SAS and ATT mode
Avoid imitations
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Yes, tottigol has it right. ATT mode fully couples the system to the aircraft's gyros.
In SAS mode there is no "right way up" or heading datums, only stabilisation of aircraft attitude.
In SAS mode there is no "right way up" or heading datums, only stabilisation of aircraft attitude.
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Its a little more complicated but essentially that. In SAS mode you can equate it like you, the pilot is the stabilization system. For example in the Jet Ranger, when you hover, you have to fine tune your cyclic and collective to keep the aircraft in a steady hover. This is SAS. Now let's add wind to the simple hover and you start to drift.To stop the drift and get back to your original hover position, you put cyclic into the wind and a little collective to compensate for the loss of lift and pedal since you added collective. This is ATT, or attitude retention. In aircraft with both SAS and ATT, if you only have SAS on and you move the controls, the aircraft will not hold the attitudes you have put the aircraft into. In ATT mode, you press your cyclic and collective trim releases put the helicopter in a nose up with a right bank and let go of the trim sync releases, the ATT mode will take those datums you set and try to hold the aircraft in that attitude (always to limits, and in some aircraft through limits, engine and/or torque). SAS won't do that for you. Now if we go one step forward and add flight director, then it changes those datum values to ones that you set through your cyclic and collective "hats". The ATT will achieve those new datums and the aircraft moves. Hopefully that helps. One could go on with describing the series and parallel actuators and there relationships, auto-trim, etc. but that is for the classroom, n'est pas?
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In SAS mode suppose you are in a hover, thus the system is continuously monitoring aircraft attitude and pilots movements, if you don't move the cyclic but the aircraft moves a few degrees from it's previous pitch and roll attitude because of an outside interference like a gust of wind, then the computer says . . . . "hey the aircraft moved, but the pilot did not move the controls, so it must be an outside interference, so I will use my actuators to return the aircraft to the position where it was before" Voila! Stabilization!!
In ATT mode, suppose you are flying straight and level and to hold straight and level the aircraft attitude is 2 degrees nose down and wings level, as soon as you release the force trim button, that aircraft attitude is "photographed" and the computers thru the actuators will hold that 2 degrees nose down and wings level attitude, which will make the aircraft more or less hold that same flight regime, it will not hold Altitude or Heading like an autopilot, but at least it will keep it still for a few seconds, in ATT mode you can make minor adjustments thru the "chinese hat" say you bump it a couple of times, that will make the attitude increase a certain amount.
A lot of newbies (Specially Robinson Newbies) to the Agusta or Bell 412 for example will fly the aircraft and keep stirring the cyclic, making a lot of control inputs, minor adjustments to keep the aircraft steady, this drives the SAS computers crazy since while the pilot is stirring the cyclic, the computers can never really get to work on stabilizing the attitude, these pilots discover that if they just stop stirring the cyclic and realize that this new aircarft does all the small adjustments for you, the aircraft becomes rock solid.
In ATT mode, suppose you are flying straight and level and to hold straight and level the aircraft attitude is 2 degrees nose down and wings level, as soon as you release the force trim button, that aircraft attitude is "photographed" and the computers thru the actuators will hold that 2 degrees nose down and wings level attitude, which will make the aircraft more or less hold that same flight regime, it will not hold Altitude or Heading like an autopilot, but at least it will keep it still for a few seconds, in ATT mode you can make minor adjustments thru the "chinese hat" say you bump it a couple of times, that will make the attitude increase a certain amount.
A lot of newbies (Specially Robinson Newbies) to the Agusta or Bell 412 for example will fly the aircraft and keep stirring the cyclic, making a lot of control inputs, minor adjustments to keep the aircraft steady, this drives the SAS computers crazy since while the pilot is stirring the cyclic, the computers can never really get to work on stabilizing the attitude, these pilots discover that if they just stop stirring the cyclic and realize that this new aircarft does all the small adjustments for you, the aircraft becomes rock solid.
Could we make it a bit simpler yet.....SAS is a "rate" controlled response and ATT uses an "attitude reference". (Thinking Sperry systems....)
SAS tries to keep the pitch, roll, and yaw movement rate equal to "zero". The system senses a rate of movement and applies a countering input to return the rate of movement to zero.
ATT tries to retain a particular attitude reference selected by the pilot. The system senses a displacement from the selected attitude and applies input to return the aircraft to the selected attitude.
SAS mode can be used with Force Trim selected "OFF" or "ON"
ATT mode requires Force Trim "ON".
In SAS mode, the Beep Trim control is inoperative, thus the pilot must manually reposition the cyclic for attitude changes.
In ATT mode, the pilot can either use the Beep Trim Switch to adjust the attitude reference, or reset the attitude reference by releasing the Force Trim momentarily and moving the cyclic to obtain the desired attitude, then set the attitude reference by releasing the Force Trim button.
Minor adjustments or temporary adjustments of attitude can be done by manually over-riding the Force Trim system once the new attitude reference is set.
By all means.....let the helicopter do as much of the flying as it can for you.
I flew with the Force Trim "ON" at all times....and used ATT almost continuouslly. I still have a big ol' callus on my right Thumb!
SAS tries to keep the pitch, roll, and yaw movement rate equal to "zero". The system senses a rate of movement and applies a countering input to return the rate of movement to zero.
ATT tries to retain a particular attitude reference selected by the pilot. The system senses a displacement from the selected attitude and applies input to return the aircraft to the selected attitude.
SAS mode can be used with Force Trim selected "OFF" or "ON"
ATT mode requires Force Trim "ON".
In SAS mode, the Beep Trim control is inoperative, thus the pilot must manually reposition the cyclic for attitude changes.
In ATT mode, the pilot can either use the Beep Trim Switch to adjust the attitude reference, or reset the attitude reference by releasing the Force Trim momentarily and moving the cyclic to obtain the desired attitude, then set the attitude reference by releasing the Force Trim button.
Minor adjustments or temporary adjustments of attitude can be done by manually over-riding the Force Trim system once the new attitude reference is set.
By all means.....let the helicopter do as much of the flying as it can for you.
I flew with the Force Trim "ON" at all times....and used ATT almost continuouslly. I still have a big ol' callus on my right Thumb!
Some good answers I think. To give a practical example that explains the previous posts, suppose you are flying along in straight and level at 100 kts and 2000 ft, with wings level and 0 degrees of pitch attitude, and encounter a sudden nose up disturbance (wake turbulence, for example.) The aircraft response will be as follows:
a) No SAS or ATT system - the aircraft will pitch nose up, and will continue to do so until a pilot input is made.
b) SAS system fitted - the SAS will sense the rate of attitude change and attempt to counter it, so eventually (quite quickly) the nose will not be rising or dropping. However, the will be a new pitch attitude, that will be higher than previously, and will lead to a deceleration and climb until the pilot adjusts the attitude himself.
c) ATT fitted - ATT systems have SAS as an integral part of their function, so the SAS will counter the pitch up, as described above, and then the ATT system will further adjust the aircraft attitude to wings level and zero degrees pitch. This pitch attitude will result in the same IAS - more or less - but the aircraft will have climbed and will now fly level at some height above the original altitude.
d) ATT with Upper modes - if you have a coupled autopilot with Altitude and IAS hold, you will get a combination of rate sensing, attitude sensing and external barometric data, all combining to give continued flight at the original datums.
Hope that makes sense!
a) No SAS or ATT system - the aircraft will pitch nose up, and will continue to do so until a pilot input is made.
b) SAS system fitted - the SAS will sense the rate of attitude change and attempt to counter it, so eventually (quite quickly) the nose will not be rising or dropping. However, the will be a new pitch attitude, that will be higher than previously, and will lead to a deceleration and climb until the pilot adjusts the attitude himself.
c) ATT fitted - ATT systems have SAS as an integral part of their function, so the SAS will counter the pitch up, as described above, and then the ATT system will further adjust the aircraft attitude to wings level and zero degrees pitch. This pitch attitude will result in the same IAS - more or less - but the aircraft will have climbed and will now fly level at some height above the original altitude.
d) ATT with Upper modes - if you have a coupled autopilot with Altitude and IAS hold, you will get a combination of rate sensing, attitude sensing and external barometric data, all combining to give continued flight at the original datums.
Hope that makes sense!
IIRC a simple SAS takes the displacement information from the gyro, integrates it giving you displacement over time = rate and then opposes that rate using a feedback loop to modify the output. The stabilisation is therefore primarily achieved by the means of rate damping thus reducing pilot workload.
Our 4 axis autopilot has force sensing links in the control run so that the ASE computer can differentiate between pilot input and real error.
Our 4 axis autopilot has force sensing links in the control run so that the ASE computer can differentiate between pilot input and real error.
IIRC a simple SAS takes the displacement information from the gyro, integrates it giving you displacement over time = rate and then opposes that rate using a feedback loop to modify the output.
An ATT system would normally use an attitude gyro signal directly, but could use the rate gyro signal as a secondary input, which it then integrates to get attitude.
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Some confusing information given earlier.
Both SAS and ATT require force trim to be on to work. If force trim is off, the SAS actuators will move the cockpit controls instead of the rotor head.
SAS typically will only damp out rates.
Force trim can be used to re-datum the attitude being held. But holding the force trim button down disables attitude hold.
If the stick is moved with the force trim engaged (and the force trim button is not held down), the helicopter will move, but return to the datum when the stick is returned to the original position (i.e. the forces on the stick are released).
If anyone wants a document on how to demonstrate the various modes to gain a better understanding of how these systems work, please pm me.
Both SAS and ATT require force trim to be on to work. If force trim is off, the SAS actuators will move the cockpit controls instead of the rotor head.
SAS typically will only damp out rates.
Force trim can be used to re-datum the attitude being held. But holding the force trim button down disables attitude hold.
If the stick is moved with the force trim engaged (and the force trim button is not held down), the helicopter will move, but return to the datum when the stick is returned to the original position (i.e. the forces on the stick are released).
If anyone wants a document on how to demonstrate the various modes to gain a better understanding of how these systems work, please pm me.
Both SAS and ATT require force trim to be on to work. If force trim is off, the SAS actuators will move the cockpit controls instead of the rotor head.
212man.....you saved me from getting confused.....otherwise the SAS light would not illuminate showing it is working (or at least selected) with the Force Trim selected off by the pilot's thumb.
Given the SAS system acutators only have about ten percent authority then it would follow the pilot's death grip on the cyclic would suffice to hold the thing against the action of the actuators as Shawn states.
Actually, the cyclic stick does not move in either mode even when the SAS Beep Trim is actuated.
Given the SAS system acutators only have about ten percent authority then it would follow the pilot's death grip on the cyclic would suffice to hold the thing against the action of the actuators as Shawn states.
Actually, the cyclic stick does not move in either mode even when the SAS Beep Trim is actuated.
212man - I clearly didn't remember correctly
Now I think about it the Gazelle SAS used rate gyros as you describe and the Sea King AFCS uses differentiated attitude info to give rate damping. Doh!
Now I think about it the Gazelle SAS used rate gyros as you describe and the Sea King AFCS uses differentiated attitude info to give rate damping. Doh!
Mark the Calendar Lads!
Strewth.....Crab admitted to an error!
He must be mellowing in his doterage.
Strewth.....Crab admitted to an error!
He must be mellowing in his doterage.
Hi Shawn
On a number of Eurocopter aircraft, the series actuators are down stream of the 1st stage hydraulic boost (autopilot hydraulics) so that they are working against a hydraulic lock unless the pilot moves the stick. On these aircraft, SAS works with or without force trim turned on, with or without the pilot holding on to the cyclic (though not holding on to it in SAS mode is not a good idea for too long!). With autopilot hydraulics turned off, there is considerable resisitive force built into the unpressurised hydraulic pack - again the SAS actuators will move the head servos rather than feed back into the cyclic even if the pilot is not holding the cyclic.
As others have said, SAS is all about the system generating control inputs to resist rates of pitch, roll and yaw. Whether the input data is from rate gyros, FOG/laser gyros or differentiated attitude/heading doesn't really matter.
ATT is about the system generating control inputs to maintain a particular pitch and roll attitude, so that when the pilot releases the controls, the pitch and roll attitude revert to the memorised datums. Typically those datums can be adjusted to new values using the cyclic coolie hat and/or by temporarily releasing the stick trim (system sets the datums to the values current at the moment the trim is reengaged)
HC
On a number of Eurocopter aircraft, the series actuators are down stream of the 1st stage hydraulic boost (autopilot hydraulics) so that they are working against a hydraulic lock unless the pilot moves the stick. On these aircraft, SAS works with or without force trim turned on, with or without the pilot holding on to the cyclic (though not holding on to it in SAS mode is not a good idea for too long!). With autopilot hydraulics turned off, there is considerable resisitive force built into the unpressurised hydraulic pack - again the SAS actuators will move the head servos rather than feed back into the cyclic even if the pilot is not holding the cyclic.
As others have said, SAS is all about the system generating control inputs to resist rates of pitch, roll and yaw. Whether the input data is from rate gyros, FOG/laser gyros or differentiated attitude/heading doesn't really matter.
ATT is about the system generating control inputs to maintain a particular pitch and roll attitude, so that when the pilot releases the controls, the pitch and roll attitude revert to the memorised datums. Typically those datums can be adjusted to new values using the cyclic coolie hat and/or by temporarily releasing the stick trim (system sets the datums to the values current at the moment the trim is reengaged)
HC
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Helicomparator:
Just goes to show that when you think you know nearly everything, something comes along to screw it all up. I guess the lesson is that you really need to know your system in order to know what is going to work when!
And at the risk of really confusing things, there is often a difference between what the R&D community calls an attitude hold, and what the rest of the world calls it.
Know your definitions clearly!
Just goes to show that when you think you know nearly everything, something comes along to screw it all up. I guess the lesson is that you really need to know your system in order to know what is going to work when!
And at the risk of really confusing things, there is often a difference between what the R&D community calls an attitude hold, and what the rest of the world calls it.
Know your definitions clearly!
On the S76, I would fly a takeoff and landing with force trim off, SAS on, Cruise and descent was FT On, and some of the modes engaged (usually HDG and ALT hold).
Stability augmentation was definitely in operation with trim off, demonstrated by turning off both autopilots and watching it try to turn itself inside out above 110kt.
Same in the 412, trim off, SAS on - unless it was night time,
when it was trim on, ATT on for takeoff and climbout.
Stability augmentation was definitely in operation with trim off, demonstrated by turning off both autopilots and watching it try to turn itself inside out above 110kt.
Same in the 412, trim off, SAS on - unless it was night time,
when it was trim on, ATT on for takeoff and climbout.
Tiger, they confused you, didn't they?
SAS, short term stabilization, YOU fly the helicopter with or without force trim (a lot more fun).
ATT, long term stabilization and requirement for coupled operations, the helipilot computers fly the helicopter, the force trim HAS to be ON to couple the Flight Director.
ATT is the required mode for IFR flying.
Just like the 412, provided you are talking about a different helicopter, like the AW-139.
SAS, short term stabilization, YOU fly the helicopter with or without force trim (a lot more fun).
ATT, long term stabilization and requirement for coupled operations, the helipilot computers fly the helicopter, the force trim HAS to be ON to couple the Flight Director.
ATT is the required mode for IFR flying.
Just like the 412, provided you are talking about a different helicopter, like the AW-139.
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Why not fly in ATT mode all the time, with the force trim on?
The helicopter is much more 'stable', and it won't be any different when you turn the upper modes on. As for the control forces, in any helicopter I've flown (which is most of them), they aren't onerous. Learn how to fly with those forces - i.e. in the hover, having trimmed the stick, make adjustments without re-trimming by just holding the forces for the brief time you need to have the stick away from trim.
You can hover just as accurately as with SAS mode and FT off, and with a lot less control activity.
You have to learn how use the system, not just fly a new helicopter like it was a Jet Ranger or other light helicopter.
But the main point is to use the AFCS / autopilot the same way all the time, not just plug in ATT mode when you're IFR.
No fixed wing airline operation would allow their pilots to choose whether they used the yaw damper and autopilot or not. Why are helicopters any different?
The helicopter is much more 'stable', and it won't be any different when you turn the upper modes on. As for the control forces, in any helicopter I've flown (which is most of them), they aren't onerous. Learn how to fly with those forces - i.e. in the hover, having trimmed the stick, make adjustments without re-trimming by just holding the forces for the brief time you need to have the stick away from trim.
You can hover just as accurately as with SAS mode and FT off, and with a lot less control activity.
You have to learn how use the system, not just fly a new helicopter like it was a Jet Ranger or other light helicopter.
But the main point is to use the AFCS / autopilot the same way all the time, not just plug in ATT mode when you're IFR.
No fixed wing airline operation would allow their pilots to choose whether they used the yaw damper and autopilot or not. Why are helicopters any different?
Shawn,
one pilot's "fine control" is another pilot's "stick stirring" !
Personally, I'm with you and Saless on this one. I have never seen the point of turning the trim off.
one pilot's "fine control" is another pilot's "stick stirring" !
Personally, I'm with you and Saless on this one. I have never seen the point of turning the trim off.