Owt?

[Richard]

RE: float type carb. Is there any validity to the idea that leaving the mixture knob/lever in the idle cutoff position will unduly stress the diaphram in the carb? And the solution would be to advance the mixture to a more rich position after shutdown.

RE: NA piston direct drive engine. Is there any validity to the idea that rotating the prop backwards will damage the vanes in the vacuum pump because of the 'directional' wear of the vanes inside the case?

 

[tonycondon]

ive never heard of number one. every airplane ive ever flown and everyone ive ever seen fly has always left the mixture in the idle cut off position, fully.

number 2, ive heard about. i dont know how accurate it is. supposedly the vanes are lubricated so why should it matter?

 

[Joe B]

number 2, ive heard about. i dont know how accurate it is. supposedly the vanes are lubricated so why should it matter?

The theory is that the blades wear at an angle and, in effect, "take a set". If you turn the pump backwards you're now rubbing the blades the wrong way, making them more likely to chip and break.

It sounds like a reasonable theory but there's no actual data behind it as far as I know. I used to follow it until one day I watched my shop doing a compression check - moving the prop both ways to get the pistons in the right place. Figured if they weren't going to worry about it I was wasting my time.

BTW the blades in a dry vacuum pump aren't lubricated, per se. That's what the "dry" means. The pump is lubricated by erosion of the blades themselves, which are made of graphite.

Regards,
Joe

 

[Let'sgoflying!]

I have done the vac pump thing.
Newly acquired a/c, needed to move the prop a smidge to get tow bar on while parking. Heard a snap, felt resistance. Yup, pump broken. Opened it up and one vane busted. Now the vanes on this pump were very short and I don't think it would be likely in a pump that was not so aged. As the pump ages the carbon vanes get short and sloppy in the block, which makes them more prone to breaking.

I cant imagine parking an airplane with the red knob anywhere but all the way out.

 

[TangoWhiskey]

Looks like the answer is "it depends". What brand of vacuum pump do you have?

http://www.sacskyranch.com/rapco.htm

To understand why pumps fail, it is helpful to know something about pump anatomy. Both Airborne and Edo (now Sigma-Tec) pumps utilize carbon-graphite rotor construction, with carbon vanes riding loose in the rotor slots. In normal operation, the vanes are thrown against the pump housing by centrifugal force, rising and falling on the elliptical walls and thus compressing the air trapped in the vane compartments. The Edo rotor has eight slots and eight vanes, it can be turned in either direction. The Airborne design, by contrast, uses six slots and vanes each at a slight angle, giving the pump a preferred direction of rotation. Airborne units thus come in 'CW' (clockwise) and 'CC' (counterclockwise) models, and for long life, the proper sense must be observed on installation.

Also, see point #7 at the same site.

Interestingly, though, RAPCO doesn't mention "don't turn it backwards" in their PDF about how to extend the life of your vacuum pump:

http://www.rapcoinc.com/pdf/How to Max Vac Life.pdf

I couldn't find anything on their site about that question, but you might consider emailing them to see if they have any empirical data/analysis.

 

[The Old Man]

Rotating the prop 1/4 turn to get it out of the way of the towbar will turn the pump how far?

On my engine, the pump is driven by the same gear(s) that drive the mags. The mags turn at 1/2 the engine rpm, so the pump would turn 1/8 turn.

Doesn't seem significant to me.

As to the mixture, My engine is FI, so I don't have a dog in the fight. I often push it (and the throttle) back in before I get out, so I don't bump them with my knees.

 

[Tom-D]

#1
there is no diaphram in the carbs. on modern aircraft. no worries. false, don't believe it.

#2

The chances of braking a vane in a vac pump are so small that risking a magneto firing while turning the engine in the direction it rotates, isn't worth it.

 

[TMetzinger]

I always make sure the mixture is off, the mags are off, and then I turn the prop in the direction of travel. It only takes an eight or quarter turn to get the prop out of the way of the towbar or tug, and I have high confidence that there's no combustible mixture in it to begin with, particularly when I'm the one who flew it last and shut it down by cutting off the fuel.

Dry vaccum pumps have their vanes at an angle to the path of rotation, so they are absolutely designed to turn in one direction only. You may not break a pump vane by making it go backwards, but you might.

Decide which is riskier based on your situation - if you don't KNOW that the airplane was properly shutdown and that no combustible mixture is in the cylinders, then you may choose not to rotate the prop in the direction of rotation.

Of course could use the starter to move the prop a tap or two, and not put anything at risk....

 

[gismo]

I have done the vac pump thing.
Newly acquired a/c, needed to move the prop a smidge to get tow bar on while parking. Heard a snap, felt resistance. Yup, pump broken. Opened it up and one vane busted. Now the vanes on this pump were very short and I don't think it would be likely in a pump that was not so aged. As the pump ages the carbon vanes get short and sloppy in the block, which makes them more prone to breaking.

I cant imagine parking an airplane with the red knob anywhere but all the way out.

AFaIK, that's exactly where this OWT comes from. If your pump is about to go, turning it backwards may end it's life a tiny bit sooner. Chances are your pump was on borrowed time already.

 

[gismo]

RE: float type carb. Is there any validity to the idea that leaving the mixture knob/lever in the idle cutoff position will unduly stress the diaphram in the carb? And the solution would be to advance the mixture to a more rich position after shutdown.

Short answer: OWT

Long answer: This OWT actually is about pressure carbs which use diaphrams to sense pressures. A conventional carburetor has no diaphrams with the possible exception of the accelerator pump and the mixture control has no effect on that whatsoever. In a pressure carb (EG PS-5C as found on early Continenal 6 cylinder engines) pulling the mixture to idle cutoff causes a pin to press against the valve operated by one of these diaphrams and this gave rise to the notion that leaving the mixture in that position would result in the diaphram "taking a set" that would affect normal operation adversely. This is still an OWT for those carbs as well since this diaphram has a fairly stiff spring pushing it the other way and during normal operation internal fuel pressure would oppose the spring keeping the diaphram and attached valve centered, but when the engine was stopped for any length of time the internal pressure would bleed off and the spring would push the diaphram farther away from it's nominal position (albeit in the opposite direction) than than the ICO pin. The reality is that when originally built these pressure carbs used older rubber materials that would stiffen over time and cause the carb to go out of spec flow wise and for some reason the idea was borne that leaving the mixture out of ICO would "solve" that problem but all it did was make it slightly better for a little while and the proper answer was to have the carb overhauled with new diaphrams (there are about 10 in the carb, only one of which has anything to do with ICO). Unfortunately this cost well in excess of an AMU so as long as the engines would run many owners would postpone this important service with the result being lean mixtures at takeoff power and hot cylinder heads.

RE: NA piston direct drive engine. Is there any validity to the idea that rotating the prop backwards will damage the vanes in the vacuum pump because of the 'directional' wear of the vanes inside the case?

Short answer: another OWT.

Long answer: There are two designs of dry vacuum pumps (this certainly has no bearing on wet pumps). One type has the vanes perpendicular to the shaft and the other angles them. The angled pumps are made for a specific rotational direction and if you install the wrong one of those, it's life will be shortened but I don't know by how much. Probably not by half. But neither type of pump will be damaged by rotating slowly backwards unless the vanes are worn beyond service limits. IOW if the pump is already worn out, turning it backwards might break a vane and end the pumps life a little earlier than would happen if it never turned backwards.

The real killers of dry pumps are contamination (solvents are particularly bad) and heat. You can just about double the life of many dry pumps by adding a cooling shroud and plumbing ambient air to it. These shrouds cost a little over $50 and should be on every airplane.

 

[cwyckham]

There's an interesting link between the two questions. If you have a problem with your magneto switch so that it doesn't turn off and you don't do a dead cut check before shutdown (not on most checklists), then leaving the mixture anywhere other than idle cut-off and then turning the prop through the right way for whatever reason could end up with an inadvertant engine start and a big ouch. Maybe not super likely, but something I'd rather avoid.

Chris

 

[gismo]

There's an interesting link between the two questions. If you have a problem with your magneto switch so that it doesn't turn off and you don't do a dead cut check before shutdown (not on most checklists), then leaving the mixture anywhere other than idle cut-off and then turning the prop through the right way for whatever reason could end up with an inadvertant engine start and a big ouch. Maybe not super likely, but something I'd rather avoid.

Chris

True, but under the right (and rather unlikely) conditions an engine that's got the right fuel mixture in the cylinders can start when turned backwards. I think you'd have to turn it pretty briskly to get the magneto to create enough voltage to spark.

 

[Tom-D]

True, but under the right (and rather unlikely) conditions an engine that's got the right fuel mixture in the cylinders can start when turned backwards. I think you'd have to turn it pretty briskly to get the magneto to create enough voltage to spark.

It does not matter if the mag will fire when turned backwards, The engine will not be on compression stroke if it does.

Secondly, Mag theory will not allow the mag to fire when turned backwards. Plus the inpulse coupling will not snap.

 

[Let'sgoflying!]

AFaIK, that's exactly where this OWT comes from. If your pump is about to go, turning it backwards may end it's life a tiny bit sooner. Chances are your pump was on borrowed time already.

Thats exactly what I was attempting to say but didn't get across...the pump vanes were very short and thus, the pump was very aged. Sorry.

But I don't think its an OWT at all....the vane DID break!

 

[Tom-D]

Thats exactly what I was attempting to say but didn't get across...the pump vanes were very short and thus, the pump was very aged. Sorry.

But I don't think its an OWT at all....the vane DID break!

Seeing as the vanes are set in the rotating block, as to slide in and out of the slots as the pump rotates, the vane would have to be shorter than the range of motion of the vane, to get it to leave the slot.

IOW it would have to be WAY TOO SHORT to be in servicable condition. Thus rotating the pump in any direction would brake the vane.

 

[Let'sgoflying!]

Seeing as the vanes are set in the rotating block, as to slide in and out of the slots as the pump rotates, the vane would have to be shorter than the range of motion of the vane, to get it to leave the slot.

It happened in summer of 99 at a time when I didn't have a digital camera - but I certainly would have taken a pic if I had one available. I had just bought the plane and the prebuy did not reveal the pump's age. When I opened it they were very short but no vanes actually left the slot. As they get short I believe they can get more slop which results in them canting instead of sliding back. Canting = a levering force = snap.

Thus rotating the pump in any direction would brake the vane.

You all know I am hesitant to contradict anyone, but the vanes did not break during the previous hours' flight....then one broke as I moved the prop backwards.

 

[gismo]

It does not matter if the mag will fire when turned backwards, The engine will not be on compression stroke if it does.
Secondly, Mag theory will not allow the mag to fire when turned backwards. Plus the inpulse coupling will not snap.

Yeah, I know it's pretty darn unlikely, but I did actually have an engine kick when I was turning it backwards. It was an experimental with electronic ignition and for some reason the ignition was on. Fortunately it didn't kick very hard. With impulse couplers and magnetos it would certainly be more difficult to ignite the charge, but I wouldn't say it's impossible.

 

[Tom-D]

Yeah, I know it's pretty darn unlikely, but I did actually have an engine kick when I was turning it backwards. It was an experimental with electronic ignition and for some reason the ignition was on. Fortunately it didn't kick very hard. With impulse couplers and magnetos it would certainly be more difficult to ignite the charge, but I wouldn't say it's impossible.

Electronic ignition is usually a reluctor type. the reluctor coil does not care which way the metal excitor passes, so it will work in both directions.

Unlike the Mag which has a lead and lag angles which will not permit the E gap to occure when the points open when turned in the opposite direction than what it is set up for.

That is why the model of a mag will have a "L" or "R"

simply because it will not work in the other rotation.

 

[gismo]

Electronic ignition is usually a reluctor type. the reluctor coil does not care which way the metal excitor passes, so it will work in both directions.

Unlike the Mag which has a lead and lag angles which will not permit the E gap to occure when the points open when turned in the opposite direction than what it is set up for.

That is why the model of a mag will have a "L" or "R"

simply because it will not work in the other rotation.

OK, are you willing to hold a plug wire in one hand and a cylinder in the other when someone turns the engine over briskly backwards with that mag on? I could be wrong, but I think the mageto will generate considerable voltage that way even though it's nowhere near as much as you get going the correct direction (even if the impulse coupler wasn't part of the issue).

 

[Tom-D]

OK, are you willing to hold a plug wire in one hand and a cylinder in the other when someone turns the engine over briskly backwards with that mag on? I could be wrong, but I think the mageto will generate considerable voltage that way even though it's nowhere near as much as you get going the correct direction (even if the impulse coupler wasn't part of the issue).

When I was in A&P school (way back when) we played mag roulette, by 9 people putting a dollar in the pot, and then spinning the mag so no one knew where the timing was, then we each held a spark plug wire, and then some would spin the mag one notch. we knew who the winner was right away.

and yes we tried to spin it backwards, nothing happened, no one jumped.

OBTW I taught mag theory for many years when the NAVY still had recips.