Originally Posted by
mrfox
From the text referenced above - locked rotor is not necessarly less drag than windmilling - it depends on the bypass ratio. In straight jets and low bypass fans the losses from the increased flow through the core more than offsets the spillage. The crossover point was mentioned to be around 5:1 bypass - right around where the last gen turbofans live.
If you carefully read what you wrote in post #59 above, your paraphrase/summary does not say that the 5:1 bypass ratio is the crossover point for a locked rotor nacelle drag being more than windmilling nacelle drag, but rather that, for some nacelle design, a 5:1 bypass ratio is the bypass ratio where the internal drag of the windmilling nacelle (due to force on the fan and force on the internal scrubbed surfaces) equals the external drag of that same windmilling nacelle. If I'm reading that correctly, that doesn't tell you anything about how a locked rotor nacelle compares to a windmilling nacelle. The locked rotor nacelle has less internal drag due to lower duct flow and a lot more external drag due to greatly increased spillage. I didn't read the actual document - only your summary from it - so I may not be understanding it correctly.
Internal drag on a windmilling pure jet would be much higher than the internal drag of a windmilling high bypass engine of the same diameter. For a given external nacelle geometry at a given spillage flow and given flight condition, the external drag is constant and has nothing directly to do with bypass ratio. Internal drag varies with significantly with bypass ratio. The higher the bypass ratio of a windmilling engine of a given fan diameter, the lower the internal drag because more of the air only has to go through a fan and EGVs and not through a whole engine core. With a duct blocked at the fan face, bypass ratio doesn't matter at all because all the air is being spilled. With a locked fan passing far less air through the duct than a windmilling engine, bypass ratio has less of an effect than it does for a windmilling engine. So expressing a windmilling nacelle drag versus locked rotor nacelle drag crossover point purely in terms of bypass ratio doesn't appear to me to make sense. I suspect absolute fan and nacelle diameter also has a significant effect due to the fan area being proportional to the square of diameter and the nacelle outer circumference being directly proportional to the diameter, with larger diameter engines having a greater overall drag difference between the windmilling and locked rotor configurations.
My recollection from an airplane program involving a very large twin with 6:1 bypass ratio engines was that the nacelles were analytically shown to have quite a bit higher overall drag with a locked fan rotor versus a windmilling rotor.