Tech Briefs

Flight tests determine whether performance standards developed for scout and attack class rotorcraft can be applied to utility helicopters.

In 1982, the U.S. Army Aeroflightdynamics Directorate (AFDD), then assigned under the U.S. Army Aviation Systems Command (AVSCOM), began development of a new handling qualities specification for military rotorcraft. This effort resulted in the U.S. Army's initial Aeronautical Design Standard–33 (ADS-33A), “Handling Qualities Requirements for Military Rotorcraft,” published in May 1987. It was initially applied to the RAH-66 Comanche Helicopter program, meaning the handling qualities requirements generally related more to scout and attack classes of rotorcraft. As more data became available and lessons learned emerged from using ADS-33A, refinements were implemented into ADS-33B and ADS-33C).

Hover MTE traffic cones course layout for UH-60A Black Hawk testing.

In 1990, ADS-33C was assessed using an AH-64A Apache by the U.S. Army's Airworthiness Qualification Test Directorate (AQTD) at Edwards Air Force Base, California. Testing was performed during the day and at night in the Degraded Visual Environment (DVE). The results of that test led to development of the next version of the specification, ADS-33D.

In 1996, the U.S. Army Aviation and Troop Command found ADS-33D met the definition of a performance specification, and issued a new designation, ADS-33D-PRF. To better understand the applicability and compliance testing issues of applying ADS-33D-PRF to utility helicopters, a flight test assessment was recently conducted with a UH-60A Black Hawk helicopter.

The objectives of this flight test were: a) to assess the required compliance testing and evaluate the criteria in ADS-33D-PRF to determine if it adequately addresses the utility helicopter mission; b) to tailor existing ADS-33D-PRF Section 4 Mission Task Elements (MTEs) in a good visual environment and develop new flight test maneuvers specifically designed to adequately evaluate the handling qualities of utility helicopters, with and without external slung loads; c) to correlate the results of quantitative testing with those from the qualitative evaluations; d) to establish a handling qualities baseline of the UH-60A (using the general criteria outlined in ADS-33D-PRF) against which the effects of future modifications to the aircraft may be better compared or quantified; and e) to document the UH-60A response characteristics in order to provide data to support refinement of AFDD's UH-60A mathematical models.

The test aircraft was a sixth-year production UH-60A Black Hawk. The UH-60A is a twin turbine engine, single rotor, semi-monocoque fuselage, rotary-wing aircraft. The main rotor system has four blades made primarily of titanium and fiberglass. The propulsion system has two T700-GE-700 engines without the hover infrared suppressor system installed. The non-retractable landing gear consists of the main landing gear and a tail-wheel. The aircraft is equipped with a cargo hook capable of carrying up to 8000-lb external loads.

Dual cockpit controls consist of the cyclic stick, collective stick, and pedals. Pilot flight control inputs are transferred from the cockpit to the rotor blades by mechanical linkages and hydraulic servos. The aircraft is equipped with an Automatic Flight Control System (AFCS), which enhances the stability and handling qualities of the helicopter. It is comprised of four basic subsystems: stabilator, stability augmentation system (SAS), trim systems, and flight path stabilization (FPS).

The research instrumentation and data acquisition system for this test consisted of sensors, signal conditioners, pulse-code modulation (PCM) encoder, time code generator, and data recorder. The helicopter sensors included air data, accelerometers, rate and attitude gyros, and control position sensors at several points in the control system. These sensor signals were passed through filters, digitized, and encoded in a PCM stream, which was recorded onboard and simultaneously transmitted to the ground telemetry (TM) station. A time code generator received the broadcast time from the TM station and supplied this to the on-board recorder and the PCM stream.

Externally-mounted test instrumentation included a nose boom incorporating a sideslip sensor and airspeed probe, a low airspeed detection system, laser reflectors attached to the left and right step fairings, and a telemetry antenna attached to the fairing below the stabilator.

The ground-based telemetry (TM) station provided not only PCM-stream recording, processing, and real-time presentation, but also video coverage from a “pan and tilt” camera located on an antenna tower. The laser tracker system provided helicopter position data with accuracies to within ±6 inches. Aircraft position data were further transformed from the laser reflector located on the side of the helicopter to the pilot's eye position by including the aircraft attitudes and distances from the reflector to the pilot. Aircraft control positions, angular rates, and attitudes, along with aircraft ground speed and position information from the laser, were presented on four 8-channel, thermal-paper strip charts.

This work was done by David R. Arterburn of the University of Alabama in Huntsville, and Chris L. Blanken and Eric L. Tobias for the Army RDECOM Aviation & Missile Center. For more information, download the Technical Support Package (free white paper) here under the Electronics & Computers category. ARDEC-0003