Air Cooled VS. Water Cooled
 

Ok Folks,
Lets be realistic here. First the basic equation Q=h A (delta)T. In the engineering world many factors have already been worked out. Q is the rate of transfer of heat. BTU's/Hr. h is the coefficient of heat transfer. interestingly the transfer from a cast iron surface (turbulent) is 1.0. Aluminum is a but better but for now lets say 1.0. Ok now our equation looks much simpler Q = (1.0) A (delta)T. So let's assume the area is one square foot. The result is greatly simplified. Q= (1.0) (1.0) deltaT. So the rate of transfer is pretty simple. The RATE of exchange is about 2.25 greater for a air cooled engine compared to a water cooled engine. There are many other considerations for a engine but let's keep this as simple as possible. That solution seems pretty obvious, BUT that equation was simplified to the same exact AREA.
Area is the other major adjustable term. h of Water to iron, or better aluminum is 60-80 times greater than to air, so the second needed transfer can practically be discounted in this discussion since the air to radiator rate is going to determine the rate of transfer.
Based on the transfer rate alone, I would say that AC (air cooled) engine has a tremendous advantage. What everyone is ignoring in the equation is A or area. On a air cooled engine the maximum AREA is practically fixed. The heat exchanger (radiator) for a WC (water cooled) engine can be selected based on the heat rejection required. The modern radiator also packages a great deal of area into a given volume. One of the problems manufacturers had with producing cylinders and heads that would hold up on the large AC engines was making enough fin area to dissipate heat at high power and boost. I've got several old War Report findings from Pratt and others that mention the need to carefully machine the fins on the large cylinders to get fine enough pitch for heat rejection. On later versions you will even see a corrigated inserts between fins to increase area and help lessen ringing noise.

What this information shows someone that looks at it rationally is there are advantages and disadvantages to BOTH.

Air Cooled
Advantages: Simpler, high rate of heat rejection, light for a given size.

Disadvantages: Fixed max dissipation, lower specific output for a given size, limited layouts for a high output, IE radial or opposed. high normal operating temperature requires better oils and fuels.

Water Cooled
Advantages: Highest specific output for a given size, freedom of configuration, IE V upright or inverted, radial, opposed, unconventional rotary or barrel engines. heat exchangers can be located remotely and sized for output. Lower normal operating temperature lessens oil and fuel requirements.

Disadvantages: more complex, weight of secondary coolant, weight of remote mounted heat exchangers. Coolant pump is a wear consideration. System must be designed to prevent trapped air. plumbing is more complex and must be protected.

Conclusion: I could make an arguement for either system. The winner in a given situation would depend on the desired characteristics of the engine package. I know this ignores ducting, plenums, and a host of other factors but an arguement can still be made for both types. While I personally like water cooled engines I didn't even put a heater on the list. I will now don my fire suit. Fire away.
Bill Jepson

When I first considered building in 1988 I was all gung-ho about doing a conversion. A fellow pilot asked me if I had ever driven or ridden in a car that experienced a coolant leak/hose/pump issue that created steam. Well.... yes, in fact, more than once. Then he asked what happened next? We pulled over to the side of the road. :(

Really well done analysis, but there is one more consideration when it comes to aircraft:

The heat exchanger (radiator) for a WC (water cooled) engine can be selected based on the heat rejection required. The modern radiator also packages a great deal of area into a given volume.

The more area exposed, and the higher volume of air required, the more cooling drag is created. This were in the aircraft application the high delta t makes a huge difference in the ability to design for low drag. This seems especially true in tractor configurations.

This could be designed around, no question about it. Ross is probably doing the most with that right now with his 10 package that has a dedicated P-51 style belly scoop and ducting. But, to overcome the issue proper inlet and outlet ducting will be required.

A dedicated airframe design might be the best way to go. At the other end of the spectrum is bolting two or three fat rads to the front of the engine. Most of these (EGG) have gotten better and better inlets to the rad, but the outlet is just the bare back of the rad...not optimum at all, but probably the easiest way to retrofit to a design originally intended for aircooled.

Q=M Cp deltaT
 

Where Q is the desired heat rejection, Cp is the specific heat capacity of air (same in either case). M is mass flowrate of air thru radiator or fins.

Assuming your engine is roughly the same efficiency true if naturally aspirated.

Then in order to get the same heat rejection the water cooled engine needs 2* more mass flow of air thru the rad.

More cooling air means more drag.

Now of course the advantage of a water cooled engine is you can put rads anywhere...But in terms of minimising drag they are starting a bit behind the pace.

Until we have true side by side flyoffs with fully instrumented engines it will be hard to come to real conclusions of how much of a detriment this issue is in terms of speed attained for unit of fuel consumed.

Frank

In my experience most coolant leaks beyond potential issues immediately after servicing or modifying a cooling system, (on the ground) is usually related to neglect or just too long a service period for the components and environment involved.

I can certainly see concern about not being able to pull over to the side of the road. One way around this would be to use non-aqueous coolants available. In fact I think I've seen this advertised within the aviation crowd I've been purusing for a short time compared to many of you. I have four gallons in my garage waiting for what I originally thought we be a ground based application. I have seen it run in a land speed application that was cooking pretty good at the end of a five mile run under power. It took care of that problem. Coolant temps got high, intake air temps got high, compression ratio was about 12.5:1. Density altitude was probably pretty high in the 7-8K range, airflow to the radiator or the radiator itself was insufficient for an extended period in the conditions we were running. That car has about a four gallon fuel tank.

In an aircraft application whether taking part in some of the benefits of non-aqueous coolant and the lack of potential steam present, or following a more traditional water based coolant mix, the hardware in the system need to be top notch and checked/replaced often, like every two or three years. Expenses in aviation tend to run high. Surely opting for highest quality material available within reason, and limiting their service life for safety, isn't going to make a big dent in the whole scheme of things.

I will add, non-aqueous coolants are generally less efficient at removing heat that traditional systems, but there is the added benefit of pressureless or very low pressure systems and no steam, theoretically just fluid loss, hot fluid at that, and an unexplained coolant temp increase that may require a precautionary landing, depending on the nature of the leak.

I believe the best solution for either an Air or Water cooled engine is for the airframe to be designed around it. If you look at a P51 that huge fighter is about the same width at the firewall as a RV-10. Much deeper vertically, but that indicates the flexibility possible with water cooling. Then again no one can argue that an F8-F Bearcat tears through the air with it's large round fuselage. I'll see if I can post a couple of the NACA war reports on the ducting. It shows that the wide open radiator rear isn't that bad, IF and it an important if, the area behind the radiator is unobstucted for at least 4x the core thickness. This isn't met with an Egg or any front mounted radiator that I have seen. That is in fact my biggest gripe about the Egg and several other conversions. That said, just because the radiator looks solid doesn't mean that the airflow is worse than in a Lyc plenum. You need to measure both with a manometer it you want to know rather than guess.
Bill Jepson

I disagree only slightly, Your assumption is that ALL the air is in contact and not in a free stream past the cooling fins. My contention is that for a given volume of air the radiator will in fact contact more of it and be much better at approaching theroetical. I was trying to keep it as simple as possible.
Bill Jepson

Correct Bill. With a diverging/ converging duct, we can also slow the air down through the rad core which results in more contact time with the fins, lower pressure drop across the tubes/ fins and can re-accelerate the air closer to free stream velocity. Despite the lower Delta T, we can use each pound of cooling air quite effectively. Is it better than air cooling? We don't know without some real world testing.

Modern rads are all aluminum and have thin tubes and pierced, louvered fins- ideal for high rates of heat transfer. The K value for aluminum is about 3 times that of steel. Steel therefore makes a poor choice for a heat exchanger say a Lycoming barrel compared to a Rotax or Porsche barrel. Copper is quite a bit better than aluminum even but is very heavy and the thickness of the tubes and effective ways to joining tubes to fins are harder without creating thermal barriers.

As far as the non-aqueous coolants like Evans NPG+ go, I've tested it back to back against 30% ethylene glycol/ 70% distilled water and a dose of Redline Water Wetter to reduce surface tension. Under the same ambient air temps and same IAS, coolant temps dropped an average of 9C in the climb using the water based mix. Evans offers a very high boiling point (375F) as does pure ethylene glycol but a much lower rate of heat transfer than water based mixtures. 9C is very significant in the climb on a liquid cooled engine. We are able to climb at 90 knots IAS at an OAT of 25C at full climb power (4600 rpm and 35 inches) without the coolant temps exceeding 92C.

With regards to rads, I've just finished flow benching 4 different common heat exchanger types for pressure drop. The often used large GM evaporator cores are the worst at 5.75 inches H2O compared to a Visteon type core with oval tubes at only 2.75 inches. Pressure drop through the coolant passages will have to measured next and then temperature drop on a test rig. This will then give us a better picture on what type and shape of HE (heat exchanger) gives the best performance per unit area and unit pressure drop.

Good thread Bill, maybe some useful info can be exchanged here.:)

Points to consider
 

This is a very valid point. I take this time to mention that MOST forced landings of experimentals are caused by NON-ENGINE systems. Fuel feed failures are greater than 50% of forced landings from the FAA stats. This is for certified and auto conversions. All this means is that we must pay attention to sub systems, JUST LIKE THE BIG THINGS. A fuel feed failure can kill you just as dead as a thrown rod if it makes your engine stop at a bad moment. We can roll through all the possibilities in a best practices thread. I am a subscriber to several Rotary engine lists. One of the guys started a Wiki page with best practices that can be updated by the guys from the forum, an excellent idea IMO.
Bill Jepson

I do a lot of historical reading of technical reports on WW2 engine development as much can be learned.

The coolant pressures used on the P51 are up to 50 psi! to raise the boiling point and Delta T. Max temps on the Merlin were 130C. The P51 used 60/40 EGW. Early Spitfires used lower pressures but pure EG. Later marks of the Hurricane used 30/70 EGW mixtures as more powerful Merlins were introduced to avoid have to redesign the radiators.

Some other tidbits include speeds with open and closed rad flaps. Typically closing the flaps at full power was worth between 10 and 24mph more speed depending on the airframe. Drag is reduced by reducing massflow through the HE (lower pressure drop).