PA-12/O-320 rear mount oil cooler?
- Thread starter T.J.
- Start date
Marc Olson
GONE WEST
Seattle, WA
The Freightliner has this mod. I'll pull the paperwork next time I'm at the hangar and let you know how it was done. The O-320 was installed under the Kenmore STC.
Scooter7779h
PATRON
Anchorage, Alaska
TJ,
I bought the STC from Charlie Center, although it is for a PA-18. STC included paperwork, oil lines and a cooling lip for the bottom cowl. We then got a field approval for the -12 noting that it was installed as noted for the PA-18 and adapted for PA-12.
http://sparky.supercub.org/photopost/showphoto.php?photo=6985
I bought the STC from Charlie Center, although it is for a PA-18. STC included paperwork, oil lines and a cooling lip for the bottom cowl. We then got a field approval for the -12 noting that it was installed as noted for the PA-18 and adapted for PA-12.
http://sparky.supercub.org/photopost/showphoto.php?photo=6985
Gordon Misch
MEMBER
Toledo, Wa (KTDO)
I have a really basic question (and after I got to writing, some follow ups also) on rear-mounting the oil cooler. It seems that a lot of attention is paid to good baffle seals inside the cowling, yet an oil cooler makes a pretty big hole - a substantial shunt for the available airflow. It seems to me that the oil cooler mounted on the rear baffle robs a considerable amount of cooling air from the cylinders, because the cross section of the cooler is a considerable percentage of the cross section of the cowl air inlet. However the front mounted cooler uses an air supply that is separate from, or in addition to, the supply to the cylinders.
So - it would seem that if high ambient air temps are expected, the rear-mount cooler might not be a good idea??
And it would seem that, as a minimum, CHT sensor(s) should be on the same side of the engine as the cooler to probably show the highest temp? Is that required? Am I all wet???
Also, what is the advantage, other than a tiny CG change, a cleaner looking nosebowl, and lack of cooler/nosebowl chafing as a result of cowl misalignment, of mounting the cooler in the back in the first place? I've never understood a good reason to make the change, and I'm wondering what I might be missing.
One more question, are there any problems with fatigue cracking of the rear baffle with the cooler mounted on it? (or does the STC require some structure other than the baffle for mounting?)
So - it would seem that if high ambient air temps are expected, the rear-mount cooler might not be a good idea??
And it would seem that, as a minimum, CHT sensor(s) should be on the same side of the engine as the cooler to probably show the highest temp? Is that required? Am I all wet???
Also, what is the advantage, other than a tiny CG change, a cleaner looking nosebowl, and lack of cooler/nosebowl chafing as a result of cowl misalignment, of mounting the cooler in the back in the first place? I've never understood a good reason to make the change, and I'm wondering what I might be missing.
One more question, are there any problems with fatigue cracking of the rear baffle with the cooler mounted on it? (or does the STC require some structure other than the baffle for mounting?)
Steve Pierce
BENEFACTOR
Graham, TX
I have mounted the oil cooler on the rear baffle in a 150 hp conversion on the PA16. The first baffle cracked at the oil cooler and took a little engineering and the inspection of several other factory installations and so far that problem has been solved. The cooling air entering the nose bowl inlets above the cylinders has to pass thru the cylinders and exit the cooling lip on the bottom cowl. Any air coming around the air box or oil cooler tends to disrupt that airflow. We did pressure differential test with an air speed indicator and got better air flows (pressure differential) by sealing around the air box so that the air coming through the cylinders passed through and out better and have not had any CHT problems on any cylinders including the one with the oil cooler behind it. We did find that putting the cooler 2" behind the cylinder cooled better. Figure it has a venturi effect with the air. I would suspect however in very hot climates the front mounted oil cooler sealed properly will cool better.
Gordon Misch
MEMBER
Toledo, Wa (KTDO)
Steve, I don't know exactly what the term 'air box' means. Is it the plenum above the cylinders, formed by the bafffle system and top cowl?
Gordon Misch
MEMBER
Toledo, Wa (KTDO)
Also, Steve - Do I understand you correctly that you found greater pressure differential between above and below the cylinders with the oil cooler mounted on the baffle?
Where were the pressure probes located, and could their orientation have been a factor (ram air)? Sounds like an interesting experiment - "One good test is worth a thousand expert opinions"!
Where were the pressure probes located, and could their orientation have been a factor (ram air)? Sounds like an interesting experiment - "One good test is worth a thousand expert opinions"!
mvivion
SPONSOR
Bozeman,MT
12 Geezer,
The Cub has a pressure cowl. The top of the cowl, above the cylinders, is pressurized by ram air from the cooling air inlets. The bottom of the cowl is "less pressurized" air, in other words, the air here is lower in pressure than the top cowl.
Piper put the oil cooler in the front of the lower cowl, with a ram air inlet. This flows a lot of air around the oil cooler, but also somewhat defeats the purpose of the pressure cowl concept, by adding ram air pressure to the lower cowl.
By modifying the oil cooler to mount on the rear baffle, you accomplish a couple of things:
1) These coolers are nearly always or always aluminum, and thus a lot lighter in weight than the brass cooler Piper used.
2) Cooling air through the cylinders is actually better organized, since the pressure differential between upper and lower cowl is re-established.
The little bit of air flow that goes over the oil cooler really doesn't affect cooling flow through the cylinders much, since its behind the cylinders.
That is a very simplistic version of the benefits I see to mounting the cooler on the rear baffle.
Of course, the negative is that you are hanging a good bit of weight on some baffles that may crack, so mounting is important.
A lot of aircraft have the oil cooler on the rear baffle.
MTV
The Cub has a pressure cowl. The top of the cowl, above the cylinders, is pressurized by ram air from the cooling air inlets. The bottom of the cowl is "less pressurized" air, in other words, the air here is lower in pressure than the top cowl.
Piper put the oil cooler in the front of the lower cowl, with a ram air inlet. This flows a lot of air around the oil cooler, but also somewhat defeats the purpose of the pressure cowl concept, by adding ram air pressure to the lower cowl.
By modifying the oil cooler to mount on the rear baffle, you accomplish a couple of things:
1) These coolers are nearly always or always aluminum, and thus a lot lighter in weight than the brass cooler Piper used.
2) Cooling air through the cylinders is actually better organized, since the pressure differential between upper and lower cowl is re-established.
The little bit of air flow that goes over the oil cooler really doesn't affect cooling flow through the cylinders much, since its behind the cylinders.
That is a very simplistic version of the benefits I see to mounting the cooler on the rear baffle.
Of course, the negative is that you are hanging a good bit of weight on some baffles that may crack, so mounting is important.
A lot of aircraft have the oil cooler on the rear baffle.
MTV
Marc Olson
GONE WEST
Seattle, WA
Mike's reply makes a ton of sense.
I was fighting high oil temps last summer in 64M and the 'fix' that made the most improvement was to put baffling around the air filter. We'd switch to a Brackett with a smaller cross section and high pressure air was getting into the lower cowl. Plugging those holes brought the temps down signficantly.
As did adding what was termed a 'seaplane lip' to the exit for the lower cowl to improve airflow out the bottom of the engine.
With the new engine I'll be taking off the lip pretty soon as it fights to get above 140 on cool days.
I was fighting high oil temps last summer in 64M and the 'fix' that made the most improvement was to put baffling around the air filter. We'd switch to a Brackett with a smaller cross section and high pressure air was getting into the lower cowl. Plugging those holes brought the temps down signficantly.
As did adding what was termed a 'seaplane lip' to the exit for the lower cowl to improve airflow out the bottom of the engine.
With the new engine I'll be taking off the lip pretty soon as it fights to get above 140 on cool days.
Mikey
FRIEND
Everett, WA
Fact or Fiction, I don't know because I haven't flown it...
But Jerry Raider at Kenmore strongly advised against the rear baffle mount as they had cooling problems with them when on floats. Mine is on the nosebowl, only downside is it is prone to cracking around the rivets in the cutout. My oil temps are on the low side, 140-150F w/ 65F ambient w/ the Pacific Oil Cooler PMA replacement for Piper original. I pondered the same thing about the additional ram effect increasing pressure on the lower deck, but it may be it actually adds to a venturi effect drawing air out the bottom and side openings. My cowling is stock PA-12 and I have not had the cooling problems w/ the standard bottom lip like Marc did.
Chris
But Jerry Raider at Kenmore strongly advised against the rear baffle mount as they had cooling problems with them when on floats. Mine is on the nosebowl, only downside is it is prone to cracking around the rivets in the cutout. My oil temps are on the low side, 140-150F w/ 65F ambient w/ the Pacific Oil Cooler PMA replacement for Piper original. I pondered the same thing about the additional ram effect increasing pressure on the lower deck, but it may be it actually adds to a venturi effect drawing air out the bottom and side openings. My cowling is stock PA-12 and I have not had the cooling problems w/ the standard bottom lip like Marc did.
Chris
Steve Pierce
BENEFACTOR
Graham, TX
The airbox I was referring to is what the airfilter is mounted to. Ram air goes around the airbox and disrupts the exit air going through the cooler. The front mounted oil cooler seems to work better on sea planes according to several people I have talked to but I would think at the expence of cylinder head temps. We used an airspeed indicator with the static mounted forward of the oil cooler and the pressure mounted behind. Greater pressure differential with the oil cooler mounted 2" aft of the rear cylinder rather than mounted against the rear cylinder. This required me making a new rear baffle. I broke a piece of 4130 90 degrees to support the left side of the oil cooler to the left rear cylinder. No more cracking problems since.
Marc Olson
GONE WEST
Seattle, WA
My cowling is of indeterminate origin. It's not stock -12, might be -18 stretched to fit on the -12 boot cowl. I have little clearance on the side of the cowling compared to other cubs I've seen. That's where the lower lip helps significantly increase the air flow through the aft part of the engine.
mvivion
SPONSOR
Bozeman,MT
First, there is a HUGE diversity in Cub oil temperatures, and in my experience, most of these aren't related to oil coolers.
For example, we bought a fresh rebuild from a well known outfit, ran hotter than hades in oil temp.
Eventually pushed the front seal out as well. Turns out it was a Comanche engine, and they'd failed to remove the rear plug in the crank when converting to fixed pitch prop. Duh.
In my experience with at least a dozen cubs with rear mounted coolers on floats, none have run hot at all.
But, maybe they're out there.
MTV
For example, we bought a fresh rebuild from a well known outfit, ran hotter than hades in oil temp.
Eventually pushed the front seal out as well. Turns out it was a Comanche engine, and they'd failed to remove the rear plug in the crank when converting to fixed pitch prop. Duh.
In my experience with at least a dozen cubs with rear mounted coolers on floats, none have run hot at all.
But, maybe they're out there.
MTV
Steve Pierce
BENEFACTOR
Graham, TX
A lot of stuff runs hot in Texas when outside temps are over 100 degrees. Makes sence to me that a front mounted cooler would cool better since it is getting all that ram air and not sharing with the cylinders. Seen some sprayers and tow planes with the cooler mounted under the firewall in the slip stream.
You can convert a constant speed crank to fixed pitch by putting a hole in the rear plug. There is a Lycoming Service Instruction with the guidelines. Almost made that mistake one time.
You can convert a constant speed crank to fixed pitch by putting a hole in the rear plug. There is a Lycoming Service Instruction with the guidelines. Almost made that mistake one time.
