By: Norm Goyer
The Curtiss JN-4 Jenny was the most manufactured US aircraft of WWI. It had a 90 hp OX-5 engine.
In Europe the rotary engine ruled the sky with all combatants using them in the majority of aircraft. The original design, from Germany licensed to England before the war. England then developed the Clerget and Le Rhone engine which was copied by Germany as the Gnome, basically all the same engine with local modifications. Back home, the only aircraft of any significance, using a rotary engine, was the Thomas Morse Scout which had a 90 hp Le Rhone. The largest produced aircraft during this time in the United States was the primary trainer Curtiss JN-4 nicknamed the Jenny. This aircraft was so underpowered with its Curtiss OX-5, V-8 90 degree engine that it barely got off the ground, and if and when it did, the job of keeping the OX-5 running until the mission was over was a full time job, even more important than “flying the airplane” Its accident rate was very high. Of course the numbers were skewed, due to the fact that the aircraft was flown by students with very low hours and many with dubious instruction. Nevertheless the Curtiss OX-5 was our engine and we used it, good or bad. Huge numbers were built and when the war was over they were sold for pennies on the dollar and were used in many aircraft until 1930 when the supply finally ran out. It was an obsolete design in 1916 and it never caught up with the products of England and Germany.
The OX-5 was a 90 degree V-8 engine with liquid cooling and putting out 90-hp. Thousands were built during the World War I era.
The OX-5 was the last Vee engine designed in by Glenn Curtiss. The first being a two cylinder Vee used for one of his 1902 racing motorcycles A modified version of one of these early designs was sold as an aircraft engine in 1906, and from then on the company’s primary market was aircraft. The basic design had slowly expanded by adding additional cylinders until they reached the V-8 in 1906. They also started enlarging the cylinders as well, but this led to cooling problems that required the introduction of water cooling in 1908. These early engines used a flathead valve arrangement, which eventually gave way to a cross-flow cylinder with overhead valves in 1909, leading to improved volumetric efficiency. The US Navy ordered a version of this basic design in 1912 for its A-1 amphibious aircraft, which Curtiss supplied as the OX. These improvements and others were worked into what became the OX-5, which was first built in 1910. By this point engine design was a team effort; the team included Charles Manley, whose earlier Manley-Balzer engine had held the power-to-weight ratio record for 16 years.
The famous Curtiss Robin was also available with a Curtiss OX-5 engine.
Like most engines of the era, the OX-5’s high-temperature areas were built mostly of cast iron, using individual cylinders bolted to a single aluminum crankcase, wrapped in a cooling jacket made of a nickel-copper alloy. Later versions used a brazed-on steel jacket instead. Cylinder heads were also attached to the crankcase, using X-shaped tie-downs on the top of the head attached to the block via four long bolts. Fuel was carbureted near the rear of the engine, then piped to the cylinders via two T-shaped pipes, the cylinders being arranged so the intake ports of any two in a bank were near each other. The cylinders had one intake and one exhaust valve, each operated by a pushrod from a camshaft running between the banks. This arrangement caused the outer exhaust valves to have a rather long rocker arm. The pushrods were arranged one inside the other, the exhaust valve rod being on the inside and the intake valve rod a tube around it. The aluminum camshaft bearings were a split type bolted together and held in place by lock screws. The pistons were cast aluminum.
The OX-5 was the forerunner of future V-12 engines which made their reputation in Mustangs and Spitfires.
The OX-5 was not considered particularly advanced, nor powerful, for its era. By this time, rotary engines such as the Oberursel or Gnome-Rhone were producing about 100 hp, and newer engines such as the Mercedes six cylinder inline were becoming available with 160 hp or more. Nevertheless the OX-5 had fairly good fuel economy as a result of its slow RPM, which made it useful for civilian aircraft. The OX-5 was used on the Laird Swallow, Pitcairn PA-4 Fleetwing II, Travel Air 2000, Waco 9 and 10, the American Eagle, the Buhl-Verville CW-3 Airster, and some models of the Jenny. The primary reason for its popularity was its low cost after the war, with almost-new examples selling as low as $20. It was often used in boats as well as in aircraft. It took America many years to rise to the top of the world’s aircraft engine producers thanks to our great engineering capability. But, it was the lowly OX-5 which led the way.
Specifications: Curtiss OX-5
- Type: 8-cylinder water-cooled 90° Vee piston engine
- Bore: 4.0 in
- Stroke: 5.0 in
- Displacement: 503 in³
- Length: 56.75 in
- Width: 29.75 in
- Height: 36.75 in
- Dry weight: 390 lb
Components
- Valve train: One intake and one exhaust valve per cylinder, pushrod-actuated
- Fuel system: Duplex Zenith Carburetor
- Oil system: Gear-pump 40 to 60 psi 3-gallon sump
- Cooling system: Water-cooled
Performance
- Power output:
- Specific power: 0.21 hp/in³
- Compression ratio: 4.9:1
- Fuel consumption: 8.0 US gal/h at 75% power
- Oil consumption: 0.5 US gal/h at 75% power
- 90 hp at 1,400 rpm
- 105 hp at 1,800 rpm for brief periods