Bell Helicopter engineers have resolved a performance anomaly that test pilots had discovered when flying the hybrid aircraft in aeroplane mode, also known as the zero-degree position of the nacelles. The BA609 is a six- to nine-passenger corporate aviation tiltrotor, a product the companies expect to begin delivering in 2011.

According to Roy Hopkins, Bell's chief test pilot for the BA609, both prototype aircraft had been exhibiting higher drag on the right side at cruise speeds.. The companies are flight testing one aircraft at Bell in the USA and the other at partner AgustaWestland's facility in Italy.

After being briefed on the quirk, Hopkins says Bell engineers homed in on the most unlikely of culprits - the placement of the exhaust port on the right engine. The aircraft's Pratt & Whitney Canada PT6C-67A turboshaft engines have a single exhaust port on the outboard side of the nacelle. The BA609's sibling, the Bell Boeing V-22 Osprey, has downward-facing exhaust ports for its Rolls-Royce engines, and has not experienced a similar issue.

Further analysis and testing proved that turbine exhaust exiting the right exhaust stack was swirling away from the engine in a way that was causing more drag than the exhaust from the left engine. The tiltrotor’s digital flight control system had compensated for the yawing motion by trimming the collective pitch of the right proprotor, but a flight-test torque sensor installed on each of the proprotor masts revealed the subtle effects of the exhaust flow.

The fix - extending the exhaust port farther down along the right nacelle - was put in place two weeks ago and tests were to resume earlier this month. If tests indeed prove the problem to be fixed, the company will install the correction on the Italian prototype, says Hopkins.

 Deliveries of the six - to nine-passenger BA609 are set to begin in 2011

Source: Flight International