Skycar Flight Control System Overview By Bruce Calkins August 14, 2012 Introduction The Skycar is a new type of personal aircraft that will rely on directed thrust produced by its engines to enable various modes of operation. This powered-lift aircraft will be able to perform vertical takeoff and landing (VTOL) operations, hover (like a helicopter) and to transition to fast forward flight (like a fixed-wing aircraft). To achieve this the Skycar requires an artificial stabilization system to monitor and control the operation of its eight engines during VTOL, hover and transition to forward flight. Additionally, once the Skycar is in high-speed forward flight where lift is provided aerodynamically, the on-board systems need to provide inputs to engines, rudders and duct exit vanes to assist the pilot with steerage and changes in altitude. This paper provides an overview of the systems Moller International has developed to provide these functions. Aircraft Stability Fixed wing aircraft are usually inherently stable with the four forces that act on an aircraft in flight, lift, weight, thrust, and drag, in balance. The motion of the aircraft through the air depends on the relative size of the various forces and the orientation of the aircraft. For an aircraft in cruise, the four forces are balanced, and the aircraft moves at a constant forward velocity and altitude. Conventional fixed-wing aircraft use their engines to provide thrust to move them forward until their wings produce sufficient lift to keep them airborne. This typically requires a long, straight section of relatively smooth ground to facilitate getting the aircraft up to speeds sufficient to generate the required lift, and once airborne to keep the aircraft moving forward sufficiently to maintain flight operations. Unlike conventional aircraft, the Skycar can lift off vertically by changing the angle of thrust its engines produce. It uses a combination of rotating nacelles and movable vanes to deflect airflow exiting from the nacelle and to redirect the thrust. The ability to change the angle of the thrust is called thrust vectoring, or vectored thrust. During a typical take off the Skycar uses its vectored thrust to lift the vehicle with little to no forward motion. As the aircraft rises and clears the immediate area s obstacles, the pilot rotates the vehicle to an appropriate heading then moves off in that direction by dropping the nose of the aircraft to gradually increase forward speed, similar to a helicopter. As forward speed picks up, lift is generated by the aerodynamic surfaces of the aircraft and less lift is needed from vectored thrust. At forward speeds of 80 mph or so there is sufficient lift from the Skycar s wings to level out the nose and start retracting the deflection vanes, which in turn allows more thrust to be used to increase forward speed. At 100 mph the aircraft is moving forward sufficiently to produce the lift required to keep it aloft aerodynamically, without the aid of vectored thrust. At this point the nacelles rotate to the horizontal cruise position and all of the available thrust can be directed rearward. During VTOL operations the Skycar does not have the effects of lift to keep the aircraft stable. In a conventional fixed-wing aircraft if a gust of wind or a nudge to one of the controls causes the aircraft to pitch, roll, or yaw, the aerodynamic design of the aircraft will tend to correct the motion, and the aircraft will return to its original attitude. Many small, fixed wing aircraft are stable enough that a pilot can let go of the controls while looking at a map or dealing with a radio, and the plane will generally stay on course. Studies of a scale model of the Skycar at simulated cruise speed in our wind tunnel suggest that it will behave in this manner.
In contrast, helicopters are always very unstable. Simply hovering requires continuous, active corrections from the pilot. When a hovering helicopter is nudged in one direction by a gust of wind, it will tend to continue in that direction, and the pilot must adjust the cyclic to correct the motion. Hovering a helicopter has been compared to balancing yourself while standing on a large beach ball. Small helicopters can be so unstable that it may be impossible for the pilot to ever let go of the cyclic while in flight. Adjusting one flight control on a helicopter almost always has an effect that requires an adjustment of the other controls. Moving the cyclic forward causes the helicopter to move forward, but will also cause a reduction in lift, which will require extra collective for more lift. Increasing collective will reduce rotor RPM, requiring an increase in throttle to maintain constant rotor RPM. Changing collective will also cause a change in torque, which will require the pilot to adjust the foot pedals. Basic Directional Control of the Skycar Skycar uses a joystick with trigger and rotational twist for directional control, both in the air and on the ground. On the ground, the twist is for left or right turns, push/pull for acceleration or braking. Once stopped, one can toggle to reverse. In the air, side to side movement of the joystick controls banking the aircraft (roll) while nose up/down (pitch) is forward and back, and twist results in Z-axis rotation (yaw). We also have an altitude selection lever a T-handled sliding switch with thumb button. To select desired altitude, one slides the handle forward and pushes the thumb button when the readout reaches the desired value. At that point the aircraft climbs in accordance with pre-programmed rates to that altitude. To descend, one does the same. Sensors tied directly to the engine control system ensure that the climb or decent rates do not exceed established parameters. The Skycar does not require any other inputs therefore we do not currently anticipate the need for foot pedals. A typical take off would require one to (1) select altitude (say 500 ft.) using their left hand on the T-handle; with their right hand on the joystick they would (2) pull the trigger to engage, (3) after clearing the ground space (above trees and buildings) assume a slight nose down attitude to gain forward speed as one continues to climb, (4) roll or yaw as required to clear the take off area and continue nose down attitude to establish forward momentum, at 60 mph IAS (5) extend wings and merge into planned flight path. Goals for Operating the Skycar Since the Skycar is capable of both VTOL and aerodynamic flight, it follows that two distinct modes of operation exist. Potentially, this could make the Skycar more difficult to fly than either a helicopter or fixed-winged aircraft. But a fundamental goal for the design of the Skycar was to make it easy to fly. Ultimately our aim was to make operation of the Skycar no more difficult than to operate an automobile. We have begun achieving this goal for an unprecedented level of ease-of-use by off-loading many of the requirements for pilot skill to computers and software and thereby simplifying the user s operation of the aircraft. Nearly all of the complicated corrections required of a helicopter pilot to maintain altitude and attitude have been reduced to automatically generated throttle commands provided by the on-board computer systems. The Skycar pilot will be able to maintain a stable, stationary hover at a desired altitude with hands off the controls. Furthermore, the availability of on-board computers allows for automation of many of the preflight checks. Many of the routine (yet often neglected) functions done during pre-flight like fuel
level and fuel quality checks, weight and load distribution checks will be accomplished by the onboard systems automatically. Any pre-flight adjustments that are required will be displayed to the pilot prior to the control system allowing one to reach the ready for takeoff stage. As one takes off on-board sensors will begin providing continuous updates to determine local air density, temperature and current altitude. Immediate feedback from this in-flight monitoring will provide hover check data as the aircraft lifts off, providing the power level adjustments as required to ensure a save takeoff. Continuous sensor feedback will be used to calculate in-ground effect (IGE) and out-of-ground effect (OGE) hover ceilings, takeoff distances, and anticipated rate-of-climb performance. In flight engine monitoring will include checks for unusual levels of vibration, sound, heat or emissions and serve to warn the pilot of a potential problem. The Skycar uses microcomputer processing power and sophisticated software to obtain reliable automatic stability control functions for the aircraft. Our continuing efforts are intended to yield improvements to the control technology, which simplify operation of the Skycar sufficiently to enable it to be flown by pilots with minimal effort and limited training. Architecture of the Flight Control System The Skycar Flight Control System (FCS) has evolved from years of research and development. Initial systems used analog computers and hundreds of discrete components. Later the system employed fewer components but used a centralized digital computer system for control. This arrangement made implementation of a truly redundant system nearly impossible. A fully redundant fail-safe system is required for manned operations, and preferably one that uses more reliable components and compatible with Commercial Off The Shelf (COTS) products. Initiated in 2000, upgrades have provided the Skycar s FCS with an architecture for discrete and continuous control that supports better integration of control mode logic with continuous control laws. In 2001, a prototype implementation of the hybrid multi-mode control software was implemented in the Skycar and was used during the initial flight testing of the vehicle and throughout 2001, 2002 and 2003. The system was designed for use in the prototype and gave rudimentary control over environmental effects (e. g., wind gusts, turbulence), which permitted safe operation even under modestly windy conditions. This technology demonstrated its potential to provide enhanced maneuverability and stability for our UAV and enhanced robustness under extreme conditions for piloted systems, increasing the aircrew survivability and decreasing pilot workload. Future implementations of our multi-modal control technology will provide bettercontrolled transitions between complex operational flight modes (inherent in vertical takeoff and landing UAVs and high performance manned aircraft), thereby reducing safety risks to the pilot, passengers and vehicle. Our latest hardware architecture is designed to work in conjunction with software developed under our Software-Enabled Control (SEC) program and will provide greater speed, more design flexibility, and higher reliability. The new hardware uses established RS-422 communications, which is a well-known communications protocol, and it is widely used in many safety-critical applications, from automotive to aircraft and aerospace electronics. In addition, the RS-422 architecture allows us to manage error confinement and the error detection making it more reliable for the computer-controlled, noise critical environment found in the Skycar. Its major advantages are reliability, simplicity and a simple message format, which supports the high-speed interoperability of our components. These new operating capabilities will enhance our ability to implement and upgrade advanced flight controls, Highway in the Sky displays, and automated air traffic separation and sequencing technologies.
The goal of the SEC program is to leverage increased processor and memory capacity to achieve higher performance and more reliable software control systems for Moller s manned and unmanned platforms. Applications include integrated avionics design and vehicle control for the Aerobot and Skycar air vehicles. The program has yielded control technology that is robust and withstands even extreme environments to enable highly autonomous, cooperating control systems. Initiated in 2000, the upgrades under the SEC program provided new open software architecture for hybrid discrete and continuous control that supported better integration of control mode logic with continuous control laws, including synchronized switching and new software scheduling mechanisms. In 2001, a prototype implementation of the hybrid multi-mode control software was demonstrated in the M400X Skycar and was used during its initial flight-testing throughout 2001, 2002 and 2003. The system is designed for single-vehicle uses, and is capable of responding to predictive modeling of environmental effects (e.g., wind gusts, turbulence) and safely controlling mode transitions under such effects. This technology provides enhanced maneuverability and stability for our UAV Aerobots and enhanced robustness under extreme conditions for piloted systems like the Skycar and Neuera, increasing the pilot and passenger survivability and decreasing pilot workload. Multi-modal control technology will provide better-controlled transitions between complex operational flight modes (inherent in vertical takeoff and landing UAVs and high performance manned aircraft), thereby reducing safety risks to the pilot, passengers and vehicle. The system architecture is divided into three hierarchical levels of control: Low-level Control (Stability & Control) This level includes the stability and control functions required to achieve and maintain vertical takeoff and landing (VTOL) operations and hover capabilities. These provide the Skycar with dynamic stability, regulation of flight parameters, as well as tracking of basic input commands. Although there are many control law architectures, the classic PID control approach augmented with online gain scheduling provides the ideal mix of robustness and performance for typical aircraft dynamics. Primary stability control for the Skycar is achieved by manipulation of the Close On RPM (CORPM) loop. This control loop queries attitude and rate sensors and commands changes to engine speed to compensate for any un-commanded or un-desired attitude change in the aircraft. This basic stability loop provides control over Pitch, Roll, and Yaw. Other features to be implemented in future versions of the control system will provide Airspeed Hold, Altitude Hold, Altitude Change Rate Hold, Turn Compensation, Turn Coordination, Turn Rate Control, Bank Angle Hold, and Heading Hold. The stability and control loops can be tuned to provide the desired performance by adjusting a set of parameters or gains. This is done through linear analysis - the nonlinear aircraft model is linearized for a representative set of flight conditions that cover the operating envelope of the aircraft. The linear dynamics of the closed-loop system (aircraft + control system) are analyzed in terms of stability and control responses (overshoot, settling time). Mid-level Control (Synthesis of Navigation & Sensing Info) The mid-level control level provides augmented flight operations, guidance and navigation information for the Skycar pilot, including assisted take-off and landing routines, climb rate control, airspeed dependant thrust vector settings via nacelle rotation and outlet vane changes, and in-flight trim adjustments. This system includes interpretation of pilot inputs and the
generation of commands to the low-level control laws. Also included in this level are systems that display information from navigational instruments and sensors. These include Highway in the Sky route displays, GPS, radar, hi-resolution and infrared cameras, ground proximity warning and enhanced obstacle/aircraft avoidance systems (TCAS, TAWS-A, EGPWS), and real-time weather advisories. The objective is to provide an augmented reality system that provides information to the pilot and/or autopilot that will ensure a safe and precise course of action. High-level Control (Autopilot/Pilot Assistant) The highest level of control is arguably the most complex and it provides a higher degree of autonomy to the Skycar thereby allowing routine operation without interaction of the pilot. This involves interpreting the flight profile objectives and safety constraints, awareness of the current aircraft and environment conditions, online updates of the flight plan such that the objectives are optimally achieved. This level also provides a certain amount of fault tolerance to the Skycar by detecting sensor, actuator, or airframe faults and reconfiguring the low and mid-level control algorithms appropriately.