Drag of a seized jet engine compared to windmilling
 

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

Am I mistaken?

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.

Thread title says jet.

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

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

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.

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/ ?

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

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.

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.

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

NACA UK Mirror report description page

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.

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.)

the bypass ratio could significantly alter the differences

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.

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

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?

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?

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.

Overall drag is higher with the fan seized.

That's interesting, would you be good enough to justify that.

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!

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.

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.

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.

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.

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.

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.

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).

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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.

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.

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.

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.

tdracer
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

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.

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.