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sink-me
Motor of the Month: Wasp .049 Engine
Atwood enters the Half-A engine picture with snappy contender; battles for top
honors in mini-motor field
A number of radical features have been incorporated in the design of the new
Class AA (or "Half-A") Atwood Wasp .049, resulting in good appearance and outstanding
performance. Some improvements such as short stroke, long crank-shaft, and cap type
cylinder head carrying most of the cooling fins bear a close resemblance to modern
European diesel engines.
Engine "whittlers" from the speed circle set will take great delight in sawing
off the gas tank and filing away excess glow plug metal to produce the shortest
and smallest frontal area currently possible in the mini-motor field. Crankcase
diameter and engine height have been reduced by using a short stroke and short connecting
rod. High rpm is achieved with a low stroke bore ratio and new 3600 intake and exhaust
porting. A circular recess bored in the cylinder acts as an intake port.
Fuel flows from the base through two bypass ducts to an intake distribution manifold.
Three large mill-cuts connect this manifold with the intake recess. The cylinder
with its many ports, passages, threads and only two cooling fins looks like the
firing chamber from a Buck Rogers ray gun. Speed flyers will also like high speed
wear resistance added by the steel crankshaft bushing.
Half-A stunt got a small start last year and should soon come to the front when
the Wasp gets into capable hands. Here is a new high in steady running. The fuel
level test of seven and one-half inches is more than double that of former designs.
Atwood undoubtedly will regain its stunt fame set with the old model JH.
Free flight and sport fans should be well satisfied with the Wasp's pulling power.
The performance chart shows 11,600 rpm on a wide blade 6/3 propeller. A built-in
fuel tank provides a very simple radial type power plant installation. Use a solid
firewall for even pressure against the back of the tank to avoid leaks. A thin aluminum
plate closes the back of the tank and is sealed with a gasket. Engine run was timed
at 1 minute, 50 seconds (11,600 rpm; 6/3 prop). The Wasp has good strength considering
its light weight. Inverted landings on the cylinder head should be avoided.
Five minutes of running was sufficient to limber up the test engine. At the end
of two hours there was no change, so the performance tests were made. Most Half-A
engines run with a sputtering noise but the Wasp had a steady clear explosion at
all operating speeds. At 15,000 rpm running was very consistent and the engine was
not sensitive to fuel adjustment.
Improved scavenging due to the 360 ° intake port probably accounts for the
steady running. Engine output increased 1,000 rpm when miniature fuel blends were
used rather than standard varieties. Starting was best with a small prime in the
exhaust and little or no prime in intake. Choking flooded the engine and usually
caused hard starting. Needle valve should be opened approximately one turn so fuel
will pick up after a burst.
Engine Data
Performance. Bare weight with tank: 1".33 oz. Propeller - 6/3
wide blade wood 116000 rpm; 5 1/2/3 wood 12900 rpm; 5 1/2/3 plastic 1400 rpm; 5/3
wood 15000 rpm; 5/6 wood 11600 rpm. Fuel: more speed from miniature blends. Fuel
level test: 7 1/2" at 14000 rpm.
Design Data. Displacement: .049 cu. in. Class; Half-A (A/2)
free flight; A for control line. Stroke: .358 in. Bore: .421 in. Stroke-bore ratio:
.846. Compression ratio head; 6.4. Compression ratio base; 1.66. Port area. intake:
.0123 sq. in.; bypass: .0126 sq. in.; exhaust: .0453 sq. in. Ignition: short glow
plug.
Construction Features, Bearings: crankshaft, steel bushing;
crankpin, aluminum connecting rod ball, aluminum and brass. Inlet and exhaust ports,
360°. Head covers most of cylinder.
A rounded crankcase adds a streamlined appearance to the Wasp. This is accomplished
by using a cone-shaped crank disk. Base compression and weight are also improved
by this feature.
The only criticism after careful testing concerns the short needle valve control
knob. Exhaust gas shoots out in all directions and is very uncomfortable on the
fingers. Adjustments can be made by holding a propeller blade against the cylinder
to deflect heat away from the fingers, A needle valve extension would be helpful
for cowled-in ships and is recommended for general use.
Wasp .049 Parts Illustrated
IMAGE HERE
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