DESIGNED AND BUILT BY R. O. WATSON
After completing and perfecting the BSA special in final form with rotary valve engine in 1995, Ralph sought a new challenge. His original interest in aviation surfaced, including a fascination with the rotary engines used during the first world war. Who else but Ralph would have seriously considered building one from scratch ?
The existence of such an engine provides a tangible answer to the question. Those who have seen it are dumfounded particularly when they learn of the limited facilities in use during production from in the main, billets or bars of solid metal. This most surely is the ultimate project and to date has been in progress for over seven years until now being very near completion. Ralph is sadly no longer in the best of health, but he is continuing the project with dogged determination, spurred on by his friends, all of whom are keen to attend and celebrate the maiden start-up and perhaps final rotation of his skills.
The engine is based on those previously in use, but as a technical exercise, incorporates significant and novel improvements in respect of the original designs, with the object of providing better reliability and performance. It is suitable for installation in a 7/8 scale flying experimental replica aircraft, the maximum size having been dictated by the constraints associated with Ralph's very special lathe.
Diagram illustrating the basics of a typical rotary aero engine of 1914-18 vintage.
The original designs were arranged with the air-fuel mixture being delivered via a passage in the crankshaft to the interior of the crankcase and then to radial external inlet tubes terminating at the inlet ports. Ralph has improved on this by discharging the mixture from the crankshaft into a closed circular passage in the crankcase, which is then connected to the inlet pipes. This system results in the body of the crankcase, being kept free of fuel.
An efficient and quite special, dry sump lubrication system is incorporated. A pump supplies oil at 25 p.s.i. to the big end bearing via a small passage in the crankshaft. The run off is scavenged by means of a fixed scoop running in a channel around the crankcase, after having been thrown there by centrifugal force which also creates a small pressure in the scoop . The oil is then returned to the oil tank, assisted by gravity, via a second larger oil way in the crankshaft. Mist within the crankcase takes care of lubricating the main ball-bearing assemblies.
Early engines used a low-pressure total loss lubrication system, whereby oil run off was retained in the crankcase to become included with the air-fuel mixture. A substantial unburned proportion, was discharged from the exhaust ports, to the detriment of the pilot and aircraft, hence the spattered images often seen on film.
The oil pump primary drive, which runs at 1.75 times engine speed, provides a take-off for a mechanical tachometer. A flexible drive is coupled via a 2:1 speed reducing right angle adaptor. A special tachometer has been constructed using a speedometer mechanism.
Estimated Power : -
60 h.p. at 1,400 r.p.m.
Capacity : -
312 Cubic Inches, ( 5,113 c.c. ). Bore 92 m.m., Stroke 110 m.m.
Physical Dimensions : -
Diameter O. A. 30 inches , ( 762 m.m. )
Depth O. A. (Propeller boss to firewall), 12 inches, ( 305 m.m. ).
Weight : -
Approximately 175 lbs.
Crankcase : -
Machined from solid 7075/T6 aluminium stock.
Machining the Crankcase. The dividing head, specially made for the lathe, can be clearly seen.
Crankshaft : -
Machined from solid 4340 Steel, assembled in two pieces. Fastened using a clamp-bolt arrangement.
Cylinders : -
Machined from 1045 steel round-bar stock.
Pistons : -
Machined from imported forgings, produced from 2618/T61 aluminium.
Each fitted with two compression rings and one stepped oil ring.
Connecting Rods : -
Machined from 4340 steel. Incorporating slipper ends, which run on the outer ring of the needle roller big end bearing. This arrangement does not use a master rod as was earlier practice.
Valves : -
One inlet and one exhaust valve per cylinder, 36 m.m. diameter, machined from stock diesel engine valves. Push rod and roller rocker operated, using roller cam followers running on twin, three lobe cam rings, located within the crankcase and driven by means of a planetary gear set.
Valve Springs : -
Custom made and designed to provide relatively light pressure due to the additional centrifugal force provided as a result of the engine rotating.
Inlet Manifold : -
Fuel mixture passes through the crankshaft into a separate circular chamber within the crankcase. then via radial tubes to the inlet ports.
Exhaust Manifold : -
No manifold is required, as exhaust gases are released directly to the atmosphere from the exhaust ports.
Carburettor : -
The intention is to fit a Bing Aircraft Carburettor, type 64, size 32.
Oil System : -
Dry sump system. Pressure 25 p.s.i.
The oil is pumped through a passage in the crankshaft , discharges from the big end bearing to be picked up from a channel in the crankcase by a stationery scoop, fed back into a second passage in the crankshaft and returned to the oil tank which is mounted below the carburettor. Mist within the crankcase takes care of the main ball-bearing assemblies. The rocker gear is lubricated at intervals using a grease gun, as is the usual practice with this type of engine.
Ignition : -
Aircraft magneto running at one and three quarter times engine speed and delivering two sparks per engine revolution. A separate distributor ring is attached to the crankcase. One spark plug per cylinder.
Engine Mounting : -
A welded frame, fabricated from 0.09 inch 4130 sheet steel, provides straightforward bulkhead mounting by means of four 5/16 inch bolts, arranged on a pitch circle diameter of approximately 15 inches. Accessories are situated on the inside of the airframe bulkhead.
Tightening figures for fastenings, expressed in pounds foot: -
Crankshaft to engine, mounting bolts, 1/2 inch. 110
Crankshaft (Maneton) pinch, cap screws, 1/2 inch. 110
Cylinder studs, 7/16 inch. 66
Rocker pedestals, 5/16 inch. 23
Oil scoop cap screws, 5/16 inch. 23
Front crankcase studs, 5/16 inch. 21
Crankcase studs, 1/4 inch. 11
Distributor set screws, 1/4 inch. 11
Inlet pipe and oil pump, cap screws, 3/16 inch, should be wired, or fixed using Loctite, thread fastening compound.
The partly assembled engine with propeller boss in place and fitted to its bulkhead mounting frame, which is fastened to an engine stand.
The timing gear end of the engine, showing the oil pick up scoop, N.B. The scoop will be uppermost when the engine is mounted and running.
A sampling of the components manufactured in Ralph's home workshop.
First stage in the machining of the conrods. The face plate in use was specially made.
The machining process continues.
The completed rods, together with one retaining ring. The other is fitted and is on the underside of the assembly.
Previous Contents Next