Lycoming R-680

Here we go. Another engine. As if the universe needed more moving parts.

1920s American aircraft piston engine

R-680

The Lycoming R-680, a name that probably conjures images of efficiency for some, is a nine-cylinder, air-cooled radial engine. It holds the rather unglamorous distinction of being the very first aero-engine ever produced by Lycoming itself. Apparently, even ancient civilizations like Lycoming had to start somewhere. This particular piece of early 20th-century engineering hails from the United States, a nation consistently keen on putting things in the air, usually with a lot of noise. Its creation was overseen by the aerospace manufacturer Lycoming Engines, a company that, rather predictably, went on to produce quite a few more engines.

The R-680 first sputtered to life, or so the records claim, in 1929. Its production run spanned a respectable 15 years, from 1930 to 1945, which, if you're keeping track, neatly encompasses the lead-up to and duration of a rather significant global unpleasantness, making it a workhorse of its era. By the time production ceased, a staggering number of these engines—over 26,000 units—had been manufactured. A testament, I suppose, to its utility, or perhaps just the sheer demand for anything that could reliably lift an aircraft off the ground during that period.

This engine was primarily offered in two main series, designated as the E and B types. Despite the distinct nomenclature, they were, in essence, fundamentally similar in their core design and function. The differences were often subtle refinements rather than wholesale redesigns, a common enough practice when you've finally landed on something that mostly works. One notable iteration was the B4E, which saw particular application as a trainer version. This variant featured a front exhaust collector "ring," an ingenious, or at least practical, design choice that allowed it to be used effectively without the need for additional cylinder air baffles. For trainer aircraft, this simplification meant reduced complexity, lower manufacturing costs, and potentially easier maintenance, allowing fledgling pilots to focus on not crashing rather than the intricacies of engine cooling. The R-680 was officially granted Approved Type Certificate No. 42 on February 4, 1930. This certification was a critical milestone, signifying that the engine had met stringent regulatory standards for airworthiness and reliability, paving the way for its widespread adoption in various aircraft models across both civilian and military sectors. This formal approval was essential for any aircraft component destined for commercial or governmental use in the nascent, but rapidly expanding, aviation industry of the time.

Its major applications, which are often the true measure of an engine's impact, included:

The Beechcraft AT-10 Wichita, a twin-engine advanced trainer that saw extensive use during World War II, preparing pilots for larger, multi-engine aircraft.
The iconic Stinson Reliant, a single-engine utility aircraft favored for private transport, bush flying, and even military liaison roles, known for its distinctive gull wings.
The Curtiss-Wright AT-9, another advanced trainer, nicknamed the "Jeep" for its challenging handling characteristics, which effectively weeded out less capable pilots.
And the Stinson Airliner, an early passenger transport aircraft that helped define the burgeoning commercial aviation market, demonstrating the R-680's versatility beyond military training.

Variants

The Lycoming R-680 wasn't a static design; it evolved, albeit incrementally, through several variants. Each iteration generally aimed to squeeze a bit more power or efficiency from the existing nine-cylinder architecture, responding to the ever-present demand for faster, more capable aircraft. This progression of variants showcases the typical engineering pathway of the era: refine, test, slightly improve, and then release another designation.

R-680: The original, unadorned model, rated at a modest 210 horsepower (160 kilowatts) at 2,000 revolutions per minute (rpm). A solid, if unspectacular, starting point for Lycoming's foray into radial engines. It was adequate for the lighter, less demanding aircraft designs of the late 1920s and early 1930s, laying the groundwork for future developments.
R-680B4E: This variant represented a slight uplift in performance, delivering 225 horsepower (168 kilowatts) at a slightly increased 2,100 rpm. As mentioned, this was a popular choice for trainer aircraft, where reliability and ease of maintenance often trumped raw power, but a little extra oomph was always appreciated. The designation itself, with its alphanumeric soup, hints at the specific modifications and applications it was designed for.
R-680BA: Pushing the envelope further, the R-680BA was rated at 240 horsepower (180 kilowatts). This steady increase in power output reflected ongoing advancements in fuel technology and internal engine design, allowing engineers to extract more energy from the same displacement without drastically altering the engine's physical footprint.
R-680E3A: A more significant leap, this variant produced 285 horsepower (213 kilowatts) at 2,200 rpm. This substantial power increase would have notably improved the performance characteristics of aircraft equipped with it, offering better climb rates, higher top speeds, and greater payload capacity. Such improvements were crucial for military trainers that needed to simulate higher-performance combat aircraft or for civilian transports carrying more passengers or cargo.
R-680-6: This specific model offered 245 horsepower (183 kilowatts). It seems to slot in between some of the earlier and later higher-powered variants, suggesting a specific optimization for certain aircraft types or operational requirements that didn't demand the absolute peak performance of the E3A.
R-680-9: Rated at a robust 300 horsepower (220 kilowatts) at 2,200 rpm, the R-680-9 marked a substantial achievement for the design. Reaching the 300 hp threshold for a radial engine of this displacement and era was a significant engineering feat, broadening the range of aircraft that could effectively utilize the R-680 series.
R-680-13: Matching the R-680-9 in power, this variant also delivered 300 horsepower (220 kilowatts) at 2,200 rpm. The different numerical designation likely indicates a specific set of modifications or a slightly different configuration, perhaps related to accessory drives, mounting points, or minor internal component changes, rather than a fundamental change in power output. It’s the kind of subtle distinction that only engineers and historians of obscure machinery truly appreciate.

Applications

The pervasive nature of the Lycoming R-680 across a diverse range of aircraft highlights its fundamental reliability and adaptability during a critical period in aviation history. From military trainers to civilian transports and even experimental designs, this engine provided the necessary thrust, proving that sometimes, the simplest solutions are the most enduring.

Beech AT-10 Wichita: As previously noted, this twin-engine advanced trainer was a linchpin in the Allied pilot training programs during World War II. The R-680's presence in such a vital role speaks volumes about its dependable performance in the rigorous environment of flight schools.
Boeing-Stearman PT-13: A truly iconic primary trainer, the Stearman biplane, in its PT-13 variant, relied on the R-680. Thousands of American military pilots learned to fly in these robust aircraft, making the R-680 an integral, if unseen, part of their foundational flight experience.
Cessna AT-8/AT-17: These twin-engine advanced trainers, known as the "Bobcat" or "Crane," also utilized the R-680. They served to bridge the gap between primary trainers and multi-engine combat aircraft, providing essential experience in managing two engines.
Curtiss-Wright AT-9: Another advanced trainer, the AT-9 was specifically designed to prepare pilots for high-performance, multi-engine bombers like the B-26 Marauder. Its demanding flight characteristics, powered by the R-680s, ensured that only the most skilled pilots progressed.
Fleetwings BQ-2: This application is a curious one, as the BQ-2 was a prototype "assault drone," an early precursor to modern unmanned aerial vehicles. The R-680 powered this experimental aircraft, demonstrating its use even in the cutting edge (for the time) of aviation technology.
Spartan NP-1: A naval primary trainer, the NP-1 also employed the R-680. This highlights the engine's acceptance across different branches of the U.S. military for basic flight instruction.
Stinson Airliner: In the commercial sector, the R-680 found a home in early passenger aircraft, underscoring its reliability for civilian transport operations where safety and consistent performance were paramount.
Stinson Reliant: A versatile aircraft used for everything from private travel to military liaison, the Reliant's enduring popularity was partly due to the dependable power provided by the R-680. Its robust nature made it suitable for operations from less-than-ideal airstrips.
Stinson L-1 Vigilant: This observation and liaison aircraft, known for its excellent short take-off and landing (STOL) capabilities, relied on the R-680 for its ability to operate from confined spaces, often close to the front lines.
Waco S Series: The Waco S series encompassed a range of popular biplanes used for personal transport, air racing, and even some light utility roles. The R-680 provided a solid power plant for these civil aviation stalwarts, cementing its reputation beyond military applications.

Specifications (R-680-E3A)

Data for the R-680-E3A variant, as compiled from sources like Jane's Fighting Aircraft of World War II, offers a glimpse into the raw technical details of this rather prolific engine. It's all very precise, as if the numbers themselves held some cosmic truth.

General characteristics

Type: A quintessential nine-cylinder, air-cooled radial engine. This configuration, with cylinders arranged symmetrically around a central crankshaft, was a hallmark of aviation powerplants in the interwar period and throughout World War II, lauded for its inherent balance, relative simplicity, and effective air cooling. Its compact frontal area, despite the large diameter, made it suitable for many aircraft designs.
Bore: 4 5⁄8 inches (117 millimeters). The bore refers to the diameter of each individual cylinder.
Stroke: 4 1⁄2 inches (114 millimeters). The stroke defines the distance the piston travels within the cylinder from its uppermost to its lowermost point. The relatively close bore and stroke figures indicate a design that favored a good balance between piston speed and cylinder volume, contributing to efficient power delivery.
Displacement: 680 cubic inches (11.15 liters). This total volume represents the combined swept volume of all nine cylinders. A larger displacement generally correlates with greater potential for power output, all other factors being equal, as it allows for the combustion of more fuel-air mixture per revolution. For its time, 680 cubic inches was a respectable size for an engine in this power class.
Length: 37.5 inches (953.1 millimeters). This measurement indicates the overall dimension from the propeller flange to the rear of the accessory section, a critical figure for aircraft designers integrating the engine into a fuselage or nacelle.
Diameter: 43.5 inches (1104 millimeters). The diameter is the maximum width across the cylinder heads. Radial engines inherently have a larger diameter than inline engines of comparable power due to the circular arrangement of cylinders, a characteristic that influenced aircraft aerodynamics and visibility from the cockpit.
Dry weight: 515.46 pounds (233.9 kilograms). This weight measurement excludes fluids like oil and fuel. For an engine producing 285 to 330 horsepower, this was a competitive power-to-weight ratio for the era, crucial for maximizing an aircraft's performance, particularly in terms of climb and payload capacity.

Components

Valvetrain: Each cylinder was equipped with one inlet and one exhaust valve, positioned at a 30-degree angle. This overhead valve arrangement, common in radial engines, allowed for efficient gas flow into and out of the combustion chamber, contributing to the engine's overall performance and reliability. The simplicity of a two-valve per cylinder design was also a factor in its robustness and ease of maintenance.
Fuel system: The R-680-E3A utilized a single-barrel carburetor. This relatively straightforward fuel delivery system, while less precise than later fuel injection systems, was standard for the period, offering reliable operation and ease of troubleshooting. It mixed air and fuel in the correct proportions before delivery to the cylinders.
Fuel type: The engine was designed to run on 87 octane rating gasoline. The octane rating indicates the fuel's resistance to knocking or pre-ignition. 87 octane was a standard aviation fuel grade in the 1930s and 40s, widely available and suitable for the compression ratios common in engines of that design, ensuring reliable combustion without detrimental engine stress.
Oil system: A full pressure type oil system was employed. This system ensured that lubricating oil was actively pumped under pressure to all critical moving parts, such as the crankshaft bearings, connecting rod bearings, and cylinder walls. This robust lubrication scheme was essential for the longevity and reliability of the engine during sustained operation.
Cooling system: As its classification suggests, the R-680-E3A was an air-cooled engine. This method relies on the airflow over cooling fins on the cylinders to dissipate heat, a design choice that eliminated the weight and complexity of a liquid cooling system. Air-cooling was particularly well-suited for radial engines, where the exposed cylinders could effectively shed heat.

Performance

Power output: The R-680-E3A was capable of producing 330 horsepower (246 kilowatts) at 2,300 rpm when operating at sea level. This peak power output, a significant improvement over earlier variants, made it a potent choice for the advanced trainers and utility aircraft it powered, providing the necessary thrust for demanding flight maneuvers and higher operational speeds.
Compression ratio: With a compression ratio of 7:1, the engine efficiently compressed the fuel-air mixture before ignition. A higher compression ratio generally leads to greater thermal efficiency and power output, but also requires higher octane fuel to prevent knocking. The 7:1 ratio was a good balance for the available 87 octane fuel and the engine's design.
Power-to-weight ratio: The engine boasted a power-to-weight ratio of 0.64 horsepower per pound (1.05 kilowatts per kilogram). This metric is crucial for aircraft engines, directly impacting an aircraft's performance characteristics such as climb rate, maneuverability, and payload capacity. A higher power-to-weight ratio means more power for less mass, a constant pursuit in aerospace engineering. The R-680-E3A's figure was quite competitive for its class and era, contributing to its widespread adoption.