Turning the Prop
 

Recently, some bloke said to me that he turns the prop on his plane to lubricate the internals before starting the engine. He reckons this helps keep the engine in fine fettle.

During my PPL I was taught to treat the prop as live at all times for what seems to me to be very obvious reasons considering the way the mags work. However, I just watched Airsport on Disco Wings and it showed an aerobatic pilot doing the same thing to his SU-aero-wotsit-big-engine-little-plane prior to starting it.

Is there any benefit?

Absolutely, although it's most common practice for 2-strokes such as the Rotaxes used on microlights. It keeps the lubricant around the engine, prevents internal corrosion, better starting next time, etc.

Of-course, the best thing is to make sure that the engine is run at-least every week or so - but occasionally that's impossible. During the Foot-and-Mouth headaches you'll have found many pilots lovingly rotating their props in hangars up and down the country.

Obviously, switches off, parking brake on, chocks in, and if it's a 2-stroke with the fuel switched on and the carb-bowl primed. And if it's an inverted 2-stroke, remove the spark plugs or they'll be utterly fouled by the next time you come to try and start the engine.

G

High Wing Drifter,
QUOTE:- An aerobatic pilot doing the same thing to his Su-aero-wotsit-big-engine etc. UNQUOTE.
Could said pilot have been turning the radial on his aerothingy prior to start-up to clear oil out of the bottom cylinders in order to prevent hydraulic lock?

Genghis,
Excellent, thanks for the info. My only worry is that according to the Thom book, if the earth is disconnected (or otherwise faulty) on a mag then the mag will be live. The thought of the consequenses of an inadvertant start make me shudder a little :uhoh:

Noah,
Ahhh! It was a radial engine yes, so possibly :confused:

Radials - and other's with inverted cylinders - are prone to hydraulic locks: When not running oil seeps past the rings into the cylinder. The fluid doesn't compress so instead of the poor piston/conrod shoving against a compressing gas during start, it slams into the fluid. End result is broken internals.

Pulling it through before start lets you feel the hydraulic lock without breaking anything. If found it can be drained by removing the plugs.

Trevor Thom is quite right, hence the precautions I'd mentioned, plus that everybody should do a mag-switch check before shutting down and if you share the aeroplane with anybody else make sure you've checked the log for any reported ignition system faults before you rotate the prop.

G

.......And always treat the prop as 'live'.

We always pull the Chippy's engine through before start as part of the priming procedure, and also to check for hydraulic locks and good compressions.

SSD

Excellent, another thing I have learnt :cool:

I shall add that to my personal SOP.

Many thanks for the replies guys :)

Look, I understand about pulling thru a radial engine for hydraulic lock prevention, but I can not for the life of me figure out how by pulling thru a prop on an engine that has been stopped for maybe a week can distribute lubricant thru the engine. Since the oil has long since drained into the sump, (assuming a wet sump engine), all you would do is redistribute what little oil remained on the surface of the cylinders. Or possibly even remove the last vestiges of it. The oil ain't going anywhere out of the sump until such time as the engine is running. The oil pumps on piston aircraft engines ain't that efficient. Ever noticed how long it can take for the oil pressure to actually show a reading sometimes? AFTER you have it idling?

Yes, turning the prop by hand before starting can have all the above advantages and is something I like to do. With anything that has one or more inverted cylinder (radials, Gipsy Major etc), I consider it a must-do every time, but with a flat-engined aeroplane that I know well or have flown the same day, I'd probably not bother unless I had some good reason to. With Gipsy it is easy to treat it as if the prop were live, you just pull it through as if you were hand-swinging the engine. With a big radial, especially with a 3 blade prop, you have to think a bit harder to keep yourself out of danger if the engine fires. Make sure you are standing in the right place, on firm footings, no loose clothing etc, and with the momentum of your body movement carrying you away from, not into, the prop arc. And keep your senses really tuned in so that you are ready for the slightest chuff (which might make the prop move smartly in either direction) Really really treat it as if it will fire, don't just think iof it as a hypothetical. Obviously check switches off, throttle closed, chocks / brakes set and any other checks applicable to type.

I would not myself turn a prop BACKWARDS except if the POH told me to, and then only with a degree of reluctance and a proper understanding why I was supposed to be doing it. Some engine ancillaries may not take kindly to it, nor may reduction gearing on some engines, and on radials without inlet manifold drains it has been known to make oil in the cylinders disappear into the inlet manifolds, only to be sucked back into the combusion chambers upon startup. This can lead to a partial or complete hydraulic lock, with possible bent con rod and subsequent engine failure sooner or later.

I've also been told that pulling the prop through a few blades as soon as you arrive at the aeroplane, checks mags off etc etc, will get some fuel vapour into the cylinders, and will make starting easier when you come to starting.

Whether there's any truth to it, I don't know!

tKF

Hi, TKF,

If the fuel is present, i.e. it's turned on and in the vicinity of the induction system via whatever priming method the type uses, then yes it will be sucked in. For a cold start it's a good tactic on Chippie, Moth, and Yak alike, though it works in clightly different ways on the different engine types. On the radials it's quite a good idea to have the prop going round if you need a lot of prime because it does not then tend to all pool in one cylinder, another potential cause of hydraulic locks.

The only time I turn over the propeller by hand is when the Flight Manual or Pilot's Handbook says that I should.

I don't remember reading in any Piper or Cesnal manual that the engine should be rotated by hand prior to flight.

Many people justify the hand turning of the prop as "limbering up the oil" or "breaking the oil". That is basically rubbish according to Piper and Cessna........if it is cold then there are two requirements....a) Use the correct grade of oil and b) Use pre-heat if necessary (very very cold temperatures).

I fly one type that the flight manual requires the prop to be turned by hand prior to first flight in order to check compression. I do this only prior to the first flight and while the aircraft is chocked and tied down.

So basically unles the flight manual requires hand turning then don't do it because.........if it chops your hand off (or worse) or even if it runs into another aircraft, the first thing the insurance company will say is you should not have done that so we won't pay. :(

Regards,

DFC

Turning the engine by hand does nothing for the lubrication - as I and others have already stated, it's to do with checking for hydraulic locks and good compression, and in the case of the Gipsy for priming (with the LH cowing open you pull a toggle that holds the carb float depressed, and work the handle one one of the fuel pumps up and down 'till fuel flows out of the manifold drain, then 'suck in' to draw it into the cylinders).

I always worried about turning the Yak 52 over before flight. You have to do it because these radial engines are particularly prone to hydraulic locks, but it's a big engine and the gearing means you are turning the engine over faster than you are turning the prop. So it's hard work, and can't be done with an arm's length flick like the Chippy. You have to get in there and really heave each blade round. Nasty if it starts :(

SSD

Mostly true for many engines SSD, but not all.

Many wet-sump engines will splash oil into the bores and onto the crankshaft when turned, all 2-strokes will bring oil-fuel mix throughout the engine when turned, leaving the oil behind as the fuel vapourises.

As to whether localised "dry spots" in a dry-sump engine will be lubricated with turning the engine over - goodness knows, but I doubt that it'll make things any worse.


DFC - whilst I have written several write flight manuals, so am duty-bound to defend them in most contexts, it's worth remembering that the manual was probably written when there were only a couple of aeroplanes of the type in existence with a few hundred hours on them. Many types have accumulated hundreds of thousands of hours since then and probably the only lessons passed into the manual as later amendments were pure safety ones. I wouldn't do anything that the flight manual specifically prohibits, but many sound practices may never find their way in there and, with caution, you can learn good techniques elsewhere. To put it another way, how much useful information will you actually find at-all in the POH of a Piper Cub?

G

Lowtimer,

I understood the primer on the Yak 52 feeds the supercharger, not the cylinders directly. You still need to pull the engine through to get the fuel into the cylinders.

An engineer of the "old school" once told me that turning the engine over by hand could damage the "vanes" in the magneto. I didn't understand it then and I still don't but I offer that for what it's worth. This refers to "flat" engines i.e.Lycoming and Continental. Possibly to do with direction of rotation?

Hi, Yakker,

That's my understanding too, but it doesn't stay in the supercharger in its liquid state - it quickly percolates down to the bottom cylinder induction pipe. If you give a full cold-weather several-shot prime, without (preferably your helpful volunteer) turning the prop between strokes, and then open the manifold drain, a highly impressive amount of liquid fuel falls out onto the path. Turning the prop between or during primes does seem to distribute it around a lot more evenly, it seems to give a much better start, and hopefully avoids what I feel (perhaps erroneously) is the risk of a large belt of liquid fuel pulling into the bottom cylinder all at once.

All the 'armstrong starters' (sounds better than hand-swingers) among us will know that turning the engine over backwards, switches OFF and throttle OPEN is an essential remedy to an over-primed engine... so I doubt that the idea that doing so damages 'vanes in the magneto' holds much water!

Actually, the whole thing of turning the motor over before swinging the prop acts as a very useful diagnostic aid. As well as hydraulic locks in those awful old Gipsy engines (lower your guns chaps - I was only joking), you can keep an eye on compression and ought to be able to feel if summat's bust inside the crankcase.

One of life's simpler pleasures is sending the flashing-blue-lights-on-their-heads brigade into orbit by solo hand-starting the old L-4 Cub whilst standing behind the prop: it is safe, for reasons I've explained elsewhere, but they don't realise it is!

Many years ago when in a rush to get airbourne I neglected to hand turn the engine pror to start on a DH Beaver It was the one time when it had hydrauliced! The mess after an attempted start had to be seen to be believed.
At least Aunty Betty paid for the new engine.

But I have pulled engines through ever since. If you assume mags are always live you wont go wrong