features Vertical Rewind: The Bell Model 48, ahead of its time

Bell’s Model 48 troop carrier helicopter was a post-war game-changer.
Avatar for Bob Petite By Bob Petite | April 16, 2020

Estimated reading time 9 minutes, 4 seconds.

In the late 1940s, the Bell Aircraft Corporation had several notable successes as it began developing early helicopters. These included the two-place Model 47 (the first commercially certified helicopter), the military two-seat Model 47A, and the six-seat Model 42.

Although the Bell Model 42 attracted little interest for a production version from the civilian market, Bell actively courted the U.S. Army Air Forces’ (USAAF) Engineering Division at Wright Field in Dayton, Ohio, to purchase a military variant of the Model 42, called the Model 48.

A Bell Model 48/YH-12B with experimental floats at Bell’s Niagara Falls, N.Y., plant. Bell/Niagara Aerospace Museum Photo

During 1946, Bell received its first procurement request for the Model 48 from the USAAF. In the form of a military prototype, the aircraft was known as the XR-12, and it was the largest helicopter Bell had manufactured at that point. Approval was initially granted for the purchase of three XR-12s from research and development funds, and 10 service test XR-12s from production funds. The contract had been arranged primarily to keep Bell’s helicopter development continuing, and it helped keep Bell in business. The helicopters were scheduled to be delivered by the summer of 1946.
Two prototype Model 48/XR-12 helicopters were manufactured based on the Model 42 civil helicopter, plus one static-test version. The two helicopters remained at Bell and were used for testing and research.

The XR-12 could carry two crewmembers and four passengers, and was to be used for liaison, courier, observation, cargo transport and personnel evacuation missions. It varied from the Model 42 in having a smaller tail boom, shorter rotor mast, and stabilizers on the tail boom. The helicopter had a Pratt & Whitney R-1340 550-horsepower air-cooled Wasp engine with a gear driven turbocharger. It was on fixed tricycle landing gear and came with a rescue hatch and winch. Floats were an additional option.

While Bell subsequently received a production order for 34 Model 48/R-12A helicopters from the USAAF, none were ever manufactured, and the order was cancelled in 1947.

A Model 48/YH-12B flies over Niagara Falls in September 1950. Bell/Niagara Aerospace Museum Photo

Instead, the USAAF decided to have Bell manufacture a much larger stretched version of the Model 48, capable of carrying up to eight people. Now called the Model 48A (and the XR-12B by the military), one aircraft was built for evaluation. This new helicopter had a wider larger cabin on a four-wheel undercarriage, and was powered by a 600-hp Pratt & Whitney engine. The nose of the helicopter was covered with transparent plastic panels.

The XR-12B had a single two-blade main rotor, with a rotor system and stabilizer bar like the Bell Model 47, but scaled up for the much larger rotorcraft. This arrangement provided the ultimate in stability for the XR-12B. A single, two-blade controllable pitch tail rotor, mounted on the left side of the tail boom, provided proper torque compensation and directional control. Flight controls — consisting of main rotor pitch control levers, conventional-type control sticks, and tail rotor control pedals — were provided in a dual installation.

The helicopter also had a hatch and rescue winch located between the pilot’s and co-pilot’s seats, which could be operated by either the pilot or co-pilot. There were large separate doors for the passengers and for loading larger cargo, and seating for up to six fully-equipped soldiers. The cabin arrangement provided enough space and facilities to transport six litter patients with a medical person in the co-pilot’s seat.

Military testing

An evaluation of U.S. military services found there was a need for a helicopter with enough cubic capacity to permit internal loading of about 1,500 pounds (680 kilograms) of personnel and cargo for reasonable distances. Flight tests of the XR-12B proved operations with useful loads of 2,300 lb. (1,040 kg) were entirely feasible, and a design study was initiated to provide the cabin arrangement and facilities to meet this requirement.

Troops enter a service-test Model 48/YH-12B. The pilot is Bell’s chief pilot, Floyd Carlson. Bell/Niagara Aerospace Museum Photo

Ultimately, Bell manufactured 10 YR-12B service-test helicopters, with the aim of delivering them in the summer of 1946, at a cost of almost $175,000 each. However, problems with main rotor blade weaving and with the rotor governor were poised to cause major delivery delays. At this point, the USAAF stepped in to help solve the main rotor concerns.

It was obvious that more development and delays would be needed before the helicopter would be ready for introduction into service. There was even talk of replacing the Model 48 helicopter with the Sikorsky S-51. The first Bell Model 48A/YR-12B helicopters were finally delivered to the USAAF in September 1946.

The YR-12B had a main rotor diameter of 47 feet, six inches (14.48 meters), a fuselage length of 39 feet, seven inches (12.06 meters), and a height of 11 feet, three inches (3.43 meters). Fully loaded, the YR-12 weighed 6,286 lb. (2,854 kg), and it had an empty weight of 3,700 lb. (1,680 kg). The aircraft’s cruising speed was 90 mph (145 km/h), and its maximum speed was 105 mph (168 km/h). It had a service ceiling of 12,800 feet (3,960 meters) and a range of 300 miles (480 kilometers).

Flight restrictions for the YR-12B included no aerobatic maneuvers, operating with caution when flying below 30 mph (48 km/h), and a maximum gross weight for flight of 6,600 lb. (2,995 kg). This was all subject to change.
Floats for the YR-12B helicopter allowed landings on ground, water and deep snow. The floats permitted taxiing on water at speeds up to 50 miles per hour (80 km/h).

Loading casualty litters into the Model 48/YH-12B during an Army practice. Six litters could be carried at one time. Bell/Niagara Aerospace Museum Photo

Initial tests with the U.S. Army ground forces as a troop carrier and cargo hauler went reasonably well, and the helicopter was used experimentally at bases across the U.S.

On Sept. 18, 1947, the United States Air Force (USAF) was formed and took over the procurement of helicopters for the Army ground forces. In 1948, the USAF changed the designations for helicopters from R for rotary-wing to H for helicopters, so the YR-12B became the YH-12B.

By 1950, with no production sales for the H-12B from the U.S. military, Bell was looking at future sales of the civilian Model 48A. Bell made plans to have a new 10-place aircraft for the production version of the Model 48.

The production Model 48 had an empty weight of only 4,200 lb. (1,907 kg) — 500 lb. (227 kg) less than the YH-12B. Its normal gross weight was 6,400 lb. (2,900 kg), with an overload gross weight of 7,200 lb. (3,265 kg). This new version could carry a pilot, co-pilot and eight passengers. The helicopter would be powered by the Canadian-built Pratt & Whitney R-1340 engine.

A drawing of YR-12, with seating arrangement for eight people. Production helicopters were to have room for 10 passengers. Bell/Niagara Aerospace Museum Image

The Model 48 had new skids, floats, and ski landing gear as options. Prolonged instrument flight with autopilot was also possible. In addition, the Model 48 was completely engineered for Arctic operations. The enhanced helicopter cruised at 90 mph (144 km/h), with a never-exceed speed of 140 mph (224 km/h). It also came with a rescue hoist and winch.

In December 1950, Bell approached the Canadian government with a proposal to mass produce the Model 48. The proposal boasted of a new helicopter capable of carrying eight troops in an assault transport role, the ability to carry six litters in an evacuation mode, and one that could move cargo and perform resupply missions.

The Model 48 had been designed so that it could be easily prepared for transport over long distances by standard cargo airplanes. It required only the removal of the rear wheels, and the main rotor and mast of the helicopter. The rotor blades, hub, mast, and wheels would be stored next to the helicopter fuselage.

A three-view drawing of the original Model/48 YR-12. Bell/Niagara Aerospace Museum Image

However, there would be no military sales to Canada, and the YH-12B was withdrawn from use by the U.S. military by the early 1950s and replaced by other helicopters. By the mid-1950s, Bell still had at least one YH-12B helicopter — the first service test aircraft — at its main plant.

Transatlantic crossings

In 1952, Bell set up a licensing agreement to build helicopters with Italian manufacturer Giovanni Agusta, starting with the Model 47D-1 two-place light aircraft. During 1956, the Bell Model 48A/YH-12B was shipped to Italy, where Agusta had begun to design and produce its own new helicopters, along with licensing Bell products. Agusta had plans to design a new and larger (up to nine-passenger) piston helicopter using the rotor system and dynamics of the Model 48. Bell engineers assisted Agusta in the engineering of this new AB-102 helicopter, which used the same Pratt & Whitney R-1340 radial engine as the Model 48A. The AB-102 prototype first flew in February 1959.

Eventually, the Bell Model 48A/YH-12B was put on display in the Agusta Museum, and was later moved to the Volandia Museum in Milan.

Only two experimental Model 48 XR-12As were built before Bell looked at coming up with an enlarged model for carrying additional passengers. Bell/Jeff Evans Collection Photo

The aircraft recently made the trip back across the Atlantic, arriving at its final home at the Lawrence D. Bell Aircraft Museum in Mentone, Indiana, where it is undergoing restoration.

The Model 48 may have been ahead of its time for both the company and the U.S. Armed Forces. It would eventually be replaced by the very successful Model 204/XH-40 turbine-powered assault helicopter.

If you have any information as to the location of other YH-12B helicopters, please contact the author through Vertical Magazine. Jeff Evans, Paul Faltyn/Niagara Aerospace Museum, Mario Bazzani, and Don Brabec contributed material to this story.

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