I have read about several propeller-driven aircraft that ran into trouble, and in many cases crashing, when the crew was unable to feather the propeller blades on a shutdown engine. This is due to a windmilling propeller significantly increasing drag on an aircraft. I know it is like comparing apples to oranges, but when a turbofan engine is shut down in flight are the fan and compressor discs stopped or allowed to spin freely? In such a condition, wouldn’t these discs create drag in the same way as an unfeathered propeller? I apologize if this is a very basic and well-known principle or if it has been asked before.

Unless there is some damage or seizure the ram air will create rotation of the fan discs. Sometimes this is enough to create residual power to stuff like accessory drives. Compared to most prop or turboprop aircraft, the level of drag and the amount of power remaining mean on the vast majority of jet aircraft, and I'll the ones, I've flown the impact is minimal.

There is no mechanism to stop the low-pressure spool from spinning. As FlapOperator says, unless it is seized somehow due to damage, it will spin.

There is indeed drag, but nowhere near that of an unfeathered propeller.

You don't want the fan to stop turning. Spinning in the breeze is much better.
Many years ago I was doing some engineering things with a Tristar simulator, and afterwards we went flying in it, and the simulator engineer simulated a seized fan. Loads of vibration and very noisy. Not recommended!

Starlionblue wrote:

There is no mechanism to stop the low-pressure spool from spinning. As FlapOperator says, unless it is seized somehow due to damage, it will spin.

There is indeed drag, but nowhere near that of an unfeathered propeller.

Are we talking about spinning unfeathered prop, or immobile?
As far as I understand, most designs have no clutch between engine and prop (is ATR-42 hotel mode unique?) - is that the cause of the difference, that unfeathered prop has to work a piston engine?

Tristarsteve wrote:

You don't want the fan to stop turning. Spinning in the breeze is much better.
Many years ago I was doing some engineering things with a Tristar simulator, and afterwards we went flying in it, and the simulator engineer simulated a seized fan. Loads of vibration and very noisy. Not recommended!

Good point! Seizure is generally far more damaging than a simple or commanded in-flight shutdown.

kalvado wrote:

Are we talking about spinning unfeathered prop, or immobile?
As far as I understand, most designs have no clutch between engine and prop (is ATR-42 hotel mode unique?) - is that the cause of the difference, that unfeathered prop has to work a piston engine?

Here is the difference between a free turbine design like a PT-6 and direct drive turboprops. Direct drive doesn't have a clutch per se, but there is lots of inertia in the engine that keeps the prop from moving once the blades are feathered. In a PT-6 design (King Air 200s/1900s) the ram air can often create some very gentle rotation, but often once the prop stops, it stops and hangs there (speaking as a person who might held the world record in King Air 200 single engine flight time, at one point.)

FlapOperator wrote:

kalvado wrote:

Are we talking about spinning unfeathered prop, or immobile?
As far as I understand, most designs have no clutch between engine and prop (is ATR-42 hotel mode unique?) - is that the cause of the difference, that unfeathered prop has to work a piston engine?

Here is the difference between a free turbine design like a PT-6 and direct drive turboprops. Direct drive doesn't have a clutch per se, but there is lots of inertia in the engine that keeps the prop from moving once the blades are feathered. In a PT-6 design (King Air 200s/1900s) the ram air can often create some very gentle rotation, but often once the prop stops, it stops and hangs there (speaking as a person who might held the world record in King Air 200 single engine flight time, at one point.)

Let me rephrase it - how free second stage with a gearbox to the prop is different from a free second stage directly connected to the fan? And what would happen with a GTF since we're at this?
my first thought is that driving pistons would be a totally different story in terms of required power and drag; but turbo driven designs should be more similar. Or I am totally missing it?

FlapOperator wrote:

kalvado wrote:

Are we talking about spinning unfeathered prop, or immobile?
As far as I understand, most designs have no clutch between engine and prop (is ATR-42 hotel mode unique?) - is that the cause of the difference, that unfeathered prop has to work a piston engine?

Here is the difference between a free turbine design like a PT-6 and direct drive turboprops. Direct drive doesn't have a clutch per se, but there is lots of inertia in the engine that keeps the prop from moving once the blades are feathered. In a PT-6 design (King Air 200s/1900s) the ram air can often create some very gentle rotation, but often once the prop stops, it stops and hangs there (speaking as a person who might held the world record in King Air 200 single engine flight time, at one point.)

Would you mind telling the story? Please

Aircellist wrote:

FlapOperator wrote:

kalvado wrote:

Are we talking about spinning unfeathered prop, or immobile?
As far as I understand, most designs have no clutch between engine and prop (is ATR-42 hotel mode unique?) - is that the cause of the difference, that unfeathered prop has to work a piston engine?

Here is the difference between a free turbine design like a PT-6 and direct drive turboprops. Direct drive doesn't have a clutch per se, but there is lots of inertia in the engine that keeps the prop from moving once the blades are feathered. In a PT-6 design (King Air 200s/1900s) the ram air can often create some very gentle rotation, but often once the prop stops, it stops and hangs there (speaking as a person who might held the world record in King Air 200 single engine flight time, at one point.)

Would you mind telling the story? Please

Just having a multitude of jobs both instructing and later instructing and line checking resulting in lots of cumulative dual given instruction in King Air series aircraft. Not all of the Single Engine time was at once. Both the US and Canadian authorities recommended (at the time, thankfully they've backed off this requirement due to the danger to aircrew/wear on engines and props) a full engine shutdown to feather and restart for flight training/recommendation for checkrides.

Doing lots of flight training in the aircraft means we pretty routinely motored around on a single engine. We had one examiner who was pretty famous for demanding a single engine approach as a "confidence maneuver." Thankfully, the vast majority of these generation of examiner have been hunted to extinction in the last two decades, along with far more King Air simulators being available at a cost point where training in the box makes more sense than the aircraft. That said, practicing single circle to land approaches in the aircraft does sharpen your skill set, build your bandwidth in single pilot operations and demonstrates to a person likely in their turbine aircraft the conditions where such a maneuver can be done safely.

FlapOperator wrote:

Aircellist wrote:

FlapOperator wrote:

Thank you very much. I imagined an incredible accident. Reality is much simpler, much more down to Earth, if I may say so (hopefully, in a very controlled way)…

Your story shows that musicians and pilots have that thing in common: training…

Aircellist wrote:

FlapOperator wrote:

Aircellist wrote:

Thank you very much. I imagined an incredible accident. Reality is much simpler, much more down to Earth, if I may say so (hopefully, in a very controlled way)…

Your story shows that musicians and pilots have that thing in common: training…

Indeed. I've had a couple of musicians (as in people who were really credible players, if not necessarily professionals) as candidates/students and there is a level of Venn overlap of skills like the ability to split attention, good hand/eye coordination, and on-demand focus.

The drag from a spinning prop is tantamount to having a disc of plywood out there that you’re pushing through the air. On a small aircraft the proportion of frontal area that said prop takes is much greater than the equivalent frontal area of a jet. The free spinning N1 also has a lot of drag, but proportionally the frontal area of the jet engine is smaller than in an equivalent prop