Dutch Roll
Dutch Roll
Dutch Roll is a combined roll : yaw oscillation. It is driven by the lateral and directional stabilities (which respectively are rolling moment due to sideslip and yawing moment due to sideslip). The DR oscillation occurs when the damping in either yaw or roll is very poor - this can be for various reasons, but a large side area forward of the CG is probably the single most common reason.
If it occurs it is driven by turbulence, or by rapid rolling manoeuvres. It's seen by the nose describing a horizontal figure of 8, or the wingtip describing an oval.
If it occurs, it's generally fixed by improving damping in the axis which has the largest magnitude of stability - that is, if the horizontal side of the oval described by the wingtip is longer then directional damping needs improving - possibly by a damper, possibly by a bigger fin. If the vertical side of the oval is longer, improved roll damping is needed.
G
If it occurs it is driven by turbulence, or by rapid rolling manoeuvres. It's seen by the nose describing a horizontal figure of 8, or the wingtip describing an oval.
If it occurs, it's generally fixed by improving damping in the axis which has the largest magnitude of stability - that is, if the horizontal side of the oval described by the wingtip is longer then directional damping needs improving - possibly by a damper, possibly by a bigger fin. If the vertical side of the oval is longer, improved roll damping is needed.
G
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See this link for a good description of what dutch roll is and how it can be handled:
www.monmouth.com/~jsd/how/
www.monmouth.com/~jsd/how/
I know of at-least one front line NATO fighter which suffers from DR so badly that it's incapable fo flying a guns fight without the yaw-damper in.
Lacking the desire to have the MoD police at my door tomorrow morning, please forgive me if I decline to say which one.
But it does make the point that DR is a serious problem if not got right in some classes of aircraft, and a serious problem in any class if it's bad enough.
Worst DR I've ever seen was in an HM293, a flying-flea derivative.
G
Lacking the desire to have the MoD police at my door tomorrow morning, please forgive me if I decline to say which one.
But it does make the point that DR is a serious problem if not got right in some classes of aircraft, and a serious problem in any class if it's bad enough.
Worst DR I've ever seen was in an HM293, a flying-flea derivative.
G
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."..could someone briefly explain what yaw dampers are and how they work? "
It's all done with "strings and mirrors" these days, Speedy...
Modern Yaw Damper systems use computers to calculate what degree of rudder deflection is required to cancel/diminish unwanted aircraft wobbling/oscillation. Inertial Reference Systems and dedicated accelerometers sense the aircraft's pitch, roll, acceleration (and what have you) and send this data to the Yaw Damper Computer(s). The computers then compute the appropriate response and then send commands to actuators in the tail of the aircraft to move the rudder(s) left or right (the movement is limited to a few degrees however and can be overridden by the pilot as required with trim and rudder pedal input).
The response of the Yaw Damper system to destabilizing influences is almost instantaneous. Other than the stable operation of the airplane, the pilot should be oblivious to the corrections supplied by the Yaw Damper system (the rudder pedals do not move when the Yaw Damper is moving the rudder).
On the most modern jetliners, the Autopilot, in most phases of flight, has no influence on the rudder (only elevators and ailerons). In this case, it is up to the Yaw Damper system to provide, damping and turn co-ordination.
Rgds.
Q.
It's all done with "strings and mirrors" these days, Speedy...
Modern Yaw Damper systems use computers to calculate what degree of rudder deflection is required to cancel/diminish unwanted aircraft wobbling/oscillation. Inertial Reference Systems and dedicated accelerometers sense the aircraft's pitch, roll, acceleration (and what have you) and send this data to the Yaw Damper Computer(s). The computers then compute the appropriate response and then send commands to actuators in the tail of the aircraft to move the rudder(s) left or right (the movement is limited to a few degrees however and can be overridden by the pilot as required with trim and rudder pedal input).
The response of the Yaw Damper system to destabilizing influences is almost instantaneous. Other than the stable operation of the airplane, the pilot should be oblivious to the corrections supplied by the Yaw Damper system (the rudder pedals do not move when the Yaw Damper is moving the rudder).
On the most modern jetliners, the Autopilot, in most phases of flight, has no influence on the rudder (only elevators and ailerons). In this case, it is up to the Yaw Damper system to provide, damping and turn co-ordination.
Rgds.
Q.
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cwatters:
That is basically correct. You use a balanced control input, called a doublet, to set the aircraft moving about the axis of interest. You use a balanced input, such as left pedal, back to neutral, right pedal, back to neutral, so you can induce pure oscillating movement, and not just a steady change in slip angle.
Then you sit back and watch, without further pilot inputs. You are primarily watching the amplitude of each successive cycle in yaw (or whatever), to see if it's getting smaller (damped) or bigger (divergent). If it is divergent, then the pilot and the test engineer try to see who can go the most cycles before chickening out and calling for control inputs. (Just joking, of course).
By the way, these are maneuvers for professional flight test people. Please don't try them at home.
That is basically correct. You use a balanced control input, called a doublet, to set the aircraft moving about the axis of interest. You use a balanced input, such as left pedal, back to neutral, right pedal, back to neutral, so you can induce pure oscillating movement, and not just a steady change in slip angle.
Then you sit back and watch, without further pilot inputs. You are primarily watching the amplitude of each successive cycle in yaw (or whatever), to see if it's getting smaller (damped) or bigger (divergent). If it is divergent, then the pilot and the test engineer try to see who can go the most cycles before chickening out and calling for control inputs. (Just joking, of course).
By the way, these are maneuvers for professional flight test people. Please don't try them at home.
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"How do you test a yaw damper system? "
Are you asking about testing for the initial certification of an aircraft type ...or simply for maintenance purposes "cwatters"?
Modern aircraft systems have automatic self testing/fault reporting as well as pushbutton testing. Yaw Damper computers monitor themselves and associated hardware for correct operation from power up.... and then store a list of faults internally and/or send the list to a Central Maintenance Computer. Also, the YD computer(s) can be made to run through a test routine with the push of a button (or two or three). For example, on a Boeing 767, there is a test switch in the cockpit for the L & R Yaw Damper Systems. You power up the Inertial Reference and hydraulic systems and simply toggle the test switch and watch the rudder cycle back and forth (there is a CRT (TV screen) in the cockpit which shows, symbolically, the movement of the rudder).
Cheers.
Q.
Are you asking about testing for the initial certification of an aircraft type ...or simply for maintenance purposes "cwatters"?
Modern aircraft systems have automatic self testing/fault reporting as well as pushbutton testing. Yaw Damper computers monitor themselves and associated hardware for correct operation from power up.... and then store a list of faults internally and/or send the list to a Central Maintenance Computer. Also, the YD computer(s) can be made to run through a test routine with the push of a button (or two or three). For example, on a Boeing 767, there is a test switch in the cockpit for the L & R Yaw Damper Systems. You power up the Inertial Reference and hydraulic systems and simply toggle the test switch and watch the rudder cycle back and forth (there is a CRT (TV screen) in the cockpit which shows, symbolically, the movement of the rudder).
Cheers.
Q.
The manoeuvre that W&CG just described is called a "Rudder Doublet" and is the standard test for Dutch Roll.
When it comes to testing, whether it's checking something's serviceable, or a device does what it should for certification what is important is the "apparent" characteristics; that is, what is seen from the cockpit. So, the DR test would be used in either case, but obviously for certification work, you'd do it at a far greater range of conditions to check it works at every corner of the envelope.
In the VERY high value end of certification work, such as something like 777 or Typhoon, then what you'd actually do is computer model it to show how it *should* work, then flight test is to prove that the computer model was correct. Necessary, but far less fun.
During certification testing, you'd also include what are known as "role relatable" tests. That is, specific manoeuvres would be flown (such as a late turn onto centreline when descending out of cloud for a final VFR approach, or flying a guns fight - the latter not being encouraged in airliner work) to determine what effect the aircraft's DR characteristics would have on the actual useability in it's intended role.
G
When it comes to testing, whether it's checking something's serviceable, or a device does what it should for certification what is important is the "apparent" characteristics; that is, what is seen from the cockpit. So, the DR test would be used in either case, but obviously for certification work, you'd do it at a far greater range of conditions to check it works at every corner of the envelope.
In the VERY high value end of certification work, such as something like 777 or Typhoon, then what you'd actually do is computer model it to show how it *should* work, then flight test is to prove that the computer model was correct. Necessary, but far less fun.
During certification testing, you'd also include what are known as "role relatable" tests. That is, specific manoeuvres would be flown (such as a late turn onto centreline when descending out of cloud for a final VFR approach, or flying a guns fight - the latter not being encouraged in airliner work) to determine what effect the aircraft's DR characteristics would have on the actual useability in it's intended role.
G
Last edited by Genghis the Engineer; 8th Sep 2002 at 10:52.
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> Are you asking about testing for the initial certification of an
> aircraft type ...or simply for maintenance purposes "cwatters"?
I was thinking of the former. I'm familure with the basic theory of control systems and the impulse test but couldn't see how you could do this for real in the air. Can the rudder on an heavy produce enough yaw to simulate heavy turbulence? I was reading elsewhere on PRUNE that rudder throw has to be limited above certain speeds for stress reasons.
> aircraft type ...or simply for maintenance purposes "cwatters"?
I was thinking of the former. I'm familure with the basic theory of control systems and the impulse test but couldn't see how you could do this for real in the air. Can the rudder on an heavy produce enough yaw to simulate heavy turbulence? I was reading elsewhere on PRUNE that rudder throw has to be limited above certain speeds for stress reasons.
Up to Va you should be able to use full rudder in any aircraft, and there should certainly be enough authority.
However, as the Queens A310 crash proved, the inertia of a heavy means that a full reversal of rudder is a bad move. In other words, you'd want approval from both aerodynamics and structures before conducting the test, and clear input rate and deflection maxima.
G
However, as the Queens A310 crash proved, the inertia of a heavy means that a full reversal of rudder is a bad move. In other words, you'd want approval from both aerodynamics and structures before conducting the test, and clear input rate and deflection maxima.
G
Back in the 80s I used to fly hang-gliders and as the developement of these a/c progressed, the first type started to appear with no vertical surface at all. So you effectivley had a flying wing. All of a sudden pilots were complaining about dutch roll as the a/c was flown at speed. It became apparent that it was pilot induced and once this was known pilots learned a different technique and all was well.
I only add this as a note of interest but it does beg the question, how does a tail-less a/c such as the B2 damp out DR?
I can only assume it is by independent and computer controlled use of aileron, am I right?
Or do the pilots just lean over one way to weight shift it back to normal?
I only add this as a note of interest but it does beg the question, how does a tail-less a/c such as the B2 damp out DR?
I can only assume it is by independent and computer controlled use of aileron, am I right?
Or do the pilots just lean over one way to weight shift it back to normal?
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Dutch Roll does seem to vary from nothing to something you simply can't practically compensate for manually. I heard of something smallish (Kingair maybe) that you couldn't control even if you were really psyched up for it. The 747 by contrast is servicable with no working yaw dampers.
I don't think you could even taxi a Trident safely without the yaw dampers operational.
(I admit, I stand to be corrected on all of this)
I don't think you could even taxi a Trident safely without the yaw dampers operational.
(I admit, I stand to be corrected on all of this)
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The King-air series seems very difficult to get to dutch-roll, even when working the rudder and ailerons, same with the EMB-120 it’s just a deadbeat oscillation. Straight wings just do not to want to do it. (At least on those models.) As M&V pointed out, lock the rudder with your feet and aileron into the rising wing solves the problem very quickly.
YAW DAMPERS
Turin
I think you will find the B-2 has split ailerons/elevons and for yaw; they open petal fashion to give drag,and I think that goes back as far as the big Northrop flying wings of the `50`s.
I think you will find the B-2 has split ailerons/elevons and for yaw; they open petal fashion to give drag,and I think that goes back as far as the big Northrop flying wings of the `50`s.
