Tethered vs. free flight force determination of the DelFly II Flapping Wing Micro Air Vehicle

Abstract

The determination of dynamic forces acting on a Flapping Wing Micro Aerial Vehicle (FWMAV) is a challenging task due to the unsteady nature of force generation mechanisms. To assure a proper force identification in future researches, this work compares two different methods to obtain the longitudinal forces acting on FWMAVs and discusses their applicability regions. The methods were 1) calculation of forces from the recordings of the FWMAV's position in a free flight condition; 2) direct force measurements in a tethered flight condition in a wind tunnel. The DelFly II is used as the FWMAV test platform in the measurements. During free flight experiments, its position and attitude were recorded at a rate of 200Hz using an external visual tracking system, whose acquired information was then analyzed to obtain the flight states and calculate the forces and moments that act on the platform during flight, under a set of kinematic assumptions. Subsequently, similar flight conditions were tested in the tethered situation. An ATI Nano-17 Titanium force transducer was used to measure time-resolved forces. The results for the most common flight regime of the DelFly, which is a slow forward flight at a high body pitch angle, are presented. It is shown that the tethered force balance tests agree with the free flight data when assessing the aerodynamic forces that are perpendicular to the stroke plane of the flapping wing. However, the forces that act along the stroke plane are coupled with structural dynamic terms, thus affecting the final lift and thrust identification. These results point to inadequate force identification in fixed point force measurements, due to effect the of the dynamic modes of the FWMAV body, thus advising proper cross-comparing between experimental methods.

Free flight force estimation of a 23.5 g flapping wing MAV using an on-board IMU

“…Recently, the development of external motion tracking systems allowed to estimate the in-flight forces from recorded temporal evolutions of body position and orientation [5], [6], [7]. In general, an agreement has been reported between the in flight estimates and wind tunnel measurements, although differences were found in the direction of the wing stroke [8].…”

Review on System Identification and Mathematical Modeling of Flapping Wing Micro-Aerial Vehicles

“…However, further study is still required to achieve the prediction of the aerodynamic forces and moments with considerable accuracy for a longer period of time and full autonomous flight missions. In this regard, very appealing developments have been reported in [132,133]. Another system identification approach that has been applied to obtain the flight dynamics equations of DelFly II is the extended Kalman filter (EKF) [134].…”

Studying the Effect of the Tail on the Dynamics of a Flapping-Wing MAV using Free-Flight Data

“…A drawback of wind tunnel experiments is the fact that the MAV is clamped to the force balance, see Figure 4. This has an eect especially on the body x b force (see Figure 5) due to the absence of oscillations in the pitch attitude which cause aerodynamic damping eects during free-ight [31].…”

“…As mentioned, wind tunnel experiments have some limitations and cannot be used to assess dynamic stability characteristics of a particular conguration [31,32]. Free-ight experiments are therefore required to thoroughly study the eect of the tail.…”

“…The wind tunnel results also show a slight trend with tail surface area, Figure 18(b), but vary quite a bit from the free-ight results in magnitude. Actually comparing these stability derivatives would require more study into the accuracy of both experimental methods (see also Caetano et al [31]). Ultimately, the results in Figure 18(b) do not show very conclusive trends and it was not considered feasible to develop a model for X u as a function of tail surface area.…”