Advanced Measurement of Angle of Attack and Sideslip

Dryden Flight Research Center, Edwards, California

Nearly every military and commercial aircraft in the United States today uses pitot-static probes for accurate, repeatable airdata measurements. Recently, local angle-of-attack- and sideslip-sensing capabilities have been added to these probes to satisfy requirements for advanced aircraft with extended maneuvering envelopes. Probes made in advanced shapes to satisfy these requirements have been evaluated in wind-tunnel tests at angles of attack up to 90°, with favorable results. Flight tests of the Advanced L-probe Air Data Integration (ALADIN) program, directed toward evaluating the performances of these probes, were recently concluded at NASA Dryden Flight Research Center.

Figure 1. Specially Shaped Fuselage-Mounted Probes offer advanced capabilities for sensing angles of attack and sideslip.

The ALADIN program is a cooperative effort of Boeing Phantom Works, NASA, and BFGoodrich Aircraft Sensors Division (BFG ASD). The F-18 Systems Research Aircraft (SRA), used as the flight-test platform to evaluate these probes, provides both high-angle-of-attack- and supersonic-test capabilities. The SRA is equipped with an on-board Airborne Research Tests System (ARTS) computer, is used to perform real-time airdata calculations. BFG ASD provided two multifunction L-probes and four dual-channel pressure transducers. The transducers measured pitot and static pressure directly from the probes (1 pitot and 2 static from each probe) and transmitted these values to the ARTS computer via an RS-422 interface. The ARTS computer executed an algorithm developed by BFG ASD to calculate the angles of attack and sideslip using pressure differentials from the probes. Altitude, mach number, and other airdata parameters were also calculated from these pressures. The ARTS computer transmitted the measured pressure values to the SRA data-acquisition system via a MIL-STD-1553 multiplexing bus, which was telemetered to the ground. All calculations were performed in real time and compared to reference data during flight.

ALADIN flight tests were conducted in two phases. The first phase of flights was used to evaluate the performance of the probes in different regions of the SRA flight envelope. These flights also provided data to fine-tune measured-pressure-correction tables in the ALADIN algorithm for computational fluid dynamics, wind-tunnel tests, and flight tests discrepancies. The second phase of flights involved reflying the test points from the first phase and included flight maneuvers that were similar to those of the first phase but in a different region of the flight envelope. These flights provided a set of data to check the refined calibration and to further evaluate system performance.

Figure 2. This is a Flight-Test Calibration Curve of right and left ALADIN probes at an altitude of 35,000 ft (10.7 km) and mach numbers from 0.24 to 0.74.

The in-flight performance of the fuselage-mounted probes (see Figure 1) for the ALADIN system was found to be excellent over the typical aircraft flight envelope, with accurate tracking of altitude, mach number, and angle of attack. The BFG ASD probes and algorithm were found to be effective over a range much greater than that of the standard F-18 airdata system. Figure 2 depicts a calibration curve of probe angle of attack vs. aircraft angle of attack, as an example of the high-angle-of-attack capabilities of the ALADIN system. The ALADIN system will provide aircraft flight-control systems with more information than was previously available, thereby creating a potential to make the aircraft both safer and more capable.

Some problems identified during the flight tests are being solved with modifications to the initial algorithm. Modifications to the algorithm are being implemented by Boeing and NASA to optimize performance for angles of attack in excess of 45°. Initial evaluation of the modified algorithm developed by BFG ASD indicates that good performance can be expected even at extended angles of attack.

This work was done by Laurie Marshall of Dryden Flight Research Center, Brian Barber of Boeing Phantom Works, and Roger Foster of BFGoodrich Aircraft Sensors Division. No further information is available. DRC-98-64