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Cooling Tests

Dateline: February 16, 2002 Shady Bend Airport; Real World Solutions Corporate Headquarters. Paul Lamar, Editor of the Aircraft Rotary Engine Newsletter, showed up yesterday with homemade water manometer in hand and proceeded to shroud the RVotter in mass quantities of duct tape and plastic tubing. The tubing was scientifically tipped with the finest quality used sponge from Laura's kitchen. Extensive flight testing yielded much interesting data. Of course, if we tell you we'll have to kill you. Feel brave? Read on. One of the most significant findings was that the right side radiator had very little pressure differential from front to back . We also found that the pressure in the cowl was higher then expected which suggested that cooling could be improved at low speeds and climb with the use of a cowl flap. We sealed the hole around the PSRU behind the prop spinner and actually increased the pressure in the cowl slightly indicating that the spinner is, in effect, pumping a small amount of air out of the cowl. However, we could not detect a big difference in cooling during this portion of the test. Tracy felt the drag was a little lower, but we have not completed sufficient tests to confirm that suspicion. The tests validated that the left side radiator and duct were working well and full dynamic pressure was recovered in front of the heat exchanger. This was 300% higher then the pressure recovery in front of the right side radiator. More rocket science will be conducted later using this highly sophisticated and expensive equipment.

Pressure tap (and sponge) in front of right radiator.	The RWS R & D Facility with certified hot gluer attaching sponge to end of tubes.

Pressure lines taped to side of fuselage from auxiliary pitot tube.	In flight photo of water manometer with secret formula fluid.

Flight test tech contemplating test results.	Entire engineering staff of RWS saying "I'm confused. How did this ever cool in the first place?"

Test Data Analysis and corrective measures: July 18, 2002

Water manometers work by measuring how high a column of water is pushed up
by air pressure. There are two columns for each manometer (with a loop at
the bottom, like 'U') to keep the water from running out of the tube. The
pressure between the two ends of the tube is the total distance between the
water level in the 'U" tube.   The sponge at the end of the tube is to
ensure an average pressure at the location.   If the air was blowing into
the end of the tube without a sponge it would read the dynamic pressure
instead of the local average pressure.   The 'magic fluid' in the tubes was
water and food coloring.

How the data is viewed depends greatly depend on which phase of flight
you want to optimize. Paul immediately zeroed in on the pressure under the cowl and saw the need for a cowl flap. The most significant finding to me was the extremely low differential pressure (1.75" H2O) across the right side radiator. The left side rad had 300% more pressure than the right rad.

The pressure under the cowl was about 2.9" H2O. Paul thought this was much
too high (he wants to see zero) and the most significant finding. The local
pressure under the plane where the cooling outlet is was 1.1" H2O.   To give
some perspective, the total available dynamic pressure at the test airspeed
(120 mph) was 7.61" H2O. The pressure at the left side cooling inlet was
8.11 due to propeller slipstream.

Paul's perspective was that of the aerodynamicist, he wants a no compromise cooling system that will operate at best efficiency in all phases of flight, from maximum power climb at Vx to top speed in level flight with minimum drag. About the only way to accomplish this is to have a cowl flap. This can be anything from a hinged slat on the bottom edge of the cowl to a sophisticated variable nozzle. The cowl flap is the only way to significantly reduce the under cowl pressure without incurring a high drag penalty at cruise speed. Lowering the cowl pressure would improve the performance of all three heat exchangers (both radiators and the oil cooler).

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.

This is the "after" shot of the reshaped inlet (left side of photo). The only change was to the bottom of the inlet which used to be a straight horizontal line. Rounding the bottom on the outboard side added only a couple of square inches of area. This inlet feeds one of the radiators and the oil cooler. This photo also shows the cleaner lines of the bottom cowl where I removed the scoop intended for Lycoming carburetors
Total time spent on inlet change: 1 1/2 hours.

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.