CHT part #5, flight data from Zenith 750


Here is some very detailed flight data from  2,850cc Zenith builder/flyer Jeff Cochran. It is a very good and useful piece of data collection, with many fine points included. I took more than an hour to examine the charts in detail. Jeff’s accompanying letter had a lot of good flyer feedback in it also. He is straight forward and methodical in his evaluation to fine tune his specific installation. I share with builders some larger perspective to put this data in a context where you may find it easier to appreciate.

Jeff and his lovely wife at CC#16. They have attended many colleges. Jeff ran his engine at CC#19, and will likely flying it back to CC#27 for it’s public debut in front of fellow builders who fully understand the achievement of completing and flying your own plane.


For a little background on Jeff and his plane, read the story by clicking on this link:

New “Zenvair-750″, Jeff Cochran, 2,850cc engine, N750ZV


One of the things that came to mind when looking at Jeff’s data was the early work that Mark Langford did in data recording in his KR-2S. (Mark was the first pilot to be awarded The Cherry Grove Trophy in 2008) Eight years ago Mark was one of the first guys to publish this kind of information from his Grand Rapids unit. It was read by many builders, and was a unique resource and sparked a lot of discussion, and also squashed a lot of pet theories among the internet armchair opinion crowd. Let me use the photo below to illustrate an interesting distinction between the data sets:

Above is Mark Langford’s plane with the cowl off in a photo from Corvair College #16. The airframe and the engine installation were unique in many ways. The plane was built as a personal expression of his creativity. Several other KR’s followed Mark’s build and utilized ideas that worked on his plane. Since this segment is focused on CHT, look at Mark’s cooling, a twin ‘plenum’ style system that worked well in his plane. His cowling was one he made a mold for, he used a rear starter and belt driven rear alternator, along with a remote cooler. These ideas served him for more than 1,000 flight hours in his KR, however some of these ideas would have limited applicability on other airframes. The 5th bearing on this plane is the same design I am using on our Wagabond, but almost all of the other subsystems on the Wagabond are common to our standard 601/750 installation. The Kr is a small fast aircraft that operates in a different flight envelope.


What makes Jeff’s data unique to me is that it is all gathered around off the shelf parts on a very popular airframe. KR airframes are highly individual. The have a choice of airfoils, wing areas and spans, different landing gear, fuselage lengths and widths,  and several canopy styles. By comparison, no such variation exists with Zeniths. What one builder learns can be directly applied to another’s aircraft with predictable result. Virtually all Corvair/750 builders utilize standard parts from our catalog, and assemble them according to our installation manual. Additionally, a 750 is the largest and slowest climbing aircraft Corvairs are commonly used on. One can be reasonably sure that anything that works in a Corvair/750 aircraft cooling/cowling system will also work on any faster smaller Corvair powered airframe, whereas the reverse is not frequently true.

In Jeff’s letter he references comparing notes with Gary Burdett. If you have not seen it, we have pictures on this site and his story is at this link: Zenith 750 Flying on Corvair Power, Gary Burdett, Illinois . Because their two airframes and engine configurations are very close to each other, They can utilize shared information to fine tune each of their planes. This goes further than just having a cowling in common. Details like both aircraft having a gold oil filter housing means that data like oil temp is taken at the same spot on both engines, giving very direct comparisons.

This effect is true for all Corvair/Zenith combinations to a degree that is not possible with individualized aircraft like KRs and to a large extent, Pietenpols. Both of those airframes have active and well run internet groups. Zenith runs it own gigantic webgroup for all of its builders. To give builders working with the Zenith/Corvair combination a specific spot where they could directly exchange data and notes, we set up a specific discussion board just for them. You can read about it by clicking on this link: ‘Zenvair’ Information board formed . The quality of discussion there is very high for several reasons. It is an invitation only group and it is very effectively organized and moderated By Phil Maxson. You can read about phil at this link: Guest writer: Phil Maxson, flying a 3100cc Corvair in his 601XL. Jeff, Gary, Phil and other ‘Zenvair’ builders can directly work with each other in a setting where everyone is a serious builder.

The two links below are the Data that Jeff refers to in his letter. Interesting to have independent confirmation and data to say that the alternator location doesn’t make much of a difference in cooling. We sell the front alternator (group 2900) and Dan sells the rear alternator (Group 2950). For many years people speculated that moving the alternator to the back would cause a huge reduction in temps. Both Dan and I told people this wasn’t likely because non-plenum cooling systems with round inlets are very good about sharing all the incoming air no mater which hole it arrives through. Jeff’s numbers confirm this and show the limitations of ‘eyeball & theory’ vs accurate back to back testing.

Many people who have never met me picture me as an opinionated zealot advocating some type of ‘my way or the highway’ mentality, unable to change perspectives. While I do have principles that I will not compromise on, 25 years of working on planes has given me the perspective to understand what is an issue of principle and what is just a matter of preference.

Many closed minded people act like zealots simply because they don’t have the experience to differentiate between these two. Picture the guy who frequently says “That will never work”; He is proven wrong by the first guy who makes a trip around the pattern with the idea. Conversely, when a guy says ” might work, but I prefer not to do it that way because….” he is speaking from experience. On matters of preference, I am open minded. I have a 5th bearing design and sell front alternators, but our production engines feature Dan bearings and mostly rear alternators. I assembled both Jeff’s and Gary’s engines. They are very similar 2,850s yet one has a Dan bearing and one has a Roy bearing.  These are all matters of preference between proven parts. I am if favor of builders making educated choices. The operative word ‘educated’ starts with real data like Jeff is presenting here. -ww.


Jeff’s data charts:


CochranPDFGraph1013 Graph Link


CochranXL101813 XL worksheet link


Jeff’s Letter:


“William, Since you are doing all of the CHT and cooling articles, I thought I might update you on my progress.

I have dropped the bottom of the cowl so that I have almost 4″ between the bottom edge of the firewall and the bend in the cowl bottom.  By my calculations this gives me a 2.4 to 1 ratio.  I still want to up this a little.  This is very close to the set-up Gary Burdett is running.  We should be almost exactly alike now with one or two exceptions.  I went ahead and ordered and installed the rear alternator kit from Dan.  The front bracket is still in place just in case I need to go back.  I am also flying without the leading edge slats.  My inlets are 5″ and still raw cut edges with no ring inlets.

Sensor set-up has been changed.  For a while I ran a thermocouple in each of the thermowells like the GM thermistors and a 10mm ring on the thermocouple bolt. I also had a 14mm ring on the corresponding plugs.  So three sensors on cylinders 1 and 6.  The plug was always the highest, the bolt the lowest with the thermowell location in the middle.  I discontinued the two bolt locations and moved those sensors to plugs 2 and 3.  Somewhere I had heard that cylinder 3 was always the hottest, but my data really does not support that as far as the plugs are concerned.


My Dynon D180 saves data on almost every possible parameter you can attach a sensor for.  I download the file after every test flight.  The first page of the attached workbook is the total raw data. On the second page I delete all of the data that is not really recorded (the Dynon seems to make up data when no sensors are attached).  Then on the short version, I delete everything I am not interested in at this time.  I chart the CHT’s and since the alternator move the electrical data. I have attached the excel workbook file.  But just in case you really are the computer troglodyte you claim to be (which I really doubt) I have converted the CHT chart to .pdf.  

The alternator move as you have often said did not seem to make much difference in cooling. Logically that was so hard to believe I just had to prove it to myself,  You probably have realized by now that some of us are hardheaded that way.  Cylinder 6 is always much cooler that cylinder 1 so I tend to concentrate on cylinder 1 numbers.  Since the 380 degree number has been posted by you and I have seen it on the car sites also, I set my goal of try to keep the temp measured in the cylinder 1 thermowell (where the car was measured) as my normal max goal.  My current set-up has been achieving that limit.  I still plan to smooth and ring the inlet some time in the future.


I’m still planning to fly to Barnwell (weather permitting).  Either way, see you there. –Jeff”

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