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Post by n3480h on Sept 6, 2011 17:47:48 GMT -5
Good suggestions, Oahupilot - and appreciated. I agree doing just one thing to reduce drag is probably worthwhile, but a bigger total benefit results form doing many things, in a prioritized way. A Honda Civic inline 4 sits in my garage, already overhauled and balanced, but it only produces about 110hp @ 5kRPM. But, I can pick it up off its cradle without a hoist or hernia. Its a bit loud with straight pipes, but kind of fun. Downside is, if I run a different engine I can't run in a VW class, lol.
Tom
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Post by heisan on Sept 7, 2011 4:59:36 GMT -5
...If you where only going to do one mod for drag reduction I would say load an inline 4 cycl water cooled engine with a 150 hp. The cooling drag from cooling the radiator will be less then cooling drag of the aircooled vw. This is one thing that has always bugged me. You often hear that water cooling results in less cooling drag than air cooling - but that doesn't make any sense to me. If you look at it, heat transfer is proportional to the difference in temperature between the object and the air. If you look at air cooled cylinders operating at ~180C, with an air temp of 25C, the difference is 155C. Now look at a radiator at ~100C, the difference is 75C. So, to remove the same thermal load will require twice the airflow through a radiator, than directly through the fins of a cylinder head. (And therefore, probably twice the cooling drag). Can anyone please explain why water cooling actually results in less cooling drag?
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Post by oahupilot on Sept 8, 2011 2:37:10 GMT -5
...If you where only going to do one mod for drag reduction I would say load an inline 4 cycl water cooled engine with a 150 hp. The cooling drag from cooling the radiator will be less then cooling drag of the aircooled vw. This is one thing that has always bugged me. You often hear that water cooling results in less cooling drag than air cooling - but that doesn't make any sense to me. If you look at it, heat transfer is proportional to the difference in temperature between the object and the air. If you look at air cooled cylinders operating at ~180C, with an air temp of 25C, the difference is 155C. Now look at a radiator at ~100C, the difference is 75C. So, to remove the same thermal load will require twice the airflow through a radiator, than directly through the fins of a cylinder head. (And therefore, probably twice the cooling drag). Can anyone please explain why water cooling actually results in less cooling drag? its not that simple here is the watered down explanation air has a specific heat capacity of .00129 Joules/(cm^3*K) while water has specific heat capacity of 4.179 Joules/(cm^3*K). So water can carry much more heat energy then air and it can also transmit the energy faster then air. A good way to think about it is it can flow more heat energy then air, this means you need less cooling surface to transfer the same amount of heat. Also a radiator is better at transferring heat then the fins of an air cooled engines. Another point to consider is the flow of the cooling air around the motor versus the radiator. Air flows straight through the radiator while in an air cooled motor it has to make turns. Every time you change the direction of the flow of air it cost you energy in the form of drag. In a sonerai it comes in straight then makes a 90 degree turn down past the cylinders, then another 90 degree turn to exit the motor/cowl. Thats a very quick and dirty explanation, but consider the fact that p51 mustang radiator actually add thrust to the airplane and has often been cite as plane with zero cooling drag. The bugatti 100p also has a zero cooling drag radiator setup.
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Post by heisan on Sept 8, 2011 3:06:54 GMT -5
This is one thing that has always bugged me. You often hear that water cooling results in less cooling drag than air cooling - but that doesn't make any sense to me. If you look at it, heat transfer is proportional to the difference in temperature between the object and the air. If you look at air cooled cylinders operating at ~180C, with an air temp of 25C, the difference is 155C. Now look at a radiator at ~100C, the difference is 75C. So, to remove the same thermal load will require twice the airflow through a radiator, than directly through the fins of a cylinder head. (And therefore, probably twice the cooling drag). Can anyone please explain why water cooling actually results in less cooling drag? its not that simple here is the watered down explanation air has a specific heat capacity of .00129 Joules/(cm^3*K) while water has specific heat capacity of 4.179 Joules/(cm^3*K). So water can carry much more heat energy then air and it can also transmit the energy faster then air. A good way to think about it is it can flow more heat energy then air, this means you need less cooling surface to transfer the same amount of heat. Also a radiator is better at transferring heat then the fins of an air cooled engines. Another point to consider is the flow of the cooling air around the motor versus the radiator. Air flows straight through the radiator while in an air cooled motor it has to make turns. Every time you change the direction of the flow of air it cost you energy in the form of drag. In a sonerai it comes in straight then makes a 90 degree turn down past the cylinders, then another 90 degree turn to exit the motor/cowl. Thats a very quick and dirty explanation, but consider the fact that p51 mustang radiator actually add thrust to the airplane and has often been cite as plane with zero cooling drag. The bugatti 100p also has a zero cooling drag radiator setup. The specific heat of water makes no difference - the water simply transfers heat from one place to another. In the end, all the heat is removed by transferring it to the air. Either via copper/aluminium fins in a radiator, or aluminium/iron fins directly on the cylinders/heads. So I suppose, in the end it comes down to the efficiency of airflow through a radiator, rather than through the cylinder/head cooling fins. Even then, most 'water cooled' homebuilt installations are a disaster, with the radiator placed directly up against the front of the cowl, with air venting through the maze of the engine compartment. I really doubt you would get any cooling drag advantage without some serious design work on the cooling ducts.
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Post by oahupilot on Sept 8, 2011 15:38:28 GMT -5
The specific heat of water makes no difference - the water simply transfers heat from one place to another. In the end, all the heat is removed by transferring it to the air. Either via copper/aluminium fins in a radiator, or aluminium/iron fins directly on the cylinders/heads. So I suppose, in the end it comes down to the efficiency of airflow through a radiator, rather than through the cylinder/head cooling fins. Even then, most 'water cooled' homebuilt installations are a disaster, with the radiator placed directly up against the front of the cowl, with air venting through the maze of the engine compartment. I really doubt you would get any cooling drag advantage without some serious design work on the cooling ducts. The specific heat of water does matter it allows for a faster rate of heat flow (also a more even rate)through the the radiator fins which also have a faster rate of heat flow then the iron/aluminum engine fins. you can move more energy faster through a radiator with water to air then through cylinder/head fins to air.
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4trade
Junior Member
Posts: 51
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Post by 4trade on Sept 8, 2011 15:47:47 GMT -5
Water transfer approx 4 times faster heat than air.
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Post by bil438 on Sept 12, 2011 21:34:37 GMT -5
I put my wheel pants on two years ago. There was an immediate airspeed gain of say 10 mph. I damaged one and did a quick fix. >I need to re-do it, because some of that airspeed gain is lost. Mine are simple pants that slide over the wheel. Bill
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hans
Full Member
Posts: 166
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Post by hans on Sept 13, 2011 16:31:45 GMT -5
an isolated view that only evaluates cooling drag might come to the conclusion that water cooling results in less cooling drag. However, as water cooling is heavier (all other things being equal), you're going to create another type of drag elsewhere, as the heavier cooling needs to be carried aloft - you're going to need extra lift to carry that, and as you know: there is no lift without associated drag.
Evaluating subsystems like engine cooling is fun, but don't forget the big picture.
cheers Hans
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Post by n3480h on Sept 13, 2011 16:55:52 GMT -5
By all means, keep your pants on Bill. ;D
Tom
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Post by oahupilot on Sept 13, 2011 19:08:18 GMT -5
an isolated view that only evaluates cooling drag might come to the conclusion that water cooling results in less cooling drag. However, as water cooling is heavier (all other things being equal), you're going to create another type of drag elsewhere, as the heavier cooling needs to be carried aloft - you're going to need extra lift to carry that, and as you know: there is no lift without associated drag. Evaluating subsystems like engine cooling is fun, but don't forget the big picture. cheers Hans thats probably a wash since water cooled engines develop more power per pound of engine weight then air cooled engines
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hans
Full Member
Posts: 166
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Post by hans on Sept 14, 2011 13:10:40 GMT -5
an isolated view that only evaluates cooling drag might come to the conclusion that water cooling results in less cooling drag. However, as water cooling is heavier (all other things being equal), you're going to create another type of drag elsewhere, as the heavier cooling needs to be carried aloft - you're going to need extra lift to carry that, and as you know: there is no lift without associated drag. Evaluating subsystems like engine cooling is fun, but don't forget the big picture. cheers Hans thats probably a wash since water cooled engines develop more power per pound of engine weight then air cooled engines Exactly for that reason I included the phrase "all other things being equal". cheers Hans
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