2020 Stewart S-51 Mustang

History

Jim Stewart designed the S51 to be a faithful scale replica of the North American P51. He decided on the scale factor of 70% based on cockpit comfort for himself (5’ 10” 160 pounds) and the available 4 bladed constant speed propeller. The 91” diameter constant speed Hartzell propeller used on the Beech King Air was deemed to be ideal for the S51 aircraft to get the P51 “look”. With a blade tip speed of 850 fps, this propeller at takeoff RPM approximates the tip speed of the full size Mustang. This Hartzell prop has a recommended maximum hub speed of 2200 RPM. In the late 70s, when the design of the S51 began, there was no 70% scale Merlin V12 available. However, due to the “horsepower wars” of the Muscle Car Era, several of the major automobile manufacturers produced V8 engines that approached 500 HP. Jim calculated that the S51 would require an engine in the 400 to 500 cubic inch displacement range. The Mark IV Chevrolet (Big Block) engine proved be the ideal choice for a large displacement engine in a compact package that would fit inside the S51’s engine compartment. A normally aspirated liquid cooled V8 engine properly designed can reasonably produce 1 horsepower per cubic inch when operated in the 4000 RPM region. To produce the 400 to 500 HP required for the size and weight of the S51, yet not turn the propeller past its RPM limits, Jim designed a two spur gear Propeller Speed Reduction Unit (PSRU) with a 2.13 to 1 reduction ratio. Though Rod’s engine is capable of 7000 RPM, the normal maximum engine speed limit is 4680 RPM, which is in fact determined by the maximum RPM limit of the propeller. Jim Stewart’s S51 prototype aircraft initially was fitted with a 454 cubic inch “crate motor” from GM Performance and provided the expected horsepower. This engine was basically “stock” with no changes to the internal components except for the camshaft which was replaced with a marine “left hand drive” unit. This was necessary since the PSRU is a two gear drive, the output shaft (prop shaft) turns in the opposite direction of the input (engine). To get the prop to turn in the conventional direction, the engine must rotate in reverse (counterclockwise when viewed from the cockpit) of the normal direction.

Chevy (BBC) engine routinely run hours on end at 4000 RPM and do it reliably and under warranty. To get as much horsepower as possible out of the engine requires the maximum cubic inch displacement that is practicable. The two factors determining the cylinder’s displacement are the cylinder bore diameter and the stroke of the crankshaft. The constraint for the maximum cylinder bore possible is the distance between the bore centers. In the Big Block Chevy engine, the largest bore size that still maintains an adequate wall thickness between cylinders is 4.500 inches. The other determining factor is the crankshaft’s stroke. There are physical limitations due to the block’s architecture, but the major constraint to the size of the crankshaft’s stroke is the mean piston speed. The larger the stroke, the higher the mean piston speed and the “G” load on the piston, wrist pin, connecting rod, and connecting rod bolts as the assembly accelerates, decelerates, momentarily stops, reverses direction and accelerates again. At 4200 RPM the mean piston speed is 49.58 feet per second and the “G” loading on the piston at Top Dead Center (TDC) is 1420 Gs. Mean piston speed is the major factor for determining the longevity of the rotating assembly. Rod’s engine was designed with a 4.25 inch crankshaft stroke (0.25 inches longer than stock). This is a compromise between the maximizing the engine displacement while minimizing the mean piston speed at the maximum engine RPM. The cylinder’s volume equates to 67.59 cubic inches which calculates out to total engine displacement of approximately 540 cubic inches. I believe that the 540 cubic inch engine is optimum for the S51. Because of the physical limitations of the BBC, the only way to increase the power output without increasing the engine’s RPM is to add a supercharger. The Vortech supercharger that is installed is capable of moving an air volume sufficient for 1000 HP.

ENGINE BLOCK/ROTATING ASSEMBLY

The cylinder block is an aluminum block manufactured by Keith Black Performance. This block is fitted with dry iron sleeves and weighs 100 pounds less than a comparable iron block. The rotating assembly consists of the crankshaft, connecting rods, wrist pins and pistons. The crankshaft is a specially ordered billet steel crankshaft built by Bryant Racing. The crankshaft was machined out of 4340 alloy steel for reverse rotation. The counterweight fairing, oiling holes and journal polishing was performed with the crank’s rotation direction in mind. The connecting rods are made by Oliver Racing Parts, with the length increased 0.25” over stock for a slightly better mechanical advantage as well as reduced side load on the piston skirts. The pistons are built by JE Pistons with an inverted dome to provide a static compression ratio of 7.5 to 1. This relatively low compression ratio was selected because the engine will be supercharged. The rotating assembly’s parts are of the highest quality available and are internally neutral balanced.

CYLINDER HEADS/VALVETRAIN

The aluminum cylinder heads are a special casting by a well known aftermarket cylinder head manufacturer who wishes to remain nameless. These heads are limited production units machined for two sparkplugs per cylinder. They are oval port heads with a 285 cc port volume and 118 cc combustion chamber volume. The narrower oval ports were selected to keep the intake charge velocity high. The intake valves are 2.25” stainless steel and the exhaust valves are 1.88” Inconel. The one weakness in the BBC engine is the stock stud mounted rocker arm retention method. This was addressed by installing a shaft mounted system built by Jesel. The spring rates are relatively mild, with a seat pressure of 150# and an “over the nose pressure” of 450#. The camshaft was custom ground for reversing the engine’s rotation. The camshaft specs are considered “mild” with a wide lobe separation angle of 114 degrees and relatively short intake/exhaust duration of 226/230 degrees, and total lift of 0.586/0.610 inches. This camshaft utilizes hydraulic roller lifters.

OILING SYSTEM

Like the full size P51, this engine has a dry sump oil system with an external camshaft driven pump that scavenges oil from the oil pan and returns it to a firewall mounted supply tank. This oiling system replaced the stock wet sump system due to the performance capabilities of the aircraft. With this engine, the S51 is capable of sustained climbs at a high deck angle. This high deck angle increases the possibility of uncovering the oil pump pick up tube in the oil pan in a wet sump system (stock internal oil pump). Some builders who use the stock oil pump system have installed a pressurized accumulator system to maintain oil pressure in case of the pickup sucking dry. However, the volume of the current commercially available accumulators would only allow 4-5 seconds of oil pressure before the accumulator discharges. Another disadvantage of the wet sump system is that it lacks the capability of removing air from the oil before it is pumped back into the oil passages.

The oiling system in Rod’s engine has provisions to supply a constant stream of oil directed at the underside of each piston to pull heat out of the piston. These piston oilers, as well as a liquid cooled cylinder head, enable the engine to run EGTs of 1500 F with no ill effects. In addition, the added piston cooling decreases the onset of detonation. The oiling system incorporates a device called a Spintric. Using no moving parts, this unit uses centrifugal force to remove air from the foamy oil that is scavenged out of the oil pan. Because of the engine’s high horsepower output and added heat due to the piston oilers, it became necessary to control the oil temperature by installing an air/liquid heat exchanger in the belly scoop ahead of the engine radiator.

COOLING SYSTEM

Rod’s engine’s cooling system uses a custom designed water pump that pumps 70 gallons per minute at 20 PSI through the Stewart provided scoop mounted radiator. This pump is directly driven by the crankshaft and turns at engine speed.This radiator has been shown to adequately cool Rod’s engine, even during a ground static check at takeoff power for 2 plus minutes. A coolant expansion tank is mounted on top of the PSRU, and no thermostat in used in this system. The coolant system maintains a 160 F coolant temperature under normal flight conditions.