Click on the links below for a brief description of some of the systems that are simulated for the BK 117 for X-Plane:
The goal with the ND BK 117 has been to create the most detailed and realistic helicopter simulation addon possible. An important step of achieving this is to simulate all significant systems of the real helicopter in great detail and to allow startup, shutdown and all other phases of flight to be performed according to the checklist.
Since default X-Plane systems and functionality is unable or too generic to reflect the exact workings of the BK 117, a custom systems plugin has been programmed to get handling and functionality as realistic as possible. Following is a selection of custom systems simulated in this addon.
Autopilot and flight director
The Sperry helicopter autopilot pilot is fully simulated in the systems plugin. This means that, to use it, you need to switch on either or both of the two AP computers (and make sure they receive power from the electrical bus), select which of them should drive the CSAS yaw damper.
Once powered up, the AP will operate in either stability augmentation mode (SAS) or attitude retention mode (ATT) as selected by the pilot. With ATT selected, the AP holds the reference pitch and bank attitude and the pilot can change this by maneuvering with the Force Trim Release switch depressed or by using a 4-way beeper switch. Autopilot "fly-through" is also realistically simulated, allowing the pilot to temporarily hand fly the helicopter by displacing the cyclic stick a certain amount.
For flight in serious IFR metrological conditions or other high workload-scenarios, the AP can also be realistically coupled to the flight director to get automatic guidance for a number of vertical and lateral navigation modes.
Force Trim Release
The BK 117 flight controls are hydraulically powered and therefor a force spring arrangement is installed to provide artificial control forces, much like the force springs in your typical gaming joystick.
Using a trim motor and magnetic brake system, the force spring housings can be moved to shift the "no force-center" to relieve the pilot of long term control forces. This is done either by the use of beeper trim switches or by the Force Trim Release switch. The beep trim works much like the default trim in X-Plane, except the 4-way also trim commands interact with the autopilot in a way the default trim won't. The Force Trim Release switch, however, is simulated in a unique way which allows the pilot to quickly make big trim changes by simply pressing his assigned "FTR"-button and centering his joystick.
Stability Augmentation System (SAS)
X-plane features "artificial stability" which can be used to approximate the stability augmentation systems (SAS) of real world helicopters, but the ND BK 117 still uses a custom algorithm to simulate some of the specific characteristics just right. What the BK 117 SAS does, like the default "art-stab", is react to angular rates in the pitch/roll/yaw axis to counter the effects of turbulence. For realism, the plugin imposes an authority restriction to +/- 5 degrees in pitch and roll and +/- 10 degrees in yaw. The real system has this to assure pilot control in case of a system failure. Another thing that is simulated is a centering function which acts to slowly re-center stability augmentation actuators during constant angular rates. This prevents this rather low authority system to saturate during turns etc. when the airframe experiences long term pitch and yaw rates. Along with this functionality, a realistic control panel is provided where the
pilot can switch parts of the systems on and off, conduct a test of the yaw axis stabilisation and select high and low beeper trim speeds.
Power Control Levers
These are two levers on the upper control panel which are used to control the engines, mostly during startup and shut down. The levers have three positions:
- OFF shuts the fuel supply to the corresponding engine
- IDLE opens the fuel supply but holds the engine RPM at idle
- FLY allows full power to be drawn from the corresponding engine
In the range between IDLE and FLY, the levers act as limiters on the power that can be drawn from the engine. Thanks to the use of "manipulators", the power levers can be intuitively dragged and positioned using the mouse cursor.
RPM/engine speed governor
On top of proper power control levers, the BK 117 also comes with a realistic simulation of the rotor RPM and engine speed governor system. This consists of fly wheel arrangement connected to the power turbine shaft, and is spring loaded to set fuel flow required for operational rotor RPM. When collective pitch is increased, however, rotor blades create increased drag and rotor RPM tends to droop. Therefor a droop compensation system transfers collective pitch changes to the RPM governor, affecting the resetting spring to increase fuel flow and engine power. On top of this, engine N2 RPM trim functionality is simulated, allowing the pilot to adjust the N2 RPM and balance the amount of torque drawn from each engine. All of this together leads to realistic handling of the BK 117 power plant. For instance, N2 RPM will correctly go from 96% to 100% as the collective pitch is increased from minimum to hover power during takeoff. Another feature is the engine overspeed trip system which can be accurately tested as part of the startup check list.
There are countless unique smaller features that contribute to the realism of this BK 117 model. One example is that both collective and cyclic sticks can be mechanically locked. Another is the Mast Moment Indicator which measures the bending moments that are transferred from the rigid rotor system to the rotor mast hub and which not only works realistically but can also be tested for correct operation before flight. There is a bunch of switches and buttons that works just slightly different from some default X-Plane ditos, but nevertheless have been customised for increased realism. Like the generator switches. In X-plane, they simply have ON/OFF positions. In the BK 117, they have ON/OFF/RESET positions. If either generator has been "tripped" using the generator trip switch (another custom switch...), the corresponding generator switch has to be set in the RESET position before that generator can be switched on again. Another example is emergency lights above cockpit exits will light up if power is lost (if you've set the switch to ARM!), or how about an ammeter gauge that can show either generator 2 or battery loads, or the fact that you can test the engine fire warning lights independently of the annunciator panel lights. We believe these little customisations, however insignificant they may seem, makes this addon a unique and immersive experience to fly.
Where relevant, all controls associated with the customs systems simulated by this addon can easily be assigned to key commands or joystick buttons in X-Planes Joystick & Equipment menu. This is particularly usefull for Force Trim Release, beep trim, engine N2 trim or other controls that the pilot need easily within reach on his flight control sticks to maneuver the helicopter. Thanks to custom commands, you can setup your joystick and hardware configuration very closely to what the real BK 117 pilot would find in his work place.