Jacobs Aircraft Engines
Compiled by Kimble D. McCutcheon
Published 6 Apr 2013; Revised 14 Apr 2018


Jacobs L-4 (R-755)
Fischer & Jacobs was organized in Philadelphia, Pennsylvania on 1 Jan 1926, and constructed an experimental air-cooled 4-cylinder, 2-stroke radial intended to replace Curtiss OX-5s. Although the engine reportedly weighed less than 250 lb, and produced equivalent power, it was never produced.

Renamed Aircraft Engine Corporation, the company moved to Camden, New Jersey for a short time, and finally settled in Pottstown, Pennsylvania when it acquired a plant from the Light Machine and Foundry Company during its 1931 liquidation. The Pennsylvania Secretary of State Corporation Registry indicates that the Jacobs Aircraft Engine Company was incorporated on 31 Aug 1932.

 

Early Engines

The first Jacobs engine, the Model LA-1, was a 7-cylinder air-cooled radial with a bore of 4.500" and a stroke of 4.750" that displaced 528.8 in³. Weighing 390 lb, it produced 180 hp at 1,800 rpm. The LA-1 received Approved Type Certificate No. 31 on 7 Feb 1929. Power was increased to 170 hp at 2,125 rpm in mid-1931, and about 60 were manufactured before the Type Certificate expired on 1 Dec 1935.

The LA-2, an enlarged version of the LA-1 with a 4.750" bore, displaced 589.2 in³. It weighed 400 lb and produced 195 hp at 2,075 rpm. This engine was awarded Approved Type Certificate No. 82 on 6 May 1932, which expired on 1 Dec 1935.

 

Jacobs briefly experimented with, but never produced, a Model B-1. This air-cooled 2-cylinder 2-stroke, with a bore of 3.000" and a stroke of 2.500", displaced 35 in³, weighed 60 lb, and produced 20 ~ 25 hp at 2,500 ~ 3,000 rpm.

 

 

The Jacobs Model L-3, which received Approved Type Certificate No. 75 on 12 Aug 1931, produced 51 hp at 2,125 rpm and weighed 189 lb. With a bore of 4.125" and a stroke of 4.750", the 3-cylinder L-3 displaced 190.4 in³. The Jacobs L-3, could be operated in either tractor or pusher configuration. Two flange-mounted Scintilla magnetos provided dual ignition, and a Stromberg NAS-3 carburetor furnished the mixture. Approximately 44 L-3s were built before the Type Certificate expired on 23 Jul 1941.

 

Classic Jacobs Engines — the L-4, L-5 and L-6

The Jacobs Model L-4, which received Approved Type Certificate No. 121 on 27 Feb 1934, incorporated many features of earlier Jacobs engines. The R-755 series military designations were built in profusion during WWII, powering numerous training and light transport aircraft. Displacing 754.8 in³, the original L-4 had a bore of 5.250", a stroke of 5.000", and a compression ratio of 5.375:1. When initially certified and weighed 505 lb, it had a normal rating of 225 hp (245 takeoff hp) at 2,000 rpm on 73 octane gasoline, but its output grew to 350 hp at 2,500 rpm in the R-755E.

Model L-4 cylinder heads, with integral rocker boxes, were heat-treated aluminum alloy castings into which forged steel cylinder barrels were screwed and shrunk. Intake valves were of stainless steel and exhaust valves were of chrome-nickel steel with sodium-filled hollow stems. Aluminum bronze was used for valve seats. Valves were actuated by a cam ring with three intake and three exhaust lobes. It ran at 1/6 crankshaft speed.

The two-piece crankshaft captured a one-piece master rod with six articulated rod. The crankshaft, master rod, and wrist pins were of alloy steel, but the articulated rods were aluminum alloy forgings machined to final dimensions by diamond boring, eliminating the need for bushings.

Forged aluminum alloy pistons had two compression and one oil ring above the wrist pin bore, and an oil ring below the pin. Full-floating wrist pins were positioned by aluminum end plugs.

Six main parts comprised the crankcase. A magnesium front case housed the cam mechanism and supported the propeller thrust bearing. An aluminum alloy diaphragm supported the front crankshaft main bearing. An aluminum alloy barrel type crankcase provided mounting pads for cylinders, a bearing diaphragm for the rear main crankshaft bearing. An aluminum alloy rear section contained a ring-type inlet manifold. An aluminum alloy intermediate rear bearing plate fed lubricating oil to the crankshaft and supported auxiliary drive gears. A magnesium accessory case provided mounting pads for ignition, generator, starter, oil pump, and tachometer.

Dual ignition typically consisted of both a battery/distributor and Scintilla magneto. A Stromberg NA-B7A carburetor was provided.

 

The Model L-5 (Military R-830), an enlarged version of the L-4, was introduced in 1936. Its bore was increased to 5.500", producing a displacement of 831.5 in³. The L-5 produced 285 hp at 2,000 rpm and weighed 515 lb.

The Model L-6 (Military R-915), an enlarged version of the L-5 with a stroke increase to 5.500" and displacement of 914.7 in³, appeared in 1938. It produced 300 hp at 2,100 rpm from 535 lb, and could produce a takeoff rating of 330 hp at 2,200 rpm for 5 minutes.

 

L-4 and L-5 Specifications
Designation L-4 L-4M L-4MB L-4MA7 L-5 L-5M L-5MB L-5MA7
Takeoff Rating (hp @ rpm) 225 @ 2,000 225 @ 2,000 225 @ 2,000 225 @ 2,000 285 @ 2,000 285 @ 2,000 285 @ 2,000 285 @ 2,000
Normal Rating (hp @ rpm) 225 @ 2,000 225 @ 2,000 225 @ 2,000 225 @ 2,000 285 @ 2,000 285 @ 2,000 285 @ 2,000 285 @ 2,000
Cruising Rating (hp @ rpm) 175 @ 1,900 176 @ 1,900 177 @ 1,900 178 @ 1,900 210 @ 1,900 211 @ 1,900 212 @ 1,900 213 @ 1,900
Cylinders 7 7 7 7 7 7 7 7
Bore 5.250" 5.250" 5.250" 5.250" 5.500" 5.500" 5.500" 5.500"
Stroke 5.000" 5.000" 5.000" 5.000" 5.000" 5.000" 5.000" 5.000"
Displacement 757.66 in³ 757.66 in³ 757.66 in³ 757.66 in³ 831.54 in³ 831.54 in³ 831.54 in³ 831.54 in³
Compression Ratio 5.375:1 5.375:1 5.375:1 5.375:1 6.0:1 6.0:1 6.0:1 6.0:1
Ignition (Scintilla) 2 Distributors 2 Magnetos 1 Distributor
1 Magneto
2 Magnetos 2 Distributors 2 Magnetos 1 Distributor
1 Magneto
2 Magnetos
Normal Fuel Consumption
(lb/bhp/hr)
0.50 - 0.55 0.50 - 0.55 0.50 - 0.55 0.50 - 0.55 0.53 0.53 0.53 0.53
Cruising Fuel Consumption
(lb/bhp/hr)
0.48 - 0.52 0.48 - 0.52 0.48 - 0.52 0.48 - 0.52 0.35 0.35 0.35 0.35
Length 38.875" 40.375" 40.375" 40.375" 36.969" 36.969" 36.969" 36.969"
Diameter 43.500" 43.500" 43.500" 43.500" 43.500" 43.500" 43.500" 43.500"
Basic Dry Weight 480* ± 3 lb 473 ± 3 lb 494* ± 3 lb 476 ± 3 lb 510* ± 3 lb 504 ± 3 lb 525* ± 3 lb 510 ± 3 lb
Specific Takeoff Power 0.469 hp/lb 0.476 hp/lb 0.456 hp/lb 0.473 hp/lb 0.559 hp/lb 0.566 hp/lb 0.543 hp/lb 0.473 hp/lb
  * Includes generator and control box

 

Specifications Pertinent
to All L-4 and L-5 Models
Ignition Timing 30° BTC
Spark Plugs BG 482
Minimum Fuel Grade 73 Octane
Normal Oil Consumption 0.018 lb/bhp/hr
Cruising Oil Consupmtion 0.014 lb/bhp/hr
Carburetor Stromberg NA-R7A
Valve Timing at Hot Clearance
(IN = 0.035"; EX = 0.040")
 Intake Opens 12° BTC
 Intake Closes 55° ATC
 Exhaust Opens 55° BBC
 Exhaust Closes 10° ATC
 Intake Opening Duration 248°
 Exhaust Opening Duration 248°
 Intake Cold Clearance 0.008"
 Exhaust Cold Clearance 0.008"
Crankshaft Spline SAE No. 20

 

- More About the Jacobs L-4 (R-755) in the Members’ Section -

The XR-1530

In early 1943, Jacobs proposed two new 14-cylinder two-row engines based on R-755 components to the US Army Air Corps, and sent draft specifications on 24 Mar 1943. These engines probably were to have had a bore of 5.25" and a stroke of 5.063", for a displacement of 1,534.3 in³. The XR-1530C, which was projected to produce 900 hp, was direct drive, and the geared XR-1530D was to produce 1,140 hp. Jacobs reported the R-1530C design to be complete and 90% of the detail drawings done in an 11 May 1943 letter to the Chief, Power Plant Laboratory.

On 25 May 1943 conferences among representatives of the Production Engineering Section and Power Plant Laboratory met to decide the fate of Jacobs R-1530 engines. The group decided that it improbable that the R-1530C could really be completed within a year, as Jacobs had promised, and that the R-1530D was too powerful for training use but not powerful enough for tactical use. With no current application or perceived future need for either engine, the group recommended that the Army Air Forces not pursue their development. These findings were summarized in Memorandum Report ENG-57-503-592 dated 17 Jun 1943, which was forwarded to Jacobs.

Despite this rather chilly reception, Jacobs Vice President Henry McFadgen and Chief Engineer CE Mines persisted, apparently convincing the Power Plant Laboratory to lend accessories for the testing of an R-1530D. It is not known whether any R-1530s were ever completed or tested.

In addition to making Jacobs-designed engines, the Jacobs firm also produced Pratt & Whitney R-985s and R-1340s under license from 1941 through 1945.

The Jacobs Aircraft Engine Company was absorbed into Republic Industries after WWII.

References

Aerosphere 1939
FAA Type Certificate Data Sheets
National Archives Record Group 342, RD2629, Jacobs R-1530-0 Engine 1943

Acknowledgements

Many thanks to Paul Christainsen, who located and forwarded the R-1530 file from the US National Archives. Thanks also to Michael Smith who provided material from which the specifications were drawn.

Nice images of the Jacobs manufacturing facility, brochures and posters can be see at the Bomber Command Museum of Canada web site.