AeroModeller – June 2018

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: