In multispool engines, rotating stall in an upstream compressor will i
mpose a rotating distortion on the downstream compressor, thereby affe
cting its stability margin. In this paper experiments are described in
which this effect was simulated by a rotating screen upstream of seve
ral multistage low-speed compressors. The measurements are complemente
d by, and compared with, a theoretical model of multistage compressor
response to speed and direction of rotation of an inlet distortion. Fo
r corotating distortions (i.e., distortions rotating in the same direc
tion as rotor rotation), experiments show that the compressors exhibit
ed significant loss in stability margin and that they could be divided
into two groups according to their response. The first group exhibite
d a single peak in stall margin degradation when the distortion speed
corresponded to roughly 50 percent of rotor speed The second group sho
wed two peaks in stall margin degradation corresponding to distortion
speeds of approximately 25-35 percent and 70-75 percent of rotor speed
. These new results demonstrate that multistage compressors can have m
ore than a single resonant response. Detailed measurements suggest tha
t the two types of behavior are linked to differences between the stal
l inception processes observed for the two groups of compressors and t
hat a direct connection thus exists between the observed forced respon
se and the unsteady flow phenomena at stall onset. For counterrotation
al distortions, all the compressors rested showed minimal loss of stab
ility margin. The results imply that counterrotation of the fan and co
re compressor, or LP and HP compressors, could be a worthwhile design
choice. Calculations based on the two-dimensional theoretical model sh
ow excellent agreement for the compressors, which had a single peak fo
r stall margin degradation. We take this first-of-a-kind comparison as
showing that the model, though simplified, captures the essential flu
id dynamic features of the phenomena. Agreement is not good for compre
ssors that had two peaks in the curve of stall margin shift versus dis
tortion rotation speed. The discrepancy is attributed to the three-dim
ensional and short length scale nature of the stall inception process
in these machines; this includes phenomena that have not yet been addr
essed in any model.