Estimated reading time 9 minutes, 41 seconds.
On the helicopter side of the aviation world, Allison, a former division of General Motors that is now owned by Rolls-Royce, built its reputation as an innovator of light gas-turbine engines. Key to that reputation was the 250 series engine, which, in 2011, celebrated the 50th anniversary of its first flight.
Most readers are aware of the success of the 250 series. But, not everyone knows the story of how the 250 was first created and then progressed into multiple, successful models that dominated the small gas turbine market and was largely responsible for developing the light turbine helicopter industry.
Starting the Journey
Now known as the Rolls-Royce M250, the original Allison 250 series engine was conceived of in 1957, when the United States Army announced during an NACA (National Advisory Council for Aeronautics, now NASA) meeting that it wanted a 250-shaft-horsepower (s.h.p.) gas turbine engine. An Army study in 1956 had determined the need for such a lightweight engine to power its future aircraft, and that the higher-power-to-weight ratio of gas turbines, over reciprocating engines, was the way to go.
Design study contracts were spread throughout the industry to many of the key players at the time, such as Garrett, Curtiss-Wright, Teledyne and Lycoming. Army specifications dictated a turbine engine configuration with 250 s.h.p., and that it be useable in both airplanes and helicopters. Additionally, a small package was required, and it had to have a low cost per unit, low fuel consumption and a 1,000-hour lifespan. Allison made a formal presentation of its design (the Model 250) in March 1958.
By that June, Allison won the contract to produce the 250 (T63 military) engines. With a household name, General Motors was seen to be a dependable contractor with vast resources on hand; along with its low projected bid, this was a likely factor in Allison’s selection.
That original 250 design had a seven-stage axial compressor, a single-stage centrifugal compressor, a singlestage gas producer turbine and a two-stage power turbine. It had a rating of 250 s.h.p. and weighed 110 pounds in turboprop form and 95 pounds in turboshaft form.
It was projected that a small turbine engine would cost $16 million to $20 million to develop, with an end unit price of around $4,000. Allison, though, bid lower than the estimate in its proposal, entering into a fixed $6.4 million cost-sharing contract with the U.S Army, with overages to be the responsibility of Allison. In the end, costs surpassed the original projections, but history would prove it to be one of the most successful engine designs.
Heading up the design and development team for the 250 series was Charles McDowall, chief of preliminary design; and Bill Castle, chief project engineer — who was also involved with the mechanical design for the compressor. Other key team members included Beauford Hall on turbine design, Bob Larkin on gearbox design, and Joseph Barney spearheading the combustor design and the aerodynamic design on the turbine and compressor. Meanwhile, Bob Wente handled control, and Russ Hall focused on aerodynamic design information for the compressor and turbine.
From the beginning, the 250 had a unique design: the gearbox, located in the center of the engine, was the engine’s primary structure and contained the hard mounts. Air was drawn through the compressor, attached to the front of the gearbox, and was sent through a diffuser, then through two tubes running down the outside of the engine to a single combustor at the engine’s tail. During combustion, gases flowed forward into the single-stage core and twin-stage power turbines, with exhaust gases leaving the engine through the bottom. For anyone who is somewhat familiar with the 250, you will probably recognize this design as being only vaguely similar to today’s 250 series engine. Part of the reason for this is due to the big challenges the design team would soon face.
After the Euphoria
After the excitement of the contract win came the reality of the project — Castle was instructed that he needed to have the new engine operational in less than a year. One of the challenges facing his engineers was the scaling down of the current turbine design and technology, which had arisen around the proportionally larger turbine engines on the market.
Early 250 test models were problematic in all areas, including poor compressor airflow performance and turbine efficiency. In fact, it got so bad that the designers came to a crossroads: should they continue with the current flawed design or begin with a new one? They chose the latter and re-engineering began in late 1959 and a new chief project engineer, John Wetzler, was soon in charge.
In the new design, there was a reworking of the exhaust — it went from single to dual exhaust ducts —while minimal changes were made to the gearbox configuration. The turbine section saw significant changes, with a second turbine stage added, and the compressor diameter was also enlarged slightly. A six-stage axial compressor design was drawn up to improve airflow ratios, and the centrifugal compressor was reengineered to improve aerodynamic design.
In 1960, initial preliminary flight rating tests (PFRTs) for the new design showed promise, but the problems continued — as did the modifications to resolve each problem.
That year, the Army announced a light observation helicopter (LOH) design competition, to satisfy its needs for a multi-role helicopter for visual reconnaissance, command and control, and acquisition of targets. The Bell OH-4A, Hiller OH-5A and Hughes OH-6A were soon selected as finalists. Each, along with the winning design, were to be powered by Allison’s engine — but the Army was becoming concerned with Allison’s slowed progress, and with a recent turbine failure, and held a competition to award a contract for an alternate engine supplier who would serve as a back-up if Allison fell short. But, Allison’s team of engineers worked countless hours to solve its problems with the T63, and was finally rewarded with a successful 50-hour PFRT.
Early flight-testing of the T63 (YT63-A-3) in 1962 was conducted by Allison and Bell Helicopter in Indianapolis, Ind., with the Bell UH-13R — basically, a converted Bell 47/HUL-1M. Unfortunately, unexpected problems occurred involving the engine’s governing system and fuel control, which controlled engine power based on the pilot’s collective control inputs. It was a problem that would prove incredibly complex in solving.
The issue had to do with the helicopter rotor system’s torsional dynamics — the flapping, feathering and lead/ lag components of normal operation. As the rotor system was turning at operating r.p.m., the governor/fuel control system was so incredibly sensitive at trying to compensate for the varying loads on each blade that the engine’s power increased beyond limits, forcing a hovering autorotation and engine shutdown. Engineers solved the issue with the Bell platform through alterations in the engine’s pneumatic dampening system, but realized that differences in rotor design (and therefore, rotor dynamics) — i.e., a three- or four-bladed main rotor vs. Bell’s twoblade system — would require a different solution for each one. The engine would need to be matched to each platform, and according to Allison’s chief test pilot, Jack Schweibold, overcoming that proved the biggest challenge they would face during these flight tests.
Other concerns led to the exhaust being needed to be turned upward (the downward exhaust was related to the Army’s idea of the engine being used in fixed-wing applications). So, the engine was basically installed upsidedown, and this became the YT63-A-5.
One of the other problems the T63 experienced at this stage of development was a delay in its response to rapid power increase requirements, for maneuvers such as aborted landings. For this concern, it was determined that a bleed valve incorporated into the compressor would increase the margins of compressor stall, allowing for faster power responses.
Finally, in December 1962, all the work paid off and the T63-A-5 met its military approval and the civilian Model 250-C10 met U.S. Federal Aviation Administration standards and was certificated. It was official; the 250 would indeed go into the winner of the LOH competition.
Deciding on a winner, though, proved almost as troublesome as the development of the 250/T63. From January to June 1964, LOH testing was conducted, and it seemed the Hiller OH-5A would come out on top. But, Hughes controversially underbid its competitors and won the Army’s contract in May 1965; delivery of the OH-6 Cayuse then began in 1966.
By 1967, even though the OH-6 was performing well in Vietnam — and had set almost two-dozen world records — problems had arisen with production. Dramatic unit cost increases and slow production began to worry the Army. Given this uncertainty with Hughes, the Army re-launched the LOH competition, which was won by Bell in 1968 with its revised OH-4A, the Bell 206A, which became the OH-58A Kiowa in military form.
For Allison, the choice of Bell was a positive one, as it was already providing the 317-s.h.p. 250-C18 for the 206 JetRanger — as well as the Hughes 500 (civilian OH-6) — and had found improvements that aided its military engines, as well. In the end, both of the Allison-powered LOH winners went on to serve the military well in Vietnam; and their civilian counterparts helped define the commercial light turbine market.
Allison next began working on the Series II 250 engines, and in May 1970 certificated the C20 version, which was rated at 400 s.h.p. Over the succeeding decades, it went on to produce a host of variants, as well as Series III and IV versions with increased power and improved features — all to great success. In fact, by the late-1980s, the T63/250 had captured 80 percent of the lightturbine helicopter-engine market share in noncommunist nations, and the 250 was also being used in light-twin-engine helicopters such as the MBB Bo.105 and Aérospatiale AS355.
In 1995, Allison was purchased by Rolls-Royce for US$525 million. And, Rolls-Royce has continued to improve upon the reliable and hearty 250 series (now M250) engines to this day. It even created a new engine, the RR300, to carry on the 250’s legacy of being a lightweight, low-cost choice for smaller light helicopter applications.