BlackShark Autopilot basics

From IvanK:

The Black Shark Auto Flight Control System (AFCS) description in the manual is I believe confusing and lacks sufficient detail to get a good understanding of its operation. This is my attempt to describe its operation in practical terms based on my experience flying the Sim. There may be errors and or omissions. I will update this document as required and would welcome feedback to make it better and or more correct.

The BS Auto Flight System is designed to do the following:

  1. Fly the Helo along a desired or planned flight path (ROUTE)
  2. Provide Stability Augmentation damping and Attitude holding.
  3. Generate Steering commands for the FD system to allow the pilot to manually fly a desired or planned flight path.
  4. Provide a rapid Auto turn in the yaw axis on to a designated target.
  5. Provide flight control inputs to the Auto Hover and Altitude hold system.

AFCS Controls.
The KA50 has a number of controls that directly interface with the AFCS system.

This switch is located on the collective leaver and should be regarded as an Auto pilot On/Off switch. It has 3 positions. In the centre position the Route mode switch is Off.In the forward position the AFCS AP is engaged and will actively fly the helicopter. In the rear position the switch is used in conjunction with the Auto Hover mode to provide a controlled descent. The Route Mode switch also works in conjunction with the Desired Heading (DH) - Desired Track (DT) switch.

The DH DT switch located on the RH sill panel provides either a Course Reference or Heading reference for the Auto pilot to maintain. In the Desired Track (DT) position the AFCS assumes that the pilot wants to fly the planned track to the active PVI800 Waypoint, that is it selects Course Line steering. A course deviation scale is displayed in the lower portion of the HUD. In the Desired Heading (DH) position the AFCS assumes the pilot wants to fly directly to the Active PVI800 waypoint. In the deselected position (centre) the AFCS assumes the pilot wants to maintain the last set reference heading.

This is located on the right Sill panel. The switch selects either Barometric Altitude (BR) or Radar altitude (RD) as the altitude source to be used by AFCS.
(it does not affect the altitude display in the HUD)

There are two methods used in the BS to provide stability augmentation Attitude Hold modes and full time damping. These are controlled by the 4 Auto pilot push buttons and are also affected by the Trimmer switch and FD switch.

There are 4 Autopilot pushbutton switches on the right panel. These switches labelled Pitch hold, Bank Hold, Alt hold and HDG hold are dual function switches. These switches control basic damping and attitude hold modes. With the switches OFF both damping and attitude hold are off. The helo is difficult to fly with no damping or attitude hold function. i.e. its raw control input with no AFCS modification.

The ON function of the switches is also affected by the Flight Director (FD) switch selection.

FD Switch OFF
With the FD Switch off and all 4 Push buttons on then Damping is active and attitude Hold modes are active. The Damping mode is running in the background to take out unwanted oscillations in the respective axis. The Attitude hold mode is a pilot assistance mode that hold the last commanded attitude. The desired Pitch,Roll or HDG attitude is set by the pilot using the trimmer switch. As the pilot releases the trimmer switch the AFCS takes a snapshot of the present Pitch,Bank attitude and heading. These snapshots are then used by the AFCS to within its authority hold these selected attitudes.

The pilot can override these attitudes but will find a certain “resistance” to his input as the AFCS attempts to maintain the last datums. If the pilot was to release the controls the AFCS would return the Helo to these last set datums. Though not a recommended technique Pressing and holding the trim switch whilst manoeuvring temporarily disables the attitude hold function (damping is still active), consequently no resistance to control input is felt. As soon as the trimmer switch is released the Attitude hold function again becomes active. In addition the pilot can select specific axes ON or OFF. In many cases whilst manoeuvring it is advantageous to Turn HDG Hold off as this provides smother dynamic heading changes… recall that damping is still present.

FD Switch ON
The FD switch does 2 things. It overrides the attitude hold functions regardless of Autopilot push button selection (Damping is still active provided switches are on). It provides HUD steering cues to the pilot to achieve the last commanded performance. The desired performance is set on release of the trimmer switch. again the AFCS takes a snapshot of the Heading (DT/DH switch in the centre) Pitch and roll attitude. These then become the Datum references used by the FD. You can see this most easily by the FD pitch reference markers appearing at the present pitch when the Trimmer switch is released. If the DT/DH switch is in either DT or DH position the heading reference used by the FD comes from the PVI i.e. a heading to go direct to the waypoint (DH) or to intercept/maintain the required inbound track to the waypoint (DT). This can lead to confusion as to what the rudder is doing. (Hint remove the confusion by placing the DT/DH switch in the centre position. In this manner the HUD diamond will always reflect the datum heading the FD and AFCS are using. A quick blip of the trimmer switch will Synch the diamond and AFCS/FD heading reference to present heading)

The Collective brake also affects the FD output. If the collective brake is used an altitude snapshot is taken by the AFCS and is used as a datum altitude. The FD then provides altitude cueing in the HUD to return to the datum attitude. This Altitude cueing will only be displayed if the ALT HOLD push button is selected on.

Note FD steering cues are only displayed in NAV HUD mode.

These switches labelled HDG HOLD and ALT HOLD provide a means for the pilot to send a reference heading and or Altitude to the AFCS to be maintained or used as a steering reference. These switches are used in conjunction with the Trimmer and Collective brake. The HDG HOLD switch when selected and provided a reference heading has been set will result in the AFCS applying rudder to yaw the aircraft towards the reference heading. Once on the reference heading rudder commands will be used to maintain this heading. This rudder action can at times conflict with immediate pilot commands and or result in unbalanced flight. If this occurs the HDG HOLD mode should be deselected.

The ALT HOLD switch should be thought off as a Mode arming switch. When selected the AFCS is armed and ready to automatically Hold a specific altitude using small adjustments to rotor pitch angle. to get ALT HOLD to work three things must be done. First ALT HOLD must be on, secondly the pilot must provide close to the required rotor pitch angle (i.e. enough collective) to more or less keep the Vs at 0, finally the pilot must then tell the AFCS to maintain the current altitude. This last step is done by pressing and releasing the collective brake. When the collective brake is released a snapshot of the present altitude (Baro or Radar) is sent to the AFCS to be used as the reference altitude. Whether the AFCS uses Radar or Baro altitude is determined by the position of the Auto pilot Altitude data source select switch on the right sill panel. In the RD position the snapshot altitude reference sent to the AFCS is Radar altitude. In the BR position the snapshot altitude reference sent to the AFCS is Baro altitude.

ALT HOLD only has a certain amount of control of the blade pitch angle, i.e is is + - a bit around the collective set pitch. If the collective lever is moved the ALT HOLD function has less blade adjustment to play with. If you move the collective outside of the ALT HOLD range then ALT HOLD does not have enough blade pitch authority to do its job. Ideally the collective brake should effectively lock the collective lever thereby ensuring that that reference blade pitch is fixed and the ALT HOLD function has enough authority to do its job. As long as the collective does not move/drift then ALT HOLD works just fine in BS.

Notes: if the ALT HOLD switch is off it will be automatically commanded on when AUTO HOVER is engaged.
ALT HOLD is only relevant when used in conjunction with the Collective brake and or Route AP/Auto Hover mode

Part 2, final.

The KA50 Auto Hover system is a pilot aid that assists the pilot to maintain a specific Hover point in 3 dimensions. There are numerous equipment interfaces to the Auto Hover system. The Auto Hover system should only be engaged at low speeds (<20Kmh). When engaged the AFCS stores the exact position at which the Hover mode was engaged. This position is used to generate the HUD hover point square and the HSI Hover point reference bar centre point. This Hover reference position can also be updated by the pilot at any time by a momentary blip of the trimmer switch (and collective brake for altitude/height). Provided the Hover point is selected (or updated) at almost 0 speed The Auto Hover system will actively fly the helicopter to the Hover reference point. The Auto Hover system simply attempts to stabilise the Helicopter at a point in space. Minute corrections are used automatically to maintain 0 speed in all axis. The Auto Hover system inputs include: Inertial, Doppler drift and ground speed from a dedicated Auto Hover doppler unit in the lower rear fuselage, Radar altitude, Baro altitude (only above 300metres radar altitude), low speed vanes, and differential static pressure between various static ports.

Altitude maintenance in the hover. Provided the pilot has a reasonably accurate Collective position the Auto Hover system uses small adjustments to the blade pitch to maintain the reference Hover Radar altitude (These can be seen by small variations in blade pitch angle, the collective stick will remain in its selected position). Recall that a quick blip of the collective brake resets the Auto Hover reference altitude. If the ALT HOLD switch is off it will automatically be commanded on when AUTO HOVER is engaged. AUTO HOVER has a minimum altitude of 4metres Radar Alt.

WARNING : The Auto Hover is only operative to an altitude of 4metres. If an inadvertent descent below 4metres or aborted landing/bounce occurs it is usual for all 4 AFCS pushbutton lights to start flashing. This indicates that an auto deselection has occurred. In this state all damping and attitude hold functions have been deselected. In this situation it is imperative to reselect the switches to regain damping and or attitude hold function.

Whilst in the hover the pilot can also use the AFCS to commence an automatic descent at 2ms. This is done by holding the Route mode switch to its forward position… Route descent. The AFCS will command a small reduction in Blade pitch to initiate the descent (The collective lever will remain stationary). The Helo will descend at 2ms whilst maintaining the Hover point) until the KA50 gets to 4metres Radar Altitude. At this point the Auto Hover system increases blade pitch angle to maintain 4 metres Radar Alt (i.e. exists descent mode). To continue the descent the pilot must manually reduce collective. If the Route descent switch is released the descent will be stopped at the present radar altitude.

In the Auto hover the pilot can make small adjustments to override the auto hover system. This is used to drive the helo to the Hover point or to a new Hover point. Small precise cyclic inputs are used to achieve this. The use of HDG HOLD AFCS mode is of great assistance in the hover. When on the desired Hover heading Engage HDG HOLD mode. If a change in heading in the hover is desired deselect HDG HOLD (prevents undesirable rudder commands) yaw to the desired Heading Re-engage HDG Hold. A quick Trimmer blip will synch the current heading and reset the current Hover reference point). An alternative is to simply override HDG Hold, then retrim when on the new heading.

This mode can be used in forward flight or the Hover and is considered a tactical mode that operates in the lateral axis only). The Auto Turn on to target mode is simply a method to align Target Line of sight with the Helos for and aft axis. When a target is designated the AFCS systems takes the angular deviation of the SHKVAL head from the aircrafts centreline and simply yaws the KA50 to drive this angular deviation to ZERO. Pilot technique is to designate the target then press the Auto Turn to target switch. As long as this mode is active the AFCS will attempt to keep the SHKVAL Line of sight to zero in the yawing plane. (For the Auto turn on target function to be operative the HDG HOLD mode select switch must be on).