Giving RTK a Whirl — GPS Aces Flight Tests

GPS World, April, 2000 by Mark Hardesty, Greg Ashe

ADS-33D describes a series of mostly ground-referenced maneuvers that are to be executed and scored per the outlined criteria. Typically, a surveyed and carefully marked runway surface is prepared to provide good visual cues. The pilot then flies the helicopter through the series of maneuvers by referencing the ground markers. Historically, judges in strategic positions have been used to evaluate how well the pilot maintained horizontal and vertical position relative to the ground markers. The judges score the maneuvers using their best visual judgment. The pilot also uses a subjective rating system known as the Cooper-Harper scale to evaluate the ease of maneuver execution. Helicopters found to be difficult to control precisely may be instrumented to monitor control activity.

Considering how much is resting on the evaluator's qualitative opinion, an objective method of documenting the aircraft performance, such as the Portable Test Range, is imperative. Our system allows the test team to immediately provide feedback to the crew regarding the performance level actually achieved. The pilot can also use the data to assist in rating the handling qualities.

One of the maneuvers used to evaluate ADS 33D helicopter controllability is the pirouette, which requires that the pilot hover at a specified altitude. The pilot points the nose of the helicopter toward a point defining the center of a circle of a specified radius. The pilot then maneuvers the aircraft around the circle defined by this radius, keeping the nose pointed at the center marker and holding a constant altitude above ground level. The pilot completing this maneuver in an MD 900 helicopter rated the task as easy, requiring small, infrequent control inputs.

Immediately plotting the maneuver on the cockpit display proved invaluable in producing accurate handling qualities ratings. Furthermore, no judges were required to participate in the exercise, as the recorded GPS data provided the necessary information (Figure 3), with the Portable Test Range assisting greatly in shifting handling qualities ratings from subjective to more objective rating criteria.

CERTIFICATION CONTINUES

FAA helicopter certification also requires that the hover controllability envelope be defined for gross weight versus density altitude up to a limiting altitude of 7,000 feet. The helicopter's ability to control heading with wind from any direction to a minimum of 17 knots must be demonstrated. Testing may be done both IGE (typically defined as a landing gear height of 3-6 feet above ground level) or out of ground effect (OGE, typically defined as 1.5 times the main rotor diameter).

Because even The Boeing Company cannot control the wind, we have developed a procedure to incrementally map the helicopter controllability by flying along a runway centerline. We artificially create headwind, tailwind, and crosswind components by flying up and down the runway at various headings relative to the direction of travel. Normal helicopter airspeed-indicating systems are only accurate for straight-ahead flight, however, and even then only begin to indicate accurately at perhaps 30 knots, so another method of velocity measurement is needed.