The Sadler Radial

Model R1765U



The Sadler Radial Engine is a compact, fuel efficient and smooth  running aircraft engine.  The 6 cylinder radial uses two banks of three cylinders with power pulses every 120 degrees of revolution.   The design uses VW cylinders and pistons for reliable inexpensive parts.  The engine is a great alternative to 2 stroke power.  Prototype testing has proven out this radial and casting patterns have been made.  The engine is ready for final pre-production development.  Our goal at the '04 Sun 'N Fun is to gauge the public's interest in the Sadler  R1765U and determine if we can take the final steps in bringing this engine to market.  Please let us know if you think the Sadler R1765U may fit any of your aircraft needs.  We think this is an exciting opportunity for uncertified aircraft owners but we need your feedback to determine the future of the project.  Give us a call or drop us an e-mail to let us know.  We look forward to hearing from you!


Configuration: Direct drive, 6 cylinder, four-cycle, internal combustion, gasoline fueled, free air    cooled engine, one gravity fed carburetor, and a dry sump lubrication system.

·         HP/Torque @ 3000 rpm--65 hp/113.4 Torque ? Displacement--1721cc (105 cu in.)

·         Stroke--50 mm (1.969 in.)    ? Lubrication: dry sump, pressurized

·         Bore--85.5 mm (3.366 in.)    ? Rotation--clockwise, view from rear

·         Compression Ratio--9.5:1    ? Option--Electric start

·         Accessories--oil cooler, oil tank (4 quart)

·         Engine weight with components (dry) - 108 lbs. (Prop. Start)

·         Engine weight with electric start/alternator systems--122 lbs.

·         Engine diameter--20.7 in. plus oil system components below engine


·         Normal rated power, T.O.  62 bhp @ 2,850 rpm

·         Max. rated power   65 bhp @ 2,975 rpm

·         Continuous rated power (90%)  58.5 bhp w/avg. fuel burn of 4.1 gph or spc=.44 lb./bhp-hr

·         Typical cruise power (75%)  49.5 bhp w/avg. fuel burn of 3.4 gph or spc=.43 lb./bhp-hr

·         Idle Speed    800 rpm

Information compiled from a review of the design drawings, test stand operation of the engine under conditions whereby accurate measurements of the related parameters could be made (torque, rpm, temperatures, pressures, etc.) and test operations in aircraft.  All figures from independent source.  Patent No. 5,150,670



·         The Sadler Radial Engine project objective was to design a reliable, lightweight, four-cycle, aircraft engine in the 65 horsepower range.  Below are the parameters, which were considered essential:

·         Air cooled

·         Duel ignition

·         Minimized design risk through utilization of off-the-shelf components, where available

·         Low R.P.M

·         Four-cycle

·         Minimum vibration

·         Low fuel consumption

·         Light weight

·         Low noise level

·         Reliable



The Sadler R1765U is the product of the above design criteria.  The "R" stands for "radial", the "17" is "1721cc" displacement, the "65" is horsepower, and "U" means "uncertified".

This engine is quiet, even without a muffler, which stems not only  from the slow-working pistons, but also from the fact that the noise produced by the SADLER engine is at a frequency much lower than the high whine of the two-strokes.

It is a four-stroke with six cylinders and runs with the smoothness one would expect from a four-stroke.  The idea of a four-stroke with its inherent reliability and fuel economy is not new.  Lawn mower engines to the VW conversions, all use the four-stroke, but all are hampered by the low power-to-weight ratio.  Thus small aircraft have been forced into the two-stroke engine, or very high-RPM auto engines.

The R1765U has a maximum width of 20.5", a maximum height of 19.4", and a length of approximately 17" depending on starting configuration, carburetion and air cleaner. 

There are two rows of three cylinders each, for a total of six cylinders.  This arrangement gives a firing impulse every 120º or three power pulses per revolution.  Each of the two, three-cylinder rows forms a system whose instantaneous mass-center or centroid, has an approximately circular locus that is exactly balanced by the equal and opposite mass-system of its adjacent cylinder row.  In plain language, we can say that one cylinder row balances the other row all of the time.  We are talking static balance here.

The slight offset between the cylinder rows results in a dynamic imbalance or a "rocking couple"  that is resolved along the aircraft longitudinal axis by the engine mounting system and a counter-weighted crankshaft.

The radial engine has excellent air-cooling characteristics.  Each cylinder is individually exposed to its own flow of cooling air without the requirement for extensive baffling and ducting.  The second cooling advantage this engine has is its low per-cylinder horsepower output making cooling demands very manageable.


The pistons and cylinders are standard Volkswagen, extensively machined.  The use of these stock Volkswagen components provides for a ready supply of spare parts, and eliminates any piston-cylinder interface design risk problems.


Individual cylinder heads are cast in 356 aluminum alloy heat-treated to T6.  Cast iron valve seat insets are cast integrally with the head.  Both inlet and exhaust valves are stainless  steel VW racing types for durability and component availability.  They ride in bronze valve guides pressed into the cylinder head.  Valve springs need only exert a pressure of approximately 26 pounds at mid travel, thereby reducing wear and tear on the valve drive train.

A 5mm deep bathtub combustion chamber is used with a 2 mm quench height area above the piston.  This allows for compression a ratio of 9.5 to 1 and the use of unleaded premium auto gasoline.  Two spark plugs per cylinder are used to improve reliability as well as combustion efficiency.


The valve train is unique in that it uses only one single small diameter cam for all inlet valves and another identical cam to drive all the exhaust valves.  Each tappet housing holds all of the six inlet or exhaust  roller tappets.  The hardened steel cams run on ball bearing integral shafts to drive the roller tappets.  The tappet housings are staggered in the rear case and drive overlapping pushrods to each rocker arm.  Because the  valves are arranged in line lengthwise along the engine (placing exhaust forward and inlet rearward) and the pushrods all come out of the engine rear case in a single plane, each rocker arm  for a given cylinder is displaced laterally to the left or right of the cylinder centerline.  This allows the front exhaust  rocker arm to miss the inlet valve and spring, as well as allowing the rear rocker arm to be similarly displaced from  center, making room for the inlet manifold to pass between the push rods on its way to the cylinder head inlet port.

Valve timing has a minimum of overlap and duration to maximize low-end torque and provide useful horsepower in the 2900 rpm range.  Valve cam accelerations have been kept low in the interest of low valve train stress and long life.


Full pressure lubrication is supplied from a rear case mounted oil feed pump running at camshaft speed.  Inlet oil to the oil pump comes directly from a remote oil tank.  A four-quart oil supply is recommended.  Because the cylinder bases protrude into the crankcase cavity, very little back oil finds its way into the lower cylinders.


Currently our plans are to use the self-contained SmartPlug* system. 

As originally tested, the engine had two completely independent electronic distributors driven from the rear case.  Their shafts are mounted with individual gears that mesh with the main crankshaft time gear.  Distributors are battery-coil type.  Solid-state breakerless contact points are used to drive the standard coils to achieve optimum reliability and simplicity.  An interesting feature of the radial engine is that the firing order proceeds sequentially around the cylinders in a direction opposite to the crankshaft rotation.

*For additional information see


Exhaust manifolds may vary according to application as the four-cycle characteristics do not require a tuned exhaust.

The inlet manifold system uses a circular cyclonic plenum chamber at the rear of the engine.  Separate manifolds join this chamber to each cylinder's inlet port.  The carburetor is mounted centrally on the plenum box.  A heat system can be fabricated using the engine's exhaust as the heat source.


We believe that the Sadler R1765U lives up to it's initial design criteria.  It is a compact, light weight, fuel efficient, four-cycle engine.  Since the project began, approximately 43 prototypes have been built.  There are almost 500 hours of dynamometer testing and the engine has also been successfully test flow on several different aircrafts.  All testing indicates great potential and, therefore, we feel this engine has an exciting future.


Constant Pressure Engines has begun an initial evaluation of the Sadler Radial Engine.  We hope to be able to determine the market's interest in the Sadler R1765U, so that we can confidently commit the necessary resources to complete the final pre-production phase of the project.

We believe that, in addition to the testing that has already been performed, we can greatly contribute to this project by enhancing the engine design through 3D CAD modeling, which will maximize and fine tune the design features and by using SmartPlugs to improve the operation of the engine, by increasing fuel efficiency and lowering emissions, while making the engine lighter, smaller and even more reliable.