Aerodynamic Measurement on the High Speed Test Track

Transcription

1 Trans. JSASS Aerospace Tech. Japan Vol. 12, No. ists29, pp. Tg_5-Tg_10, 2014 Topics Aerodynamic Measurement on the High Speed Test Track By Daisuke NAKATA 1), Kenji NISHINE 2), Kaoru TATEOKE 1), Nobuhiro TANATSUGU 1) and Kazuyuki HIGASHINO 1) 1) Muroran Institute of Technology, Muroran, Japan 2) Kawaju Gifu Engineering, Kagamihara, Japan (Received June 28th, 2013) This paper describes onboard balance system used in the rocket sled test on the high speed test track. Newly developed acceleration-compensated balance mechanically cancels inertia force and enables us to use suitable size load cells for expected aerodynamic force. Lift force was evaluated by another internal 6-axis balance. With a flat plate and AGARD-B model, system verification tests were conducted to clear the accuracy of this system. Although the cancellation of the onboard G-force was almost accomplished, uncertainty of the wind speed or mechanical friction in the system caused severe error to the measured drag coefficient. Key Words: High Speed Test Track, Rocket Sled, Balance, Wind Tunnel, AGARD-B Nomenclature a : acceleration, m/s 2 C D : drag coefficient C L : lift coefficient F air : aerodynamic force acting on the sting, N F net : net force detected by the load-cell, N L counter : the distance between counter-weight side pivot and the knife edge, m L sting : the distance between sting side pivot and the knife edge, m M counter : mass of the counter weight, kg M model : mass of the test model, kg M sting : mass of the sting component, kg Re : Reynolds number u : sled velocity, m/s : angle of attack, deg 1. Introduction 1.1. High speed test track facility High speed test track (HSTT) facility and rocket sleds on the track have been widely used since 1950s in United States for the purpose of aerodynamic measurement, anti-g test, the parachute deployment or canopy separation under high speed condition 1). This is also useful for the confirmation of aero elasticity or drag force measurement of real flight model. HSTT is considered to be a missing link between wind tunnels and free flight test. In 2009, Muroran Institute of Technology constructed the Japanese first HSTT for academic use in Shiraoi town, Hokkaido prefecture. This facility is opened to the researchers of another university or research organization. Although the detail of this facility is described in our previous reports 2,3), this paper especially focuses on the onboard balance system equipped on the rocket sled The importance of aerodynamic measurement at real raynolds number The effect of the Reynolds number on drag coefficient is significant. For example, the drag coefficient of NACA0012 wing 4) at an attack angle of = 10 is with Re = but with Re = This way, the difference is almost 50 %. Usually some correction method is used to predict the drag coefficient of full-scale aircraft from the results of wind-tunnel tests, but the importance of the validation test at real Reynolds number is still large. Nowadays huge subsonic wind tunnels which enable more than Re = 10 8 condition are available in the world, but a chance to use them is limited. In a relatively small project, simple and low-cost validation method is needed. Burt Rutan used car-topped balance at a low-cost and successfully manufactured manned aircraft 5). Compared to the car-top test, rocket sled provides us much more stable and higher speed condition with reasonable cost. Especially it should be addressed that we can test several-meter span models at supersonic condition on the sled facility. As an example, Aerial Corporation used rocket sleds to check their laminar flow wing of supersonic business jet 6). Although the test condition realized by the rocket sled facility is quite useful, the accuracy of the data is usually worse than that of the wind tunnel test. Especially the acceleration effect acting on test models are considered to be one of the biggest error factors. For this purpose, the acceleration-compensated balance was newly introduced. Whereas the final target accuracy is 1 % of the measured value, the purpose of here is just evaluate the principal error factors quantitatively. 2. Experimental Setup 2.1. Acceleration-compensated balance Generally the accuracy of the force sensor is related to its full-scale range (usually % of F.S.). Taking care of acceleration/deceleration G force during the sled run, we have Copyright 2014 by the Japan Society for Aeronautical and Space Sciences and ISTS. All rights reserved. Tg_5

2 Trans. JSASS Aerospace Tech. Japan Vol. 12, No. ists29 (2014) to choose much larger load-cells than estimated air drag level. Resultantly the accuracy of onboard drag measurement becomes poorer. Acceleration-compensated balance enables us to use a suitable size of load-cell for expected air drag. Figure 2 shows the schematics of acceleration compensated onboard balance. Both of sting side and counter-weight side are laid on the linear sliders. The counter-weight side is completely covered by wind shield so that the air drag is applied only to sting side. Load-cell (KYOWA LMBT-A-200N, official accuracy is 0.5 % of F.S.) feels the following net force; (1) 2 and Fig.6). Although AGARD-B model is usually used in supersonic wind tunnels, subsonic data (at Mach 0.6) is also available in literatures 8). Our purpose here is to find critical error factors which detract the validity of this system, although the models might not have perfectly same C D or C L with the referenced literatures. More precise discussion (taking into consideration the surface roughness and so on) is beyond the scope of this report. Size Table 1. Specification of Flat Plate. 400 mm 400 mm Weight 2.24 kg(plate) kg(sting) 7) C D 1.17 Assuming the following ideal condition, (2) (3) Fig. 3. Flat plate used for drag force measurement. Fig. 2. Acceleration compensated force measurement system which consists of knife edge, sting side and counter-weight side. Fig. 4. Schematics of flat plate attachment. Strictly speaking, perfect alignment in terms of arm length L or the mass of counter weight M is impossible. Relatively it is easy to set M less than 1 % of error, but it is hard to set L (7.5 cm) within 1 % error axis balance Acceleration-compensated balance is necessary only for drag measurement. The other components such as lift force or pitching moment are measured via internal 6-axis force sensor (Nitta IFS-90M40A100-I50-ANA) attached on the tip of the sting. Full-scale (F.S.) of the sensor is 800 N and officially assured accuracy is less than 1 % of F.S. This way, the intrinsic error of the 6-axis load-cell is much bigger because it should be stand for the big G-force Test models For the validation of acceleration-compensated balance, vertical flat plate 7) was used (Table 1, Figs.3 and 4). For the validation of the lift force, AGARD-B model was used (Table Table 2. Specification of AGARD-B model. Length mm Body Diameter Wing Span 165 mm 660 mm Wing Area m 2 Weight Mean Aerodynamic Chord kg(agard-b) kg(sting) mm Re at u = 30 m/s C D 8) at = Tg_6

3 D. NAKATA et al.: Aerodynamic Measurement on the High Speed Test Track Fig. 5. AGARD B model used for lift force measurement. Fig. 8. Sled run with AGARD-B model. Fig. 6. Schematics of AGARD B model attachment Test platforms Rocket sled RS-702 (Fig.7) was used as a test platform. Specification of the sled is shown in Table 3. The sled was propelled by 4 hybrid rockets along the 300 m-long rail track and decelerated by water braking system. The detail of propulsion and braking system is written in our past reports 2,3). Total weight at the experiment was kg including propulsion system, balance, test models and counter weights. It is addressed that car-top test was also conducted in the verification test of the G cancellation system. Table 3. Specification of rocket sled RS-702. Length m Width Dry Weight m 75.2 kg The sampling rate of onboard data-logger was 100 Hz. 3-axis acceleration sensor was attached on the sled to measure onboard G level. Sled speed and running distance were calculated by integrating the output of the acceleration sensor. Integration error was not significant because the running distance was limited (< 300 m). Although the dynamic pressure during the run had been measured by a pitot tube, it was found that the obtained real-time data was not reliable. Here we estimated the maximum natural wind speed during the run by the data of an anemometer in the test field. 3. Results and Discussion 3.1. Cancellation of the acceleration At first, the ability to cancel the acceleration force was verified with car-top test. Instead of the test models, same weight was attached on both of sting side and counter weight side. Whole balance was covered by wind shield to avoid any aerodynamic effect acting on the balance. Figure 9 shows that the acceleration and net force F net profile during run. Figure 10 shows the relative output level to the inertia force. If the counter weight side was removed, sting side felt 100 % of inertia force. The result showed that the inertia force was successfully cancelled but some dynamic response remained. Between t = 18 and 23 s, the car was at a condition of free running (acceleration is closed to 0 G). Without this range, the cancellation error (F net / Ma) was almost less than 5 %. Fig. 7. RS-702 rocket sled with 4 hybrid rockets. The white box fixed at the left-front corner of the sled is the onboard data handling unit. Fig. 9. Detected force by load-cell at an acceleration cancellation test. Tg_7

4 Trans. JSASS Aerospace Tech. Japan Vol. 12, No. ists29 (2014) Fig. 10. Detected force divided by expected inertia force (Ma) at an acceleration cancellation test. The vertical axis is written in the unit of %. Fig. 12. Velocity profile at a drag measurement test of the flat plate Drag coefficient of the flat plate Table 4 and Figs are experimental condition and the results of drag measurement of the flat plate with acceleration-compensated balance and 6-axis force sensor. As for the 6-axis force sensor, expected acceleration force Ma was numerically subtracted after the experiment. All of plots are smoothed by 25-point moving average since the original raw data had severe oscillation. The error bars in Fig.15 are of natural wind factor which is considered to be 3 m/s at maximum during experiment. As for the 6-axis balance, the referenced value (C D = 1.17) was within the error bars concerning the natural wind effect. As for the acceleration-compensated balance, the measured value was usually higher than the referenced value. The difference is caused by some hysteresis or friction force on the sliding of sting side. Fig. 13. Drag force of the flat plate measured by 6-axis balance and acceleration-compensated balance. Table 4. Experimental condition at the drag force measurement with flat plate model. Outside Temp. Ambient Pressure Air Density Wind Speed (before the test run) Attached Angle of Attack 278 K 1014 hpa kg/m3 < 3 m/s 0 (vertical alignment) Fig. 14. Drag coefficient of the flat plate measured by 6-axis balance and acceleration-compensated balance. Fig. 11. Acceleration profile at a drag measurement test of the flat plate Lift coefficient of AGARD-B model Table 5 and Figs are the results of lift measurement of AGARD-B model with 6-axis force sensor. All of plots were smoothed by 25-point moving average since the original raw data had intense oscillation. The error bars in Fig.18 are of natural wind factor which is considered to be 6 m/s at maximum during experiment. The drag of AGARD-B model is not discussed here because the expected drag is much lower than the F.S. of onboard force sensors. Taking into consideration the error bars of natural wind effect, obtained lift coefficient is almost corresponding to the referenced value. One may concern the low-frequency Tg_8

5 D. NAKATA et al.: Aerodynamic Measurement on the High Speed Test Track oscillation appeared in Figs.17 and 18. This point is discussed in the next section. Table 5. Experimental condition at the lift force measurement with AGARD-B model. Outside Temp. 270 K Ambient Pressure 998 hpa Air Density kg/m3 Wind Speed < 6 m/s Attached Angle of Attack 8 Fig. 18. Lift coefficient of AGARD-B model measured by 6-axis balance. Fig. 15. Acceleration profile at a lift measurement test of AGARD-B Error factors of the onboard balance Time-variable natural wind was the dominant error factor of the experimental results. This factor will be eliminated by adopting air-data sensor equipped on the sled or negligible at a higher speed test. However, this does not perfectly explain the higher drag coefficient obtained by acceleration-compensated balance. The measured C D was about 1.5 whereas the reference C D in literature was Here we consider another error factors acting on the onboard balance system. As for the acceleration-compensated balance, acceleration cancellation error was considered to be less than 4.5 N/G as discussed in section 3.1. The conceivable error in the setting of L sting or L counter was 1 mm, resulting 2 N/G as a measurement error. These contributions to the total error were less than 5 %. Friction force of the slider affected to the result and considered to be significant error factor, especially at lower velocity (< 20 m/s). The friction force was checked by static-load test. As a consequence, it was found that the friction force was very larger than expected due to the big momentum force acting on the sliders. In order to solve this issue, 2 sliders at a large distance should be used for both of sting side and counter-weight side (Fig.19) Fig. 16. Velocity profile at a lift measurement test of AGARD-B. Fig. 17. Lift force of AGARD-B model measured by 6-axis balance. Fig. 19. Top view of the modified measurement system supported by tandem linear sliders. Tg_9

6 Trans. JSASS Aerospace Tech. Japan Vol. 12, No. ists29 (2014) As for the 6-axis balance, intrinsic error of the force sensor had been considered as a significant error factor because we choose large-scale balance (F.S. 800 N) to stand for G-force during the run. According to the official data sheet, the error is 1 % of F.S. corresponding 8 N. After solving these error factors, it is expected that the acceleration-compensated balance will show higher accuracy than usual balance system even in the large-g condition Oscillation analysis As shown in Figs.17 and 18, the sled is under severe oscillation caused by propulsion system. Each system component was affected by this oscillation during the run at their characteristic frequency. Figure 20 is a result of Fourier analysis during free running phase at AGARD-B test run. One can see there are two peaks at around 2 Hz and 12 Hz. The latter one is explained as a characteristic frequency of sting part (cantilever structure). The former one might be explained as a characteristic frequency of the basement structure of acceleration-compensated balance. Although the amplitude in Fig.20 is written in arbitrary unit, actual amplitude range was 2 in attack angle. This oscillation amplitude should be minimized not to cause harmful problem for the aerodynamic data. Amplitude [A.U.] Frequency [Hz] 4. Conclusions The onboard balance used on the rocket sled is considered to be very practical device to check the drag coefficient of the small aircrafts. The authors newly developed an acceleration-compensated onboard balance in order to remove the inertia force acting on the balance. The performance of the balance was checked with a flat plate and an AGARD-B as testing models. It was confirmed that the balance successfully canceled the inertial force within 5 % error but the accuracy of the air speed, friction force on the slider, alignment error and oscillation factors caused significant errors on this system. For example, measured drag coefficient of the flat plate was 1.5, whereas the reference C D in literature was Since it was found that the friction force was very large due to the big momentum force acting on the sliders, tandem linear sliders at a large distance is proposed as one of the solution. References 1) Holloman High Speed Test Track Facilities and Capabilities, AAC/PA , ) Nakata, D., Kozu, A., Yajima, J., Nishine, K., Higashino, K. and Tanatsugu, N.: Predicted and Experimented Acceleration Profile of the Rocket Sled, Aerospace Technology Japan, 10, ists28(2012), pp. Ta_1-Ta_5. 3) Nakata, D., Yajima, J., Nishine, K., Higashino, K. and Tanatsugu, N.: Research and Development of High Speed Test Track Facility in Japan, AIAA ) 5) 6) Peter, S.: Extensive Supersonic Natural Laminar Flow on the Aerion Business Jet, AIAA ) Murata, S.: Koukuu Uchu Binran, Maruzen (2005), pp.14,85. 8) Damljanovi, D., Viti, A. and Vukovi,.: Testing of AGARD-B Calibration Model in the T-38 Trisonic Wind Tunnel, Scientific-Technical Review, 56, No.2(2006), pp ) Fig. 20. Fourier analysis during free running phase at AGARD-B test run. Tg_10