Wright Aeronautical

Articles by Carl Kuhns

Wright Cyclone Licensees

Wright Aero Helicopter Engines

The Cyclone 7


Turbocompound Blower Drive Failures

Articles by Jay Smith

R-2600 Lockland Investigation

R-2600 Case History

Curtiss-Wright Cyclone 22 (Military XR-4090)

Curtiss, Lawrance and Wright Engine Specifications

The Curtiss-Wright SGV-1800



"Hex" Engine Firing Orders
by Kimble D. McCutcheon

Both Curtiss and Curtiss-Wright built unique 12-cylinder "Hex" engines arranged as double-row radials with six cylinders per row. The air-cooled Curtiss H-1640 "Chieftain" was intended to be a commercial engine, but was also tested by the U.S. Navy. The liquid-cooled H-2120 was built exclusively for the U.S. Navy. We have all been taught that a four-stroke radial engine must have an odd number of cylinders per row in order to achieve even firing, yet these two Hex engines were even firing. How was this accomplished?

    For engines such as these with 180° crankshafts and overhead camshafts for each cylinder bank, three possible schemes will provide even firing:
  1. Fire all the cylinders in one row successively, then all the ones in the other row;
  2. Fire a cylinder in one row and then fire the opposite-row cylinder situated 240° in the direction of crankshaft rotation. Twice in each cycle, fire two adjacent cylinders in the same row. This was the scheme used by the H-1640
  3. Fire two adjacent cylinders in the same row, followed by an opposite-row cylinder 240° in the direction of crankshaft rotation. This scheme was used by the H-2120.

The H-1640 was tested twice by the U.S. Navy in the late 1920s. According to Navy reports, each had a different firing order and different master rod placement. Both test reports complained of excessive vibration. Neither test was successful. A new liquid-cooled hex engine, the Curtiss-Wright H-2120, was tested by the Navy in the 1930s. Again, the master rod location and firing order was changed. Again, the engine was not accepted for Navy use.

The tinkering with firing order and master rod location suggests that the engine designers were trying to resolve some underlying vibration issue. Master rod placement allows some control over the amplitude, frequency and direction of vibration. It is also possible to obtain good torsional vibration characteristics at the expense of overall linear "shake", and vice versa. With the Curtiss Hex engines, three master rod phasing schemes were tired: 0°, ~90°, and 180° apart.


Wright R-2160 Tornado Restoration
Raymond Zanella Kindly Contributed the Following Tornado Restoration Images