Last week I wrote a report of the weight & balance and performance of Bob Lester’s Pietenpol going from a 65 hp Lycoming to 100 Corvair ponies. Here are some photos of the conversion, and an important improvement Vern and I made in the plane at my hangar. Bob flew the plane over in the morning, we did the work, and he flew it home in the afternoon. In my book, this type of gear is an improvement over bungees. Bob had just replaced his bungees, but had a very hard time getting enough tension on them, so his plane sagged. Bungees have worked on a hundred thousand light aircraft, but here is a look at an alternative system that I have used for many years.
Above, Bob’s aircraft on the ramp in our front yard at 10 am. It is easy to see that the gear is splayed out terribly. The bottom of the fuselage at the front gear attach fitting was only 22″ off the ground. I was motivated to fix this for Bob first because it is dangerous due to poor handling and the potential for a prop strike. Bob isn’t a welder, and I didn’t want him to keep flying it until something happened, The plane did have safety cables under the bungees, but they turned out to be boat cable, something I wouldn’t trust. Beyond all this, aircraft that have the gear like this look as if they are massively overweight. I didn’t want anyone to come to the false conclusion that it was the additional weight of the electric start corvair that was doing this. The only light aircraft that has the gear like this is the ugly “Texas Taildragger” Cessna 150 STC’ed conversions. Besides, gross wheel camber like this reminds me of lowered Honda cars driven by teenagers with their underwear sticking out of their pants, booming rap songs and 4″ diameter mufflers with the exhaust tone of a flatulent elephant sitting in a mud puddle. If you compare this with the photos of the aircraft when it still had the Lycoming, keep in mind that Bob had tried to re-wrap the bungees in between the photos.
Above, on the welding bench in my hangar. The red part is the original bungee strut. Above it is the steel die spring and the tube that houses it. We cut the ends off the red part and grafted them on to the new tubes. The springs are available from A/C spruce for about $100. Their rating is 1200 pounds per inch. They are 1″ ID, 2″ OD, and 6″ long. the spring works in compression. The 2.25″ outer tube is fixed to landing gear frame at the top. The 1″ tube works as a plunger. It passes up the middle of the spring to a washer welded on it above the spring. When the plane lands, this strut elongates by compressing the spring. Many people have seen the reverse of this system with external springs with complex machines slots. I made that style for my Pietenpol in 1997. It required a mill and some precision work. After making it I got a chance to see an aircraft that Bernard Pietenpol himself had built. It used this external tube system. Brilliant, just as you would expect from the patron saint of homebuilding. The tube method has no milling, has no high shear points, is self aligning because the two ends are free to rotate., and had obviously worked for a few decades. But be forwarned, if you choose to put this on your plane, you will still have a handful of people tell you it will never work. (take their photo and name down, send it in and we will all make fun of them together.)
Above, I trim the ends of the main tube square in the lathe before welding. This makes it easy to get a great vee notch for penetration on the end caps. The 1″ bit is sticking into the tube to keep it from going into low earth orbit if it get lose in the chuck. We trim the tube to 6.25″ overall. This length has a great harmonic”ringing” sound if you get the tool shape just so, it gives the feeling of the fillings in your teeth falling out.
Above, a 2,25″ heavy-duty washer gets welded on each end of the main tube. The hole in the middle of it has to be drilled for a slip fit on the 1″ tube. The first weld is done without the spring or 1″ tube in place.
Above shows the 2″ washers welded onto the 1″ tube. The spring has to be inserted before assembly.
Above is a head on view of what Vern and I were working on curing. Bobs plane flies great, but letting this go would not show well and potentially end up putting the word “Corvair” in an accident report some where. I have devoted more than 20 years of my professional life to Corvair powered planes. I am in this for the long run, and letting something like this go, even if it never broke, projects a poor image of the Corvair movement. Besides I have been friends with Bob for 13 years, and I want him to have a sharp-looking, safe plane.
Above, the spring is inserted into the tube, the plunger goes through it, and the top cap gets welded on the spring gets a slight compression to prevent it from being noisy. In a ground loop, this system goes metal to metal in compression, just like a bungee system does. The part in the vice grips is the top cap, with the fitting taken from the red strut and blasted clean and pre welded on. If I was building from scratch, I would use a rod end or a fork. (off the topic, but Irwin made vice grips in the USA for decades. Today they are made in China. This disgusts me. CEO’s that make decisions like that should have their citizenship revoked and be sent to China themselves, but then we would have to send the new president of the EAA to Mao Se Tung land also. He was the CEO who sent the Cessna 162 to China. Google his name and the words “60 minutes, CEO, fraudulent engineering degree” for a nice look at his integrity.)http://www.aero-news.net/index.cfm?do=main.textpost&id=d75a19ed-4ce3-4ddf-abc4-7b2da56dc1db
Above, top cap being welded on. You could not gas weld this without hurting the spring inside, but it isn’t an issue for a tig welder.
Above, one side down, one to go. This is a significant drag reduction when you also consider the bungees bags. Heat and some oil are the enemies of the bungees. However, the steel spring would only consider getting baked and a fine mist of oil a form of love and care.
To save people who live for worry, speculation and internet drama the hassle of thinking up their own troubling questions, I provide them the questions pre-made:
Why does that long-haired, opinionated mechanic William Wynne think this system would even work? What makes him think springs are tough enough? Maybe they are no better than bungees. William’s thinking of selling kits to do this, so if he might make 25 cents, and doesn’t that mean we can’t trust him to give us an impartial review? Wouldn’t I be better off taking the advice of a guy on a Matronics list who has never flown nor built one of these? That guy wouldn’t ever write back with an unsubstantiated claim would he?
OK, look above. This is the underside of the hangar gang wagabond. I welded this gear up in 2004. It has flown many hours and landed at over 1600 pounds gross. It works well period. I am not a great pilot, but I was well schooled by two masters. The only time I have in my log book in any tricycle geared aircraft is a single 30 minute flight in a 1963 Cessna 150 in 1999. I went to the airport that day and my old school instructor said “There are some things you should do once in your life, but not speak of….Today you fly the Cessna one filthy.” It was hardly the trip to adult entertainment section of Bogota Colombia I first thought he was hinting at.
Seriously, I have flown a number of classic conventional geared aircraft to know what a well-engineered, well-behaved one feels like. A far better evaluation is that a number of super-skilled pilots flew both my Pietenpol and the Wagabond, and pronounced them to have excellent handling. How tough are the springs? The ones in the Wagabond gear are actually salvaged from my Pietenpol wreck in 2001. They worked for hundreds of hours in the Piet, got smashed into the ground at 15 or 20 G’s, were BBQ’ed by the plane burning to a cinder, laid in a rainy field for 6 months, and then I cleaned them off and saved them for the next project, which turned out to be the Wagabond in 2004. Until they invent the elastic asbestos bungee, you can’t claim a bungee is this tough. (actually someone will, but ignore them.) Bungees are date coded. and they are to be taken out of service every few years. It may creep you out, but steel springs will out live you.
Above, in the fitting process, we held the aircraft up with the hoist and telescoped the original tube bottom into the 1″ tube. We tacked it with a mig welder and rolled it back and forth to have it settle. We checked every thing with levels, and actually cut the tacks on the right side twice. On the third try the plane sat perfectly level. The struts turned out to not be the same exact length because the original builder in the 1970’s didn’t make everything symmetrical. It takes a small error here to make the plane have a wing tip an inch lower on one side. In other planes this could be completely avoided by having adjustable rod ends or forks on the ends. When perfect, we pulled them back out and TIG welded them on the bench with the proper rosette welds. Graces’s Taylorcraft and our 1-26 sit on the front lawn in the background.
Above, the finished product at 4pm. Ugly and dangerous duckling transformed in 6 hours, including a lunch siesta. The new ride height is 26.5 inches under the fuse at the front fitting. While it looks tall, scratch built gear on a piet can be much taller. Look at Photos of my blue Piet and you will see that the gear was 30″ under the fuselage. This extra angle of attack available in the three-point attitude allows the plane to land far slower, and stop shorter. It also looks a lot better. If anyone says “but you will not be able to see over the nose.” suggest that they go ask the guy they paid for a conventional gear check out for a refund. Ask an old school instructor if looking over the nose in the flare is a “technique,” and he will probably slug you like Buzz Aldrin decking a conspiracy nut job.( http://www.youtube.com/watch?v=tRBesDx1WQc )
Very important to handling, when the plane is empty and at rest, the tops of the tires should be slightly further apart than the bottoms. Only when the plane is actually taxing at full gross weight should the tires come close to vertical. When the plane lands, the gear, if it is steel leaf, steel coil, bungee or Aluminum, has spring rate, but no damping itself. All the damping comes from the action of scrubbing the tires from the orignal camber, through vertical and then a little bit further. This is a very powerful damping force, especially on pavement. If the tires start out as vertical, the system forfeits much of the available damping, and starts heading back to the original incorrect camber. I know arcane details like this because I spent 5.5 years of my life at Embry Riddle. (If I had only known that I could one day be president of the EAA with a fake engineering diploma for credentials, imagine how much time and money I could have saved.)
Above, Bob with his re-done aircraft. This is a radically different aircraft than it was a few months ago. It when from barely having a positive rate of climb with two people on a hot day to being able to climb 800 feet per minute at the same load and conditions. It now has safe and smart-looking gear that works correctly. The plane is 25 mph faster now on the top end. For reference, Bob is 5’7″ or so.
Above Grace, Chris Welsh, Terry and Vern stand at the edge of our lawn and the runway. They are watching Bobs test taxi and flight on the new gear. The plane looked so much better than it had a few hours before, I was filled with a sense of pride in the skills, understanding, tools and capability I had painstakingly worked for over the last two and a half decades.
If you have never met me, but read this and think that I am charmed with myself, you got it all wrong. I know countless humans who are better people than I. They are kinder, smarter, and harder working. I can’t sing nor dance, I learn slowly, and I can’t stand to hear my recorded voice nor see my image on film. If I was once handsome, all trace of it is gone along with my uncorrected eyesight. I can be a conversational bore, and I deeply wish I had given my parents more moments to be proud of me. At 50 I look back on my life with a very critical eye and stand on the far side of a very wide gulf from the heroes of my youth. Even our dog, impeccably honest and loyal as canines are, Loves Grace and only tolerates me.
Honest evaluation leads to harsh thoughts like this. I spend a lot of time alone and have long bouts of insomnia, which can lead to thinking about things excessively. But the secret I would like to share with anyone who at times feels the same way, is that I have a sanctuary where I am insulated from much of my self-criticism, and a have a front, where at 50, I am much better on than I thought possible in my youth. When I am building things with my hands in my shop, I rarely feel poor. Although I now need glasses to do any close work, and my hands have lost a lot of dexterity, I am a far better craftsman than I ever was in my youth. I am not a great craftsman, but over a very long time I have worked to develop these elements in my life, and I compete with no one except who I was last year. While all else fades, these things flourish. It is a gift I am most thankful for.
I was aware of this in my youth, but it did not come into focus until 1999, the worst year of my life. (getting burned was 2001, but it was a picnic compared to ’99.) Feeling dangerously low, I sought the council of a guy I knew. He had come back from such a year. He is an artist, working as an incredibly detailed wood carver. He tells me to forget everyone and everything else, go back to your tools and work with your hands. Give up your apartment, but never your hangar. Explore all the things you can’t forget, have stolen, give away or loose. At the moment, I was having a hard time picturing another week, and I asked him how long it took him. The thought with great care a slowly said “two, no really three..” I was jolted and blurted out “Three months?” he looked me in the eye and said “No. Years. It’s probably your only way out.” It turned out to be a painfully accurate prediction.
In the years since I have read letters or posts from many people in a tough spot, who have sold their project or tools. I often think their ship is sinking and they have just traded their life jacket for five more minutes on the deck. I have also met a number of successful builders who have said that when everything else in there lives was broken, they had a place of refuge in work and creation. Of the thousands of people I have met in aviation, these people are truely brothers, for we share the same salvation.
For these reasons, I can honestly say that the only time I allow myself to feel anything most people would call pride, is when I exercise my skill knowledge to make something good. Much of the time, this is far better in the company of Grace or friends like Vern. It’s not real often that I indulge myself for an hour and give in to being pleased with something. Maybe two or three times a year. Watching Bob’s plane fly away with the gear fixed was one of these moments. There was stuff to do, but instead we sat around outside and enjoyed the evening. -ww.
Mike Schwab is a Vision builder working on a 3,000cc Corvair for his plane. He and his wife attended Corvair College #23, and they are regulars at Sun n Fun. In his day job, Mike is an expert on Marine engines, and specifically knows Yanmar Diesels.
Why should you care? Because he offers a great deal through his business to any Corvair builder in need of a Dynamo. At $125 he is less than 1/3 the price that John Deere wants for the same part. This unit directly bolts on our front alternator brackets and is flying on countless Corvair powered aircraft. The unit is also the basis of the direct drive rear alternator the Dan and I are working on.
I am fully aware that publicly revealing that smaller models of the sacred Green and Yellow divine brand of tractor comes with a Yanmar engine can result in having a Fatwa against me issued from the sacred headquarters of the divine Green tractor in Moline Illinois. When I have pointed out that dealers for Deere have massively marked up their prices, I have been attacked by rabid fans of the green brand, who actually claimed that Deere parts are better, but this is the dogma of the indoctrinated, a mantra they were taught to repeat at the mention of other brands. I have held both dynamos in my hands at the same time, and they are both made in the same place in Japan by Kokosan-Denki. The only difference is the box and the price. (Full disclosure: I own a Case tractor, drink Dunkin Donuts coffee and drive a Chevy truck. No kidding, the last time I was trying to buy a dynamo at the Deere lawn care dealer, a guy drove in a Range Rover with a Starbucks travel mug in hand, wearing a cardigan. jokingly I said “One of us is in the wrong place.” When I refused to pay $396 for the dynamo, Mr. Starbucks leaned over and whispered “It’s you.”)
Mike also has a good deal on the matching voltage regulator. You can check the parts out on the link to Mike’s site below:
Mike is headed to Corvair College#24 and has offered to bring Alternators that are ordered in advance with him. For builders not able to make it to the College, Mike is willing to ship for a modest fee. Every one interested should contact Mike directly at:
Mikes regular Email and Vision building sites are listed below:
Hats off to Mike Schwab for making aircraft building a little more affordable. -ww
Pictured below are 5 powder coated Zenith 750 mounts. I took the photo in our back yard today. We rarely offer items on sale, but here is a special offer for 750 builders.
At the Zenith open house, we brought 12 mounts, six 601/650 models and six 750 mounts. All of the 601/650 mounts went home with builders, but we still have five 750 mounts. (oddly enough, the exact reverse happened at last years Zenith event.) Because our mounts are made from CNC tubing sets we have machined 12 at a time, and because our powder coater likes to only do quantities, we are motivated to put these 5 mounts in the hands of 750 builders and get another batch welded and coated before CC#24.
We sold the mounts at Zenith for the normal $739 price, without the $70 shipping cost. Powder coating is normally a $100 option. For this sale, These coated mounts are $799, and we will cover the cost of shipping in the US. Any builder who would like one can send us a quick email note with you name address and phone number. Payment can either be by pay pal or by check. The mounts are ready for immediate shipment, and they take only 1 to 2 days to arrive. This is a good opportunity for a 750 builder to save some money and get an importiant part now. -ww
A number of people have raised questions about Corvair Cooling on the Internet. A lot of theory can be debated, but you can learn a lot more from a good example and a positive discussion focused on the details of installation the way we have done them. The photo below was taken in 2005, just east of our old hangar in Edgewater. In the background is the Atlantic ocean. The plane is N707SV, the Wagabond built as a joint project by members of our old Hangar Gang.
The Wagabond we built was based on a PA-22 airframe. It has 4″ longer gear legs than most Pacer conversions. This airframe is the same size as the four place, 150hp certified PA-22. As far as Corvair powered planes go, it is huge. Yet it is flown efficiently on a direct drive 100 hp Corvair. The plane weighs 804 pounds empty, because we were very careful not to put things in it like an interior beyond seat cushions. It has a full electrical system including a starter. The plane isn’t a speed demon, but combined with the Corvair it did something few experimentals can do: As a test we actually flew the plane with an 820 pound Payload on board. Yes, it has flown more than its own empty weight as a payload.
Above, the Wagabond sits on the flightline at Sun N Fun 2007, the second year it was there. The plane is a well-known flyer, as many people saw it at shows and at our old hangar. YouTube films of it flying have thousands of views, and it was seen in our DVDs. It has been publicly demonstrated to work well over the past 7 years. It has had a long and trouble-free existence, despite being produced on a $9,000 budget for both the engine and airframe. No money was spent on electronics, flashy paint, or an interior. The money all went to a solid airframe and a sound engine. (In the background is Dan Weseman’s Wicked Cleanex.)
The Wagabond has excellent cooling. Florida is known to be a warm place, but the plane never has had any kind of cooling issue. When discussing something like cooling, you can often appreciate a point best by examining an extreme example. The size and lifting capability of this plane, combined with a slow climbing speed should make this plane the most difficult Corvair to cool, yet it has no such issues, it just works. The plane uses our standard components and a standard aircraft approach to cooling design. All the concepts are nothing new, they appear on countless certified planes like Cessna-150s and 172s. There could be no more proven cooling concept in general aviation. Out of the 60,000 150s, 152s and 172s built you never hear about one of them frying an engine from a cooling issue, despite the fact that they are most often flown by students, and almost none of them have CHT gauges. They work because they have generous inlet areas, well thought out exit areas, their timing is set correctly, they have a regular aircraft carb that doesn’t lean out, and they have a large cooling plenum that covers the whole top of the engine, sharing air to whichever side needs it. The Wagabond also works because it shares all these characteristics.
The above photo shows that the plane uses one of our regular Nosebowls, with the air inlets trimmed to 4-7/8″ diameter. Notice that the plane not only has a front mounted alternator, it actually has an oversized pulley mounted on it. Although this inlet lets in slightly less air than the other side, it doesn’t matter because the air is freely shared by both sides of the engine.
Above is a photo of the bottom of the firewall, looking in from the side of the cowl. Note the rolled piece of sheet metal that smooths out the airflow exiting the cowl at the base of the firewall. Notice that the exit area is twice as large as the inlet. The carb on the plane is a Stromberg NAS-3, a regular aircraft carb. These are set to go slightly rich at full power, a very big part of preventing detonation on a full power take off and climb out. Motorcycle carbs, particularly CV ones that are subjected to even slight amounts of ram air pressure, almost always go lean, making the engine detonate. The number one reason people don’t use aircraft carbs is cost. This one in the picture was bought out of a flymart and set up with old bits and pieces: Total cost, about $200. Even a guy buying an overhauled one outright from Russ Romey at D&G will only spend $800. Pricey, but a whole lot less than damaging your engine with detonation. When building a plane, buying a great carb comes before glass cockpit stuff, avionics or paint jobs. The airbox on the bottom of the carb is a standard aircraft one. Carb heat air that is not used exits the bottom of the box and immediately heads out of the cowl rather than heating anything up.
The above photo shows the underside of the cowl. Notice that the plane has a smooth ramp for the air to flow out, and it has a very crucial fixed cowl flap, the lip on the leading edge of the exit opening. The sheet metal roll from the photo above is visible in this photo. The 5/8″ tube hanging down is the crankcase vent.
To give a good idea of how big this plane is, note the crankshaft centerline is 62″ above the ground in the three-point attitude. The top of the cabin is 80″ from the ground. This plane’s best climb speed is only 65 mph, but it can climb at this speed, at full weight, at full power, on a 100F day without overheating, for any length of time.
The above photo shows an overview of the cooling baffles. We have dozens of Corvairs flying with this same arrangement. It works, period. In the back of the photo is the Distributor. If I wanted to make a Corvair overheat quickly, I would just set the timing by ear to the smoothest running setting. This sounds cool, but it will cause the engine to detonate and overheat on the first full power climb out. Timing set by ear in a Corvair always has too much advance. Builders must use a timing light: They are cheap, there is no excuse, but still 15-20% of builders with running engines have never set the timing on their engines with a light at full static RPM.
If a builder is having an issue, the first question to ask is “What is different from the Wagabond?” a plane which clearly works. In many cases the plane that isn’t working is trying a combination of using low-grade car gas, not setting the timing, having a carb that leans out at power, and no cowl exit design. Builders can eliminate these issues by taking advantage of the things we have learned over the years. -ww
Below are photos of our latest evolution on front starters. Since 2002, the only starter arrangement that we have worked with is the front starter. Originally we used hand prop engines, moved to front starters in 1992, Developed and flew the rear starter in 1998-2001, and then came back full circle. In the last 10 years we have refined the front starter several times to make it lower profile (2003), use machined brackets instead of welded ones (2006), switched from welded on ears to bolted on ones (2010), and now we have changed the ear to a single piece bracket with an adjustable slot which eliminates the previous drilled aluminum link. installing a starter now takes a few minutes with three wrenches. No drilling or fitting. If you crank it up and it doesn’t mesh with the sound of prefect engagement, a minute of loosening the bolts and resetting the adjustable bracket will make it correct in no time.
Above, the new bracket bolted on a starter mounted on a 3,000cc Corvair equipped with a Dan Bearing. In the last two years, starters we have sent out have had the front ear bolted on instead of welded on. any of the bolt on ear starters can be retrofitted with the new bracket. The above photo is of the same engine we had on display at Sun n Fun. A number of builders asked about the starter, saying it looked smaller than previous models. It was an illusion; This is the same EA-81 based ND starter we have used for the last 10 years. The main visual difference was that I painted this one black and spent 15 minutes on the band saw removing the shroud around the starter gear. The new bracket has a cleaner look than the previous system, but the concept is the same one that has started 100’s of Corvair builds in the last 10 years. The improvements have been small and evolutionary. I have had much better experiences with things in aviation that are refined and slowly evolved to be what the are rather than things that are new and revolutionary. We can all think of things that are revolutionary success stories like the Vari-eze, but you have to remember that it was the exception. In the 1970s there were dozens of other new revolutiary airframes that didn’t work out, like the BD-5. I like to read about new and exciting things, but I have been much better served by things that are old and proven.
Above is a photo of two dozen of the new brackets. They are CNC machined for accuracy from 1/4″ 6061-T6 plate. At the bottom of the photo are two different spacers. These move the starter forward slightly when installing a Dan bearing. If you look at the top photo, you can see the small spacer between the new bracket and the left hand side gold anodized starter bracket. We have these spacers and pre-machined tail brackets for engines that are assembled with Dan bearings. The tail bracket is visible in the top photo.
Jeff Cochran, CH-750 builder from Alabama with a running 2,850 cc engine, writes:
Welcome back to the world wide web. You have been missed. Questions about the installation of your SS exhaust pipes. First, if ceramic coating of mild steel is bad, what about wraps on the SS system (except for the heat muff section)? Next, do the Heat Muff Box Ends need to be attached to the pipe and if so what is the best and worst method? And last, you say the pipes do not require tail pipe brackets, but the 601 Installation Manual calls for a steel tubing brace across the ends?
The new site is great, keep blogging.
Good to hear from you. The photo above is the first run of your engine at Corvair College #19.
Wrapping the pipes is bad for mild steel for the same reason why ceramic coating the outside of mild steel is bad: It keeps heat trapped in the steel, and mild steel can’t take this. If you look at the pictures of our Pietenpol in the late 90s at our http://www.flycorvair.com/carbice.html page, it had wrapped exhaust. I learned my lesson then. As a concept, it is worse than ceramic coating steel because when it cracks or disintegrates, you can’t see it. The only Corvair builder who I can think of who found this out the hard way was 601 builder and pilot Scott Laughlin. His wrapped mild steel exhaust gave in in about 100 hours, but he initially didn’t see it because it was wrapped. Wrapping the exhaust had its heyday in drag racing 25 years ago before coatings were available. Today they are a fashion statement on custom motorcycles. I can attest that it doesn’t work all that great either. My motorcycle, a Buell XB12X Ulysses came secondhand with a wrapped exhaust right where it passes my right thigh. It still radiates enough heat to be very uncomfortable. Sooner or later I am going to send the header pipes out to Jet Hott in Texas to have them ceramic coated inside and out. The best way to secure the heat muff ends it to get the box built and fitted right where you want it and then let a local welder put two tack welds on each end. The welds don’t have to be very big, two spots 1/4″ in diameter will do it. Other builders have used a hoseclamp above and below the box. Avoid anything that would puncture the main exhaust tube like a rivet or a screw. Your Zenith Installation Manual is an early one where we experimented with tying the ends of the pipes together aft of the nose gear. Subsequent experience has shown that this isn’t necessary.
This follow up came in from Gary Burdett, 750 builder from Illinois, also building up a 2,850 cc engine:
I take it that the short stubs are the place for the egt clamps.
If you’re planning on 6 egts, the stacks are the place to go. However, a majority of Zenith builders are using just 2 egts, one in each pipe, allowing them to monitor each side of the engine. In this case, they mount it about 6″ past the last stack.
Our Friend Rob Schaum, who is building a Murphy Rebel to be powered by a large Corvair, wrote us this note with a number of welding questions related to his quest to build his own motor mount. Rob bought one of our Motor Mount Trays and Spool Sets to get started, and then did some very impressive motor mount calculations that he ran past me. (His work turned out to be very well organized and accurate, best I have seen from a homebuilder.) His questions are far outside the simple scope of building a Corvair engine, but the engine by itself isn’t going to fly your plane. Unlike many people who market engines, I actually know how to mount them on planes. Over the years, I have seen a number of companies say things like “liability prevents us from commenting on that.” In many cases this is a face saving way of dealing with the fact that they sell imported engines, but don’t know any of the details of how you would do a custom installation. I have built more than 50 different Motor Mount designs for the Corvair, and I stand ready to help anyone with a question they may have with their installation, even if it is a one of a kind.
So I’m ready to start fitting tubes, as the motor mount jig is all ready to go and firmly attached to the work bench (see pic).
In preparation, I have read Finch’s book, and also L.S. Elzea’s WW2-era bible on aircraft welding. I am now all set up to start practicing on the “problems” at the back of the book (essentially practice exercises). I have to say, it’s a great book, and the “problems” do a great job describing techniques specific to specific steel tube structures and configurations. I expect that the most challenging parts to this will be joining the tubes to the heavy spools, due to the thicker spools sucking up all the heat. Also the 2-tube cluster (see above) might be challenging. I have some spool “stock” on hand to practice those specific welds, and plan to practice the exact cluster a few times before doing the real thing. Have a look at my set-up in the picture and let me know if there are obvious flaws. It is actually really sturdy front-to-back and laterally, but I am most suspicious of the twisting loads created while tacking-on the tubes. Some questions I had on the actual welding of the motor mount:
1) I can’t find any closeup photos of your 2-tube clusters at the lower firewall mount points. It definitely looks like the short tube is fitted first, followed by the long one, saddled primarily on the first tube due to the acute angle with the spool. However, I can’t determine whether the shorter tube is completely finish-welded before welding-on the longer tube, or if the shorter tube is tack-welded first, followed by the long one, then the cluster is welded as a unit (the latter appears to be standard practice in the literature). Can you please describe this procedure/area to me?
When you weld a cluster, it is not required to weld the parts of the cluster that are covered by the tubes placed later in the cluster. Basically, you are just welding the seams of the cluster that are visible on the outside of the completed joint. If you think about it, the forces on the joint are going to be transmitted through the outer surfaces of the tubes, and the welds that would be hidden inside would not be doing much work. I have cut apart a lot of welds in certified planes that have been around for decades, and none of the planes had the interior layers of the tubing clusters welded, even in the motor mounts or landing gears. Your assessment of the order of placing the tubes in the jig is correct.
2) Your motor mount 101 writings also refer to finger-straps on the 2 top tubes…do you still advocate that? I like the extra insurance at this location. Seems like one should have them on both ends (spool and tray).
Yes, they are a good idea. If you look at the November update on our website at www.FlyCorvair.com/hangar1111.html you can see photos of our personal Tailwind mount, and if you look closely, you will see that I put the tabs in where the upper tubes contact the tray. In our case, you have to remember that a Tailwind’s mount also has very high gear loads going through it. If you are an amateur gas welder, it can’t hurt to put the reinforcements in as outlined in our Conversion Manual ( available at the www.FlyCorvair.com/manual.html link). They are there to absorb tension loads on that joint. We do not use them on Mounts like our Zenith designs because I can get the full strength of the tube out of the joint by having it wrap slightly around the Tray at the contact point, and using 30 years of welding experience and a top of the line TIG welder to make our production Mounts. If this is your first mount, and you’re using gas, put them in. We don’t have them at the top as commonly because the top joint to the spools wraps around the spool, putting a lot more of the weld bead in shear, which is much less failure prone than a straight tension weld.
3) Is there an overall sequence to attaching the tubes to minimize distortions in the geometry due to weld-cooling stresses? Tack weld everything then finish weld? Or tack and finish each tube (or each matching set of tubes) then move on to the next set?
Tack weld everything then finish weld. To minimize distortion, work your way around the mount; it is good to do part of one cluster and then part of the next. There is no harm in this as long as you heat up and cool down the joint you’re welding each time. Something like 60 seconds leading in and 120 exiting in still air. Do not gas weld in a room that has air currents in it.
4) I had planned on tack-welding everything, then taking the thing off the wooden jig and test fitting on the plane. If all looked well, I was going use the tacked mount to construct a steel jig (I’ve been studying photos of yours), and do all the finish welding on the completed steel jig to avoid distortions that might otherwise occur using the wooden jig. Obviously, it would also enable better positioning of the work for the finish welding process. Is a tack-welded mount sufficiently strong to act as a “jig” for a jig, or will distortions generated during the construction of the jig end up “popping” tack welds on the mount itself?
If you are reasonably gentle with it, it will be fine. Try to put at least two tacks on each joint, but three is better. Try to space the tacks around the joint so they are not bunched up on one side. I would resist trying to gas weld a jig. It it is made out of strong enough material, it will be hard to get enough heat into it for welding without distorting the structure though warpage. If you have a buddy with a Mig or stick welder, burn the jig together using one of these techniques; they produce instantaneous heat which keeps distortion in check. Consider bolting your jig together. If you do weld it, pulling the tacked mount off it and checking it on the plane again isn’t a big step.
5) If I screw anything up, how structurally sound is it to cut the offending tubes off the tray, grind flat, and start over?
A lot of books act like this is a big deal, but it isn’t. If you couldn’t do it, then how would repairs be accomplished on steel tube planes? If you don’t like something, just cut the tube out, grind the weld bead away, and start again. The main thing that you want to avoid doing is running the flame set to an oxidizing flame (too much O2); this will BBQ the steel and it will take on a slightly rough, baked texture. If you keep going over a weld area with a flame like this, you are harming the base metal. Use good sense, and if you don’t like the way things are going, stop, take pictures and send them to me and we will figure it out.
6) Time permitting, are there useful weldments I should be attaching, or are Adel clamps the norm? What about attach points for the SS 1/8″ safety cable?
A safety cable can be threaded through the mount and bolted back onto itself, I would not weld tabs on for it. I would weld a battery ground cable strap onto the right rear corner of the tray; this will go to the back of the right hand cylinder head. Most of the other stuff will use adel clamps.
7) Much has been written about ambient temperatures for welding. How strict must one be in maintaining the ambient around 70 degrees, or can careful withdrawal of the flame compensate for virtually any environment? My garage is unheated, and it is currently 24 degrees F right now….perhaps I should do the finish welding in the basement?
In 1981 I was rabid about motorcycle drag racing. I lived in New Jersey, a state that regards drag racing as a birth right and a modern form of dueling. Englishtown was only 22 miles from my house, but like most young guys we were drawn to the “you can’t break the rules, we don’t have any” attitude of Atco, a track that was sanctioned by IDBA, the non-family entertainment version of the NHRA. That winter, my friend Ben and I welded up a new frame for our 830cc Kawasaki H-2. It had an all out Denco engine and in a fairly stock chassis had run 11.22 in the quarter. We were hoping that a new frame and an air shifter would get us in the 10’s at 120+mph. ( If you ride a 1,000cc Japanese sport bike that may not sound quick, but we are talking about an era where bikes handled like shopping carts with a bad wheel and a $29 Avon Speed Master II was considered a great tire because they usually stayed in the front rim when they went flat.) We welded up the frame using a gas torch in a 30 degree garage. When it was all done it looked as stout as the Pulaski Skyway. A dopey friend asked how we knew it wouldn’t break. I considered the question a serious insult. To demonstrate how strong it was I picked it up to chest height and dropped it on the floor in front of him. I was stunned that two of the welds had cracked! It was ugly to think about what might have happened if they had popped in the top end of the first run. No matter how you’re dressed, no one wants to think about how far you will slide at 100 mph on pavement.
Do not weld anything in a shop that is below 70F if you are building your own stuff for the first time. Pros can stretch this to a much cooler number, but it is a very bad idea to try to get away with this in your first go around. Find a warm spot and stack the deck in your favor.
The steel jig would go to you in the hope you’ll be able to save the next Rebelvair builder some time.
That is the kind of thinking that I have always found to be the best thing about the Corvair movement. I am glad to take the time to help any builder learn something, but it is especially rewarding when I can tell that the guy is already thinking of other builders who will follow him. Most other things you can do in aviation don’t have very much of this element anymore. It is unfortunate, but I recognize that I can’t change the commercial direction of aviation. The good thing is that I don’t need to, I am happy to just make our corner of Corvair power an oasis where builders who are here to learn, create and have fun have a place to be among friends.
Your help/wisdom here would be greatly appreciated.
You’re welcome, keep us posted on the progress.
Have a good night,
One of the most popular products we sell are Stainless Steel Exhausts for Corvair powered planes. We have been continuously making them since 2005. In this post we will cover the different systems that we make, talk a little about the pros and cons of certain designs, and look at some applications.
Prior to stainless, we built systems out of mild steel and had them ceramic coated. They looked great, but actually had a shorter life than plain painted steel. This is a surprise to many people, but here is why: Ceramic coating really works. It is a great heat barrier. A normal mild steel exhaust lives for a while as long as it can run cool. Ceramic coating the outside of it makes it look good, but it is actually hurting the system because it is trapping the heat in the metal. All affordable ceramic coating is done on the outside of parts. Very high end shops like Jet Hott charge several hundred dollars for a system because they use special tools to apply the coating on the inside of the pipes.
In 2003, I built an exhaust for our 601XL, N1777w. It was made from mild steel, but it was ceramic coated by the Moore Brothers, a very high end shop in Florida. The coating alone cost $300. It worked, and the best evidence of this was the fact that the heat muff for the carb didn’t work because the coating prevented any useful heat transfer. The system also racked up a lot of time on our plane, and it held up well. When other Zenith builders wanted to follow our success, I began to look at stainless as a better material for production exhaust systems.
Stainless is inherently a better material for exhausts because it is stronger at elevated temperatures and it is very resistant to corroding. Both of these are a big deal in aircraft because you can’t tolerate any kind of an exhaust leak in a plane. Everyone first thinks about carbon monoxide getting in the cabin, but my real concern is the possibility of starting a fire in the engine compartment. It is remote, but it is something that experienced aviators actually consider more of an emergency than having an engine quit on you. The strength and rust resistance of stainless, combined with good materials and welding techniques, applied to a design that has been flight proven not to resonate or crack on your airframe is the answer to minimizing your risk.
The stainless we use is an alloy called 304. It is the standard alloy of certified exhaust systems. The main tubes of our systems are bent for us by a shop in Florida that specializes in robotically bent tubing. They actually make the OEM systems for Lycoming and Continental, and make STC’d systems for companies like Powerflow. The head pipes on our systems are CNC machined from solid 304 bar stock. (They are made in the same shop in Florida that produces our Gold Prop hubs.) We have a separate shop that produces the Heat Muff Box Ends and another company that makes the tight radius front pipes. All of the systems are TIG welded in our hangar with 308L rod while they are pressure back purged with argon gas. Getting very expensive American made subcomponents from four different shops together in one jig and welded is something of a logistical challenge, but the end product is well worth the effort. In the past seven years we have produced about 250 Stainless Exhaust Systems for the fleet of Corvair powered planes. Chances are, most of the Corvair powered planes you have seen in person or seen in photos have a stainless Exhaust System that came out of our shop. Virtually every Corvair powered Zenith has an Exhaust System of ours on it.
To get a look at one of our Stainless Exhausts in action, watch this video of Jeff Moore’s Corvair powered Merlin on floats:
Jeff is from Newfoundland, Canada. His aircraft previously flew with a Rotax, but he has opted to repower his plane with a Corvair that he built with our conversion parts, http://www.flycorvair.com/products.html. His engine is a 2,700 cc 100 hp engine with all of our Gold Systems and a Weseman bearing. Jeff built his own mount utilizing one of our pre-welded trays. The Exhaust seen in the video is one of our Universal #2 Systems.
Below are three of the four production Stainless Exhausts we make. Universal #1 is the Exhaust System that is used on KR-2s and Cleanex airframes. Chris Smith’s “Son of Cleanex” was the first aircraft to fly with this system. The Universal #2 is the system that we make for aircraft like Jeff Moore’s Merlin. It fits a broad variety of planes like John Pitkin’s Kitfox 5 and Russ Mintkenbaugh’s Wagabond. It combines good motor mount clearance with the ability to work with a high thrust line. It is also a good match for a Pietenpol. Universal #3 is specifically bent for aircraft with a very low thrust line, like a Tailwind. Ordering information is on our Exhaust System page, http://www.flycorvair.com/uniexhaust.html
If you have any questions about which model is correct for your plane, just send an e-mail or give me a call on the shop line, (904) 529-0006.
Our fourth production system is our Zenith 601-650-750 System. This is specifically engineered to fit in the Zenith’s engine compartment, which has plenty of room, but the Exhaust has a sophisticated shape because it passes through the mount and clears the nose gear installation. This has proven to be the most popular system we sell. The Zenith has a particular motor mount geometry that requires this Exhaust to fit the engine correctly to the airframe. While some aircraft like Pietenpols utilize stock car exhaust manifolds, this is not an option on a Zenith because the car manifolds actually hit the upper tubes of the motor mount. Thus, a stainless system is an upgrade on a Piet, but a requirement on a Zenith.
One of the first things people ask about the systems is if they would make more power if they looked like aftermarket headers for cars. The magic answer is no. I tell them that you don’t have to take my word for it, you can just ask our Dynomometer. Before we came to the design we use, we tested lots of prototypes and systems. The technical reason why these compact systems do not restrict performance has to do with the camshaft pattern and the rpm range we use. The OT-10 cam has very little overlap, which is one of the reasons why it makes good torque. Engines like this, especially ones with 3,500 rpm power peaks, don’t see the same benefit from a full tubular exhaust system that a 7,000 rpm V-8 car with a high duration cam does. There are three basic goals served by making the most compact stainless system: First, it is lighter than something elaborate. Second, it is structurally stiffer, and therefore it is not prone to vibration damage (our Exhausts are cantilever off the bottom of the engine, they do not require tail pipe brackets nor secondary mounts). And last, it has a lot less surface area to radiate heat into the engine compartment. This last one is a bigger point than many people suspect. If you operate your aircraft in a very hot climate, this makes a difference on whether it is susceptible to vapor lock. I saw an experimental that had terrible trouble with vapor lock, yet when the builder looked in the engine compartment he missed the concept that his flat black mild steel tubular exhaust pipes were radiating the vast majority of the heat that was bothering his carb and gascolator. A poor exhaust system choice can easily put as much heat into the engine compartment as the engine itself. You are far better off having this heat run out the exhaust pipe. Stainless is a poor conductor of heat, and it does not radiate heat well. Combine this with a compact design, and you have the making of a cooler engine compartment.
The second most common question is about how loud the system is without mufflers. You can watch a number of videos, but they don’t give you a good feeling about the level of the sound. In person, most people are very impressed with the sound of the engine; throaty, but it doesn’t have a harsh bark. Some of the video shorts near hangars sound harsh because any aircraft turning a prop makes metal hangars resonate like steel drums, and microphones are very good at exaggerating this frequency. Out in the open, the engine is not loud. A subjective comparison; a Cessna C-172 in the pattern of your airport is a lot louder than a Corvair powered plane. It is also an effect of the engine’s cam timing; the low overlap means the cylinder is done burning by the time the exhaust valve opens. This short duration also has a secondary effect: Only one of the exhaust valves in each head are open at the same time. Each side of the exhaust system only has to serve one cylinder at a time, contributing to low back pressure. If you would like to use a muffler, you will end up with an exceptionally quiet aircraft. We flew our Pietenpol with a muffler for many years, and many people thought it was one of the quietest aircraft they had heard. Although many people think of quiet engines as being down on power, this is only true on engines that have a long duration camshaft design. Taking the muffler on or off our Piet only reduced the static rpm by 20-30 rpm.
On the subject of custom exhausts, I have produced a number of one-off designs for builders. Most of these were for engines equipped with 140 hp heads (they have a different size exhaust stack), or for a one-of-a kind airframe. If you find yourself heading in this direction, give me a call, and we will talk it over. It is also worth mentioning that we can install oxygen sensor bushings in the exhausts for builders who want to use an air/fuel meter. This is a concept with some appeal, but 90% of our builders still opt to use EGT probes, which are placed into the exhaust system after it is installed by drilling a small hole in the tubing.