PEER PRESSURE, well its a bitch. While at Oshkosh 2010, the subject of engine mounts became the conversation with a close friend; A friend who happened to be finishing building a custom dynafocal mount for his one of a kind racer. When I told him that I was planning on buying my engine mount from a supplier, he scoffed at my plan. His words were something like, “well, I happen to know this guy who kinda has a dynafocal engine mount jig that we can modify to build a Long-Ez mount.” Long story short, it was decided that it would be much more educational for me to build my own rather than write out the check. Only problem, the workshop and jig are in Texas and I am in Minneapolis. Good thing I work for the airlines and can get around fairly easily. We started in October, and were just about done end of April 2011. Ideas always sound so much better in Oshkosh! All joking aside I am really proud of my engine mount, and am glad I was nudged into this project.
1. The Dynafocal jig: This Jig was constructed for a different project and modified for Long EZ dimensions. The aft & forward plates are 1/4” steel plate, and the uprights are 1.5”x.25” square tube. There is a hole in the center of the forward represent the thrust centerline. Dynafocal donut placement was found utilizing plans from a T-18 engine mount.
2. Jig Upright Ends: The aft plate did not need to be modified, the dynafocal mount geometry will stay the same. The only things that needed modification were the uprights between the dynafocal ring from the forward plate (represents firewall). These were made at such lengths to create the 2 degree down thrust and also align the firewall tubes with the longerons.
3. Jig Upright Parts: These are the parts for the uprights. Each end is .5” thick and tapped to assemble the jig. I welded the caps onto the end of the tubes. This welding I actually did. I spent some time practicing with scraps since this was my first welding project. My buddy who talked me into this project is a very experienced welder and welding instructor.
4. My Welding: You can see that my welding was coming along. This was the best one I got. There are still some problems with the welds. My consistency and control are not there yet. The width of my weld beads varied quite a bit, the width of the heat affected zone was too large in many places, and it varied whether I had convex or concave beads. We decided it best to let a skilled welder work on the flying parts.
5. Dynafocal Ring: This is how the dynafocal ring kit comes when you order it from Spruce. It is three stamped pieces and 3 tubes which comprises the dynafocal ring once welded. The difference between getting a welded ring and one you have to weld is $50. I don’t know how much your time is worth, but it was about 5 or 8 hours to prep and weld the ring alone.
6. Welding the Dynafocal Ring: Each part was sand blasted. Then, using large hose clamps, the donut gets clamped together to resemble its shape. This is first tacked, then finish welded for four donuts. The donuts are then placed into the jig. I turned aluminum pucks in the lathe to be the inside diameter of the donut to accurately center each one on the jig. (below left)
7. Welding the Dynafocal Ring: Each tube from the ring needs to be fish-mouthed to match the donut. These were done using nothing but a few grinders and belt sanders. Needless to say the fit was not very accurate with this and our experienced welder needed to fill a few gaps. Each tube was fit to be approximately parallel with the aft plate.
8. Finished Dynafocal Ring: This is the view from firewall looking aft with the finished dynafocal ring. Note that the bottom of the circle is not closed yet as that tube is not part of the kit from A.S.S. That comes later in the build. This was all welded without the forward plate attached to the jig as it is not necessary at this point.
9. Test Fit Dynafocal Ring: This was a visual mile stone! We took the dynafocal ring out of the jig and slipped it on a spare case using old vibration isolators my buddy had around. The bolts slid in slicker than $h!+. I have been party to working on engine mounts where you need to use a large motivator (read sledge hammer) to get the bolts in. Not so with my mount!
10. Back at it: Then it was back to work. We put the dynafocal ring back into the jig and assembled the jig. We had to spend some time figuring out how we were going to locate the four stub tubes that attach to the firewall mounts.
11. Locating the Firewall Attach Points: We transfered the center hole (centerline of thrust) from the aft plate to the forward (fire wall) plate (shown). Using the long ez plans and our new center hole, we were able to measure and drill holes at the center of each stub firewall mount tube location.
12. Milling Angles: The drilling was done on a mill with the firewall plate at the 2 degree angle to generate the proper down thrust angle once the engine is mounted. We measured the angle using a prop protractor, it amazing how accurate that measuring device is, we have 2 degrees precisely! The holes were then counter-bored to allow the bolt head to fit flat.
13. Oops: At this point we realized we made an error. We had the lengths of the four corner posts wrong. We forgot to take the .25” firewall into account. Luckily the corner posts were too long so we space the firewall mount tubes off the forward (firewall) plate. I turned these fittings on the lathe as spacers and to ensure the mount tubes located perpendicular to the firewall.
14. Ready for tubes: The four mount tubes bolted to the firewall plate, the dynafocal ring, and the aluminum angle for the firewall/spar mounts. Note that the angles are the 1×1.5” for the O-320. Also note that the bolt holes are in the mount tubes. I am not sure if this will bite me, but precisely drilled in the mill for both the tubes and angles. I think it will be ok.
15. Fish Mouthing Tube after Tube: Here is where I started getting lazy with photos. We had to fish mouth 9 more tubes to connect the ring to the mount tubes. Many of these required bends. Fish mouthing was done a little bit at a time using grinders of appropriate diameters and as I made mistakes I gradually transitioned to using a lathe to fish mouth.
16. More fish Mouthing and Tacking: We set up a jig in the lathe to hold the tube at accurate angles and turned the tubing sized endmill in the head stock. It was slow due to cutting angles but gave much more precise fits on the fish-mouths. Bending the 5/8”x.049 tubes was pretty easy, but getting all the fish mouths to line up properly at oddball angles was a task.
17. Bends and Welds: The bends were done using a tubing bender borrowed from my buddies neighbor. Thanks Dave! This photo shows almost all the tubes installed and tack welded. It was really rewarding to get accurately fitting tubes. The bottom tube (right in photo) is, per plans, 7/8”x.049 tube. We tried 3 different methods of bending, and didn’t have success.
18. Bending 7/8 4130 Steel: We tried 1. Cold (twice), 2. Cherry red hot, 3. Cold, packed the tube with compacted sand, and used high pressure lubricant. All these methods were unsuccessful. We then switched to 7/8”x.060 tube, welded a cap on the end, packed it with sand, welded the other end shut, used high pressure lubricant, and finally success. The key is .060 wall.
19. Finish Welding: I am planning on using an O-320 so notice that there are two extra 5/8” tubes for extra strength. This way the top of the dynafocal ring is not cantelevered. The 7/8 tube was installed and once that tube was in, there were three more 5/8” tubes to go and we were all done. It was just a matter of finish welding, jig assembled.
20. Whats This?: If you have been studying the photos, you have noticed this Schroder valve. All of the tubes have ventilating holes between them. Air can pass freely into all the tubes of the mount. This valve will allow me to inflate the engine mount. At annual, if you check the pressure and it reads zero, you need to investigate for cracking. It has been tested, & holds 100 PSI for weeks.
21. Stress Relieving: While welding we build up internal stresses and harden the steel. So it is necessary to normalize the weldment. We took it to a heat treating facility and utilized a well known aircraft manufactures standards and processes for normalizing 4130 in an atmosphere chamber. It was $99 compared to using an oxy-acetylene torch, but worth it IMO.
22. Stress Relieving: Since the pucks that hold the dynafocal ring to the jig were made out of aluminum, those were remade out of steel before normalizing. The melting temperature of aluminum is significantly lower than the normalization temperature of steel. After back from normalizing it was prep for paint by sandblasting.
23. Priming: We used the Stits Aero-thane products to finish the mount. It was primed and painted in my buddies dedicated paint room. There are about three light coats of primer on the whole thing.
24. Painting: Another first for me. Priming and painting with a “real” paint gun and paint system. All the prep sure is a lot of work, but it pays off. Of course I am not a professional, but the runs are minimal and there is one spot where I didn’t do a great job covering the primer, as such you can see it looks a little dusty in this area.
25. Let it Dry, and get it mounted: This is the finished engine mount in all its glory hanging in the paint room drying. I left it there reluctantly, I wanted to bring it home, but figured I would let the paint harden for a few weeks. Besides I need to use the mill to accurately drill the angles to match the mount, might as well do all that before I bring it home.
26. The paint I used: My buddy uses Stits products because he has a good source for the stuff. He actually didn’t have the paint he thought he did on my last day at his house. I thought we were sunk for this trip, but low and behold, a quick phone call and trip to his neighbors, and we were re-stocked ready to go.
PEER PRESSURE, well, yup it certainly is a bitch. It took me about 8 months of trying to arrange time in my work schedule to travel 800 miles to work on my engine mount. Cost: Well, lets just say that my guess is I only saved a couple hundred dollars, IF that. But the education was awesome and completely enjoyable. I really enjoyed learning welding, fish mouthing tubes, heat treating processes, and a bit about painting too. I am really glad my buddy nudged me to make my own mount. Every time I look at my engine mount, I am proud that we built it. I can’t wait to have it hanging on a fuselage!
BTW: My buddy created a monster, I bought a Lincoln tig welder on Craigslist and have it wired up in my shop. Now I just need a bottle of argon and a reason to melt metal!