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Reduced CHTs by ~17 degrees with a Cowl Flap

Triumph1974

Well Known Member
Hi Everyone,

Just wanted to report out that after installing a cowl flap from Anit-Splat, I have been able to keep my 90 degree day climb out CHT temps at less than 395 degrees.

I have a good baffle seal (no light coming through with the spotlight test) but would have to lower the nose and reduce climbs to around 350 ftm to keep temps in from getting above 415 degrees on hot days.

One possible recommendation for those considering the cowl flap option to lower CHTs - you may want to install a regular sized switch if you have the room vs. the little micro switch that comes with the kit....it is just hard to see and work with when soldering wires.

We don't have many 90 degree plus days any longer, so I was not able to get a detailed set of test data in since installing the cowl in early Aug, but for the non scientific test flights I have done, I am happy with the CHT temp reductions using the cowl flap. I installed it on the passenger side of the cowl.

Paul

RV7A, 3 blade Catto/0360
 
I ordered and plan to install the VANS cowl louvers during the winter -fighting a similar problem.

Will try and report out next summer!
 
Before you cut holes...

I hope my response doesn't create an endless debate worthy of popcorn :)

Firstly, I am a big fan of Alan's products & services, however, in this case I think it's a solution in search of a problem and/or the wrong application of a solution (think: injecting bleach to cure a head cold).

Yes, Van's cowling/baffling is very tight compared to their contemporaries (Piper, Cessna, et al). However, most of us are able to make it work with comfortable margins in all climes...

Some things to check before cutting large holes in the airframe:

1. Timing -- is it set to 25°BTDC (8.5:1 pistons), 20°BTDC (8.7:1 - 9.5:1 pistons, 17° BTDC for 10:1, etc. This is worth A LOT temperature wise...ask me how I know ;)

2. Baffles -- top, sides, bottoms, inter-cylinder, the gap behind #3, the other gap behind #3, etc. Use a flash light and friend who vapes to blow smoke and see where it goes. Red RTV (hi temp silicone) is your friend, don't be afraid..

3. Inlet ramps -- Are they installed? Are they sealed on the sides?

4. Leading edge of inlets between the cowl halves -- is daylight visible between the halves? Fix this...

5. If "A" model, is the front & rear bottom plate and fairing around the gear leg present? -- If not...fix this...

6. Other stuff -- look for items that trip up the smooth airflow from the inlets, leaks around the baffles, seals going the wrong way or missing along the top, rivet mandrels missing, etc.

The cooling system for air/oil cooled engines is dependent upon airflow in/through/around the cylinder heads and oil cooler -- if the "Delta P" (the difference in pressure between the top side and the bottom side) isn't high enough, air won't move fast enough, suck heat away, and so on...


Good luck!
 
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I hope my response doesn't create an endless debate worthy of popcorn :) Good luck!

Well said. Good advice. Agree do other things to lower temp before cutting extra holes.

I would add what jug has +400F? Is it the #3 Jug? Do you have a gap between baffle and cylinder to allow airflow where cylinder fins are shallow or non existent.
 
Another partial solution for #2

Well said. Good advice. Agree do other things to lower temp before cutting extra holes.

I would add what jug has +400F? Is it the #3 Jug? Do you have a gap between baffle and cylinder to allow airflow where cylinder fins are shallow or non existent.

Here is a 10F reduction for those who have built/bought and pondered #2 CHT reduction. I did mine in build differently but this was a friends Phase II solution.

Choose the hole locations wisely.

IMG_6027.jpg
 
Increasing exit area reduces lower cowl pressure, thus increasing deltaP. That much is obvious, but there are other interesting effects.

Way back, I modified a stock cowl, removing the entire "coal shovel" exit and placing nutplates in hard points around the perimeter of the opening. I then laid up several screw-on exit panels forming progressively smaller exit areas.

Early in testing (2011), one experiment was to remove the lower cowl exit panel and go fly with a HUGE exit area (see below).

CHT responded with a large drop, as expected. I was then averaging 322F CHT with the exit panel installed...an exit area considerably less than stock. It dropped to an average of 274F CHT under the same conditions with the panel removed.

The interesting point was air temperature rise across the cylinder. I had a thermistor hung just below the #1 cyl head baffle wrap exit. Normal air temperature was around 240F. Given very high mass flow, it dropped to 135F. I was flowing a lot more air, and removing more total heat...but transferring considerably less energy to that air on an energy per pound basis. In other words, it was way less efficient.

Spin the concept around. Low cowling exit temperature is a yardstick for the quality of the sealing and baffle wraps. If they are effective (no leaks and good heat transfer) cowl exit temperature will be higher. Conversely, very low exit temperatures indicate leakage, and/or poor baffle wraps, or simply more mass flow than really necessary.

Caveat; the above is an angle valve cylinder, which has better cylinder head finning than the parallel valve. It is easier to get low CHT with an angle valve engine, and the CHT's reported above were too low, in my opinion. Ultimately I shrunk the cowl's fixed exit further, and installed a variable exit door (not really a cowl flap) for temperature control. The challenge with the angle valve was shrinking the cowl exit for velocity while holding the line on oil temperature.
 

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