Displacement: 39.96cc/2.439 cu. in.
Bore: 40.0mm/1.575 inches
Stroke: 31.8mm/1.252 inches
Practical rpm: 1,800-9,000
Output: 3. 75 hp at 8,600 rpm
Weight: 3 pounds, 2. 2 ounces
Engine: 42. 65 ounces
Silencer: 3. 98 ounces
Ignition: 3. 53 ounces
Fuel consumption: Approximately . 75 to . 8
ounces per minute at full
Street price: $799.99
• Easy starting.
• Reliable idle.
• Smooth midrange and acceleration.
• Strong power.
• Great sound.
• None noted.
AT A GLANCE ...
The lightweight ignition requires between 4. 8 and 8. 4 volts, giving many options with
NiCd, NiMH, LiPo, or LiFe batteries.
This view displays the machining quality
for which O.S. is known. The CM- 6
spark plug is included with the
an attractive, finely textured finish. My intention was to tear
down the engine for a closer look, but the socket head cap
screws securing the major castings were all tighter than I was
comfortable forcing. I suspect they are using a thread-locking
compound during final assembly. The fit and finish is so
precise that I could barely detect the parting line between the
backplate and crankcase castings.
The two-piece, cast-aluminum silencer has a baffle plate
sandwiched between the main shells. These shells are easily
separated, allowing you to reposition the angled exhaust port
to best suit your installation. Soft aluminum gaskets minimize
leaking at this
mounts to the
cylinder on a
15mm of linear
pair of lock nuts
secures the silencer
in your desired position.
The rear carburetor features two readily
accessible needle valves as well as traditional
throttle and choke levers. And here’s a nice
touch: one of the mounting lugs has a ring cast
onto it to support and guide a pushrod for
manual choke operation if desired.
Setting a Baseline
It just so happened that my 1/4-scale SR
Batteries Eindecker had been gathering dust
in my hangar and was a ripe candidate for an
upgrade. The 17-pound World War I model
was originally designed for a popular 26cc gas
engine and it flies well on its large wing, but
more power never hurts.
Before swapping engines, I made one last
trip to the field to gather some reference data with its original
engine. I didn’t expect excess power to noticeably speed up
this draggy WW I kite, but I did know it could significantly
boost the rate of climb.
To watch that trend, I installed a ZLog recording altimeter
from Hexpert Systems and headed out to the field. This nifty
gadget tracks altitude across time, making it easy to calculate
rate of climb. The original engine pulled the Eindecker off the
ground in slightly less than 100 feet and gives a maximum
climb of 450 feet per minute.
Time to head home and install the new O.S.
Mounting the GF40 is straightforward with mounting tabs
integral to its backplate. The rear carburetor configuration
requires 1.5-inch standoffs. Because none were provided, and
I hadn’t thought to order them with the engine, I grabbed a
length of 6061-T6 from my metal stash and then spent some
quality time with my Sherline lathe. Hobbico offers standoffs
in several sizes and I could have just ordered a set, but at that
point making them was
72 Model Aviation APRIL 2014