Question for Paul McBride, the General Aviation News engines expert: I have a 1978 Warrior with an O-320 Lycoming engine.
I try very hard to fly it every two weeks, but winter weather has kept me on the ground for about five weeks.
The engine has an oil pan heater and is covered and cowl plugs are installed. The aircraft is hangared.
At the end of your column “Should I pre-oil these Lycoming engines?” you said do not turn the prop by hand. What is the harm in this action?
I have been doing this on my plane, as well as flight school airplanes, after being advised years ago that it helps to get the oil to the cylinders.
Thank you for your advice.
Mike Holmberg
Paul’s Answer: Mike, thanks for submitting your question. I would guess that you’re not the only person who would like to learn more about this subject.
First of all, I’d like to congratulate you on the fact that you make every attempt to fly your 1978 Piper Warrior at least every two weeks. There is no doubt that this is the best thing you can do for your aircraft and its engine. It appears that from the information you provided that you make every attempt to do what is best and it appears to be working out for you.
The reason I advise people not to turn the prop when the aircraft is idle is because we know that when the engine was last shut down, there was some residual oil left on the cylinder walls.
The thinking is that if the prop is turned, there is the potential for the piston rings to scrub that small amount of oil off the cylinder walls, allowing outside air into any cylinder that may have had both valves closed.
With the valves now open, there is a chance of air entering the cylinder that has less oil film, as a result of moving the prop, which may in turn allow corrosion to begin.
While most people may shake their head at this statement, it’s important to factor in the geographical location of the aircraft and the environment that the aircraft resides in.
If you live in a northern climate vs. a southern climate where salt air and high humidity is common, you can imagine this may not be too much of a concern because the air is colder, but it’s more than likely that it’s drier as well.
Now that you have my thoughts, I’ll let you decide what approach you’ll take in the future.
Again, I commend you for the way you have been looking after your aircraft and wish you all the best in the future.
For those who have vacuum driven steam gauges, years ago there were articles that
claimed turning a prop backwards could damage the vacuum pump. Vacuum pumps are designed to turn clockwise or counterclockwise and if they are turned in the opposite direction that they were designed to turn, damage /breakage of the vanes can be the end result.
Is this no longer true—or was it ever?
I had often wondered the same thing, and one day I disassembled an old dry vacuum pump from a 1969 Twin Comanche, to understand exactly how it worked. I honestly could see nothing that would be harmed by rotating the pump in reverse. The individual vanes themselves were not tapered or angled, the inlet and outlet ports were oriented at 90° to the axis of the vanes, and in short, I think the warning is a holdover from alternate style pumps. However, I still avoid the practice of pulling a prop through in either direction, primarily because I see no advantage to it. Additionally, besides the oil scraping potential mentioned in Paul McBride’s article here, there’s always the potential for an unknown “hot mag”, which I have encountered in the past.
Continental oil pressure relief valves, when we where playing around with getting our STC for the oil cooler diverter block for the C75 thru 0-300 series engines, we had a couple issues with the coolers due to start up pressures with the C-125 thru 0-300 engines. The primary problem is the pressure relief valve is at the end of the lubricating cycle, which means it regulates the residual oil after it goes through the engine. The pick-up point for the gauge is also at this point. Now what happens is with that 3 inch wide positive displacement pump pushing cold heavy oil at the beginning of the cycle, with any excessive rpm trying to keep that cold engine running, the oil pressure in the accessory case can build up to 400 pounds pressure until things get warmed up and flowing. That is the reason you do not see pressure readings on the gauge immediately on a Continental. as you are reading residual pressure ( 30-60 lbs.) Lycoming on the other hand, is exactly the opposite, it’s relief valve and gauge points are before the cycle ( 60-100 lbs ) and shows on the gauge quicker.
If you really feel the need to rotate the prop, do it with the starter and mixture lean.
I am not sure about lycoming, but for rotax engines, dominant in the LSA comunity, turning the prop “the normal way” is part of the preflight in order to return oil to its tank for quantity check.
As to the humidity issue, as a mechanical engineer, I would trust a well documented quantitative research rather than, what I feel is mainly assumptions.
The Rotax 9xx are unique in the way the dry sump system works, so clearing the crankcase of residual oil is necessary to determine the oil quantity on the oil tank.
Also, there is no risk of ignition since the series uses an electronic ignition,with no possibility if a cylinder firing.
As another commented, radial engines need to purge the lower cylinders of any excess oil that could cause a hydraulic lock.
Moisture does exist in the crankcase after shutdown, as I know when I open the oil access door and remove the oil filler cap, a quantity of steam rises from the filler neck for a few minutes. [ I’m an engineer, pilot and aircraft owner ].
Except, never ever turn a Rotax backwards.
Same subject…Different motor…
I have a Waco UPF7 biplane with a 300hp Jacob’s radial engine. Radials were (and still are) know for oil leaking down into the cylinders if the updates are not installed to drain the oil from te combustion chamber.
Only way to confirm that the cylinders are clear prior to starting the engine is to hand prop the plane 14 blades…7 full revolutions of the prop….this ensures every cylinder gets a full cycle of the engine stroke prior to cranking the engine to verify the cylinders are not full of oil and hydro lock when cranked…believe it or not the starter is stronger than the connecting rods.
While i do not cycle the engine regularly to move oil and see no value in it, One would have to understand that most of the lubrication of an engine is done by oil splashing around while an engine is running…a cold engine with cold oil (and i mean “Not running temps”) will not splash the oil around the entire circumference of the pistons and will cause premature removal of essential oil…How much is a more appropriate question and depending on the oil or manufacturer and degree of viscosity breakdown and too many other variables that are way above my pay grade.
Turning the engine by hand during the periods between flights once in a while will not damage the engine nor help it…
My best answer is….If it makes you fell better than i suggest you do it. A couple turns now and then will be fine. My best advise…Good oil, Good additives if long term storage is done…
For what it’s worth, my Continental takes about 15 seconds, after engine start, to show oil pressure.
So, at 1,000 rpm, that’s 16.67 rev/second X 15 seconds, is over 250 revolutions of the crank before the oil pressure comes up. So…turning the crank by hand is worthless and does the harm mentioned.
As far as starting an engine and running it for a few minutes, the current wisdom is that a lot of fuel is washed past the pistons and an amount of blowby of combustion produces will produce an acid in the crankcase with the moisture existing.
Without getting the oil to 180 degrees to boil off the moisture, corrosion can result.
So, go fly the aircraft and enjoy the privilege of your certificate…!!!
Was the question asked “not turn the prop period” or ” not turn the prop for lubrication purposes.” For aircraft with no starter, turning the prop by hand is rather essential, and anyone whom needs to, should always treat the engine as ignition “HOT” regardless as to switch position. The same goes for any engine that prop is turned by hand. Now, if you are turning the engine for basic lubrication, it does not work that way. Paul and Terry are correct in stating that what lube is there is wiped, and can introduce more rusting potential. Both the cylinder walls and camshaft are splashed lubricated from the crankshaft bypass pressure oil while the engine is running. Now here is where I disagree with Terry, in any engine started and ran for a short period is going to survive longer than one just sitting and or rotated by hand. It needs that splash lube to relube the parts that need the oil shield. As for the ” moisture” in the oil, in my opinion based on 57 years of experience working on engines that set a lot, I do not agree with the theory that “moisture” in the oil is the culprit. My observation relative to water and oil is oil usually sits on top of the water, and that is down in the oil pan. The crankcase vent system is a bigger culprit, as the engine cools, it breaths, sucking in cooler moisture laden air that is not mixed with the oil. The air keeps breathing. Draw your own conclusions
I agree with Mr. Mcbride. But more importantly than the wiping of oil from the cylinders is the wiping of oil from the camshaft, especially with the Lycoming engines. Any moisture in the crankcase tends to condense at the top. I suggest that more importantly than frequent flying is to make any flight prior parking long enough to heat the oil and evaporate any moisture. Short flights around the patch can cause moisture to build up. I’ve worked on engines that were started and ran for frequent short quick flights and subsequently found to be full of rust. On the other hand, I have opened engines that sat for years having flown a couple hours before being parked and found them clean and rust free. Basically, we need to cook the moisture out of the oil.
Perhaps Jim R should do a mag check prior to shutting engine down to determine if there is a grounding issue. Then treat every propeller like a gun, assume it’s loaded. The use common sense caution and turn until your hearts content. Be turning and hand propping for 55 years.
I’ve hand propped vintage planes with no starter and am comfortable with that. And doing a mag check at shutdown is great advice. But I’ll never casually move a prop. Ask my friend who nearly lost a leg about moving a prop during maintenance on a conventional engine, when the mags were accidentally hot.
I maintained a fleet of training aircraft and taught a preventive maintenance course for 35 years. Everytime the chief flight instructor finished a flight he would instantly get out and turn the prop horizontal for the tow bar. To hear the impulse couplings fire sent chills down my spine everytime. He would also stand right in the arc of the propeller when I would do a compression test, scary!
I checked magneto grounding at every maintenance interval and taught to turn propeller backwards.
Electric pre heaters are great if used properly. Same chief instructor demanded these to be plugged in 24-7.
Not a problem if the aircraft flies everyday but it will ruin your engine from rust if not, as I have seen multiple times.
Flying the aircraft at least every other week is the best for controlling corrosion.
Never, EVER, turn a prop in the “normal” rotational direction, even slightly, for any reason. People have been severely injured or killed when the engine fired, either because the mag switch was accidentally not turned completely off, or the switch was faulty, or the switch was indeed turned off, but the “P” lead was loose or disconnected, allowing the engine to fire. If you must turn the prop, such as to facilitate installing the cowling, turn it backwards.
Yes, never move the prop, but if you need to , as I do to hook up the tow bar, move it in the reverse direction….the mag impulse coupling[s] will not fire then.!!
I’ll accept the possible damage to the vacuum pump.
An Engine Dehumidifier is cheap insurance, if you have electricity at hand. You can buy a BlackMax for about $600. Or you can use a fridge and aquarium pump. My dorm fridge ($134@Walmart) and $30 aquarium pump gives me an unlimited supply of12% Relative Humidity 40F desert dry air.
I have a BlackMax and highly recommend it. How do you rig a fridge and aquarium pump to serve the same function?