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Test Data Analysis and corrective measures: July 18, 2002
Water manometers work by measuring how high a
column of water is pushed up
On the other hand, I'm thinking about what the easiest and quickest thing I can do to improve cooling without taking the RVotter out of service for a long time. Even a simple flap requires significant fabrication and some means of actuating it from the cockpit. And cockpit space is at a very high premium on an airplane like the -4.
My priorities had me zeroing in on the poor pressure recovery on the front of the right side radiator. Correcting this could be as simple as reshaping or slightly enlarging the cooling inlet.
So, what's the down side of going the simple way out? My fixed cooling outlet was more or less optimized for minimum drag at cruise flight conditions. This meant that it limits cooling at low speeds such as in a climb. On a hot 95 degree day I have to limit full throttle climb to about 1 minute. This sounds like a severe limitation unless you look at the full picture. At full throttle rate of climb, this means I am more than 2000 feet agl before coolant temps reach my conservative red line of 205 deg. F. At this point I reduce power to keep the temps from going higher but I'm still climbing at around 1000 fpm. This is better performance than most factory built aircraft can give at full throttle. At this rate of climb the plane is soon in the cool air at cruise altitude and the coolant temperatures are coming down fast. Bottom line is that while the airplane is not performing at it's best in all phases of flight, it doesn't have much effect on flying the mission.
Having rationalized my way out of fabricating a cowl flap system, I began hacking away at the right side hole in the fiberglass cowl. I like to document changes with before and after pictures but I got carried away and butchered the cowl before taking any pix.
July 14 2002 turned out to be a perfect day for cooling tests. The temperature was around 94 degrees with matching humidity. I could see that the cooling was improved even before takeoff because the engine took longer than usual to come up to minimum temp for takeoff. My cooling system has no thermostat so the radiators start cooling right away.
When the oil temperature reached 135 degrees I firewalled the throttle and was airborne in a matter of seconds and climbing at 2050 fpm. At 3000 feet the coolant temperature was only at 200 deg F and I was ecstatic with the improvement. The air was already cooling off a bit at this altitude so I reduced power to cruise and descended down into the hot sticky air at 1000 feet. When the temperatures stabilized, I measured an 8 to 10 degree reduction in coolant temp (compared to temp prior to cooling inlet change) and a 2 degree reduction in oil temp.
The next question in my mind was how much drag the change had added. I'm sure that there is some added drag but it was not measurable. Both cruise speed at 7 GPH and top speed at full throttle was unchanged as close as I could tell.
It is very rare to be able to get this much improvement from such a small change. The key to its success was identifying the deficiency in the system. It was Paul's emphasis on the importance of pressure tests that accurately pointed to where the improvements could be made so he deserves full credit for dragging me kicking and screaming into doing them.
I'll probably have to add Paul's beloved cowl flap too. But that can wait till this winter when the cooling improvements will result in engine temperatures never getting up to normal operating levels! A cowl flap will fix that problem too.
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