Drag of a seized jet engine compared to windmilling

[beardy]

I understood that the aerodynamic drag of a windmilling engine was greater than that of a seized engine.

Am I mistaken?

[Tomaski]

Not my area of expertise, but I'm pretty sure it's the opposite - a seized turbine engine should create more drag than a windmilling one. This is also assuming no unusual damage that would effect the drag profile.

[B2N2]

We talking piston or jet?

Drag windmilling piston > drag seized engine.

[beardy]

We talking piston or jet?

Drag windmilling piston > drag seized engine.


Thread title says jet.

[lomapaseo]

Spinning or not, the area between the blades remains the same and at near ambient pressure. The most significant blunt area is the inlet cowl lip. Windmilling your compressors will build up pressure (that's how you restart) . There is little difference in realistic discussion since jet engines don't seize in most flight conditions

[QuarterInchSocket]

There is an academic book written on jet engine performance characteristics that talks about a windmilling engine, written assumedly for engine designers learning their trade (uni qual).

"Gas Turbine Performance" by Philip Walsh and Paul Fletcher (Rolls Royce Engineers I believe)

The windmilling jet engine is least efficient

[oggers]

I understood that the aerodynamic drag of a windmilling engine was greater than that of a seized engine.

Am I mistaken?

You are definitely not mistaken. The drag is greater in the windmilling case.

[Spunky Monkey]

I read recently a simple explanation, the energy that the aircraft is using to push itself through the air is being reduced as some of that energy is being used to spin the windmilling blades.

[NutLoose]

Hence why when you ferry an aircraft back on three etc you gag (fit a locking device to prevent it turning) the fourth engine to stop it windmilling and also to prevent it causing further damage.

[lomapaseo]

Hence why when you ferry an aircraft back on three etc you gag (fit a locking device to prevent it turning) the fourth engine to stop it windmilling and also to prevent it causing further damage.

Can I read this OK by removing the "why" word/ ?

[B2N2]

Thread title says jet.

My mistake.
Consider the added drag of the accessory gearbox driving hydraulic pumps and electric generators.

[mrfox]

Think of a helicopter. Which would come down faster? Windmilling or stationary rotor?

[Centaurus]

During my time in the RAAF we were taught how to conduct practice turn-backs following engine failure after takeoff in Vampire jet fighters. Looking back now and with the benefit of hindsight, I realise it was a potentially dangerous manoeuvre since so much depended on height and airspeed at instant of engine failure as well as wind. The engine failure was simulated by closing the throttle and much also depended on angle of bank.

At RAAF Base East Sale where this manoeuvre was taught at Central Flying School, we would practice this stuff and most of the time when it became clear an undershoot looked like happening, power had to be added to get over the fence. At Sale, there was no shortage of suitable fields surrounding the aerodrome so that most of the time it was better to force land straight ahead rather than risk stalling in a steep turn while trying to turn back.

One dual Vampire did have an engine failure and both pilots were killed when they tried to turn back and hit short of the runway. Landing straight ahead would have been a safer option but we were conditioned to turn back if there was any possibility of success. As part of the accident investigation, qualified test pilots from ARDU (Aircraft Research and Development Unit) flew a series of test flights and proved that turn backs were real edge of the envelope flying. It was during tests where engine seizure was simulated that it was found the drag from the seized engine was so significant as to change the airspeed and height parameters needed to make a successful turn back.

[beardy]

Centaurus,
Thanks for your anecdote, it would be interesting to know how a seized engine was simulated and compared to a windmilling and idle engine.

[oggers]

This paper explains how it was done in the lab at least

NACA UK Mirror report description page (http://naca.central.cranfield.ac.uk/report.php?NID=5103)

[beardy]

This paper explains how it was done in the lab at least

NACA UK Mirror report description page (http://naca.central.cranfield.ac.uk/report.php?NID=5103)
Thanks for that. It seems quite definitive. And thanks for find this paper. This time Google was not my friend and I found nothing like this.

[old,not bold]

I guess the fundamental reasons apply equally to a wind-milling propeller and a windmilling turbine; wind-milling consumes energy. Mrfox is on the button.

Which is why we were told, in the olden days, to stop the propeller of a SEP so as to extend the glide after it all goes quiet, if you need to. (I found out the hard way that doing that, for the first and hopefully only time, in cloud with a basic panel is challenging.)

[lomapaseo]

This paper explains how it was done in the lab at least

NACA UK Mirror report description page (http://naca.central.cranfield.ac.uk/report.php?NID=5103)


the bypass ratio could significantly alter the differences

[lomapaseo]

I guess the fundamental reasons apply equally to a wind-milling propeller and a windmilling turbine; wind-milling consumes energy. Mrfox is on the button.

Which is why we were told, in the olden days, to stop the propeller of a SEP so as to extend the glide after it all goes quiet, if you need to. (I found out the hard way that doing that, for the first and hopefully only time, in cloud with a basic panel is challenging.)

There differences are most significant if the pitch changes between conditions . If nil difference in pitch than there should be nil difference in drag for large prop driven planes.

[beardy]

the bypass ratio could significantly alter the differences
A bypass engine is just a turbojet with a big multi bladed propeller at the front. So the figures may be different, but the principle is the same.

[lomapaseo]

A bypass engine is just a turbojet with a big multi bladed propeller at the front. So the figures may be different, but the principle is the same.

Don't agree on this subject

The large bypass engines have large inlet cowls which make up the majority of the residual drag. They typically have fixed inlet blade angles and lower bypass ratios for their outer dia then prop jets. Thus a lot more number crunching

[Centaurus]

it would be interesting to know how a seized engine was simulated
It was too long ago although I recall seeing the results of the trials. Maybe extension of the dive brakes in the Vampire were used as a drag equivalent?

[Uplinker]

Not being an aerodynamicist, my untrained mind would imagine there to be less drag if air was allowed to go through a rotating fan, than if the fan was held fixed in the airflow - presenting a huge “airbrake”. As several have said, turning the fan and engine and gearbox will take power to do, but would that be more power than the equivalent “airbrake” would absorb though?

I suppose that a ‘plug’ of nearly stagnant air might build up in the intake in front of a non moving fan, which might produce a virtual fairing, around which the airstream could flow relatively smoothly?

Wasn’t there a once a four engined jet from somewhere that landed at Heathrow with one engine fan held immobile with ratchet straps - or have I imagined that?

[Bergerie1]

I did many 3-engine ferry flights on 747s and VC10s. If I remember rightly, we always flew with the engine 'spragged' so that it wouldn't rotate. I have no idea about the drag question, but it was important to prevent the engine windmilling, especially if it was damaged in some way.

[Dave Therhino]

Overall drag is higher with the fan seized.

[beardy]

Overall drag is higher with the fan seized.
That's interesting, would you be good enough to justify that.

[Vessbot]

Overall drag is higher with the fan seized.

A paper was just posted a few posts up describing the results of an experiment showing the opposite -- and it really seems the eggheads had done their work properly, it has Greek letters and everything!

[Fly Aiprt]

Not being an aerodynamicist, my untrained mind would imagine there to be less drag if air was allowed to go through a rotating fan, than if the fan was held fixed in the airflow - presenting a huge “airbrake”. As several have said, turning the fan and engine and gearbox will take power to do, but would that be more power than the equivalent “airbrake” would absorb though?

Just to get a feel of it, the post above, of which helicopter descends faster, rotor turning or rotor stopped seems a fairly good analogy.

[Vessbot]

Not being an aerodynamicist, my untrained mind would imagine there to be less drag if air was allowed to go through a rotating fan, than if the fan was held fixed in the airflow - presenting a huge “airbrake”. As several have said, turning the fan and engine and gearbox will take power to do, but would that be more power than the equivalent “airbrake” would absorb though?

Think of the blades like a clutch that engages the airsteam to the rest of the engine, and unstalling them (by letting it windmill) as the engagement of that clutch. With them unstalled and windmilling (clutch engaged) now it's like you're engine braking downhill in a car. Locking the engine from turning stalls the blades and disengages the clutch, allowing the car to roll downhill freely. (Yes they still make more drag than if they weren't there at all, but less than they would if they're transferring energy to the engine.)

Talking about prop engines, some people think of the feathering function as a way to reduce drag by reducing the frontal area, but that's only a fraction of their benefit. By far most of the benefit is in preventing windmilling and transferring energy from the airsteam to the engine. If the engine seized and the blades failed to feather (i.e., stuck flat) the drag would be slightly higher than if they were feathered, but still far less than if it was windmilling.

[Vessbot]

Don't agree on this subject

The large bypass engines have large inlet cowls which make up the majority of the residual drag. They typically have fixed inlet blade angles and lower bypass ratios for their outer dia then prop jets. Thus a lot more number crunching

All these particulars are true, but that doesn't change that fundamentally the fan is still a prop attached to the shaft. Including in the context of this discussion, transfer of energy to the shaft via windmilling.

[Chu Chu]

Energy transfer doesn't seem like the whole explanation. As a thought experiment, mount an (unfeathered) propeller on a shaft running in ball bearings on an airplane wing. Mount an identical setup on the other wing, but lock the shaft so it can't rotate. The energy transfer will be (essentially) zero on both sides. Will the drag be the same?

Here's how I'm thinking about it: The angle of attack of the propeller blades depends on the airspeed (of the aircraft) and the propeller's RPM. Under power, the propeller advances forward on each rotation by an amount less than its pitch. This means that the blades operate at a positive angle of attack and make positive lift -- aka forward thrust. If the propeller advances by an amount equal to its pitch on each rotation (perhaps during a dive), the blades operate at zero AOA and make zero lift and zero thrust (or a little, depending on the airfoil).

A windmilling prop advances more than its pitch on each rotation. This means it operates at a negative AOA and makes negative lift (i.e. drag as seen from the perspective of the aircraft). Lock the propeller from rotating, and the AOA goes so massively negative that the blades stall. They then stop making negative lift (but there's still significant drag, of course). Or better, feather the prop and reduce (or is it increase?) the AOA to near zero.

If a windmilling propeller was mounted in bearings and was 100% efficient aerodynamically, it would rotate fast enough to advance an amount equal to its pitch, bringing the AOA to zero. But there's always drag, so it can't spin that fast. Add a rotational load to the aerodynamic drag, and the propeller spins even slower, increasing the negative AOA and negative lift/drag. At least to up to the point where the blades stall. So the energy transfer does matter.

This all seems relevant to the fan in a jet engine. But I know there's a lot of other stuff going on that I don't understand.

[Dave Therhino]

A paper was just posted a few posts up describing the results of an experiment showing the opposite -- and it really seems the eggheads had done their work properly, it has Greek letters and everything!

That paper was talking about the internal drag on the engine only, as stated in the paper. As many here have pointed out, the drag on a propeller or on the fan itself is lower when the prop or fan is seized. However, in that condition the air flow through the inlet is significantly reduced, resulting in increased spillage from the inlet. My recollection from the discussions back when the reserve fuel requirements were being developed for ETOPS was that the overall nacelle drag was greater at single engine cruise speeds with the fan seized due to the effect of the spillage. However, enough of you are saying the opposite to make me question whether my memory is accurate. It was 30 years ago. I'll check some of the aircraft operation documentation I have when I'm back in the office on Tuesday and post what I find.

[tdracer]

Just to get a feel of it, the post above, of which helicopter descends faster, rotor turning or rotor stopped seems a fairly good analogy.

Not really an applicable comparison - when a helicopter looses power, the pilot is constantly varying the pitch of the rotor blades during the descent - initially it's to accelerate the rotors, then at just the right moment it's changed to convert the rotational energy of the rotor to lift in order to cushion the impact. Helicopter pilots constantly practice a simulation this maneuver since if they need to do it, they only get one chance to get it right...
Dave, I also have an old memory that a seized fan was a critical case for EROPS/ETOPS range due to increased overall aircraft drag - I can't readily resolve that against some of the comments here.
I do remember a reason the fan was locked in place during an engine failed ferry flight was that there may not be sufficient oil pressure generated to protect against bearing damage during an extended windmill.

[Vessbot]

Energy transfer doesn't seem like the whole explanation.

I would say that energy transfer is the whole story, but not the only story ;)

What I mean is that many phenomena have more than one explanation, from different aspects, simultaneously, of what's going on. For example, rocket propulsion can be explained either by conservation of momentum (mass times velocity of the expelled propellants must equal the mass times velocity of the vehicle going the other way) or unbalanced force (pressure of the hot propellants at the nozzle times its). Both ways account for the phenomenon 100%, but looking at it different ways. Another example (not to derail this on a tangent) is the multitude of explanations for the lift of a wing. People who champion one of them, but understanding (at least somewhat) some of the others, get bent into pretzels in considering how much each one contributes to lift, where they all actually contribute 100%. Or, what makes a car go? Is it he combustion of fuel and oxidizer in the cylinders, or the compressive force on the connecting rods, or the torque on the driveshaft, or the friction of the tires, etc.? Which camp do you fall into? :}

Anyway here, we have the energy transfer view, and also the the particular forces (as a result of the flow vectors and AOA on the blades that you detailed, and with which I agree completely).

----------------

As for the seized jet rotor drag. Now two separate guys are grasping at muddy memories of ETOPS data saying overall drag may be greater on a seized rotor, so I'm prepared to change my position. Dave Therhino brought up the notion of airflow spilling over the inlet inside-to-outside making its own drag, and I suppose it's possible that in the sized case that drag contribution could outweigh the savings of the reduce drag on the rotor itself.

[Vessbot]

Not really an applicable comparison
I see it as perfectly applicable, if you take a bigger scale view of things than all the control transients, different areas of the disk, etc. What's important is that by turning, the rotor puts enough disk area into AOA's that becomes usable to 1. drive the rotor and 2. accelerate air down; in essence doing what I said in my earlier post in "engaging the clutch" between airstream and airframe, transferring energy from the former to the latter, and thus holding it up (ish). Were the rotor still, none of this would be happening and the vehicle would merely pass through the air with no interaction.

[Uplinker]

.....Were the rotor still, none of this would be happening and the vehicle would merely pass through the air with no interaction.

Well, an airbrake has interaction with the airstream - it slows the aircraft down. So a locked fan will extract energy from the airflow because it is, in effect, a large circular airbrake. But I don’t know how to calculate if that energy would be greater than if the fan was rotating.

[Chu Chu]

I agree that energy transfer is one way of telling the whole story. I guess my real point was that energy transfer into the engine isn't the whole story -- you'd have to consider energy transfer into the atmosphere through vortices and such as well. That makes the analogy to a clutch on a car running down the hill less useful.

[lomapaseo]

tdracer
Dave, I also have an old memory that a seized fan was a critical case for EROPS/ETOPS range due to increased overall aircraft drag - I can't readily resolve that against some of the comments here.

Yup ! therein lies the crux of this discussion throughout this whole thread :)

you can lead a horse to water .....

Like many other fun threads on this net, we often end up voting our answers. That's why in the end I just rely on what the FCOM manual says since that at least has been carefully thought out

btw, I also recall the documentation provided to the FAA regarding seized rotor drag (due to icing between the blades and casing) in the event of a forced descent into icing conditions in an ETOPs event. So it seems that it wasn't enough of a problem to stand in the way of granting ETOPS

[racingrigger]

All good stuff about drag etc but one of the main reasons to stop an engine windmilling is, as already said, to stop further damage. Another consideration is that if the HP fuel cock is closed as one would expect and the engine is allowed to windmill the engine driven fuel pump will more than likely overheat and ultimately self destruct! Could be a recipe for an engine fire.

[tdracer]

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).

Vessbot, the helicopter analogy is invalid because of the way the blade pitch is varied during an auto-rotation. If you measured the rate of descent of a chopper with a fixed rotor, you'd find it initially would descend slower than one performing an auto-rotation maneuver. With a fixed rotor and a vertical descent, the air will be hitting the rotor blade nearly perpendicular - making the rotor basically just a big flat plate to the air (the drag coefficient of a flat plate perpendicular to the airflow is nearly 1.0 - which is roughly the same as a parachute. In short fixed rotor blades become a huge drag source during a vertical descent. In contrast, the blade pitch during an auto-rotation is controlled to specifically prevent it from stalling - using the resultant rotational lift component to accelerate the rotors to a high speed so that the kinetic energy an be used to create lift and slow the descent in the last second before impact. There is some induced drag associated with creating the lift that provides the rotational acceleration, but it's much less than the drag created by a fully stalled rotor blade.

[lomapaseo]

[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

[CCA]

A fan producing thrust is obviously providing a forward lifting force towards the front of the engine (+ thrust) while a fan being spun by the relative airflow is now providing a fan force towards the rear of the engine due to a lift force on the rear side of each blade (-thrust)

Windmill drag is obviously significant enough that aircraft now use thrust on descent to reduce drag. While I remember the A330 having it from what I imagine was service entry, you’d descend with an EPR of 1.0 or so, the 747 only got thrust on descent with the NGFMC and the introduction of the 747-8 ie circa 45 years late.

So we’re back to the beginning is the interference drag reduced more or less than the negative thrust component caused by a windmilling engine?

I guess you could plot windmill N1 / EPR vs airspeed and see if it’s a direct correlation or find a aerodynamicist.

I’m going with windmilling has more total drag or penalty than a locked fan.

Logs added to fire.....

[oggers]

If you measured the rate of descent of a chopper with a fixed rotor, you'd find it initially would descend slower than one performing an auto-rotation maneuver. With a fixed rotor and a vertical descent, the air will be hitting the rotor blade nearly perpendicular - making the rotor basically just a big flat plate to the air (the drag coefficient of a flat plate perpendicular to the airflow is nearly 1.0 - which is roughly the same as a parachute. In short fixed rotor blades become a huge drag source during a vertical descent. In contrast, the blade pitch during an auto-rotation is controlled to specifically prevent it from stalling - using the resultant rotational lift component to accelerate the rotors to a high speed so that the kinetic energy an be used to create lift and slow the descent in the last second before impact. There is some induced drag associated with creating the lift that provides the rotational acceleration, but it's much less than the drag created by a fully stalled rotor blade.

No. There is so much wrong with that paragraph that I am led to the conclusion you have virtually no understanding of rotary wing principles of flight. Debunking the misconceptions contained is worth a thread in itself so I will just debunk the first line: as soon as there is a rate of descent airflow, even with the blade at zero pitch, combined with with the rotational airflow from any reasonable Nr, the relative airflow will result in the blade producing lift (aka rotor thrust in rotary parlance). The total rotor thrust of the disc will be orders of magnitude higher than any 'drag' from non-rotating blades subject to the rate of descent airflow only. There is absolutely no way that a helo will descend slower with the blades stationary, neither initially nor in a steady state auto.

[jmmoric]

Seen from an energy point of view...

To get the engine spinning, you need energy.... and that energy comes from "stored energy" (in case of a dead engine), that stored energy will be the altitude below the aircraft, or the fuel in the tanks etc.

So no matter how you twist and turn it, if you have an engine windmilling, you're using energy to get that engine windmilling, that requires extra thrust on the other engine(s), hence extra fuel consumption.

I have to think a little more about the drag thing though, but I'm confident oggers has it nailed here.

[TURIN]

Does this help?

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/747x458/autorotation_f6f7ebcb6dfb0aefbcdf642cb58bd0949b1d46bc.jpg

[cattletruck]

I read recently a simple explanation, the energy that the aircraft is using to push itself through the air is being reduced as some of that energy is being used to spin the windmilling blades.

Another way to look at it is the potential drag energy 1m in front of either a stationary or windmilling jet engine is the same. You could say that if some of that potential drag energy is then used to windmill an engine then a windmilling engine has less drag than a stationary one, however, turbine blades are designed to push air backwards rather than be pushed by the incoming airflow so their may actually be no real benefit at all, in fact it could even be worse.

A lot would also depend on the engine size, design, location, airspeed, etc, an interesting question nonetheless.

[jmmoric]

Does this help?

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/747x458/autorotation_f6f7ebcb6dfb0aefbcdf642cb58bd0949b1d46bc.jpg

So what you are saying, since the relative wind in relation to a windmilling engine, is that the AoA is actually "inverse", and the lift generated by the windmilling engine, is actually opposite direction of the lift generated by a normal operating engine? So the lift goes towards the rear, hence increasing the drag??

Thinking a bit about it, I can buy that, looking at the example of the gyro copter.

[Centaurus]

In a CM56 jet engine if the core of the engine seizes completely for whatever reason, will the N1 fan still rotate freely?
In the 737-300 simulator, actuation of "turbine seizure" selector on the instructor panel causes significant noise and heavy vibration and eventually the N2 indicates zero but the N1 still indicates rotation; albeit around 10 N1 only. Does turbine seizure mean engine core seizure? Or are they both different parts of the engine?

[Dave Therhino]

It sounds like imprecise language is used in those simulator controls. The N1 (fan) shaft and the N2 (core) shaft both have turbine stages at the back end. The rotation of the two shafts is not mechanically connected. The core being seized or simply not turning due to accessory drag at low airspeed will not directly mechanically affect rotation of the fan shaft. It sounds like the "turbine seizure" selector you describe simulates a stopped core shaft and tries to simulate some sort of N1 system mechanical failure. It's not clear what actual mechanical failure the designers were trying to simulate. The heavy airframe vibration they simulate with N2 at zero obviously is intended to represent damage to the N1 system, like out of balance or bearing failure. Failure of the N2 turbine that releases parts in the gas path often will cause heavy damage to the downstream N1 stages.

[BARKINGMAD]

.........Another consideration is that if the HP fuel cock is closed as one would expect and the engine is allowed to windmill the engine driven fuel pump will more than likely overheat and ultimately self destruct! Could be a recipe for an engine fire.

Doesn’t that put an end to 180 & 240 minutes ETOPS then in the event of IFSD?

Let alone continuing on 3 of 4 assuming adequate terrain clearance when the next one fails?

I’ve been P2 on 6 3-engined ferrys all of which had the fan etc tethered by proper engineering kit, especially the one with the remains of a borescope in it!

And yes, there was a 747 transiting through Europe with the front end tethered by a collection of leather belts and other Heath Robinson improvised devices. It was impounded by the relevant xAA until a more airworthy solution was found and the subsequent dispatch proven as legal and compliant.

[Vessbot]

Well, an airbrake has interaction with the airstream - it slows the aircraft down. So a locked fan will extract energy from the airflow because it is, in effect, a large circular airbrake. But I don’t know how to calculate if that energy would be greater than if the fan was rotating.

You're right and I was painting in broad strokes. I meant "interaction" beyond that which a brick would have in its fall.

I agree that energy transfer is one way of telling the whole story. I guess my real point was that energy transfer into the engine isn't the whole story -- you'd have to consider energy transfer into the atmosphere through vortices and such as well. That makes the analogy to a clutch on a car running down the hill less useful.

You too are right, and I shot my gun too fast. Energy goes into turning the engine, and energy goes into accelerating air - into eddies, vortices, etc., and into adding a bit of overall movement in the same direction as the craft. (Decelerating, in a reference frame fixed to the aircraft)



Vessbot, the helicopter analogy is invalid because of the way the blade pitch is varied during an auto-rotation. If you measured the rate of descent of a chopper with a fixed rotor, you'd find it initially would descend slower than one performing an auto-rotation maneuver. With a fixed rotor and a vertical descent, the air will be hitting the rotor blade nearly perpendicular - making the rotor basically just a big flat plate to the air (the drag coefficient of a flat plate perpendicular to the airflow is nearly 1.0 - which is roughly the same as a parachute. In short fixed rotor blades become a huge drag source during a vertical descent. In contrast, the blade pitch during an auto-rotation is controlled to specifically prevent it from stalling - using the resultant rotational lift component to accelerate the rotors to a high speed so that the kinetic energy an be used to create lift and slow the descent in the last second before impact. There is some induced drag associated with creating the lift that provides the rotational acceleration, but it's much less than the drag created by a fully stalled rotor blade.

I'm not following. Drag coefficient (i.e., shape) is important but much more goes into it than that: namely the opposing force. By your argument, a beach ball full of air will reach the same terminal velocity as a beach ball full of lead.

In reality they both have the same drag coefficient, but the latter will have much more weight, which will oppose drag and accelerate downward faster, reaching a speed (terminal velocity) where the much higher weight is balanced by equally much higher drag.

I still maintain that an autorotating helicopter is a good analogy.
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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

The relative wind during windmill is on the convex side which renders the AOA negative, but not necessarily stalled. Think cambered-wing airplane doing inverted flight.
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It sounds like imprecise language is used in those simulator controls. The N1 (fan) shaft and the N2 (core) shaft both have turbine stages at the back end.

Thank you for also being bothered by this! I flew a bit in a Citation where the engine synch selector positions (for which set of spools you want to synch) were labelled "Fan" vs. "turbine." I wanted to murder someone over that.
​​​​​​​

[Dave Therhino]

I was able to dig out the drag analysis from several years ago supporting a three engine ferry performance data approval which showed that, for a nacelle mounted high bypass engine, the nacelle with a non-rotating fan has appreciably higher drag than that same nacelle with a windmilling engine.

[beardy]

I was able to dig out the drag analysis from several years ago supporting a three engine ferry performance data approval which showed that, for a nacelle mounted high bypass engine, the nacelle with a non-rotating fan has appreciably higher drag than that same nacelle with a windmilling engine.
That's really interesting, would you care to share a bit more detail?

[Dave Therhino]

Unfortunately, because I don't personally own the data, I have to be careful not to release information that might be considered proprietary.

However (and this is now just me talking about the physics as I understand it), I hope most of us can agree that a fully plugged nacelle (entire fan duct and core path blocked) would have more drag than a nacelle with a windmilling fan due to the effects of 100% spillage. The nacelle with a stopped fan behaves a lot like a nacelle with a fully plugged fan because the flow through the fan duct is greatly reduced versus a windmilling engine. The stream of air that actually flows through the nacelle is greatly reduced in diameter when the fan is stopped, unlike when a propeller is stopped, due to the high solidity of the fan and the exit guide vanes relative to the flow direction and the total effect of the tortuous path the air has to follow to get around the fan blades and the exit guide vanes (which are shaped and angled all wrong for this flow case). This "spilled" air, which has to get out of the way of the nacelle rather than pass through it, causes an appreciable amount of what I believe is referred to as interference drag, but I am not an aerodynamics engineer so that may be the wrong term.

[mrfox]

In the B-36, the jet engines are only used for the high speed dash over the target area. During cruise on piston power, nacelle doors cover the unused jet intakes. This design seems to run counter to the assumption that a blocked intake is draggier than windmilling engines.
https://images.dailykos.com/images/148091/large/Convair_B-36_5373054244_b510f96f9b_b.jpg

[Dave Therhino]

That shape is very different from the shape of an 8 foot diameter high bypass engine with a blockage 4 feet back in the inlet. I know nothing about that airplane except for admiring it's extremely complex cockpit in a panoramic viewer you can find on the internet. Try to find the jet engine controls if you don't already know where they are!

[Vessbot]

In the B-36, the jet engines are only used for the high speed dash over the target area. During cruise on piston power, nacelle doors cover the unused jet intakes. This design seems to run counter to the assumption that a blocked intake is draggier than windmilling engines.


Dave Therhino already said it, but just to be more clear: there's at least 3 configurations under the discussion:

1. Blocked/faired intake
2. Open intake with windmilling engine
3. Open intake with seized engine

And I've listed them in order of what I now think is least to most drag. (At the beginning of the thread I thought 2 and 3 were reverwed, until the few guys referenced the ETOPS data)

[Dave Therhino]

Your item 1 is really two configurations that I suspect are the lowest and highest drag configurations, fully faired inlet being lowest drag, and nacelle with inlet fully blocked at the fan face being highest. The fully faired inlet is a theoretical case.

[mrfox]

Further research on the topic of spillage drag has led me to the text Gas Turbine Performance (P.Walsh/P.Fletcher) where it mentions spillage drag can range from 10 - 20 percent of internal drag in a pure turbojet, to appoximately equal to internal drag at a bypass ratio of 5:1. This does corrolate with the disconnect between the data from the old NACA reports using straight jets against what is being put forward here.
Its a good day when one sets off to prove someone wrong only to demise by his own sword!
The text references data from ESDU 81009, 84004 and 84005 - if anyone has access and could summarize it would be greatly apperciated.

[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.

[Tomaski]

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]

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]

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]

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]

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]

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

https://cimg4.ibsrv.net/gimg/pprune.org-vbulletin/1183x782/147064_52ca138b8e073nw_20b727_20251_20tpa_20n280us_20cn_2021 159_201179_2001_2004_201990_fe02392fa26cf159c005a4abf90c25e9 064c0fbe.jpg

[QuarterInchSocket]

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]

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...