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Adventures in Cooling Land
This is the original cowling with no flap. Exit area about 2 x 23". Radiators are standard 1981 Caprice/Impala evaporator cores. I believe they are Harrison GM code 60-61. Core is 10.5" x 9" x 3.625", and outside dimensions are 13.5" x 9.125" x 3.625". Core volume is about 342 cubic inches. Oil cooler is straight off the car.
It became obvious after the first flights that cooling would be inadequate, so I cut out a flap from aluminum and hinged it from a piano hinge at the forward edge. Actual flap size was about 7 x 23 inches. Control was from a "T" handle on a double cable (1/4 turn to lock type) from Aircraft Spruce that pulled up on the rear of the flap. Has about a 2" range of movement.
This opened up the exit to about 4 x 23. Cooling was good for the power
available at the time.
When I modified (improved) my intake manifold to this one, with a 70mm
aftermarket throttle body designed for the 5.0 L Mustang, I picked up 300 rpm
static, which translates into a VERY noticeable power increase :-)))
Power also equals heat. Now the cowl flap wasn't doing the job.
Takeoff power for more than a minute or two would put the oil and coolant temps
over 210F. What to do??
I decided to copy the process that Paul Lamar used on Tracy's RV-4. If you leave your tubes open to the cockpit, be sure to check the difference between that location and the static ports. I found 1" more at the static port than in the cockpit.
Intakes are standard RV at the top, and standard "Ed" for the oil
cooler. (Thanks Ed Anderson for saving me this development time.)
I found that pressure recovery was quite good even toward the edges of the radiators. I then concentrated on what was going on inside the cowl. I found that there is a fairly even pressure drop starting at the radiator and ending at the cowl exit. I figured that if I could minimize the pressure at the exit, I had done my best.
I checked it with the old flap, no flap at all, a small 1-inch "flipper" flap, and a flap opened up to 6 inches. The 6 inch opening dropped the pressure significantly, so I elected to try that route. With the original hinge line, it was basically a giant speed brake, so I cut (ouch!!) another chunk of cowl out and moved the hinge line forward. The new flap is 13 x 23 inches.
I had already removed my "rounding" fairing from the base of the firewall. It has been there since first flight, but due to its radius, it was reducing the size of the exit with the old cowl flap. I may reinstall it now that I have cut the opening in the cowl larger.
Open and closed, opening is 6" to 2.5" x 23"
My first control attempt included a fancy linkage to reverse the action of the cables and to get enough throw. Hello Mr. Goldberg. After destroying two cables, it was obvious that the aerodynamic forces on this flap necessitated a different approach. Contrary to my intuition, the highest force was not required to go full open, but to get it closed! The trailing edge opening is adjustable from 6 inches to 2 inches.
Next version was an electric control using a flap actuator jackscrew and a bell crank mounted at the base of the firewall. I used one of the broken cables as a position indicator, and now I have complete control. I am on my second flap actuator, though. The first one unscrewed itself out the end and fell in two. It was my good fortune to have this happen just as I arrived at Van's for homecoming. I simply removed it, took it to the counter for a warranty replacement, and reinstalled. No FedEx required! This one is holding up much better.
This is not the end of the story, as the cowl flap creates turbulence which can be felt in the belly skin just below the rudder pedals. The turbulence goes completely away with the flap fully closed (2" opening), but that is only possible with OAT below 50F. This opening is WAY bigger than it needs to be in theory, so I am sure more research is in order.
I finally have started leaning this thing like I'm supposed to, by going way lean of peak and controlling power with the mixture rather than the throttle (for the most part). Fuel flow is now in line with others' experience, under 8 GPH at cruise. I am running a Warp Drive 3-blade ground-adjustable prop (Rusty's old one) because it is smoother, not because it is faster. It has allowed me to increase rpm for takeoff and climb. Click here for comparison charts.
The cooling worries have been put to rest by simply climbing at a higher speed! Duh! It will overheat on a 90+ day at full throttle if you try to climb at 120 mph or less, but if you let it run out to 140 mph, the cooling will stabilize and the climb rate is as good or better!
If you would like more info, let me know.