Now, as far as I can tell, the Rotary Engine page shows that those engines had a top speed of ~1200 revolutions. So let’s imagine someone is flying with that big hunk o’ steel spinning at 1000rpm.
That means that a propeller blade crosses in front of the gun 2000 times per second.
They say on the synchronizer page that the first device that worked would trigger a round when the prop was horizontal.
The time of the free space only seems to be 1/2000 sec, and they only use half of that, if you assume the bullet went out in the same gap.
Now, the only missing variable is the lock time of a Maxim machine gun, which I couldn’t find. Considering how difficult it is to get a manual camera to operate 1/2000 sec, I can’t imagine that the lock time could possibly be anywhere near that speed.
In other words, between the synchronized triggering of the hammer fall and the bullet passing through the propeller, I imagine that the prop has spun at least a full revolution, if not more. They had to plan ahead. I can buy that — just dial it in.
But…
How can the burn rate of the powder be accurate enough that the total firing time from trigger to bullet leaving the gun doesn’t vary more than 1/2000 sec?
I can’t see it working, but it did. What did I miss?
I once saw a cartoon showing how this great invention was invented. As the frantically firing machine gunner swung the muzzle around to the front of the airplane, it gradually reduced the propeller to a smaller propeller, then a very small propeller, then a propellerless hub. This was, of course, followed by the standard cartoon take a la Coyote where the pilot looked at the camera, was suspended in midair while he contemplated his fate, then plummetted to the inevitable.
Wasn’t long after that when the sync gadget made its appearance.
I remember seeing that cartoon at the Wright-Patterson Air Force Museum in Dayton when I was a kid. Is that where you saw it? I always think about that same cartoon whenever I see or read anythng about those WWI prop synchronizers.
The interrupter was cam driven, and synched with the prop. As noted, at 1000rpm, this isn’t difficult. Perfected by Anthony Fokker, it would become standard on all German aircraft.
*Major Charles J. Biddle, described the principle of the synchronized machine gun.
There is no mystery about a machine gun firing through a propeller without hitting the blades. Nearly everyone understands the principle by which the valves of a gasoline motor are timed so as to open and close at a given point in the revolution of the engine. In the same way a machine gun may be timed to shoot. On the end of the cam shaft of the motor is placed an additional cam. Next to this is a rod connected with the breech block of the gun. When the gun is not being fired the rod is held away from the cam by a spring. pressing the trigger brings the two in contact , and each time the cam revolves it strikes the rod which in turn trips the hammer of the gun and causes it to fire. The cam is regulated so that it comes in contact with the rod just as each blade has passed the muzzle of the gun which can therefore fire at this time only. The engine revolves at least 1,000 turns per minute and as there are two chances for the gun to fire for each revolution, this would allow the gun to fire 2,000 shots per minute. The rate of fire of a machine gun varies from about 400 to 1,000 shots per minute according to the type of gun and the way in which it is rigged. The gun therefore has many more opportunities to fire between the blades of the propeller than its rate of fire will permit it to make use of. Consequently, the gun can work at full speed regardless of ordinary variations in the number of revolutions of the engine.*
Rate of Fire: 400 rounds per minute, or 6.66 per second.
Muzzle velocity: 2953 ft/sec
Distance from gun muzzle to prop arc: 5 feet?
Bullet will take .0016931 seconds to travel that distance. Once fired, the bullet is out and gone in a two, maybe three milliseconds tops…
An RPM of 2000 is 33.33/sec, or 66.6 blade passings/sec. I don’t know the width of a prop blade, nor the circumference of the prop arc, so I’m not sure how far a prop blade moves in 3 milliseconds. Some bright young German figured out that math, though, and set the interrupter gear to take all that into account. The gun will be prevented from firing (by the gear) as long as the prop blade will/would be in the way, and the interrupter will have to work for a range of RPM’s.
That means that there was around 30ms to play around between blades, and not 1/2000sec.
1000rev/min * 2 blades * 1min/60sec = 33-1/3 blades/sec
That gives 0.030sec/blade, which is 30ms.
Still, is the burn time and lock time of that machine gun accurate enough that it won’t vary by 30ms?
I was referring to rotary engined planes, not radial engined planes. In those WWI planes, the prop was bolted directly to the engine, which spun around the stationary crankshaft at max speed all the time, more or less. Hence the “hunk o’ steel” comment, referring to the engine block, and the lack of any mention of gearing.
More like 0.5 to 2 feet, depending on the plane. The entire DR. 1 was only 19 feet long and the E-I through E-III series and the Albatrosses and Pfalzes were only slightly longer. Five feet would have put the machine gun armature behind the pilot (making it difficult to clear the frequent jams) and might have placed the muzzle next to his ear or face.
Actually, it was a bright young Dutchman. Anthony Fokker, himself, devised the interrupter gear.
Hehe. I am just a little over 5 feet tall… I mentally measure many things in mlees lengths. That plane is almost four of me long.
If the cockpit is 2/3’s of the way forward from the tail, there is still 6 feet from pilot to nose. But I like you number of 2 feet from muzzle to prop arc better.
The big secret, not revealed at the time due to censorship and since largely ost to history, is that every pilot lost in WW1 actually shot himself down.
Presumably on this theory, the Red Baron was actually the worst pilot in WWI. He spent his time doddering around as an easy target in front of enemy pilots, thus ensuring that more of them shot themselves down in his vicinity than any other pilot. And a legend was born.
Didn’t someone first just bolt metal plates to the prop to deflect any bullets? I’m thinking the British, it’s the sort of “bright” idea they would consider.
*wanders away muttering about the time I tried to remove the starter motor from an Austin Gypsy and found that each of the 3 bolts had a different size head. Gave up when I couldn’t find a socket to fit the really hidden bolt (and we had every known socket type found in the Empire *
According to Wikipedia (and various books I read a long time ago) this was the invention that inspired Mr Fokker to do the job properly. Google eventually led me to a page crediting it to Frenchman Eugene Gilbert. Coincidentally, it’s also quite an interesting read.
Am I right in thinking that there were incidents when the interruptor chain (or gears, or whatever) jumped a tooth and this resulted in propellers being shot off?
*