Another Angle on Oil Temps?

[207WF]

Its hot air temp season again in SoCal, and a new idea has emerged about the persistently high oil temperatures that some of us -- but not all of us -- experience in the climb. Please chime in with the data for your bird. 1. Do you have the problem of high oil temps in the climb? 2. What is the gap between the top of your radiator and the top of the large hole in the front of the cowling?

 

We recently readjusted mine to close my gap a bit, and I want to believe that it lowered my oil temps in the climb, but some more data would help. I know that some CTs have a large gap there and others do not.

 

WF

[Roger Lee]

Hi Wayne,

 

The cooler opening can and is sometimes misaligned. It can easily be loosened and adjusted to fit the hole in the cowl better. I'm with you as far as the better the alignment you should get a few degrees better cooling.

[Rogerck]

I'm thinking about temporarily adding duct tape between the top of the oil cooler and the cowling bottom. Curious to see if this reduces climb out oil temp. If this works, perhaps a permanent rubber baffle could be added (we have on ELSA.) My concern is that the tape might block some otherwise required air flow. On the other hand, with oil and water cooling, the engine might not be sensitive to small changes in air flow.

 

Opinions?

 

Roger Kuhn

[Roger Lee]

Hi Roger,

 

What coolant are you using? (Evans or a 50/50 mix) What flap setting and speed on take off? If temps climb on take off then use zero flaps and not 15 and take off a little flatter and faster for better air flow. It may also be a kink or a reduced flow from a coolant line or the oil line. The oil line coming off the bottom of the engine and back up into the oil tank may be a little to sharp a bend and the reduced flow would cause temps to rise. The re-positioning of the cooler would help only slightly and that's if it were way off, which I doubt or the hoses would rub terribly on the cowl. If it has been after a hose change on the plane then check the coolant lines on top of the engine. Rotax is using a thin walled coolant line and FD has changed it's original routing which can cause the hose to have a reduced bend in certain spots. This is one reason FD put springs in a couple of hoses.

Other people aren't having a serious issue so you have a small problem with one of these listed. My day time temps are running 105F here in Tucson and we aren't having any over heat issues. If the prop is over pitched then that is the cause. Your full throttle flat and level rpm should be 5500-5600. If it is less than 5500 then each 100 rpm lower will raise the temp. If the prop is too course you need to unload the engine with less pitch.

[Mike Koerner]

I'm sorry folks. I don't think closing the air gaps around the edges of the radiator is going to amount to much more than a hill of beans.

There's a ton of air available, even at 60 knots, to pour through the cowl opening. The radiator presents a huge restriction to the air flow so the air flow is spreading outward from the center of the radiator anyway. You essentially have 100% ram recovery and full stagnation pressure across the whole front surface air of the radiator, even with some air spilling around the edges. I suspect the leakage only affects the upstream air pressure in a very narrow band around the gap.

You might try working on the problem from the other end instead. Recognizing that the flow through the radiator is a function of the delta P across it, and that at any speed and altitude you have a fixed pressure on its upstream face, you might try to reduce the pressure on the downstream side by making it easier for the air to flow through, and out of, the engine compartment.

The lower edge of the rear of the cowl hangs down below the bottom of the rest of the fuselage. I think this step causes a pressure drop which draws air through the engine compartment. You might try increasing the size of the step or opening the gap up a bit as a way of increasing the flow through the radiator.

I would try this myself, but I don't really have a problem with high oil temperatures. I lower the nose if I'm climbing and otherwise back off the throttle a bit if I see the oil temperatures getting hot.

Mike Koerner

[Runtoeat]

Mike, you've made a good point. A lot of aricraft deal with oil cooling by using cowl flaps which provide enhanced airflow thru the engine compartment. I recalled a previous post where CT owner Waggles in Australia enhanced his CT's airflow by modifying his cowl in the area you mention:

My link

[S3flyer]

Airwolf came out with an oil chiller product last year that I was tempted to try: Oil Chiller. The basic premise is to create a heat sync similar to what you find on hot running computer CPUs which creates more surface area subject to air cooling. Although the intent was to wrap the device around the oil filter, they said that Rotax installs are putting it around the oil tank. Anecdotal results for Rotax were around 3-5 degrees cooler. YMMV.

 

I don't believe I can fit the chiller on my Sting but E-LSA CT's might be a different story.

[Roger Lee]

You can help, but not cure some heat build up by wrapping your exhaust pipes. I have had this done for 5 years as other CTer's have not had any issues. My OAT's are as hot and hotter than many and I never over heat and neither do the other 5 CT's at my field.

If you land and pull your cowl would you grab your exhaust pipe. Most likely not and keep your skin, but I can. The wrap Mfg claims a 70% heat reduction and it may not be that, but it sure is a lot. It keeps the heat down, keeps the radiated and convective heat off hoses, wire, engine mounts and plastic parts. It helps.

[Runtoeat]

Dave, my friend uses the aluminum chiller on his Stinson oil filter and says it decreased oil temps about 7 F. He's got a big filter on the Stinson (Franklin engine?) so I don't know how much effect this might have on our small Rotax filter. The use of the cooler on the oil reservoir sounds like a better application and the evidence shows this does work per your post.

 

Roger, I'm not troubled by oil temps here in Michigan but I have the header wrap on my short list. This can't hurt and probably would lower the temps as you point out and looks cool too.

[mocfly]

Mike, you've made a good point. A lot of aricraft deal with oil cooling by using cowl flaps which provide enhanced airflow thru the engine compartment. I recalled a previous post where CT owner Waggles in Australia enhanced his CT's airflow by modifying his cowl in the area you mention:

My link

 

 

This modification did not work on the ctsw,it did work on the mc.

[Runtoeat]

Mocfly, I wasn't aware that this mod was tried on the CTSW. I see that the exit openings in the MC cowl have the rear edges lower than the front and thus would be scooping air so it seems logical that the mod works here. Still, I would think that the mod would provide greater scavenging and would move more air thru the engine compartment on the CTSW's than the modest opening of the factory cowl which should mean cooler temps. Perhaps the mod doesn't work here because the small air entrance in the front of the cowl is the limiting factor? Did you try this on a CTSW yourself or know who tried this?

[207WF]

None of you guys are measuring the gap or answering my question. Please measure the gap and chime in with your experience. I want data, not speculation here! WF

[mocfly]

I believe its is more related to the angle of the radiator relative to the opening, and the fact that FD has that piggy back configuration.

I am trying to figure out how to remotely measure the airflow behind the radiator. Once that is done i will ask FD what they use as a design figure.

[Adam]

Wayne, I'll measure my gap next time I'm at the airport. I flew to Flabob (breakfast), Big Bear (lunch) and Thermal (off the back side of Big Bear) this weekend. Coming out of Thermal (Palm Springs area for those not familiar with KTRM) was the first time I encountered climb temperature issues. 210 to 225 is good on my bird (green), once above 225 or so I start getting into the yellow. To combat this I was step climbing. Every thousand feet or so as I started to get into the yellow I'd level off for a few minutes. As I picked up airspeed the temp would come down. Once back to about 214 I'd go up another 1000. Still early in the season. I can see my new plane will be one of those that has this issue! In level cruise I am typically running 200 to 210. I heard you talking to So Cal coming through the Banning pass as I was headed into KTRM. Looks like you were playing out in the desert too!

[Roger Lee]

Hi Adam,

 

First don't sweat a climb oil temp up to 245F. 225F to 240F is nothing. Second make sure your prop pitch rpm is setup to see 5500-5600 for your average altitude. 5600 rpm would be better. Don't set this at sea level and then always fly at 8500-10K as an example. Set the prop pitch rpm for your average altitude. Do more climbing at zero flaps and don't try to gain as much altitude in a short distance as you might in the winter. I have 6 CT's at my field in Tucson and we have hotter temps than most and none of have over heat issues. It's all pilot technique and prop pitch setup and I have all these setup to see 5600 rpm WOT at our average altitude which is 4500' MSL.

 

 

p.s.

In your video I saw you landed at zero flaps (no issue), but then I heard the CFI saying to pull back the stick more and have the mains hit first and hold off the nose wheel. Typixcal GA CFI. That is a good way in zero flaps to have a tail strike. In zero flaps the mains will barley hit before thefront or almost at the same time. Too much aft stick in zero has had many scrape a tail. Just curious (no right or wrong), with all the weight in the plane and low winds why not use 15 flaps?

[mocfly]

None of you guys are measuring the gap or answering my question. Please measure the gap and chime in with your experience. I want data, not speculation here! WF

 

 

Here is a picture of the gap of the cowling to the cooler and the cooler mount which doesn't look quite right. It looks to be about 1.0 to 1.25 total. Another interesting thing was the angle of the radiator assembly. I am adding that also. Maybe some of the guys who don't have high oil temps can measure the angle on their aircraft.

 

Flew from CHA to TAZ. 3.9 hours at the redline the whole way. I a really starting to dislike this aircraft. Pitch is good, CHT is good, oil pressure is good and the GD oil temp is driving me crazy.

 

I am ready to try anything at this point. Including a match.

[Roger Lee]

The radiator looks a little low in the cowl from the first picture. It can be loosened some and pulled upward. You would only need an inch to make a difference. You need to loosen both side attachment points on the radiator and the one to the rear on the left side. Pull the radiator up and forward as much as possible and make sure it is as close to center left to right as possible. The radiator can have a gap around it and is usually wider at the bottom, that's why you have the rubber skirt on the bottom.

I take it you are using 50/50 coolant and your absolutely sure you have no reduced flow line? The one that FD has coming off the bottom of the engine and up to the oil tank is the oil return line. Because of the way they route this hose it can get kinked slightly which won't stop the flow, but the flow reduction causes temps to go up.

[mocfly]

Roger,

I will make the adjustments this weekend and let you know what the outcome is. I am running 50/50 and based on what i see there are no obvious restrictions. I would assume that if there was a slight restriction an increase in pressure would follow?

 

What do you make of the radiator angle? I am having a german buddy of mine send a letter in German to the guys overseas about what the angle should be. I would be curious what they come back with.

Thanks again

Chris

[Roger Lee]

It might be worth sliding the fire sleeve off that lower oil hose and then hook it up and really see what's going on under the fire sleeve. It wouldn't take that long to do this. Another thing you can try, but I don't hold a lot of hope is to switch the oil temp sender with the #2 cy. CHT sender. They are the same and see if you get the same readings. Don't put anything on the threads of the oil sender.

[207WF]

Mocfly:

 

Thanks for the measurement and the photo of the radiator gap. Your gap is much greater at the top than mine. Mine used to be about 5/8" but I have it down to about 1/3" now and that has helped somewhat. I would like to raise the radiator a little further, as there is a lot of radiator coil hiding below the bottom of the hole in the cowl that is likely not being used to its full effect.

 

I wonder if the clamps on the side of the radiator can be turned 180 degrees to raise the radiator further. It looks like it might be necessary to replace one of the water hoses with a 90 degree banjo type to make that work. Any thoughts anyone?

 

WF

[Roger Lee]

Oil and or radiator hose length can be easy changed if need be. The limiting factor may be the bracket length to the rear on the left side. Loosen everything thing first and try to re-position first.

[mocfly]

WF,

After some calculations I believe a huge contributor is the angle of the radiator to the relative wind and the pressure/velocity in FPM thru both the coolers. I believe that a few degrees angle will make a huge difference and i am building a test rig to try some things. Since air has a lower heat capacity and density than liquid coolants, a fairly large volume flow rate (relative to the coolant's) must be blown through the radiator core to capture the heat. There is a fine line between increasing the angle to a point that creates excess drag and the condition that we/I have now. Not having any other data I can only assume that this is relevant.I would like to hear from those who do not have oil temp issues to measure that radiator angle and post it up.

I will post information as I get it.

Mocfly

[Ed Cesnalis]

I suspect that the angle means little. I can fly at 3 different flap settings and a variety of pitch attitudes and still no joy. The only thing that gets me into the green is climbing above 72 degrees F. Once I cross that altitude I am fine below it I'm in the yellow even in level cruise.

[Roger Lee]

I think angle may have some an impact. I never get hot on take off no matter what the temp outside or how hard I climb because this climb angle puts my radiator in line with the incoming air flow. When I level off the angle is a more swept back position with the air flow through the radiator. This angle is severe enough to reduce air flow compared to climb angle air flow direction. The temps all go up no matter what the throttle setting in cruise. I have been thinking of playing with the angle some, but haven't done so because climb is where I usually don't want to get too hot. On a 95F day I will see around 195F-200F on climb and 218F in cruise. I think the more unobstructed air flow with the radiator face square with the in coming air does make a difference. When the radiator is tilted bac far enough the air flow is reduced. The angle could be changed by swapping out the short piece of metal that attaches the rear bracket of the radiator support on the left side of the engine and the rubber vibration isolator is located. This would swing the bottom edge forward to be more square facing with the air flow. You could then shorten the rubber air dam (after some testing of course). That rubber isn't cheap so don't trash it before you know what is going on. Hose may have to be changed too.

The question then begs: What would this angle change do to climb temps?

[ozairangel]

Dont forget to check the cleanliness off the heat dissapating surfaces, ie, the airflowthru the radiators which can become dirty with time, depending on your flying conditions, and if perhaps some oil got spilled when changing the filter, which can result in a nice insulating build up of dust etc.