Configuration Features

The Sikorsky S-56 featured a five blade 72 foot main rotor and a 4 bladed 15 foot anti torque tail rotor.  The engines were mounted in nacelles at the tip of a short high wing.  The wings provided lift in forward flight which unloaded the main rotor which delayed blade stall, allowing an increase in top speed.  Fuel was carried in wing and nacelle tanks and the retractable main landing gear was located in the nacelles, leaving the cabin clear for loading or unloading troops or cargo.  The lower portion of the nose consisted of “clam shell” type cargo loading doors and a cargo loading ramp.  The cockpit was located in the nose of the helicopter above the cargo compartment, slightly aft of the cargo loading doors allowing downward visibility during approaches through the windows in the cargo doors.

S-56 Models

CH-37A (H-37A) Mojave

U.S. Army version of the S-56 featured a manually folded main rotor and a hydraulically folded tail rotor pylon.

CH-37B (H-37B) Mojave

U.S. Army CH-37A helicopters factory remanufactured by Sikorsky Aircraft to the CH-37B configuration.  Major modifications included:

• Automatic Stabilization Equipment (ASE) which allowed hands off flying and instrument flight.
• Stabilizer moved from both sides of rear fuselage to the top right side of the tail rotor pylon.
• Increase of engine oil tank capacity from 13.3 gallons to 30 gallons per engine.
• Replacement of rear split cargo door with a sliding cargo door.

CH-37C (HR2S-1) The Deuce

U.S. Marine Corps version of the S-56 which featured an automatic fold main rotor and a manual hydraulically folded tail rotor pylon to reduce the size of the S-56 to fit on the aircraft carrier elevators.  Automatic Stabilization Equipment (ASE) allowed hands-off flying and safe operation at night and in instrument flying conditions.

Keystone FAA Certified CH-37B and CH-37C Helicopters

Keystone Helicopters purchased 40 CH-37B and C helicopters and obtained a FAA Restricted type certificate which allowed carrying10,000 pound external loads. The helicopter was restricted to carrying only the required number of crew members internally.  For commercial operations Keystone made the following modifications:

• Removed all unnecessary cowlings
• Removed heater system
• Main landing gear was locked down and hydraulics removed
• The hydraulic system to open/close the nose doors and ramp were removed
• The ASE system was disabled, adding to pilot workload.
• Any system not required for the external lift mission was removed to save weight and increase lift capability.

Fuselage

The S-56 fuselage was constructed of an aluminum structure with aluminum and magnesium skins and reinforced plastic or Fiberglas removable panels.

Stabilizer

Three different stabilizer configurations were installed on S-56 helicopters:

Adjustable Stabilizer  Early CH-37A helicopters had an adjustable (trim-able) stabilizer on the aft fuselage to aid in maintaining longitudinal trim during forward flight.  The stabilizer was adjustable in flight from a 15 degree positive angle to a 10 degree negative angle. For best hovering performance the stabilizer was feathered relative to the main rotor downdraft.

Two Position Stabilizer  On later CH-37A and CH-37C models, a two position stabilizer was installed on the aft fuselage. In the flight position it was positioned horizontally with a 2 degree nose down position and in the hover position it dropped to a vertical free floating position in the main rotor downwash with a hydraulic damper to reduce vibration.

Tail Rotor Pylon Stabilizer  On CH-37B models the stabilizer was moved to the top of the tail rotor pylon on the right side.  This 24.5 sq. ft. stabilizer had 10 degrees of dihedral and a zero degree angle of incidence.

Main Rotor Assembly

A 5 bladed fully articulated main rotor head with a 24 inch offset flapping hinge was installed on the S-56 helicopter. The 72 foot diameter main rotor was controlled by a 2 stage hydraulic irreversible servo control system. On CH-37B and CH-37C helicopters Automatic Stabilization Equipment (ASE) with an auxiliary servo was installed to permit safer night flying and instrument flight.  A rotor flapping restrainer allowed rotor engagement in winds of up to 60 knots with the helicopter headed into the wind.  Normal operating range for the main rotor was between 164 rpm (618 ft. per sec.) and 204 rpm (769 ft. per sec.).

Six Bladed Main Rotor

In March 1959, a six bladed main rotor was test flown on an Army H-37A helicopter to investigate the flight characteristics.  Although the tests indicated lower vibration levels, an appreciable delay in blade stall, and no increase in power requirements it was never used on the S-56 helicopter.  However, the six bladed main rotor was used on the S-64 (CH-54A) helicopter.

Main Rotor Blades

The all metal main rotor blades were constructed of aluminum alloy except for steel cuffs.  The main rotor blade was constructed of a hollow extruded aluminum spar with 27 honeycomb filled, aluminum skinned pockets bonded to the spar.  A NACA 0012 airfoil was used. The original main rotor blades had a 21.5 inch chord with 14 degrees of negative twist and weighed 335 pounds.  Wide chord blades with a NACA 0010.9 airfoil, a 23.65 inch chord, and 8 degrees of negative twist were offered on a retrofit basis which provided increased lift and an increase in Maximum speed from 110 to 130 knots on the CH-37C and 80 to 120 knots on the CH-37B.  The wide cord blades weighed 350 pounds.  The wide chord blades were the first use of an airfoil other than the NACA 0012 on a production Sikorsky helicopter.

Stall Indicator/Performance Indicator

A stall indicator later called a performance indicator was mounted on the top of the instrument panel to assist the pilot in obtaining optimum performance from the helicopter.  This was accomplished by a transducer mounted within the piston of the right lateral main rotor servo which measured the vibratory loads imposed on the stationary star (swashplate).  The vibratory loads on the stationary star were influenced by varying conditions of rpm, gross weight, altitude, temperature, load factor, airspeed, and fuselage attitude.  These loads were a direct indication of the onset of blade stall or the degree of blade stall encountered during a flight. By September, 1960, the stall indicator had been removed from the Marine CH-37C helicopters.

Main Gearbox

The main gearbox consisted of magnesium housing with a 2-stage planetary reduction system plus a spiral bevel input gear mesh for each engine input which all together provided a reduction ratio of 14.05 to 1. This reduced engine rpm from 2700 rpm to 193 rpm.

Tail Rotor Assembly

A 4 bladed 15 foot diameter all metal fully articulated tail rotor was installed.  To prevent the tail rotor blades from flapping when the helicopter was parked, the tail rotor blades were coned against their stops by an electrically operated coning ring located on the tail rotor gear box.  The tail rotor pitch change was accomplished by a 2 stage hydraulic servo.

Twin Tail Rotor 

In 1956, a Vee shaped twin tail rotor was installed in a Marine XHR2S-1 helicopter to improve the cg (center of gravity) range and to increase forward speed.  Both 4 blade and 5 blades twin tail rotors were tested.  With two canted tail rotors the individual tail rotor thrusts could be adjusted to provide both the required anti-torque  thrust and different levels of lift.  In this way if the center of gravity was too far forward or aft it could be compensated for by increasing or decreasing tail rotor lift. The improvements were less than expected and the modifications would increase the complexity of the flight control system and complicate the design solution to the pylon fold requirement.

Engine

Two Pratt and Whitney Double Wasp  2100 hp R-2800-54 18 cylinder twin-row engines were installed with a 80 degree turn and a 12 ½ degree upward slope in the drive system to align with the main gearbox input. They were located in a nacelle supported by a stub wing.  The engines were plagued by excessive oil consumption until late in its service life when baffles were added to the oil tanks to eliminate oil foaming.  The non-standard engine configuration probably contributed to the oil consumption problem.  The engines were run at high power settings almost continually which brought a 2,000 hour time between removals in fixed wing applications to 350 hours in a helicopter.

Auxiliary Power Unit (APU)

The S-56 helicopters were equipped with a portable APU which was secured to the rear cabin floor cargo tiedown fittings.  The APU used the helicopter battery for starting power or could be hand cranked when necessary. The APU starter-generator was driven by a 10 hp V-2 cylinder gasoline engine.  The APU starter-generator provided 28 vdc power primarily to power the cargo winch for helicopter loading.  It could also provide power to start the engines and power all electrical equipment installed on the helicopter.

General Arrangement Drawing

Mission Systems

Cargo Hoist

The S-56 helicopters were equipped with a 2,000 lb capacity electric cargo hoist. A monorail on the cabin overhead could be extended out the cabin door to allow self-loading of the helicopter. An Auxiliary Power Unit provided electrical power for loading operations.  The cabin floor under the rotor head contained a hatch “Hell Hole” which allowed observation of cargo hook operations and allowed  rescue  hoist operations (hoist weight was limited to 600 lbs. when hoisting personnel). 

MEDEVAC

The S-56 helicopter could be configured to carry 24 litter patients during a Medical Evacuation mission.

Cargo Hook

A 10,000 pound capacity external cargo sling was available for use on the S-56 helicopter. A crewmember was able to view the external load through cargo hatch in the cabin floor and had a pendant with the ability to drop the load in an emergency.

Troop Accommodations

Troop seats for fully equipped troops were provided for 36 troops in Marine CH-37C helicopters and 23 troops in Army CH-37A/B helicopters.

Mid Air Retrieval System (MARS)

In the 1960s spy satellites ejected film capsules which were captured in mid-air to prevent destruction of the film from a hard landing on earth.  Recoveries of packages up to 800 pounds were performed by helicopters. In 1962, White Sands Missile Range identified a need to recover missiles, boosters and capsules using this type of soft recovery.  U. S. Army CH-37B was used to recover packages of up to 2,500 ponds.

The technique involved installing a winch and equipment designed by All American Engineering, Wilmington, Delaware.  The winch was installed in the cabin forward of the cargo hatch.  Two 24 foot arms extended below the helicopter which caught the drogue chute on the package.  The main chute was then jettisoned and the package reeled into the helicopter.  The winch was like a giant fishing reel and the cable slipped when the load was applied allowing a controlled deceleration. When the speed of the load and the helicopter were matched, a brake was applied and the load reeled into the belly of the helicopter.