Like the Wankel rotary engine, the horizontal piston engine seems
like a good idea on paper. Both engines were masterpieces of outside-the-box
thinking, and were successfully implemented in both prototype and
production. Unfortunately, both engines suffered from technical
issues that ultimately excluded them from being widely adapted.
This report on the Aero 35 horizontal piston engine appeared
in a 1964 edition of American Modeler, and also happens
to be the year (maybe the only year) that it was available from
its manufacturer, Aero Research and
of Buffalo, New York. The ingenuity that went into designing and
implementing this engine is amazing. Here is an incredibly in-depth
analysis on the
Aero 35 on the Model Engine News website that is worth
your time reading if you want more details on how the engine works
and how an actual example performs.
Netzeband Reports on the Aero 35 Horizontal Piston Engine
The dream of numerous powerplant experimenters seems to have
come true with the Aero 35; Wild Bill gives you his personal reaction.
At infrequent intervals during the life of our hobby someone
takes an outstanding technological step upward. The Aero 35 is an
example. We went to the Buffalo plant for the following observations.
Mr. Auggie Savage and John Piston (who better to design engines?),
worked 20 years and built 45 test engines before obtaining the perfection
necessary for a production version. This engine has been thoroughly
bench run, endurance checked, crash and flight tested.
Basic desire behind the departure from standard was a truly streamlined
engine, to be used by scale builders and special types. However,
the engine is also exceptionally vibration free. Since the piston
reciprocates parallel to the shaft, its vibration is applied along
the fuselage, rather than normal to thrust line. It is quite easy
and natural to absorb this vibration. R/C and Stunt types will reap
benefits from this. The engines we ran at the plant were exceptionally
smooth, compared to the finest balanced standard engine.
The basic transfer motion of piston reciprocation to crankshaft
rotation is unique, but not radical. It is well engineered, based
on sound geometry and entirely practical. It goes something like
Connecting rod is an L shaped part with connection into the piston
via a slipper block, a ball socket mating with a fixed bearing in
the crankcase and a pin bearing through a slanted hole in the crank
shaft (see sketch). In the piston joint, 3 degrees of freedom exist
- vertical rotation, vertical linear motion and horizontal rotation.
Simply stated, the piston pressure forces bottom end of rod (1)
to move in an arc about the ball socket (2). Geometry is established
to minimize the vertical travel of (1). This motion is translated
into vertical force at the crankpin (3), which becomes rotary motion
since the pin is restrained by the shaft.
"A brand new concept In miniature two-cycle engine design" sez
WBN. This version Illustrated at the left is the R/C Throttle model.
Look for smaller and larger motors from same outfit.
Actually this same force transfer pattern is applied in a conventional
engine, except the Aero formula translates it another 90°. Ball
joint (2) is on shaft center line so the crank pin travels as an
element of a cone (sweeping about·300). The bottom end (1) travels
vertically approximately 0.050" total and the piston rotates about
10° each side as the plane surface (1, 2. 3) reciprocates. These
slight extra motions do not appear to decrease power and all points
of wear and strain have been adequately designed.
Bore/stroke ratio is 1.21, which is in line with racing configuration.
However, intake bore of 0.177 inch diameter (0.0246 sq. in. area)
is in line with stunt design. Intake shaft timing and cylinder port
timing are conservative to make the engine easy to operate for everyone.
Compression ratio is a mild 8:1. Basic version performance typical
of stunt 35 performance, 10,000 rpm on 10x6. Top Flite, mild fuel.
However, there are tremendous possibilities here for hop-up.
Construction: Basic crankcase is a one piece aluminum die casting.
Alloy steel cylinder liner is accurately located and cast into the
case giving intimate physical contact for best heat transfer and
rigidity. Shape and method of coring yields a clean casting with
very little flash, which is neatly trimmed. Shaft end is precision
machined to receive ball bearings with a light press. Cylinder cooling
fins are cast in a line parallel to crankshaft center line. Twin
exhaust ports are vertical and normal to engine center line.
The Aero 35's crankshaft departs from standard practice in several
ways. It is machined from 2024-T4 aluminum bar stock. The thru portion
is ground to 3/8" diameter to press lightly thru the ball bearings.
Rear portion has a concave cavity, an angular hole with pressed-in
bronze bushing to receive the connecting rod pin and a milled recess
to form the intake rotary valve. Rear diameter is ground to close
tolerance since it runs in close contact with crankcase walls.
Static balance is simply effected by drilling a (3/16) diameter
hole along side the rod bearing. The wall is relieved, except around
the intake port which has a raised machine pad to effect sealing.
Forward end contains a flat which registers the 1/4-28 set screw
that locks the propeller drive shaft. Prop drive shaft is a 2024-T4
screw machine part utilizing a 5/16 shaft. Rear boss carries through
the crankcase diameter, smoothly fairing this line. Prop nut is
an aluminum piece shaped and threaded to mate with prop shaft. It
is tightened with a 1/8 diameter rod through a drilled hole. Total
effect of front end is a continuous streamlined shape requiring
The cylinder head is an aluminum die cast part, utilizing four
4-40 attaching screws, an asbestos-composition gasket and receiving
a long reach glow plug. Internally the head fits into the cylinder
forming a cone-shaped combustion chamber.
The innards of the Aero 35 tested by Netzeband.
Note crankshaft shape, connecting rod on right. simple throttle
in foreground, twin ball-bearings and neat castings. Ten and one-half
ounce powerplant sells for $34.95.
The piston is an aluminum die casting. Machining consists of
2 annular ring grooves, OD grind, and a 7/16" dia. reamed hole to
receive the rod slipper block. Perfect Circle piston rings are installed.
Piston is flat top configuration with no baffle plate or dome shaping
involved. Stiffness is assured by cast-in webs and relatively husky
side walls. Piston rings are installed with fairly high expansion
loading, so the manufacturer recommends rich running for the first
hour until the rings wear in. Top bypass is directly into crankcase,
the other is on bottom of engine. Fuel transfer distance is much
shorter than on conventional mills.
The back plate is an aluminum die casting. It contains a pressed-in
ball socket of special sintered-oil impregnated porous bronze to
support the transfer point of the connecting rod. The mating surface
is precision machined to flatness and the assembled joint requires
no gasket. The backplate reaches into the crankcase, effectively
packing the unused area for highly efficient fuel transfer. Engine
is mounted by two #6-32 screws into a firewall type mount. Backplate
also contains a screw adjustment for final assembly of connecting
The connecting rod, which is the heart of this engine, is an "L"
shaped 4130 steel forging, heat treated for extra strength (see
sketch). Piston-end connection is through a 5/32 diameter hole and
a roll pin thru the slipper block. At the transfer point a precision
ball is pressed onto the rod, and the crankshaft pin is precision
ground to 7/32" dia. The slipper block is machined from alloy steel
ground stock, receiving a longitudinal milled slot and a radial
5/32" diameter hole. The roll pin supporting the rod is pressed
into the slipper block, so all motion occurs between hardened steel
surfaces of roll pin and connecting rod.
The intake body is extremely well designed featuring a clean-unobstructed
venturi throat connected on each end to rectangular opening of generous
size. The needle valve does not have the usual small diameter needle,
rather it utilizes a tapered end from 0.220 to 0.190 inch dia. in
approximately 0.150". It meters very accurately. Anti-rotation device
is a nylon set screw driven into the side of the needle valve threads.
This arrangement allows smooth settings, and positive lock. Fuel
enters the throat thru a 0.018 inch dia. hole. There is sufficient
material to open the venturi throat to 0.25 square (0.063 sq. in.
area), which should gulp enough fuel for any combat or R/R type.
Intake body is attached to crankcase with two 4-40 screws with Nylok
buttons utilizing a thin composition gasket for perfect sealing.
The needle valve is 2-3/4" from the prop eliminating barked knuckles.
Piece de resistance is an exceptionally simple and effective
throttle. Basically a rod of 3/16" dia. containing a needle valve
down its center line installed in a hole parallel to the regular
fuel inlet. This is parallel to shaft center line. Fore and aft
motion restricts the venturi throat and the throttle needle can
be adjusted for proper low speed mixture. Two Elastic stop nuts
adjust open and closed positions. Travel is same as throat diameter
but a simple bellcrank can adjust the throw of your throttle system
to match. The test engines throttled very well, with no foolishness.
So, what have we here? It is different, but with a purpose. The
Aero 35 will probably be scaled up and down, It fits a fuselage
1-5/8" wide and 2-1/2" deep. It is 3.85" from back to prop. As built,
it is docile, rock steady and easy to handle. However, hop-up artists
should be able to make a tiger from her since: the aluminum shaft
is easily worked, the bypass distance is exceptionally short, the
bore-stroke ratio is there, the light piston and rings are right
and balance is no problem. The engine has something for all, even
if you're just interested in things excitingly different.
One last note. We just barely have loosened up our sample. It
runs easily, starts well and shows good potential. With a close
press deadline we selected to tell you about it right away. We witnessed
factory runs, tach'ed them and observed said running.
The demand for the first engines is brisk, with good reason.
You will have to change some of your ideas on nose shapes, 'cause
this engine can be exotically cowled, beside being very good looking
out there in the breeze.
This 10-1/2 ounce engine is $34.95 in regular version, $40.90
with throttle. The patents in U.S. and 6 foreign countries have
been approved. Manufacturer is Aero Research and Development Co.,
Inc., 51 Great Arrow Avenue, Buffalo 16, N.Y.
Posted July 11, 2015