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.