Abstract: The purpose of this investigation is to find out the effect of shear layers with various strength on stability of an axisymmtric external compression supersonic air intake.Although A.Ferri's theory on stability of an air intake is well known,it does not tell how strong the shear layer has to be in order to produce a noticeable effect on an air intake.S.A.Fisher's work shows that for a side intake,a shear layer with a strength about 6% to 7% is required to induce a buzz in the two-dimensional intake.Here "strength" is defined as the ratio of total pressure difference between the two sides of a shear layer to the total pressure of incoming flow.But,for an axisymmetric intake,it is still necessary to find out how it responds to action of a shear layer with a specific strength. Experiments were carried out in a free stream wind tunnel with a fixed Mach number M=1.72.Six central bodies with different cone angles were used to produce shear layers with strength ranging from 5% to 11% of total pressure of upstream flow.Experiments indicated that shear layers with strength lower than 10% were strong enough to cause separation of the boundary layer from the inside surface of the cowl.They also showed that shear layers with strength lower than 11% were unable to induce buzz in the axisymmetric intake tested,while,according to literature,a shear layer with much lower strength could cause a two-dimensional intake to buzz.The experiments also testified that the effect of a shear layer depended on its distance from lip of the cowl,and the effect was the most noticeable at its distance equal to 10% to 18% of the cowl radius.It is possible to conclude that a two-dimensional intake is more sensitive than an axisymmetric one to the effect of a shear layer.