Contra-rotating props and Bernoulli etc
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2 vs 4 engines
The way i understood it, the 4 engine layout has 2 advantages.
1 - could spread the weight of the engines over the a larger section of the wing, hence making the wing lighter to construct and
2- in the advent of engine failure at V1 you only lost 25% of the thrust not 50%. I know the climb gradient required is larger, but the net result was you put smaller engines on.
A small ps on the CR story, i like the theory of energy 'recovery' as u smooth out the helical flow, kinda like stators behind the N1 fan.
1 - could spread the weight of the engines over the a larger section of the wing, hence making the wing lighter to construct and
2- in the advent of engine failure at V1 you only lost 25% of the thrust not 50%. I know the climb gradient required is larger, but the net result was you put smaller engines on.
A small ps on the CR story, i like the theory of energy 'recovery' as u smooth out the helical flow, kinda like stators behind the N1 fan.
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A small ps on the CR story, i like the theory of energy 'recovery' as u smooth out the helical flow, kinda like stators behind the N1 fan.
Is there any reason a propeller aircraft cannot use stators?
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The "shrouded stator" bit works best when it's a convergent duct, yielding a higher exit velocity for greater cruise efficiency. This can be done with a smaller rotor diameter, bringing the drag down.
And by the time you do all this, you might as well drive it with a gas turbine and call it a turbofan.
But if you want really efficient lower-Mach cruise, the counter-rotating fan (e.g. GE36) is the record-setter.
And by the time you do all this, you might as well drive it with a gas turbine and call it a turbofan.
But if you want really efficient lower-Mach cruise, the counter-rotating fan (e.g. GE36) is the record-setter.
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The "shrouded stator" bit works best when it's a convergent duct, yielding a higher exit velocity for greater cruise efficiency. This can be done with a smaller rotor diameter, bringing the drag down.
And by the time you do all this, you might as well drive it with a gas turbine and call it a turbofan.
And by the time you do all this, you might as well drive it with a gas turbine and call it a turbofan.
Why do turbofans find the swirl-compensating stators to be efficient, rather than a source of extra drag?
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Why do turbofans find the swirl-compensating stators to be efficient, rather than a source of extra drag?
But to merely call the stators "swirl-compensating" misses the point. They convert the swirl or vortex energy (which would otherwise be wasted in the slipstream) into increased static pressure, which results in a higher nozzle pressure and velocity, which can in turn be optimized for best cruise thrust & SFC.
A propeller designer doesn't have so many "knobs to twist" in optimizing his design, so he is stuck in a lower-speed cruise regime.
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A correction
I think I made a faux pas, should have used the word 'guide vains' not stators. Stators in the engine have a totally different function.
All engines have guide vains behind the N1 fan
All engines have guide vains behind the N1 fan
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All engines have guide vains behind the N1 fan
But vains, veins, and vanes all have distinct and separate meanings.
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Like everthing else, there's a bit of tradeoff (airfoils have lift AND drag, don't you know).
But to merely call the stators "swirl-compensating" misses the point. They convert the swirl or vortex energy (which would otherwise be wasted in the slipstream) into increased static pressure
But to merely call the stators "swirl-compensating" misses the point. They convert the swirl or vortex energy (which would otherwise be wasted in the slipstream) into increased static pressure
They take the swirl downstream of a (shrouded) fan, and convert it into (nonswirling, more direct) jet blast. They spend some of the energy for drag, but they still gain more energy by converting swirly slipstream to direct than they lose to drag.
Why cannot open propellers also install open stators behind the propeller? Converting swirl to more direct slipstream?
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As a seeker after knowledge, let me put this to you:
Considering the airflow through the prop as in a tube, the prop generates thrust by accelerating the air in the direction of flight. The only way for the stators to generate extra thrust is to further accelerate the air. How do they do this?
Dick W
Considering the airflow through the prop as in a tube, the prop generates thrust by accelerating the air in the direction of flight. The only way for the stators to generate extra thrust is to further accelerate the air. How do they do this?
Dick W
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Crackin' thread this one. Thanks for a great read so far.
As to the 4 Vs 2 bit on the 146, all I know is that having 4 hair dryers means an engine failure won't cause one of our old skippers a heart failure! Just a minor inconvenience.....I look forward to more of this,
Thanks again!
As to the 4 Vs 2 bit on the 146, all I know is that having 4 hair dryers means an engine failure won't cause one of our old skippers a heart failure! Just a minor inconvenience.....I look forward to more of this,
Thanks again!
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Considering the airflow through the prop as in a tube, the prop generates thrust by accelerating the air in the direction of flight. The only way for the stators to generate extra thrust is to further accelerate the air. How do they do this?
1) Capture the vortex energy by turning the air back parallel to the direction of flight. In the process of doing this, the stator vane array has a larger exit area than its entrance area, so the air is slowed. The energy capture is reflected in the static pressure rise.
2) Re-accelerate the air through an annular nozzle, maintaining the axial flow direction.
Both steps are necessary!
Last edited by barit1; 29th Mar 2007 at 12:42.
