Continental R-670

The Continental R-670, officially designated W670 by its manufacturer, was a seven-cylinder four-stroke radial aircraft engine that emerged from the Continental Motors workshops in 1934. With a total displacement of 668 cubic inches, or approximately 11 litres, and a remarkably lean dry weight of 465 pounds (211 kg), this engine was less a technological marvel and more a workhorse – a necessary component in the relentless, often unglamorous, pursuit of flight and ground mobility during a tumultuous era. Its power output, which modestly ranged from 210 to 240 horsepower at a consistent 2,200 revolutions per minute, served as a foundational power plant for numerous aircraft throughout the 1930s and 1940s.

This particular radial engine didn't spring from a vacuum; it was the direct, albeit somewhat improved, successor to Continental's inaugural radial design, the 170 horsepower Continental A-70. The R-670 carried the torch of its predecessor, finding widespread adoption in critical roles. Perhaps its most notable aerial application was powering the ubiquitous PT-17 Stearman biplane, which served as the primary training aircraft for countless pilots within the U.S. military during World War II. It was, in essence, the mechanical heart that taught a generation to fly, a thankless task that few machines truly appreciate.

Beyond the skies, the R-670, with its robust design and relatively compact footprint, proved surprisingly adaptable. It was pressed into service, rather unceremoniously, within a range of light armored vehicles that saw action during World War II. This dual utility speaks volumes about the engine's reliability and the pragmatic design philosophy of the era, where an engine wasn't just for one purpose, but for any purpose that could squeeze it in and make it run. It seems even engines weren't allowed to specialize in those days.

Variants

The Continental W670 series, as documented in the rather definitive pages of Jane's All the World's Aircraft 1938, wasn't a static design, but rather a family of incremental improvements and adaptations, each variant meticulously tweaked to meet evolving operational demands or, more likely, to accommodate the peculiarities of various airframes and fuel availabilities. It's almost as if engineers, in their infinite wisdom, understood that one size rarely fits all, even for something as fundamentally simple as converting fuel into thrust.

The primary distinctions among these variants revolved around their fuel system (carbureted versus fuel-injected), compression ratio, the requisite octane rating of their fuel, and the configuration of their exhaust systems, which, surprisingly, made a difference to aircraft designers who had to contend with things like cooling and structural integration.

W670-K: This foundational variant featured a conventional carburetor and operated with a compression ratio of 5.4:1, requiring 65 Octane fuel. Its exhaust system was configured for front exit, delivering a solid 225 horsepower (168 kilowatts). A reliable, if somewhat uninspired, starting point.
W670-L: Identical to the -K in its carburetor, compression ratio, and power output, the -L variant distinguished itself by demanding 73 Octane fuel and routing its exhausts to the rear. This subtle change in fuel requirement reflected a gradual shift towards higher-performance fuels becoming more widely available.
W670-M: Stepping up the performance ladder, the -M variant retained the carburetor but pushed the compression ratio to 6.1:1, necessitating 80 Octane fuel. With its front exhausts, it managed to squeeze out an additional 15 horsepower, reaching 240 horsepower (179 kilowatts). A modest gain, but in aviation, every bit counts, or so they say.
W670-N: This variant mirrored the -M's increased compression and 80 Octane fuel requirement, also delivering 240 horsepower (179 kilowatts). Its key differentiation was the rear exhaust configuration, offering designers another option for integration.
W670-K1: Marking a significant technological leap, the -K1 introduced fuel injection, a more precise method of fuel delivery that offered improved efficiency and reliability compared to carburetion. While maintaining the 5.4:1 compression ratio and front exhausts, it required 73 Octane fuel and produced a slightly enhanced 230 horsepower (172 kilowatts). Progress, even if it was just a few more horses.
W670-L1: The fuel-injected counterpart to the -L, this variant also boasted a 5.4:1 compression ratio and consumed 73 Octane fuel, but directed its exhausts to the rear. Its output matched the -K1 at 230 horsepower (172 kilowatts).
W670-M1: Combining the benefits of fuel injection with the higher 6.1:1 compression ratio, the -M1 was designed for 80 Octane fuel and featured front exhausts. This configuration yielded the highest power output in the series, a respectable 250 horsepower (186 kilowatts).
W670-N1: The zenith of the W670 line, this variant shared the -M1's fuel injection, 6.1:1 compression ratio, and 80 Octane fuel requirement, delivering 250 horsepower (186 kilowatts). Its distinguishing feature, once again, was the rear exhaust layout, providing maximum power with flexible installation.

Applications

The Continental R-670 truly earned its reputation for versatility, proving itself adept not only in the air but also on the ground, a testament to its robust and adaptable design. It wasn't the flashiest engine, but it was reliable, and in the volatile decades of the 1930s and 1940s, reliability was far more valuable than glamour. Its applications read like a roster of workhorses, from primary trainers that shaped future aviators to armored vehicles that slogged through the mud of global conflict.

Aircraft

The R-670 powered a diverse array of aircraft, predominantly in roles where steadfast performance outweighed cutting-edge speed or altitude. These machines were the unsung heroes, performing essential tasks that kept the aviation world turning.

American Airmotive NA-75: A post-war conversion of the Stearman, showcasing the enduring nature of the R-670.
Boeing-Stearman Model 75 (PT-17, N2S): This is perhaps the R-670's most iconic application, serving as the backbone of Allied pilot training. Millions of hours were logged with this engine humming away.
CallAir Model A: A utilitarian agricultural aircraft, where reliability and low maintenance were paramount.
Cessna 190: A larger, single-engine transport aircraft, indicating the engine's capability for more substantial airframes.
Eagle Aircraft Eagle 220: A biplane designed for aerobatics and agricultural work.
Fairchild PT-23: Another critical primary trainer, solidifying the R-670's role in foundational flight instruction.
Funk F-23: A light aircraft, demonstrating the engine's suitability for smaller, civilian applications.
G class blimp: A rather unexpected application, highlighting the engine's ability to provide sustained, reliable power for lighter-than-air craft.
Grumman G-164 Ag Cat: A dedicated agricultural aircraft, emphasizing the need for robust, dependable power in demanding environments.
Kellett K-2A, K-4: Early autogiros, showcasing the engine's adaptability to novel rotorcraft designs.
Morane-Saulnier MS.317: A French trainer aircraft, indicating the engine's international reach.
Timm N2T Tutor: Another trainer, emphasizing the engine's widespread adoption in flight schools.
Waco 240-A: Part of the diverse Waco biplane family.
Waco Standard Cabin series (UEC, UIC, UKC, UKC-S, UKS, VKS): A popular series of civilian biplanes, where the R-670 contributed to their reputation for reliability and comfort.
Waco Custom Cabin series (UOC, VQC): Further examples of the R-670's use in higher-end civilian aircraft.
Waco A series (UBA, ULA): More variants of Waco's versatile aircraft.
Waco F series (UBF, UMF, UPF): A comprehensive range of Waco aircraft, all benefiting from the R-670's dependable power.

Armored fighting vehicles

Perhaps less glamorous than soaring through the air, the R-670's contribution to ground warfare was equally significant, providing motive power for a variety of light armored vehicles when other, more specialized engines were either unavailable or unsuitable. It seems this engine was destined to be a utility player, whether in the sky or in the mud.

T2 Combat Car: An early prototype of light armored vehicles.
T4 Combat Car: Another experimental combat car.
M1 Combat Car: One of the first standardized light armored vehicles used by the U.S. Army.
M2 Light Tank: A precursor to more famous light tanks, it relied on the R-670 for mobility.
M3 Stuart: An iconic light tank of World War II, the R-670 powered many of its early variants, providing the essential mobility for reconnaissance and flanking maneuvers.
Landing Vehicle Tracked (LVT-2, -4; LVT(A)-1, -2, -4, -5): These amphibious assault vehicles were crucial for island hopping campaigns in the Pacific, and the R-670 provided the robust power needed to propel them through water and over land.

Specifications (R-670-K)

Delving into the cold, hard numbers of the Continental R-670-K variant reveals the pragmatic engineering choices that defined this engine. These aren't just figures on a page; they are the distilled essence of its operational character, a blueprint of how it managed to convert fuel into the necessary force for flight or ground movement. The data, meticulously compiled from Jane's All the World's Aircraft 1938, offers a glimpse into the mechanical heart of a bygone era.

General characteristics

Type: A quintessential seven-cylinder air-cooled radial engine. This configuration was a popular choice for aircraft of the era, offering a good power-to-weight ratio and inherent cooling efficiency due to the cylinders being exposed directly to the airflow. It's a simple, elegant solution that worked.
Bore: 5+1⁄8 inches (130.2 mm). The diameter of each cylinder, a critical dimension that, in conjunction with the stroke, dictates the engine's displacement and potential power.
Stroke: 4+5⁄8 inches (117.5 mm). The distance the piston travels within the cylinder. The relatively short stroke compared to the bore suggests an engine designed for higher rotational speeds, albeit within the limits of the technology available at the time.
Displacement: 668 cubic inches (10.95 liters). This total volume swept by all pistons in a single cycle is a primary indicator of an engine's potential power output. For its time, it was a mid-range displacement, suitable for trainers and light transport aircraft.
Length: 34+3⁄16 inches (868.4 mm). The overall length from the mounting flange to the front of the propeller shaft, a crucial dimension for aircraft integration, particularly in terms of cowling design and center of gravity.
Diameter: 42+1⁄2 inches (1,079.5 mm). The maximum width of the radial engine, another significant factor for aerodynamic cowling design. Radial engines, by their very nature, tended to be wide, presenting a challenge for minimizing drag.
Dry weight: 450 pounds (204.1 kg). This measurement, without fluids like oil or fuel, is critical for aircraft designers. For an engine of its power class, the R-670-K offered a respectable power-to-weight ratio, contributing to its suitability for trainer aircraft where performance and handling were important.

Components

Valvetrain: Each cylinder was equipped with one inlet and one exhaust valve. This relatively simple two-valve-per-cylinder configuration was standard for many engines of the period, offering a good balance of manufacturing simplicity, reliability, and performance without the complexity of multi-valve designs.
Fuel system: The R-670-K relied on a single Stromberg NA-R6 Carburetor. Carburetors were the dominant fuel delivery system for piston engines before the widespread adoption of fuel injection, mixing air and fuel before it entered the cylinders. The Stromberg unit was a common and reliable choice for aircraft engines of this type.
Fuel type: This variant was designed to operate on 65 octane gasoline. This lower octane requirement made the engine less demanding on fuel quality, a practical consideration during periods when high-octane aviation fuel was not universally available or was reserved for higher-performance combat aircraft.
Oil system: A dry sump system was employed, featuring one pressure pump and one scavenge pump, with an enclosed valvetrain. Dry sump systems are beneficial in aircraft as they ensure a consistent supply of oil regardless of the aircraft's attitude (e.g., during aerobatics) and allow for more flexible engine mounting due to the external oil tank. The enclosed valvetrain offered protection from contaminants and improved lubrication.
Cooling system: As with most radial engines, cooling was achieved through direct air-cooling. Fins on the cylinders dissipated heat directly into the airflow generated by the propeller and the aircraft's forward motion. This simple, robust system avoided the complexities and weight of a liquid-cooling system.

Performance

Power output: 225 horsepower (168 kilowatts) at 2,175 rpm. This figure represents the continuous rated power, a reliable output for sustained operation, rather than a peak burst.
Specific power: 0.337 hp/in³. This metric indicates how much horsepower is produced per cubic inch of displacement, offering insight into the engine's design efficiency in terms of power generation relative to its size.
Compression ratio: 5.4:1. This relatively conservative compression ratio was suitable for the 65 octane fuel, balancing power output with reliability and preventing engine knock or pre-ignition.
Fuel consumption: 13 US Gallons per hour (49 liters per hour) at cruising rpm. While not optimized for modern efficiency standards, this was a typical consumption rate for an engine of its power and era, a necessary cost for keeping things airborne.
Specific fuel consumption: 0.54 lb/hp/hr (0.328 kg/kW/hr) at rated rpm. This value indicates the efficiency with which the engine converts fuel into power, measured by the weight of fuel consumed per unit of horsepower per hour.
Oil consumption: 0.4 US Gallons per hour (1.5 liters per hour) at cruising rpm. A tangible reminder that engines of this vintage, unlike their modern counterparts, openly consumed oil as part of their operational cycle.
Power-to-weight ratio: 0.49 hp/lb at cruising rpm. This is a critical performance indicator for any aircraft engine, highlighting how much power the engine produces relative to its own mass. A higher ratio generally translates to better aircraft performance.