Questions regarding Sikorsky S-70 SAS

[Vee1.rotate]

Dear Rotorheads,

firstoff, I 'm myself from a fixed-wing background , so please forgive me if my questions are put in a simplistic or stupid way.
I read a bit about helicopters and out of Interest would like to ask (in more or less simple terms):

In what way do Stability Augmentation Systems typically help/support the pilot during manual flight?

Do they eliminate the need to compensate for torque-induced yaw moments with the pedals?
Is the SAS normally switched on during the entire flight or only during specific flight phases?

I guess there probably many different variants of SAS available depending on the manufacturer of the helicopter.
If somebody could answer my questions specifically for the S-70/UH-60 that would be quite nice as I have a personal interest in that helicopter model.
(I read on the Sikorsky archives, that it's SAS went through several iterations , from fluidic to analog&digital electric to dual digital electric)

However any general or other-model related answers are also very welcome,

Thanks in advance,

Kind regards,

V1R

[mixing lever]

Here is some study material that might help you.


http://aerostudents.com/files/automaticFlightControl/stabilityAugmentationSystems.pdf

http://www.helicoptermaintenancemagazine.com/article/understanding-helicopter-automatic-flight-control-systems-afcs

http://nams.usra.edu/NAMS/assets/AFDD/AHS_2004_Tischler.pdf

http://www.helicoptermaintenancemagazine.com/article/sas-autopilots-and-flight-directors-what%E2%80%99s-name

http://aasf1-ny.org/4-Standards/Documents/PUBLICATIONS/StudentHandouts/UH60%20AFCS.pdf

[ShyTorque]

Hopefully John Dixson or Nick Lappos will see your request and contribute, seeing as they were very closely involved with the project.

I only have about four hundred hours on the S-70, so I am no expert. But the SAS is normally on for all flights, as in most SAS equipped helicopters.

[212man]

The other simple answer to your question is that the SAS effectively takes a machine with negative dynamic stability and makes it positive. Most aeroplanes have inherent positive dynamic stability.

[Jack Carson]

The S-70 flight control system can be explained in 4 parts.
1. The S-70 incorporates an irreversible hydraulic flight control system. As such there is no direct mechanical control of the rotor systems without hydraulics. The system incorporates a mechanical mixing unit that provides collective to yaw coupling. I believe that it also provides collective to pitch and roll coupling as well. “John and Nick help me here”!
2. Since the basic system is dynamically unstable, a Stability Augmentation System (SAS) is provided to provide basic rate referenced limited dynamic stability. Control is limited to plus or minus 10% of total flight control authority by what Sikorsky terms inner loop control. Inner loop control is provided in the AFCS without moving the cockpit controls. As such it is transparent to the pilot.
3. On top of this, the S-70 is equipped with varying levels of autopilot through the trim system. Basic S-70s have attitude and heading retention as functions of the trim system. This works as an outer loop function and as such provides control by moving the flight controls. Trim control movement rate of input is limited to 10% of total control per second but can move the controls full throw. Trim can be easily overridden by the pilot. Advanced models of the S-70 also incorporate Airpeed hold, altitude hold, hover position hold and automatic approach departure functions.
4. The S-70 also incorporates a digitally controlled Stabilator that provides for improved maneuver stability and a more level fuselage attitude during approach to hover.

I hope this helps. I am sure John D and Nick N may be able clarify any discrepancies and add more details and background.:8

[Vee1.rotate]

Thank you very much to all of you for your answers!

Most of it has become clearer now for me.Especially thanks Mixing_Lever for the many links with study material. It has helped getting a better understanding/overview of things .

Shy Torque and Jack Carson, since you both seem to have some on-type experience may I ask you again the one thing that still is not clear to me from reading the material (sorry probably its just me being dumb):

Does the SAS compensate for torque-induced yaw moments or do you have to counter it with the pedals?
I mean,for example during take off, can you raise the collective without simultaneous pedal input and still maintain constant heading?

Most links I read seem to agree that the SAS' purpose is to provide dynamic / short-term stability in the pitch/roll/yaw axes by countering disturbances and smoothening out pilot inputs.


However does the torque-induced yaw moment count as a disturbance in this context?


Thats still not entirely clear to me. One of Mixing_lever's links mentioned specifically mentions that the system prevents "fish-tailing" in the yaw axis. Does this refer to torque related yawing movements or is fishtailing something else in helicopter speech context?

Thank you very much again for all your answers and explanations!

Kind regards,

V1R

[[email protected]]

Vee1 - the mechanical mixing unit that Jack describes will compensate for a certain amount of torque-induced yaw - as the collective is raised the mixing unit moves the anti-torque (power) yaw control. It is effectively putting in left pedal for you as you raise the lever (assuming counter-clockwise rotation of the rotor when viewed from above). Other mixing of collective to cyclic can offset some effects of flapback (pitch) or Tail rotor drift (roll).

However, in order to lift to the hover with feet off the pedals, you require a heading hold function which is the higher mode or outer loop autopilot function - something beyond the capability of a SAS.

In simple terms, a SAS uses series actuators in the control runs (fast speed but limited authority) but SAS only senses a rate/displacement error (using the Vertical and Horizontal Gyros) and opposes it, improving the handling qualities of the helicopter by damping the inputs from the pilot. When the series actuator gets to its limit of travel (usually 10%) it can't go any further (saturation).

The autopilot needs to be able to re-datum the series actuator to give 100% authority and can only do this by physically moving the flight controls using a parallel/trim actuator (slow moving but 100% authority). As this happens, a signal is sent to the series actuator to re-centre itself, thus giving another 10% authority but around a new datum. This is called open loop.

You need SAS for handling stability and rate damping but an autopilot computer for heading/ height/airspeed etc holds.

Some helicopters do without physical mixing inputs to counter aerodynamic effects and do it electronically using the autopilot computer instead.

The main reason for all this mechanical and electronic assistance is, as 212 man says, to make an unstable platform stable - it reduces pilot workload and allows him to use the helo as a tool to perform specialised functions.

Hope that helps.

[Lonewolf_50]

We practiced SAS off when I flew Seahawks and Blackhawks.

The trick to doing this was to be your own SAS: anytime you put in a control input or correction, you then put in a counter correction. (A bit like teaching a newbie how to fly formation, in terms of technique). The other technique item is to not make large control inputs. I may be able to find an old diagram from a pdf version of a NATOPS manual to amplify a few points, but Jack Carson covered most of it.

It's flyable SAS off, but it's a bit more work, and a variety of AP features will of course be disabled.

[JimEli]

The UH-60 mechanical controls incorporate a mixing unit that performs (among other forms of mixing) collective-to-yaw mixing that is a basic compensation (on a fixed ratio basis) for torque induced yaw. It was designed to work properly at 16825lbs, so at other weights the mix ratio is wrong. It obviously cannot correct for external disturbances.

This mixing cannot be disabled.

Basic UH-60 AFCS is composed of:
1. SAS is short term rate damping providing P/R/Y correction for disturbances like wind, turbulence, etc. SAS does not compensate for torque induced yaw. SAS has no inertial reference, so it only opposes a disturbance (it doesn't know what heading/attitude to return to).

2. FPS is long term stabilization like rudimentary A/P functions (airspeed, attitude, heading hold, etc). FPS incorporates inertial references and will allow a pilot to perform a UH-60 takeoff with his feet off the pedals.

A pilot can disable SAS and/or FPS.

With mechanical mixing and SAS (no FPS), a basic UH-60 can maintain takeoff heading within ~15 degrees without pilot pedal input. With FPS engaged, it improves to about +/-3 degrees.

This is from memory (so take with grain of salt).

[Vee1.rotate]

Crab, LoneWolf and JimEli,

thank you all very much for the additional explanations and for taking the time to explain the system and its use in more detail!

Thats very kind of you.

I think I understand it now.

Kind regards,

V1R