A Staff Report from the Straight Dope Science Advisory Board

How did World War I fighter planes keep from shooting off their own propellers?

April 14, 2009

How did WWI planes shoot their machine guns so that it didn't destroy the propeller? I have heard that they were on timers, or that there was some sort of marking on the back of the prop blade, but none of this seems to make sense.

When World War I broke out in 1914, the British engineering and manufacturing firm Vickers was trying to figure out how to equip a plane with a machine gun. The problem was that the gun had to be mounted on the plane's fuselage, as the wings were too fragile to support a gun's weight, but mounting the gun directly ahead of the pilot meant that the plane's forward-mounted propeller was right in the line of fire. Vickers's first attempt at a solution – the Experimental Fighting Biplane 1, or E.F.B. 1 – had a machine gun mounted in its nose; the propeller was moved to the rear, where it pushed the plane forward. Unfortunately, the weight of the machine gun affected the plane's center of gravity too much for it to be stable, and it crashed on its first test flight.

So while Vickers and the other large aircraft makers theorized and planned, the pilots themselves improvised. Pilots began to carry pistols to shoot at other planes; soon they started carrying rifles. Some even carried bricks, with the idea of dropping them onto enemy aircraft from above. Eventually, double-seated airplanes had a strap added to the observer's chair so the observer could stand up and fire a swivel-mounted machine gun over the propeller.

By the end of 1914, Vickers had updated their E.F.B. 1 into the F.B. 5 Gunbus. Still using a front-mounted gun and rear-mounted propellers, the Gunbus at least wasn't as crash-prone as its predecessor. But it was slow and inherently stable – the latter a term of aeronautic art describing a craft that tends to right itself after being disturbed by turbulence. As you might guess, an inherently stable plane pays a huge price in maneuverability, but the UK's Royal Flying Corps, believing at this time that the airplane's role in warfare was primarily one of observation and reconnaissance, insisted that all its aircraft display this property. Thus the Gunbus: an armed plane too clumsy to get out of its own way. In practice, inherent stability meant that by the time the Gunbus could turn to aim at an enemy plane, the enemy would be long gone, and the lumbering Gunbus had little hope of catching up. It also meant that an awful lot of British pilots and observers died before the RFC abandoned its pro-stability policy.

And so inventors and engineers turned to the idea of taking a much smaller and more nimble front-propellered single-seat airplane and giving it a machine gun. But figuring out how to avoid shooting off the propeller remained a problem. One English aviator tried mounting the gun on top of the upper wing of his plane, which was a general success – or least it was until the ammunition drum jammed during reloading. The pilot had to stand up on his seat to pull out the drum, whereupon the plane stalled and went into a dive. He would have been pitched out of the plane had he not been holding so tightly onto the drum and had the drum not been quite so badly jammed.

It would be a French military hero (insert joke here) who would come up with the first step toward modern fighter planes. Roland Garros was already a famous aviator by 1914, having made the first nonstop flight across the Mediterranean the year before. (His name remains familiar to tennis fans – the tournament popularly known as the French Open and the stadium complex where it's held are named for him.) Garros was teaching military aviation in Germany when the war began and smuggled himself into Switzerland so that he could join the French Air Service.

Garros and aircraft manufacturer Raymond Saulnier experimented with forward-mounted machine guns and propellers, and found that only 10 percent of the bullets fired actually struck the propeller. This was far less than had been expected, and it meant that if something could be done to keep that 10 percent from doing too much damage, the remaining 90 percent could actually bring an effective strength to bear upon the gun's target. So Garros and his mechanics mounted steel wedges onto the backs of the propeller blades, which served to deflect the copper-coated bullets safely away from the prop.

On April 1, 1915, Garros took to the air in his modified Morane-Saulnier monoplane and easily shot down a German plane; two more kills followed over the ensuing weeks. Unfortunately for Garros, his plane went down behind enemy lines on April 18, and he spent much of the rest of the war in a POW camp. Unfortunately for the Allies, his plane was captured nearly intact and sent to Anton Herman Gerard Fokker.

Fokker, the aviation pioneer nicknamed “the Flying Dutchman," had offered his services to both sides when the war came, and it was the Germans who accepted. When Garros' plane was sent to him, he immediately rejected the idea of deflectors on German propellers; the steel-coated German bullets would not be so easily rerouted. But he would later claim that viewing this plane would give him the inspiration to create a new system: a “synchronization gear" enabling the machine gun's rounds to be fired between the blades of the propeller.

In truth it's hard to determine whether Garros's design provided any inspiration for Fokker or not. The story would always be that Fokker had his crucial flash of insight immediately after seeing Garros’s plane and had a successful design less than a week later, but members of Fokker's team report working on the device as early as 1914. And other engineering teams had already designed their own synchronization systems – Franz Schneider of the German manufacturer Luftverkehrsgesellschaft developed one in 1912, Raymond Saulnier of Morane-Saulnier had a working model in 1913 (though it wasn't fully debugged, hence Garros's deflectors), and a pair of brothers named Edwards took out a patent on such a device in England in 1914. In fact, Fokker would later be successfully sued by Schneider for infringing on his patent.

Whether he invented the synchronization gear or merely worked a few of the kinks out, Fokker was the first to obtain actual government support and approval of his design. By June 1915, the first of the Fokker E-series were arriving on the western front with synchronization gears and forward-mounted machine guns. The results weren't pretty – for the Germans. Pilots had difficulty preventing the Fokkers from "porpoising" (bobbing up and down uncontrollably) in level flight, but a bigger problem was that the synchronization gear would periodically fail, causing the gun to fire direcctly into the propeller. Fokker set up a special training school to help German pilots control his planes, but the German Air Service cancelled the training and banned the planes after three fatal crashes.

On August 1, however, Max Immelmann jumped into a Fokker E-series to avenge the bombing of his airfield by the British and scored the first kill recorded by a plane using the synchronization gear. This began what would be called “the Fokker Scourge”. By the end of the year, Immelmann had scored seven kills in his Fokker, his friend Oswald Boelcke had six kills in his, and both were awarded the Pour le Mérite, the highest German honor for bravery (known familiarly as the “Blue Max"). Now every German pilot wanted a Fokker. Suddenly, the tables were turned, and the Germans controlled the skies while Allied pilots feared to go on even the simplest runs.

However, the only thing the Fokker Eindecker really had going for it was its synchronized machine gun. Otherwise, it was an unremarkable and ill-handling (though not inherently stable!) airplane with a record of structural failures (a problem with all Fokker models of WWI); it was quickly matched by the British Airco D.H. 2 and Royal Aircraft Factory F.E. 2b, both “pushers” like the Vickers E.F.B. 1, and the French Nieuport Bébé, a conventional “tractor” biplane with a gun mounted above the top wing. (OK, technically the Bébé was a sesquiplane, as the lower wing was much narrower than the upper one.) Advantage shifted back and forth until 1918, when Allied aircraft production simply swamped the Germans.

So how did the synchronization gear work? It was pretty simple, really. One of the engine's crankshafts was fitted with a cam wheel – a wheel with a slight bulge in its circumference. The bulge was aligned to the rotation of the propellers, set to push up on a rod just before each blade came in front of the gun. The rod linked back to a mechanism connected to the gun's trigger. If the rod was in the down position and the trigger was pressed, the machine gun would fire. If the rod was in the up position, it wouldn't.

In fact, this system proved to be a little too simple; because firing rate depended on how fast the crankshaft was turning, fewer bullets were shot at slower engine speeds. The British eventually dropped this mechanical system for the hydraulic Constantinesco synchronization gear, which allowed firing rates nearly as high as those delivered by unsynchronized machine guns.

By World War II, the development of stronger airplane wings allowed guns to be mounted on them, outboard of the propeller arc; after the first couple of years of combat, few British and American fighters still had fuselage-mounted guns. Other nations, though, continued to use them (and synchronization gears) throughout the war, and even into the Korean War. Why? In order for wing-mounted guns to fire at a target directly ahead of the pilot, they need to be “harmonized" – that is, they need to be aimed slightly inward so that their fire meets at a specific point some distance out in front of the plane's nose. Fuselage-mounted guns, by contrast, fire directly along the axis of the airplane and thus are naturally aimed wherever the airplane is headed.

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