Cowl carbon baffles, CHTs

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Si_Zim
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Cowl carbon baffles, CHTs

Post by Si_Zim »

Hi

Been watching the CHTs as the OAT has been quite high recently. I've had to keep things richer than I would like to keep it cool.
I noticed that the carbon fiber baffles (or whatever they are called) in the cowl openings - especially the left one (facing the prop) - were rather high. This is probably by design to force airflow up over the vains on the top of the cylinders and the down thru them... but I just thought I would check in case this is not typical.
The photo of the left one is very dark - apologies. But you can just make out where the carbon fiber ends. The right one is much clearer but the carbon ends a fair bit lower than the left.
If anyone remembers - can you check yours and maybe post a pic so I know if this is normal?

Thanks
Simon
leftCowl1.jpg
rightCowl1.jpg
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smoss
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Re: Cowl carbon baffles, CHTs

Post by smoss »

mine looks the same as that.
Steve
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Chris B
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Re: Cowl carbon baffles, CHTs

Post by Chris B »

smoss wrote:mine looks the same as that.
As do mine.

On hot days (and/or with a strong inversion), you may need to increase airspeed during climbs in addition to running rich mixtures. On *really* hot days I have occasionally needed to back-off on the throttle even in cruise to keep CHTs under 380F.

Chris
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Re: Cowl carbon baffles, CHTs

Post by Si_Zim »

Guys - thanks for confirming
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David B
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Re: Cowl carbon baffles, CHTs

Post by David B »

Mine too are the same. On hot days at nearly full gross I too have to keep the mixture a little richer than I would like to maintain CHT on number 4 at less than 420 on climb. Generally, I prefer to keep it less than 400 as a rule but at full gross in the summer heat that is nearly impossible without a 100 fpm climb. At altitude (8,000 feet or more) with throttle to the max and leaned to 7.5 gph I see the CHT's drop to 350 - 380 depending on the OAT.
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Re: Cowl carbon baffles, CHTs

Post by Antoine »

My recipes for keeping CHTs as low as possible. Note: this is stuff I have learnt from others here, credit to them!
1) Plug all of the gaps between baffling and engine
2) Use airspeed to manage CHTs in climb
3) lower RPM makes a BIG difference. I find it better to use 2300 RPM and wide open than 2500 and throttled back.

I use fuel as a very last resort (in practice - never) to cool my engine. But then I dont fly in Arizona!
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Chris B
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Re: Cowl carbon baffles, CHTs

Post by Chris B »

Antoine wrote:3) lower RPM makes a BIG difference. I find it better to use 2300 RPM and wide open than 2500 and throttled back.

I use fuel as a very last resort (in practice - never) to cool my engine. But then I dont fly in Arizona!
Hi Antoine -

I agree with your first two points, and your third point if you are talking about cruise power. But climbs are where CHT is most challenging for me, and low RPM there can cause big problems.

Everything I have learned from the APS guys (Deakin, Atkinson & Braly) is that at high power settings in a climb you want the prop at high RPM. Otherwise the detonation margins are *greatly* reduced, and peak internal cylinder pressures are very high. The best analogy I have heard is a bicycle: a strong cyclist can climb in low gears, but puts huge strain on the drive train.

OTOH, at cruise power, running with relatively high MP and slow RPM is perfectly safe, since the pressures are greatly reduced. The Lycoming IO-360 has lower power per cylinder than the big bore engines, and can tolerate abuse that would destroy a Continental IO-550. So it could be that "high power" for our engines is closer to cruise power for the big engines. But I wouldn't recommend low RPM climbs as a best practice.

Separately, the rich mixture is not cooler because of the chilling effect of additional fuel. The engine runs cooler simply because the extra-rich mixture burns less efficiently and the engine develops less power (CHT is a good proxy for power). It's just an expensive way to run at reduced power. ;)

Chris
Last edited by Chris B on Tue Aug 25, 2015 3:21 pm, edited 1 time in total.
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David B
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Re: Cowl carbon baffles, CHTs

Post by David B »

From my recent experience spending the past 2 weeks flying across Canada in hot weather and especially through the mountains, I've had almost 40 hours of PIC time to "study" CHT-power combinations. I tend to agree with Chris that climbing at higher RPM leaning initially to about 10 gph and eventually down to 8 gph with altitude and max power has been the best way to keep the CHT's below 400. In cruise I never have an issue and find it quite easy to keep them all under 350 at all times even at 2200 rpm's. At higher altitudes even down to 320.

During my trip through the mountains I took off from a few higher altitude airports and one in particular at 3,200 ft with 4,000 ft of runway and mid day temperatures at 30 degrees Celsius (90 degrees Fahrenheit) at near full gross. To climb out of this airport initially amounted to 200 fpm. I kept the climb at 200 fpm with 2300 rpm and 10 +/- gph flow to keep the CHT's below 420 but it was a challenge and a very long climb to 8,500 feet. However, the beauty of this plane is the ability to monitor all of this so closely as opposed to my first couple of planes with only a one probe EGT.
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Re: Cowl carbon baffles, CHTs

Post by Rick »

David B wrote:I kept the climb at 200 fpm with 2300 rpm and 10 +/- gph flow to keep the CHT's below 420 but it was a challenge and a very long climb to 8,500 feet. However, the beauty of this plane is the ability to monitor all of this so closely as opposed to my first couple of planes with only a one probe EGT.
I am curious, what is the reasoning behind reducing RPMs to 2300 during the climb, as opposed to climbing at full RPMs?
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Re: Cowl carbon baffles, CHTs

Post by CFIDave »

If you read the Lycoming documentation for the DA40's IO360 M1A engine, at sea level the engine produces 180 hp only at the full 2700 RPM. If you bring the RPM back to 2400-2500 (I forget which) you'll obtain only 160 hp out of the engine even at full throttle.

Obviously density altitude above sea level reduces the horsepower even more. So to climb, don't reduce RPM below 2400, and lean as you get higher to maintain EGTs 100-200 ROP to obtain max hp. By climbing at a "cruise climb" airspeed of 90 knots this keeps CHTs from going much above 400F (maybe 405F) on hot days -- this is not going to hurt the engine when climbing for only 10 minutes or so. (With the GFC700 autopilot this is where FLC set to 90 knots is wonderful). For IFR flights, ATC likes to see a climb of at least 500 fpm and this is a way to minimize climb time.

Once you level off, CHTs below 380 will result from the increased airflow in cruise and decreased power from reduced fuel-air at higher altitudes.
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