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On the subject of shimming a dynafocal mounted engine by inserting washers between the engine and the isolator, there is a small detail of which you should be aware from my "Been there, done that" file. The mounting geometry is going to be just a teensy bit off. It's nothing that can't be dealt with as long as you don't have to shim very much, but if you do shim, the engine mount bolts - always a bit of a hassle to get into place even without shims - will be even more reluctant to go quietly. I found out the hard way by screwing up when I fitted the cowl to the spinner back-plate, forcing me to shim the engine forward by about 0.090" or rip into the finished cowl to fix things. I elected to shim with one AN970 between each isolator and the engine, but getting the bolts in place allowed me to invent a whole new vocabulary! For dynafocals, the more you shim, the more the mounting geometry will be off, since as you add shim thickness, the centerline of the mounting bolt exit holes (on the mount) moves toward some central point. It doesn't take a whole lot to get to a point where the mounting holes in the engine and those on the mounts won't align no matter how many new words you invent!

To Bill Gradwohl, I doubt that gravity-feed from the minimal head available at the height of the wing tanks would be able to keep up with the fuel rate demand of even my small O-235, let alone a 320 or 360, especially for takeoff. I don't remember the Advisary Circular number that treats minimum fuel rates, but I believe there is a general rule of thumb that says you should be able to flow at least 5 times (sombody out there correct me if that's wrong) the takeoff flow rate by whatever means your fuel system feeds the engine. Your proposed 1-pint header tank would be drained by the engine demand (by the engine pump) far faster than gravity could refill it. A high-wing like a Cessna (I'll wash my mouth out later) can gravity feed because they have at least 5 or 6 feet of head from the wing to the carb - plenty to keep the fire going, but it's unlikely that the head-pressure in a low wing would be enough.

Also, to Bill, I'm sure I remember some articles a few years back in the EAA magazine about an engine development for a replica P-51 that did run the output shaft along the top of the engine, as you suggested. If I remember correctly, the fellows doing it mounted the engine "backwards" in the airframe (actually, in the same orientation that it would be in a car - with the engine output shaft toward the rear - ignoring front drive, transverse mounted engines for now). The the output shaft was driven at the aft end via belts (where the reduction took place), with the prop shaft running forward along the top of the engine thru suitable bearing mounts. It sounded kinda neat at the time, wonder what ever happened to it? It also sounds right, that a long, suitably supported and designed shaft should be able to absorb the torsional impulse that the very short, stub shaft cannot. Though it may be, that just by using a belt reduction, with it's inherent "softness" (for a better term) instead of gears, is the real answer.

Cheers,

Dan Schaefer


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