Originally Posted by
Mad (Flt) Scientist
Higher drag with a seized engine.
All discussion of how the windmilling fan is extracting energy and thus must be creating more drag ignores the fact that a seized engine is essentially a giant blunt body with all kinds of intake spillage going on. That effect is way more than the windmilling. (All the windmilling needs to do is overcome the bearing drag once the engine is stabilized, which is not that big)
Am aware of an incident where significant engine damage caused one of a twin to seize. Max attainable altitude dropped from the theoretical ~25kft for a normal OEI case to below 15kft.
Great discussion. I was starting to question my intuition with the back and forth. My initial response way back at the start was from simply observing that any system (in this case a turbine engine exposed to oncoming air) is going to naturally seek the low-energy, low-drag state. Think about sticking your arm straight out the car window - it takes an input of energy from your muscles to maintain the higher drag configuration instead of letting it fall back. Likewise, given the option, an unpowered turbine that is free to rotate will - because it would requires more energy to resist the force of the oncoming air. That energy is proportional to the increased drag of the seized condition. This more of a physics/thermodynamics argument, but I think it still gets us to the right answer.