Technicalities: Why Left?

Why we turn left in the pattern when we could turn right.

Sopwith Camel
The spinning engine of the Sopwith Camel gave it quirky, and often lethal, flying qualities.Peter Garrison

Thirsting for knowledge, I Googled why we drive on one side of the road rather than the other. I found a lot of obvious rubbish about quarrelsome knights and Roman charioteers. I suspect that what really happened was Henry Ford flipped a quarter and William Morris a shilling, and they came up different. Nevertheless, I propose to ­offer an explanation of why the pilot of an airplane sits in the left seat, and why we normally make left turns in the traffic pattern.

Actually, this theory is not original with me. It comes from Javier Arango, a collector, student and flier of World War I-era airplanes, who delivered a paper on it to the 2016 meeting of the Society of Experimental Test Pilots in Anaheim, California. He does not claim that it is anything more than a guess, but it’s a plausible one.

The root of our leftist tendencies was, in ­Arango’s analysis, the rotary engine. The ­rotary was the engine type most commonly used on “scouts,” or, as we would now call them, “fighters,” in World War I because it was exceptionally light and powerful and, for the era, reliable.

In the broad principle of their operation, ­rotaries were similar to the radials that became dominant later; what was different was that the crankcase and cylinders spun while the crankshaft was fixed to the airplane. How this inside-out idea occurred to anyone in the first place is a mystery to me. It may have had to do with the fact that some of the earliest uses of rotaries, in the 1890s, were in bicycles; a little engine was integrated right into a wheel.

The rotary arrangement had several ­advantages. Since the cylinders were always moving, they enjoyed built-in air cooling. A rotary didn’t need a heavy flywheel; the engine itself was enough. It didn’t need an oil pump; centrifugal force did the job. Remarkably, considering that aero ­engine technology was in its infancy and that rotary engines were made entirely of steel, they achieved outputs and power-to-weight ratios comparable to those of current four-cylinder Lycomings and Continentals.

One inconvenient peculiarity of the rotary was that, as a heavy, rapidly spinning mass, it imposed gyroscopic forces upon the airplane. Fortunately, these forces were only moderately strong, for a number of reasons. The single-engine fighters were very compact, and their engines were close to the airplane’s center of gravity. The rotating mass of the engine, furthermore, was concentrated near the crankshaft because the cylinder walls and heads were remarkably thin. Finally, rotaries did not spin very fast; 1,200 rpm was typical.

Such a slow-turning engine called for a large propeller, and the propellers of early scouts were huge by modern standards. That of a Sopwith Camel, for example, was almost 9 feet in diameter; although, being made of wood, it weighed only 32 pounds — about the same as today’s 6-foot aluminum fixed-pitch props. The gyroscopic contribution of the Camel’s propeller was about half that of the engine.

It happens that aviation rotaries spun, like most modern aero engines, clockwise when viewed from behind. The reason for the choice of direction of rotation, like that of which side of the road to drive on, is buried in history’s junk pile. But the effect in flight was that when a rotary-engine airplane turned to the left, its nose tended to swing upward, and when it turned to the right, the nose went down. (The gyroscopic force is always 90 degrees off the direction of pitch or yaw, in the direction of ­rotation of the upper part of the propeller disk.)

Camels rolled equally rapidly in either direction, but the downward slicing of the nose in a rapid right turn, and the consequent acceleration, made pilots feel that they “turned better” to the right than to the left, and gave rise to the canard, repeated in Wikipedia, that ­Camels could make a 270 to the right more quickly than a 90 to the left.

Peculiar turning behavior was not confined to Camels. In a 1919 book titled How to Fly and Instruct in an Avro — the Avro 504 was a single-engine trainer widely used by the Royal Flying Corps — the author, one Lt. F. Dudley Hobbs of the 2nd Life Guards (who probably never dreamed that his name would be mentioned in a ­magazine nearly a century later), suggests securing a toy gyroscope to the nose of an airplane model to use as a teaching tool. “No amount of explanation of gyroscopic action is worth so much to a pupil,” he writes, “as five minutes spent playing with a little gyroscope.”

A paradoxical consequence of the gyroscopic force was that whichever way you turned, you needed left rudder. In all pre-World War II airplanes, and some postwar ones, considerable rudder, applied in the direction of the turn, is normally needed to overcome adverse yaw. But a ­rapid, steeply banked turn in a rotary-engine airplane was different. In a left turn, left rudder was needed both to overcome adverse yaw and to keep the nose from slicing upward. In a right turn, ­after an initial application of right rudder against adverse yaw, left rudder was needed to keep the nose from dropping.

Camels had a high accident rate in the hands of novice pilots. As Hobbs explains: “A pilot starts a gliding turn to the right and finds his nose dropping. Instead of holding up his nose with a little left rudder, he tries to hold it up with his stick, with the result that the machine spins.” It was this propensity to spin out of a right turn, Arango suggests, that led to a preference for approaching the landing with left turns. That habit eventually led to the pilot ­occupying the left seat in side-by-side cockpits.

A couple of Hobbs’ other observations are striking. One is that to loop a rotary-engine airplane you must use left rudder because the pitch-up produces a gyroscopic pull to the right. A more surprising statement is that “a Camel will not roll properly to the left because the gyroscopic action of the engine swings the nose to the right, with the result that the machine sits up on its tail.” This opaque assertion becomes clear when you realize that when Hobbs says “roll,” he means not what we call an aileron roll, barrel roll or slow roll, but rather a snap roll, a horizontal spin that begins with an abrupt pitch-up.

In the now-proverbial words of the novelist L.P. Hartley, “The past is a foreign country; they do things differently there.” And they speak a different language too.