Evaluating Follower Wing Performance in Formation Flight Using the Power Balance Method

Formation flight is used to reduce the drag of the follower and increase range and endurance. However, quantifying how much of the potential energy in the leader’s wake is available to, and used by, the follower is poorly understood. This paper explores how the power balance method (PBM) may be applied in the context of formation flight to evaluate not only classical measures of performance, such as the spanwise efficiency factor, but also to quantify what proportion of the potential energy in the leader’s wake is recovered as drag savings. A finite NACA0012 wing in subsonic flow is modeled downstream of an identical leader wing. The spanwise efficiency factor is compared to the PBM at the far field in predicting induced drag savings: the former predicts induced drag savings of 37.0–61.0%, while the latter predicts 41.5–63.0%. The PBM shows that, for the inviscid case, approximately 73.3% of the wake potential energy was recovered. This demonstrates that the power balance method agrees with traditional momentum-based analyses of performance and can also be used to gauge the efficiency of extracting potential energy from the flowfield. Future work is recommended to separate induced drag from profile drag in the follower’s far field in the viscous regime.