HJT-36 Flight Test Resumption Renews Hope for AL-55 Engine

 - May 1, 2019, 9:06 AM
S3474 was the first of two HJT-36 prototypes. Following a considerable redesign a third aircraft is now flying. (photo: Vladimir Karnozov)

Following a three-year break, Hindustan Aeronautics Ltd (HAL) has resumed flight tests of the HJT-36 Sitara twin-seat aircraft developed for the Intermediate Jet Trainer (IJT) program. The latter aims to produce a direct replacement for the Indian Air Force Kiran, of which production was terminated in 1989 with nearly 200 built. The resumption took place in late April involving a prototype aircraft serialed S3854. “The flight was flawless and its success is an important step in the IJT program,” reported the manufacturer.

With over 800 flights performed since March 2003 on the PT-1 and PT-2 prototypes (serialed S3474 and S3466, respectively), the IJT effort appeared to be well on its way to serial production, with the Indian air force looking to procure between 200 and 250, until unsuccessful spin testing in 2016. Issues necessitated a major redesign of the aircraft’s tail section. The aircraft that flew recently features a repositioned vertical fin moved aft and other design changes. It is understood that, if the new configuration proves up to customer expectations, the Indian air force may buy 73 serial examples. Although far below the initial requirement, already met in part through the purchase of 146 BAE Systems Hawk Mk 132s contracted in 2004-2012, such an order would bring HAL a business worth Rs 5,000 crore ($719.2 million).

Working out remedies to improve spin characteristics for an otherwise promising and mature design required extensive wind testing on scale models. During the past three years, HAL also addressed issues of excessive airframe weight, while its program partners in Russia had more time to improve the engine that powers the HJT-36. Back in 2005, NPO Saturn won a $300 million Indian contract to develop the AL-55I, a version of the baseline AL-55 customized to match the airframe. Three engine examples were shipped in December 2008, including one for the first prototype, which was refurbished and flew in May 2009.

Replacing the previously fitted Snecma Turbomeca Larzac 04-30 (04-H-20) engine reduces fuel burn by 10- to 13 percent through higher turbine temperatures (by 15 to 45 degrees C) while keeping the same mass flow (at 28kg/62 pounds per second). The maximum thrust increase from 14.24 to 17.27 kN (3,200 to 3,880 pounds) is important in view of airframe overweight issues, and is necessary to meet the customer requirements for rate-of-climb, ceiling, and weapons load. Developing the AL-55I required considerable changes to the baseline design, chiefly to reduce weight by more than 50 kg (110 pounds). This was necessary to match the respective figure for the 295-kg (650-pound) Larzac, and thus eliminate the need to re-balance the airframe and onboard equipment for center-of-gravity position.

Russia insists that the 2005 contact was fulfilled in 2013, when AL-55I pre-production specimens demonstrated a 300-hour lifetime. Further plans called for the extension to 600, and then to over 1,200 hours, the latter being an Indian Air Force requirement. According to developers, the engine’s cold section is designed to withstand 6,400 hours and the hot section 4,000 hours. So far, however, Russia has delivered only about 20 engines, due to the HJT-36 production program being halted by the spin issue.

For NPO Saturn and its patron United Engine Corporation, the IJT program resumption means a boost to the AL-55 effort. If the engine goes into quantity production, unit costs would decrease to a level affordable for more foreign and local customers. Russian aircraft makers are yet to place their orders, with the AL-55 fit to power the 3,100-kg (6,830-pound) SAT SR-10 jet trainer with forward-swept wings and the 2,300-kg (8,380-pound) Argument UCAV derived from it. With more than 85 flights performed on a single SR-10 prototype, the project won a positive MoD assessment, but experiences financial issues related to establishing serial production. Apart from SAT, there are some other Russian companies interested in the AL-55 version generating 17.35 to 19.6 kN (3,900-4,400 pounds') thrust at military power and up to 29.4 kN (6,600 pounds') thrust when fitted with an afterburner.