AeroModeller – June 2018

(C. Jardin) #1

Power Trip

10 AeroModeller - June 2018

smelly and cantankerous. Sometimes
brought out by the old druids in his
Club and mounted on the test bench,
just to show they can get it started.
Au contraire! Smell aside, this one is
satisfyingly simple. Open the needle
valve by one extra turn from the running
setting, three carburettor chokes once
the fuel line is full and four or fi ve turns
without choking to pump mixture to the
cylinder. It took us six fl icks to start from
new (on E.D.’s settings) and thereafter
almost unfailingly one fl ick. Exhaust
priming is unnecessary. We put this
excellent starting down to the nicely
free ball race shaft mounting, minimal
crankcase volume arising from the disc
induction system and very modest
transfer passage area.
There were no signs of stress, right
from the start, but we ran it around
10,000 RPM on an easy setting in short
bursts for 30 minutes. We noted a hum
from the bearings, so primed the front

bearing with oil before each of these
initial runs, just to be sure it was well
lubricated. Those old style ball races
do take a bit of settling down, so a fuel
with high oil content is sensible at this
early stage. Compression and mixture
adjustments worked as they should and
held their settings well.
Vibration remains at a low to moderate
level throughout the useful operating
range. Thereafter, it builds noticeably.
By 14,000 RPM, tuning became tricky,
unless the needle setting remains
somewhat rich. Actually, Super Hunter
rather likes running a bit rich and RPM
doesn’t drop much by doing so. Fuel
for operating speeds up to 12,000 need
only contain 1 percent ignition improver.
We upped that to 1.5% for higher speed
running and our RPM checks, but kept
with a generous 30% castor oil content.
Alan suggested E.D.’s Super Zip fuel
formula, for extra power. This contained
20% high quality 2-stroke mineral oil and

2% Iso Amyl Nitrate. Spot tests with our
equivalent of this had the Super Hunter
running happily and with markedly less
exhaust goo. The extra ignition improver
smoothed out running in the 14,000 RPM
range, but no increase in measured RPM.
Perhaps that gain will come after more
running time?
The four cylinder retaining screws
needed regular tightening during running
in, as the paper gaskets found their fi nal
seated shape. Easily done (of course)
after loosening the compression screw.
Throttle performance with our
Graupner 9x5 propeller was excellent.
RPM could be reduced by two thirds,
to a safe 3200-3500 RPM. Response
when opened again is unmatched by
any diesel with throttled carburettor, in
our experience. Alan’s reasoning is well
justifi ed and any amount of prolonged
idling seemed to have no ill eff ect.
Throttling range is non-linear, with
increasing eff ect as the barrel opening
almost closes the exhaust. In fact, the
fi rst half of the nearly 90 degrees rotation
does very little. Adjustment is simplicity
itself. Set the engine as usual at full
throttle and then fi ne tune idle speed with
the barrel set screw.

Performance Analysis
Torque rises slightly from 30 oz-in at
truly slogging speeds to 32 oz-in at
12,000 RPM. Thereafter torque drops
quite rapidly, perhaps as a combination
of vibration and quite restrictive transfer
passage areas. This is refl ected in the
power curve, which does not extend
much further before its decline. Despite
generous eff ective choke area and rather
racy port durations. Even so, just topping
0.4 BHP between 13,000 RPM and 14,

Bore 16.68 mm (0.657 in)
Stroke 15.97 mm (0.629 in)
Swept Volume 3.49 cc (0.213 cu in)
Weight 272 g (9.6 oz)
Exhaust duration 164 degrees
Transfer duration 146 degrees
Intake opens 45 deg. ABDC
Intake closes 40 degrees ATDC
Intake duration 175 degrees
Sub-piston induction 75 degrees
Eff ective choke area 16.3 sq mm
Height (mounting face to top of cylinder) 62 mm (2.4 4 in)
Length (prop face to rear of venturi) 82 mm (3.23 in)
Bearer spacing 32 mm (1.26 in)
Mounting hole pattern 14.4 x 39.5 mm (.567 x 1.56 in)

Propeller RPM
APC 8x4 15200
Graupner 8x5 14200
APC 8x6 13900
APC 9x4 13300
Graupner 10x3 12500
Graupner 9x5 11700
APC 9x6 11100
APC 10x4 10500
APC 10x6 9100
APC 11x6 7300

POWER TRIP 055.indd 10 26/04/2018 15:

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