AeroModeller – June 2018

(C. Jardin) #1
enterprise in 1985, keeping it ticking
over as part of his current business.
The Super Hunters sit among his West
engines, exhaust systems and other
off erings.

The original Super Hunter revamp carries
over the Hunter’s basic core, but has
new nylon “back door” assembly, revised
cylinder porting and an exhaust throttle,
replacing the carburettor throttle. Alan
reasoned that a water-cooled cylinder
soon cools below 70 degrees C when
running at low throttle. This delays
pick-up when the throttle is opened. An
exhaust throttle gives better cylinder
temperature control by restricting
exhaust gas fl ow at lower speed settings,
resulting in rapid throttle response.
Conversion to aero specifi cation involved
prop driver and nut, a new fi nned
cylinder jacket and piston/cylinder
reversal from front to rear exhaust.
Normally, that leaves just enough full skirt
depth covering the exhaust port width
(to avoid sub-piston induction) with the
remainder cut away to clear the induction
rotor and ease gas fl ow to the transfer
ports towards BDC. Ours is unusual,
having only a small nib of metal, the gaps
either side towards the top of the stroke
giving signifi cant SPI.
The crankcase has E.D.’s later

tubular front section. It looks heavy,
but much of it is hollow behind the
front bearing, leaving only enough
around the crankshaft to seal crankcase
compression. The solid unbalanced
crankshaft rides in two English 3/8 inch
by 7/8 inch ball races. As there is no
step down to a smaller front bearing,
crankshaft position is maintained by a
decent press fi t into the bearings, which
in turn are fi rmly held in the crankcase.
The shaft’s tapered seat for prop driver
is knurled – a truly belt and braces
Induction is via a nylon rear rotary
disc fi tted to a hardened steel pin, riding
in a thin-walled brass bearing in the
nylon “back door”. A small E-clip and
washer limit rotor end-fl oat. Forged
aluminium conrod with unbushed 3/
inch big and little end bearings. Novel
wrist pin is retention by a simple wire
circlip engaging grooves in one wrist
pin end and piston boss. A special
tool is required to compress the circlip
while guiding the pin into the piston, so
do not disassemble unless absolutely
The generous aluminium cylinder
jacket retains the hardened steel cylinder
by bearing on its base fl ange. The
cylinder itself is quite thin-walled, even
at its larger, lower section, limiting the
internally milled transfer passage depths.

Super Hunter has a single exhaust
port and three transfers – one opposite
the exhaust with inclined top and two
perpendicular, having square fl at tops.
Somewhat like Schnuerle porting, but the
opposing side transfers direct incoming
mixture towards the bore centre, not
away from the exhaust. This can’t form
to the true loop scavenging pattern at
the core of the Schnuerle design, but
has worked quite well with other general
purpose model engines.
The throttle is a straightforward aff air
with brass rotating barrel housed in an
aluminium manifold. A spring-loaded
stop-screw sets the minimum exhaust
aperture, controlling idling speed. It’s
quite large, for adequate outlet area
when running at full speed. Not a
problem for marine use, but at 1.35 oz
(38g) it is rather hefty by aero standards.
It can of course be easily removed if
throttling is not required.
Before putting the Super Hunter to
work, it had to be “thawed out”. Usually
warming the whole thing up to soften the
old castor oil residue and lay-up oil does
it. Ours took a little more coaxing to free
up the intake rotor pin in its bearing.

On the Bench
I recently read an amusing comment by
someone on an internet forum, regarding
model diesel engines. Said they were


Exhaust throttle is out of the rut. Has excellent throttle response, but there’s
a catch. We explain and offer a simple fi x.

Generous fi n area moderates cylinder temperature. Outer diameter belies
modest steel liner dimension.

POWER TRIP 055.indd 9 26/04/2018 15:

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