Todd, the cylinders are aluminum, the air is air, the rads are aluminum, so the coefficients are constant. Given that, and given a similar quantity of heat to dissipate, at lower Delta T it takes a higher quantity of air.
Of course if you could design a ducting system which was enough better to make up for all of it, you would have an overall system which was better. However, such a design has been elusive for decades now. In all probability Ross's 10 will be a good test of a dedicated duct design approach.
In the end, you may get to laugh last here....when your plane is weighed and flown, we will easily be able to tell the drag/power/fuel issue.
But, pumping water through an engine, and then into a radiator does not magically change the quatity of air required to dissipate a given quantity of heat, nor does it magically reduced the amount of heat there is to dissipate.
For Subaru, we find 23 in the same period and zero internal catastrophic failures. All these failures were due to supporting systems and poor maintenance- very similar to the majority of certified engine failures. The point here is that there were no core failures.
First, when the supercharger blows the engine up, is that a core failure? When an overheated sub went down with cooked rods, was it core?
Second, if all the ancilliaries are critical to keeping the prop spinning, and it stops doing so, does it really matter? Ross, did you crash any less, because the core was still good. David, did you?
Third, 23 power failures out of a fleet of 500 or less flying versus 75 failures in all GA using recip engines...how is this a sobering statistic about the choice of a certified engine? You should also add yours to the number Ross, because as you know, the NTSB database does not include yours or any of the other Canadian crashes, although my fleet number of 500 does include all of North America, and is probably generous, at that.
I had previously posted general MTBF numbers, which you did not respond to, I will say that the very istant the fleet generates a MTBF in terms of flight hours which is half of the "sobering" failures statistics of the traditional piston recip, I might try it.
For a given mass flow, the rate of heat transfer is not only dependent on Delta T but the surface area, turbulence within the boundary layer next to the fins, turbulence within the tube (radiator), the amount of time the air is in contact with the fins (which we can adjust with duct shape and exit aperture) and temperature gradient.
Drag is a result of pressure drop across the system. Coincidentally, I was actually running some flow bench tests on different radiator cores today for a magazine article and I'll add to this some data for air flowing around a typical air cooled cylinder. My hunch is that the cylinder will offer much higher pressure drop than a rad core even with no diffuser, but I'll wait for the scientific confirmation on this one. Any time you turn air sharply, you have a pressure drop. Air goes straight through a rad but turns 180 degrees in most air cooled setups.
Duct shapes have been well understood for decades just not applied on civilian aircraft for the most part. Reg Clarke in particular has done some fine work with belly mounted diffusion ducts on his turbo Sube Dragonfly showing how little air and how little rad is required to cool an engine if done correctly.
Core means just that, engine core. I was trying to be fair to the certified engines as well by not counting external parts like mags, carbs etc. or pilots operating the engine with no oil. The 72 I came up with were 90% rod failures and crank failures, unrelated to oil starvation or improper maintenance.
Nobody is debating that ancillary systems are important to keep engines running, more so in the case of auto engines but if you wish to count non core failures bringing down certified engines, then the figure is about quadruple with magneto, carb and carb icing thrown in as reasons the prop stopped turning. For the 5 year period, the following search strings are related with numbers of accidents (connecting rod 143, power loss 195, engine failure 278, carb ice 141). Clearly while there are tens of thousands of piston powered aircraft flying in the US accumulating hundreds of thousands of flight hours, there have been at least 278 accidents caused from engine failure in the last 5 years.
There have been over 650 RAF gyros sold, nearly 200 Groen Gyros sold and there are several other designs out there, most of which are Subaru powered. Add to this all the NSI, Eggenfellner and private Subaru powered creations and we have closer to 750-1000 probably flying today. There were only 16 actual power loss conditions (out of the 23 total Subaru accidents) and several of these were undetermined for reason.
I don't think either stat is particularly comforting. I would say that given the numbers of the two engine types flying, you do appear to have a higher likelihood of a power loss condition with a Subaru than a Lyconental but a much lower likelihood of a core engine failure. (not that that matters when it happens to you!)
There are no accurate MTBF stats for either engine type. Should anyone wish to compile this information somehow, it would prove very interesting.
My original point of the exercise was to was to counter the notion that turning Subarus at 4000-5000 rpm was in some way detrimental to the engine. Can't find a single case here to support that idea. The bulletproof term may be more deserved by the Subaru than Lycomings and Continentals.
I think much has been learned and applied in the last 5 years to make auto conversions more reliable overall, backup batteries, standardized fuel systems, better PSRUs. The biggest problem I see that may forever remain is poor wiring practices and electrical design. This is very critical with EFI/ EI and electric fuel pumps.