Installing a WAM-120 diesel in an RV-9A

[Steve Brown]

Speed is not a function of liquid verses air cooling, its a function of HP verses drag.

You only have to look as far as cars to get the answer on which works best. They're all liquid cooled, they all have low drag numbers. Before you start yelling about the realitely low average HP used on public roadways, look at F1 racers (per SvingenB). Weight-to-HP and drag are all important and they run HP settings that dwarf ours. I don't see any of those guys using air cooled engines.

Liquid cooling has some obvious advantages in terms of having a more even temperature throughout the engine, eliminating shock cooling, etc. Another advantage alluded to by ergie63 & SvingenB is that you can move the cooling to wherever makes the most sense.

Many liquid cooled RVs are sporting automotive engine conversions that are probably not putting out the HP expected - so - they are slower. Probably drag issues as well.

The RVs were designed and optimized around air cooling. Trying to plug & play a liquid cooled engine in that airframe is not likely to have optimal results.

What if the an RV cowling was aluminum - same thickness as the rest of the plane - and that tiny radiator tubing was spread all over the inside of that cowling?

Then just close the cooling holes entirely.

Anyway, there are probably 20 reasons why that is a dumb idea, but the point is to engineer a liquid cooled solution from the ground up. I don't possess the skills to do that, but I suspect some on this site do.

[Dave_Boxall]

Chaps

The standard wilksch pack is not the ideal solution for low drag cooling, but it does allow Wilksch to produce an engine with a finished and tested cooling solution that is applicable to a wide variety of airframes. It's not the only solution, and you're not restriced to using that solution.

As things go I'd have to describe the Wilksch cooling pack as "pragmatic" rather than ideal or elegant. Similarly I have to say that my choice to build an installation around the wilksch cooling pack is also pragmatic. Designing and building a new engine installation is hard work, and I didn't want to make it any harder than I had to.

Dave

[Andy_RR]

Originally Posted by Steve Brown

You only have to look as far as cars to get the answer on which works best. They're all liquid cooled, they all have low drag numbers. Before you start yelling about the realitely low average HP used on public roadways, look at F1 racers (per SvingenB). Weight-to-HP and drag are all important and they run HP settings that dwarf ours. I don't see any of those guys using air cooled engines.

Cars and aircraft makes for a falacious comparison. They neither must produce lift, nor thrust and they are surprisingly unaerodynamic compared to aircraft. Cooling drag is a pretty insignificant part of the total drag on a car.

200hp will make a car do approximately 140mph with no lift (or downforce) to speak of. 200hp in an aircraft will see it doing 220mph (nearly 60% faster) and generating 1g of lift!

Cooling drag is much more important on an aircraft, where the drag coefficient is around a tenth of that of a typical road car, which in turn is much more aerodynamic than an F1 car.

Originally Posted by Steve Brown

Liquid cooling has some obvious advantages in terms of having a more even temperature throughout the engine, eliminating shock cooling, etc.

liquid cooling is invariably thermostatically regulated, whereas air cooling invariably isn't, but that's not to say it cannot nor should not be done.

Originally Posted by Steve Brown

Another advantage alluded to by ergie63 & SvingenB is that you can move the cooling to wherever makes the most sense.

...at a cost - usually paid for in pounds, and not pounds sterling!

Originally Posted by Steve Brown

Many liquid cooled RVs are sporting automotive engine conversions that are probably not putting out the HP expected - so - they are slower. Probably drag issues as well.

The RVs were designed and optimized around air cooling. Trying to plug & play a liquid cooled engine in that airframe is not likely to have optimal results.

Most piston engine cooling systems are hardly optimised be they air or liquid cooled. A comparison of fully optimised air and liquid cooled aircraft would show that the air-cooled aircraft has less cooling drag - however neither aircraft actually exists as yet...

[ergie63]

Originally Posted by Dave_Boxall

pragmatic

Coming from someone in the midst of it all makes me laugh at myself.

[Steve Brown]

Originally Posted by Andy_RR

..... A comparison of fully optimised air and liquid cooled aircraft would show that the air-cooled aircraft has less cooling drag - however neither aircraft actually exists as yet...

Andy,

Some airplanes only get 140mph or less out of 200hp.

Anyway, your conclusion is surprising to my simple mind.

Assuming you are correct, what is it then that drives every auto manufacturer and every racing team to a liquid cooled conclusion? Aerodynamics & weight are both important. As important as an airplane? No, but engineers don't give away performance for nothing.

Serious question

[Andy_RR]

Steve,

I don't speak for the race teams or the auto manufacturers, but I would say the reason race teams don't use air cooling (though Porsche did!) is due to the specific power that they require from their engines.

Auto manufacturers choose liquid cooling because it makes NVH (noise) refinement much easier. Also they have legislated drive-by noise targets which arguably killed the air-cooled boxer engine in the 911.

There's no doubt that in certain circumstances, liquid cooling is an advantage, but looking at it purely as rejecting heat to the environment, liquid cooling will dump heat to a higher total volume flowrate of air than direct air cooling will.

For a more relevant motor racing comparison, look at intercoolers. Air-air intercoolers are generally chosen because they are more efficient and lighter...

[rv6ejguy]

Specific hp, emissions, noise, longevity, proper cabin heat all all reasons why the air cooled engine is dead in modern cars. Many of these criteria are either not so important or non-existent in aviation.

Saying that more mass flow is required to dissipate the same heat energy on a liquid cooled engine is a simple and perhaps inaccurate way of looking at the issue. This may be true but as the total mechanism of heat transfer from the critical high flux areas of an engine goes to actually transferring that energy to the atmosphere with the lowest drag, it is far more complex.

As I've said before, because of the high mass and far superior conduction of water in close proximity to the cooled parts, combined with the alumimum radiator being many times more efficient than a steel barrel or finned aluminum head at radiating heat per unit area and volume, finally combined with a diverging/ converging duct which makes maximum use of the available mass flow of the cooling air and being able to efficiently re-accelerate the air to near free stream velocity with lower pressure loss- simple delta T does not adequately cover the whole process from a drag perspective.

Without hard data, we simply don't know. I haven't seen anything that would suggest that air cooling is superior in this respect. I invite anyone who can supply such data to share it here so we can stop speculating.

Intercoolers are of course exchanging heat between two mediums with nearly equal mass. Routing is simple and we don't have any parts likely to melt from inadequate heat transfer. These factors make air to air intercoolers the clear choice for most land or air bound vehicles which have adequate space. However when we have tight packaging like say under the cowling of a Spitfire or P51 or a boat, liquid to air intercoolers are employed. Just like with engines, both types have been proven to work well.

[Jconard]

Without hard data, we simply don't know. I haven't seen anything that would suggest that air cooling is superior in this respect. I invite anyone who can supply such data to share it here so we can stop speculating.

Hard data: every RV with a liquid cooled, horizontally opposed engine and the same cowling as the aircooled, is slower as a function of HP/fuel flow. Since the cowls are the same, and the configurations are the same, it appears that drag is the issue.

Now, the problem is that the need for converging/diverging ducts has not been answered. It would be difficult to do so on an RV, I think. My personal thought is that perhaps the belly scoop approach offers a great opportunity.

In looking at TSweezy's install, I also wonder if it will work pretty well, the side radiator/side exit concept seems like it would allow a diverging duct into a plenum on the inboard surface of the rad, and perhaps the a converging duct on the outboard side, leading to the side exit....will wait to see how it works.

It seems clear that on an RV, the front mounted rad, dumping eventually into the lower cowl exit, it a very draggy configuration....probably designed with other goals in mind (ease of installation...selling plug and play installations).

In the end I do not think, without a specific airframe design approach, weight and drag parity will be accomplished, but the above two configs seem like they will get closer than we have been thus far.

[ergie63]

Originally Posted by rv6ejguy

...a diverging/ converging duct which makes maximum use of the available mass flow of the cooling air and being able to efficiently re-accelerate the air to near free stream velocity...

I agree. What strikes me about the longitudinal cross section of diverging/converging duct systems is that they are, broadly speaking, the same as a jet engine. In sequence: 1) Build up static pressure, 2) add heat, and 3) extract work.

Since the amount of mass and heat in a cooling system are orders of magnitude less than a jet engine, we're not talking about useful thrust. What seems to be possible to me (a lay person) is that the careful management of pressure and heat can be used to offset the penalty for having to make air move.

Turbulence in cooling passages, whether the engine is directly (air) or indirectly cooled (liquid), works the air which is to say adds heat. We experience that transfer of energy as drag. But all you engineers already know that.

Thoughts on shape (answering to jconard):
The respective cross sections of the WAM (narrow) and RV-9 (wide) together offer an opportunity not possible with other combinations. There is enough room for ducting roughly in the shape of a beetle's pincers on either side of the engine. In my mind, the fellows at the start of this thread were on the right track with their initial effort. It seems to me that they didn't appear to be mindful of managing their pressure gradients evidenced by the location of the engine air inlet in front of the radiator. That configuration steals precious mass and pressure from where it is needed the most. It would have been interesting to see the effect of changing that one thing alone.

[SvingenB]

Originally Posted by Andy_RR

Cars and aircraft makes for a falacious comparison. They neither must produce lift, nor thrust and they are surprisingly unaerodynamic compared to aircraft. Cooling drag is a pretty insignificant part of the total drag on a car.

200hp will make a car do approximately 140mph with no lift (or downforce) to speak of. 200hp in an aircraft will see it doing 220mph (nearly 60% faster) and generating 1g of lift!

Cooling drag is much more important on an aircraft, where the drag coefficient is around a tenth of that of a typical road car, which in turn is much more aerodynamic than an F1 car.

The downforce of an F1 car is enough to enable it to drive upside down at already at 80 mph or so. F1 teams spend billions on making as much downforce as possible with as little drag as possible, cooling drag included. As I wrote earlier, the cooling drag destroys much of the downforce and creates drag. Aerodynamics is much more than drag coefficients. The main thing is that F-1 cars use 2.4 litre normally aspirated V8 engines producing up to 800 hp. If this engine was to be air cooled, it would probably max out at 300-400 hp if it would last a GP race. There simply is no way you can get the needed 250 kW of heat transported out from the core of that little engine by conduction through aluminum alone.

If the same amount of money spent on aerodynamics on F1 cars were spent on airplanes with an F1 engine installation, this airplane would be the size of a -3 and cruise around MACH 0.9 It would probably also have underbelly cooling and look very much like a mini P-51