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

[rv6ejguy]

Originally Posted by ergie63

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.

The Meredith Effect was to have produced a net gain in speed by using the energy from the expanding cooling air exiting the radiator duct at higher velocity. Theory proved somewhat different from practice and later wind tunnel testing showed that it was not possible below about 350-400 knots to achieve unity in cooling drag at the typical 110C coolant temperatures and state of the art radiators in the late 1940s.

It has been fairly clearly shown that a well designed radiator system such as the P51 or leading edge setups like the Westland Whirlwind, Tempest I, or Hornet were quite a bit more efficient that the underwing rads employed on many aircraft and could offset some of the cooling drag associated with these setups.

Obviously most under cowling setups on an RV are not going to be as efficient as what could be done with a lot more work but the drag of a belly scoop must be figured into the equation with its extra frontal area. Clearly cheek mounted rads are the least efficient with poor wetting of the HE surface and poor pressure recovery, not to mention willy nilly air exit paths and poor velocity recovery at the cowl exit- but they are convenient and simple as far as minimal changes to the cowling and plumbing runs go.

[Steve Brown]

Originally Posted by 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...

Andy,

I'm not a Porsche fan, but my recollection is that Porsche made the jump to water cooling to support performance and reliability unobtainable with air cooling.

In other words, the performance didn't go down due to water cooling, it went up.

Also, with your "specific power" comment I think you are conceding that liquid cooling is more efficient at getting the heat out.

To me that equals better performance, if the airframe is optimized to take advantage of it.

[rv6ejguy]

Emissions and noise were the main factors in Porsche going water cooled in production cars.

In racing, where the 962 reined supreme for so many years in IMSA GTP, it was simply an issue with power density and specific output once the Nissan VG30s and AAR Toyotas entered the picture. Even with later water cooled heads, the Porsches were no match for the water cooled narrow engines in the hp department or in the downforce area where engine width was a detriment.

The same thing is seen in Unlimited racing at Reno where in years past, the Dwight Thorne Merlins were making about the same hp (about 4000 ) as the best R3350s with only HALF the displacement. The P51 Dago Red still holds the fastest race lap ever run at Reno by a substantial margin at 512 mph.

The latest F1 engines are unbelievably small packages for the 800hp they produce. An 800hp air cooled engine would probably have to be triple the volume of a liquid cooled one even allowing for the radiator volume to be included.

The Sport Class Lycomings and Continentals are nearing their thermal and mechanical limits now even with spray bars and ADI. These will probably not be able to exceed much over 750-800 hp no matter what with any reliability. The liquid cooled Falconer V12s and Chevrolet V8s being raced and developed in the wings will supplant the air cooled engines in the not too distant future IMO.

Bottom line, liquid cooling works just fine and we just have to find better ways to package them into an RV airframe.

I was just uploading some photos to Flysoob and noticed some new photos of a Subaru installation on a Europa there. Details a dedicated Kuchemann/ Weber derived radiator duct with the rad mounted underneath the oil pan. The inlet is relatively small and the duct diverges to the radiator and then converges aft of it to the exit. I was thinking of a similar arrangement when I redo my 6A cooling system. I should have built a -6 rather than a -6A as the nose gear structure makes a nice duct shape more difficult. I'll have to split the exit flow.

As for the intercooler- I like the setup in the P47 which used a dedicated duct to feed an aft fuselage mounted intercooler with nice exit ramp exits. The aircraft was designed around the turbocharger and intercooler installation much like the P51's cooling system was designed into the airframe rather than onto it. The P51 integrated the liquid to air intercooler heat exchanger into the radiator duct. This was also a pretty clean installation.

A liquid cooled, turbo charged and intercooled RV presents some real challenges in efficient packaging to fit it all into a low drag setup. Ask me how I know.