Trans. Japan Soc. Aero. Space Sci. Vol. 53, No. 182, pp. 250–257, 2011 Comparison Study and Sensitivity Analysis of Flight Test Techniques for Air Data Position Error Correction in Small Aircraft By Sang-Jong LEE,1Þ Jae Won CHANG,1Þ Jeong Ho PARK,2Þ Byoung Soo KIM3Þ and Kie Jeong SEONG1Þ 1ÞFlight Control Team, Korea Aerospace Research Institute, Daejeon, Republic of Korea 2ÞPGM R&D Lab, LIG Nex1 Co. Ltd., Yongin, Republic of Korea 3ÞSchool of Mechanical and Aerospace Engineering, Gyeongsang National University, Jinju, Republic of Korea (Received May 18th, 2009) The flight test is important in the development and certification phases of an aircraft. It is composed of various tests, but the position error correction test should be performed first to determine error of the pitot-static measurement system that is the basis for evaluating flight characteristics. This paper investigates and compares recent test methods using real flight test results. The ground course and arbitrary heading method are both considered using GPS and DGPS to measure ground speed. The arbitrary heading method was most efficient and precise. In addition, new method is proposed and successfully used to determine accurate position error by comparison with DGPS results. Finally, sensitivity analysis was performed to analyze the effect of error sources. It shows that the most important error is the measured indicated airspeed following by measured true airspeed, outside air temperature, and pressure altitude. Key Words: Flight Testing, Aircraft, Airworthiness, Position Error Correction, Analysis Nomenclature CAM: measured by camcorder DGPS: measured by DGPS aSL: speed of sound at sea level GPS: measured by GPS HI: indicated pressure altitude s: standard day condition HIC: indicated pressure altitude corrected for instrument W: wind error x: vector component of x-axis ÁHPC: altimeter position error y: vector component of y-axis Pa: ambient pressure PSL: pressure at sea level 1. Introduction qc: dynamic pressure t: time The flight test plays a key role in evaluating the per- Át: time taken flying between waypoints of ground course formance of an aircraft and its subsystems. The ICAO VC: calibrated airspeed (International Civil Aviation Organization) certifies new ÁVC: compressibility error commercial aircraft using its own certification system. Con- VE: equivalent airspeed sequently, every country has its own certification regula- VG: ground airspeed tions and government certifying agency, such as the FAA VI: indicated airspeed (Federal Aviation Administration) in the USA, JCAB (Japan VIC: indicated airspeed corrected for instrument error Civil Aviation Bureau) in Japan, and EASA (European ÁVIC: instrument error Aviation Safety Agency) in the EU. At certification, the ÁVPC: airspeed position error flight test is the final compliance test for evaluating flight VT: true airspeed characteristics and certifying that design, flight safety, and 1) VW: wind speed structural safety meet all requirements. The flight test : pressure ratio includes various tests, including performance, handling, : temperature ratio aero-elastic/flutter stability, structural loads, and avionics/ a: ambient air density equipment performance. The position error correction SL: ambient air density at sea level (PEC) flight test is the starting point to process the other : air density ratio of test day tests because aircraft performance is based on airspeed s: air density ratio of standard day and altitude from the pitot-static measurement system. : heading angle Errors are generated by the pressure field and flow angular- Subscripts ity as a result of the position of the pitot and static port. This A: one waypoint of ground course creates a difference between the readings and true airspeed B: one waypoint at ground course and altitude. Therefore, this error must be defined at the PEC flight test and corrected. Ó 2011 The Japan Society for Aeronautical and Space Sciences Feb. 2011 S.-J. LEE et al.: Comparison Study of Flight Test for Air Data Position Error Correction 251 The classic PEC flight tests use a test boom, trailing cone, 2–4) ðVGA þ VGB Þ tower-fly-by, pacer aircraft, ground course method, etc. VT ¼ ð1Þ GPS (Global Positioning System) is used with the ground 2 course method to measure ground speed during reciprocal This method has several limitations: (1) timing error, (2) test flying along a track. This implementation reduces error from altitude, (3) crosswind. Use of GPS or DGPS removes lim- measuring time to fly a known distance. Lewis5) of the itations (1) and (2), but (3) cannot be removed because exact NTPS (National Test Pilot School) proposed a flight track true airspeed can be calculated from components parallel to along the wind direction and Bailey6) of the AFFTC (Air the flying track as defined in Eq. (2). The magnitude as well Force Flight Test Center) suggested a 90 track to the wind as the source of the error is described by Rogers,7) and we in both directions. However, both methods require knowl- describe the derivation of Eq. (2) in Appendix A. edge of the wind direction before the test flight. Rogers7) V GA À V GB VGAx þ VGBx calculated the error due to variation in wind speed and angle VT ¼jV Tj¼ ¼ ð2Þ 2 2 for the ground course method using GPS. New test methods using GPS have been proposed by several researchers and 2.2. Arbitrary heading method institutes. All are based on multi-track flying of least This paper uses Gray’s method11) for the PEC flight test, three legs. In addition, no information on wind direction is and it is the recommended FAA certification method.14) needed. Fox8) first proposed an accurate method for deter- Both GPS and DGPS were used to measure ground speed mining true airspeed using GPS. The test aircraft is flown and the results are compared. It is necessary to fly three along three ground tracks 90 to each other. It is more con- legs to determine three unknown parameters such as the venient for pilots to adjust the heading instead of the ground true airspeed and two components of wind speed. Since track on the fly, so a three-orthogonal heading technique ground speed is a vector sum of true airspeed and wind was suggested.9,10) This is known as the horseshoe heading speed, they can be solved from the simultaneous equations method and extends from Fox’s method. However, these defined in Eqs. (3), (4), and (5). Gray provided a spread- techniques are limited to successive 90 headings. Although sheet to calculate the unknown parameters and this paper the true airspeed calculation is more complex, the arbitrary shows the mathematical procedure to reach the solution in heading method proposed by Gray11) can be applied easily Appendix B. to the PEC flight test. Finally, some of these methods are ðV À V Þ2 þðV À V Þ2 ¼ V 2 ð3Þ compared with classical methods.12,13) G1x Wx G1y Wy T 2 2 2 This paper investigates the flight test results for various ðVG2x À VWx Þ þðVG2y À VWy Þ ¼ VT ð4Þ PEC test methods using GPS or DGPS (Differential GPS) ðV À V Þ2 þðV À V Þ2 ¼ V 2 ð5Þ and compares their effectiveness and usefulness. Further- G3x Wx G3y Wy T more, sensitivity analysis is used to analyze error due to 2.3. Single camcorder method indicated airspeed (IAS), true airspeed (TAS), outside air This method is similar to the ground course method for temperature, and indicated altitude. DGPS is common today, eliminating the timing error. With the progress of digital so it deserves investigating. Five PEC methods were com- technologies, a small camcorder is easily installed on an pared: (1) arbitrary heading using DGPS, (2) arbitrary head- aircraft to record video with time synchronization. In this ing using handheld GPS, (3) ground course using DGPS, (4) paper, one camcorder was installed under the left wing. ground course using GPS, and (5) single camcorder method. By using this technique, ground observers and other equip- The last method records a video using an under-wing ment are not required and the exact time required to fly camera. Analysis of time frames gives accurate ground between known waypoints can be determined. speed without the timing error of classic ground course methods. 3. Flight Data Reduction 2. Flight Test Methods and Mathematics The same data reduction procedure can be applied to three methods explained in the previous section. Several To find position error, all PEC methods using GPS calcu- kinds of airspeed are used for calibration of the pitot-static late true airspeed from ground speed measured by GPS and system as shown in Fig. 1. Using ground speed, the true then compare it with indicated airspeed. Hence, the key role airspeed can be determined by a flight test and equivalent of GPS is how it can be used to obtain the accurate true airspeed (EAS) can be obtained by correcting the density airspeed under the existing wind conditions. altitude. This EAS is the calibrated airspeed (CAS) corrected 2.1. Ground course method for compressibility. Assuming that instrument error is cor- To find true airspeed, the test aircraft is flown back and rected (ÁVIC ¼ 0), the indicated airspeed (IAS) can be used forth between two fixed waypoints (A and B) to eliminate directly to obtain CAS. Therefore, the reduction procedure wind effects. Usually, observers on the ground use a chro- provides CAS and airspeed position error can be defined nometer to measure time and true airspeed is calculated in Eq. (6). simply using Eq. (1). 252 Trans. Japan Soc. Aero. Space Sci. Vol. 53, No. 182 Never Exceed VI (IAS) : Indicated Airspeed from instrument in aircraft MTOW 2,850 lbs Speed 180 kts ∆ VVVIC=+I V IC Max. Cruise Max. Fuel 68 gal Speed 160 kts VIC (IAS) : Indicated Airspeed corrected for instrument error ∆ Length 22 ft Stall Speed 61 kts VVVCICPC=+ V 1,100 Width 34 ft Max.