[QUOTE=tdracer;10604835]I've been thinking about this, and I think the answer is that when a fan is windmilling, the fan blade airfoils are basically working as intended - the angle of attack of the blades is within the range that the flow won't separate and the airfoil stall. Hence the pressure loss through a windmilling fan is small. If the fan is locked, the incoming air is hitting the fan blade at a very high angle of attack - beyond the stall angle - so the fan blades are effectively stalled with the associated large pressure drop (i.e. drag). So there is a greater pressure loss through a locked fan than a windmilling fan, and that equates to more aerodynamic drag. I'm unaware of any engines that drive accessories off of the LP rotor so that doesn't come into play (the core would be a different question). {/quote]
Not quite. In both cases, locked and windmill, the angle of attack is on the convex side rendering the blade stalled.The differences in drag are due to what is going on in the compressor which is working to pressurize the burner sufficient for re-start, if you're going fast enough.Kinda like a ramjet