Blade Flapping
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Blade Flapping
I have dredged the archives for an answer to my following question and could not find an adequate solution, so apologies if this topic has already been discussed and strangled to a long, slow painful death.
I am aware that some blade flapping takes place in a semi rigid rotor head within the actual blade, as opposed to the teetering head. As a %, how much of the flapping to equality is carried out by the blade and how much by the rotor head?
I am also of the belief that a rigid head does not have any tolerances for flapping to equality, all flapping is carried out by the blades, is this 100% correct?
Finally, how much flapping is carried out in a fully articulated rotor head by the flapping hinges and how much by the blades?
Thanks
I am aware that some blade flapping takes place in a semi rigid rotor head within the actual blade, as opposed to the teetering head. As a %, how much of the flapping to equality is carried out by the blade and how much by the rotor head?
I am also of the belief that a rigid head does not have any tolerances for flapping to equality, all flapping is carried out by the blades, is this 100% correct?
Finally, how much flapping is carried out in a fully articulated rotor head by the flapping hinges and how much by the blades?
Thanks
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I cannot give you percentages but the amout of flap of the blade itself on a system that has a hinge to allow flapping is minimal to nil. The blade will only flap enough until it overcomes the friction of the flapping hinge. Take a helicopter on the ground for example (r22 in my view)
As you push up on the blade the blade will flex until you overcome the friction of the tettering bearing and which point the whole unit will move. Now pretend that the blades do not have any flex, almost as though they are spinning and centrifugal force is holding them solid. If you try and push up the opposing blade the resistance of the hinge is not enough to overcome the rigidity of the blade.
Are you perhaps confusing it with the blade absorbing the lead and lag forces?
My not so engineering background opinion
As you push up on the blade the blade will flex until you overcome the friction of the tettering bearing and which point the whole unit will move. Now pretend that the blades do not have any flex, almost as though they are spinning and centrifugal force is holding them solid. If you try and push up the opposing blade the resistance of the hinge is not enough to overcome the rigidity of the blade.
Are you perhaps confusing it with the blade absorbing the lead and lag forces?
My not so engineering background opinion
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dog-
It is my belief that the individual flapping for a teetering rotor blade is nearly nil, as the opposite blade would have to restrain the flapping force to make the blade bend, and this is unlikely. Of course, the coning of the blades would be appreciable.
For rigid systems like the Boelkow and the Comanche, we speak of equivilent offset, which equates the rotor head moment created by a rigidly attached blade to that moment an articulated blade would impart. We speak of the offset as a percent of radius, as if a hinge were at that place and the blade was attached to that flapping hinge. For a Boelkow, the equivilent hinge would be at 12 to 15% of span, for Comanche it would be at 11%. Most newer articulated systems have flapping hinges at 4 to 5% of span, so "rigid" systems are considerable stiffer than articulated ones.
It is my belief that the individual flapping for a teetering rotor blade is nearly nil, as the opposite blade would have to restrain the flapping force to make the blade bend, and this is unlikely. Of course, the coning of the blades would be appreciable.
For rigid systems like the Boelkow and the Comanche, we speak of equivilent offset, which equates the rotor head moment created by a rigidly attached blade to that moment an articulated blade would impart. We speak of the offset as a percent of radius, as if a hinge were at that place and the blade was attached to that flapping hinge. For a Boelkow, the equivilent hinge would be at 12 to 15% of span, for Comanche it would be at 11%. Most newer articulated systems have flapping hinges at 4 to 5% of span, so "rigid" systems are considerable stiffer than articulated ones.
wossupdog, don't let the terminology confuse you, a semi-rigid or rigid rotor system simply lacks some or all mechanical hinges; the blade must still flap, drag and twist but the rotor head is made of materials that deform sufficiently to allow this. For example on a Lynx (semi rigid with about 17% effective hinge offset) the feathering is permitted by a mechanical sleeve and bearing and flapping and dragging are permitted by bending of the titanium rotor head and the blade root respectively. The blades still flap to equality but the movement of the blades is controlled by the materials of the rotor head and not hinges.
The main advantages of semi rigid/rigid heads are ease of manufacture/maintenance and control power; control power can be defined as how much the fuselage responds to a given control input ie snappier pitch and roll response. The blades act as a lever on the fuselage and they force it to follow their flapping - the bigger the lever the faster the fuselage response to a control input. This is what the percentage of effective hinge offset indicates - the higher the percentage the faster the response.
The main advantages of semi rigid/rigid heads are ease of manufacture/maintenance and control power; control power can be defined as how much the fuselage responds to a given control input ie snappier pitch and roll response. The blades act as a lever on the fuselage and they force it to follow their flapping - the bigger the lever the faster the fuselage response to a control input. This is what the percentage of effective hinge offset indicates - the higher the percentage the faster the response.
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Hopefully, Nick or someone else here with real engineering knowledge will correct everything I get wrong in this answer, so here goes...
It seems to me that the each blade would flap on the "coning" hinge, and then the balance of forces applyed to each coning hinge would then teeter the hub on the teetering hinge.
At the factory course (if my memory serves) they claim that all the movement on the coning hinges ceases after the disk is coned, and that all further flapping (flapping to equality) to takes place on the teetering hinge.
The Robinson rotorhead is always called semi-rigid, but I prefer the term semi-articulated.
It seems to me that the each blade would flap on the "coning" hinge, and then the balance of forces applyed to each coning hinge would then teeter the hub on the teetering hinge.
At the factory course (if my memory serves) they claim that all the movement on the coning hinges ceases after the disk is coned, and that all further flapping (flapping to equality) to takes place on the teetering hinge.
The Robinson rotorhead is always called semi-rigid, but I prefer the term semi-articulated.
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There is around 10 tons of force pulling across the coning hinges of the R22 - the offset of the hinges means there will be little (if any) flapping going on there. OTOH, there is nothing to resist the flapping at the teetering hinge.
Robinson Rotors
The rotor teeters about the teetering hinge in the middle. The coning hinges simply allow the rotor to cone and hence reduces blade bending loads and control loads.
If you do a search on the US Patent site you'll find Frank Robinsons original patent (and explanation) of the design and how it works.
Hope this helps
CRAN
If you do a search on the US Patent site you'll find Frank Robinsons original patent (and explanation) of the design and how it works.
Hope this helps
CRAN
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Retreating blade flapping down
Regarding dissimetry of lift and blowback.
It makes sense to me why the advancing blade flaps up on a teetering rotor but I can't seem to get my mind around what makes the retreating blade flap down.
The only thing I can see that would make the retreating blade flap down is that the plane of rotation is tilted upwards by the advancing blade flaping up, which wound make the retreating blade flap down only because it is staying in it's plane of rotation.
I think this theory is riddled with holes.
First post here, not sure if it is in the right place.
It makes sense to me why the advancing blade flaps up on a teetering rotor but I can't seem to get my mind around what makes the retreating blade flap down.
The only thing I can see that would make the retreating blade flap down is that the plane of rotation is tilted upwards by the advancing blade flaping up, which wound make the retreating blade flap down only because it is staying in it's plane of rotation.
I think this theory is riddled with holes.
First post here, not sure if it is in the right place.
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The retreating blade is going very much slower, isn't it? (often by around 200 kts) Therefore it isn't producing so much lift and can't stay up anyway.
phil
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Once you have the rigid and semi-rigid and not mentioned fully articulated heads under control, you realise you have to change all you understand due the spheriflex and starflex heads. The mentioned 3 depends on mechanical hinges limited by a mechanical travel stop or limit. Simply put. Blade must take it from here or you feel it complain in the cockpit as it can not compensate.
How much the blade flap itself depends on its design and construction-metal vs metal spar vs composite. On a 135, 105 etc the blade does it all. On the 350, 120, 365, 332l2 for instance with the starflex/spheriflex heads, the composite blades as well as the head flap and drag. Their are no mechanical hinges in the head, just a laminated spherical stop which balances aerodynamic force vs its own tension torsion design. Simplist way to explain the laminated bearings used extensively is: flexible in shear, stiff in compression. Thus almost like the leaf springs on the rear suspension of a older pick up truck. Together with the laminated spherical stop onto which the blade/ blade sleeves mount is a droop ring that allows the head to tilt around the mast to further allow for movement and correction on the other side. Working of the concept is easiest if you pull down on a 350/120 blade and let it go. You will see the 3 dimensional movement of the droop ring, sleeves and the heart of the system the sperical stop. Add a composite blade and their is loads of movement and compensation for wind, imbalance, etc in the design.
How much the blade flap itself depends on its design and construction-metal vs metal spar vs composite. On a 135, 105 etc the blade does it all. On the 350, 120, 365, 332l2 for instance with the starflex/spheriflex heads, the composite blades as well as the head flap and drag. Their are no mechanical hinges in the head, just a laminated spherical stop which balances aerodynamic force vs its own tension torsion design. Simplist way to explain the laminated bearings used extensively is: flexible in shear, stiff in compression. Thus almost like the leaf springs on the rear suspension of a older pick up truck. Together with the laminated spherical stop onto which the blade/ blade sleeves mount is a droop ring that allows the head to tilt around the mast to further allow for movement and correction on the other side. Working of the concept is easiest if you pull down on a 350/120 blade and let it go. You will see the 3 dimensional movement of the droop ring, sleeves and the heart of the system the sperical stop. Add a composite blade and their is loads of movement and compensation for wind, imbalance, etc in the design.
Remember, though, that "flapping to equality" is negated as soon as you put some cyclic in to stop the blade flapping away from the relative wind and to maintain forward speed.
In forward flight, when the disc is tilted forward, the advancing blade is actually flapping down and the retreating blade is flapping up - or else the disc would not be tilted forwards , would it? The cyclic feathering overcomes the desire to flap to equality. Go back to neutral cyclic, and you are back in the dynamically unstable flapping to equality.
In forward flight, when the disc is tilted forward, the advancing blade is actually flapping down and the retreating blade is flapping up - or else the disc would not be tilted forwards , would it? The cyclic feathering overcomes the desire to flap to equality. Go back to neutral cyclic, and you are back in the dynamically unstable flapping to equality.
But you have tilted the control orbit to make the tip path plane try to follow it. Although the rotor disc is inclined forwards, the blades are still flapping relative to the control orbit (as that is what dictates each blades pitch angle) not the horizon.
Therefore the retreating blades are flapping down with regard to the control orbit even though they appear to be going up with regard to the horizon. How else do you explain the high AoA on the retreating side (which we know is true) and the phenomenon of retreating blade stall?
Therefore the retreating blades are flapping down with regard to the control orbit even though they appear to be going up with regard to the horizon. How else do you explain the high AoA on the retreating side (which we know is true) and the phenomenon of retreating blade stall?
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Advancing blade-relative wind. Retreating blade- no relative wind. Result=dyssemetrical lift. Solution-flapping or teetering head opposite equal reaction. Other heads-retreating flap in order to increase pitch thus produce same as advancing blade having a holiday due relative wind. Purpose of ddrag hinge is to absorb increase in drag on retreating blade due increase in pitch thus blade allowed to "fall behind" and catch up again when it enters the advancing halve of the disc. Retreatingblade stall due higher pitch on retreating blade so higher the pitch on the disc, the smaller the stall margin on the retreating side.
Could have it completely wrong, but thats how I understand it in simplicity?
Could have it completely wrong, but thats how I understand it in simplicity?
Therefore the retreating blades are flapping down with regard to the control orbit even though they appear to be going up with regard to the horizon. How else do you explain the high AoA on the retreating side (which we know is true) and the phenomenon of retreating blade stall?
At 90 degrees right, the blade is at its minimum pitch, and has its maximum rate of flapping down. It reaches its lowest point of flapping down at the front, while the pitch angle has started to increase via the swash plate.
At 90 degrees left (retreating side) the blade has its maximum pitch angle, and has its maximum rate of flapping UP. It reaches its highest point on the orbit at the rear, where the blade pitch has started to decrease again, and then back to 90 right where it has minimum pitch and max rate of DOWN flap.
If you need convincing, look at the sine wave depictions of the pitch angle and the blade position, separated by approximately 90 degrees - phase lag.
Because of the huge differences in relative wind in forward flight, the advancing blade has to throw away huge amounts of lift (smaller pitch angles / alpha) to match the pitiful amount produced by the retreating side (with bigger and bigger alpha)- a ratio about 7:1. Eventually, the tiny part of the retreating blade with a positive airflow reaches its stalling alpha, and away we go. THAT is why you have retreating blade stall, not because it is "flapping to equality".
This is why the coaxial counter-rotating blades are so efficient - they can operate with a smaller rotor diameter and higher forward speeds because there is no dependence on the retreating side to produce lift.
You assume that the tip path plane and the control orbit are parallel but the final position of the disc is a combination of the control orbit and the aerodynamic effects.
The blade loses lift on the retreating side and tries to flap down - the cyclic input and therefore the control orbit adds pitch to combat that - the resultant is the high AoA on the retreating side.
Max pitch is at the 9 o'clock as you say but the RBS occurs after that position (hence the pitch up along with the roll) as pitch is starting to decrease so the blade must be flapping down with regard to the relative airflow.
The opposition of the blade trying to flap down and the pitch change arms trying to force it up is why the aerodynamic back load are at the highest just aft of the 9 o'clock (it is why jackstall occurs on gazelle just past the 3 o'clock position - French direction of rotation).
The blade loses lift on the retreating side and tries to flap down - the cyclic input and therefore the control orbit adds pitch to combat that - the resultant is the high AoA on the retreating side.
Max pitch is at the 9 o'clock as you say but the RBS occurs after that position (hence the pitch up along with the roll) as pitch is starting to decrease so the blade must be flapping down with regard to the relative airflow.
The opposition of the blade trying to flap down and the pitch change arms trying to force it up is why the aerodynamic back load are at the highest just aft of the 9 o'clock (it is why jackstall occurs on gazelle just past the 3 o'clock position - French direction of rotation).
The tip path plane is the real judge of what is going on.
At the front in forward flight, the tips and the TPP are at its lowest position - relative to ANYTHING - and it then climbs around the retreating side to reach its highest point over the tail boom. It has flapped UP to get there, and then it flaps DOWN to get back to the front. Relative to the horizon, relative to the aircraft, relative to your maiden aunt, still flaps down at the front and up at the back.
The advancing blade has to dump most of its lift to match the 25% of the retreating blade that is still producing lift.
As you say, the TPP doesn't necessarily match the swash plate - I think the BO105 has its swash plate working at 90 degrees to the direction of travel? All depends on where the pickoffs are.
At the front in forward flight, the tips and the TPP are at its lowest position - relative to ANYTHING - and it then climbs around the retreating side to reach its highest point over the tail boom. It has flapped UP to get there, and then it flaps DOWN to get back to the front. Relative to the horizon, relative to the aircraft, relative to your maiden aunt, still flaps down at the front and up at the back.
The advancing blade has to dump most of its lift to match the 25% of the retreating blade that is still producing lift.
As you say, the TPP doesn't necessarily match the swash plate - I think the BO105 has its swash plate working at 90 degrees to the direction of travel? All depends on where the pickoffs are.
Assume you are in a perfect hover, disc level, no wind.
No cyclic is applied, no flapping anywhere.
Along comes a rogue puff of wind, for convenience it is from the front. There is now a relative wind, and an advancing side and a retreating side.
The advancing side has a higher relative airflow, the lift generated is proportional to the square of the wind velocity, so it develops more lift than the retreating side. The force generated makes the blade start to fly up - the balance between centrifugal force and rotor lift has been upset. As it flies upwards, the induced flow component increases until it reduces the angle of attack and thus the lift, until the balance is restored again.
On the retreating side, as well as being pushed down by the teetering effect of the advancing blade flying up, there is less relative airflow, so the blade falls downwards. As it does so, the induced airflow is reduced, increasing the angle of attack and the lift, until the balance is restored between rotor lift and centrifugal force. But this flying up and flying down takes time - the force on the blade is instant, but acceleration up or down takes time to establish itself - about 90 degrees - and it is likened to the precessional property of a gyroscope, to allow us simple pilots to understand what is happening.
The end result of this puff of wind is that the blades have flapped to equality, but now the disc is tilted up at the front and down at the back. Where is the rotor thrust now pointing? Backwards! So, the aircraft, which was in a steady hover before, will now move backwards away from the wind.
It moves backwards more and more, until there is now a relative wind from behind. The disc flaps away from that, but the pendulous effect of the fuselage hanging under the disc makes the reversal of direction a bit more pronounced, and the disc is now tilted even further down to the front.
This is the start of dynamic instability.
All this time, the cyclic has been held in a neutral position. The pilot decides he doesn't want this to continue, so he stops the oscillation, and returns to a steady hover in the new headwind. The fuselage is stationary, but the disc is tilted forward to overcome the wind, and the cyclic feathering all around the disc is stopping the disc from flapping to equality. The equality of lift is achieved by the cyclic feathering.
No cyclic is applied, no flapping anywhere.
Along comes a rogue puff of wind, for convenience it is from the front. There is now a relative wind, and an advancing side and a retreating side.
The advancing side has a higher relative airflow, the lift generated is proportional to the square of the wind velocity, so it develops more lift than the retreating side. The force generated makes the blade start to fly up - the balance between centrifugal force and rotor lift has been upset. As it flies upwards, the induced flow component increases until it reduces the angle of attack and thus the lift, until the balance is restored again.
On the retreating side, as well as being pushed down by the teetering effect of the advancing blade flying up, there is less relative airflow, so the blade falls downwards. As it does so, the induced airflow is reduced, increasing the angle of attack and the lift, until the balance is restored between rotor lift and centrifugal force. But this flying up and flying down takes time - the force on the blade is instant, but acceleration up or down takes time to establish itself - about 90 degrees - and it is likened to the precessional property of a gyroscope, to allow us simple pilots to understand what is happening.
The end result of this puff of wind is that the blades have flapped to equality, but now the disc is tilted up at the front and down at the back. Where is the rotor thrust now pointing? Backwards! So, the aircraft, which was in a steady hover before, will now move backwards away from the wind.
It moves backwards more and more, until there is now a relative wind from behind. The disc flaps away from that, but the pendulous effect of the fuselage hanging under the disc makes the reversal of direction a bit more pronounced, and the disc is now tilted even further down to the front.
This is the start of dynamic instability.
All this time, the cyclic has been held in a neutral position. The pilot decides he doesn't want this to continue, so he stops the oscillation, and returns to a steady hover in the new headwind. The fuselage is stationary, but the disc is tilted forward to overcome the wind, and the cyclic feathering all around the disc is stopping the disc from flapping to equality. The equality of lift is achieved by the cyclic feathering.
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Narrowing in
Flapback is what happens if the pilot does not relocate the cyclic stick to counter Dissymetry of Lift.
Flapping to equality is how the lift is kept equal around the disk if the pilot does not move the cyclic.
Flapback serves as feedback to inform the pilot that the stick must be moved.
Once the pilot has moved the stick to the correct place (running Dissymetry of Pitch) the Dissymetry of Lift has been cancelled and Flapback no longer occurs.
(Pleased to see this unconventional approach becoming common ground here now)
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A simple way to think about a rotor blade is that (almost) ANY effect that persists for a half cycle would cause a change of plane of rotation and need to be cancelled with asymmetric pitch probably from the cyclic stick.
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The matter of the Plane against which flapping is measured is again going to confuse the hell out of everyone.
Measuring flapping against the Control Orbit ( or Control Axis or Zero Pitch Plane) is utterly pointless and throws up incorrect statements (like "Max pitch is at the 9 o'clock" - not relative to the Zero Pitch Plane it isn't !)
I believe the most common definition of Flapping is concerned with how a blade would move around it's flapping hinge (if it has one).
The other useful definition of Flapping concerns the movement of the blade relative to the TPP - which is useful because it gives you an idea of how the plane of rotation will change - as in Flaback for instance.
Measuring Flapping against the Control Orbit has little practical use
Flapping to equality is how the lift is kept equal around the disk if the pilot does not move the cyclic.
Flapback serves as feedback to inform the pilot that the stick must be moved.
Once the pilot has moved the stick to the correct place (running Dissymetry of Pitch) the Dissymetry of Lift has been cancelled and Flapback no longer occurs.
(Pleased to see this unconventional approach becoming common ground here now)
++++++++++++++++++++++++++++++++++++++++++++++++++++
A simple way to think about a rotor blade is that (almost) ANY effect that persists for a half cycle would cause a change of plane of rotation and need to be cancelled with asymmetric pitch probably from the cyclic stick.
++++++++++++++++++++++++++++++++++++++++++++++++++++
The matter of the Plane against which flapping is measured is again going to confuse the hell out of everyone.
Measuring flapping against the Control Orbit ( or Control Axis or Zero Pitch Plane) is utterly pointless and throws up incorrect statements (like "Max pitch is at the 9 o'clock" - not relative to the Zero Pitch Plane it isn't !)
I believe the most common definition of Flapping is concerned with how a blade would move around it's flapping hinge (if it has one).
The other useful definition of Flapping concerns the movement of the blade relative to the TPP - which is useful because it gives you an idea of how the plane of rotation will change - as in Flaback for instance.
Measuring Flapping against the Control Orbit has little practical use