Technology update
Dunlop upgrades
 Dunlop Aviation's 163 MJ dynamometer is being upgraded. |
Carbon brakes are finding increasing acceptance in the aerospace industry, according to Dunlop Aviation, which is enhancing its manufacturing capacity with the installation of new machining facilities and greater furnace capacity to meet requirements. Dunlop is a pioneer of carbon brake technology, which offers enhanced resistance to thermal shock and, unlike metals, offers a strength and braking efficiency that increase with operating temperature. Dunlop's carbon/carbon disc brake system is comprised of a number of stators and rotors, each made from several discs of prefabricated polyacrylonitrile (PAN), which are converted to a solid assembly in furnaces using a chemical vapor deposition process. Post-processing machining operations are necessary to prepare the blanks for assembly.
Dunlop Aviation is also upgrading its 163 MJ dynamometer to increase its load and mechanical energy capacity and is adding a new test-head to facilitate direct measurement of drag. When the work is completed in the early fall, the dynamometer will be capable of operating under radial loads as much as 150,000 lb. According to Dunlop, the new inertia configuration allows for incremental increases between the main and secondary flywheels. These increments also provide additional capacity when used in conjunction with both flywheels, giving a maximum mechanical inertia equivalent of 80,000 lb.
 P&W's PW6000 powerplant is to use Dunlop Precision seals. |
The dynamometer will be suitable for testing brakes and systems for very large aircraft now being developed or considered, including the Airbus A380 (formerly A3XX) and the latest variants of the Boeing 777. A secondary mechanical drive will allow programmable static torque tests to be carried out that will simulate engine run-up before takeoff. Two data acquisition systems will gather brake temperature/vibration and performance data for post-processing and analysis. Sound and video of the test can be recorded digitally and time-indexed to the performance data.
Another part of the Dunlop Group—Dunlop Precision Rubber—is supplying four different seals for Pratt & Whitney's PW6000 powerplant, which will be an option for the 100-seat Airbus A318. The seals comprise a fabrication of silicone rubber reinforced with glass fabrics and are designed to resist extremes of temperature without loss of performance. Seals of this type, according to Dunlop, will continue to provide an effective fire barrier even when the polymer layer has burned away. In FAR tests they have proved capable of withstanding 1100°C for a minimum of 15 min. The PW6000 engine has a 16,000 to 24,000 lb thrust range.
- Stuart Birch
Integrated circuit measures time-of-flight
 Ken Condreva, Engineer at Sandia National Laboratories, has developed an integrated circuit to accurately record critical timing signals in weapon test flights. |
Sandia National Laboratories has developed an integrated circuit called Falcon, which is capable of measuring time-of-flight to an accuracy of 125 ps to record critical timing signals in weapon test flights. New telemetry systems required a compact, lightweight, low-power device for this purpose.
"The only things I could find that had this resolution were tabletop instruments packaged in a box," said Ken Condreva, Engineer at Sandia National Laboratories and inventor of the device. "They were too big and used too much power."
Condreva developed the "pulse stretcher" technique to increase resolution up to 200 times for a low-power electronic clock using 300 mW at 40 MHz. The circuitry provides greater resolution by lengthening the duration of the output signal, making it last from 64-200 times longer than the input signal. Although the input pulse is "stretched" in real time, the technique can be compared to recording a sporting event with fast-action film and replaying it at slow speed to clearly see what happened.
The integrated circuit is also designed to operate in extremely rugged and harsh environments—high and low temperatures, high vibration and shock, and high and low humidity. According to Condreva, it uses standard, commercially available CMOS technology and could be inexpensively manufactured by most semiconductor businesses.
- Frank Bokulich
Flutter testing at Dryden
 The ATW was mounted on a special ventral flight test fixture and flown on board NASA Dryden's F-15B Research Testbed aircraft. The ATW was intentionally flown to the point of structural failure to enable engineers to record the effectiveness over the entire regime of flutter testing. |
NASA's Dryden Flight Research Center in Edwards, CA, has begun demonstrations of a new software data-analysis tool, the flutterometer, which is designed to increase the efficiency of flight flutter testing.
Flutter is the rapid and self-excited vibration of wings, tail surfaces, and other aircraft parts that can damage or destroy an aircraft component. It is caused by the airflow around the surface of a structure; the aerodynamic forces couple with structural bending and twisting to result in the vibration. Flight-flutter testing is the process of determining an aircraft's flight envelope. According to NASA, traditional approaches to flight flutter testing do not accurately predict the onset of instability, resulting in increased costs and testing time.
The flutterometer is an online software tool that enables flight data to be analyzed immediately to determine the aircraft's stability properties. It is designed to predict the flight conditions at which the onset of flutter may occur, enabling the aircraft's operating envelope to be determined more quickly and safely than traditional approaches.
Dryden engineers demonstrated the flutterometer during the Aerostructures Test Wing (ATW) experiment, which consisted of an 18-in. carbon-fiber test wing with surface-mounted piezoelectric strain actuators. The test wing, which was designed by NASA Engineer Cliff Sticht and manufactured by Fiberset, Inc. in Mojave, CA, was mounted on a special ventral flight test fixture and flown on Dryden's F-15B Research Testbed aircraft.
 Research objectives of the ATW experiment included validation of the new flutterometer and the aerodynamic load predictions on the test wing as well as analytical strain-gauge-calibration techniques. |
The five flights consisted of increasing speeds and altitudes leading to the final test point of Mach 0.85 at an altitude of 10,000 ft. At each Mach and altitude, stability estimations of the wing were made using accelerometer measurements in response to the piezoelectric actuator excitation. The test wing was intentionally flown to the point of structural failure, resulting in about a third of the 18-in. wing breaking off. This experiment enabled engineers to record the effectiveness of the flutterometer over the entire regime of flutter testing, up to and including structural failure.
The actuators were moved at different magnitudes and frequency levels to induce wing vibrations and excite the dynamics during flight. Mercedes Reaves, Engineer at NASA Langley Research Center in Hampton, VA, determined the placement of the piezoelectric actuators for maximum effectiveness. The ATW experiment represents the first time that piezoelectric actuators were used during a flight flutter test.
Potential benefits of this research include reduced time and cost associated with aircraft certification by lowering the number of flights required to clear a new or modified aircraft for flight, and provision of a structural dynamics database for industry and university flutter research.
The concept of a flutterometer was initially conceived by Dryden Structural Dynamics Engineer Len Voelker. Fellow Engineers Rick Lind and Marty Brenner developed the flight data algorithms that made the flutterometer concept a reality. NASA was recently awarded a patent for the tool. The flutterometer software was previously evaluated using simulations and wind tunnels along with flight data from several aircraft types, including NASA Dryden's F-18 Systems Research Aircraft.
- Frank Bokulich
