Bell's new Model 427 is in development flight testing. The eight-seat, light twin turbine has gained weight, hover, speed and range since its maiden flight in December 1997, but the manufacturer aims to reduce the weight by May





 Bell is fine-tuning the performance of its new Model 427, as development flight testing of the eight-seat helicopter nears completion.

The company hopes to certify and deliver its first production model by early May, marking Bell's inaugural entry to the light twin-turbine helicopter market.

The Texas manufacturer has clocked up more than 500h of flight time and amassed another 137h of ground testing, using two prototypes and three production machines.

The flight-test programme has included more than 800 engine starts and over 1,250 landings since the helicopter made its maiden flight in December 1997 at Mirabel in Canada.

During this time, the 427's empty weight has increased marginally to 1,670kg (3,680lb), shaving off just over 80kg from the helicopter's 1,135kg target payload.

The helicopter's price has also edged up, from $1.9 million to $2.2 million in 1999 dollars, but this has not dampened Bell's enthusiasm for its new machine.

"We did get more hover, speed and range, but we let the weight grow on us," says Max Wiley, Bell commercial applications manager. "We're going to get it all back, plus some. You're going to have greater than a 2,500lb useful load when we're through. It's probably going to end up around 2,700lb."

The solution is to increase the 427's maximum take-off weight by nearly 6%, to 2,885kg and to certificate the helicopter at this new limit in parallel with US Federal Aviation Administration Category A approval, targeted for the end of 1999. The company says it is "comfortable" with the new weight, having conducted much testing at gross weights of over 2,950kg.

Bell has improved the 427's performance by switching from the Pratt & Whitney Canada PW206D turboshaft to the more powerful PW207D. Product manager Bob Barr says: "The key to a good twin is a good Cat A one engine inoperative [OEI] performance. We wanted to improve this and had the option to go with the 207D. We now have a take-off rating of 1,420shp [1,060kW] and an 800shp transmission, meaning good hot and high performance."

The 207D has a maximum continuous rating of 465kW, compared to the 206D's 425kW. OEI can produce 610kW for up to 30s and 580kW for 2min. This has translated into a marginally faster cruise speed of 136kt (250km/h), but the real difference has been a 4,000ft (1,220m) improvement in hover-out-of-ground-effect to 14,000ft and nearly an 8% increase in range to 700km (380nm).


Cost Target

With the 427 containing 33% fewer cabin components than the single-engined Model 407, and 40% fewer transmission gears than the Model 430 twin, Bell has set a direct operating cost (DOC) target of $416 per flight hour. Its projected DOC factors in 10,000h main rotor blades and 5,000h tail rotor blades.

The time between overhaul (TBO) for the 427's new "flat pack" main transmission has been set initially at 2,500h, and that for the engine at 3,000h. Bell plans to increase these TBOs to 5,000h and 3,500h, respectively, through a fleet leader programme. "We're looking for a high-time customer to generate the maximum hours in the minimum time. We'll announce at HAI [Helicopter Association International Show] who we'll do it with," says Wiley.

Bell has taken several steps to improve the 427's handling and safety. The inclusion of a Hamilton Standard full authority digital engine control protects against engine surge and rotor droop, by maintaining a constant rpm. A serial port allows for the downloading and diagnosis of flight data, in the event of a suspected torque exceedence.

The cockpit's integrated instrument display system is colour coded to warn of actual or potential exceedences, while there is the option of a collective stick shaker. Wiley says: "If the pilot gets a shake in his hand, he knows he is at maximum torque and to back off, while keeping his eyes out of the cockpit. It's pilot friendly."

The Rogerson-Kratos liquid-crystal display system can also be used to practise for OEI situations, without the pilot having to shut down an engine and risk an overtorque. One screen is used to show OEI conditions and the simulated response of a single engine, but will revert to normal twin-engined operations in the event of a threatened exceedence.

Initial FAA visual flight rule (VFR) certification has been pushed back from February to April, largely as a result of the switch to the 207D engine. European Joint Aviation Authorities VFR approval is running three months behind that, while FAA/JAA instrument flight rules and JAR Ops 3 certification is targeted for December.

Source: Flight International