POWERED BY THE CONTINENTAL A-40 ENGINE

BY MARK W. STEWART

WHEN THE CONTINENTAL A-40 engine was introduced in 1931, it changed everything in general aviation. Few people know of the A-40 engine or the pivotal role it played. This little engine gave birth to general aviation as we know it today.

The 37-hp A-40 completely changed the cost of flying. It was now economical for flight schools to purchase, operate, and maintain these little engines. As the world’s first horizontally opposed four-cylinder engine, it launched a whole new engine design platform that is still in production today. The compact design of the A-40 also allowed aircraft designers to reduce the drag of the engine and streamline the entire aircraft to achieve 200 mph for a fourplace airplane.

Prior to the A-40, flight schools were few and far between because they were not profitable. Flying heavy and expensive radial-engine-powered airplanes was only for the rich, especially in the early 1930s and the Great Depression. The lightweight, low-cost A-40 engine, paired with the introduction of lightweight aircraft, changed all of that. Now, for the first time, flight schools could be profitable and the average person could not only afford flying lessons but also their own airplane. Piper issued a promotional poster for its Cubs in the summer of 1936 with the headline “Count the Cubs.” The claim of 10,000 student pilots was probably an exaggeration, but it did emphasize the fact that flight schools were popping up all around the country and that they were profitable flying the A-40 engine, especially on Cubs.

Not only did the A-40 make flying affordable for the general public, but also it started a whole new engine design platform, the horizontally opposed four-cylinder engine. Think of the many hundreds of thousands of horizontally opposed four-cylinder engines that have been built over the past nine decades, and they all owe their start to the A-40. Manufacturers like Piper, Beech, Cessna, Aeronca, Mooney, and many more all used the flat four-cylinder horizontally opposed engines on their aircraft, from engine makers like Continental, Lycoming, Franklin, and Menasco. Whether fourcylinder, six-cylinder, or even the 400-hp, eight-cylinder horizontally opposed engine, the A-40 is the grandfather of them all.

Lightweight Aircraft Designers Were Frustrated

A lightweight engine that could lift two people into the air was the longtime dream of many flyers and aircraft designers. After World War I and through the 1920s, the war surplus 90-hp Curtiss OX-5 engines powered many different airplanes. Although few in number, these engines had notorious reliability issues and high maintenance costs. The antiquated and heavy V-8 engine design was barely a suitable powerplant, even for large biplanes.

The radial engines of the late 1920s and early ’30s — built by Wright, Kinner, Warner, Continental, and Lycoming — were big, heavy, and expensive to buy and maintain. The options for a lightweight radial engine were limited to the threecylinder, 45-hp Szekely SR-3, or the 40-hp Aeromarine AR-3 engines. Both of these engines had terrible vibration characteristics and marginal reliability. Even the French nine-cylinder radial, the 50-hp Salmson engine, was complex and outrageously expensive.

A couple of inline engines were available: the 25-hp Heath-Henderson four-cylinder and the 20-hp Light Manufacturing Co.’s Tiger Kitten. The Tiger Kitten was an inverted two-cylinder design. However, neither of these engines could get two people airborne, and they achieved little success in the marketplace.

Aeronca Aircraft (the Aeronautical Corp. of America) had purchased the rights to the Poole-Galloway twocylinder engine in May 1929, which evolved into the Aeronca E-107. The design was a horizontally opposed, two-cylinder, four-cycle engine, which meant the propeller was only powered half the time. To have a continuous power stroke on a four-cycle engine, the engine must be designed with four cylinders. The little E-107 developed 26-29 hp and was used on the Aeronca C-2; about 170 C-2s were built.

Aeronca modified the design in 1930 with a bigger bore and overhead valves, and it became the 36-hp E-113 engine. The Aeronca C-3 and Aeronca K used this engine up through early 1938. Aeronca had decided early on not to sell its engines to any other aircraft manufacturers, and as a result the engine had limited success. Aeronca had manufactured about 1,000 of the E-series engine from 1929 until it was abandoned in 1938.

What Was an Aircraft Designer to Do?

The lack of a lightweight engine powerful enough get two people airborne for flight instruction was an insurmountable obstacle in the aviation world. So, there were many airplanes in search of an engine. Enter Continental. The Continental Motor Co. started in 1902 manufacturing two- and four-cylinder engines for automakers. The engine business boomed, and by 1912 Continental was producing 35,000 automobile engines each year. By 1929, it sold more than 330,000 automobile engines!

At that point, Continental launched into the aviation market, with the introduction of a seven-cylinder radial engine, the A-70. Most people don’t know about the A-70 because it was not very successful. It only developed a pitiful 160 hp and was priced at $2,700. Later, this engine would go on to become the famous W670, powering many aircraft, most notably the Boeing Stearman PT-17. Sales of the A-70 in 1930 were dismal, and with the crash of the stock market, Continental was in serious financial trouble and was virtually bankrupt. That was the impetus to start the design of the lightweight and low-cost A-40 engine.

Introduced in May 1931 with approved type certificate No. 72, the A-40 was rated at 37 hp at 2550 rpm. The engine incorporated a horizontally opposed four-cylinder design with the flat-head technology of the day, known as the L-head, with the valves in the cylinder block. This is the same technology that the Ford Motor Co. used when Ford introduced the first V-8 flathead engine in 1932. To reduce the weight for an aircraft engine, the one-piece engine case was constructed from cast aluminum, and the heads, which spanned two cylinders, were also cast aluminum with large cooling fins. The crankshaft shafts were completely machined from a caststeel billet. The engine was equipped with a single Scintilla mag and a Stromberg NA-S2 carb, weighing just 147 pounds and priced at $400.

As with many new products, the A-40 suffered from a host of design issues. The most significant was the fracture of the crankshaft after just 100 hours in service. Additionally, the cooling fins on the cylinders were inadequate, causing major overheating problems. Also, the head gasket would frequently fail and the exhaust valves would burn out. From May 1931 through August 1933, Continental only produced 98 engines, all of which were total failures and had to be replaced. The A-40 engine was reintroduced in August 1933 as the A-40-2, starting over with serial No. 300. It included a forged-steel crankshaft, more cylinder cooling fins, and an external aluminum heat sink to cool the exhaust valves. This was a viable, cost-effective, and reliable aircraft engine. Along with its successors, the A-40-2 went on to power lightweight aircraft for the next 90 years.

Several design improvements to the A-40-2 were incorporated into the dash 3 engine, released in May 1935. The most significant change was to the camshaft timing. The dash 3 cam fired at 22 degrees before top dead center from the previous 27 degrees. This change finally resolved the overheating problems and slightly increased the horsepower.

The vast majority of the A-40 engines were the dash 4 version, starting with serial No. 880, in 1936. The dash 4 engine was a dash 3 with the addition of automotive-type steel connecting-rod bearing inserts. All engines prior to serial No. 880 had Babbitted bearings in the connecting rods, which limited the engine rpm to 2550 for continuous operation. The steel insert allowed the continuous operating rpm to be increased to 2575 rpm. The dash 4 version, along with all the previous modifications, meant that the engine could now be rated at 38.9 hp, but Continental kept the 37-hp rating on the A-40.

The final variant of the A-40 was the dual-mag dash 5. The dual-mag design did nothing to increase the horsepower, nor did it improve the engine reliability. Unfortunately, it did increase the weight by 11 pounds and added $125 to the cost of the engine. As a result, fewer than 300 dash 5 engines were produced. Continental finally reached the 40-hp rating for the A-40 by increasing the rpm to 2700. This resulted in a significant loss of propeller efficiency, and so the design was never implemented.

Continental manufactured more than 2,600 A-40 engines from 1931 through 1938, at which time it was succeeded by the 50-hp Continental A-50 engine. For six long years, the A-40 was the only engine available for light aircraft and it had no competitors.

A Fleet of Light Aircraft Powered by the A-40

Several aircraft makers used the A-40 to power their planes, but none more than Piper Aircraft. Piper Aircraft, including the Taylor Aircraft company’s airplanes, built more than 1,800 aircraft with the A-40 engine from 1931 to mid-1938. The Taylor Aircraft Model E-2 Cub was the first to employ the A-40 engine, building a total of 353 aircraft. After C.G. Taylor and Bill Piper separated in December 1935, Piper went on to manufacture 1,196 A-40-powered J-2

Cubs, followed shortly by more than 356 J-3s, all using the same engine. Most of the J-3 Cubs employed the dual-mag version of the A-40.

C.G. Taylor moved to Butler, Pennsylvania, in January 1936 and opened a new factory, which became the Taylorcraft Aviation company. The first design from the Taylor factory was a sideby-side, enclosed-cabin airplane designated the Taylorcraft Model A. Taylor would manufacture 606 A-40-powered Model A’s from 1936 to 1939.

In the early 1930s, following on the wave of enthusiasm from the Lindbergh solo flight across the Atlantic Ocean, many entrepreneurs jumped into the light airplane market. Some were more successful than others, but all adopted the A-40 as the powerplant. The Heath Parasol achieved some success with the A-40, as did the Alexander Flyabout, the Rose Parrakeet, the Porterfield Zephyr, the Buhl Bull Pup, and the Ercoupe, just to name a few. All together, these small-quantity aircraft manufacturers used another 200 Continental A-40 engines.

Interestingly enough, Aeronca joined the party in 1938 and began selling its new Model K airplanes with the Continental A-40 engine as an option. These airplanes were designated the Model KC (C for the Continental engine), but only 35 were completed.

In total, the fleet of A-40-powered aircraft grew to more than 2,500 airplanes that were launched during the height of the Great Depression. This fleet led the way to more than 10,000 lightweight airplanes by the outbreak of the war in December 1941.

The Unreal A-40 Legacy

The Continental A-40 has left us with an incredibly robust and rich legacy. The A-40 was the cornerstone engine for all lightweight aircraft engines that followed. All manufacturers of horizontally opposed engines modeled their designs on the A-40. Starting with the A-50 in January 1938, and continuing through today with the high-performance four- and six-cylinder engines, the fundamental design is unchanged.

The 50-hp Continental A-50 (ATC No. 190) was a redesign of the A-40, with large single cylinders, a split case, overhead valves, and an up exhaust. Optional accessories on the A-50 included an electric starter and a generator. The A-50 led to the long line of ever-increasing horsepower engines through the next several decades, including the A-65, A-75, A-80, C-85, C-90, and the O-200. This fundamental design platform went all the way to the 400-hp, eight-cylinder Lycoming O-720 engine. Lycoming even went on to build one 1,200-hp, 12-cylinder O-1230 horizontally opposed test engine.

Lycoming, Franklin, and Menasco joined the lightweight engine market with the release of three new engines in 1938: the O-145-A, the 4AC-150, and the M-50, respectively. These engines were identical to the Continental A-50 design, and each was rated at 50-55 hp. The most successful competitor was Lycoming; Franklin had only moderate success, and Menasco only made a few engines of this size, as they were of a single-mag design.

Today, Continental and Lycoming are the dominant players in the lightweight aircraft engine market. Each has evolved the horizontally opposed engine into a highpower-to-weight-ratio engine, with unbelievable performance and spectacular reliability. The reliability of this design is nothing short of unbelievable. This simple, robust design is as reliable as the sunrise, and time between overhauls is now at 2,000 hours. The Continental TSIO550-N and the Lycoming iE2 TEO-540 engines are at the pinnacle of this design platform. By adding equipment and functionality like twin turbochargers (with intercoolers), direct fuel injection, and full authority digital engine control (FADEC controls), these engines now produce 350 hp. The 550-N develops 10 times the horsepower of the A-40-2. But, look closely at the TSIO-550-N, and the camshaft is still under the crankshaft, just like it was on the A-40. From the basic O-200 to the iE2 TEO-540, there is an amazing diversity of engines and an incredibly robust legacy.

The new FADEC-equipped engines will do everything for the pilot, simplifying the engine control to a single lever for engine, prop, and mixture. Additionally, thousands of data points can be downloaded in real time, covering every detail of the engine’s performance and condition while in flight — something unimaginable to the designers of the A-40 in 1930.

A Round of Applause

The A-40 was “the little engine that could,” and it did, giving birth to general aviation with 2,500 airplanes in the 1930s.The unique design of the A-40 completely eliminated all other engine designs. The other engines include the three-, five-, and seven-cylinder radial engines; the inline four-cylinder engines; the inverted inline four-cylinder engines; and the two-cylinder horizontally opposed engines. They all disappeared from the lightweight aircraft general aviation market by the early 1950s.

Aircraft design was radically changed by the A-40 as well. The four-cylinder horizontally opposed engine was a natural fit for the side-byside seating in airplanes. The design provided an unobstructed view out the front, reduced the drag of the aircraft, and offered a practical environment for flight instruction. Can you picture a five-cylinder radial engine on the nose of a Cessna 172? The inline engines of the day made the nose of the airplane long, like the inverted 200-hp Ranger on a Fairchild 24. The compact and lightweight design meant the aircraft designer could substantially shrink the nose area and the cabin width, thereby streamlining the aircraft. These design changes also led to the aerodynamic pressure cowling that is ubiquitous on aircraft today. This pressure cowling enables a four-place airplane to cruise at 200 mph, while burning just 18 gph.

Why the Passion for the A-40?

The A-40 played a pivotal role in my own family’s life too. Piper, Taylorcraft, and Aeronca were producing lightweight and low-cost aircraft that sold for about $1,295 in the late 1930s. This meant that the average worker could afford flying lessons and afford to buy an airplane. Case in point, my father, Bob Stewart. He had graduated from high school as an automobile mechanic in 1938 and got a job in a local repair shop. He earned $12/week! He took a $2 ride in a 37-hp J-2 Cub and was hooked on flying. Bob came home from that ride and told his brother Donald that they had to buy an airplane. Donald had never been up in an airplane, nor did he have any money. So, Bob and Donald borrowed $300 from their dad and bought a used 1936 Taylor J-2 Cub,

NC16667, for $600. Bob and Donald soloed in the J-2, got their private pilot certificates in it, and Bob went on to get his CFI rating in it. He sold the J-2 in January 1941 and bought a 60-hp Franklin-powered Piper J-3 Cub.

Bob then moved to Maryland in January 1941 and taught in the Civilian Pilot Training program. By the end of 1942, he was commissioned in the Air Transport Command as a second lieutenant. From that little A-40 Cub, he went on to fly B-17s, B-24s, B-25s, B-26s, C-46s, and C-47s, flying across the South Atlantic, North Africa, Southern Europe, the Middle East, and all the way to India.

After the war, the A-40 continued to impact Bob’s life. In 1993, Bob repurchased his J-2 Cub, NC16667, and restored it. Amazingly, 59 years after his solo flight in the 37-hp J-2 Cub, he witnessed his 16-year-old granddaughter solo behind that same A-40 engine, in that same J-2 Cub.

How that $2 ride changed Bob’s life forever! I have inherited NC16667, and I have purchased a 1935 Taylor E-2 Cub, also A-40 powered. My goal is to fly each airplane on its 100th birthday. Keeping the A-40s flying, and helping seven others restore A-40-powered airplanes, has been very rewarding.

Come See the A-40 This Summer at AirVenture

Stop in the Vintage Hangar to see a cutaway of the A-40 engine and learn more about its history. Also, be sure to walk through the first row of Vintage Aircraft parking and inspect more than a dozen A-40-powered aircraft from the 1930s — Cubs, Taylorcrafts, and Heaths, all flown in to Oshkosh for your enjoyment.

In Recognition of Chet Peek

Chet Peek wrote many books on the early days of aviation, including Flying With 40 Horses: A History of the Continental A-40 Aircraft Engine and the Planes It Flew, published by Three Peaks Publishing, which this author used extensively in the research for this article. Chet also wrote The First Cub and The Taylorcraft Story, which provided great detail about the application of the A-40 to these early aircraft.