Ah, the Jacobs R-755. A relic, really. Not that relics aren’t without their… uses. This particular radial engine, a seven-cylinder, air-cooled specimen, carries the company designation L-4. It hails from the United States, a product of the Jacobs Aircraft Engine Company , a name that used to mean something more than just a footnote in aviation history. Its first breath, its inaugural run, was in 1933. And remarkably, it persisted, clinging to life and production lines well into the 1970s. A testament to stubborn engineering, I suppose, or perhaps just a lack of imagination elsewhere.

Design and Development

The R-755 was conceived in an era when aircraft engines were more art than science, more brute force than refined efficiency. Its design, finalized around 1933, featured a bore of 5.25 inches and a stroke of 5 inches, a rather pedestrian combination that yielded a displacement of 757 cubic inches, or a more manageable 12.4 liters. Power output, as you might expect from an engine that spanned decades, varied, ranging from a modest 200 horsepower up to a more spirited 350 horsepower, translating to 150 to 260 kilowatts. The construction itself was typical of its time: robust steel cylinders topped with aluminum-alloy cylinder heads. A particular variant, the R-755E, was even adapted for the peculiar demands of helicopter propulsion. Helicopters. Imagine.

Variants

The R-755 wasn’t a monolithic entity; it evolved, or at least it was tweaked.

• R-755A1: This was the standard, the baseline. Direct drive, the workhorse of the family. Nothing particularly exciting, but it did the job.
• R-755A2: A step up, offering a more robust 300 horsepower. For those who felt the A1 was… lacking.
• R-755A3: Largely similar to the A1, but distinguished by its use of Scintilla magnetos. A minor detail, perhaps, but details matter.
• R-755B1: A de-rated version of the R-755A, specifically intended to drive a fixed-pitch airscrew. Think of it as the engine for those who preferred simplicity over complexity.
• R-755B2: Another de-rated variant, this one designed for the slightly more sophisticated variable or controllable pitch airscrew. For pilots who liked to fiddle.
• R-755E: This was the one for the helicopters, as mentioned. Up-rated and equipped with reduction gearing. A necessary compromise for vertical flight.
• R-755EH: A specialized iteration, developed with the sole purpose of powering the Jacobs Type 104 Gyrodyne. Quite niche.

Applications

This engine found its way into a surprisingly diverse range of aircraft, a testament to its reliability, if not its cutting-edge technology. It powered the Waco YQC-6 , a rather charming cabin biplane. You’d also find it on the elegant Beechcraft Model 17 Staggerwing in its B17L, C17L, and E17L configurations. The Boeing-Stearman PT-18 Kaydet , a trainer known for its robustness, also utilized it. For twin-engine trainers, the Cessna AT-17 Bobcat was a common sight. Then there was the Cessna 195 , a larger, single-engine offering. The Funk F-23 , a rather obscure light aircraft, also employed the R-755. For agricultural work, the Grumman G-164 Ag Cat was a familiar sight, often powered by this very engine.

It was also fitted to the Kellett KD-1 , an early autogyro. The Lascurain Aura and the Morane-Saulnier MS.505 Criquet , a liaison aircraft, were other applications. WACO, a significant manufacturer of the era, used it in various models, including the F series (YMF, YPF), the Custom Cabin series (YOC, YQC), and the Standard Cabin series (YKC, YKC-S, YKS-6). There was even a prototype, the Waco PG-3 , a twin-engined powered version of a glider, that featured this engine, though it never progressed beyond the experimental stage.

Engines on Display

If you have a peculiar interest in such things, you can find preserved examples of the R-755 on display. The Arkansas Air Museum has one. The Aerospace Museum of California also boasts a Jacobs R-755. In Medicine Hat, Canada, Super T Aviation Academy has a preserved unit. And at the House of Whitley, a restored Jacobs R-755A can be found. Museums. Where old things go to be forgotten, or perhaps, appreciated by those with too much time on their hands.

Specifications (R-755-A1)

Let’s delve into the rather uninspired technical details of the R-755-A1.

General Characteristics

• Type: A seven-cylinder, air-cooled radial piston engine. Simple, direct.
• Bore: 5.25 inches (133 mm).
• Stroke: 5 inches (130 mm).
• Displacement: 757 cubic inches (12.4 liters).
• Length: 37 inches (940 mm) overall. Not exactly compact.
• Diameter: 43.5 inches (1,100 mm). It’s a radial, after all.
• Dry weight: 505 pounds (229 kg). Heavy, but not excessively so for its era.

Components

• Valvetrain: Two valves per cylinder, operated by pushrods. The exhaust valves were sodium-cooled. A common, if somewhat crude, method.
• Fuel system: A single Stromberg NA-R7A carburetor. Uncomplicated.
• Fuel type: 73 octane. Readily available, unremarkable.
• Oil system: Standard setup with one pressure pump and two scavenge pumps.
• Cooling system: Air-cooled. The most basic form of thermal management.
• Reduction gear: Direct drive. No fancy gearing to complicate matters. It was a right-hand tractor configuration.

Performance

• Power output: 225 horsepower (168 kW) at 2,000 rpm at sea level. Adequate for its intended purpose.
• Specific power: 0.32 hp/cu-in (14.8 kW/L). Not groundbreaking, but functional.
• Power-to-weight ratio: 0.48 hp/lb (0.8 kW/kg). Again, serviceable.

See Also

For those who wish to compare or contrast, similar engines include the Armstrong Siddeley Cheetah and the Wright R-760 .

Related Lists

A more comprehensive overview can be found in the List of aircraft engines .