|Sikorsky S-52-2 / HO5S / H-18|
The S-52's performance was certainly 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. 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. 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.
S.Harding "U.S.Army Aircraft since 1947", 1990
The Sikorsky S-52 was a four-place utility helicopter that was evaluated by the U.S. Army as the YH-18 and saw service with the U.S. Navy as the HO5S-1. It was the first American helicopter to have all-metal rotor blades and, in its earlier S-52 civilian version, set three international helicopter speed and height records. 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.
The semi-monocoque fuselage was a pod-and-boom type constructed of aluminum-magnesium alloy, with a three-bladed, all-metal main rotor and a two-bladed anti-torque tail rotor driven by a six-piston, 245hp air-cooled engine. A quadricycle fixed landing gear was fitted.
The U.S. Army evaluated four YH-18s in 1950 but did not procure any for service use. The U.S. Navy purchased 89 S-52-3s as the HO5S-1 for general utility duties, all of which were delivered by 1953.
Norman Polmar, Floyd D. Kennedy, Jr. "Military Helicopters", 1985
The first US helicopter with all-metal rotor blades, the Sikorsky S-52-1 two-seater was first flown on 12 February 1947, powered by a 133kW Franklin engine. In 1948 it established three international helicopter records for speed and altitude and was developed into 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 НO3S. Deliveries of the HO5S-1 began in March 1952, and the type served also with the US Coast Guard as the HO5S-1G.
D.Donald "The Complete Encyclopedia of World Aircraft", 1997
Although little known in Great Britain, the Sikorsky S-52-2 has earned for itself in the U.S.A. a well-deserved reputation for being a great little helicopter. It has been used extensively in Korea as a rescue machine for the evacuation of wounded, and as a command transport vehicle. In these capacities it has ably demonstrated that the comprehensive background of research behind its production has resulted in flight characteristics of which its makers may be justly proud.
Analogies have been drawn between the helicopter and the horse; it is not without precedent, therefore, that the pedigree of the S-52-2 should be examined in equestrian terms. Sired by the S-52-1, of 1948 vintage, it had its origins in the stable which produced a helicopter that was for a number of years the mainstay of rotating-wing flight, the Sikorsky R-4. In fact, it was the R-6A series which was born directly out of the R-4; but this series, although it achieved a high quality of rotor control, suffered numerous drawbacks - associated with maintenance - which made it clear that a re-design was necessary.
Thus the S-52-1 emerged, incorporating all the lessons learned from the R-4 and R-6 series, together with many features of its own, and it showed its worth quite early in its development by establishing a new world helicopter speed record of 208.89km/h.
As may be expected, it is from improvements in rotor head design that the S-52 derives its pleasant handling characteristics. The basic change involves the location of the blade flapping hinges so that their action is coincident with the drag hinges. This results in a decreased control lag and also allows greater range of control. Movements of the stick are light and easy, and the response in the machine is lively and accurate.
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.
From that time development proceeded steadily. A project was then already in hand to enlarge the cabin space to provide three/ four seats, and thus utilize the Franklin engine's 245 b.h.p. in a more practical manner.
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 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.
Of the many features embodied in this admirable little helicopter, the most important is undoubtedly the metal construction of the rotor blades. These blades were originally incorporated on the S-52-1 and now have had five years of flight experience on the series. 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 S-52-2 is a happy compromise in size between the small two-seater helicopter, the utilization of which is rather limited, and the somewhat larger 4/5-seater machine in the 500hp class. The latter is often uneconomical for many charter operations. The current military versions are in use by the U.S. Army, Coast Guard and Marine Corps, but the series has now been returned to the stable for another "breeding" spell while further power-plant research and development is carried out. When the new "filly" eventually emerges it is to be expected that it will uphold the reputation of its distinguished forbears.
"Flight", 7 August 1953