Sikorsky S-52 / HO5S / YH-18T
Design work on the Sikorsky Model S-52 began in late 1945, and the craft made its first flight in the summer of the following year. The first American helicopter to be equipped with all-metal main and anti-torque rotor blades, the S-52 had a semi-monocoque, pod-and-boom type fuselage, a single 175hp Franklin engine, quadricycle wheeled landing gear, and a fully-enclosed cabin that could seat up to three people. The machine's performance was impressive by the standards of the day. The commercial S-52 set three international speed and altitude records in 1948. These 1948 records were 208.57km/h over a 3-kilometer course, 197.55km/h over a 100-kilometer course, and 6468 metres absolute height.
Shortly after gaining the speed record, the little two-seater demonstrated its manoeuvrability, during the course of a routine test flight, by performing a series of successive "loops," gaining height slightly after each in the approved manner. The pilot was Harry Thompson of Sikorsky's flight-test department, and the date was May 9th, 1949. Later he repeated the performance - carrying Ralph Alex, then project engineer, as passenger.
A two-seat helicopter, featuring all-metal rotor blades. The two-seat S-52 (YH-18 Serial number 49-2888) first flew on Feb 12, 1947, powered by a 133kW Franklin engine.
The S-52's performance was a factor in the Army's 1949 decision to purchase four examples of the slightly modified Model S-52-2 for service test and evaluation. It is the H-18 that holds the distinction of being the first Sikorsky helicopter to be procured for service evaluation by the Army Ground Forces, as distinct from the USAAF.
The Army's four YH-18As (serials 49-2888 through -2891) were essentially similar to the standard commercial S-52, differing primarily in their ability to carry a fourth passenger. Extensive testing showed the YH-18A to be quite capable in the light utility and observation roles, but the Army ultimately decided not to procure the type in quantity.
The S-52-2, a three/four-seater with a 183kW Franklin O-425-1 engine which was ordered by the US Marine Corps as a replacement for the HO3S. Deliveries of the HO5S-1 began in March 1952, and the type served also with the US Coast Guard as the HO5S-1G.
The use of a high-speed rotor allows a wide range of permissible rotor revolutions in flight, which is of particular advantage in the autorotative-landing case. Where an engine-off landing with zero forward speed is desired, it is possible, by using the kinetic energy stored in the rotor, to hover the machine momentarily in the flare-out before touching down. The touch-down itself is perfectly steady with the quadricycle landing gear.
The rotor blades were originally incorporated on the S-52-1 and the main spar is an alloy extrusion which is itself the leading edge of the blade. The trailing edge is made up in sections, each of which is attached to the spar individually. Thus the sections towards the blade tip are relieved of carrying the centrifugal load exerted on the inner sections, resulting in greater overall strength. The blades are fully interchangeable and their uniformity is a considerable contribution to the smoothness of the machine in flight.
The enlarged version, designated S-52-2, was granted its C.A.A. certification in the early part of 1951. A few civil machines were built, but the main production up to the present has been for the U.S. Navy; the type has been designated HO5S-1. The machine is officially classified as a "three-place" helicopter but a fourth occupant can be carried on shorter flights. The large fuel-tankage capacity allows for 227 litres.
With a pilot and three passengers aboard sufficient fuel can be carried for a flight of approximately one-and-a-half hours - or a distance of between 193 kilometres and 225 kilometres at the cruising speed of 153km/h. With only three occupants the fuel tank may be filled, giving an endurance of 3.5 hours and a range, with fuel reserves, of 530 kilometers. This "built-in alternative" increases the machine's versatility and has been proved to be of much practical value in the field. For delivery flights, auxiliary fuel tanks may be fitted to provide for a maximum distance of 1287 kilometres with pilot only aboard.
In its role as a rescue helicopter the S-52-2 normally carries two stretcher cases alongside the pilot. The canopy opens forward, in a manner reminiscent of the Bristol Freighter, to allow ease of loading and unloading.
Improvements in stability have been effected by the addition of large ventral stabilizing fins to the tail boom. These fins are fixed surfaces, as they have recently been so made on the American S-55. In earlier versions of the S-55 the fins were adjustable by the pilot to provide fore-and-aft trimming in flight, but this is no longer necessary. Additional vertical fin area has also been provided by an extended fairing on the tail-boom elbow to compensate for the increased keel surface forward of the rotor pylon with the longer nose.
The U.S. Navy purchased 89 S-52-3s as the HO5S-1 for general utility duties, all of which were delivered by 1953.
One aircraft was later converted into the sole H-39 turbine-powered research helicopter and the S-52-2 eventually served the Navy, Marine Corps and Coast Guard as the HO5S.
On Jul 24, 1953 the S-52T (YH-18B) turbine powered version of the S-52 was first flown. It used a french Turbomeca Artouste I engine. A developed version designated YH-18B and powered by a T51-T-3 (Artoustc) turbine is now re-designated XH-39, and has set up an international helicopter speed record of 156 mph.
Engine: Franklin 6V6-245-BI6F, 245 hp
Rotor diameter: 33 ft
Rotors: 3-blade main; 2-blade tail
Fuselage length: 27 ft 5 in
Loaded weight: 2,700 lb
Max speed: 110 mph
Ceiling: 22,200 ft
Typical range: 415 miles at 95 mph