Turbocharging Corvair flight engines, Pt #1

Builders,

I am going to sweep together much of the info we have on turbocharging Corvairs here and have it as a reference page for builders, with links to other previous information I have written on the subject.

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Why put a turbo on a Corvair Flight engine? For more power. Corvair cars were the first mass produced turbocharged passenger cars. Many people who know little about cars mistakenly think it was the Porsche 911, but the Corvair Spider beat the Carrera to the market by a full 12 years. The Corvair was designed from the start with the possibility of boosting the output by putting a turbo on it. Above all, it has the cooling for this. Engines that barely have the cooling to run naturally aspirated don’t stand a chance with a turbo.

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 Our work with turbos on flying Corvairs: Most of our flight test work was done in 2005. The information was of personal interest to me, and many builders expressed an interest also. But as a reality check, A turbo on a flying Corvair was not really something 95% of builders needed. Also, bringing our test bed aircraft to airshows and speaking with builders taught me that the great majority of people who expressed interest had little appreciation of the complexity and often they had very unrealistic expectations. The best example of this was the majority of people saying “I don’t want a boosted engine, I just want it turbo-normalized” Clearly some of the sources of information on turbocharging of planes that people were reading was not written from a practical experience. Having a flying plane was done, but there was a lot of work to go before builders could understand what the motor would entail, what it would be good at, and what it could not practically do.

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Above is a 2005 overhead shot of  our test bead aircraft the Turbo-Skycoupe.  It is easy to see the stainless heat shield over the hot side of the turbo in this view. You can see more photos at this link: More Turbo Skycoupe photos

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Turbo-normalizing engines: Picture a naturally aspirated 100 hp Corvair powered plane climbing out from an airport at sea level. The pressure is 29.92″ there. Now picture the same plane taking off from Leadville CO, at 9,927′. The air there is has only 65% of the density it does at sea level. A turbo could easily put this right back, but here is the in-escapable issue: You can only do this with an inflight adjustable prop. If you tried it with a fixed pitch prop that worked at sea level, the prop would radically over speed at altitude.  If you put on a fixed pitch prop that absorbed 100 hp at 9,927′ and then tried to take off from sea leave without boosting the engine past 29,92″ on take off, the plane might not even spin the same prop to 2,500 rpm. performance would be very poor, less than a naturally aspirated 100 hp motor with the right prop. The bottom line is you can’t turbo-normalize any plane unless it has an in flight adjustable prop. They exist, but they cost nearly as much money as your motor will. The good news is that a turbo-boosted engine still makes sense in some applications, and it works with a fixed pitch prop.

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The Vne problem: Many people who like the idea of a turbo say “I want to get up high and go fast.” OK, this can be done, but here is a very real issue: Many light planes, especially experimentals, can already operate near their Vne (Velocity Never Exceed) speed. If you add a turbo to them, they will be able to fly right through it in level flight, a very bad idea. People debate this, but here is the reality that the educated side of the argument knows: Vne is based on TRUE airspeed not indicated. If you are in a plane with a Vne of 200 mph, and you are at 10,000′ and indicating 170 mph on a naturaly aspirated engine, you have no where to go. This is because your true airspeed will be 199 mph, and that is 1 mph below your Vne. Put a turbo on that plane and you can’t use it to increase the high altitude cruise. This is a very common condition for Van’s RV aircraft, and it is a big part of the reason why you don’t hear about them being usefully turbocharged. With Corvairs, the common example is the KR-2s, which can fly very near it’s Vne naturally aspirated.  If someone around the airport tells you I am wrong about this, look it up for yourself in Aerodynamics for Naval Aviators. I can’t sing nor dance, but I did learn some things in my 5 1/2 years at Embry-Riddle.

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Above, Arnold Holmes and I stand behind the engine installation on a V-8 powered Lancair IV-P. This is an EngineAir package that I helped develop from 1993 to ’98. It’s 450hp, geared, injected, intercooled and very heavily turbocharged.

 Most of the people commenting on turbocharging piston planes have little experience with it. In 1996 We took a Lancair IVP like this one on a test flight to 32,500′  I have a number of hours aloft above 29,000′ in these planes. Very few people have flown that high in light piston planes, and truly very few have worked on the engines and system that worked in this environment. You can learn a lot; example, you can easily overheat an engine even when it is 30 below zero outside because the air density is low, and it can’t take many BTU’s out of the cooling system.

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There are also many practical things that directly relate to turbocharged Corvairs such as techniques of welding 321 stainless tubing. Many new guys like to talk about selecting the turbo itself, but my experience says that the reliability of the system has a lot to do with details like how large the radius in the exhaust bends are, if the welders are really careful to come off the Tig pedal slowly and not to leave tiny ‘craters’ on the ends of weld beads, and a bunch of other details. Dozens of companies have on line catalogs to pick turbos, and people regurgitate that info all the time, but real installations have to be very carefully fabricated by experienced people.

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Get a good look at the size of the 5-blade MT propeller. Air is thin at 30,000′ and to absorb 400 hp there, you need blade area, speed and lots of pitch change . Contrary to what some people think, even though this engine was geared 2.19 to 1, it only needed 74″ of diameter to be optimized for the task.

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on to part #2…..

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