The widely used improved delayed detached eddy simulation (IDDES) method is supposed to accurately predict the mean skin friction when it acts as wall-modeled large eddy simulation (WMLES), as claimed by the original authors. This property has been validated in subsonic flows but not in supersonic flows. In this article, IDDES acting as WMLES was assessed in the simulation of supersonic flat plate boundary layers, accompanied by two other hybrid Reynolds averaged Navier Stokes (RANS)/large eddy simulation (LES) methods which were also employed as WMLES, namely zonal detached eddy simulation (ZDES) and a hybrid RANS/LES method with the blending function proposed by Edwards and Choi (HRLMEC), for comparison. The underprediction of mean skin friction is existent in the results of IDDES but negligible in the results of ZDES and HRLMEC when all other settings are identical, presenting a contrast to the original idea of IDDES and some former research in subsonic flows. With the analysis of the shear stress, the underprediction of mean skin friction of IDDES is attributed to the underprediction of total Reynolds stress and turbulent kinetic energy production in the inner layer. Further research shows that, as for the present supersonic flow cases, IDDES can be capable of precisely predicting skin friction, but only if under the same condition the relevant LES, which uses the same subgrid-scale model, can precisely predict skin friction. This suggests that the problems met by IDDES may be essentially due to the intrinsic difficulty of LES to resolve the inner layer of a supersonic boundary layer: when resolving the inner layer of a supersonic boundary layer, LES should be tuned subtly and needs high computational cost, and may be damaged by any faulty factor, including but not limited to numerical resolution.

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