In ww2 we have films of airmen doing what was called walking the propelers on radial engines.?

Question

I was told it was to lubricate the cylinders before start up because the oil would pool at the bortton cylinders,Could they not turn the engines on the starter without ignition and then switch ignition to start,This would be on the B17s or any radial engine.

Answer 1

Radial engines don't have oil sumps.  Any oil left in the engine on shut-down tends to run down into the lower cylinders and pools below the pistons.  If you were to attempt to start the engine with this oil still in the cylinder, it could damage the engine.   It is a common practice on all radial engines to turn the prop by hand through several compression cycles to blow excess oil out of the cylinders before attempting to start the engine.

Answer 2

Mentorman's got it spot on.  Those engines complete with huge prop on the front take a massive amount of effort to turn over and the aircraft, frankly, has little to spare for blind cranking, external generator/compressor or not.  Plus, more importantly, the turning over needs to be done gently in order to purge the cylinders of oil without risking a hydraulic lock - that would wreck the cylinder involved. Walking pace is just right.

Even now, Yak aircraft usually have radials, air-started, and they  most emphatically need pulling over before you dare risk engaging the starter.  Even then there's a huge amount of coughing, banging, farting and gigantic clouds of smoke before it settles down.

Answer 3

I am in agreement with what everyone has stated here. I can only add the reason for the possible engine damage. This is due to the fact that liquid (as in oil) does not compress. The result would be more oil in the cylinder than space available for it, during the compression stroke of the piston. The same risk occurs when trying to start a flooded automobile engine.

Answer 4

Those big klunky engines had relatively loose piston/cylinder clearances and a LOT of oil could collect in the headspace of the lower cylinders. The starters were not the gear engaged electric motors like an automobile; it would take a HUGE electric motor (& batteries) to turn a big radial! They were different schemes, some used compressed air and some used flywheels brought up to effective speed by a relatively small motor and then clutched to the engine. In either case the rpm for starting is too fast to squeeze the oil out of the chambers pretty much the same way it got in and a lot of stress would be put on the connecting rods. "Walking" the engines gave the oil time to squish out and a man pushing on a propeller blade could not exert too much force.

Answer 5

The tendency of oil to accumulate in the lower cylinders can cause what's known as hydrolock. As the piston reaches "top center" on a compression stroke, depending on how much inertia the prop has at that point, the damage caused can be impressive.

Aircraft cylinder jugs are built with a threaded, shrink fit. The head is heated and the cylinder is cooled before being threaded together. The equalization of teperature provides for a very tight seal. Among other tributes to dumb airmen, my powerplants instructor had a head, and a cylinder from a Pratt&Whitney R985. The threads of the head were still with the cylinder, and he made a point of explaining that the demonstration model he had was courtesy of a pilot that didn't check his Beech 18 for hydrolock before trying to start the engines.

Among other items on the cold engine start checklist, there is a count of either 5 prop blades for 2 blade props, or 8 blades for engines swinging three blade props, before the manetos or shower of sparks are used. This ensures that the engine has gone through two full revolutions and at least one compression stroke for each cylinder of the engine.

As the magnetos, which supply the spark to the plug, are ungrounded to supply the spark and grounded to kill the spark after the mixtures have been cut at shut down, a loose wire anywhere between the magneto switch(es) and the magnetos could allow a few cylinders to fire prematurely.

Which is why the props are turned through two full rotations before engine start is attempted. The possibility is entirely to real of ruining a perfectly good jug as well as necessitating a time consuming and expensive inspection of the rest of the engine.

Taking the airplane out of service through failure to follow the engine manufacturer's recommendations sort of defeats the purpose of flying.

JT

Answer 6

This was to get rid of oil which would have run down into the lower cylinders, and may have caused an hydraulic lock, therebye damaging the engine.

Answer 7

Most of the points hit:  

Radials do have sumps like that gray thing inbetween the bottom two cylinders on my 1340.  

They don't always have to be turned over by hand per the P&W Manual "The Aircraft Engine and it's Operation", copywrite 1951 (Part Number PWA 109702).  Some count 4, 8, etc.  I fly a DC-3 and we count 12 before turning on the mags.

They do use electricity, compressed air (tanked or Kaufmann starter) to turn over.

Spark provided by the mags are stopped by turning the mags to OFF, pulling the mixture control to IDLE CUTOFF turns off the gas.

It is done to get the oil out of the bottom cylinders and prevent a hydraulic lock, which even if motored past might still bend or stress a connecting rod and, after that, it's only a matter of time.

Answer 8

An electric, compressed gas, or inertia starter turns the engine too forcefully, too fast, and may put damaging pressure on the cylinder bodies, valves, or other joints.

Walking the props clears out oil gently without applying too much pressure too soon.

Answer 9

Agree with Warbird pilot.. Radials do have sumps where the oil collects and is scavenged back to the oil tank by a pump..