World War II, a lot of leisure activities were sacrificed due
to unavailability of raw materials for manufacturing products needed
to pursue them. We have all see photos of kids collecting scraps
of metal, rubber, and other materials for recycling as components
of airplanes, guns, canteens, ships, etc. Aircraft modeling took
a hit along with most other hobbies since metal for engines and
wood for airplane kits (balsa was popular for shipping contain packing)
were scarce. It wasn't until the late 1940s that Roy Cox was able
to begin mass producing his famous line of miniature engines, most
notably the .049 family. Other manufacturers - like Allyn - were
following suit. This article reviewing the Allyn .049 Sky Fury came
Allyn .049 Sky Fury Engine Review
Allyn .049 Sky Fury reveals new, important improvements, runs steady,
wide rpm range.
By E. C. Martin
The design of the Sky Fury affords big-prop, high-torque performance
as well; good for free flight.
A novel and most interesting engine, the Sky Fury is a refreshing
departure from conventional Half-A design in almost every detail,
and justifies its difference with a sparkling performance. The Allyn
engineers have obviously tossed convention out of the window and
made a fresh start, with a result that embodies several functional
improvements which are doubly attractive because they also simplify
The basis of the Sky Fury is a neat
aluminum pressure die casting which incorporates intake, main bearing,
crankcase, the entire cylinder including head and fins, and large
double opposed exhaust stacks. Three equally spaced radial mounting
lugs of a full 1/8 in. thickness do much to eliminate the broken
lug bug, and the absence of a cylinder flange thread takes most
of the risk out of a careless plug change, whereas the widely used
screwed cylinder assembly is prone to distortion when tightening
of the plug or cylinder is not carefully done.
barrel is accurately bored to a close slip fit for a drop-in type
cylinder liner, and a shoulder at the bottom provides end location
in one direction, while a plug screwed into threads at the top of
the bore constrain it in the other, thus sandwiching the liner in
a manner which directs all stress axially along the cylinder walls.
In this way, all the bore distortion from radial thread loading
common to normal screwed assemblies is removed.
and rather small diameter crankshaft rotary intake provides unusual
flexibility and potent fuel suction, but somewhat restricts top
end performance. Several other design features tie in with this
carburetion characteristic to provide a great deal more torque than
usual among Half-A glow engines which are almost exclusively in
the high bhp at high rpm category, and the resulting big prop performance
combined with usefully dimensioned exhaust stacks, long jet needle
and single vent large capacity tank, makes the Sky Fury of interest
to free flight scale enthusiasts. Viewed in this light, a slightly
longer intake would probably improve carburetion still further,
and greatly facilitate handling and the attachment of an extension
for cowled installations.
nickel plated brass spray bar, with a split female thread frictioning
device, is press fitted into substantial bosses on either side of
the intake, and the delivery side has a bore that is smaller than
the maximum diameter of the needle taper so that a positive fuel
shut-off is available if required.
Every effort has been
made to reduce crankcase Volume and charging loss to a minimum.
The bearing end of the crankcase is accordingly hemispherically
shaped with a small annular projection to sustain the thrust loading
of the crank disc, and the disc itself is heavily chamfered to suit,
and also to clear the overlap of the piston at bottom dead center.
The two alternative backplates supplied are arranged to butt up
as close to the crankpin as possible and are stepped for piston
Aluminum pressure die castings of substantial
proportions comprise the combined backplate and radial tank mount,
and alternative regular backplate, both being formed to match the
crankcase mounting lugs. The tank blends into a bell shape in order
to provide a mounting face of large area, and to accommodate a large
fuel supply without excessive overhang. Only one vent tube is fitted
and this extends down into the tank in such a way that fuel level
when filled is only slightly above the jet with the engine in a
horizontal position. A boss is cast into the bottom of the tank,
however, which can be easily drilled for a lower vent tube, and
the existing upper tube can be replaced by one of appropriate length
to produce a simple stunt tank. The plastic delivery pipe extends
through the usual hole in the top to the tank bottom, and owing
to the lack of a second vent it is wise to use the delivery pipe
for filling if carburetor flooding is to be avoided. Three mounting
bolts are supplied which also serve to retain the tank and tank
backplate. Vellumoid gaskets are used at both joints.
case hardened steel crankshaft is of average dimensions for the
displacement, having 7/32 in. outside diameter, 5/32 in. dia. gas
passage, and 1/8 in. dia. crankpin. A circular valve port of 5/32
in. dia. is used, which is ample for the size of air intake. Apart
from a most excellent surface finish, the crankshaft is interesting
because it is hollow through its entire length, and the prop retaining
bolt is the only agency which opposes crankcase leakage. In practice
there is slight oil seepage of no consequence, apart from which
the system seems very satisfactory. The payoff on this feature goes
to the man and machine that tap the hole. It is considerably quicker
to tap than the usual blind variety. A fairly abrupt taper provides
keying action for the aluminum prop driver and an aluminum washer
and blued steel bolt retain the special 6 x 3 Sky Fury (Top Elite
manufactured) prop supplied.
A machined aluminum conrod
with ball jointed small end and outstanding crankpin fit and finish
is retained on a recessed boss inside the hardened steel piston
by a cup which is swaged over rod and boss to form a permanent assembly.
piston skirt is about 30 percent longer than usual for the displacement,
and as a result, exceptional life may be expected from both piston
and cylinder, as loading is distributed over a larger area and better
guidance reduces piston rock. However, we seldom get something for
nothing, and the above benefits are offset, to some extent, by the
necessarily longer cylinder and the increased total crankcase Volume
arising from the greater internal Volume of the piston. This is
actually a limiting factor for ultimate performance which has already
been prevented by the intake and shaft valve, so in the final reckoning
the piston dimensions may be taken as an asset, as it is under heavily
loaded conditions that the Sky Fury excels.
cylinder liner is extremely interesting as it employs a bypass porting
system that is unique in this country, while paradoxically being
almost universal among European engines. It takes the form of four
grooves which appear to be holes with an imaginary center inside
the bore diameter, which is indeed exactly how they are made. A
plug is inserted in the bore, during manufacture, which has four
holes in it that break out of its outside diameter. An end mill
inserted in each hole then chews a crescent shaped groove out of
the cylinder wall to a depth corresponding to the height of a normal
bypass port. The final result is a port and bypass passage produced
in one machining operation. The piston, of course, provides the
inside wall of the passage. The advantages are smooth gas flow and
excellent piston lubrication, while the disadvantages devolve on
the manufacturer in the form of worn or broken end mills and bore
The cylinder wall has to be slightly
thicker and heavier than usual to accommodate the grooves, but,
as a result, has a resistance to distortion and heat conducting
quality that would be an asset to most of the conventional designs.
Two opposed exhaust ports, each covering about 160°
of the bore circumference, are located just above the bypass grooves
to provide valve events of typical nature for a radial ported engine.
Piston crown and combustion chamber, however, have flat faces, which,
contrary to expectations, did not seem to produce excessive charge
loss or dirty exhaust.
The head, as already mentioned, is
an aluminum plug which screws into the main casting to bear on the
top lip of the cylinder liner, with a plasticized gasket between,
and has a pair of small flats projecting about 1/16 in. above the
casting for wrench application.
An interesting glow plug
is supplied. It has a very large Volume recess and an element in
the form of a "W" pointing downward. During the test, conventional
plugs were substituted and it was found that few gave such good
results at any speed and none at very low speed, where a point could
be found at which no other would sustain running unless the battery
It was also apparent on the test that the
Sky Fury has remarkable range of speeds over which it will run steadily.
A sticking tendency between piston and cylinder may be in
evidence in a brand new engine. This is a common peculiarity of
the materials used and will disappear eventually. It has no effect
on performance and occurs only during hand cranking.
Allyn Sky Fury .049
Plug: 1/4 x 32, Short reach as supplied,
1-1/2 volts to start; Fuel: O & R AA; Running Time Prior to
Test: 2 hours; Bore: .390 in.; Stroke: .400 in.; Weight: 1-3/4 oz.
6 x 5
6 x 4
6 x 3
5 1/4 x 5
5 1/4 x 4
5 1/4 x 3
6 x 5
6 x 4
6 x 3
Sky Fury prop supplied
Posted March 1, 2014