turning prop backwards

[Bluebeard777]

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.

[Circuit Basher]

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.

[Aim Far]

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

[Shaggy Sheep Driver]

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

[bluskis]

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?

[Aim Far]

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)

[Davidt]

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 .

[Keef]

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.

[TheKentishFledgling]

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

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.

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

tKF

[Shaggy Sheep Driver]

KF

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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.
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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

[TheKentishFledgling]

Thanks SSD!

Got it now :D

tKF

[A and C]

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.

[bluskis]

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

[Keef]

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.

[Bluebeard777]

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 .........

[Sensible]

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.

[compressor stall]

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

[Onan the Clumsy]

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 (http://se.cessna.com/who.chtml) is from the Cessna web site]

[Airbedane]

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

[TANGOWHISKEYINDIA]

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.

[Circuit Basher]

As well as everything said so far, my reasons for turning a prop by hand in past years have been:

- to clear the nosewheel tow bar (on the club Fuji, which has long prop / short nose leg) - the prop pretty well always stops in the vertical and requires turning horizontal (having checked I've got the keys in my pocket!)

- the PA28 which I had a share in at Bournemouth was parked outside. It also had a very small / slightly k*ackered battery. In cold weather, my standard procedure was to prime the engine, pull it through 4 blades, one more squirt of primer and then attempt to start. This worked most times and saved having to prop swing a live engine, which I try to avoid if possible.

[A and C]

I except what you say about turning the prop backwards damaging a vac pump but only if it is worn almost to the point at which it is about to fail.

The carbon vanes in the pump are so lightly loaded at the speeds that you can turn the prop by hand that I cannot see how any damage could take place except if the vane had excecive play in the rotor IE a high time pump. I can see how damage could be done if the pump was run at high speed backwards.

The latest pumps on the market have an inspection port to monitor vane condition this should put the end to the lie that " the pump failed because it was turned bacwards " when the engineers remove pumps with worn vanes before the pump fails.

I suspect the warning that came with your new pump is due to smart thinking on the part of the pump maker in using this bit of aviation folklore to keep the warranty clames down and to muddy the waters in case of leagal action after a pump failure , now that it is posable to see inside a pump and check the state of the vanes i see the truth coming out.

What I dont see is this disappering from the folklore quickly after all only last week I was told that the engine in a PA28 should not be leaned below 5000 ft ( this is a tiger moth limitation ) .........................it never fails to amuse me how these things become folklore !!!.

[poetpilot]

Circuit Basher, I use a similar technique for cold starting the Archer which I fly occasionally from Barton.

It has a rather lazy starter(engineers have looked at it & the battery many times but it always behaves the same), as well as being a bit temperamental on cold and warm starts.

I've found the almost guaranteed way to get a quick fire-up in the morning is -

prime six pumps

all switches OFF, handswing through 6 compressions with throttle cracked open a tad.

Jump in, flick the switch and she's away first turn.


Once she's warmed up it's a different story - great has to be taken not to flood her on warm starts.



Incidentally, I used to take a masochistic delight in helping people handswing props to start them (on wooden prop PFA jobs mostly). However I gave it up after I got bitten with a backfire by a Luton Minor. Nasty gash, but I was lucky, it fully healed. Others have not been so lucky.

It's a horrible feeling when your hand goes numb and you daren't look at it to see what the damage is.....

[Onan the Clumsy]

Hard to follow that post

but here goes...
prop pretty well always stops in the vertical and requires turning horizontal You might consider getting the prop remounted on the flange.

With a four cylinder engine for instance, the crank is likely to stop in one of four positions. So the prop will tend to stop in one of two positions - in your case, this sounds like vertcal or horizontal. But if the flange has 8 bolt holes, you can remove the prop and relocate it one hole over and it'll usually stop on the disgonal.

[Windy Militant]

Onan,
This may not be possible. On many aircraft the prop mass is included in the engine balance so can only be fitted in one position.

I'm surprised at the lack of knowledge of engines and systems shown here. Having been in engineering for many years and being a bit of an anorak a lot of this stuff seems to be pretty basic to me. I'd have thought, especially if you own an Aircraft learning how the systems work would be an essential thing to do.

With modern cars so reliable and complex people no longer tinker so don't learn how things work. Even with Motorcycles a lot of new riders these days can't even do simple things like adjust the drive chain.

This seems also to be the case with many pilots. They treat aircraft like cars, just jump in and go. I'm not saying that you should have to be a licensed engineer before you're let lose in an aircraft but some time spent with the manuals would be a good thing. (Something I must do myself shortly!)

At least people are asking questions here which is a good thing because It makes you think about what you're doing and that may just save your life. (Or an awful lot of money)
:ok:

[Onan the Clumsy]

WM, I don't know enough to respond, but if it's the case, It would be news to me as I always thought the prop balance was a separate issue. I'll look into it though. Thanks.

Your other comments are pretty much spot on though and I feel like it's time I stuck my nose back in the books too.

Jackie Cochrane in her biography said a similar thing. Her comments were that when she started off flying, it was pretty rare to make the journey in one hop and quite common to have to find a field, set down and clean the plugs. Not sure I'm ready for quite THAT level of education.

Cheers.

[Windy Militant]

OTC
It's something I'd not really thought about until we got rain erosion on the prop on the aircraft I have a share in. When we took the prop off it was doweled to the boss and the boss was indexed by a master spline to the crank output shaft. So it would only fit in one position. Thinking a bit more about it, this could be more to do with the fact that its not fitted with a starter motor. This would fix the firing position of the impulse mag with the prop so that the blade is in the right position for swinging perhaps.

See what I mean about making you think;)

[IO540]

I don't think the "instant vac pump damage if turning brop backwards" is universal, or even common.

Recently when searching for a vac pump which is STCd for my plane (to keep around as a spare) I read some details of a fairly common pump which was rated for 50 hours of reversed operation at cruise RPM!

One may ask how on earth could that be achieved; apparently it can be achieved if fitting a pump to the "wrong" engine in a twin. Or, in a case of a vac pump failure (perhaps in a twin but not necessarily), one may have to fit the "wrong" kind of pump as an emergency measure.

[Bluebeard777]

After some rounds of emails with my vacuum pump manufacturer the conclusion is that the prop may be turned backwards without compromising pump life. However it should not be run backwards.

So that's it. The folklore had a grain of truth, but not the full story, for this particular pump.

[RW-1]

From AvWeb:

In the early 1970s, general aviation switched to dry vacuum pumps manufactured principally by Airborne and relative newcomer Sigma-Tek. Although there are differences between the two, Airborne and Sigma-Tek pumps use self-lubricating graphite vanes which are both the solution (no engine oil required for lubrication) and the problem (the pumps tend to produce spec vacuum throughout their lives and then disintegrate internally in a cloud of carbon dust).
Compounding the sudden failure mode is a frangible coupling that is designed to shear abruptly in the event of overstress or sudden stoppage. In most cases, the aircraft operator will see no sign of any problem until the vacuum gage reading falls precipitously to zero.

Perhaps backwards pulling of the prop causes the vanes to dig in, and refer to the latter para ...

however a further search of AVWeb got me:

http://www.avweb.com/news/maint/182905-1.html

Backwards is Bad

The hub slots of Airborne pumps are canted in the direction of rotation. For this reason, Airborne offers different pump models for clockwise and counterclockwise applications. The most common model numbers are 211CC and 441CC (for counterclockwise rotation) and 212CW and 442CW (for clockwise rotation). It's not difficult to break the code.

Installing a wrong-direction pump is a sure prescription for premature failure. Most Continental engines require a clockwise pump, and most Lycomings require a counterclockwise pump. But not always. In fact, twins with counter- rotating props need one of each!

so it would appear that turning one backwards WILL cause problems depending on it's installation.

[Onan the Clumsy]

Like pushing your fingernails over an uneven surface then?