Originally Posted by
Uplinker
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.
Originally Posted by
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.
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)
Originally Posted by
tdracer
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.
Originally Posted by
lomapaseo
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.
Originally Posted by
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.
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.