Nowadays,applications of Unmanned Aerial Vehicles(UAVs)are becoming increasingly prevalent.Biomimetic perching has been one of the interesting topics in the field of UAV-related research.As the first stage of the perching procedure,approaching flight requires effective and efficient guidance to achieve successful perching.Tau Theory has been found applicable to guidance of approaching locomotion in humans,pigeons,insects and so on.Instead of adopting the classic assumption of a constant tau-dot of 0< <0.5 in Tau Theory,varying tau-dots have been found in parrots approaching flight during perching and applied as a fuzzy control strategy for UAV perching flight in our previous research.The feedback of tau information was realized through an indoor motion capture system,which is not applicable in general scenarios.As such,the biomimetic estimation approach of tau information via visual cues for guidance of approaching flight of UAVs is addressed in this paper.Firstly,the advantages of visual guidance for approaching flight are discussed and the potential visual cues are covered.Secondly,a scale-dependent expansion model(SEM)is proposed for tau-dot estimation,taking the visual cue of object scale into account.Also,the classic pinhole camera model is utilized as the imaging model.Theoretical derivation of the relationship between the tau/tau-dot and the identified object scale is presented.