Sonex-A purchase - What to look for

[Hambone]

Thanks to all for the interesting and useful information!

In retrospect, I have only myself to blame for this situation. Knowing now what I didn’t know then (ain’t hindsight grand?), I should have insisted on a leakdown check. The seller apparently didn’t do one when he performed the conditional inspection back in October, claiming that the engine wouldn’t generate full power if there was a compression problem.

Live and learn, they say…

[flyguy0609]

Pilots never have a bad day, we collect war stories to tell the unwary.

[daleandee]

Still something has to be done with those engines that are cooking their heads and/or are overheating their valves. So I wonder what the Corvair community has done to get on top of this?

Kai,

I can't speak to every Corvair conversion but on the Sonex airframes overheating is not a problem. In fact too much cooling could be sited as a concern.

When I flew to my first Corvair college I asked Dan Weseman why my Corvair ran so cool. He asked what problem I was trying to solve! He used to fly aerobatics in his "Wicked Cleanex" which was a 3.1 Corvair conversion. Reported the highest CHT he ever recorded was 375ºF. Early on during my first few hours in the summer of 2013 I recorded 365ºF. These days I see 225-250ºF during cruise. My sensors are under the two rear plugs so the center cylinders might run a bit higher but all the plugs look amazingly the same as far as the mixture & burn goes. I can even see the proper burn line on the ground strap to show they are in the correct heat range.

Chris Smith as well as a few others I've known with Corvair powered Sonex aircraft have all had very cool temps. In a slower draggy plane (Zenith 701 for instance) they make a cowling with larger openings and a cooling lip on the bottom of the cowling. Corvairs are verboten on Sonex airframes because they are heavy ... but the extra weight must be in more cooling fins on the heads. 8~)

Keep in mind that the VW was never designed to offer the kind of power being asked of it. The turbo Corvairs were rated at 180 HP @ 5600 RPM. Now that's not constant power but asking an upgraded conversion engine to put out 90 HP @ 2800 RPM is highly derating the engine.

Sorry to give such a long answer to how the Corvair guys deal with overheating. It's basically not an issue.

Link to EAA Corvair college (yes I'm in there): https://www.youtube.com/watch?v=wvXAX0C2q5c

Dale
3.0 Corvair/Tailwheel

[Kai]

Dale,

Yes- thanks: I think that is some of the essence. The 2170 cc VW-1 Is very highly loaded when asked to produced 70 plus HP at 3000 rpm. The Corvair on the contrary is derated when asked to produce slightly more at 2800 rpm.

As an aside to your verboten: once when we still had Jeremy among us, I mentioned that a Corvair would be of interest in my build. As you say, he strongly adviced against it. When pressed for why, he believed that a sufficiently strong engine mount could very well be welded up to take the strain of this somewhat heavier power unit. His concern were the angular brackets (this was of course for the Legacy) in the fuselage front designed to accept the mount: they had never been calculated/tested for more that 200 lbs.

[Kai]

Thanks to all for the interesting and useful information!

In retrospect, I have only myself to blame for this situation. Knowing now what I didn’t know then (ain’t hindsight grand?), I should have insisted on a leakdown check. The seller apparently didn’t do one when he performed the conditional inspection back in October, claiming that the engine wouldn’t generate full power if there was a compression problem.

Live and learn, they say…

Now I am am confused- what happened to the small dia fuel line in an ear?

You take a 1/4 in clear (or blue) soft flex fuel line of some length. Then you take off the cowling. Then ignition OFF and fuel OFF and throttle to full. Take the dipstick out of its tubing/crankcase hole and stick in the fuel line. Stick the other end in your ear. Tell the owner to pull the prop over a few compression strokes.

If you hear a hissing sound in your ear you know that at least one of the piston rings is leaking. The engine will require an overhaul.

The same procedure with the flex line in the exhaust outlet. If it hisses, one/more of your exhaust valves is/are leaking.

Then the same with the hose in the carby throat: hissing and one/more of you inlet valves is/are leaking.

Of course there are nuances here-especially the piston rings: but surely everybody does at least that before the purchase of a second hand engine??

[Scott Todd]

"...they had never been calculated/tested for more that 200 lbs." Seriously? I'm an Aerospace Engineer with some background in Structures. That's a pretty lame excuse. People put their 180 lb VW out there and then add the Turbo, big exhaust, cabin heat, battery, etc. Then its rated for 6g's. So move your battery (it will have to go back anyways) and de-rate it to 5g's without any calculations. Who among us are doing the kind of aerobatics to 5g's anyways? Most of us would pass out before that if not used to it.

FWIW, I do a compression check before every flight. Just pulling it thru you can tell the difference between 50 and 70 pounds. I teach this to my students and they are all amazed at how different it feels once you know to look for it.

[flyguy0609]

His concern were the angular brackets (this was of course for the Legacy) in the fuselage front designed to accept the mount: they had never been calculated/tested for more that 200 lbs.

Well not exactly. A couple of yeas ago I cam across a table containing weights and engines and landing gear on a very old thread here. So I downloaded the data. The average empty weight of a nose roller is roughly 25 lbs. heavier than the tail dragers. The heaviest WW Corvair conversion weighs in at 225 lbs. I consider that a very fortuitous coincidence. In my opinion. the reliability, low cost of maintenance and purchase as well as the extra 5 knots from exchanging the nose wheel for a tail wheel, makes the Corvair engine a good choice.

[Kai]

In my opinion. the reliability, low cost of maintenance and purchase as well as the extra 5 knots from exchanging the nose wheel for a tail wheel, makes the Corvair engine a good choice.

I can offer no arguments- I only repeat what Jeremy verbally told me during an Osh convention.

But there is an aside to this-as an EXP builder you can hang on any engine you want- in a lot of countries as long as you can document to the national CAA that the airframe designer will indeed accept the deviation from original plans (which at that time included only Aerovee and Jabiru). Since no such data for the swap to Corvair were at that time available, they would have to be produced to satisfy the CAA- and Sonex was happy (this was back in 2004, as far as I recall) to do so. They suggested a sum of USD 20K was transferred to their account for an initial feasability study.

Count your blessings living in the US!

[Hambone]

Thanks to all for the interesting and useful information!

In retrospect, I have only myself to blame for this situation. Knowing now what I didn’t know then (ain’t hindsight grand?), I should have insisted on a leakdown check. The seller apparently didn’t do one when he performed the conditional inspection back in October, claiming that the engine wouldn’t generate full power if there was a compression problem.

Live and learn, they say…

Now I am am confused- what happened to the small dia fuel line in an ear?

You take a 1/4 in clear (or blue) soft flex fuel line of some length. Then you take off the cowling. Then ignition OFF and fuel OFF and throttle to full. Take the dipstick out of its tubing/crankcase hole and stick in the fuel line. Stick the other end in your ear. Tell the owner to pull the prop over a few compression strokes.

If you hear a hissing sound in your ear you know that at least one of the piston rings is leaking. The engine will require an overhaul.

The same procedure with the flex line in the exhaust outlet. If it hisses, one/more of your exhaust valves is/are leaking.

Then the same with the hose in the carby throat: hissing and one/more of you inlet valves is/are leaking.

Of course there are nuances here-especially the piston rings: but surely everybody does at least that before the purchase of a second hand engine??

Great information!

I would certainly never buy any kind of piston engine again without including a comprehensive leakdown check, no matter what the seller tells me.

[Hambone]

New heads on order from Sonex. A friend who is an expert VW air-cooled racing specialist will be helping me with the head installation.

I hope that new heads, along with tightening up of the baffling, will be all it takes to get flying.

Edit: with now-daily >6,000 ft density altitudes at KGOO, I’ll need all 80 hp!