This paper investigates the phonetic nature of flapping for /d/ and /th/ in intervocalic pre-unstressed and pre-stressed positions at a normal speech rate in Xiangxiang Chinese. The resulting data were approached in two perspectives: acoustic and aerodynamic. It was found that the acoustic and aerodynamic patterns were significantly correlated with each other, and both show that /d/ and /th/ exhibit considerable intra-speaker variation which constitutes a continuum from typical [d]s and [th]s to typical flaps in intervocalic position, indicating an articulatory continuum from long and complete oral closure for typical [d]s and [th]s to short and incomplete oral closure for typical flaps. This mirrors a gradient process which springs from a single mechanism common to each speaker. A model of consonant-vowel co-articulation is proposed and other related problems raised by these results are also discussed.