HAVE YOU SEEN that guy next door with a Corvair engine mounted on a stand, turning a propeller with a gearbox taken from a Volkswagen truck? He runs the thing wide open hour after hour. It is so loud that he has to wear earmuffs and sits inside his car parked behind the stand operating the throttle out of the car window with a long bamboo pole.
He just sits there hour after hour, burning gas and churning up the air as if he were going somewhere. What a nut!” Thoughts like these must have run through the minds of my neighbors as they witnessed me testing a gearbox I had for the Corvair engine.
What my wife thought was “something else”. The concussion from the propeller blast rattled the windows, trembled the floors and literally shook the whole house. It soon became apparent that it was going to be “engine or wife” so testing was done while she was out shopping or just out gathering pity from friends.
Even then I expected each test run to be interrupted by the possible intruder of a black and white police car which miraculously never appeared.
After about thirty-five grinding hours of the screaming monster which, by the way, was gobbling up twelve gallons of gasoline per hour, I began to wonder if I were a worse nut than even other people thought, and ended testing convinced that the gearbox modification was a success.
Weighing only 25 pounds, the VW gearbox allowed the 1962 turbocharged Corvair engine to develop 125 hp at its rated 4400 rpm, while turning the propeller at a respectable 2600 rpm. It also gave the propeller three more inches of ground clearance and turned it the “right way”, since the Corvair engine turns opposite of most aircraft engines.
The complete engine/gearbox/prop combination weighed only 272 pounds including the heavy cast iron headers and muffler.
The Corvair engine was originally to be mounted in my flying Cougar, but the thought of giving up even one sweet hour of flight for the installation of a new engine was more than I could bear, and the FAA said the plane would have to be restricted again for another 75 hours. Well then why not build another plane for the engine?
OK, but while I was starting construction on a new design, the word was getting around about the gearbox success. Soon all my spare time was spent answering letters and phone calls and making drawings of how it was done instead of working on the new plane.
The only thing left to do was to print up all the information and advertise it in SPORT AVIATION magazine under the “engines” section of the classified ads. This, along with a couple of articles, satisfied everyone’s curiosity and I was soon back working on the new airplane design.
There was only moderate interest and response to the gearbox articles and ads and after about a year interest seemed to drop off even further so advertising was discontinued. That was a mistake. Letters started pouring in again asking: “What was wrong? Did the gearbox fail? Was it unsafe?” etc., etc. So back with the advertising just to keep everybody calm and happy.
That all happened about three years ago. Recently there has been renewed interest in the gearbox idea, probably due to the fact that some are showing up on airplanes here and there and that one was on display at Oshkosh by Roger Davenport (SPORT AVIATION January, 1974).
In the meantime, what ever happened to my airplane design? Well, I made the mistake of going into business for myself. Now instead of working for someone else 8 hours a day, the work is never done. You self-employed people will know what I mean.
Consequently, the airplane project has bogged down and now other people are beating me at my own game of mounting a geared, turbocharged Corvair engine in an airplane. In fact, it now appears that there are about twenty of the gearbox conversions flying around or operating in airboats.
It is difficult to keep tabs on them because people just don’t seem to want to take time to communicate — unless something goes wrong. In this case it appears that the old axiom is true, “no news is good news”. Two letters did arrive from people who had inflight problems with the Corvair engine.
Both were using the gearbox, but felt the gearbox held up fine and was not responsible for the problems. I will relate both of these incidents in hopes of letting other people learn from the experience derived.
The first letter was from a man whose engine gobbled a valve seat in the left center cylinder while he was still flying off his 75-hour restriction about 20 miles from his airport.
This was a Pientenpol type airplane using the standard 80 hp engine. The piston crunched the ring-shaped seat into dozens of bits that somehow became distributed inside the other cylinders on that side of the engine.
The man said he never heard such a clatter in his life and instinctively let up on the power, but as the bits were munched up, spit out, and embedded into the piston heads, the racket quieted down so he opened the throttle and found he had enough power to get back to the airport.
In fact, he said, he gained more power as he approached the airport as the valves in the other two cylinders stopped hanging up on valve seat bits.
I believe valve-seat failure is quite rare and personally don’t know of any fail-safe cure other than close inspection at overhaul time. The second letter was from a man who burned a piston about two minutes out on takeoff. He was burning regular gasoline in a newly overhauled 1963 turbo 150 and was pulling about 55 inches of mercury at the time.
He said he was unable to hear any detonation (pinging) which blew a hole about the size of a 50-cent piece exactly in the center of the top of the right rear piston. Despite this the engine continued to run smoothly with only slight power loss and he was able to come around to a safe landing, leaving a very impressive trail of oil smoke.
I could have almost predicted that incident since I had duplicated the failure on my test stand using regular gasoline and pulling high manifold pressures. In my case it was the left center piston that burned through at the top front edge.
The lessons learned here are:
Use PREMIUM gasoline on the turbocharged and high compression engines.
Limit takeoff power to 50 inches of mercury, unless water injection is used.
Hold down manifold boost to 30 inches of mercury as much as possible on the first 100 hours of operation after an overhaul.
Those seating rings create lots of heat themselves and since they are NOT fully seated, there is more blowby than usual at high boost pressures.
This blows the oil from between the piston wall and cylinder wall causing less lubrication and hence more heat. Both of these cases show the benefit of having six cylinders instead of two or four, in times of trouble, and the benefit derived from an inherently balanced engine such as the Corvair.
One can pull three plug wires off one side of the engine and it will keep right on running with uncanny smoothness. They also point out what a rugged little cuss it really is and that the piston is the weak link during intolerable boost pressures — perhaps fortunately so.
There have not been any inflight gearbox problems brought to my attention so far. One man did have a problem just after assembly, but it was due to an initial mistake in my plans. Actually there were two mistakes (I’m only human, you know) but the first was so obvious that apparently everyone caught it.
On sheet 3 a brake-shoe-mount replacement spacer plate was labeled 0.50″ thick instead of 0.05″ thick. Actually the existing shoe-mount can be cut off and used as is. The other mistake, located on sheet 1, was more serious.
It was the dimensions of the press fit between the Volkswagen axle stub-end and the Corvair crankshaft. It was mistakenly called out 1.000 O.D. and 1.001 to 1.003 I.D. (which is a slip fit) instead of 1.000 I.D. and 1.001 to 1.003 O.D.
Apparently everyone caught the mistake by the time I sent out a correction notice, except one man. When he started up his engine with the slip fit, he could hear the fitting knocking against the key at idle, which would have eventually sheared the key.
We decided that the best cure was either to shim or chrome the undersized shaft oversize. He had it chromed 0.001″ thick (0.002″ overall diameter thickness) pressed it on, and has had no trouble since.
Some of the airboat people were unhappy about having to disassemble the Corvair engine to machine the crankshaft, so an alternate system was devised which utilizes a 4130 steel coupler that allows the gearbox to be mounted right onto an assembled engine by bolting the stub-axle to the existing bolt ring.
Now wouldn’t you know that no matter how clever you think you are there is always someone else who can beat you to it. I’m speaking in particular of K. C. Bunnel, of Hopewell, Virginia.
Instead of fabricating the coupler out of expensive 4130 stock, he simply cut the bolt-ring end off of a second discarded Corvair engine, (they’re all alike), pressed the VW shaft into it and bolted it backwards onto the good engine bolt-ring, using a % inch thick spacer between the two.
See Figure 1. Other people have used round keys in the press-fit which are much simpler to fabricate than square ones, with no problems and one man suggested using massive set screws.
There were a couple of reports of problems with the gearbox moving on the engine (causing oil leaks) in spite of the alignment pins called out in the plans, although most people, including myself, never experienced that problem.
For people who are interested in the plans there is an information package available for $1.00, which includes a price list and a list of material kits that are supplied from Herb Statz, of Dane, Wisconsin, at a very reasonable price. The original gearbox plans still sell for $20.00 and are very explicit.
They include a complete parts list, a parts breakdown assembly drawing with a bolt code and a complete step-by-step description of how to fabricate and assemble the gearbox onto the Corvair engine as well as full-size template blueprints.
The blueprints show how to modify a Volkswagen brakedrum (which contains a spline) into a propeller hub and how to mount a Corvair starter ring-gear onto the gearbox so a Corvair starter can be used (which, by the way, weighs only a couple of pounds more than a comparable aircraft starter).
There is also a complete description on how to preload the bearings in the gearbox, but it was later decided that the ball-type bearings were really meant to float and that there was no particular advantage of preloading. That is nice to know because it makes an already easy project just that much easier.
It also means that the gearbox can be used in either pusher or tractor installations with equal reliability. The alternate system plans sell for $10.00 and are to be used in conjunction with the original plans.
I don’t believe any of these are flying as of this writing, but a couple are operating well in airboats. See Figure 2. The turbocharged Corvair engine is probably the most economical way a homebuilder can achieve turbocharged power.
A 125 hp turbocharged Corvair engine operating at 10,000 feet will put out about the same amount of power that can be derived from an 180 hp normally aspirated engine at the same altitude. Some people are installing the turbochargers on the high-compression engines and plan on using a waste gate to control manifold boost pressures at lower altitudes.
These engines should be able to put out about 160 hp at altitudes up to at least 25,000 feet. A 160 mph airplane would turn out at over 220 mph at tha t altitude with turbocharged power. Well, I believe that about takes care of this update on the Corvair engine syndrome for now.