OVERHEATING? CONSIDER A THERMOSTAT By Donald B. Wall, EAA 218017 Winter Park, Florida
Overheating problems sometimes happen at unexpected times. After about 175 hours on my normally aspirated O-320 Glasair RG, I have experienced at least four times when I couldn't restart after a recent shutdown. Each restart could perhaps be called a "hot start." The fuel system is considered conventional. My Glasair has an electric primer system which takes its source from the output of the electrical fuel pump. The electrical boost pump is immediately upstream of the mechanical pump and just downstream of the gascolator. All three are blast tube cooled and the plumbing is fireshielded. While flying, the cylinder head temperature runs about 350 degrees and the oil temperature about 180 degrees. I have never had any inflight heating problems, only when I shut down, say after about 15-30 minutes. Why does the fuel not get to the carburetor after a shut down? I suspect a fuel vapor lock. Closely cowled fiberglass aircraft appear to contain engine heat longer than metal cowled aircraft, unless a vent system is provided. My electric primer does not work when this happens. My boost pump does nothing. No fuel drains from the carburetor. My accelerator pump in the carburetor is dry. I can't flood the engine. Some of the solutions I considered were installing either electric fans or pilot operated doors. I ultimately decided to provide a vent or louver system to the aft portion of the upper cowl which would operate independently and only on thermal demand, this demand being a compartment temperature in excess of 110 degrees. The temperature choice of 110 degrees appears nominal, at least for Florida temperatures. As you may know, vapor pressure of aviation fuel is about 7 psi at 100 degrees (water has a vapor pressure just under one psi at 100 degrees and needs to be heated to about 175 degrees to reach a vapor pressure of 7 psi). Fuel vapor lock then can occur
PHOTO 1 - Inside view of engine cowling showing "power pill" and louver door.
after shut down if the fuel lines become heat soaked. Any hot fuel tends to stay hot due to the fireshield covering. I selected an automotive thermostat rated at 110 degrees as my sensor and actuator. The thermostat, manufactured by Robertshaw Controls, Inc. of Knoxville, TN, is filled with a solid which changes to a liquid at the desired temperature. This phase change from solid to liquid causes a sudden expansion, pushing a piston to actuate the valve. I removed the extraneous parts surrounding the thermostat and kept the "power pill", as it is called. My "pill" gave a stroke or push of just over a quarter inch when heated. Testing can be performed using a heat gun or hair dryer. Photo 1 shows the "pill" pushing on a U-bar lever. The position of the fulcrum was selected to cause a mechanical advantage of about three times. This selection was needed for the movement of the louver control horn. When the "pill" expands and pushes the U-channel bar, the cable pulls the control horn mounted on the spring loaded louver door. The door is designed to open
about 60 degrees giving an effective opening of about nine square inches (there are six louver slots, each 1/2 by 3 inches). The effect of these louver slots or vents is to create a chimney type draft to occur in the engine compartment. Heated air rises rapidly and exhausts quickly. The incoming makeup air enters principally through the nose gear door opening and cooling air exit. To help transfer the heat from forward of the baffling to the louvers, I installed a gravity/pressure operated door in the engine baffling. Photo 2 shows this door installed on the starboard engine baffling. When in flight, this door closes due to air pressure. After shutdown, an eccentric weight or rat trap spring opens the door allowing air to convect the heat to the aft compartment. In this manner, the engine forward of the rear baffling is permitted to cool more uniformly and faster. There is less time for the fuel lines to become heated. Flight tests indicate no changes from pre-louver flight temperatures. The louver door is always closed and, of SPORT AVIATION 43
PHOTO 2 - View of the baffling door.
course, is flush with the cowling. Sometimes the "pill" will actuate the louver when taxiing in from a landing. Photo 3 is a view of the upper portion of the engine cowl. Note the location relative to the oil access door. The cap
PHOTO 3 - Location of vents. Note the fuel cap for the fuselage tank.
for the reserve header fuel tank is shown just left of the six louver vents. I have started my engine everytime after shutdown since I installed the vent system. I have restarted after ten minutes, 20 minutes, and even 45 minutes and
have not experienced any problems with restarts. I feel confident that if I have not cured the after shutdown vapor lock problem, I certainly minimized the time the fuel system is locked up.
About the time EAA was being formed by Paul Poberezny, he was also building up a little sportplane out of the remains of the legendary Howard Pete racer. The result was the Little Audrey. Recently, Bill Turner bought Little Audrey with the intention of extracting the original tubular fuselage frame and tail surface to recreate the Howard Pete. He was, however, loath to discard the work done by Paul in the early '50s, so a Soloman-like decision was made to cut the fuselage frame in half. Bill is using the aft portion and the tail to recreate Pete, and Riverside, CA's EAA Chapter 1 is using the front portion to recreate Little Audrey . .. for display in the EAA Air Adventure Museum. This photo taken in late May shows the status of Little Audrey at the time. The airplane had cut down Luscombe wings, which are on hand, so the restoration is even further along than it appears here. 44 JULY 1990
