pulling a prop through on a radial engine
Sun Apr 20, 2008 8:30 pm
this is posted in the main hangar, but thought it could be X-posted here for a wider audience. Please answer in the main Hangar.
Thanks!
"Basic question for the pros on radial engines: after a recent 1820 accident due to a failed rod that was bent, it has posed a question:
In a geared engine, is there any value in pulling through the prop prior to starting. I did this with a direct drive 1340 for years. The mechanical advantage seems to work against bending the rod in a geared engine, and I have always heard to use the starter motor which will not bend a rod from hydraulic lock.
Many thanks"
Thanks!
"Basic question for the pros on radial engines: after a recent 1820 accident due to a failed rod that was bent, it has posed a question:
In a geared engine, is there any value in pulling through the prop prior to starting. I did this with a direct drive 1340 for years. The mechanical advantage seems to work against bending the rod in a geared engine, and I have always heard to use the starter motor which will not bend a rod from hydraulic lock.
Many thanks"
Sun Apr 20, 2008 9:21 pm
A starter motor will not feel the resistance of a hydraulically locked cylinder: to pull a radial through by hand is the best method.
Hydraulic lock from the oil in the bottom cylinders will do quite a good bit of damage to the engine by causing a rod to bend after attempting to compress an incompressible liquid... The starter motor, as I mentioned, with a geared engine may have enough torque to bend that rod- when you turn through by hand, you will feel that resistance, and know there is a lock probem: the starter don't feel a thing..
The other method would be to leave a bottom cylinder lug out, and make a drain piece to allow the run-down a place to exit the cylinder. Of course, you'll have to reinstall the plug each time you wish to fly!
Sorry to hear about the rod-
Robbie
PS- Sounds like a mistake in the NATOPS manual: might check for a revision. Unless the gearing makes the feel go away, come to think of it...
Hydraulic lock from the oil in the bottom cylinders will do quite a good bit of damage to the engine by causing a rod to bend after attempting to compress an incompressible liquid... The starter motor, as I mentioned, with a geared engine may have enough torque to bend that rod- when you turn through by hand, you will feel that resistance, and know there is a lock probem: the starter don't feel a thing..
The other method would be to leave a bottom cylinder lug out, and make a drain piece to allow the run-down a place to exit the cylinder. Of course, you'll have to reinstall the plug each time you wish to fly!
Sorry to hear about the rod-
Robbie
PS- Sounds like a mistake in the NATOPS manual: might check for a revision. Unless the gearing makes the feel go away, come to think of it...
pulling a prop through on a radial engine
Sun Apr 20, 2008 11:51 pm
Actually,the starter clutch on the R-3350's on DC-7's and P-2's will slip enough to stop turning if you have a cylinder that has hydraulic lock.It gets very obvious,even if you try a couple of times when you aren't sure if it's slipping or if you just accidentally let go of the starter switches.Then you go out and start pulling lower front spark plugs.The DC-7 manual recommends using the starter if you need to move a prop for more than a couple of blades because the starter will slip in the case of hydraulic lock.Fortunately,hydraulic lock is very rare in DC-7 engines.
With the B-17,we used to pull the prop backwards a blade or two and wait a couple of minutes if it stopped turning due to a slipping clutch.One flight manual recommended doing that,and it worked if you had drain lines from the lower intake pipes,which would allow the oil to drain overboard.I've also seen B-17 and DC-3 flight manuals that said never to pull the prop backwards in that senario because you could force an additional cylinder with oil in it back into a compression stroke.You can usually feel the resistance on an R-1820 with hydraulic lock if you pull the blades through by hand,but it is possible to bend a rod if you really force it.If you're strong enough that you can pull a cold R-3350 through by hand enough to bend a rod,you won't need a fork lift to move heavy objects.
With the B-17,we used to pull the prop backwards a blade or two and wait a couple of minutes if it stopped turning due to a slipping clutch.One flight manual recommended doing that,and it worked if you had drain lines from the lower intake pipes,which would allow the oil to drain overboard.I've also seen B-17 and DC-3 flight manuals that said never to pull the prop backwards in that senario because you could force an additional cylinder with oil in it back into a compression stroke.You can usually feel the resistance on an R-1820 with hydraulic lock if you pull the blades through by hand,but it is possible to bend a rod if you really force it.If you're strong enough that you can pull a cold R-3350 through by hand enough to bend a rod,you won't need a fork lift to move heavy objects.
Mon Apr 21, 2008 12:15 am
excellent post - many thanks!
Mon Apr 21, 2008 2:06 pm
snj5, you might want to PM Gary about the benifits of pulling a prop. He had some info posted on the B-24 link awhile back about proper way and way.
Kurt
Kurt
Thu May 01, 2008 1:25 pm
The clutches on the starter of the C-54 will also let you know there's a liquid lock before any damage occurs. The leverage advantage you gain by pulling on the prop can damage a rod, so you'd be better off to use the starter.
However, to add to an earlier post, a C-54 also has the drain manifold in all of the lower the intake pipes, so pulling it backwards a few turns will push the oil into these and it should drain overboard.
Nevertheless, the danger there is you should turn it over a decent number of times after clearing the oil out, to be certain the cylinders are clear before hitting the mags...or you will suck the oil slugn in and if it fires at that instant, things could get ugly and expensive.
However, to add to an earlier post, a C-54 also has the drain manifold in all of the lower the intake pipes, so pulling it backwards a few turns will push the oil into these and it should drain overboard.
Nevertheless, the danger there is you should turn it over a decent number of times after clearing the oil out, to be certain the cylinders are clear before hitting the mags...or you will suck the oil slugn in and if it fires at that instant, things could get ugly and expensive.
Thu May 01, 2008 2:11 pm
And sometimes the clutch on the starter does not protect everything! It isn't just rods that suffer. This is from a Wright R975. Note the spline twist. The starter drives the shaft/gear on right and that power is fed thru the splines to the large gear on left, and on to crank gear. Watch that torque setting on the starter!
