Dissymmetry of lift, flapping and blowback...
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It will be interesting to see if the CarterCopter is able to take its blades supersonic. Particularly, since they will probably go from subsonic, to supersonic and then back, each revolution.
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Time Index Table
I worked up this time index table for the first complete rotation of the rotor blade in the video clip. The clip includes 2 complete rotations. If anyone desires, they can use this time index table to determine where the rotor blade is at a given time in the clip. You can see that the rotation is counter-clockwise.
The table starts the time index at zero degrees, where the leading edge of the rotor blade first visually aligns with the tail rotor disk. The ending time index occurs when the leading edge again aligns with the tail rotor disk. The table's values are calculated and assume a constant rate of rotation.
Degrees Time Index
-------------------------------
0 3.50 (start index)
10 3.76
20 4.02
30 4.28
40 4.54
50 4.81
60 5.07
70 5.33
80 5.59
90 5.85
100 6.11
110 6.37
120 6.63
130 6.89
140 7.16
150 7.42
160 7.68
170 7.94
180 8.20
190 8.46
200 8.72
210 8.98
220 9.24
230 9.51
240 9.77
250 10.03
260 10.29
270 10.55
280 10.81
290 11.07
300 11.33
310 11.59
320 11.86
330 12.12
340 12.38
350 12.64
360 12.90 (end index)
Enjoy.
The table starts the time index at zero degrees, where the leading edge of the rotor blade first visually aligns with the tail rotor disk. The ending time index occurs when the leading edge again aligns with the tail rotor disk. The table's values are calculated and assume a constant rate of rotation.
Degrees Time Index
-------------------------------
0 3.50 (start index)
10 3.76
20 4.02
30 4.28
40 4.54
50 4.81
60 5.07
70 5.33
80 5.59
90 5.85
100 6.11
110 6.37
120 6.63
130 6.89
140 7.16
150 7.42
160 7.68
170 7.94
180 8.20
190 8.46
200 8.72
210 8.98
220 9.24
230 9.51
240 9.77
250 10.03
260 10.29
270 10.55
280 10.81
290 11.07
300 11.33
310 11.59
320 11.86
330 12.12
340 12.38
350 12.64
360 12.90 (end index)
Enjoy.
Last edited by Flight Safety; 2nd Dec 2003 at 09:35.
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Flight Safety,
Using your plotting, the tip is at its maximum height when the blade is at 270-degrees. This would make sense since the lift on a retreating blade is concentrated near the tip. The tip is lowest a 0-degrees and this may be the combined result of the lift moving inward on the blade while the tip is springing back down.
Nick,
Basically, the text says that the camera is mounted on the leading edge of the blade (presumably, this means outboard of the flapping hinge). It also says that if the blade were not allow to flex, the rotorhead, gearbox and/or fuselage would be destroyed.
I have difficulty with this statement about flexure; when applied to future rotorcraft. It would appear that an absolutely rigid rotor will cause little more vibration then do the wings of a plane ~ after blade-to-blade, rotor-to-rotor and rotor-to-fuselage aerodynamic interactions have been addressed, and the tail rotor has been discarded.
Using your plotting, the tip is at its maximum height when the blade is at 270-degrees. This would make sense since the lift on a retreating blade is concentrated near the tip. The tip is lowest a 0-degrees and this may be the combined result of the lift moving inward on the blade while the tip is springing back down.
Nick,
Basically, the text says that the camera is mounted on the leading edge of the blade (presumably, this means outboard of the flapping hinge). It also says that if the blade were not allow to flex, the rotorhead, gearbox and/or fuselage would be destroyed.
I have difficulty with this statement about flexure; when applied to future rotorcraft. It would appear that an absolutely rigid rotor will cause little more vibration then do the wings of a plane ~ after blade-to-blade, rotor-to-rotor and rotor-to-fuselage aerodynamic interactions have been addressed, and the tail rotor has been discarded.
I have found this translation in another forum:
""The rotortips absorb the turbulence with their flexibility. This recording was made, during flight, by a camera mounted to the leading edge of the rotorblade. At every turn you can see the tailrotor. If the flexible rotorblades wouldn't absorb the the forces, they would be transmitted to drive mechanism and fuselage and the vibration would destroy the helicopter.""
Regards.
""The rotortips absorb the turbulence with their flexibility. This recording was made, during flight, by a camera mounted to the leading edge of the rotorblade. At every turn you can see the tailrotor. If the flexible rotorblades wouldn't absorb the the forces, they would be transmitted to drive mechanism and fuselage and the vibration would destroy the helicopter.""
Regards.
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Dissymmetry of lift, flapping and blowback...
Greetings all!
First post from a CPL-H (FAA) student:
Had an interesting discussion today on what should be basic aerodynamics, leaving a couple of unanswered questions.
I have looked in my books (Wagtendonk, Coyle, RFH) and searched the Rotorheads forum but haven't found a clear answer so far.
I'm sure someone here will come up with a reasonable explanation:
1. FLAPPING AND BLOWBACK
Statement 1: "Flapping will automatically and perfectly balance the dissymmetry of lift between the retreating and advancing side. The blowback is a side-effect of the flapping caused by the blade reaching it's highest position over the nose and lowest over the tail."
Statement 2: "Flapping is only partially eliminating dissymmetry of lift, the remaining difference in lift causes the blade to start rising on the advancing side and the blade to reach its highest position over the nose and result in blowback."
Hopefully one of the statements is close to the truth. I would very much appreciate a clue to which one is most correct and why.
2. DISSYMMETRY OF LIFT AND FLAPPING
If Statement 1 above is the most correct, can someone describe the mechanics behind the blades flapping the exact amount necessarry to achieve equal lift on both sides?
Looking forward to read your responses!
First post from a CPL-H (FAA) student:
Had an interesting discussion today on what should be basic aerodynamics, leaving a couple of unanswered questions.
I have looked in my books (Wagtendonk, Coyle, RFH) and searched the Rotorheads forum but haven't found a clear answer so far.
I'm sure someone here will come up with a reasonable explanation:
1. FLAPPING AND BLOWBACK
Statement 1: "Flapping will automatically and perfectly balance the dissymmetry of lift between the retreating and advancing side. The blowback is a side-effect of the flapping caused by the blade reaching it's highest position over the nose and lowest over the tail."
Statement 2: "Flapping is only partially eliminating dissymmetry of lift, the remaining difference in lift causes the blade to start rising on the advancing side and the blade to reach its highest position over the nose and result in blowback."
Hopefully one of the statements is close to the truth. I would very much appreciate a clue to which one is most correct and why.
2. DISSYMMETRY OF LIFT AND FLAPPING
If Statement 1 above is the most correct, can someone describe the mechanics behind the blades flapping the exact amount necessarry to achieve equal lift on both sides?
Looking forward to read your responses!
Last edited by Trimstring; 24th Jun 2005 at 02:07.
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I'd venture saying that statement 1 isn't 100% true. Flapping will not balance dissymmetry of lift perfectly. You'de still have a rolling tendency according to the FAA RFH.
The way I understand it, the advancing blade would still provide more lift as it rises towards the 12 o clock position, causing the left roll in a anti-clockwise system.
Happens in the R22. Thus the trim pulling thingy.
By no means an expert answer, more part of the discussion.
The way I understand it, the advancing blade would still provide more lift as it rises towards the 12 o clock position, causing the left roll in a anti-clockwise system.
Happens in the R22. Thus the trim pulling thingy.
By no means an expert answer, more part of the discussion.
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Statement 1 is pretty close to the truth as I understand it, although the word perfectly would be best left out. The pilot balances minor errors in the system with a little cyclic input.
The mechanism of this is that when the blade has a faster relative airflow, it generates more lift and so flaps up.
As the blade flaps up, the angle of attack is reduced so the lift is reduced.
Low airspeed on retreating side, less lift. Blade flaps down so angle of attack is increased to increase lift. this more or less balances things out.
Blowback is what you describe. (Flapback in UK) Pilot counters it by applying forward cyclic.
Simple innit.
(actually there are a few more things to throw into the equation but thats basically it- unless someone shoots me down in flames)
Edit to remove misleading and irrelevant reference to pitch horn placement.
The mechanism of this is that when the blade has a faster relative airflow, it generates more lift and so flaps up.
As the blade flaps up, the angle of attack is reduced so the lift is reduced.
Low airspeed on retreating side, less lift. Blade flaps down so angle of attack is increased to increase lift. this more or less balances things out.
Blowback is what you describe. (Flapback in UK) Pilot counters it by applying forward cyclic.
Simple innit.
(actually there are a few more things to throw into the equation but thats basically it- unless someone shoots me down in flames)
Edit to remove misleading and irrelevant reference to pitch horn placement.
Last edited by Gaseous; 24th Jun 2005 at 02:32.
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One of my favourates, part of the myths
Statement 1 : false if it means blade lift is equal.
Equilibrium
For the sake of simplicity think of the rotor as a theatering rotor. That means hinges are free to move. As a pilot you know the rotor-disk settles quite fast in a given situation. In this situation forces of aerodynamic and inertial origin are in equilibrium. Flapping means blades go up and down when going around exactly to achieve this equilibrium. In each state an overall trust (= time average over one revolution) results that is almost perpendicular to the disk. That does not mean the lift of each blade is the same, because the lift is only part of the equilibrium, the inertial forces that makes the blades go up and down account for the other part. In many other threads intrincancies of this dynamic system have been discussed. In forward flight the forward blade will produce much more lift while maintaining equilibrium, that is not roll the disk and as such the heli.
So 'neutralising' the dissymetry of lift does not mean each blades produces the same lift, it means aero- and inertial forces are in equilibrium.
Blow back
Changing one parameter, namely increasing the wind speed, while keeping all other parameters (such as cyclic input) equal creates a new equilibrium. It creates more lift an the forward blade, less on the retreating blade and this makes the whole disk tilt backwards (again this is a simplification, it may not go precisely backwards but at a certain angle, but again that is a long story)
Statement 2 : correct if interpreted in the above sense.
It is important to make a difference between stable flapping (that means flapping of a stable disk) and a change, that is a transient between two stable positions.
Delta3
Equilibrium
For the sake of simplicity think of the rotor as a theatering rotor. That means hinges are free to move. As a pilot you know the rotor-disk settles quite fast in a given situation. In this situation forces of aerodynamic and inertial origin are in equilibrium. Flapping means blades go up and down when going around exactly to achieve this equilibrium. In each state an overall trust (= time average over one revolution) results that is almost perpendicular to the disk. That does not mean the lift of each blade is the same, because the lift is only part of the equilibrium, the inertial forces that makes the blades go up and down account for the other part. In many other threads intrincancies of this dynamic system have been discussed. In forward flight the forward blade will produce much more lift while maintaining equilibrium, that is not roll the disk and as such the heli.
So 'neutralising' the dissymetry of lift does not mean each blades produces the same lift, it means aero- and inertial forces are in equilibrium.
Blow back
Changing one parameter, namely increasing the wind speed, while keeping all other parameters (such as cyclic input) equal creates a new equilibrium. It creates more lift an the forward blade, less on the retreating blade and this makes the whole disk tilt backwards (again this is a simplification, it may not go precisely backwards but at a certain angle, but again that is a long story)
Statement 2 : correct if interpreted in the above sense.
It is important to make a difference between stable flapping (that means flapping of a stable disk) and a change, that is a transient between two stable positions.
Delta3
Welcome Trimstring,
You may need to learn to search a little better, as this topic has been discussed at length. Both statements are partially correct, but both are incomplete, and statement 1 oversimplifies to the point of innaccuracy.
For the detailed explanation, delta 3 is good. For a simple explanation (though suffering simplification): flapping evens out dissymetry of lift due to the pitch horn placement. As the blade flaps up, pitch angle is reduced, thus angle of attack, thus lift. Reverse for retreating side. This, coupled with cyclic corrections to keep the aircraft level is cyclic feathering. So dissymetry of lift is overcome by flapping to equality and cyclic feathering.: ie the equilibrium described by delta 3.
Blowback, or Flapback is very accurately described by delta 3. Not to be confused with inflow roll.
It is discussion like these where we really miss Lu!!
You may need to learn to search a little better, as this topic has been discussed at length. Both statements are partially correct, but both are incomplete, and statement 1 oversimplifies to the point of innaccuracy.
For the detailed explanation, delta 3 is good. For a simple explanation (though suffering simplification): flapping evens out dissymetry of lift due to the pitch horn placement. As the blade flaps up, pitch angle is reduced, thus angle of attack, thus lift. Reverse for retreating side. This, coupled with cyclic corrections to keep the aircraft level is cyclic feathering. So dissymetry of lift is overcome by flapping to equality and cyclic feathering.: ie the equilibrium described by delta 3.
Blowback, or Flapback is very accurately described by delta 3. Not to be confused with inflow roll.
It is discussion like these where we really miss Lu!!
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Is it not the change in relative airflow that change the angle of attack, and not the pitch angle?
Look at several helicopters for example schweizer 300, or cobra and so on, they have the pitch link mounted at the trailing edge, which would make an upflapping blade get an even higher pitch angle thus increasing angle of attack and lift, and make dissymetry of lift even worse?
Look at several helicopters for example schweizer 300, or cobra and so on, they have the pitch link mounted at the trailing edge, which would make an upflapping blade get an even higher pitch angle thus increasing angle of attack and lift, and make dissymetry of lift even worse?
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Thanks for the responses so far!
I found some similar threads from the past but the one I read seemed to drift a bit from the questions I had.
My understanding of this has so far been based on what MortenT says, that the flapping works based on changing the relative airflow because of the up/downward movement. This is the description most textbooks give but it does not seem to be the explanation you guys are giving.
Why is this way of explaining it not correct?
The concept of the flapping movement changing the actual pitch angle of the blade is something I have read is only used to limit flapping as an alterntative to the delta3 hinge(Wagtendonk, page 92).
And when it comes to the S300 which is the helicopter I am training in, I was told that it has a "flapping neutral" pitch horn. This would mean that the pitch links are attached to the blades in line with the flapping hinge and therefore the flapping movement should not change the pitch angle during flapping.
Bonus question: What is the aerodynamic reason for having to apply right force to the cyclic in the R22 in forward flight?
My understanding of this has so far been based on what MortenT says, that the flapping works based on changing the relative airflow because of the up/downward movement. This is the description most textbooks give but it does not seem to be the explanation you guys are giving.
Why is this way of explaining it not correct?
The concept of the flapping movement changing the actual pitch angle of the blade is something I have read is only used to limit flapping as an alterntative to the delta3 hinge(Wagtendonk, page 92).
And when it comes to the S300 which is the helicopter I am training in, I was told that it has a "flapping neutral" pitch horn. This would mean that the pitch links are attached to the blades in line with the flapping hinge and therefore the flapping movement should not change the pitch angle during flapping.
Bonus question: What is the aerodynamic reason for having to apply right force to the cyclic in the R22 in forward flight?
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Well spotted, MortenT. I have altered my original post to make more sense. Too much alcohol. It doesnt matter which side of the blade the pitch horn is on as the effect depends on whether it ends inboard or outboard of the flapping hinge and there are heads with both designs. forget pitch horns in this context.
Well spotted trimstring - it is indeed the change in relative airflow that causes the AoA to change and therefore the amount of lift the blade produces. The delta 3 hinge (or other arrangements that mimic the effect) is most commonly found on tail rotors to limit the amount they can flap - it does reduce pitch as the blade flaps.
Bonus question - flapback (blowback) is one part of flapping to equality caused by differential airspeed between the advancing and retreating sides of the disc (maximum at the 3 and 9 o'clock position) and is overcome with forward cyclic as the aircraft accelerates.
Inflow roll (transverse flow) is also flapping to equality but caused by different amounts of induced flow between the front of the disc and the rear. As the disc is tilted and moves forward, the air entering the forward part of the disc from above has less time to be influenced by the disc than the air that has to cross the whole disc and reach the back of the disc. The angle that the air reaches the blades at the front is therefore closer to the horizontal than the air reaching the back. This means that the Induced Flow (vector diagram) is reduced at the front giving a greater AoA and increased lift. So the blade flaps up until the AoA is reduce again by the new relative airflow (the rest is the same as for flapback regarding flapping) The difference is that the blade high point is in the 9 o'clock (for american rotation) and the aircraft wants to roll towards the advancing side of the disc. This is countered with lateral cyclic. Although the effect is most noticeable during the transition to forward flight, it is still there at all speeds above that and pilots subconciously correct with lateral cyclic.
Since the R22 has no trim capability, Frank introduced a system (a weight if memory serves) that reduces the lateral cyclic load felt by the pilot in the cruise. I think that you apply left cyclic in the R22 and not right.
Bonus question - flapback (blowback) is one part of flapping to equality caused by differential airspeed between the advancing and retreating sides of the disc (maximum at the 3 and 9 o'clock position) and is overcome with forward cyclic as the aircraft accelerates.
Inflow roll (transverse flow) is also flapping to equality but caused by different amounts of induced flow between the front of the disc and the rear. As the disc is tilted and moves forward, the air entering the forward part of the disc from above has less time to be influenced by the disc than the air that has to cross the whole disc and reach the back of the disc. The angle that the air reaches the blades at the front is therefore closer to the horizontal than the air reaching the back. This means that the Induced Flow (vector diagram) is reduced at the front giving a greater AoA and increased lift. So the blade flaps up until the AoA is reduce again by the new relative airflow (the rest is the same as for flapback regarding flapping) The difference is that the blade high point is in the 9 o'clock (for american rotation) and the aircraft wants to roll towards the advancing side of the disc. This is countered with lateral cyclic. Although the effect is most noticeable during the transition to forward flight, it is still there at all speeds above that and pilots subconciously correct with lateral cyclic.
Since the R22 has no trim capability, Frank introduced a system (a weight if memory serves) that reduces the lateral cyclic load felt by the pilot in the cruise. I think that you apply left cyclic in the R22 and not right.
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The idea that rotor dynamic hinge placement will make more stability is only partly correct. The pop-corn texts that discuss this and make such sweeping statements as those in the first thread are so woefully short of the facts as to make them misleading.
Flapping is a way of partially relieving the unbalancing aerodynamic forces, but it cannot make a perfect response (as in "automatically and perfectly balance") because the flapping itself introduces pitching and rolling moments that upset the aircraft. Regardless of the flapping, if the pilot or stability system does not correct the controls, the helo will pitch and roll, flip upside down and destroy itself. Also, the position of the flap hinge (percent hinge offset) and the geometry of the hinge (alpha one and delta three) will decide what the rotorcraft does when a flap occurs. Remember (or learn, please) that the lift of the blade is only one of the forces it delivers to the aircraft. There are powerful moments that the blade delivers, and these moments are often more powerful than the lift. When a blade flaps, it delivers these moments, and a control force is generated. Think about it, most real helicopters can maneuver at zero G, where the blade lift is zero. Where does that control capability come from? Do not let the limited controllability of a teetering rotor define for you the typical helicopter control paradigm.
Forget that bunk implied in either statement above, mostly, the hinges are there to help the rotor structure withstand the unbalanced forces of flight, and the pilot is there for stability. All else is mostly rubbish.
Flapping is a way of partially relieving the unbalancing aerodynamic forces, but it cannot make a perfect response (as in "automatically and perfectly balance") because the flapping itself introduces pitching and rolling moments that upset the aircraft. Regardless of the flapping, if the pilot or stability system does not correct the controls, the helo will pitch and roll, flip upside down and destroy itself. Also, the position of the flap hinge (percent hinge offset) and the geometry of the hinge (alpha one and delta three) will decide what the rotorcraft does when a flap occurs. Remember (or learn, please) that the lift of the blade is only one of the forces it delivers to the aircraft. There are powerful moments that the blade delivers, and these moments are often more powerful than the lift. When a blade flaps, it delivers these moments, and a control force is generated. Think about it, most real helicopters can maneuver at zero G, where the blade lift is zero. Where does that control capability come from? Do not let the limited controllability of a teetering rotor define for you the typical helicopter control paradigm.
Forget that bunk implied in either statement above, mostly, the hinges are there to help the rotor structure withstand the unbalanced forces of flight, and the pilot is there for stability. All else is mostly rubbish.
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Relationship between collective and cyclic
Hi folks
Can someone explain to me in plain english why lowering the collective requires aft cyclic and viceversa, cos i am having an hard time to visualize the mechanics behind it.
Also and correct me if i am wrong, during a hover you normally pull, depending on the circumstances, takeoff power which is whatever you get from the table plus a factor of .9 in the r22. As you accelerate down the runway during the takeoff roll, as you reach 50kias you establish the 60kias attitude and raise the collective to takeoff power, if you have any left, which should give you about 500fpm, while when at 10% roc prior to your assigned altitude, 50ft at 500fpm, you reduce the pitch and set the power to 21 inches for the r22 which should equal approximately 70kias or if you want to go at the best perf speed of 83 you set mcp.
Am i correct?
Many thanks
Baobab72
Many thanks
Can someone explain to me in plain english why lowering the collective requires aft cyclic and viceversa, cos i am having an hard time to visualize the mechanics behind it.
Also and correct me if i am wrong, during a hover you normally pull, depending on the circumstances, takeoff power which is whatever you get from the table plus a factor of .9 in the r22. As you accelerate down the runway during the takeoff roll, as you reach 50kias you establish the 60kias attitude and raise the collective to takeoff power, if you have any left, which should give you about 500fpm, while when at 10% roc prior to your assigned altitude, 50ft at 500fpm, you reduce the pitch and set the power to 21 inches for the r22 which should equal approximately 70kias or if you want to go at the best perf speed of 83 you set mcp.
Am i correct?
Many thanks
Baobab72
Many thanks
I'm guessing you're referring to maintaining a constant pitch attitude with power changes/collective movement. It's all due to dissymmetery of lift. There is an excellent thread on this somewhere.
Hi Baobab,
It's not mechanics as much as aerodynamics
It's caused by flapback. Dissymmetry of lift causes flapback... dissymetry of lift increases with forward speed and also increased angle of attack i.e. pitch angle from collective.
For power requirements you pull what you need to whist keeping within the limitations. There is no hard and fast rule to what power you pull doing different things as it varies greatly with changes in weight, temperature and height.
AM
It's not mechanics as much as aerodynamics
It's caused by flapback. Dissymmetry of lift causes flapback... dissymetry of lift increases with forward speed and also increased angle of attack i.e. pitch angle from collective.
For power requirements you pull what you need to whist keeping within the limitations. There is no hard and fast rule to what power you pull doing different things as it varies greatly with changes in weight, temperature and height.
AM
Not to mention increased or decreased downwash on the tailboom and aerodynamic surfaces aft of the mast as opposed to those in front, so the effect will also be noted in the hover.
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Flapback
Hi
So if i get it right, as airspeed increases the horizontal component of the relative wind on the advancing blade is comprised of the rotational velocity plus the aircraft velocity, while the horizontal component on the retreating blade is comprised of the rotational velocity minus the aircraft velocity, resulting in a greater velocity thus lift on the advancing blade side than on the retreating, however due to gyroscopic precession the increase in lift is sensed 90 degrees away - at the 12o'clock position, causing the rotor disk to flap back, and the nose to pitch up. As the nose pitches up a portion of the thrust vector is riderected forward and away from the forward direction ultimately resulting in a decrease in speed which must be compensated for by a forward movement of the cyclic.
So this applies to forward flight and explains the tendency of the helicopter to picth up as you accelerate.
How is it related to changes in pitch with the collective, since the rrpms are kept constant so even though the pitch is increased, the rotational velocity should remain the same and thus the dissimetry of lift? Or am i missing something? Or better what i am missing cos the facts prove me wrong!
Thanks
Baobab72
So if i get it right, as airspeed increases the horizontal component of the relative wind on the advancing blade is comprised of the rotational velocity plus the aircraft velocity, while the horizontal component on the retreating blade is comprised of the rotational velocity minus the aircraft velocity, resulting in a greater velocity thus lift on the advancing blade side than on the retreating, however due to gyroscopic precession the increase in lift is sensed 90 degrees away - at the 12o'clock position, causing the rotor disk to flap back, and the nose to pitch up. As the nose pitches up a portion of the thrust vector is riderected forward and away from the forward direction ultimately resulting in a decrease in speed which must be compensated for by a forward movement of the cyclic.
So this applies to forward flight and explains the tendency of the helicopter to picth up as you accelerate.
How is it related to changes in pitch with the collective, since the rrpms are kept constant so even though the pitch is increased, the rotational velocity should remain the same and thus the dissimetry of lift? Or am i missing something? Or better what i am missing cos the facts prove me wrong!
Thanks
Baobab72