Corvair Cooling

Friends,

A number of people have raised questions about Corvair Cooling on the Internet. A lot of theory can be debated, but you can learn a lot more from a good example and a positive discussion focused on the details of installation the way we have done them. The photo below was taken in 2005, just east of our old hangar in Edgewater. In the background is the Atlantic ocean. The plane is N707SV, the Wagabond built as a joint project by members of our old Hangar Gang.

The Wagabond we built was based on a PA-22 airframe. It has 4″ longer gear legs than most Pacer conversions. This airframe is the same size as the four place, 150hp certified PA-22. As far as Corvair powered planes go, it is huge. Yet it is flown efficiently on a direct drive 100 hp Corvair. The plane weighs 804 pounds empty, because we were very careful not to put things in it like an interior beyond seat cushions. It has a full electrical system including a starter. The plane isn’t a speed demon, but combined with the Corvair it did something few experimentals can do: As a test we actually flew the plane with an 820 pound Payload on board. Yes, it has flown more than its own empty weight as a payload.

Above, the Wagabond sits on the flightline at Sun N Fun 2007, the second year it was there. The plane is a well-known flyer, as many people saw it at shows and at our old hangar. YouTube films of it flying have thousands of views, and it was seen in our DVDs.  It has been publicly demonstrated to work well over the past 7 years. It has had a long and trouble-free existence, despite being produced on a $9,000 budget for both the engine and airframe. No money was spent on electronics, flashy paint, or an interior. The money all went to a solid airframe and a sound engine. (In the background is Dan Weseman’s Wicked Cleanex.)

The Wagabond has excellent cooling. Florida is known to be a warm place, but the plane never has had any kind of cooling issue. When discussing something like cooling, you can often appreciate a point best by examining an extreme example. The size and lifting capability of this plane, combined with a slow climbing speed should make this plane the most difficult Corvair to cool, yet it has no such issues, it just works. The plane uses our standard components and a standard aircraft approach to cooling design.  All the concepts are nothing new, they appear on countless certified planes like Cessna-150s and 172s. There could be no more proven cooling concept in general aviation. Out of the 60,000 150s, 152s and 172s built you never hear about one of them frying an engine from a cooling issue, despite the fact that they are most often flown by students, and almost none of them have CHT gauges. They work because they have generous inlet areas, well thought out exit areas, their timing is set correctly, they have a regular aircraft carb that doesn’t lean out, and they have a large cooling plenum that covers the whole top of the engine, sharing air to whichever side needs it. The Wagabond also works because it shares all these characteristics.

The above photo shows that the plane uses one of our regular Nosebowls, with the air inlets trimmed to 4-7/8″ diameter. Notice that the plane not only has a front mounted alternator, it actually has an oversized pulley mounted on it. Although this inlet lets in slightly less air than the other side, it doesn’t matter because the air is freely shared by both sides of the engine.

Above is a photo of the bottom of the firewall, looking in from the side of the cowl. Note the rolled piece of sheet metal that smooths out the airflow exiting the cowl at the base of the firewall. Notice that the exit area is twice as large as the inlet. The carb on the plane is a Stromberg NAS-3, a regular aircraft carb. These are set to go slightly rich at full power, a very big part of preventing detonation on a full power take off and climb out. Motorcycle carbs, particularly CV ones that are subjected to even slight amounts of ram air pressure, almost always go lean, making the engine detonate. The number one reason people don’t use aircraft carbs is cost. This one in the picture was bought out of a flymart and set up with old bits and pieces: Total cost, about $200. Even a guy buying an overhauled one outright from Russ Romey at D&G will only spend $800. Pricey, but a whole lot less than damaging your engine with detonation. When building a plane, buying a great carb comes before glass cockpit stuff, avionics or paint jobs. The airbox on the bottom of the carb is a standard aircraft one. Carb heat air that is not used exits the bottom of the box and immediately heads out of the cowl rather than heating anything up. 

The above photo shows the underside of the cowl. Notice that the plane has a smooth ramp for the air to flow out, and it has a very crucial fixed cowl flap, the lip on the leading edge of the exit opening. The sheet metal roll from the photo above is visible in this photo.  The 5/8″ tube hanging down is the crankcase vent.

To give a good idea of how big this plane is, note the crankshaft centerline is 62″ above the ground in the three-point attitude. The top of the cabin is 80″ from the ground. This plane’s best climb speed is only 65 mph, but it can climb at this speed, at full weight, at full power, on a 100F day without overheating, for any length of time.

The above photo shows an overview of the cooling baffles. We have dozens of Corvairs flying with this same arrangement. It works, period. In the back of the photo is the Distributor. If I wanted to make a Corvair overheat quickly, I would just set the timing by ear to the smoothest running setting. This sounds cool, but it will cause the engine to detonate and overheat on the first full power climb out. Timing set by ear in a Corvair always has too much advance. Builders must use a timing light: They are cheap, there is no excuse, but still 15-20% of builders with running engines have never set the timing on their engines with a light at full static RPM.

If a builder is having an issue, the first question to ask is “What is different from the Wagabond?” a plane which clearly works. In many cases the plane that isn’t working is trying a combination of using low-grade car gas, not setting the timing, having a carb that leans out at power, and no cowl exit design. Builders can eliminate these issues by taking advantage of the things we have learned over the years. -ww

About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

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