Flying 2700 cc Zenith 601 XL(B), Alan Uhr


Below is the 601-xl of Alan Uhr, of central Florida.  About a week ago I got on the motorcycle and rode 150 miles down to his airport to pay a house call. I gave Alan and his son a hand installing an E/P distributor to replace the dual points model that they originally used. The trip gave me a chance to see the plane first hand. In our industry, very few companies have the HMIC, (head monkey in charge – me) pay house calls to builders. The goal of most companies is to just sell things, and they place very little value on inspecting progress of builders. Conversely, our goal is to teach builders, and thus personally inspecting their progress in the field is critical to understand their needs and evaluating how well our instruction methods are working.

In a recent conversation, a magazine writer told me that he “Had his finger on the pulse of experimental aviation” because he read “all the important web discussion sites” and went to many airshows. I have been part of experimental aviation for more than 20 years, and I a pretty sure that no one ever completed a plane by sitting at a computer, and the only plane I have ever seen built at a show was when Zenith used to build a plane in a week at Sun n Fun. There are some useful things on the web, but they are most often lost in a sea of disinformation and negativity, and the Zenith factory team is not typical of homebuilders working in their shops.  Reading sites and going to shows gives a very distorted view of experimental aviation. You can find out what people are talking about or buying, but this has little to do with actual building. We don’t call it homeTalking nor do we call it HomeBuying, experimental aviation is HomeBuilding, and to find out what people are building, you have to go to their shops and see it in person, and listen to them. Over the years, I have made several hundred house calls to builders. This is a good measure of my actual contribution to the success of fellow builders, and of my understanding of the state of rank and file homebuilding.

Above, A side view of Alan’s 2700cc corvair. It has flown about 40 hours. Initial progress was slow because Alan mistakenly had a MA-3 from a 145hp-O-290 Lycoming instead of the MA-3 from an O-200 contential, as we specify. The correct model number is a 10-4894.  Alan went direct to D&G in Niles MI for the correct carb, and instantaneously the engine ran vastly better. Many people don’t understand that too large a carb on an engine will often run way too lean, not too rich. A very large venturi and oversized idle circuit tends to have a weak pressure signal to the fuel in the bowl. Before the incorrect carb was diagnosed, Alan was besieged by local “Experts” who tried to talk him into changing every other aspect of his installation, even though none of them had ever seen a Corvair in a plane before. To me this is akin to a veterinarian offering neo-natal advice to a mother. Children and dogs are both mammals, but the detail advice is crucially different on some points. Corvairs and Lycomings are both engines, but the details differ, often in ways that are not good for your heath to ignore. If any one needs advise on a Corvair installation, I am a better resource than your local “Expert.” If your child has fleas, I am of no help, but I would suggest the MD over the Vet.

Above is the rear view of his installation. It has a complete heavy-duty gold oil system and a Niagara cooler. Alan’s aircraft is very much like the other 45 Corvair powered Zeniths that have been flown by our builders. Let me use this to point out a very basic, but critical concept in homebuilding.  Woody Harris and Phil Maxon, who each have nearly the same corvair powered Zenith as Alan, can and have flown their planes literally around the USA without issue. Either Woody or Phil could push their planes out of their hangars today and fly to the opposite coast of our continent at will. We have plenty of other Zenith flyers Like Lynn Dingfelder, Dave Garda and dozens more who can do the same.

Here is the critical point: When Alan was having an issue, His local “experts” ignored the fact that plenty of other people are very successfully operating the same combination of airframe, engine and systems. They all wanted to redesign everything. Conversely, the approach of any actual mechanic is to look at a proven plane like Woody or Phils, and then carefully study what is different about Alan’s, and then only change that to make the plane identical to the proven working ones. Aircraft do not love you, and they do not play favorites, and the have no prejudices nor pet peeves. They are machines, and they owe 100% of their loyalty to Physics, chemistry, metallurgy and aerodynamics. If plane A works perfectly, and plane B does not, all you need to do to fix this is to detect the differences between A and B and then make B just like A. Understand that this strategy isn’t just likely to work, or even almost certainly going to work, It is absolutely going to work

Most people in aviation have flown in a Cessna 172, and know they have a 50 year track record of being a trustworthy aircraft. When a 172 comes in for an annual, all the mechanic is doing is looking to see how that particular 172 is different from the specifications from the type certificate and then he goes about making the plane back to the known accepted standard that has proven to work. It is a skill, but the concept isn’t deeply philosophical. The mechanic doesn’t get creative and offer to reconfigure the fuel system, he just makes it identical to the ones that are known to work. Just because a plane has the word “Experimental” painted on it, doesn’t mean that the rules of reliability go out the window. The plane can’t read the word experimental, but it is smart enough to know that it is only going to respond to the laws of physics, not the psychology of local “Experts.”

Above, a side view of Alan’s plane. He has a very good background in Gyrocopters and is a pretty good craftsman. The aircraft demonstrates very nice workmanship in person. I have known Alan for a number of years, and he is a very good guy, exemplified by his positive attitude.  Although he had some frustration moments in getting is aircraft squared away, he never lost sight of the big picture, that he had persisted to achieve what many people start, but far fewer finish, building a plane with ones own hands. Hats off to Alan Uhr, Corvair builder and pilot.

In Search Of … The Economical Carburetor


There are a lot of smaller research projects that we work on during a year. One of these that I had kicked around in discussions with Dan Weseman was the potential to use an off the shelf, one barrel automotive down draft carb. Although both Dan and I have made a lot of high-end stuff for Corvair conversions, we are both still interested in keeping the engine and it’s applications affordable. Ma-3 and Elison carbs run about $950 overhauled or new. The are great and serve an important purpose. Stombergs are $250 to $550, and are a very good choice for gravity feed planes. Aiming lower, we are moving into the possibilities of  Zenith 268 carbs and motorcycle units. Neither of these offer a significant savings over Strombergs.  I am not a huge fan of carbs without float bowls. Aerocarbs, Revfows and Posas have all flown on Corvairs, and they have their fans, but most builders would like a carb that has a float, or a design like the Elison or Rotec that stops flowing fuel when the engine is shut off.  I have owned and operated all of the above carbs, and I will gladly say that float carbs are much more accurate at metering fuel. 

Below, what we are looking at is a Carter 1 barrel from a 170cid Ford straight six. I bought this carb in a box from Discount auto parts for $149, and paid an additional $10 core charge. This is about as inexpensive as you will ever see for an overhauled carb. The concept here isn’t new. 15 years ago we tested one single barrel Corvair carb and found that it could produce more than 85HP on the right manifold.  This Ford carb is significantly better flow potential than the Rochester HV. If you go to our main website and look at the category of flying planes, Dale Jorgensen’s VP2 has been flying for more than 10 years on a single down draft taken from a Chrysler slant six.

Above is a shot of the manifold on the test stand. The test engine is a 3,000cc, 120 hp engine. It makes a good test here because we want to know if the carb is operating near its flow potential. I am guessing that most of the people who may be interested in a Ford carb would be building a 100hp engine, and if the cab demonstrated that it could produce a solid 90 or 95 horses, this would be plenty. This manifold was made from one of our regular cnc bent manifold tubes. The box under the carb is 1.5″ x 3″ tubing. There is a lot of initial reaction to guess that such a flow pattern would be restrictive, but in practice, it matters a whole lot less than most people suspect. We are building a manifold for a 3,200 rpm engine making .65 HP/cid, which is very different from making a manifold for a 160HP sport bike that needs instantaneous throttle response.

Above, the carb mounted. The small red lever is a lawn mower throttle arm rigged to run the choke, in a plane this would be done by a cable. Looking at the carb, think of these thoughts: It can take fuel pressure; the inlet fitting is 1/8″NPT, not a brass nipple for fish tank tubing; It has a choke; It has an accelerator pump; It has robust construction and has a low parts count. The only downside it that it doesn’t have an easy way to control the mixture in flight.  If you want to fly at 15,000′, this is an issue. If you want to use this on a Pietenpol, it probably isn’t. In the years we had our Piet, I can’t recall ever having the plane over 6,000′ MSL, and I would guess that it spent 95% of it life below 3,000′ AGL. It had a Stromberg, but the mixture was wired in the full rich position. I would lay a serious bet that 100hp Corvair in a Piet with no mixture control would have a much higher service ceiling than a an A-65 or 75 Piet with the most elaborate mixture control.

Above is the carb running on the engine. Initial results are very positive, It starts right up and runs well. It idled very smoothly at 650rpm, and I could probably get that a lot lower with some carb heat and a finer adjustment. I am going to install a different EGT set up and a 5 psi electric pump to do a little more testing. We will have more results and the system on display at Corvair College #23 next month.