David-aviator
Well Known Member
This thread is in the FWIT category. Some guys are having cooling issues, some are not.
I seem to have stumbled on a method of providing excellent cooling for the cylinders and oil and not appreciably increasing drag. Looking at the set up would make one suspicious of that claim but the numbers seem to support the conclusion. The machine cranked up to 202 mph yesterday at 8500'.
The next subject is RAM air into the fuel injection system. It is without doubt worth considering. It took some cowl adjustment to provide for a connection to a filter by-pass in the left intake area and one would think messing with Vans cowl is asking for trouble with regard to drag, but that does not seem to be the case.
I am convinced the key to good cooling is air flow across the oil cooler. The type of cooler may be a factor also, but air flow comes first. A RV-4 guy reported poor cooling with a James inlet arrangement with upper cowl pressure at 10:1 with outside air. That's a lot air packed into the upper cowl area. That ratio would seem important but I believe what is more important is the lower cowl ratio to outside air pressure. If the ratio in the lower cowl is the same as outside air, there has to be tremendous flow from the upper high pressure area down and out and cooling ought to be very good. But that is not the result. Cooling is not good and it can only mean air is not flowing down and out because the pressure down there is higher than outside air. And that is caused by a restricted exit area.
Vans has cut the inlet to exit area ratio very close out of a consideration of cooling necessity and drag imposed by exit area dimension. As near as I can calculate, that ratio is about 108% exit to inlet area when considering the exit area occupied by exhaust pipes and a NG leg with -A models. My exit area is about 226% to inlet with the 5x15 bottom exit and 2 Bonanza side vents at 14 inches of area each. That is overkill on exit area but may explain way I have not seen an oil temp above 180 even with a climb to 12.5 yesterday with heat soaked engine at take off and the OAT at 92F. The gross exit area is left over from the Subby adventure trying to get a couple too small radiators to work.
Beyond cooling issues, I believe the RAM air inlet to the AFP fuel injection system is worth considering. It provides for squeezing max performance out of the engine. There is a manifold pressure penalty pulling air through a filter.
The Subaru cooling set up was a compromise from the get-go. I did some informal tuft testing to confirm my suspicion that air was not going through the radiators as much as it should and thought the tuft test proved it. Some other guys asserted the test was not valid because the airplane was not inflight. I believe to this day the cooling set up did not work efficiently because the back side of the rads was within an inch of the engine block and air flow in that area was total chaos. Air should be plenum ducted to and from the radiators with gradual area diffusion as was proven with a number of WWII fighters.
The lack of performance of my airplane with the H6 led to some very black days for me in concluding there was really something wrong with my machine and how I built it. But switching to the IO360 has pretty well vindicated the matter. The same airplane is now performing about as well as can be expected and is at or over Vans numbers. I have not had any black days with the Barrett Lycoming IO360X engine and the Catto prop.
That's not to conclude the alternative engine effort is for naught. It is working well for some guys but it did not work for me. I had one of the first H6 engines and its application has been much improved from 6 years ago.
I seem to have stumbled on a method of providing excellent cooling for the cylinders and oil and not appreciably increasing drag. Looking at the set up would make one suspicious of that claim but the numbers seem to support the conclusion. The machine cranked up to 202 mph yesterday at 8500'.
The next subject is RAM air into the fuel injection system. It is without doubt worth considering. It took some cowl adjustment to provide for a connection to a filter by-pass in the left intake area and one would think messing with Vans cowl is asking for trouble with regard to drag, but that does not seem to be the case.
I am convinced the key to good cooling is air flow across the oil cooler. The type of cooler may be a factor also, but air flow comes first. A RV-4 guy reported poor cooling with a James inlet arrangement with upper cowl pressure at 10:1 with outside air. That's a lot air packed into the upper cowl area. That ratio would seem important but I believe what is more important is the lower cowl ratio to outside air pressure. If the ratio in the lower cowl is the same as outside air, there has to be tremendous flow from the upper high pressure area down and out and cooling ought to be very good. But that is not the result. Cooling is not good and it can only mean air is not flowing down and out because the pressure down there is higher than outside air. And that is caused by a restricted exit area.
Vans has cut the inlet to exit area ratio very close out of a consideration of cooling necessity and drag imposed by exit area dimension. As near as I can calculate, that ratio is about 108% exit to inlet area when considering the exit area occupied by exhaust pipes and a NG leg with -A models. My exit area is about 226% to inlet with the 5x15 bottom exit and 2 Bonanza side vents at 14 inches of area each. That is overkill on exit area but may explain way I have not seen an oil temp above 180 even with a climb to 12.5 yesterday with heat soaked engine at take off and the OAT at 92F. The gross exit area is left over from the Subby adventure trying to get a couple too small radiators to work.
Beyond cooling issues, I believe the RAM air inlet to the AFP fuel injection system is worth considering. It provides for squeezing max performance out of the engine. There is a manifold pressure penalty pulling air through a filter.
The Subaru cooling set up was a compromise from the get-go. I did some informal tuft testing to confirm my suspicion that air was not going through the radiators as much as it should and thought the tuft test proved it. Some other guys asserted the test was not valid because the airplane was not inflight. I believe to this day the cooling set up did not work efficiently because the back side of the rads was within an inch of the engine block and air flow in that area was total chaos. Air should be plenum ducted to and from the radiators with gradual area diffusion as was proven with a number of WWII fighters.
The lack of performance of my airplane with the H6 led to some very black days for me in concluding there was really something wrong with my machine and how I built it. But switching to the IO360 has pretty well vindicated the matter. The same airplane is now performing about as well as can be expected and is at or over Vans numbers. I have not had any black days with the Barrett Lycoming IO360X engine and the Catto prop.
That's not to conclude the alternative engine effort is for naught. It is working well for some guys but it did not work for me. I had one of the first H6 engines and its application has been much improved from 6 years ago.
