Chts too cold? Any suggestions?

[syfhmasta]

Hello all. First post.

I'm having an issue with CHTs being to low in cruise. Relatively new 10, flew the 40 off last early summer, and then took 8 months to paint it. So this is my first winter, if you can call it that, flying in the south.

The aircraft has a standard cowl, but a custom built plenum that is super tight. Cowl flaps for climb.

Climb temps are normal, with #2 being my hot cylinder, and gets to about 390 in the climb at 115kts. A little cooler if I go faster. I climb at 2500rpm WOT, and start leaning above 5k. Cowl flaps are open for takeoff, and get closed somewhere around 4-5k.

Cruise is my issue, at least in winter. It seems too cold when talking with others. I'm usually running at 2200rpm WOT at around 8.5-9.5k, which, when leaned to LOP will give me sub 300 degree CHTs. If I richen it to peak, it'll run around 310-320, with at least one cylinder around 290 (#6). #5 is my hot cylinder in cruise. This is with OAT around 36 degrees at altitude. Typical cruise speed is about 172-175 TAS at around 11.2-11.4 gph around peak, closer to 10gph if running full LOP. Egts between 1350-1400. I'm running SDS and my peaks are very close, and if running full LOP I'll advance the timing to 29 degrees.

Decent is even worse. Cylinders cool to around 250, with power at 40%, 500fpm down, and 155kts indicated. Cowl flaps stay closed. My cylinders actually warm up after landing during taxi to around 300...

I use a TCW butterfly for my oil cooler to keep the oil temp up near 180, but the more I close it the cooler #6 gets.

During summer, climb temp was about the same, but cruise temps were 340-350, and the oil cooler didn't need to be damped much if at all.

So my questions:

1. How cold is too cold given oil temp is good and egts are healthily hot? I know the limits, but worry about lead buildup and sticky valves in my future.
2. If I wanted to make a servo controlled bypass hole on the baffle/plenum (say 3 inch), where is the best place to put it? Near #6 (the coldest cylinder), or #5 (the hottest)? Not much side baffle space forward of those cylinders. I may also be able to fit a 3 inch hole on the back baffle, offset towards #6.
3. Does anyone have another solution?

The engine is an older 540, with 5 chrome cylinders, one nickel, but runs like a champ. Eats a quart every 8-9 hours. It's a big investment, and Id like to treat it right.

Not the typical CHT thread, but I appreciate any input!

Roy

 

[rv8ch]

Does Lycoming even have a lower limit to CHT? I've read some say CHTs too cool can cause lead or carbon buildup, but I don't see how lower CHTs will substantially change the temperatures in the combustion chamber. Sounds like you have an excellent setup, and I would not change anything.

 

[syfhmasta]

Does Lycoming even have a lower limit to CHT? I've read some say CHTs too cool can cause lead or carbon buildup, but I don't see how lower CHTs will substantially change the temperatures in the combustion chamber. Sounds like you have an excellent setup, and I would not change anything.

Thanks Mickey,

Yes, per the manual lyc recommends chts between 150-435 for service life. Obviously, there are tons of discussions about the upper number (I personally use 400) but very little about the lower one, with recommendations running the gambit. I have found a couple of discussions about air inlets and cooling drag where folks post cht's around where I'm running, always on 4 pistons, but nothing about whether that truly has an effect on service life or not. In principle, I lean (pun intended) towards your line of thought wrt healthy egts mitigating the possibility of lead or carbon buildup creating an issue. The one ground truth I know involves Connie IO550's that can develop ring issues when running cold chts in the range I'm running, (had a buddy run into this issue), nothing heard about Lycs. Just trying to separate the facts from the heresy.

I've thought about more variable exit modification to reduce it further, but honestly, I can't envision shrinking the exit area much more being that the cowl flaps sit there and I have heater bypass lines (branching off from just before the heater valves to drive hot air that was dumping from heater valves into the accessory area) jammed in there too. Prepaint Pic attached, unfortunately in the middle of my text!

Thanks for the input!

 

[Toobuilder]

Cylinders are machined with a taper to anticipate a certain expansion and “final shape” at the nominal temps. Too cold is off nominal as is too hot. I would suggest that you install a “block off plate” to your cowl. does not have to be complex - you are just looking to spoil the flow out of the cowl a little bit. A simple flat “winterizing plate” held with a few screws should move the needle a little bit. Anything to get you back toward the mid 300’s in winter months is good.

 

[Jesse23]

I’d love to have your issue, especially in FL.

 

[supik]

I'm not of much help, but I would like to see more pics of your setup (inlets, plenum..). I do have exactly the opposite problem.. thanks

PS: What's the function of the 2 scat tubes in your picture?

 

[Bavafa]

The numbers you have given are hardly too cold, many would wish to have your numbers especially that these are the winter time numbers. You can bring them up by closing the exit area in your cowl but I don't believe you are close to having any issues.

 

[RV10inOz]

Hello all. First post.

I'm having an issue with CHTs being to low in cruise. Relatively new 10, flew the 40 off last early summer, and then took 8 months to paint it. So this is my first winter, if you can call it that, flying in the south.

The aircraft has a standard cowl, but a custom built plenum that is super tight. Cowl flaps for climb.

Climb temps are normal, with #2 being my hot cylinder, and gets to about 390 in the climb at 115kts. A little cooler if I go faster. I climb at 2500rpm WOT, and start leaning above 5k. Cowl flaps are open for takeoff, and get closed somewhere around 4-5k.

Cruise is my issue, at least in winter. It seems too cold when talking with others. I'm usually running at 2200rpm WOT at around 8.5-9.5k, which, when leaned to LOP will give me sub 300 degree CHTs. If I richen it to peak, it'll run around 310-320, with at least one cylinder around 290 (#6). #5 is my hot cylinder in cruise. This is with OAT around 36 degrees at altitude. Typical cruise speed is about 172-175 TAS at around 11.2-11.4 gph around peak, closer to 10gph if running full LOP. Egts between 1350-1400. I'm running SDS and my peaks are very close, and if running full LOP I'll advance the timing to 29 degrees.

Decent is even worse. Cylinders cool to around 250, with power at 40%, 500fpm down, and 155kts indicated. Cowl flaps stay closed. My cylinders actually warm up after landing during taxi to around 300...

I use a TCW butterfly for my oil cooler to keep the oil temp up near 180, but the more I close it the cooler #6 gets.

During summer, climb temp was about the same, but cruise temps were 340-350, and the oil cooler didn't need to be damped much if at all.

So my questions:

1. How cold is too cold given oil temp is good and egts are healthily hot? I know the limits, but worry about lead buildup and sticky valves in my future.
2. If I wanted to make a servo controlled bypass hole on the baffle/plenum (say 3 inch), where is the best place to put it? Near #6 (the coldest cylinder), or #5 (the hottest)? Not much side baffle space forward of those cylinders. I may also be able to fit a 3 inch hole on the back baffle, offset towards #6.
3. Does anyone have another solution?

The engine is an older 540, with 5 chrome cylinders, one nickel, but runs like a champ. Eats a quart every 8-9 hours. It's a big investment, and Id like to treat it right.

Not the typical CHT thread, but I appreciate any input!

Roy

There is NOTHING WRONG with your CHT in cruise LOP or descent. If anything the climb speed of around 120-125 knots would be better.

Ignore any advice that says they are too cool. That is yet another Old Wives Tale.

 

[pa38112]

I have flow with those temperatures and lower for 1,100 hours. Wobble tests look good, so no issue with lead fouling. You could maybe pick up a knot or two by restricting your cooling air, but it seems like your warm weather temperatures are good, so I would not change a thing.

 

[jcarne]

Almost identical temps to what I see in my -7A. No issues

 

[syfhmasta]

I'm not of much help, but I would like to see more pics of your setup (inlets, plenum..). I do have exactly the opposite problem.. thanks

PS: What's the function of the 2 scat tubes in your picture?

First off, to everyone who made a reply, Thank you! You put my mind at ease!

Supic, the scat tubes you see in the exit area come from a y tube connector that I have just prior to the heater valve, they "predivert" the air from the heater valve down to the exit when I have them in the closed position. I did this because I was getting hot air from the heater valves stuck up in the accessory area forward of the firewall, around 200 degrees, which was causing my main alt fuse to burn out prematurely. This modification dropped my accessory area temps about 30 degrees (pretty much spot on with oil temp and what I suspect is crank case temp) and alleviated having to upside my alternator fuse. It has a side benefit of removing any tunnel heat, as very little hot air makes it through to heat soak the firewall heat valves. My tunnel heats werent hot before, but they were a little warm, now they are cold. Required an extra switch, to open or close the two y tube valves. They were the ones you can buy off of spruce with a small linear actuator connected directly to each on. I'll try and attach a pic, but it is hard to get a good one.

As for the plenum, zero rocket science. I splashed the upper cowl for shape with carbon and made a box that closes off at the top of the aluminum baffles with minimal clearance to the upper cowl. The trick is is the inlets. Took some inlet lip off the top cowl, made the bottom cowl have a ring, so it butts up to the plenum. Mates together with neoprene ziptied to it. The neoprene is from an old wetsuit. There is a hole is the oil door area that has a rubber seal (home depot window trim, same stuff I used for my door seals, works great) that presses on the upper cowl for a seal. Finished off with a hidden oil door latch, the wire is put through the gap between the plenum and lower cowl, through the neoprene, and velcroed to the upper inlet in the plenum. The plenum is only 4 ply, so there is some foam for structural rigidity across the middle and near the door hole, but not much. Basically it's just a baffle, with near zero leakage.

 

[syfhmasta]

First off, to everyone who made a reply, Thank you! You put my mind at ease!

Supic, the scat tubes you see in the exit area come from a y tube connector that I have just prior to the heater valve, they "predivert" the air from the heater valve down to the exit when I have them in the closed position. I did this because I was getting hot air from the heater valves stuck up in the accessory area forward of the firewall, around 200 degrees, which was causing my main alt fuse to burn out prematurely. This modification dropped my accessory area temps about 30 degrees (pretty much spot on with oil temp and what I suspect is crank case temp) and alleviated having to upside my alternator fuse. It has a side benefit of removing any tunnel heat, as very little hot air makes it through to heat soak the firewall heat valves. My tunnel heats werent hot before, but they were a little warm, now they are cold. Required an extra switch, to open or close the two y tube valves. They were the ones you can buy off of spruce with a small linear actuator connected directly to each on. I'll try and attach a pic, but it is hard to get a good one.

As for the plenum, zero rocket science. I splashed the upper cowl for shape with carbon and made a box that closes off at the top of the aluminum baffles with minimal clearance to the upper cowl. The trick is is the inlets. Took some inlet lip off the top cowl, made the bottom cowl have a ring, so it butts up to the plenum. Mates together with neoprene ziptied to it. The neoprene is from an old wetsuit. There is a hole is the oil door area that has a rubber seal (home depot window trim, same stuff I used for my door seals, works great) that presses on the upper cowl for a seal. Finished off with a hidden oil door latch, the wire is put through the gap between the plenum and lower cowl, through the neoprene, and velcroed to the upper inlet in the plenum. The plenum is only 4 ply, so there is some foam for structural rigidity across the middle and near the door hole, but not much. Basically it's just a baffle, with near zero leakage.

And here you can see the lower cowl loops, sort of, and the plenum off the plane. And once it's all together, pretty clean look.

Looks like the plenum pic has to be opened, won't do a thumbnail.

I did split the lower cowl to make all this work. The engine inlet comes completly off, it's just laying on the cowl in that pic. When putting everything on the plane, it's easiest to put the plenum on first, then the lower cowl, but it can be accomplished the other way, just difficult up near the inlet area. In this configuration I can do oil changes without removing the lower cowl or plenum, so usually, if I'm doing something that requires removing the plenum, I'd be removing the lower cowl anyway.

Hope that helps.

 

[supik]

Roy, that's a unique approach I haven't seen before. Very cool ideas! I might steel some in future.

Many thanks for the explanations and pictures!

I wonder if directing the excess hot air from the heat valves would help to increase the Delta P between the upper pressurized part vs the lower part. It would indicate to me that this way you're not pressurizing the lower cowling part with the heated ram air as you're directing it outboard.