The following is a brief look at how human nature intersects experience with aircraft cooling systems. The Corvair has outstanding cooling, and this has been documented countless times, but the poor performance of a handful of badly thought out planes colors the perception of some people. I can present logic, engineering, experience and specific data. In cases of popular aircraft like Pietenpols and Zeniths, we have literally dozens of examples of how the Corvair, as we teach people to build it, has an excellent cooling track record. Most builders will listen and learn. A handful will not, and they will spend a lot of time ignoring all the success stories and focus on the few that willingly chose not to follow the models we painstakingly developed as good proven examples.
Above is the Panther engine, a solid 120hp power plant. Because the corvair in the car made up to 180HP from 164 cid at 5,200 rpm and we are only asking 120HP from 183 cid at 3,200 rpm, the Corvair is essentially “flat-rated” for reliability. The engine performance number that best reflects this is the outstanding cooling of the corvair flight engine. Flight Corvairs operate at a fraction of their 575F CHT limit rating. We have countless aircraft flying that verify the validity of flat rating an auto engine, although we are the only popular engine in the experimental market place that does this. Running an engine at 60% of its rated load is not a new nor clever concept, it is an aircraft engineering principal long used in certified aviation.
At Oshkosh 2011 a LLC was introducing an auto engine conversion that used the same power rating at 6,000 rpm for its tiny 4 cylinder engine as the car did. This was accompanied by a brochure that claimed that this engine “Was the worlds most reliable engine turning a propeller.” (This made me think about the propaganda axiom that people are far more likely to question a simple exaggeration than a giant outright lie.) In spite of the clear reality that an engine with only one example that had exceeded 100 hours could only make such a claim as a joke, plenty of people looked at it, because Unicorns have that special magic about them. Among professionals who live in reality, there is little argument that the PT6A is the most reliable engine to ever turn a prop: http://en.wikipedia.org/wiki/Pratt_%26_Whitney_Canada_PT6
If you study them, you can learn that ‘flat-rating’ is a concept that was central to the PT6 success story. Even in gas turbine engines, de-rating is the understood method of reliability. Only a Unicorn breeder would tell you otherwise.
Above, a real world proven Corvair system, the Wagabond cowl. Note that the air inlet is a simple 4.875″ hole in the cowl. This aircraft has flown at the record gross weight for Corvairs, it has always lived in Florida, it has a very large airframe with plenty on drag to spare, and yet it never ran hot, even with a front alternator and no inlet cooling rings. Why? because Corvairs have excellent cooling. builders can either utilize this success or they can ignore my suggestions. If they chose the latter and it doesn’t work, they rarely see the problem as a people issue. For some reason, a fraction of builders will focus on stories of people who has trouble with one-off ideas rather than looking at all the people who are flying proven ideas without issue.
Here is A good example. Yesterday, I spoke with a person on the phone who has been to colleges, Oshkosh and sun n fun, reads our website (or claims to) and has a slow-moving airplane project. In spite of the fact that he once actually flew in the Wagabond years ago, he told me that he had been speaking with another builder and they were looking at heat “problems” with slow flying planes. Really? Yes, he tells me that he thinks there is some issue. His perspective is not derived from the big picture of what works, it comes from focusing on a single plane that didn’t, and ignoring all the planes that work.
The plane that didn’t was the first highlander to fly on a Corvair. Instead of taking any of the designs that we have proven or any of the systems I know to work, the highlander builder opted to use a cowling from a Jabbaru and a Bing carb that ran lean enough in test runs to melt an egt probe. After ground running it without a cooling baffle box, the highlander was flown once and it was revealed to have $3,000 worth of detonation damage to the engine. Let’s contrast this with the experience of Zenith 750 builder/flyer Jeff Cochran. I spoke to Jeff just yesterday. He is rapidly accumulating time on his aircraft and now has 28 hours on it. He is thinking about flying it the 350 miles to CC#24 in a week. This is possible because his firewall forward is exactly how we teach people to build them, with our cowl and a good old MA3 aircraft carb. Jeff didn’t think it would be an issue to fly a couple of hours off a day because his aircraft works great and has moderate 345F CHT’s and the ideal 220F oil pre-cooler. Jeffs only question was if the weather was going to cooperate.
One man has a completely reliable machine, the other has a very expensive self-inflicted lesson on what will not work. A 750 is heavier, has more drag and clibs slower, so it is actually harder to cool than a Highlander by a long shot. Yet the 750 works well, and the one off highlander didn’t work at all. Jeff’s 750 is typical of the experience of our builders. I can point to people who have flown hundreds of hours in very high local temps, I can point out planes like the turbo Skycoupe, the Wagabond, etc, but some people are still going to perceive that cooling a Corvair is an unknown mystery. I have no explanation for this human issue. One thing is for sure: any one who wants to take advantage of what we have shown to work will be rewarded with success. Thermodynamics and chemistry don’t play favorites. If the set up is proven to work, it will work with the same reliability for you if you make a clone of it. The corollary is that thermo and chem don’t hate the guy who’s engine gets BBQ’ed from a Jabbru cowl and a german motorcycle carb, they are just as willing to cook the engine of anyone one who chooses to emulate the failed design. People may have distorted perceptions of what worked and what didn’t, but the laws of nature and the physical world never change in any of these experiments. Use our examples and have them work for you, and they will always work. Think you’re going to prove physics wrong on round two? Good luck with that. Most people would guess that a guy who has lots of trouble with his own path, would be more receptive to input the next time, but 22 years of working with homebuilders has conclusively shown me that the handfull of builders that have issues with their own work almost never switch over to proven systems. They are more likely to keep trying to find a way they were right all along.
Above is the nosebowl from our Zenith 601XL. We started flying this in 2004, and it has worked on dozens of Zeniths and other airframes. As seen in the photo, this is how the inlets looked in 2006. Before this they looked just like the Wagabond inlets, just slightly smaller. The Wagabond cowl was done in 2005, and we never had any cooling issues. The inlet rings above were a result of seeing Dan Weseman put them on his wicked Cleanex. They function as an improvement, but they also make the cowl look a lot better. Note that I put them in a lot of planes in the last 7 years, but we never went back and put them in the biggest, heaviest plane in our fleet. Why? because the Wagabond didn’t have a need for them to function. The Wagabond, just like Jeff Cochran’s 750 has normal cooling even in a slow climb. (note that both aircraft have front alternators.) The cooling on corvairs isn’t marginal, it is very good and we have long proven this.
Above, How the inlet rings are made. The part in my hand is a PVC coupler for joining a 4″ iron pipe to a 4″ schedule 40 pipe. The ID of the big end is just below 5″ I trim these down in the lathe to thin them out and produce an inlet ring that gets bonded into the nose bowl. Even though the 601 and Wagabond cowls shown are original one piece models, we have ways of doing this on late-model splitting nose bowls. If you look at our main page, you can see photos of many planes with inlet rings and split nose bowls like Lynn Dingfelders 601, Louis Kantors, and Andy Elliott’s. I am going to convert this nose bowl to a two piece model after I install the rings and I will post pictures.
Above is a 4″ PVC coupler, one for schedule 40 to schedule 40 pipe. It has an ID of 4.75″ but I open it up to 4.875 or so. These inlet rings are generally 1″ deep and have a slight taper on the engine side. This is the size that I use on all aircraft other than STOL planes. The one above is for things like the Wagabond or a 750. notably Jeff Cochran’s 750 is now flying with the smaller size, and they work fine.
The story here is bigger than air inlet holes. The central question is about choosing to use proven information or needing to go a different path. There have always been a small minority of builders who for human reasons, want to “show people” or prove me wrong somehow. Little I say will deter these people from their path, even repeated lack of success. These are a small number of people who are more attached to proving something I am promoting as wrong then they are attached to being successful themselves. These people are not going to point out how well Jeffs 750 is working. If there is no issue, then there is no justification for their great effort, expense and time. The great majority of people only owe their allegiance to what has proven to work. If you plan on going flying with as little drama as possible, chose a path that has long proven to work.-ww