Lycoming cam wear power loss

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Toobuilder

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Considering the cam turns at 50% speed of the crank and is primarily lubricated by oil thrown out of the rod journals, building the lobe/tappet oil wedge while "on the starter" is pretty much fantasy. I'm of the opinion that any rotation of the engine with flat tappets below running speed and full oil pressure is killing the the cam due to scuffing. The debate really is: does the cam die first from environmental rust or scuffing damage?
 
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Dan Thomas

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If you use airplane oil it does not have zink in it. That is an ash.
Right. Zinc has been known to attack magnesium, I think it is, and some aircraft crankcases are magnesium or have lots of it in the aluminum alloy. I did some reading on it when I wanted to try (much cheaper) automotive 15W40 in my A-65.
 

Dan Thomas

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Considering the cam turns at 50% speed of the crank and is primarily lubricated by oil thrown out of the rod journals, building the lobe/tappet oil wedge while "on the starter" is pretty much fantasy. I'm of the opinion that any rotation of the engine with flat tappets below running speed and full oil pressure is killing the the cam due to scuffing. The debate really is: does the cam die first from environmental rust or scuffing damage?
Scuffing can happen to a new cam. Corrosion can happen to any cam, and corrosion accelerates scuffing damage by reducing the contact area and overloading the remainder.

In any case, my general advice is to fly it or leave it alone.

The cam is only one victim in the engine. I've seen rusty gears in the accessory case. Rusty magneto bearings. Rusted starter drive bearing balls that fell out into the case. Rust pitting in the cylinders. Rust pitting on the valve stems. All of it suffers, and some of it could fail long before the cam. One of the problems with the Bendix dual magneto used on some Lycomings was corroding impulse coupling springs. The spring would break and both sides of the magneto would instantly go to full retard and the engine's power was just about gone. There were a few accidents.
 
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TiPi

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I didn't see what the minimum cranking rpm was for hydrodynamic pressure?
Or if hydrodynamic pressure happens on open cam lobes?
Hydrodynamic lubrication happens whenever there are sliding surfaces and a lubricating fluid. The "speed" when that hapens is varies with oil viscosity, load and primarily, the speed of of the contact area of the surfaces (relative movement).
Some engine builders use an assembly lube (very high viscosity and solid lubricant particles) on the cam lobes and lifter to get them undamaged through the start-up phase. Running-in is then done at mid-range rpm to maintain the lube film until the lifters and lobes have bedded in.
 

PiperCruisin

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Great discussion.

On the original reason for this post, I was trying to estimate the power loss from that much cam wear. Just before teardown, I recorded the takeoff distance at about 1540 feet. The POH says about 1100. Based on that I was estimating 20-30% power loss. However, I have not found a good equation for the takeoff distance and don't feel like deriving it (my memory of calculus is failing me). Takeoff distance equations, anyone?

On the cause and prevention of cam wear, it could be rust, operating practices, oil chemistry, metallurgy, etc. Or, there could be a certain amount of bad luck involved / probability. I suspect that a little bit of metal finds itself at the wrong place at the wrong time and gets "welded" to the cam or tappet face. Once this ultra hard piece breaks off, it leaves a pothole. The pothole gets bigger and others form. Pretty soon one is through the hardened layer that has residual compressive stress and moves on through the residual tensile stress like a hot knife through butter. Pretty soon you are flying through the canyon wondering why you are not climbing anymore despite good compression.

We try to control everything, but sometimes stuff happens. In my case, I'm guessing the infrequent use/rust was what did it in.
 

Dana

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For a fixed pitch prop, power is proportional to the cube of rpm. If static rpm is 2200 when it should be 2300, (2200/2300)^3 = 88% power. But if you're expecting 2350 and "2200 or a little less" means 2150, that's 77%.
 

Vigilant1

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For a fixed pitch prop, power is proportional to the cube of rpm. If static rpm is 2200 when it should be 2300, (2200/2300)^3 = 88% power. But if you're expecting 2350 and "2200 or a little less" means 2150, that's 77%.
IMO, this approach will yield more accurate before/after comparative power info than trying to use takeoff roll. Of course, all depends on the accuracy of the static RPM data provided.

Even differences in air density matter little when using the approach above, since the engine power reduction produced by reduced density is almost entirely compensated by the lower power needed to turn the prop in air of lower density.
 
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PiperCruisin

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For a fixed pitch prop, power is proportional to the cube of rpm. If static rpm is 2200 when it should be 2300, (2200/2300)^3 = 88% power. But if you're expecting 2350 and "2200 or a little less" means 2150, that's 77%.
I like the idea of using fan law as a first pass, but was not sure how indicative the static rpm was of power available. So, I wanted to check both, but was unsure of the takeoff equation results.
 

Dana

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I like the idea of using fan law as a first pass, but was not sure how indicative the static rpm was of power available. So, I wanted to check both, but was unsure of the takeoff equation results.
Static rpm is indicative of how much power the engine is actually producing and delivering to the propeller.
 

Dan Thomas

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We taught our students to know the airplane's POH. They were to know the takeoff distance expected, and to identify, right before opening the throttle, the point where they should have liftoff speed. If they were approaching that point and weren't getting the speed, closing the throttle and stopping on the remaining runway was the SOP. They were also taught to glance at the tach early in the takeoff roll to make sure the engine was up to snuff.

There have been enough accidents where the engine just wasn't doing its job and the pilot either didn't notice or was hoping it would fly anyway. Not good piloting. Probably from casual training procedures.
 

PiperCruisin

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We taught our students to know the airplane's POH. They were to know the takeoff distance expected, and to identify, right before opening the throttle, the point where they should have liftoff speed. If they were approaching that point and weren't getting the speed, closing the throttle and stopping on the remaining runway was the SOP. They were also taught to glance at the tach early in the takeoff roll to make sure the engine was up to snuff.
Good advice. Most people don't know what the type certificate says and the POH for the Cherokees is a bit sparse. I knew mine, but had enough experience in a similar plane to suspect something was not quite right, but wasn't sure if the engine wasn't right or it was the fact that it had a cruise prop. I'm a CFI, so not completely green, and my MEI/IA thought they just flew that way. Usually, for mountain flying the rule was 75% Vy by 50% of the runway. The home base is almost 1 mile long, so it had no problem with takeoff, just noticeably longer, and climb seemed a bit weak. Used more fuel than usual and leaning didn't seem to do much.
 

BBerson

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I put a climb prop on my Cherokee and used it to tow gliders since it was at TBO anyway.
Did the overhaul with a local mechanic for $3000 and then sold it.
 

PiperCruisin

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I put a climb prop on my Cherokee and used it to tow gliders since it was at TBO anyway.
Did the overhaul with a local mechanic for $3000 and then sold it.
Standard for the PA28-140 is a 58" pitch. Mine is 60. I decided to keep the cruise pitch since I'm using higher compression pistons (to 160 hp) and the tuned exhaust. I would have preferred the lower expense of a field overhaul, but none of the local A&Ps would do it. Most of the other shops in the area are only interested in working on the big expensive stuff. Unless MOSAIC allows me to do more work, I'll probably sell it in a few years since it is such a pain. Currently I do some work on it, supervised by my A&P IA, but he does it in his off hours from his regular job. Meaning my plane has been down for 6 months and probably won't fly for another month. Sighhhhh....
 

pfarber

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Power loss maybe 10%.

When I was instructing we taught partial-power takeoffs to simulate engine trouble or high density altitude. Right after liftoff the instructor would reduce the RPM by only 100; it was amazing how much performance suddenly disappeared. And it's amazing how, later on, owners would sometimes comment on how much better their airplanes flew, and it was usually after doing the mag inspections and reinstalling them at just the right timing to the engine. The internal inspection usually included new points and getting them set so the E-gap was just right. And sparkplugs cleaned, gapped and tested under pressure. I liked to replace a bad plug set with new Tempest REM37BY plugs wherever they were approved for the engine. That plug has extended electrodes that get the spark farther out in the mixture, rather than so close to the plug well that might be harboring spent gases. And they don't foul easily, either.

Some folks think a spark is a spark. Nope. A HOT spark, at the right point in the cycle, gives improved performance.
And you expect a magneto to do this? EI all day long over your more prone to preignite because the wrong temp range plugs were used.
 

Pops

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I owned a 1968 Cherokee 140 for 5 years . Wife and i did a lot of traveling in it. Put 7500 miles on it one summer. Had a 56" pitch climb prop. Cruised a lot at 7500' or 8500'. Did a good job. 2 people and lots of baggage. Low maintenance airplane.
 

Dan Thomas

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BJC

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We didn't have fouling issues, either, though we didn't have long taxis to do. Leaning it way back limits the fouling, and installing UREM37BY plugs in any engine approved for them pretty much eliminates it altogether.
Dan:

What is the difference between the REM38E plugs that I use in my Lycoming 180 HP IO-360-EXP?

Thanks,


BJC
 

BJC

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Thanks, Dan.

Am I correct in my thinking that the electrodes are in a hotter area and that, plus the more open area around the electrodes, make then less susceptible to fouling?


BJC
 

TFF

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Those plugs were invented to stop lead fowling on O200 0235 when 100LL became to only option. I would not stick them in an engine not rated for them so you don’t knock a hole in your pistons or create lots of detonation
 
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