Experimental studies were carried out on aspect-ratios of 2 and 3 rectangular polyethylene terephthalate membranes to clarify the effects of aspect-ratio on membrane flapping dynamics and strain energy distribution for Reynolds numbers from 3228 to 9732. Flapping modes similar to our previous study are identified and an increase in aspect-ratio is observed to promote transition of membrane flapping from limited cantilever-like mode to quasi-periodical traveling wave mode. This leads to corresponding increases in the flapping amplitude and its growth rate. On the other hand, both membranes achieve “lock-in” states at the same Reynolds number with nearly identical flapping frequency growth rates. Increasing the membrane aspect-ratio leads to strong variations in the strain distributions and reduces its increments with the Reynolds number Lastly, estimated membrane strain energy increases more with the Reynolds number for the higher aspect-ratio membrane. Compared to the lower aspect-ratio membrane however, it reaches an asymptotic level once it attains “lock-in” condition due to the more three-dimensional flapping motions.