PAUL LEWIS / WASHINGTON DC & GUY NORRIS / LOS ANGELES

Stability and control problems with modernised AH-1Z/UH-1Y helicopter lead to further delays and cost overruns

The US Marine Corps Bell AH-1Z/ UH-1Y helicopter modernisation has slipped another year due to a horizontal stabiliser redesign.

The rejigged programme calls for a first low rate initial production contract around October or November 2004 - a two-year overrun -Ê and the first squadron of six AH-1Zs and three UH-1Ys to reach initial operational capability in September 2007, 12 months later than the original schedule and overrunning a pre-planned cushion.

The USMC says the hold-up is "due primarily to engineering drawing release delays and parts shortages". Bell's revised estimate for completing development is $742 million, with $12.7 million extra costs exposure for the US Government. The programme is already late and $100 million-plus over budget as the result of problems with the H-1's avionics suite.

A reconfigured tail is being flight tested at the US Navy's Patuxent River test centre after longitudinal and lateral flying-quality problems in early test flights. The issue surfaced during tests of the AH-1Z structural and aerodynamic demonstrator equipped with the new four-bladed composite main and tail rotors, new drive systems, new wet wings, beefed-up landing gear, new hydraulics and flight controls, and a strengthened structure.

The new stabiliser is extended by 530mm (21in) on each side of the tailboom, increasing area by 60% and span by 35%. Tests include its effect on angle-of-attack, static longitudinal and manoeuvring stability, the former described as "unsatisfactory" with the original tail and the latter two as "weak". The effect of the bigger stabiliser on pitch-rate control will also be evaluated. "The rotor was over-powering the pitch axis," says Bell test pilot Herb Moran. He adds that this meant the pilot was operating under high workloads in most regimes and that "pilots had to exercise caution in open-loop testing".

It is hoped the larger tail surface will improve directional flying. Tests revealed "dead-band" areas in straight and level flight in which larger pedal inputs were needed for small directional changes. The hope is that by extending the horizontal tail the vertical endplates will be moved into cleaner air outside the wash of the larger weapons pylons and wing, improving lateral and directional stability.

Source: Flight International