Van's Air Force
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Lycoming Cooling Chart
- Thread starter Pilot8
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They're empirical, data obtained with a set of baffles described in a book no longer available from Lycoming. Better baffles would improve the values, worse would be, well, worse. I would not treat the numbers as absolute, but the relationships hold true.
Specific to your question, take a look at the parallel valve chart in post#2. The red line says with 4" H2O difference between the upper and lower cowl volumes, at 5000 ft there will be about 1.5 lbs of air per second flowing through the fins. If the OAT at 5000 ft is 60F, and power is 75%, CHT will be around 460F.
The green line says a delta of 5.25 or so would drop CHT to 435F under the same conditions.
Study it a bit and you soon see it takes about 2lbs/sec through the fins to get a decent CHT at 75%, again assuming the mysterious "standard" baffling. The good news is at 5000 ft and 165 KTAS, dynamic pressure is about 15" H2O. A good cowl and baffle setup should make about 80% of it available as pressure above the engine, so an open exit on the lower cowl should result in 10 to 12 inches across the fins. Climbing through 500 at 125 knots should net about 7", maybe a little more. The 5.25" delta would be a 110 KTAS climb.
Specific to your question, take a look at the parallel valve chart in post#2. The red line says with 4" H2O difference between the upper and lower cowl volumes, at 5000 ft there will be about 1.5 lbs of air per second flowing through the fins. If the OAT at 5000 ft is 60F, and power is 75%, CHT will be around 460F.
The green line says a delta of 5.25 or so would drop CHT to 435F under the same conditions.
Study it a bit and you soon see it takes about 2lbs/sec through the fins to get a decent CHT at 75%, again assuming the mysterious "standard" baffling. The good news is at 5000 ft and 165 KTAS, dynamic pressure is about 15" H2O. A good cowl and baffle setup should make about 80% of it available as pressure above the engine, so an open exit on the lower cowl should result in 10 to 12 inches across the fins. Climbing through 500 at 125 knots should net about 7", maybe a little more. The 5.25" delta would be a 110 KTAS climb.
Last edited:
Marc Bourget
Well Known Member
Dan,
I tried copying the image into my computer and the resolution is poor.
Any way to get copies that will print in 8.5 x 11" Landscape format?
I tried copying the image into my computer and the resolution is poor.
Any way to get copies that will print in 8.5 x 11" Landscape format?
