Nick - another why does this happen...
Thread Starter
Nick - another why does this happen...
Cyclic position vs roll rate.
In a US configuration helo when you roll rapidly to the right (wingover, downwind quickstop etc.) the roll rate increases rapidly and requires cyclic input considerably left of the wings level position to stop the roll and even more to reduce it. To the left - more cyclic is needed to produce the same rate of roll which is easily arrested with a central cyclic position. Why? does this have to do with the fact that the disc is being loaded during the manoeuvre and is therefore a cross coupling or is it something else. A helo with the blades rotating clockwise produces the opposite effect - eg to achieve a given roll rate less cyclic is required towards the advancing side than the retreating.
In a US configuration helo when you roll rapidly to the right (wingover, downwind quickstop etc.) the roll rate increases rapidly and requires cyclic input considerably left of the wings level position to stop the roll and even more to reduce it. To the left - more cyclic is needed to produce the same rate of roll which is easily arrested with a central cyclic position. Why? does this have to do with the fact that the disc is being loaded during the manoeuvre and is therefore a cross coupling or is it something else. A helo with the blades rotating clockwise produces the opposite effect - eg to achieve a given roll rate less cyclic is required towards the advancing side than the retreating.
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I havn't responded mostly because I am baffled. I have not seen the difference in control power or damping in left vs right in the first place, and intend to look some data up to see.
The tail rotor is a possible answer if the effect is real, but the typical Jet Ranger has a very low tail rotor, so there is little roll effect into it. For an S-76, the TR is large and high, but the roll effect is not apparent to me.
I would subscribe to the opposite effect for US helicopters at altitude and at mid to high speed. Left rolling maneuvers unload the retreating blade, and so are much snappier than right rolls, which demand more lift from the retreating side.
The tail rotor is a possible answer if the effect is real, but the typical Jet Ranger has a very low tail rotor, so there is little roll effect into it. For an S-76, the TR is large and high, but the roll effect is not apparent to me.
I would subscribe to the opposite effect for US helicopters at altitude and at mid to high speed. Left rolling maneuvers unload the retreating blade, and so are much snappier than right rolls, which demand more lift from the retreating side.
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Nick, I don't believe the TR is an issue but I am prepared to be proved wrong. The rate of roll is not as noticeable as the amount of opposite cyclic required to oppose the roll and recover to wings level. In the Lynx in a 60 degree AoB downwind quickstop to the right or a wingover using 90 degrees AoBto the right, almost full left cyclic is required. The same sort of input is required in the Sea King during similar manoeuvres. One test pilot once muttered something about manoeuvre stability boundary but I didn't fully understand his explanation which I am sure didn't differentiate between the direction of turn.
The Gazelle definitely responds in the same, although opposite way and rolls left much more readily than right and requires a lot of right cyclic to roll out from a left wingover/quickstop.
All the above relates to flying stab out so AFCS/SAS saturation is not a player.
I hope you can find an answer or tell me I am imagining it.
The Gazelle definitely responds in the same, although opposite way and rolls left much more readily than right and requires a lot of right cyclic to roll out from a left wingover/quickstop.
All the above relates to flying stab out so AFCS/SAS saturation is not a player.
I hope you can find an answer or tell me I am imagining it.
Avoid imitations
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I have noticed a similar phenomena in the S-76; in a fairly tight turn at about 60 kts, the aircraft tends to "tuck in" to the turn and the nose tends to drop. It needs opposite cyclic to control it.
roll rate vs cyclic posn.
10 p`nnth ,which could also be rubbish.. If you are measuring roll-rate in level flight there may be a slight difference due to the rigging of the head/stick to offset translational effects/sideslip,and any gearbox offset/tilt to give a levellish fuselage attitude.If it`s only noticeable during transients,like d/w quickstops,then I think it is a flap-back/roll interaction,with t/rotor interaction from the "attitude" in relation to the rotor and fuselage c of g--ie the a/c wants to change ends ,but quicker than you thought.I`ve seenGreenham Common runway thru` the roof panels of a Sea-king with CO`s and training officers as Display Pilots and thought-I hope the big arrow "L" knows which way it is pointing!! I also saw a new young Sqn.Ldr practicing a d/w q/stop at Ternhill,unfortunately,he didn`t get all the mighty Sycamore`s 550 hp under his armpit ,and ended up running on at about 50 kts and spreading t/r bits all over the airfield,and he didn`t get ground-resonance.We lesser mortals on the course we nt and had another coffee,and contemplated our future.!!! Very observant of you anyway Crab,you should apply for ETPS,and you`ll probably be asked at the interviews"what about roll-rate------??Keep your balls in the middle,I say..
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Fortyodd, Bloo was my boss on 671 and, as excellent a pilot as he is, did not have the answer to this one.
Sycamore - I don't think the starboard lateral lead of the Sea King mixing unit is a factor, the Lynx doesn't have this and still needs loads of left cyclic to roll out from a quickstop/wingover to the right. Upside down in a Sea King - rather you than me, no problem in a Lynx however. As for ETPS - too old and crap at maths!
Nick - just a thought about the slope of the CL curves on the opposite sides of the disc - the retreating side is at high AoA and a one degree pitch change would produce a bigger increase in CL than the decrease on the advancing side which is operating at low AoA (and therefore presumably on a shallower part of the CL slope). The only problem with this thought is that it ought to work the same way in a roll to the left.....ah well, back to the drawing board!
Sycamore - I don't think the starboard lateral lead of the Sea King mixing unit is a factor, the Lynx doesn't have this and still needs loads of left cyclic to roll out from a quickstop/wingover to the right. Upside down in a Sea King - rather you than me, no problem in a Lynx however. As for ETPS - too old and crap at maths!
Nick - just a thought about the slope of the CL curves on the opposite sides of the disc - the retreating side is at high AoA and a one degree pitch change would produce a bigger increase in CL than the decrease on the advancing side which is operating at low AoA (and therefore presumably on a shallower part of the CL slope). The only problem with this thought is that it ought to work the same way in a roll to the left.....ah well, back to the drawing board!
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crab: i recon the tail rotor would be the magor factor in the roll rate problem. the helicopter naturaly wants to tip over right with the main rotor turning clockwise.
nick: wouldnt the low bell tail rotor have more of a rolling moment than the high 76 one? (if the tail rotor was behind the main rotor, no rolling would take place).
and the pitching up you talk about i would put down as gyroscopic pressesion. with the disc spinning clockwise and right cyclic applied the disc would start tilting right and pressesion would advance the rolling movement in the direction of rotation (tending to a pitching up attitude) but the blades would feather because of the cyclic position leaving the disc at a aft of right rolling position. of cause as soon as the bank angle is reached and the roll rate stopped, the pressesion stops?
nick: wouldnt the low bell tail rotor have more of a rolling moment than the high 76 one? (if the tail rotor was behind the main rotor, no rolling would take place).
and the pitching up you talk about i would put down as gyroscopic pressesion. with the disc spinning clockwise and right cyclic applied the disc would start tilting right and pressesion would advance the rolling movement in the direction of rotation (tending to a pitching up attitude) but the blades would feather because of the cyclic position leaving the disc at a aft of right rolling position. of cause as soon as the bank angle is reached and the roll rate stopped, the pressesion stops?
Thread Starter
Vorticey, the Lynx and Sea King have rotors which rotate Anti-clockwise when viewed from above so your theory does not hold water. BTW see other threads for my thoughts on precession and rotors systems but don't ask Lu Z.
If the tail rotor was such a major player, each time you put an input in yaw you would get a roll in the opposite direction - in fact the opposite happens (eg yyaw left - roll left)due to changing the relationship of the control orbit to the direction of travel.
If the tail rotor was such a major player, each time you put an input in yaw you would get a roll in the opposite direction - in fact the opposite happens (eg yyaw left - roll left)due to changing the relationship of the control orbit to the direction of travel.
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Every helicopter I've ever flown rolls one way more quickly than the other depending on rotor system direction of rotation. Pilots who haven't noticed this either have very little flight time/experience or haven't been paying attention very closely. I haven't yet figured out exactly WHY they do it, only that they do.
I have noticed this tendency as well, particularly evident on the UH 60.
To roll right, as you argue Crab, the CL changes roughly the same on the right as the left, albeit in oppposite directions. It is your assertion that they therefore equal out that I dont agree with. For example, when you roll toward the retreating blade in forward flight, you require more power, and this “roll induced torque spike” is readily demonstrable. This is because the reduction in drag created by the reduction in lift (AoA) on the retreating blade is exceeded by the increase in drag created by the increase in lift on the advancing side. This has the overall net effect of increasing total rotor system drag, thus requiring more torque until the roll rate stops. The imbalance is due to the fact that V is squared in both the lift and drag equations, thus the effects of changing AoA between the advancing blade and retreating are disproportionate. The V squared creates an exponential curve, not a linear one, and if I could remember the correct chart to use, it can be plotted out and demonstrated diagrammatically. Damn those memory cobwebs.
Do you think that this roll induced torque spike may have anything to do with your roll rate question? It would seem logical (though most helo subjects ARE NOT
) that it would be easier and faster to roll in the direction of less power. Same as it is easier to begin a rate of descent, than it is to start a climb.
Sorry if I have confused the issue.
To roll right, as you argue Crab, the CL changes roughly the same on the right as the left, albeit in oppposite directions. It is your assertion that they therefore equal out that I dont agree with. For example, when you roll toward the retreating blade in forward flight, you require more power, and this “roll induced torque spike” is readily demonstrable. This is because the reduction in drag created by the reduction in lift (AoA) on the retreating blade is exceeded by the increase in drag created by the increase in lift on the advancing side. This has the overall net effect of increasing total rotor system drag, thus requiring more torque until the roll rate stops. The imbalance is due to the fact that V is squared in both the lift and drag equations, thus the effects of changing AoA between the advancing blade and retreating are disproportionate. The V squared creates an exponential curve, not a linear one, and if I could remember the correct chart to use, it can be plotted out and demonstrated diagrammatically. Damn those memory cobwebs.
Do you think that this roll induced torque spike may have anything to do with your roll rate question? It would seem logical (though most helo subjects ARE NOT
) that it would be easier and faster to roll in the direction of less power. Same as it is easier to begin a rate of descent, than it is to start a climb.Sorry if I have confused the issue.
Thread Starter
Flare Dammit - I don't know why it does it either - that's why I posted the question to Nick.
Helmet Fire - I completely agree with the business about torque spikes and the overall change in rotor drag (see my post on the Aerodynamics thread). However, in searching for an explanation for the disparity in roll rates I have tried to consider all sorts of different ideas. It is a fact that the slope of most blades CL curve is not constant - generally at low AoA the slope is shallower, increasing in steepness towards CL max. My suggestion therefore, was that as the retreating side is operating at higher AoA than the advancing side, then the increase of CL on the retreating side during the roll was greater than the reduction in CL on the advancing side.
I will be very happy if Nick can come up with an explanation.
Helmet Fire - I completely agree with the business about torque spikes and the overall change in rotor drag (see my post on the Aerodynamics thread). However, in searching for an explanation for the disparity in roll rates I have tried to consider all sorts of different ideas. It is a fact that the slope of most blades CL curve is not constant - generally at low AoA the slope is shallower, increasing in steepness towards CL max. My suggestion therefore, was that as the retreating side is operating at higher AoA than the advancing side, then the increase of CL on the retreating side during the roll was greater than the reduction in CL on the advancing side.
I will be very happy if Nick can come up with an explanation.
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crab
my mistake i thought "us configiration" was clockwise rotors.
you said: If the tail rotor was such a major player, each time you put an input in yaw you would get a roll in the opposite direction - in fact the opposite happens (eg yyaw left - roll left)due to changing the relationship of the control orbit to the direction of travel.
not realy! a yaw in direction of rotation even in hover will cause a roll in the same direction, not opposite. the reason it affects roll so much in flight is flap back. if the cyclic is held at the same compass direction during a yaw roll wont occer.
you said: If the tail rotor was such a major player, each time you put an input in yaw you would get a roll in the opposite direction - in fact the opposite happens (eg yyaw left - roll left)due to changing the relationship of the control orbit to the direction of travel.
not realy! a yaw in direction of rotation even in hover will cause a roll in the same direction, not opposite. the reason it affects roll so much in flight is flap back. if the cyclic is held at the same compass direction during a yaw roll wont occer.
Thread Starter
Vorticey, If you start at the beginning of the thread you will see I have been talking about an aircraft rolling into a turn - not exactly a manoeuvre carried out in the hover is it? You have clearly misread the part in the post you quote where I point out that a yaw input in forward flight (without moving the cyclic or collective) produces a roll in the SAME direction - this is the secondary effect of yaw. I have only demonstrated effects of controls a few thousand times but I seem to remember that the whole point was NOT to mask the secondary and tertiary effects of moving a control in isolation.
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In forward flight, the helicopter's equilibrium is affected by CG, lift vector, drag, tail rotor effects, etc.
To roll that helicopter you tilt the disk so the lift vector and weight create a couple. Once they no longer create that couple, rolling stops. Of course, the attitude may not have been level in equilibrium. This means the couple may be a different magnitude for equalt left vs right tilting of the lift vector.
When you input cyclic, you induce a cyclic pitch change that results in the disk tiliting. As pointed out previously, the amount of that change is subject to the blade pitch while in equilibrium, relative flow velocities, amount of flapping, etc.
There's also a rigging factor to consider. If roll in the collective does not create a linear change in blade pitch, then displacement left vs displacement right would only cause the same pitch change if the equilibirum were at a point of symmetry in the roll rigging.
That's some of the stuff I considered may explain this phenomena. After thinking through that list I realized it would be more incredible if the same input resulted in the SAME roll rate vice resulting in different roll rates.
I think the only way that roll rates could be balance would be if it were designed that way...that may come at the cost of something else.
To roll that helicopter you tilt the disk so the lift vector and weight create a couple. Once they no longer create that couple, rolling stops. Of course, the attitude may not have been level in equilibrium. This means the couple may be a different magnitude for equalt left vs right tilting of the lift vector.
When you input cyclic, you induce a cyclic pitch change that results in the disk tiliting. As pointed out previously, the amount of that change is subject to the blade pitch while in equilibrium, relative flow velocities, amount of flapping, etc.
There's also a rigging factor to consider. If roll in the collective does not create a linear change in blade pitch, then displacement left vs displacement right would only cause the same pitch change if the equilibirum were at a point of symmetry in the roll rigging.
That's some of the stuff I considered may explain this phenomena. After thinking through that list I realized it would be more incredible if the same input resulted in the SAME roll rate vice resulting in different roll rates.
I think the only way that roll rates could be balance would be if it were designed that way...that may come at the cost of something else.
Last edited by heedm; 18th Jul 2002 at 07:15.
Thread Starter
Heedm, this rate of roll situation (I am very reluctant to call it a phenomenon) is apparent in every helicopter I have flown and many more besides - witness some other posts on this thread. Each of these helicopters is designed differently with different control systems, different number of blades, different direction of rotation, different hinge offsets etc. etc. The only common factor is that they all exhibit a tendency to roll better/faster towards the advancing side which rather indicates it is a generic helicopter fact of life and not due to specific rigging. The sort of thing you are probably alluding to is Sikorsky's 'starboard lateral lead' , evident on the Wessex and Sea King where the output from the mixing unit is modified so that on application of collective, the starboard lateral jack is displaced slightly more than the port to help counter increased TR drift and roll as AUM and therefore hover power increase. On winching platforms such as these, more TR drift requires more left cyclic to maintain station producing more roll thus affecting the proximity of the winch wire to the fuselage (presuming the winch is on the right).
I know that test pilots carry out a stick plot for sideways flight as well as roll response to ensure sufficient control margins remain for safe flight which is why I asked Nick if he (as a mega test pilot God) could shed some light on what I and others have noticed.
Inherent sideslip (due to the TR thrust) in forward flight would tend to produce an opposite roll due to dihedral effect and flapback which works in opposition to the roll rate 'phenomenon' so I am still stuck for an answer.
This might be one of those sort of questions I have been asked by students where the final answer is " It just does, OK"
I know that test pilots carry out a stick plot for sideways flight as well as roll response to ensure sufficient control margins remain for safe flight which is why I asked Nick if he (as a mega test pilot God) could shed some light on what I and others have noticed.
Inherent sideslip (due to the TR thrust) in forward flight would tend to produce an opposite roll due to dihedral effect and flapback which works in opposition to the roll rate 'phenomenon' so I am still stuck for an answer.
This might be one of those sort of questions I have been asked by students where the final answer is " It just does, OK"
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crab
would it have somthing to do with the position of the cyclic during foward flight? ie, right and foward of the hover position (anti clockwise rotation?
would the positions you are talking about be the same distance from the flight position??
would the positions you are talking about be the same distance from the flight position??
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Crab wrote:
(snip)
Whoa! Hold on! Crab, it has been my experience that in many small helicopters, a slight yaw to the right will absolutely produce a roll to the LEFT due to the extra drag caused by the the cabin being out of trim. And vice-versa. And it's very dangerous at high a/s in underslung systems because it plays hell with the flapping angle and mast/hub clearances. ...As many dead Cobra pilots can attest.
This is the opposite of the "dihedral effect" we've come to know and love in airplanes (where you can pick up a dropped wing with opposite rudder). Remember, our tail rotors are attached to the airframe, so yaw changes do not immediately effect the rotor...which is why it's harder to hold a localizer with pedal adjustments as you can do in a plank.
It is quite easy to maintain straight and level flight in a helicopter with the cyclic frictioned down tightly. Trouble is, yawing also makes the nose drop, so pedal inputs have to be tiny.
To Vorticey. You wrote:
Sorry, matey, but the roll rates actually don't have anything to do with cyclic position.
(snip)
...I point out that a yaw input in forward flight (without moving the cyclic or collective) produces a roll in the SAME direction - this is the secondary effect of yaw.
Whoa! Hold on! Crab, it has been my experience that in many small helicopters, a slight yaw to the right will absolutely produce a roll to the LEFT due to the extra drag caused by the the cabin being out of trim. And vice-versa. And it's very dangerous at high a/s in underslung systems because it plays hell with the flapping angle and mast/hub clearances. ...As many dead Cobra pilots can attest.
This is the opposite of the "dihedral effect" we've come to know and love in airplanes (where you can pick up a dropped wing with opposite rudder). Remember, our tail rotors are attached to the airframe, so yaw changes do not immediately effect the rotor...which is why it's harder to hold a localizer with pedal adjustments as you can do in a plank.
It is quite easy to maintain straight and level flight in a helicopter with the cyclic frictioned down tightly. Trouble is, yawing also makes the nose drop, so pedal inputs have to be tiny.
To Vorticey. You wrote:
would it have something to do with the position of the cyclic during foward flight? ie, right and foward of the hover position (anti clockwise rotation)?
would the positions you are talking about be the same distance from the flight position??
would the positions you are talking about be the same distance from the flight position??
Sorry, matey, but the roll rates actually don't have anything to do with cyclic position.