Drag of a seized jet engine compared to windmilling

[Tomaski]

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.

[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.

[Fly Aiprt]

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.

Concerning props, it appears there is also a crossover point :
http://www.peter2000.co.uk/aviation/misc/prop.pdf

[Tomaski]

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.

For now, I'm going stick with the thermodynamics model, and to that point I would say that the minimum drag stable point of a low vs high bypass would show up in the speed of rotation.

[tdracer]

Originally Posted by mrfox

The crossover point was mentioned to be around 5:1 bypass - right around where the last gen turbofans live.

Actually, 5 to 1 is around where the first generation of 'big' turbofans lived - JT9D, CF6-6/CF6-50. It's gone up considerably since then with the latest generation approaching 10.

[Dave Therhino]

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.

[BARKINGMAD]

Going Down.

At 20W over the wet bits I don’t think I’d be trying to do the maths on this one, thoughtful formulae kindly provided from this thread!

I’d be hoping all the ETOPS fiction dreamt up by the airlines and eagerly certified by the xAAs actually worked in practice?

And if it actually had seized, was it still on the pylon or were the terrified pax filming the remains of fuel, electrics, hydraulics and pneumatics systems on their mobile phones hoping loved ones would see their recovered last filming efforts?

I’m still waiting for an authoritative answer to the likelihood of a “dry” HP fuel pump overheating snd causing a potential engine fire.

As Monty Python used to say when a sketch had run out of steam........”This sketch is silly, stop it at once!”.

[Winemaker]

..... or were the terrified pax filming the remains of fuel, electrics, hydraulics and pneumatics systems on their mobile phones hoping loved ones would see their recovered last filming efforts?

Well, there's not much else we can do is there? Might as well make a movie; it could relieve the tension while waiting for the final lurch.......

[cujet]

Interesting thread. It seems that conditional drag is dependent on engine design, more specifically, bypass ratio.

[megan]

And if it actually had seized, was it still on the pylon

Makes the locked rotor/windmilling issue mute. Torque imposed by the rotor locking up tears the engine off.

[QuarterInchSocket]

Originally Posted by mrfox

Further research on the topic of spillage drag has led me to the text Gas Turbine Performance (P.Walsh/P.Fletcher)

You mean the reference book I quoted all the way up at post #6? 😒

im not sure what to say... everything is in that text to arrive at a sound conclusion and I’m sceptical of the references that arrive at the differing conclusion that the windmilling case is more efficient than the locked case.

From the referenced text, it is fairly evident that it’s not so much what happens with the fan (for turbofans), but what happens in the core

Excluding the aerodynamic implications of a windmilling core/fan vs a locked core (as applied to single or dual spool), in practical terms it would be best to lock the core of a malfunctioned powerplant to alleviate risk of <further> damage to the powerplant and/or it’s accessories.

[India Four Two]

Torque imposed by the rotor locking up tears the engine off.

I've often wondered what would have happened if it was the No. 2 engine that locked-up.

[lomapaseo]

Best not to get into torque and locked rotors and just stick to aero loads in this thread. Much of what has just been posted is wrong in analogy. But hey, it's a free for all so just take it where you want

[tdracer]

Originally Posted by India Four Two

I've often wondered what would have happened if it was the No. 2 engine that locked-up.

It wouldn't have been much different. Remember where that center engine is - in the very back on the fuselage centerline - it's not in the tail. The 727 engine mounts had 'shear points', designed to kick the engine away from the fuselage in the event of a structural overload.
Besides, remember what was believed to have caused the engine seizure - blue ice ingestion. It's pretty hard to envision a scenario where a big chunk of blue ice ends up going through the center inlet...