Question about WW2 aircraft

I was watching a show about WW2 tonight & I noticed that before they started the bombers
the air crews would manually rotate the propellers before the engines were started.
Why were the props rotated manually before start up? Just curious, thanks.

It's to make sure there aren't any cats sleeping in the engine.

Those were radial cylinder engines.

Sitting for a while, oil would accumulate in the lower cylinders.

"Walkin the Props," displaced the oil.

As mentioned, oil collects in the lower cylinders of a radial engine when at rest. If enough oil is accumulated in the lower cylinders and an engine start is attempted with out "walking the props through" hydraulic lock can occur. Hydraulic lock is when the piston on the compression stroke, is attempting to compress the oil in the cylinder. "Walking the props through" ensured this oil was drained off through the valves before start.

In the photos below, a professional model on site for a photo shoot, "walks the prop through" on the B17G "Aluminum Overcast" while under the supervision of a crew member....... then one of her associates gives the crew member a helping hand with his engine.

the consequence of this happening is that since oil won't compress, that cylinder gets damaged. The spark plug blows out, the cylinder head blows out, or the piston breaks. These things are not conducive to a successful mission; the cylinder and/or engine needs to be replaced.

Yep. Blow a jug, or bend a crankshaft or connecting rod, and you won't be getting one of these from your Crew Chief.........

My dad was a bomber pilot in WWII. He told me about having to do this before starting the engines. It was as the folks above have said, to move the oil around.

I never stand in line with the rotation of the propeller, sorry, but just something I would never do.

You gotta do what you gotta do. Take precautions such as ensureing that the mags are off to mitigate the risk.

We never did that, in the T-28. We did however, use the starter to turn the engine over three complete revolutions, before actually introducing fuel to start it.

Large radials are pretty hard to start, to begin with. B-17s, for instance, had an inertia starter, where the starter motor actually spun a large flywheel. Once it was spun up, the flight crew would mesh the starter flywheel to the engine, and the engine would turn over at a speed high enough to fire. (Most of the time.)

The starter can still directly turn the engine, if you hold the mesh switch while the starter turns, (either before spinning the flywheel all the way up, or after it's expended its energy and is turning at the same speed) but it is unlikely to start, at that point. The limited duty-cycle on the starter makes this a fun affair.

Bottom line, as long as you take your time, you are INCREDIBLY unlikely to accidentally hand-prop an R-1820, even if the fuel and mags are on.

You will also notice the clouds of smoke on start up on those huge radial engines, even though pulling the props through circulates the oil and checks for hydraulic lock, there still tends to be some accumulation of oil in the lower cylinders, which is burned off as the engine starts.

I helped "walk the props through" on that aircraft when it was in town a few years ago, its not an easy feat. I really had to put my back and legs into it.

I think it was 12 blades around per engine maybe?

The model in the leather jacket was struggling and everyone was teasing her for not being able to push the prop through. It was all in fun and a good time was had by all. She piped up at one point, stating that her problem was that she only weighed 95lbs......

Aluminum Overcast Photos

Great photos. The models are fantastic!

Thanks DzlBenz.

QUIB...love your black and whites of the B-17, very classy.

Thanks.

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