+4 kts -30 CHT Possible with Baffle Sealing?

[larrynew]

Is it possible to get a 4-5 kt increase and a 30 degree CHT decrease with extremely thorough baffle sealing?

I decided to get serious about sealing the baffles while replacing baffle parts that cracked in the corner at #4 where the oil cooler hangs. I was also installing an oil cooler shutter. Hopefully the prop balance by Walt will cure that.

Baffles are plans built and what I thought were adequately sealed. 400+ hours in Texas and no real problems with CHT other than having to lower the nose during climb after a quick summer turn to keep the CHTs below 400°. Oil temps have always stayed 185-190.

The biggest change I believe was from completely filling the large gap in the baffles behind the prop. Using standard red RTV in a Semco sealant gun with 4" nozzle made it easy but I used a LOT of RTV. I also did the RTV impregnated fiberglass mat wrap on the lower #4 Cylinder under the baffles. I then went crazy with the RTV and sealed absolutely every air path I could find that didn't go by cylinder fins. I even squirted it in the tiny gaps between the pop rivets sealing the baffle fabric to the baffles. I did not do a pretty job by any measure but will happily take the results.

I didn't do a strict before and after test as I wasn't expecting such dramatic results. A lot of the 400+ hours have been long cross countries so I have a very good feel for the numbers plus Dynon Skyviews and IFR Garmins make it easy to accurately measure everything. Sunday was about 6 hours of flying to Brownsville, TX (KBRO) and Cypress River (24F in Jefferson, TX) with ground temps around 100°. We were after BBQ! Google "Texas Monthly BBQ Passport". Never saw CHTs above 400° during hard climbs and they were consistently about 30° lower during cruise. Cruise TAS was consistently 4-5 kts faster at the same LOP fuel flows as always. CHT and EGT temps were much better balanced. Also, fuel pressure stayed steady with the boost pump off during the climbs to 14.5 where before it would occasionally get low. I suspected it was a cooling problem but hadn't done anything about it. Is this possible?

[Bavafa]

Originally Posted by larrynew

The biggest change I believe was from completely filling the large gap in the baffles behind the prop. Using standard red RTV

Great to hear your result, especially if it can be confirmed with multiple flight. Pictures would be great especially in this area.

Over the years I have also worked to seal any little air gap that I can find and know my baffles touch the cowl in complete and all around since there is a good markings left on the baffle material yet have seen little improvement in CHT which is very close to your numbers including oil temp.

[RV Jerry]

I experienced almost the exact same results in my Rocket although instead of using a lot of RTV I made new baffles that fit tighter, had increase in speed 3-5 knots and 30 degrees lower on CHT AND 10-12 degrees lower oil temp now CHT in cruise 350-360 oil temp 185- 188 on of the best things done.

[DanH]

Can't swear to specific numbers, but theory says speed gain and lower CHT should be the expected result.

A reduction in mass flow or an increase in exit velocity means a reduction in cooling drag.

Eliminating leakage increased restriction to total flow, which increased upper cowl pressure. The larger pressure difference between the upper and lower cowl volumes increased mass flow through the fins, which improved cooling for several reasons. However, the additional mass through the fins is probably not as large as the previous leakage mass flow, which was doing nothing other than adding to the drag total.

Exit velocity will not change, or might decrease a little, which would be counterproductive. However, two steps forward and a quarter step back is still an improvement.