DIY

DIY - ENGINE MOUNT ~ made simple

Originally this mount was created for a GlaStar the same principle will work for almost any plane.

Having decided to put an auto engine in my GlaStar, I found that I was completely on my own to supply a suitable engine mount. After lots of head-scratching, I fabricated brackets to adapt four Lycoming style conical mounts to my motor. The problem now was how to hang my engine package in front of the airplane so that it was sitting exactly where I wanted it.

The first thing I did was to determine my engine and redrive weight. I then hung the whole engine drive assembly with all its accessories from my engine hoist and shuffled it around until it hung evenly. This gave me a weight and center of gravity for the unit. I got in touch with the factory and had them send me all of the weight and balance sheets that they had to date for the various GlaStar/engine combinations that they had tried out. With this information, I put together a "design" weight and balance sheet for my airplane by subtracting the factory engine and prop and inserting my Subaru package figures. All of this was made very easy by putting together a simple weight and balance program on my computer. I could then shift the engine forward an inch and see what happened to the balance. Or, I could toss on a heavier prop and see what happened to the figures. Although all of this was theoretical, I did end up with a fairly good idea of just where I wanted my engine package to be located.

The next step was to build a big alignment jig. I duplicated the firewall profile and its engine mount holes on a piece of 3/4 inch plywood. The horizontal and vertical center lines of the airframe were drawn on this piece. This wooden firewall was then mounted on a heavy frame which had four parallel vertical "posts" running up from it at exactly 90 degrees. (See photo on left.)
A cross-frame was fabricated (see photo below) which held a wood socket to pick up on the propeller flange. This frame was made so that its arms exactly fit the spacing of the vertical posts and that the propeller pick-up flange coincided with the firewall center lines. The cross-frame produced the desired end result, that of holding the prop flange exactly on center regardless of where the engine itself was placed - forward, aft, left, tight, up and down. I could easily remove the engine from the jig and put it back without having to do a whole bunch of re-measuring and fitting.

I then turned a wood spacer to give me the desired distance I wanted my engine to be from the firewall. The turned piece gave me the advantage of easily picking up on the crankshaft damper puller and giving me an accurate center at the other end. Next, I calculated the offset I would need at the pulley in order to achieve the required GlaStar propeller offset angles. In my case, I needed the engine to be tilted 1.5 degrees to the left and 1.8 degrees down. For my airplane, this calculated out to be a crank pulley set-over of 13/16 of an inch to the right and 15/16 inch up. This was marked on the dummy plywood firewall and here I inserted a pin to pick up on the center of my wood spacer. The close-up photo (above left) shows the locator pin centered on the offset marks.

The jig was then leveled, the round wood spacer plugged in and the engine/redrive combo lifted and placed on the spacer. The cross-frame was then clamped to the vertical posts with the center picking up on the prop flange. The engine was then rotated so that it would be level with the airframe. This was done by dropping a plumb line down from a common point on each valve cover to catch the horizontal reference line in the firewall. Everything was given a last check and I was ready to start cutting and fitting mount tubes.

The rest is pretty straightforward. I used 3/4 inch .049 wall 4130 for the tubes. I found that it was far easier to simply tack weld the tubes in the jig and then bolt the mount to my fuselage where the final welding was done. This way I didn't have to worry about my plywood pulling or warping and I was guaranteed that my mount was "dead on" with the fuselage.

I also found that replacing my rubber engine mounts with temporary sold wood replicas kept everything in better alignment for welding. Finally, for the one or two areas that still were pulled a little bit out of alignment, I simply fired up the oxy-acetylene torch and did some gentle tweaking. A little spot of heat caused the tubes to shift into perfect position.

I have resisted the temptation to do any final painting or powder coating on the mount as I plan to weld on small attach tabs for all the accessories and last minute goodies that I am sure will be stuffed into the engine compartment.

As a final note, this system will also work with a belt redrive. All one has to do is establish two centers on the "firewall" - one for the prop and one for the engine crankshaft.

Thanks go to Jeff Liot for his method on making an engine mount

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