Transient aerodynamics effect on v-tail aircraft in lateral stability

Abstract

The trend in applying unconventional aircraft configurations for aerodynamic efficiency has caused some problems in flight dynamics especially in aircraft stability. Although, the mathematical modelling of flight dynamics has been established, however the accuracy of aerodynamic data, normally in the form of aerodynamic derivatives may affect the actual motion responses of the aircraft in design process for stability and control simulation. The aerodynamic derivatives may differ from small to large aircraft motion amplitude and may also vary in transient conditions. Clear that it is important to establish methods in estimating the aerodynamic derivatives accurately. This research work presents the effort in introducing a reliable method in estimating the aerodynamic derivative for V-tail aircraft in lateral motions focusing on Dutch roll mode using wind tunnel testing technique. Two methods of estimation are presented, first the steady-state measurement by static wind tunnel test and second transient measurement by dynamic oscillatory test. CAMAR UTMUAV has been used in this study for several V-tail configurations with dihedral angles of 35°, 47°, 55° (later use V35, V47, V55 respectively) including a conventional tail for reference. In static wind tunnel test, the static derivatives of C?ß and C?ß were measured for different tail configurations within ±25° yaw angle with range of wind speed from 10 m/s to 40 m/s. Meanwhile in dynamic oscillatory test, the transient condition was simulated at range of reduced frequencies, ?m of 0.05 - 0.25 by varying oscillation frequency through various spring stiffness, ?s. Hence, the dynamic oscillatory test was measured within yaw angle (±10°). Static wind tunnel test results showed that within ±10° yaw angle, all configurations possess positive yaw stability. When compare with conventional tail, found that V47 and V55 have higher degree of stability except for V35. For yaw angles more than ±10°, the Vtail showed better stability as it reaches neutral stability later than the conventional tail. However, measurements by static wind tunnel tests indicated there are discrepancies in representing the derivatives during transient condition and unable to measure dynamic derivatives of C?r and C?r. Meanwhile, aerodynamic derivatives of C?ß , C?ß, C?r and C?r were measured in the dynamic oscillatory test. The result from the dynamic oscillatory test are then compared with static wind tunnel test results and presented in the form of amplification factor. Within tested reduced frequencies, C?ß measured dynamically for all Vtail are more than static measured (amplification factor more than unity). This factor highlights the existence of the transient effects in the estimation of aerodynamic derivatives where it indicated the steady-state measurement underestimated the derivatives. At the same time, the steady-state derivative has also overestimated the aerodynamic damping in Dutch Roll simulation with crosswind input about 50%-90% depending on tail configurations. Meanwhile, through dynamic simulation using state-space equation of Dutch roll motion resulted the V55 has a higher sensitivity in response to crosswind followed by V47 and V35 respectively.