turning prop backwards
 

When moving the prop, I've always turned it backwards so the mags wont fire if the grounding is faulty. Now with my new O-320 I'm being advised not to turn backwards because the vacuum pump seals might be damaged.

My recollection is that there was a thread about this here many moons ago - I seem to recall consensus was that the dont-turn-vacuum pump-backwards story is the aviation equivalent of an urban myth ?

Cant find the old thread using the search function, maybe someone can confirm.

Can't confirm or deny myth status - I do, however, know that the PA28 of which I was (in fact, still am!!) a share owner went away for annual with serviceable vaccuum pump a few years back. 2 wks later, we were advised that the pump had to be changed as it wasn't working. The senior group member (who was a CFI of around 40 yrs standing) ran around muttering under his breath about 'mechanics turning the damn prop backwards' :D

I have since then been extremely cautious about turning the prop the correct way, but have never seen technical arguments for or against.

Please excuse my ignorance on this but why would you want to turn the prop by hand?

Certain aero engines of the flat sort can have their vacuum pump damaged by turning the prop backwards.

Why would you want to turn a prop by hand?

1) It's imperative to do it to check for hydraulicing (oil in the cylinders) on a radial or inverted engine. Not to do so risks destroying the engine.

2) To check for good compressions.

3) To suck fuel into the cylinders as part of the priming process on some engines.

3) On Gipsies, to check that the impulse mag is working (it clicks - no click, no start, so you tap the mag to free it).

4) On many aeroplanes, to start the engine.

SSD

Vacuum pumps on Lycomings have brittle carbon vanes, and can definately break if an engine kicks back under power. I believe an engine can also fire if turned back while the mags are live.

Unless your engine falls into categories SSD para 1 to 4, why turn it backwards?

Thanks SSD - the only reason I ever came across before was that the CFI liked to see the props all vertical so the engineer would turn them to the horizontal (or vice versa, can't really remember)

One more reason to add to Shaggys list; some folks advocate turning the prop by hand to" break the oil film" especially when its cold.

I too had the expense of a new vac pump recently and was warned not to wind the prop backwards the pumps vanes are designed to go in one direction, repeated winding the wrong way will break them I was told .

I was shouted at by the CFI when I was a student pilot, for turning the prop backwards. He drew me a picture of the vac pump and explained what would happen. Lesson not forgotten!

If the mags aren't grounded, nasty things can happen whichever way you turn the prop. Always treat it as "live"!

Our checklist says the prop must be pulled through four compressions before the first flight of the day, to confirm no hydraulic lock.

SSD - might be missing something obvious here, but you said...

How do you know when the oil is nicely in the cylinders? Will it simply feel "smoother"?

tKF

KF

--------------------------------------------------------------------------------
It's imperative to do it to check for hydraulicing (oil in the cylinders) on a radial or inverted engine. Not to do so risks destroying the engine.
--------------------------------------------------------------------------------

Hydraulicing is a situation when excessive oil is present in the cylinder bore. It happens easily on inverted engines and the lower cylinders of radial engines because after shut down it is easy for oil in the crank case to run down the cylinder walls, seep past the piston rings, and pool in the cyclinder heads.

If there is sufficient oil in the heads, and the engine is started, the piston on the compression stroke will come up against an incompressible layer of oil and will stop dead. This usually results in a bent or broken con rod, and the condition is called hydraulic lock.

The results of 'lock' can be more insious than instant engine destruction - if the lock is relatively minor, the engine may continue to run OK with a slightly bent con rod. At some indeterminate time in the future, this damaged con rod will fail, and the engine will eat itself - usually when at full throttle, like take off for instance.

If you pull the prop through by hand before starting, and there is a 'lock', you will be unable to turn the engine over and may have to remove the spark plugs to get the oil out of the cylinder(s). But see next paragraph....

The Yak 52's Vendeneyev radial is particularly prone to locking, and great care must be taken to avoid the condition. On this engine, it is posible to pull it through, get no 'lock', but still suffer one on start up. This is becuase the inlet ports extend beyond the level of the inside of the cylinder heads. On the lower 3 cyclinders, oil can pool in the 'U-bend' at the bottom of the inlet pipes, and remain there during manual pull-through. On start up, the induction gases blow the oil into the cylinder on the inlet stroke, and you get a lock on the compression stroke. To prevent this, there are drain plugs for these 3 inlet pipes - if you even suspect there might be oil there, you have to unscrew the drain plugs, which will allow it to drain out.

There are, I beleive, mods availabale to automatically do this, without having to manually remove the drain plugs. If I still had a Yak share, I would insist on having this mod fitted.

SSD

Thanks SSD!

Got it now :D

tKF

urban myth !
 

Without doubt this vac pump damage thing is almost an urban myth.

If the carbon vanes on a vac pump fail when the prop is turned backwards then the vac pump is so worn that it sould not be flying.

The new breed of vac pumps with inspection ports should put the lid on this bit of aviation folklore.

An urban myth perpetuated by the warning paperwork which arrives with your $1000 replacement pump.

I'd say an "urban truth" rather than "urban myth". Carbon vanes turned backward will dig in, potentially with disastrous results.

Compare pushing and pulling a pram across the sand. Pushing, it digs in more, the more you push. Pulling, it's (relatively) easy.

The simplest reason for turning the prop is to get it horizontal for manoeuvring by hand on the ground, or for clearance reasons while working on the engine or otherwise. Any prop movement in either direction while doing so is to be avoided.

Absent a clear view on this thread I guess I will ask the manufacturer .........

I was actually told to turn the prop backward four revolutions if the PA28 201 Arrow fuel injected engine didn’t start after a few turns on the starter. It was always a beast to start when it was hot although a cold start seemed no problem for some strange reason. Turning the prop backward as instructed did seem to do the trick and didn't seem to do any harm.

Vac Pumps
 

If it's a dry vac pump, DO NOT turn it backwards - it will shorten the life of your vac pump significantly.

If it's a wet vac pump, then it will have no effect on it.

CS

Why hand turn the prop?

(1) So it's not horizontal then the birds won't sh1t all over it.

(2) I learned at a school that used the prop position to indicate fuel status, vertical meant needing fuel, horizontal meant full to the brim.

(3) I've heard some poeple like to leave the prop horizontal to lessen the chances of someone taxiing past and hitting it.

Note I didn't say it was necessarily a good idea though.


I too have heard the warnings about rotating backwards and vacuum pumps but have no idea if it's really true. I've heard going backwards reduces the chances of an actual firing due to the timing. Don't know if this is true either.

Perhaps (as SSD suggests) rotating dates back to the days of the big radials, the idea being to rid the lower jugs of oil and thus prevent hydrolocking. I've seen films of three people turning the blades of a B24. It looked a little like a Maypole dance.

However, I have read that if you do the mathematics, you can bend the rods just by turning the blades by hand, due to the tremendous mechanical advantage produced, so it really helps to have those lower plugs removed..


Anyway, fooling with the prop cant' be THAT bad. This is from the Cessna web site]

Nice thread with a few good points on props. Here's a couple more for interest:

Turning prop backwards: it's an essential procedure to clear some flooded engines, and to enable some engines to start when hot - for a practical demo, watch the Chief Engineer at Shuttleworth trying to re-start the Tiger and Maggie after the show

Horizontal props: wooden props should always be left horizontal. If left verticle for a long time, over winter, say, any sap or liquids in the prop may drain to one end leaving the prop un-ballanced.

Final thought: Always treat the prop as live and never walk through the disc - how many pictures do we see of proud pilots smiling at the front of their aeroplanes, cuddling the prop. I wonder what the pictures would look like if the engine fired...............??

A

A

I would say that the POH or engine manual should have the last word on this.

In my case I can say that our Diamond Katana DV20 has had a technical directive from Swedish CAA to not turn the prop backwards, du to air bubbles forming in the oil pump.

The Super Cub manual on the other hand states that if engine start can't be accomplished due to flooding, the engine should be rotated so and so blades backwards to clear up fuel.

Hand propping is done on all our club aircraft at the start of the day, to insure compression is available on all cylinders, and also to "get the oil going" in cold weather conditions.