In this note we're gonna discuss an (almost) antique engine: the venerable ol'
McCulloch 4318, more commonly thought of as the Mac 72 - - 72/90 horsepower @
about 6,000 RPM, 72 cubic inches displacement, 72 pounds [devoid of all/any
accessories] . . . . . . . . Four cylinder, horizontally opposed, air cooled
like classic VW 'boxer' engines.
A loooong time ago these engines were designed, manufactured and used for one
activity ONLY - - McCulloch sold 'em to go on the firewalls of gunnery-target
drones and were intended to be flown only once and even then only very briefly.
Ha! Having been through Naval Aviation gunnery training as a NavCad student
pilot myself, and having watched a lot of ground/sea-to-air gunnery training
among all the branches of military, it was a rare machine that was actually
blown out of the sky on its first meager tank of fuel! The classic phrase of
'shitty shots' comes to mind -- yours truly included! Anyway . . . . . . . . .
The engines had a few . . . . well, 'idiosyncrasies' . . . . to say the least.
Two cycle/two stroke fuel/air mixture - - 50 parts avgas to one part oil,
caused plug-fouling quite soon. Single mag ignition. Really weird ignition
firing sequence: front two cylinders fired simultaneously, then aft two
cylinders fired simultaneously - - I have never read or heard an explanation
for this oddity, but HAVE read & heard that the lil four-bangers vibrated so
badly that within just a few minutes the bushing/bearing areas on the carb
butterfly valve would literally disintegrate.
Probably the worst aspect of the engines was the absence of or very poor
quality of all bearings &/or journals used throughout the engine! About forty
(40) hours was the max I have heard was the most hours one ever obtained on an
otherwise un-modified engine!
However. With only minor machine-shop work and the installation of decent
bearings & journals, and the re-timing the ignition (of course) TBOs of up to
almost 400 hours were not uncommon. Many of the surplus engines were modified
and ended up on gyrocopters and more than a few homebuilt fixed wing flying
machines.
I have heard a rumour -- from several very diverse sources -- that there was a
retired machine-shop owner somewhere in Kansas who was also an
experimental/homebuilder of aircraft. Supposedly, he took two of the Mac 72
engines and reworked them to an extreme extent: ball bearings were installed
at both ends of the crankshaft, needle bearings were used on the rest of the
crank support as well as both ends of the connecting rods! Dual ignition was
installed and precisely timed. Injector-type carbs (two) and tuned dual intake
manifolds and tuned exhaust manifolds were also used. Etc., etc., etc.
According to the sea-stories/fairy tales, he got over two thousand hours total
time on the two engines and upon tear-down and precision inspection he found
absolutely NO appreciable wear . . . . . . . just a rumour, though . . . . . . .
[Ya know the difference between a sea-story and a fairy tale? Well, a fairy
tale starts out, "Once upon a time . . . . . . . " A sea-story usually starts
out with the adamant phrase, "Now, this is NO shit! . . . . . . . "]
OK, we're finally at the crux of this missile: There are several thousand
homebuilt, experimental flying machines, all over the world. AND there are
darned near that many engine variations! Most are outlandishly expensive, most
are of questionable reliability, most are one-of-a-kind, etc. etc. etc.
How many readers would be interested in a revival of a ('modernized') up-dated
Mac 72-type engine???
Let me know, please. We're getting ready to contact the myriad folks who hold
title to the old Mac drone engines.
WxBY
ORL/MCO
