William Wynne

"The Corvair Authority"
5000-18 HWY 17 #247
Orange Park, FL 32003 USA

Motor Mounts and Cowlings for The Corvair Powered Zenair 601XL

November 30, 2003

Here's this month's update. We've been very, very busy in the hangar, but still found the time to make progress on the 601. We've received a lot of e-mail from people with questions focusing on the availability of motor mounts and cowlings. This month, we'll direct the update to those two aspects of the Corvair installation.

Above is a photograph of my airframe with the completed tail in place. In the past month, we've buttoned up a lot of the stuff in the seat and center console area, and made a lot more progress on the wings. As we progressed more with the Zenair kit, I still have to say that this is one of the most straightforward and fastest to build kits I've ever worked with. Coming from a traditional A&P background, I had some questions about things like Avex rivets, but I've got to say that all my questions have been answered in the affirmative. The airplane really is a completely worked through concept. From the plans to the methodology to the parts, the 601 works as an entire concept, and the more we work with it, the more I appreciate this as one of the best packages in aviation.

Corvair Motor Mounts for the 601

I've designed, tested and built motor mounts for many airplanes - not just for Corvairs, but for Lycomings and V-8s as well. Structurally, the 601's Corvair motor mount is a simple, straightforward design with favorable geometry. But my design of the mount takes into consideration innumerable small details. My years of working with the Corvair engine have taught me to search out these details and address them very closely in the exact positioning of the engine. Without making major alterations in the thrust line of the aircraft, or any change in the firewall location of the motor mount hardware, it is still possible to come up with a location that will produce a more aesthetically pleasing cowl, an easier exhaust installation, more room for an alternator, allow removal of the oil pan with the engine in the mount, etc. Additionally, the generous size of the Zenair fuselage allows enough room to be reserved for a later turbo installation. With all these factors weighed in, some of the final revisions to the motor mount design involved lowering the engine a seemingly insignificant 1/8". In the end, I came up with something I feel is truly idealized.

Weight and Balance and Cowling Considerations

A motor mount design which does not address the availability of cowlings leaves behind many builders who have not developed their Fiberglas skills. I've studied the Zenith cowlings, and my motor mount will easily work with their O-235 cowling. The Corvair, a much smaller engine, will easily fit inside that cowling. I've designed my mount so that the prop flange is in the same spot as Zenair's O-235 installation. Although the Corvair motor is far lighter, the weight & balance will work out because we're going to have the battery in a different location.

A Purpose Built Cowling

Right now, we're working on our own highly optimized 601/Corvair cowling. Taking advantage of the Corvair's smaller physical size, our cowling will be much sleeker than the 235 cowling. I have a lot of experience making high end composite parts, and this does not represent a huge challenge.

The Corvair motor, at only 28" wide, will fit into a very tight cowling.

Since our cowl will be built in a female mold, we'll obviously be able to produce them for other Corvair/601 builders. They'll cost about the same as Zenair charges for the parts they build for people who have chosen other engines. In a later installment, I'll show you exactly how the cowl is made.

Motor Mount Fabrication and Detail

My 601 motor mounts are constructed of 4130 aircraft tubing. I weld them using a combination of Mig, Tig and gas, where appropriate. As you can see from the photo above, they're built in a very sturdy jig. Owners of my Corvair Conversion Manual know that many pages were devoted to an in-depth discussion of motor mount building. The Manual contains drawings for a motor mount very similar in layout to the 601's. I'll be happy to provide any Corvair/601 builder with the exact dimensions I used in this motor mount, provided you've purchased a Conversion Manual from me, and returned your Manual Registration and Liability Statement for my files. Although I'm sure most guys will want to buy a mount from us, I'm always willing to help out our customers who are real do-it-all-yourself types.

This is a 38mm Firewall Spool for a 601.

Each motor mount contains eight spools. The spools are used where the bolts pass through the firewall, and where the engine mounting bolts go through the tray. Each spool is made of a piece of 1/2" .058 4130 aircraft tubing welded to an area washer. In the case of the 601, the tubes in the firewall spools are exactly 38mm long. This allows the Zenith O-235 Dynafocal firewall mounts to be used as shown in their drawings. The washer on the firewall spools is an extra thick, CNC hydrocut, 3/16" x 1 1/8" diameter custom job. While the thickness may seem to be overkill, it ensures that the tube maintains its position perpendicular to the firewall throughout the welding process. This in turn means your motor mount will be far easier to install on Zenith's stud type mounts. The four spools that mount the motor to the tray have 1 1/8" tubes and 1 1/4" washers. I cut the tubes to an exact length using a parting tool on my semi-automatic lathe. I weld the tubes to the washers on a variable speed motorized turntable. The lathe-cut ends allow for accurate jigging. And the feed rate of the motorized turntable is optimized for my 220 volt Mig welder. If you've heard that Mig welders are not used in aircraft fabrication, you've only heard half the story. Mig welders can be used for specific parts as long as the setup is proper, and adjustments are made to the methodology. My motor mount design takes these factors into account, and utilizes Mig welding where appropriate. The techniques have proven themselves in static, destructive and flight testing.

The Corvair motor is a bed mounted engine. This means that it is supported from the underside of the motor like a six cylinder Continental or a Franklin motor. The beginning of any mount is the part that we refer to as "The Tray." The tray is directly underneath the engine. Shown in the photo above is my tray jig. Although it's not beautifully painted (because it's used too often), this very heavy-duty and accurate jig weighs about 50 pounds and is immune to distortion during the welding process. The tray is about 90 percent welded in the jig to ensure accuracy.

Drilling a hole in the tray while it's in the main jig.

All the separate tubes of the mount are interconnected with 5/32" drill holes. When the mount is welded, it is air tight if it's done correctly. Without air, it will not corrode inside. Aircraft tubing is fairly oily on the inside from the factory. The drill holes allow the tubes to be welded without "blowing out" as you try and close off the last little bit of weld bead on a sealed tube. This effect of the expanding air in the tube bothers Tig welders more than gas welders. If your friends haven't experienced this with a Tig welder, they're probably not making airtight welds.

The main jig for the 601 weighs about 70 pounds. It may look a little funny, but it takes into account the 601's sloping firewall, which is why the tray appears to be angled when mounted in the jig.

No motor mount can be built more accurately than the jig from which it came. The white diagonal bracing seen on the jig is put in place to counter stresses in the jig assembly process. My 23 years of welding experience allow me to take a lot of subtle factors into account to produce very accurate parts.

Side view of the 601 Motor Mount.

This is a brief overview of the motor mount/cowling situation. We have a few more photos to insert here, so check back in a week or so.

Thank you.

William Wynne

November 2004 601 Page at www.FlyCorvair.com

September 2004 601 Page at www.FlyCorvair.com

August 2004 601 Page at www.FlyCorvair.com

May 2004 601 Page at www.FlyCorvair.com

March 2004 601 Page at www.FlyCorvair.com

February 2004 601 Page at www.FlyCorvair.com

January 2004 601 Page at www.FlyCorvair.com

October 2003 601 Page at www.FlyCorvair.com

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