Adequate Cooling

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ChuckerF14

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Nov 1, 2013
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741
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Prescott Valley, AZ
I've been waiting for summer weather before finalizing the cooling exit ducts. Today it was 90 degrees on deck departing from a 5,000' MSL airport. At full power and 80 mph I got up to 419 degrees on the CHT and 215 degrees oil temperature. I then accelerated to 100 mph (still climbing at full power) and dropped about 10 degrees. Then I pulled the RPM back to 3200 (still climbing at 100 mph) and the CHT dropped below 400.

The engine is a Revmaster 2100-D running 8.5:1 CR. My cooling configuration is....well...different. I have stock VW cooling shrouds which draw the air up through the cylinders and heads then port it directly overboard via exit ducts on top of the cowl. I'm trying to decide if the ducts are large enough or if I should expect better cooling and need to enlarge them some.

Your experiences are invited. Thanks,

Chucker
 

ChuckerF14

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Nov 1, 2013
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Prescott Valley, AZ
True statement. However, VW spent millions of R&D dollars on the shape of the shrouds. I don’t think any of our home grown designs route the air as efficiently. I’m not sure the metal cares where the air molecule is headed....just that it passes close enough to transfer some heat. My cooling exit area is about 1/2 that of a stock cowl that has had the exit opened a few inches. I will increase the exit size as necessary. The point of my thread was to get some relative numbers from others flying in high temperatures and (if available) high elevation airports.

Thanks,
Chucker
 

n109jb

Active Member
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Jun 9, 2013
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32
Location
Morris, IL
I tried a few different box designs, with and without little tubes going elsewhere with flat back walls, curved back walls, etc. I finally wound up with baffles reminiscent of most of the lycoming or continental baffles we all see. Simply flat aluminum slabs that tuck into the stock VW lower cylinder shrouds across the back , and screw to the outside fins of the cylinders with 8-32 screws. The baffles have silicone baffle seal to seal against the underside of the cowl. Since I went to this, I have no more cooling problems, access to the engine is easier, fitting inside the cowl was easier, etc. BTW I run a 2276cc engine.
 

kennyw

2nd and current "Caretaker" N994SP
Joined
Aug 27, 2017
Messages
160
Location
Mount Vernon, WA
I've been waiting for summer weather before finalizing the cooling exit ducts. Today it was 90 degrees on deck departing from a 5,000' MSL airport. At full power and 80 mph I got up to 419 degrees on the CHT and 215 degrees oil temperature. I then accelerated to 100 mph (still climbing at full power) and dropped about 10 degrees. Then I pulled the RPM back to 3200 (still climbing at 100 mph) and the CHT dropped below 400...
Thats what I see on a warm day. I try to limit time climbing at 80mph. 4SP seems to climb just as well at 95mph. So, as soon as I clear the tree tops, I drop the nose slightly and let the speed increase to 95-100mph.

[Side note: I used to fly with a CFI who was a stickler for not climbing at Vx any longer than absolutely necessary. If I wasn't accelerating, at least, to Vy by the time I cleared the trees, he was fussing at me about heat killing engines.]
 

eschrom

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Jan 18, 2006
Messages
799
Location
Manchester, PA
Chucker, your numbers sound pretty good to me. I'm not able to fly at full cruise power on a hot day because the OT continues to creep up beyond 210 so I'm now in the middle of installing a bigger cooler. Just curious, how long are you climbing at 80 before you see 419/215?

Ed
 

eschrom

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Jan 18, 2006
Messages
799
Location
Manchester, PA
That's similar to my experience except, as I said, at 3200 the OT keeps going. But that was with the 24-row cooler and now I'm moving up to the 48-row. In a couple of weeks (I hope!) we shall see.
 

ChuckerF14

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Nov 1, 2013
Messages
741
Location
Prescott Valley, AZ
The CHT probe is mounted in the boss for the top aft most cylinder head stud on the right hand side (cockpit view)... just above the exhaust port. It is not actually under the nut and washer. According to the Revmaster manual, I removed the ring from the probe and drilled the boss. The probe is held in place by a set screw threaded into the boss at a right-angle to the probe wire.
 

Magisterol

Member
Joined
Nov 26, 2019
Messages
8
Hey guys. Just a 2 cent info. Normally the exhaust area is 0.76-0.80 of intake area for efficient cooling. Lager will overcook, smaller will heat up. This is one of the empiric formulas NACA came up when they started to design their NACA scoops.
 

mhflyit

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Joined
Jun 2, 2012
Messages
854
Location
Jupiter, FL
Chucker, your numbers look OK. Remind me - is it a new engine?

My temps on a FL summers day run in the 340-365 range at cruise and oil no more than 190 but my cooler works particularly well. Its bottom mounted with 'smiley' air ducted directly through it.
 

mhflyit

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Joined
Jun 2, 2012
Messages
854
Location
Jupiter, FL
I changed the cowl looking for the optimal sized exit and added a small 'flap' underneath to disturb the airflow and help the exiting hot air. Smokey has some good posts on this. I think he used VG strips. This dropped my CHT's 20+ degs. Also sealing the cowl gap behind the spinner took off another 10 degs.
At 25hrs and still 'breaking in', your temps look OK.
 

Smokyray

www.iamanet.org
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Aug 6, 2015
Messages
234
Location
TX32
Chucker,
Having ducted many cowls over the years including the custom plenums on 994SP's Ultimate 1835, I found beyond a shadow of a doubt that cooling plenums and stand alone high pressure oil cooler ducts work, period. Remember the 1:1.2 ratio equation for air-in-air out for cowling inlet/exit area. Measure your inlet/exit ducts, make sure your oil cooler lines are at least #6, place a light inside your cowl at night and make sure your cowl seals seal also. As Matt mentioned, I placed VG strips forward of my cowl exit to excite the air and improve exit airflow, a trick learned on my RV4.

Higher airflow via climb airspeed as mentioned by Kenny and Matt is also very effective and safer forced landing energy management if you lose power on takeoff. I would install a second temp source for CHT/OT just for a second opinion and test data. I used a bluetooth BBQ grill meat sensor linked to my iPhone for a few undercowl temp tests! You can purchase complete iPhone bluetooth engine sensor packages as well.

Your updraft cooling isn't a new concept as the original Thorp T-18 employed it with a 0-290 GPU. The Super Cub and later Thorps use side cowl exits. As mentioned, your numbers aren't bad but could be a little better.

Take note, make adjustments, keep testing!
:)
V/R
Smokey
 
Last edited:

ChuckerF14

Well-Known Member
Joined
Nov 1, 2013
Messages
741
Location
Prescott Valley, AZ
Smokey, Matt, et.al.,

Thanks for the input. Since it is easier to take fiberglass than it is to give it back, my original exits were only about 80% of what they could be. Based on the numbers I gave above, I opened them up to maximum capacity. To open them further would require reworking the cylinder shrouds. That is possible, but not something I am anxious to do right now.

Opening the exits resulted in 36 inches of exit area and a 10-15 degree drop in maximum CHT and Oil Temperature (406/204 in an 80 mph climb with an OAT just under 90 degrees). Accelerating to 100 mph takes another 10 degrees off those numbers.

The next modification will be a smiley shroud to duct air over the bottom of the sump. As for oil lines, I have none. My cooler is top-mounted and the exit ducts suck air through it.

Everything outside the cylinder shrouds is in cool air. I have a probe on my magneto and the highest temperature I have seen is 46 Celsius (115 F)...and there is no cooling air directed to it.

As for the 120% rule of thumb, I believe that I will need to reduce the size of my inlets to get there. ;) I don't know if anyone else has done the math. The best I can guess, the cheek inlets are about 12 inches each and the smiley is about 10 (34 inches in total). I will need to lose 4 inches of intake air to hit the 1.2:1 ratio.

Keep the cards and letters coming. I learn something from every post.

ATB,
Chucker
 
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