Thacher CX-5 and SPA Cougar, 2 seaters


Following EAA magazine coverage of the thacher CX-5 prototype flying, two people wrote me today asking about the possibility of powering one with a Corvair. It is a understandable sentiment. Mr. Thacher is universally regarded as a fine gentleman, and his CX-4 single seater is a very popular design. Many people were waiting to see how the two seat -5 would turn out, and some of these people are also Corvair fans. This said, I would like to take a minute to explain why being fans of both doesn’t mean they would make a good marriage.

Above, Mr. Thacher and his new design, the CX-5. I have met him in person a number of times, and I will attest that he is a very knowledgeable but modest guy, and 100% airplane builder. Several years ago Grace and I heard that he was working on a two seater, and  supplied him with a complete set of information on the Corvair. Although I thought there was small chance he would design around the Corvair, we provided them as a small gesture to simply say that we liked his work in experimental aviation.

The CX-4 and CX-5 are both designed around large VW engines. By all accounts, they fly very well, and their efficient airframes do as well on these engines as any other similar design developed for the VW. This said, I am going to say that it would probably take a lot of work, and would compromise the designer’s intent, to put a Corvair on a CX-5.

There is a lot to installing an engine besides weight and balance. There are mounting and structural issues, fuel supply and a host of small details. Even a very skilled builder is challenged by being the first guy to put a Corvair in the place of say, an O-200. But going the other way from VW to Corvair is far harder, if you are the first guy doing it.

There are four airframes that were originally intended for VW power that work very well with Corvairs they are; ….(you can click on the color link to read about one of each of them.)

1) The ” Cleanex ” (Corvair powered Sonex)

New 3,000 cc Cleanex, Dale Williams, SC

2) KR-2 and KR-2S:

KR-2S at 700 Hours – Joe Horton

3) The Dragonfly,

Corvair powered Dragonfly, Charlie Johnson, aka ‘One Sky Dog’

4) VP-2

Two More Flying Planes: Merlin and VP-2


Notably, each of these planes is a side by side configuration. There are a number of complex factors that go into this, and they vary with each of the designs. On the other side of the coin, there have also been a number of tandem seat planes that were originally intended to VW power that later flew on a Corvair, that didn’t work out nearly as well. The first one that comes to mind is the Sonerai II. At least four of these flew on a Corvair, but they were a poor match. I have not seen the CX-5 in person, but I suspect that it isn’t a good candidate for a Corvair, simply from a design perspective.

The second part of the equation is the builder perspective. People find it hard enough to get planes done without adding a very challenging engine installation, one that the designer may object to. Considering these factors, it is my best advice to people who really like the CX-5 to plan on following the designers advice and put a large VW on it.


What if you are a committed Corvair guy, but you are looking for a two seat tandem metal kit plane? Then there is something on the distant horizon that you may wish to keep an eye on. It is called a Cougar.

By now, almost everyone has heard of Dan Weseman’s design, the “Panther.” It is a single seater with a 3000 cc Corvair in the prototype. Things are going very smoothly with the introduction of the Panther: it is actually on the cover of both Kitplanes and The Experimenter this month. The same way the CX-4 lead to the -5, Dan is going to follow the Panther with the Cougar, a two seat plane in the same design family.

The Panther was designed 100% on CAD, so the drawings and CNC manufacture were integrated from the start. This also allows Dan to easily make the drawings below. The planes have many design points in common such as the fuselage cross section and the tail cone assembly, but they do have different wing cords and other detail so that each design can be optimal.

The LSA version of the Cougar is slated to use engines from 100-130 HP. Dan tells me that a 3000 cc Corvair will be an option on the Cougar. People who have seen the expansion of Dan’s capability have no doubt that the plane will follow the Panther.

For anyone who likes Mr. Thacher’s CX-5, my best advice is to stick with the intended VW power plant. For anyone who likes Corvairs, but is looking for an appropriate airframe in the same category, follow along on Dan and Rachel’s website and watch the development.

You can read about it by clicking on this link:




One Reply to “Thacher CX-5 and SPA Cougar, 2 seaters”

  1. William,
    I rummaged through my junk draw and finally found an old pair of cardboard 3D glasses. The image of the Panther and Cougar overlays just popped off the screen.

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