View Full Version : Turning propellers by hand
I read in the March Pilot magazine that if you do have to move a propeller that you should turn it backwards which is what I advise my students.
However, the article in the magazine continues with a caution that turning a prop backwards can 'shatter the vanes in the vacum pump'.
This is new to me so can anyone offer any more information on this possibility? I have had a thorough look in our PA28 POH and can find no mention of it.
Whopity
7th Mar 2006, 14:50
Why turn it backwards? The engine is designed to run forwards and timed accordingly. If you turn it forwards and it were to fire at least you know which way its going to go and arrange your hands accordingly. If you turn it backwards it could fire either way and chop your hands off. If you are checking the compression as required in the A Check CAP411 LAMS Schedule you need to turn it the correct way.
The Vacumn pump is designed to operate in one direction only, if you turn it backwards you run the risk of damaging the connecting rods which can snap off if they are pushing when they should be pulling.
DO NOT TURN AN ENGINE BACKWARDS!
Say again s l o w l y
7th Mar 2006, 15:07
A new one on me. If you have to turn the prop, do it in the correct way i.e. forwards.
Honestly, these magazines are comics sometimes.... And then people complain when we give advice contrary to what was in a mag, as if they are the be-all of aviation knowledge.
high voltage
7th Mar 2006, 15:16
Unless you are hand swinging I wouldbnt be rotating the prop, other than a few degress to eg attach the pull bar to the nose wheel. Too many people have no respect or understand how to handle the prop, therefore I always tell them to leave it alone! I can't see it ever firing by rotating it counter-clockwise due to the timing set-up but you could easily destroy the suction pump.
Hmmm, obviously a contentious issue this.
I was taught by my FIC instructor to turn the prop backwards. The reason given being that if you turn it the correct way then when the impulse magnetos operate, (which you can hear by the click they make) the engine could spring into life. I was under the impression that this is 'less' likely to happen if the prop is turned backwards.
(the reason why he was turning the prop at all was his request that props be left vertical to avoid the possibility of water pooling and freezing in the of the spinner thus causing imbalance/vibration)
Whopity, I'm no engineer which is why I am asking about this, but I find it hard to believe that the con rods are so delicate that they could snap when I turn the prop backwards, but I will certainly bear it in mind! Regarding the suction pump and potential damage, well if anyone can offer a technical/mechanical explanation of why this is so then it would be much appreciated - just so I can understand it all a bit better, (and incase any clever dicks ask why!)
So I suppose good advice for students would be 'if you absolutely have to turn the propeller by hand, do so in the normal direction of rotation, but expect the engine to fire at any time'.
EGBKFLYER
7th Mar 2006, 17:17
Vac pump vanes need to form an air-tight seal with the pump housing so they develop the vacuum the pump is supposed to supply.
Old vac pumps did this via an oil film but the downside is the oil contamination of the vac supply. Modern pumps have an aluminium housing which is highly polished. The vane ends are made of graphite (think pencil 'lead'). The graphite provides a soft bearing surface that conforms to the pump housing, thereby sealing the pump but it is fairly brittle material and doesn't like being loaded in ways other than designed (think of your pencil again)
The vac pump is available in clockwise and anti-clockwise models (for different engine rotations), the difference being due to the vanes being angled in the direction of motion.
Turning the prop backward, against the angle and contra to the profile the vane ends have worn to, runs the risk of breaking up the graphite vane ends in the pump, breaking the seal and making the pump u/s.
My view (as an engineer but not on aircraft) is that turning a prop slowly backward as you describe will probably not do any damage. However, the risk is there.
My 2p-worth: props should always be treated as live which ever way you turn them. Backwards is not a no-no, so long as it is carefully done and the risk is understood!
Hope that helps
Edited to add http://www.avweb.com/news/maint/182905-1.html - which explains still more and confirms the above...
Whopity
7th Mar 2006, 17:38
Dude "Con Rods in the Vacumn pump as described above!
If you compress a fuel air mix and get a spark, the direction it will initially go will depend on where the piston is in relation to TDC; it will always go down the bore and could be in either direction depending which side of TDC it is. I have seen an engine kick back so I would never dream of putting my hands on the wrong side of the prop.
EGBKFLYER
7th Mar 2006, 17:44
Listen to Whopity - I have seen the same. The wooden prop broke several bones on the back of the engineer's hand and this was with the plugs out of half the cylinders and one mag off. Honestly!
Props are always live...
As somebody who has experienced being struck by a propeller ( which shattered both bones in my arm) I cannot stress enough that ALL props are live. In my case, there was an intermittant live mag and I was careless pulling the prop through. Just by moving a prop by a few degrees may be enough to trip the impulse with disasterous results. Prop swinging aircraft that are normally started by starter motor are potentially lethal too - the prop is rarely in the correct position for hand swinging . Magnetos do not generate a spark when turned backwards and certain engines are regularly turned backwards to "blow out" an over rich mixture. I have heard that turning an engine with a vacum pump backwards is not recommended- likewise Rotax engines with a reduction gearbox should not be turned backwards either.
coolape
7th Mar 2006, 20:44
Im no certified engineer, but I've been educated by certified engineers, and theres a rule they keep screaming at me when I do something.
"never stick your hands where you wouldnt sticky your willy" I swear by that rule.
172_driver
7th Mar 2006, 21:11
If anyone understands Swedish, here you go:
http://www.pilotmagazinet.se/aid/report_view.asp?aid_id=32
Keeping in mind that the majority of you don't understand Swedish this is an occurence report as a result of lost vaccum pressure during climbout over a dark wooden area. A situation where you most need the horizontal gyro. Prior to start-up, the prop had been turned backwards when it opposed to fire off. The pump was quite fresh, only 170 h old.
OpenCirrus619
8th Mar 2006, 08:13
Dude "Con Rods in the Vacumn pump as described above!
OK - I'll show my ignorance. I was under the impression that Vacuum Pumps were like this inside: http://www.sacskyranch.com/vacuum_pump_inside.jpg
Where do con-rods come into it? I always thought they connected cranks to pistons - but there are no pistons here.
OC619
P.S. Yes I do realise this is a rather poorly example
'Chuffer' Dandridge
9th Mar 2006, 12:04
Disregarding the vac pump subject completely (I have also been told never to turn an aircraft fitted with a vac pump backwards), isnt this just another example of magazine 'experts' getting it wrong AGAIN! A lot of these so called experts are just mates of the Editor:yuk:
Opencirrus 619,
That Vac Pump looks $hagged!
OpenCirrus619
9th Mar 2006, 14:29
Opencirrus 619,
That Vac Pump looks $hagged!
P.S. Yes I do realise this is a rather poorly example I couldn't find a photo of a good one.
Actually, from what I've read in various places, there seems to be a lot of debate on the question of whether rotating vacuum pumps backwards is really likely to cause problems.
Firstly if you have a wet pump then its no problem which way you turn it.
If you have a dry pump (as above) then there are 2 variants - one which has the vanes arranged radially and the other at a slant (as above). It seems pretty well everyone is agreed that roatating a pump with radially arranged vanes backwards will not cause a problem.
So, the only type likely to cause a problem iif turned backwards is one with the slanted vanes. I would put my vote in the "It won't cause a problem unless the vanes are so worn they are almost out of the slots in the central rotor, or there is another problem with the pump already" camp. Since, in either case the pump should either have already been changed, or is about to fail anyway, I'm not going to get excited (at least as far as the vacuum pump goes) seeing someone turning a propellor backwards.
Should I admit here that nothing I fly regularly has a vacuum pump fitted?
OC619
P.S. If you're going to turn propellors learn how. Join a club (http://www.tigerclub.co.uk) where part of the checkout is hand-swinging - if you don't have a starter how else are you going to start it?
AerocatS2A
10th Mar 2006, 15:15
Unless you are hand swinging I wouldbnt be rotating the prop, other than a few degress to eg attach the pull bar to the nose wheel. Too many people have no respect or understand how to handle the prop, therefore I always tell them to leave it alone! I can't see it ever firing by rotating it counter-clockwise due to the timing set-up but you could easily destroy the suction pump.
You should turn the prop on some aircraft particularly those with upside down cylinders (radials, some in-line 4s and 6s) to clear any oil and make sure you don't have any hydraulically locked cylinders.
Best to teach them how to do it properly rather than tell them to leave it alone.
A and C
10th Mar 2006, 18:25
The chances of a Mag producing a spark when the prop is turned backwards are nill if the prop is turned by hand at slow speed. The reason for this is that the impulse unit wont work, in fact to check the timing you HAVE to turn the prop backwards to dis-engage the impulse otherwise the timing will be set incorrectly.
So the bottom line is that each time the igniton timing is set the engine HAS to be turned backwards, I don't see vac pump failures after each 150 or annual check so it would seem that the thing about turning the vac pump backwards slowly is just so much hogwash.
If the vac pump was turned backwards at high speed it would be another matter.
But please remember Props Bite
stevef
11th Mar 2006, 23:10
I have an e-mail from Parker Aerospace Customer Services in answer to a recent maintenance query about this:
Turning the engine over backwards will NOT harm an Airborne vacuum pump. A problem WOULD arise if the Air Pump was installed on a pad, which rotated in opposite direction than the pump and the engine run for a period of time.
They're the experts!