Recent design changes to the Bell 429 development prototypes could land Bell Helicopter in hot water with helicopter performance specialist BLR.

Unlike Bell's product brochures, the latest 429 sports an efficiency-boosting strake on the upper port side of the tail boom and a BLR-trademarked "fast-fin" cut out on the vertical fin. Bell "froze" the design of the helicopter in October 2007, and plans to complete certification of the twin-engine medium helicopter by year-end.

BLR president Robert Desroche tells Flight that BLR worked with Bell for two years on similar design improvements to obtain a supplementary type certificate (STC) for the Bell 212.

He says Bell had signed a non-compete agreement with BLR regarding the use of the technology on other products. "We have our lawyers looking into this," Desroche says.

For the Bell 212, Desroche says the combination of the reduced-area vertical fin ("fast-fin") and NASA-developed dual strakes increase payload by 500kg (1,100lb).

The retrofits boost tail-rotor effectiveness and overall helicopter stability by reducing airflow blockage from the vertical fin and cancelling the sideways lift and turbulence caused by main rotor downwash over the tail boom.

The company has STCs for the Bell 206, Bell 412 and other Bell models and is working on STCs for existing products built by other rotorcraft manufacturers.

It is unclear whether a BLR challenge could impact Bell's schedule to complete certification, which is 50% complete according to Bell programme manager Neil Marshall.

Bell has more than 270 orders for the $4.8 medium helicopter, many of which are coming from the emergency medical services community, and says it is "not aware on any patent or agreement that would prevent us from making the design changes we have done on the 429".