BMW Rolls-Royce has resolved a compressor blade cracking problem in the BR715 engine. The problem has delayed the start of flight testing of the Boeing 717 regional airliner for several weeks.

According to research and development director Günter Kappler, the cracks arose during altitude chamber tests to determine the engine's surge margin. A small incidence adjustment to the engine's stage three compressor blades has resolved the problem.

The tests, required before certification to prove the engine's operational limits, call for the compressor to be "-detuned in a way which can only take place on the ground", says Kappler. The engine reached its surge margin without unexpected problems, but ran into trouble during rapid deceleration, when the airflow across the third-stage rotor blades separated, causing flutter which led to cracking.

Kappler says that the separation could only arise with such a detuned compressor, so the flutter would never have occurred under operational circumstances.

The problem has now been cleared up with a 0.3° adjustment to the rotor blade incidence, combined with a change in the variable guide vane schedule to maintain stage loading. He adds that the problem was "academically interesting", having only been seen to date on wings and highly loaded research compressors.

The engine is scheduled for certification in September, says BMW R-R, following further validation testing of the modified compressor and blade-off tests. Boeing is hoping to have carried out the first flight of the 717 by August, with delivery to launch customer Air Tran due in June.

The engine manufacturer is planning to move its corporate headquarters from Oberursel, near Frankfurt, to the Dahlewitz BR700 factory, near Berlin.

Production of the smaller BR710 engine, which powers the Gulfstream V and Bombardier Global Express corporate jets, is scheduled to increase from 75 engines in 1997 to 115 units this year. Turnover is expected to reach DM800 million ($450 million), says Nittinger, but cautions that a good deal of money is still expected to be spent on completing BR715 certification.

Source: Flight International