibration at 2,200 rpm. This did not apply to the 36-cylinder engines. The 3-cylinder was reassembled, run in for two hours and failed again with a broken link rod and badly damaged piston due to a piston pin circlip coming out; it had run 22.5 hrs at maximum cruise power. Engine #101 knuckle pins arrived and assembly would start next week. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

9 Apr 1947. Carpenter reported that the 4,000 hp dynamometer bearing pad casing had been off for about two weeks. He expected #101 to be in operation in about a week with the 0.56 reduction gear and torquemeter. The 3-cylinder engine crankshaft and crankcase was serviceable and would be rebuilt next week. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

12 May 1947. Lycoming's C.H. Wiegman submitted preliminary XR-7755-1 dynamometer calibration data, which demonstrated a minimum carbureted bsfc of 0.38 lb/hp/hr with the minimum bsfc points limited by detonation. Lycoming surmised that better mixture distribution and lower-cylinder charge temperatures would be obtained with cylinder head fuel injection. One friction reading at 1,600 rpm and 22 inHgA manifold pressure yielded a friction horsepower of 305. This all pointed to a lower multi-cylinder fuel consumption than was anticipated from single-cylinder data.

[J.G. Blackwood Notes. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Engineering Data.]

Engine #101 was coming off the propeller stand for inspection after 50-hr endurance test. Pinto was helping try to figure out a way to mount a Curtiss controllable propeller on the 0.56 reduction gear, which had a No. 70 shaft and a No. 80 nose. The problem was how to mount the electric propeller brush holder housing onto the nose bolt flange. Pinto had ruled out the existence of an experimental adapter and was hoping to rework a No. 80 nose casting to work. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

Preliminary Mixture Control Data
rpm	1,000	1,000	1,300	1,300	1,600	1,600	1,600	1,600
bhp	890	1,000	910	1,090	1,410	1,550	1,810	1,970
bsfc (lb/hp/hr)	0.390	0.380	0.420	0.405	0.405	0.395	0.385	0.380
bmep (psi)	91	102	72	86	90	99	116	126
damp (inHgA)	22	24	18	20	22	24	26	28

2 Jun 1947. Experimental Department Report, Liquid-Cooled Section for 16 May – 1 Jun 1947.

Multi-Cylinder Engine Tests
1. XR-7755 #101, Project T-223 Sup. Item 4, Running Time 43:36 hrs. The scheduled 50-hr endurance test at cruise conditions was completed. Inspection of the reduction gear showed these parts to be in good condition. The engine was to be removed from the stand for 100% visual and magnaflux inspections.
2. XR-7755 #102, Project T235 sup. Item 12-11, Running Time 0:00 hrs
  1. Preliminary Induction System Temperature Survey: report was being written.
  2. Preliminary Calibration: report was being written.
  3. Temperature Survey and Calibration: The reassembly of this engine was about 50% complete. Testing was to begin during the week of 9 Jun 1947.
1-Cylinder Test Engines
1. SC-215-3, Project T-235, Sup. Item 2, Account 194, Running Time 19:54 hrs. This test was about 80% complete. Runs were first made to determine the cylinder vibration characteristics with the detonation pickup adapter located on the cam cover and the intake port compared to the standard location in the fuel injector boss. Knock limit runs of 3 – 4 points were then made with the pickup located on the intake port, cam cover and standard location. Results showed that the cam cover pickup location was comparable to the standard location at high cruise and takeoff conditions but it differed at low-cruise operating conditions by about 10 – 20 imep. The new Sperry knock meter was calibrated for single-cylinder engines with the pickup located in the injector boss.
3-Cylinder Test Engine
1. 3C-645 No. 7, Project T-230, Sup. Item 1 Account 204, Running Time 0:18 hrs. Simulated 50-hr Preliminary Flight Test Rating According to Specification No. XR-28398-A. The engine was completely assembled and installed on the test stand. Run-in of the new engine parts was underway in conjunction with study of the crankshaft and rod mechanism movement.
Special Problems Laboratory
1. Fuel Injection Lines for the Multi-Cylinder Engine. Injection line support clamps were received and work fabricating the finished injection lines had started. Lycoming expected the Bendix fuel injection nozzles and American Bosch fuel control unit would arrive during the first week of June.
2. Oil Scavenge Pump, Project T-193, testing had been discontinued; a report was to be written.
3. Multi-Cylinder Valve Gear, Project T-190, results were being typed.
4. Torquemeter and Pump Test initial torquemeter report was being typed; report on the second unit was ready for editing.
5. Supercharger Laboratory
  1. Performance and Distribution Tests of the X-7 Supercharger Housing using the F-8053-Y Baffle, Project T-233, Contract Item 8, report was being edited.

[J.G. Blackwood Notes. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Engineering Data.]

Engine Time at Power, 16 May 1947 to 1 Jun 1947
Test Engines Time at Power
imep at rpm	264 at 2,800	241 at 2,500	155 at 2,200	143 at 1,600	90 at 1,300	Misc	Period Time	Total Time
Eng No. SC-215-3 Time	0:00	2:54	3:14	1:45	0:00	12:01	19:54	250:06
imep at rpm	231 at 2,600	205 at 2,300	155 at 2,200	143 at 1,600	80 at 1,300	Misc	Period Total	Total Time
Eng No. 3C-645 Time	0:00	0:00	0:00	0:00	0:00	0:18	0:18	118:01

XR-7755 Time at Power
Rating, Power Range (hp)	Takeoff, ≥ 5,000	Normal, 5,000 – 4,000	High Cruise, 4000 – 3,000	Medium Cruise, 3,000 – 2,000	Low Cruise, 2,000 – 1,000	Misc < 1,000	Period Time	Total Time
XR-7755-101 Time	0:00	0:00	0:00	31:28	6:50	5:08	43:26	118:00
XR-7755-102 Time	0:00	0:00	0:00	0:00	0:00	0:00	0:00	22:30
XR-7755 Total Time								140:30

Lycoming XR-7755 Performance Data

X-7 Fuel Consumption	XR-7755-102 Performance	XR-7755-1 Minimum Fuel Consumption

2 Jul 1947. In a Lycoming telcon, Carpenter related that the 0.56 reduction gear had been mounted on the dynamometer and run-in up to 1,600 rpm. It had then uncoupled due to pinions being stripped from their carrier. An oil hole lubricating the pinions had become misaligned during assembly; this problem was only applicable to the 0.56 reduction gear. It would be rebuilt as soon as possible. The 3-cylinder engine had been rebuilt with old connecting rods but no added weights. This was equivalent to new rods in the 36-cylinder. Lycoming planned to run it 50 hrs at full power, 2,600 rpm and 5,000 equivalent hp. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

15 Jul 1947., PPL civilian employee A.H. Goyne sent Lycoming's Robert B. Ingram a schedule for XR-7755-101 calibration, Project No. T-239, Item 12-11, Contract No. W-33-038 ac-564.

Objectives
1. The engine was to be calibrated using direct fuel injection
2. The oil quantity remaining in the engine after shutdown was to be determined.
3. Induction system temperature was to be surveyed and factors affecting same were to be identified.
Test Equipment
1. The engine was to be mounted on the No. 1 dynamometer stand.
2. Torsiographs were to be obtained using equipment for studying torsional vibration installed under the direction of Mr. Faust.
3. Additional thermocouples were to be installed in Banks 3, 5, and 8: six in the intake manifold runners and eigt in the intake manifold elbow sections.
4. The tests were to be performed with the 0.56:1 propeller reduction gearing installed.
5. A sight gage with drain valve and sight glass shutoff valves was to be installed on the No. 5 scavenge pump.

Running Conditions

The following conditions were to be held constant during all testing:
- Camshaft Position = cruise
- Spark Advance = 20°
- Fuel = AN-F-33 (grade 115/145)
- Oil = AN-O-8 (grade 1120, Sinclair)
- Coolant = 70% AN-E-2a (not less than 95% ethylene glycol plus AC Spec. 14108 inhibitor treated with WBT inhibitor) plus 30% H<sub>2</sub>0.
- Oil Inlet Temperature = 185 ± 5°F
- Coolant Outlet Temperature = 250 ± 5°F
- Coolant Pressure at Pump Inlet = 15 psi
- Fuel Pressure at Carburetor Inlet = 18 psi
- Carburetor Entrance Pressure = 29.92 inHgA
- Carburetor Inlet Temperature = 80 ± 2°F
Run-In
1. Torsiograph equipment was to be hooked up for the run-in
2. Oil level readings were to be taken at all stops until enough information was obtained to warrant discontinuing the readings.
3. A small combustion air flow orifice was to be used to measure air flow throughout the test.
4. A ram blower was to maintain approximately 29.92 inHgA and 80°F at the carburetor entrance.
5. The four-hour run-in was to use the following schedule:
  
  rpm time
  (min) beam bhp
  
  1,200 20 1,266 570
  
  1,300 20 1,570 722
  
  1,400 20 1,714 900
  
  1,500 30 1,971 1,110
  
  1,600 30 2,247 1,350
6. At completion of the 1,600 rpm run-in period a mixture control run was to be made at 1,600 rpm, and 22 inHgA, 60°F carburetor inlet temperature, and fuel air ratio varied from 0.09:1 to 0.06:1. Note that the 'beam' value in the following table was apparently the force exerted by the test stand torque beam.
  
  rpm time
  (min) beam bhp
  
  1,700 30 2,440 1,620
  
  1,800 30 2,833 1,915
  
  1,900 30 3,126 2,240
  
  2,000 30 3,516 2,640

rpm	time (min)	beam	bhp
1,200	20	1,266	570
1,300	20	1,570	722
1,400	20	1,714	900
1,500	30	1,971	1,110
1,600	30	2,247	1,350

rpm	time (min)	beam	bhp
1,700	30	2,440	1,620
1,800	30	2,833	1,915
1,900	30	3,126	2,240
2,000	30	3,516	2,640

Calibration Running. After installing the fuel injector pumps, the following mixture control curves were to be run, starting at the specified manifold pressure and raising the manifold pressure in 1 inHgA increments until detonation limited operation or when best economy mixture strength was achieved. Data was to be taken only at the highest manifold pressure setting that allowed a mixture control curve to be completed without detonation.

rpm	ADMP (inHgA)
1,000	22 to detonation at best economy
1,300	20 to detonation at best economy
1,600	26 to detonation at best economy
1,800	26 to detonation at best economy
2,000	28 to detonation at best economy
2,200	32 to detonation at best economy

Oil Level Survey. The following procedure was to be used for the oil pressure survey:
1. When the engine was stopped, the oil level in the engine was to be measured by means of the No. 5 sight gage and the time required for the level to rise 3, 5, 7, 9, 11, 12 and 13 inches from the oil sump bottom. The time required for all remaining oil to drain was also to be recorded along with the level above the oil sump bottom.
2. A thermocouple or plug was to be removed from the bank No. 4 and 5 manifold elbows and the oil quantity that had leaked into the lower manifolds measured.
3. The drain valves were to be opened and oil weighed as it passed the levels specified in 4a. Total weight of all oil drained from the engine was also to be recorded.
4. After the final stop at the end of a day's running, the procedures specified in 4a, 4b and 4c were to be followed, the results recorded, and the drain valves in the cam cover closed. The same procedure was to be repeated the following morning to determine if oil is leaking into the engine after a long time period.
5. Valves in the No. 5 cam cover were to be close and safety wired when the engine was in operation.

Inspection
1. At run-in completion the 0.56:1 propeller drive assembly (dwg 81498) was to be removed for visual inspection. If warranted, this inspection was to be repeated after 10 additional hours and at 20 hour intervals thereafter.
2. Upon completion of the four hour run-in or at the end of the first day, the No. 5 sump magnetic plug was to be removed and checked for chips and the first oil draining was to be screened for metal particles. [RG 342 RD2311. XR-7755, 503-602 Current]

26 Aug 1947. Carpenter reported the engine #102 would be ready to test with new rods in about a week. The 3-cylinder engine would be running at week's end with full-floating pins. In fatigue tests past 10 million cycles, over 1 in² silver came off the unloaded side; Lycoming was making up bronze bushings. The 36-cylinder engines had logged between 130 and 135 hours. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

Engine #102 Knock Limit Data
Speed	bsfc	Power
1,800 rpm	0.40 lb/hp/hr	2,300 hp
2,000 rpm	0.41 lb/hp/hr	2,700 hp
2,200 rpm	0.415 lb/hp/hr	4,000+ hp

1 Oct 1947. In a Lycoming telcon, Don McKay reported the engine #101 was being assembled for endurance running and would be complete in about two weeks. Engine #102 was on the dynamometer undergoing runs to establish its knock limit. A scavenge out tube leak had caused a fire with damage apparently only to the ignition system. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

Lycoming's J.H. Carpenter sent the PPL a curve showing XR-7755 No. 102 dynamometer performance to date. He noted that this engine was being removed from the test stand for tear down inspection following a test cell fire, which was believed to have originated because of a cracked scavenge pump outlet flange. Lycoming did not think the engine was damaged, but was being disassembled to remove any water that may have gotten inside as the fire was extinguished. Full-floating link pins were to be used when the engine was reassembled. Engine No. 101 was partially rebuilt and was to be mounted on the rigid test stand for endurance running by 15 October. This engine had all the latest revisions including full-floating link pins and stronger master rods. [RG 342 RD2311. XR-7755, 503-602 Current]

13 Oct 1947. Carpenter telephoned to report that the one bank of master rod link pin bearings had started to crack. Although not stated, these were apparently in #101, which had recently been rebuilt. This engine had only been run to 4,000 hp at 2,300 rpm. New rods with floating pins were prescribed. The 3-cylinder link pins had accumulated about 20 hrs, 11 of which were at takeoff power. Its pins were satisfactory. On an unspecified engine, about 6 hrs into a 50-hr test, a coolant jacket cracked at a coolant output fitting. A new bank design included a gusset to reinforce this area on all banks. The test would be resumed next day. In a later report, the coolant pump had cracked with 9 hrs left to go. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

15 Oct 1947. Lycoming's Carpenter submitted Monthly Report No. 63-7, a summary of Contract No. W33-038 ac-564 work covering September 1947.

Item 1 – Building and Performance Testing of XR-7755 Single-, Three- and Four-Cylinder Engines.
All work completed including 976.2 hrs testing, except for building the four-cylinder engine, the work on which was stopped because of the revised contract.

a. Torquemeter and Pump Test – Report on a torquemeter pump with increased clearances was being published.

b. Link Pin Fatigue Test – A link pin installed in the No. 4 position completed 10 million fatigue cycles at a load corresponding to 1,500 psi peak cylinder pressure without a failure. This position induces the greatest stress on the pin. A report was being written.

Items 10 and 11 – Fuel Injection Equipment. No additional equipment was received from suppliers during September.

Item 12-8 – Multi-Cylinder Engine Finished Parts. All parts for engine Nos. 101 and 102 were complete. About 95% of a third parts set was finished and in stock; replacement parts were being drawn from this stock.

Item 12-9 – Multi-Cylinder Engine Finished Spare Parts. Further work on this item was stopped by the revised contract.

Item 12-10 – Finished Parts for Two 2-Speed Reduction Gear Units. Part sets for two reduction gear units were complete except for six parts undergoing inspection and seven being machined.

Item 12-11 – Build-Up and Initial Test of Two XR-7755 Engines. During this period 16.7 test hrs were completed for an 88.5 hour total. XR-7755 No. 102 was run-in through 2,200 rpm. Additional data were collected for the induction system temperature survey. Variable manifold pressure runs were made at 1,800, 2,000 and 2,200 rpm at a fuel-air mixture bordering on incipient detonation. These data were being analyzed. During this running two cylinder barrel jackets cracked at or near the elbow. Two oil drain seal bushings worked loose from the cylinder head covers, resulting in an oil leak. These conditions were corrected. Another shutdown occurred when exhaust gasses apparently leaked in to the coolant passages; this was still being investigated. XR-7755 No. 101 was about 25% assembled. This engine's link pins were revised to the full-floating design. This engine was expected to be running in two to three weeks.

Item 12-12 – XR-7755-1 and XR-7755-5 Acceptance Tests. Total operating time under this item was 32.3 hours; further running was discontinued as per the revised contract.

Item 12-2 (Supp. 7, Item 1) – XR-7755 Single- or Three-Cylinder Endurance Testing. A total of 1,108.1 hrs testing had been accomplished under this item. With its 6.375" bore, 6.75" stroke and 646.36 in³ displacement, three-cylinder 3C-645-7 had run 26.7 hrs. After 5 hrs endurance running the engine was shut down due to high crankcase pressure and blowby. Inspection showed that the No 5 pistion No. 2 compression ring was broken and that all compression rings on the No. 2 piston were broken. This was blamed on coolant entering the oil system when the engine was hooked up. New rings were installed and run in. An additional 11 hrs running at takeoff power were accomplished before the engine was disassembled and the floating link pins found to be in good condition.

Item 12-4 (Supp. 7, Item 2) – XR-7755 Single-Cylinder Performance. A total of 1,544.7 test hours had been completed on engine SG-215-3 on the No. 3 dynamometer under this item; no further testing had been scheduled during this period.

Item 13 (Supp. 7, Item 3) – XR-7755 Nose-to-Nose Reduction Gear Test. No activity during this period.

Item 14 (Supp. 7, Item 4) – XR-7755 Flight Approval Test. Test time under this item remained at 61.4 hrs. Work was expected to resume once preliminary work under Item 12-11 was complete. [RG 342 RD2311. XR-7755, 503-602 Current]

The other engine (presumably #102) was on the dynamometer and would be run early next week. Its run-in would be up to 5,000 hp, along with other calibration activities. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

17 Dec 1947. Carpenter reported that link pins were satisfactory and that a secondary balancer quill shaft had broken, but running continued. This presumably referred to engine #102. [Penciled notes by J.G. Blackwood. RG 342 RD2311 XR-7755, 1943-1948: 503-602 Conf and Tel Notes.]

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Item 2	(2,271.6 hrs test)	Item 12-1	(122.3 hrs test)
Item 3	(773.6 hrs test)	Item 12-2	(1,988.7 hrs test)
Item 4	Design	Item 12-3	(562.9 hrs test)
Item 5	Mock-Up	Item 12-4	(1,000 hrs test)
Item 6	Parts, Finished	Item 12-5	(299.1 hrs test)
Item 7	Parts, Rough	Item 12-6	Design
Item 9	(302.6 hrs test)	Item 12-7	Design
		Item 12-13	(500 hrs test)
		Item 12-14	(500 hrs test)
		Item 12-15	(200 hrs test)


These early-1947 Blackwood sketches suggest that Lycoming was still being troubled by and tinkering with its XR-7755 knuckle pin and supercharger diffuser designs.