November 2004


PUTTING SENSORS TO WORK
Table of Contents

Flight Testing the
Black Hawk

A sophisticated software program is helping Army test pilots and data analysts evaluate helicopter airworthiness.

The flight test engineering team in the U.S. Army Aviation Technical Test Center (ATTC) has a dual mission: Optimize aviation war fighting capability, and give the fighting troops a full-spectrum dominance of the battlefield. The ATTC plans, conducts, analyzes, and reports on the developmental and airworthiness qualification of aircraft. The tests focus on the handling qualities of the aircraft and its performance in various maneuvers, including operation under icing conditions.

The team consists of experimental test pilots and engineers working together to create a well-documented and highly graphical report that serves both as an archival reference and a record of the tests that were conducted. The collected data are reviewed by the U.S. Army Aviation Engineering Directorate at the U.S. Army Aviation and Missile Command, which ultimately determines the airworthiness of the helicopter and the specific system(s) being evaluated. Among the aircraft tested are the UH-60 Black Hawk, AH-64 Apache, CH-47 Chinook, OH-58 Kiowa Warrior, MH-6M Mission Enhanced Little Bird, and the RC 12D Huron.

The Black Hawk: A Typical ATTC Test Vehicle
The Sikorsky Aircraft Black Hawk helicopter has been used by the Army since 1978 and is among the most frequently tested. These aircraft have logged four million flying hours, including combat and rescue missions. Equipped with digital avionics and a flight crew of three, the helicopter can carry 11 combat-loaded air assault troops, and transport a 105 mm howitzer and 30 rounds of ammunition. It also has provisions for mounting an external stores support system that can carry sixteen Hellfire missiles. The cabin's double doors are designed for transportation of cargo and passengers.

Two General Electric turboshaft engines power the craft, and its internal crash-resistant and self-sealing fuel system's 1360-liter
figure
Figure 1. The Black Hawk model UH-60L, one of a long line of helicopters used by the Army since 1978, is shown here undergoing airworthiness testing. The orange boom collects data on airspeed, ambient conditions, and other pertinent parameters.
capacity is good for more than three hours of flight time. The Black Hawk can maintain a normal cruising airspeed of 120 knots with maximum dash airspeed of 178 knots. A digital automated flight computer system is available to simplify pilot workload, and an electronic flight information system provides the air crew with primary pilotage and navigation displays.

The Black Hawk design is periodically updated and revised to new standards. The most recent model, the UH-60M, is currently undergoing testing that will extend its service life until 2025. Its precursor is the UH-60L model, shown in Figure 1 in a typical test configuration. The orange boom on its nose captures airspeed data, external ambient conditions, and other parameters.

Airworthiness Testing
As helicopters' systems are upgraded, the test teams perform additional airworthiness evaluations to ensure that the aircrafts' handling qualities have not changed. For example, the addition or modification to aircraft controls, fuel systems, or other avionics requires an aircraft to go through the same testing that it went through previously to demonstrate the airworthiness of the new system. It is therefore critical that the data are consistent and easy to compare to previous tests so that changes can be quickly identified. When the external profile of the aircraft has not been significantly altered, airworthiness testing is strictly a confirmation that aircraft's performance has not changed for other reasons.

Acquiring the Data
The basic test data include plots for control positions in level and diving flight to ensure that the handling qualities have not changed and to ensure that the aircraft has "positive static stability"-if the stick is pushed forward, the aircraft speeds up. Anywhere from 10 to 20 measurements may be taken. Other tests plot different parameters such as airspeed or torque to see how power
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Figure 2. In a plot of low-speed sideward flight performance, the control excursions are indicated with vertical lines and horizontal bars. Winds are 5 knots or less. The automatic flight control system is on during both this test and the one plotted in Figure 3.
affects the handing qualities. The majority of the report relies on instrument readings, but the test pilots still add personal comments regarding any uncomfortable operations or challenging flying problems. The pilot's subjective data are combined with the test data as part of the complete report. At the end of the day, the pilot still has to say the aircraft flies like a Black Hawk, or he or she could have comments such as, "I had to work hard on the pedals to maintain heading," or "I had to make continuous inputs in the lateral axis to keep the aircraft level," or "I was fighting it."

The low-speed sideward flight measurements are plotted in Figure 2. The start point is at a hover and the cyclic (or stick) is pushed to the left to achieve some airspeed to the side. If the goal is to fly 35 knots to the left and at 30 knots the stick cannot be moved any farther left, then there is a problem-the craft has run out of control margin. Control margin is one of the measurable criteria used to evaluate the aircraft. The error bars in Figure 2 show how hard the pilot had to work to maintain that flight point. These I-bars are maximum and minimum control excursions and are recorded for many of the measurements. Stabilator position, roll attitude, pitch attitude, collective control position, directional control position, lateral control position, and longitudinal control position are all plotted vs. the true airspeed in knots.

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Figure 3. Trim points in the measurements for static lateral directional stability are indicated by the solid circles.
Static lateral-directional stability data are shown in Figure 3. These plots are of average torque, stabilator position, pitch attitude, roll attitude, longitudinal control position, lateral control position, and directional control position, all plotted against the slide slip angle. Figures 2 and 3 represent just a few of the control positions in level and diving flight that are part of the overall document package.

Documentation
Making sense of all the data is a monumental task, and the method of presenting the test results is of extreme importance. ATTC uses a software tool that provides the ability to add control to the plots so the data can be shown in formats appropriate for comparison and archiving purposes. The way the plots are displayed assists subsequent analysts to obtain specific data points.

As part of one special project, structural analysis was performed. Fast Fourier transforms were used to calculate the frequency content of the signal, important in helicopters because of the vibration that occurs.

Every airworthiness evaluation includes data similar to those shown in Figures 2 and 3. Because different teams generate reports, organizing and presenting the data in a standard format is important to maintaining uniformity.

Summary
Because aircraft are continuously being modified and upgraded to provide the U.S. Army with the capabilities required in the 21st century world, it is important to ATTC to minimize training time for personnel. The ease of learning the software program allows ATTC to get new flight test engineers up to speed quickly and ensures that standardized processes are used to compare and analyze data accurately. As a result, the challenge of ensuring that these aircraft are the best they can be is rewarding as well as demanding.



 
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