Blade Flapping
AnFI - would it placate you to state that flapback and inflow roll occur as a result of inequality of lift and that they are overcome by the pilot positioning the cyclic to achieve the desired disc and fuselage position?
This both accepts that the aerodynamic effects exist (especially when trying to explain how the rotor behaves to students) and acknowledges that by using the cyclic, the aerodynamic effects are negated.
This allows the understanding of flapping to equality, removes confusion about which plane of reference we are are using or which rotor system or how much phase lag or many other variables that have muddied the waters in such discussions.
Then it is clear that whatever the situation, a change in circumstances (such as a gust of wind or some turbulence) will first create an inequality of lift and secondly, that the pilot will correct it to remain in his desired attitude/stage of flight.
We can then allow the idea that if the pilot changes the collective position (say in forward flight, without moving the cyclic) then an inequality of lift will be produced that he must again correct with cyclic.
In summary - air tries to do stuff to rotor, pilot does stuff to controls to oppose it.
This both accepts that the aerodynamic effects exist (especially when trying to explain how the rotor behaves to students) and acknowledges that by using the cyclic, the aerodynamic effects are negated.
This allows the understanding of flapping to equality, removes confusion about which plane of reference we are are using or which rotor system or how much phase lag or many other variables that have muddied the waters in such discussions.
Then it is clear that whatever the situation, a change in circumstances (such as a gust of wind or some turbulence) will first create an inequality of lift and secondly, that the pilot will correct it to remain in his desired attitude/stage of flight.
We can then allow the idea that if the pilot changes the collective position (say in forward flight, without moving the cyclic) then an inequality of lift will be produced that he must again correct with cyclic.
In summary - air tries to do stuff to rotor, pilot does stuff to controls to oppose it.
Crab - yeah, now yer talkin'!
In the real aircraft, as distinct from the theory classroom, the student's job is to select and hold an attitude. Various things are going to try to change the attitude, and Bloggs' job is to do whatever is necessary with the cyclic to maintain the attitude.
A bit like balancing a stick on your fingertip - he is trying to keep the tip of the stick steady, and moves his hand in whatever way is needed to achieve the result - no thinking necessary, just bluddy DO IT!
In the real aircraft, as distinct from the theory classroom, the student's job is to select and hold an attitude. Various things are going to try to change the attitude, and Bloggs' job is to do whatever is necessary with the cyclic to maintain the attitude.
A bit like balancing a stick on your fingertip - he is trying to keep the tip of the stick steady, and moves his hand in whatever way is needed to achieve the result - no thinking necessary, just bluddy DO IT!
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Progress
Sure Crab - it's significant progress.
It's been hard to get this far, enduring insults and untrue rudeness.
Thanks AC - you seem to have a grip on what's going on.
Which reference plane (or axis) do people use when they talk about flapping?
It's been hard to get this far, enduring insults and untrue rudeness.
Thanks AC - you seem to have a grip on what's going on.
Which reference plane (or axis) do people use when they talk about flapping?
AnFI;
Always a point of discussion in bearingless rotor design circles at Bell. I was responsible for structural analysis of main and tail rtrs and I used the angle between the flat of the blade attach joint and the axis of the main rotor mast.
Always a point of discussion in bearingless rotor design circles at Bell. I was responsible for structural analysis of main and tail rtrs and I used the angle between the flat of the blade attach joint and the axis of the main rotor mast.
You might find that no-one really cares
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Best practical example of the worlds first true rigid rotor
The worlds arguably first true rigid rotor and the real life example of what happens to uncorrected flap back and in-flow roll is the Aust Aboriginal returning boomerang.
As the boomerang is thrown, the increasing airspeed coupled with the rotational velocity causes the advancing blade to flap back and make the boomerang to both climb - (resulting in a decreasing airspeed).
Simultaneously, the disymetry of lift on the advancing side of the disc causes the boomerang to turn toward the retreating blade side (inflow roll).
As the IAS decreases to zero at the boomerang's apex, it momentarily hovers until gravity takes over and potential energy of height is converted once again into airspeed as it dives toward the ground. Once again as IAS increases, the flap back/blowback pulls the boomerang out of its dive and miraculously levels it out with a slight climbing attitude as energy progressively is lost from the system.....now heading back toward where the thrower was standing originally.....
Some might say "white man magic" .....or is it simply a fantastic practical demonstration of aerodynamics at work and its effects on a true rigid rotor???....and perhaps the best practical demonstration flap back and in-flow roll (disymetry of lift)
As the boomerang is thrown, the increasing airspeed coupled with the rotational velocity causes the advancing blade to flap back and make the boomerang to both climb - (resulting in a decreasing airspeed).
Simultaneously, the disymetry of lift on the advancing side of the disc causes the boomerang to turn toward the retreating blade side (inflow roll).
As the IAS decreases to zero at the boomerang's apex, it momentarily hovers until gravity takes over and potential energy of height is converted once again into airspeed as it dives toward the ground. Once again as IAS increases, the flap back/blowback pulls the boomerang out of its dive and miraculously levels it out with a slight climbing attitude as energy progressively is lost from the system.....now heading back toward where the thrower was standing originally.....
Some might say "white man magic" .....or is it simply a fantastic practical demonstration of aerodynamics at work and its effects on a true rigid rotor???....and perhaps the best practical demonstration flap back and in-flow roll (disymetry of lift)
See what I mean? No mention of reference plane or cyclic feathering at all
A child's bamboo rotor on a stick, sold at fairs or markets, is another example of a rigid rotor being unable to cope with a relative wind. (the Chinese had these as a toy about the same time the darker brothers were hunting kangaroos for a living)
Spin it in nil wind, it rises a bit, then flutters to the ground.
Do it in a wind and the flapback makes the oscillations look really ugly.
Spin it in nil wind, it rises a bit, then flutters to the ground.
Do it in a wind and the flapback makes the oscillations look really ugly.
AnFI - you are the one with all the answers (allegedly), enlighten us with your consistent and proven theories
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Alledged by you? (not me) All answers too much...
I did already - but don't think you were receptive then...
1 Flapping as defined by the hub plane is great for seeing what a flapping hinge (or equivalent) has to achieve - but the flapping thus defined doesn't show us what is going on aerodynamically - it just shows us the difference between Tip Path Plane and Hub Plane.
2 Defining it against the Control Plane just shows us what the flapping would be if the pitch were not varying cyclically (Pitch Change and Flapping just being equal and opposite equivalents by that definition).
3 Personally I like Flapping as defined against the Tip Path Plane - just because it more purely indicates what is going on in a helicopter. This way any half-cycle asymmetries, which are not cancelled by cyclic pitch change, result in Flapping (to equality) - the beauty being that a pilot does not NEED to know this - he just sits there and makes the attitude look the way he thinks it should look - but he is infact (secretly
) making the lift equal around the disk, and de-facto applying asymmetric pitch which accounts for (almost) all effects known (and unknown - (DR))
4 Flapping as defined by the Horizon is a possibility but doesn't make much sense in a wingover !! AC!
The problem is - a helicopter is just much simpler than suits everybody - I think you (Crab) agree in cases like the (bogus) gyroscopic precession doctrine?
1 Flapping as defined by the hub plane is great for seeing what a flapping hinge (or equivalent) has to achieve - but the flapping thus defined doesn't show us what is going on aerodynamically - it just shows us the difference between Tip Path Plane and Hub Plane.
2 Defining it against the Control Plane just shows us what the flapping would be if the pitch were not varying cyclically (Pitch Change and Flapping just being equal and opposite equivalents by that definition).
3 Personally I like Flapping as defined against the Tip Path Plane - just because it more purely indicates what is going on in a helicopter. This way any half-cycle asymmetries, which are not cancelled by cyclic pitch change, result in Flapping (to equality) - the beauty being that a pilot does not NEED to know this - he just sits there and makes the attitude look the way he thinks it should look - but he is infact (secretly
) making the lift equal around the disk, and de-facto applying asymmetric pitch which accounts for (almost) all effects known (and unknown - (DR))4 Flapping as defined by the Horizon is a possibility but doesn't make much sense in a wingover !!
AC!The problem is - a helicopter is just much simpler than suits everybody - I think you (Crab) agree in cases like the (bogus) gyroscopic precession doctrine?
Last edited by AnFI; 31st Oct 2013 at 23:55.
