Nick - another why does this happen...
Thread Starter
Flare dammit - no comprendez senor 'roll due to drag cause by the cabin being out of trim'?Que? The back of the aircraft is untidy and causes a roll? no it must lose something in the translation.
I can explain 'yaw left-roll left, yaw right-roll right' which is simply because the control orbit is no longer coincident with the direction of travel and the high point of the flapping back disc has moved. Can you explain yaw left roll right a little more succinctly?
I am no fixed wing guru but I believe dihedral effect is the roll in the opposite direction to a sideslip due to different angles of attack between the 2 wings in this condition. As I remember, the picking up of a dropped wing with opposite yaw makes one wing go faster and the other slower, therfore giving the wing with the higher Vsquared more lift.
As for holding the localiser with yaw, unless you physically move the whole aircraft in space eg turn onto a new heading and change it's position relative to the centreline, you will continue to sideslip left or right of the centreline.
I can explain 'yaw left-roll left, yaw right-roll right' which is simply because the control orbit is no longer coincident with the direction of travel and the high point of the flapping back disc has moved. Can you explain yaw left roll right a little more succinctly?
I am no fixed wing guru but I believe dihedral effect is the roll in the opposite direction to a sideslip due to different angles of attack between the 2 wings in this condition. As I remember, the picking up of a dropped wing with opposite yaw makes one wing go faster and the other slower, therfore giving the wing with the higher Vsquared more lift.
As for holding the localiser with yaw, unless you physically move the whole aircraft in space eg turn onto a new heading and change it's position relative to the centreline, you will continue to sideslip left or right of the centreline.
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Crab wondered:
Hooooo-kay, here we go.
Not just the back of the aircraft, the WHOLE aircraft (fuselage). Imagine a big rectangular slab of wood (longer in length than height), suspended by one attach link at its midpoint, flying through the air edge-wise. Streamlined, right? Now turn (yaw) the piece of wood to an out-of-trim condition. Which way is it going to lean (bank)? I can assure you that it will not lean in the direction that it is turned.
Speaking mainly of underslung systems (but it happens in even the multi-blade systems I've flown), this is what causes the fuselage of a helicopter to bank in the opposite direction of yaw. Try it! It has nothing to do with the stability or lack of stability of the rotor, which is another story altogether. The rotor will eventually but not immediately respond.
Well...in a helicopter, yeah. But we have two separate entities flying through the air: the fuselage and the rotor. Sometimes they act with surprising independance. You could yaw the fuselage quite a bit and the rotor will happily continue along on whatever heading it was "trimmed" for (and by that I mean whatever position the cyclic was in that kept things in equilibrium).
But in a plank, yawing the aircraft without a corresponding opposite aileron input will cause the ship to wander off on its new heading, more or less. Making a yaw input in an airplane with positive dihedral effect will cause the ship to roll in that direction. With no ailerons, a fixed-wing can still be controlled well enough.
Hope that clears it up.
Flare dammit - no comprendez senor 'roll due to drag cause by the cabin being out of trim'?Que? The back of the aircraft is untidy and causes a roll? no it must lose something in the translation.
Hooooo-kay, here we go.
Not just the back of the aircraft, the WHOLE aircraft (fuselage). Imagine a big rectangular slab of wood (longer in length than height), suspended by one attach link at its midpoint, flying through the air edge-wise. Streamlined, right? Now turn (yaw) the piece of wood to an out-of-trim condition. Which way is it going to lean (bank)? I can assure you that it will not lean in the direction that it is turned.
Speaking mainly of underslung systems (but it happens in even the multi-blade systems I've flown), this is what causes the fuselage of a helicopter to bank in the opposite direction of yaw. Try it! It has nothing to do with the stability or lack of stability of the rotor, which is another story altogether. The rotor will eventually but not immediately respond.
As for holding the localiser with yaw, unless you physically move the whole aircraft in space eg turn onto a new heading and change it's position relative to the centreline, you will continue to sideslip left or right of the centreline.
Well...in a helicopter, yeah. But we have two separate entities flying through the air: the fuselage and the rotor. Sometimes they act with surprising independance. You could yaw the fuselage quite a bit and the rotor will happily continue along on whatever heading it was "trimmed" for (and by that I mean whatever position the cyclic was in that kept things in equilibrium).
But in a plank, yawing the aircraft without a corresponding opposite aileron input will cause the ship to wander off on its new heading, more or less. Making a yaw input in an airplane with positive dihedral effect will cause the ship to roll in that direction. With no ailerons, a fixed-wing can still be controlled well enough.
Hope that clears it up.
Thread Starter
FD, Now try deliberately sideslipping the aircraft eg cross controlling on an approach - put left pedal in to hold a heading 20 degrees off your line of approach and which way will you have to move the cyclic to keep flying the approach? Yes... to the right because your rotor disc is trying to flap back along the axis of your travel and has produced a LEFT roll relative to the fuselage.
I agree that a slab sided fuselage will tend to increase drag when presented to the airflow but not enough to roll the aircraft into a turn (maybe 1 or 2 degrees AoB). Otherwise, if you tried to hover in a 30 kt crosswind from the right in your slab sided helo, you would run out of LEFT cyclic trying to oppose the fuselage roll to the right - this clearly doesn't happen.
Some helicopters with more fuselage area forward of the rotor mast than behind it have directional stability issues (Puma for example) but this is going away from the original point of this thread.
Of coures it's easier to fly an ILS in a plankwing, that's why helicopter pilots are so much better (well one of a multitude of reasons really!!!)
I agree that a slab sided fuselage will tend to increase drag when presented to the airflow but not enough to roll the aircraft into a turn (maybe 1 or 2 degrees AoB). Otherwise, if you tried to hover in a 30 kt crosswind from the right in your slab sided helo, you would run out of LEFT cyclic trying to oppose the fuselage roll to the right - this clearly doesn't happen.
Some helicopters with more fuselage area forward of the rotor mast than behind it have directional stability issues (Puma for example) but this is going away from the original point of this thread.
Of coures it's easier to fly an ILS in a plankwing, that's why helicopter pilots are so much better (well one of a multitude of reasons really!!!)
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This thread is getting off the topic of torque increase due to roll rate, but to continue the discussion....
You need to consider the position of the vertical CG and the side area of the fuselage above and below the CG. Put any light helicopter on floats (the permanent kind, as opposed to pop-out) and side slip and you will see a negative dihedral effect- right pedal, which generates a left sideslip will roll you to the left as the greater area below the CG overcomes anything the rotor may do. This could be a problem in a lightly loaded AH-1, but probably not when loaded with weapons as the CG is much lower.
Food for thought.
You need to consider the position of the vertical CG and the side area of the fuselage above and below the CG. Put any light helicopter on floats (the permanent kind, as opposed to pop-out) and side slip and you will see a negative dihedral effect- right pedal, which generates a left sideslip will roll you to the left as the greater area below the CG overcomes anything the rotor may do. This could be a problem in a lightly loaded AH-1, but probably not when loaded with weapons as the CG is much lower.
Food for thought.
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Crab, my point was that so many factors are involved, that assuming the roll rates to be balanced left and right is erroneous (my opinion).
Helicopters aren't symmetrical. There are some symmetries, but overall nope. When we find things about them to be unbalanced, we shouldn't be surprised.
Helicopters aren't symmetrical. There are some symmetries, but overall nope. When we find things about them to be unbalanced, we shouldn't be surprised.
Thread Starter
Shawn, I appreciate that it might be possible to end up with a negative dihedral effect but the majority of helos display positive dihedral effect in sideslip which is just as well for safety's sake. BTW this thread is not about torque changes with roll rates but different roll rates in left and right turns on most helicopters.
Heedm, I agree but I would really like to know why it happens and why no-one can answer the question.
Heedm, I agree but I would really like to know why it happens and why no-one can answer the question.
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Crab, your analogy about what happens when helicopters hover in brisk winds is off-base. We're talking about cruise flight here.
You wrote:
Crab, you won't even take the word of a guy who works for the National Test Pilot School? Isn't that being just a teensy bit closed-minded?
Once established in a stable sideslip, yes, the aircraft will behave in the manner you describe. It's getting to that point that's the fun part. If you're cruising along in just about any helo without artificial stabilization (with the possible exception of the 222UT or B or whatever that peculiar model was), squeezing in some pedal will result in a roll in the opposite direction. Helicopters do not exhibit "positive dihedral effect" Might I point out - even though it should be obvious - THEY HAVE NO DIHEDRAL! And in any event, big changes take too long to manifest themselves. "Ruddering" a helicopter around to pick up a "wing" or something is an exercise in futility and will only upset things more because of the undesireable yaw/pitch coupling that many helicopters exhibit.
You wrote:
Shawn, I appreciate that it might be possible to end up with a negative dihedral effect but the majority of helos display positive dihedral effect in sideslip which is just as well for safety's sake.
Crab, you won't even take the word of a guy who works for the National Test Pilot School? Isn't that being just a teensy bit closed-minded?
Once established in a stable sideslip, yes, the aircraft will behave in the manner you describe. It's getting to that point that's the fun part. If you're cruising along in just about any helo without artificial stabilization (with the possible exception of the 222UT or B or whatever that peculiar model was), squeezing in some pedal will result in a roll in the opposite direction. Helicopters do not exhibit "positive dihedral effect" Might I point out - even though it should be obvious - THEY HAVE NO DIHEDRAL! And in any event, big changes take too long to manifest themselves. "Ruddering" a helicopter around to pick up a "wing" or something is an exercise in futility and will only upset things more because of the undesireable yaw/pitch coupling that many helicopters exhibit.
Thread Starter
FD, Shaun was talking about a light helicopter with floats on in his example of negative dihedral - a situation where the amount of drag caused by the fuselage is much more than normal and the CofG is unusually high. I agreed with him completely in this situation (I've even bought his book!) I think he would agree that most helicopters do not display this effect and instead display positive dihedral EFFECT (ie the same as a fixed wing would despite the fact helicopters don't have wings).
It is the positive dihedral EFFECT that makes the helicopter roll in the opposite direction to the sideslip (therefore in the same direction as the yaw).
As for the boll*cks you wrote about ruddering around to pick up a wing - you were the one who said this was dihedral effect, not me!
Ask Ray Prouty if helicopters exhibit positive dihedral effect and I think you will get an affirmative answer (I've got his books too!!!)
PS My analogy using a crosswind hover was accurate because it highlighted the error in your argument that a sideslip produced a roll in the same direction. A crosswind hover is only an extreme sideslip.
It is the positive dihedral EFFECT that makes the helicopter roll in the opposite direction to the sideslip (therefore in the same direction as the yaw).
As for the boll*cks you wrote about ruddering around to pick up a wing - you were the one who said this was dihedral effect, not me!
Ask Ray Prouty if helicopters exhibit positive dihedral effect and I think you will get an affirmative answer (I've got his books too!!!)
PS My analogy using a crosswind hover was accurate because it highlighted the error in your argument that a sideslip produced a roll in the same direction. A crosswind hover is only an extreme sideslip.
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Crab wrote:
Man! You Robbie pilots sure can get testy! Look, I don't care what any engineer says about what heliochoppers are supposed to do. All I can say is what I've observed based on my many, many, many years of experience and my tens of thousand of hours at the controls. And when blokes like you attain the same level of acheivement and knowledge as the great Flare Dammit!, you too will understand so that I won't have to keep repeating it ad nauseum. Do you little people think that I enjoy hearing myself talk?
You can use the what appears to be "dihedral effect" to your advantage, but it must be understood that the opposite control (anti-torque) input must be made.
Now listen and listen closely.
1) Take a helicopter...a 206 say (your choice whether it's an L or a B). It does not matter whether it's on "standard" floats or pop-outs or low-skids.
2) Set up stabilized cruise flight at MCP.
3) Put enough friction on the cyclic so that it will not move.
4) Squeeze in a little LEFT pedal.
5) Notice that the ****** banks decidedly to the RIGHT!
6) Repeat.
Works every time.
Keep feeding the pedal in and the nose will drop. Do the same thing to the right and you'll notice that the nose drops more quickly.
You can quote all the books you want from now until kingdom come. But I challenge you to go up in an actual aircraft...you know, like an actual, thinking, rational pilot...and set aside your preconceived notions about how someone *says* that the ship will behave...and go see for yourself. Then, and only then, come back to me with your observations. Because I grow tired of this thread and your unbelieveable, galactic ignorance.
It was Nick Lappos who long ago postulated that helicopter rotors didn't actually employ "gyroscopic precession" because the blades (analogous to the vanes of a gyroscope) were not solidly attached to the mast. Further, he said that a rotor blade's reaction to a pitch input was not ninety degrees, but something more or less, depending on a variety of factors. I believe that he mentioned that the Sikorsky S-76 had a 77 degree "precession" or something like that.
When I mentioned this to other, supposedly experienced and knowlegeable helicopter pilots, they acted like I was loony. Why? Because they learned from their original texts and instructors that a helicopter rotor possesses "gyroscopic precession" and that blades react ninety-degrees later than where the force is applied. They learned it that way, they read it that way, and that's the way it bloody is. So there.
Not everything that we read in books is true.
It is the positive dihedral EFFECT that makes the helicopter roll in the opposite direction to the sideslip (therefore in the same direction as the yaw).
As for the boll*cks you wrote about ruddering around to pick up a wing - you were the one who said this was dihedral effect, not me!
As for the boll*cks you wrote about ruddering around to pick up a wing - you were the one who said this was dihedral effect, not me!
Man! You Robbie pilots sure can get testy! Look, I don't care what any engineer says about what heliochoppers are supposed to do. All I can say is what I've observed based on my many, many, many years of experience and my tens of thousand of hours at the controls. And when blokes like you attain the same level of acheivement and knowledge as the great Flare Dammit!, you too will understand so that I won't have to keep repeating it ad nauseum. Do you little people think that I enjoy hearing myself talk?
You can use the what appears to be "dihedral effect" to your advantage, but it must be understood that the opposite control (anti-torque) input must be made.
Now listen and listen closely.
1) Take a helicopter...a 206 say (your choice whether it's an L or a B). It does not matter whether it's on "standard" floats or pop-outs or low-skids.
2) Set up stabilized cruise flight at MCP.
3) Put enough friction on the cyclic so that it will not move.
4) Squeeze in a little LEFT pedal.
5) Notice that the ****** banks decidedly to the RIGHT!
6) Repeat.
Works every time.
Keep feeding the pedal in and the nose will drop. Do the same thing to the right and you'll notice that the nose drops more quickly.
You can quote all the books you want from now until kingdom come. But I challenge you to go up in an actual aircraft...you know, like an actual, thinking, rational pilot...and set aside your preconceived notions about how someone *says* that the ship will behave...and go see for yourself. Then, and only then, come back to me with your observations. Because I grow tired of this thread and your unbelieveable, galactic ignorance.
It was Nick Lappos who long ago postulated that helicopter rotors didn't actually employ "gyroscopic precession" because the blades (analogous to the vanes of a gyroscope) were not solidly attached to the mast. Further, he said that a rotor blade's reaction to a pitch input was not ninety degrees, but something more or less, depending on a variety of factors. I believe that he mentioned that the Sikorsky S-76 had a 77 degree "precession" or something like that.
When I mentioned this to other, supposedly experienced and knowlegeable helicopter pilots, they acted like I was loony. Why? Because they learned from their original texts and instructors that a helicopter rotor possesses "gyroscopic precession" and that blades react ninety-degrees later than where the force is applied. They learned it that way, they read it that way, and that's the way it bloody is. So there.
Not everything that we read in books is true.
Thread Starter
FD, I can only assume you are a 'wind-up' merchant as you now doubt the words and wisdom of Nick Lappos and flatly refuse to hear any application of theory that might oppose your own ravings on this topic.
As for experience - well your profile doesn't give any information whatsoever - it is even possible you are a wannabe with very limited experience indeed.
As for me, well as a military A2QHI with 6000 hours in helicopters and 2000 hours instruction - I guess I must be Mr Thickie here as I have only taught effects of controls a few thousand times and probably haven't got enough experience to match your awesome abilities.
PS the nose drops because you are moving the horizontal stabiliser into different ares of downwash (the downwash is stronger behind the retreating side of the disc than it is behind the advancing side).
PPS I have actually flown a 206 and it does not do what you claim.
PPPS. If you believe so strongly in precession as the cause of phase lag (yes that's the phrase you were looking for) then a certain Mr Luckerman might want to add you to his Christmas card list.
I started this thread hoping for an answer to a question but it seems there isn't one.
As for experience - well your profile doesn't give any information whatsoever - it is even possible you are a wannabe with very limited experience indeed.
As for me, well as a military A2QHI with 6000 hours in helicopters and 2000 hours instruction - I guess I must be Mr Thickie here as I have only taught effects of controls a few thousand times and probably haven't got enough experience to match your awesome abilities.
PS the nose drops because you are moving the horizontal stabiliser into different ares of downwash (the downwash is stronger behind the retreating side of the disc than it is behind the advancing side).
PPS I have actually flown a 206 and it does not do what you claim.
PPPS. If you believe so strongly in precession as the cause of phase lag (yes that's the phrase you were looking for) then a certain Mr Luckerman might want to add you to his Christmas card list.
I started this thread hoping for an answer to a question but it seems there isn't one.
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FD is dead wrong about the yaw effect, at least in every helicopter I have flown. If the aircraft responded the way he describes, it would be illegal, since the basic Federal Aviation Regulations demand positive dihedral.
In short, at cruise speed, left pedal makes left bank, and requires right cyclic to trim out that bank. The opposite is also true, right pedal makes right bank.
This effect was invented by the Wright brothers, and is true for all aircraft I have tested. On helicopters it is a natural outcome of the coning that the disk has, since the sideslip produces an increase in the angle of attack on the side of the disk that "leads", and a decrease on the side that "trails" when sideslip is applied.
A high tail rotor tends to wash out this effect, since it produces a roll in the opposite direction of the yaw, but most helicopters have relatively low tail rotors.
In short, at cruise speed, left pedal makes left bank, and requires right cyclic to trim out that bank. The opposite is also true, right pedal makes right bank.
This effect was invented by the Wright brothers, and is true for all aircraft I have tested. On helicopters it is a natural outcome of the coning that the disk has, since the sideslip produces an increase in the angle of attack on the side of the disk that "leads", and a decrease on the side that "trails" when sideslip is applied.
A high tail rotor tends to wash out this effect, since it produces a roll in the opposite direction of the yaw, but most helicopters have relatively low tail rotors.
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flair damit
you wrote
Sorry, matey, but the roll rates actually don't have anything to do with cyclic position.
BUT, the first thred from crab was entirely related to the cyclic position and roll rate!
if the cyclic is allready off centre duing flight then more cyclic is neaded to make a roll in the same direction. then centering the cyclic would stop the roll.
centering the cyclic duing forward flight would cause a roll to the oposite side. to stop the roll, an awfull lot of cyclic is going to be needed.
CRAB
in flight or hover, whats the difference with tailrotor roll?, its always going to be there when power is applied. sure less peddal is required but the anti torque is still in the same place.
Sorry, matey, but the roll rates actually don't have anything to do with cyclic position.
BUT, the first thred from crab was entirely related to the cyclic position and roll rate!
if the cyclic is allready off centre duing flight then more cyclic is neaded to make a roll in the same direction. then centering the cyclic would stop the roll.
centering the cyclic duing forward flight would cause a roll to the oposite side. to stop the roll, an awfull lot of cyclic is going to be needed.
CRAB
in flight or hover, whats the difference with tailrotor roll?, its always going to be there when power is applied. sure less peddal is required but the anti torque is still in the same place.
Thread Starter
Vorticey, absolutely right, hence when you have wings level and ball in the middle you are actually sliding a right a bit - this is inherent sideslip.
The only way to fly straight ie fuselage exactly pointing the way you are going is with a little left bank and the ball slightly out. Or you could have a piece of string (wool in the case of the Gazelle) mounted centrally on the outside of the cockpit which will indicate the true direction of the airflow (except in a climb where the whirl of the rotating downwash can alter it's accuracy). Or you could tilt the rotor mast in the opposite direction to TR thrust.
Back to the point - I hope we have sufficiently thrashed out that sideslip in one direction, almost always gives roll in the other direction (positive dihedral effect again) and that therefore, other than with a very high TR position, there is unlikely to be any rolling couple caused by the TR.
Nick, I guess the original question regarding roll doesn't have an answer - it just does it?
The only way to fly straight ie fuselage exactly pointing the way you are going is with a little left bank and the ball slightly out. Or you could have a piece of string (wool in the case of the Gazelle) mounted centrally on the outside of the cockpit which will indicate the true direction of the airflow (except in a climb where the whirl of the rotating downwash can alter it's accuracy). Or you could tilt the rotor mast in the opposite direction to TR thrust.
Back to the point - I hope we have sufficiently thrashed out that sideslip in one direction, almost always gives roll in the other direction (positive dihedral effect again) and that therefore, other than with a very high TR position, there is unlikely to be any rolling couple caused by the TR.
Nick, I guess the original question regarding roll doesn't have an answer - it just does it?
Crab,
I am still waiting to hear your thoughts on the power requirement slowing down the roll rate.
Also, if you have an aircraft available to check, does the roll rate speed difference vary with forward speed? I.E. the difference in roll rate is relatively easy to notice at 120KIAS, but how about at 20KIAS?
I am still waiting to hear your thoughts on the power requirement slowing down the roll rate.
Also, if you have an aircraft available to check, does the roll rate speed difference vary with forward speed? I.E. the difference in roll rate is relatively easy to notice at 120KIAS, but how about at 20KIAS?
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Crab,
Regarding the original question of assymetrical roll rate response to cyclic:
I had a chance to check some data, and at least on the aircraft I fly regularly, there is no significant difference between the roll rate due to 1 inch of cyclic from trim for left or right turns. This holds until you get very much toward blade stall, whence (wow, I am starting to sound British!) there is much less right roll control power than there is left.
I will get some data and post it on my web site in a day or so.
The rate damping is the measure of the amount of opposite stick you need to stop the rate once started - low damping means you have to stop the roll rate with opposite stick, high damping means it stops when the stick is returned to the trim position. Teetering rotors have very little damping, and I seem to recall that right rolls take more left stick on recovery than left rolls take right stick. I touched the left stops in a Cobra in a right rolling maneuver, and that cured me of playing around (for about a week or so, anyway).
Articulated rotors have higher damping by far, and need very little opposite stick, and "rigid" rotors have very high damping, and stop crisply when the stick is recentered.
Nick
Regarding the original question of assymetrical roll rate response to cyclic:
I had a chance to check some data, and at least on the aircraft I fly regularly, there is no significant difference between the roll rate due to 1 inch of cyclic from trim for left or right turns. This holds until you get very much toward blade stall, whence (wow, I am starting to sound British!) there is much less right roll control power than there is left.
I will get some data and post it on my web site in a day or so.
The rate damping is the measure of the amount of opposite stick you need to stop the rate once started - low damping means you have to stop the roll rate with opposite stick, high damping means it stops when the stick is returned to the trim position. Teetering rotors have very little damping, and I seem to recall that right rolls take more left stick on recovery than left rolls take right stick. I touched the left stops in a Cobra in a right rolling maneuver, and that cured me of playing around (for about a week or so, anyway).
Articulated rotors have higher damping by far, and need very little opposite stick, and "rigid" rotors have very high damping, and stop crisply when the stick is recentered.
Nick
Thread Starter
Nick, Thanks for persevering with this question. I think it is definitely the differences in rate damping which I have found most noticeable in manoeuvres. Strangely enough the Lynx was ths worst offender and it has a very high effective hinge offset - I wonder if the amount of left cyclic required to stop a roll right is partly due to the rate of roll you can achieve thanks to it's huge control power.
Helmet fire, I don't know whether rolling towards the low power side (torque spikes in roll) makes any difference - the rate of roll seems to be purely decided by control power and damping. My original question had to do with manoeuvres like downwind quickstops and wingovers where the typical speed during the middle of the manoeuvre would be 40 - 60 kts. I could try experimenting next time I'm on shift but the rearcrew moan like bugg**y when you try to roll their yellow SAR bus upside down!!!
Helmet fire, I don't know whether rolling towards the low power side (torque spikes in roll) makes any difference - the rate of roll seems to be purely decided by control power and damping. My original question had to do with manoeuvres like downwind quickstops and wingovers where the typical speed during the middle of the manoeuvre would be 40 - 60 kts. I could try experimenting next time I'm on shift but the rearcrew moan like bugg**y when you try to roll their yellow SAR bus upside down!!!
