Mar 26, 2022
It's time for the puzzler.
You've all seen those old movies? Those old movies with Jimmy Cagney, you know, "You dirty rat. You're the guy who sent up my brother!"
And he's got a leather jacket and a scarf around his neck climbing into a plane. Remember the great line? "I can fly the crates they come in."
Everyone's seen those movies. So, he climbs into the plane and his assistant prepares for takeoff.
And all those planes were started by hanging on the prop and pulling it, starting it much like you would crank an old-timey Model T Ford. And of course, they had electric starters, later on. But that was the way they were all started back then. And once the plane was started, they'd remove the chocks from the wheels and they'd fly off into the sunset and do whatever they did.
But the question is about the starting procedure. You'll notice if you watch those movies that the assistant always takes a hold of the prop. And when he starts it, what he's going to do is he's going to snap the prop, and that's what gets the crankshaft turning and gets the pistons all moving and that's what starts it. But before he snaps the prop, he will turn the prop, sometimes an entire revolution, very slowly. And then once he has done that, then he will start the engine. So, the question is, why?
Why what? Why did he always turn the prop very slowly at first?
The reason they start the plane this way is because these engines on all these old planes, or at least many of these older planes, were radial engines. And it's possible if there's something wrong, for oil to work down past the rings and get into one of the cylinders, one of the bottom cylinders.
Because radial means that the cylinders, instead of being like an inline engine in a car in a row or in the shape of a V, they're all around the periphery of the circle. The crank shackles to the middle and there's a connecting rod connected to a whole bunch of different cylinders. And it's possible for oil to get down past the rings on some of these older engines and collect in there and cause the thing to hydro lock.
If the oil were inside the cylinder and the engine started, since oil is not compressible, hardly, then what would happen is it would put such intense force on the piston that would actually bend the connecting rods or blow a head gasket or nose one crack a head on some some such thing.
And that's why he turns the propeller first.
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