Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
7Development and Flight Testof Multi-aircraft Coordination Control Technology

YOICHIRO MASUKO*1

HIDETO FUJII*2

The number of aircraft operations continues to increase, and unmanned aircraft operations
are gaining momentum. The increased number and complexity of aircraft in the same airspace has
thus become a major issue. To deal with this issue, Mitsubishi Heavy Industries, Ltd. (MHI) is
studying cooperative aircraft swarm control, which treats a number of aircraft as a swarm byutilizing a mutual information network formed among the aircraft. MHI applied this technology tothree small flight-demonstration aircraft to conduct an automatic formation flight test. These
included one small actual flight-demonstration aircraft and two small virtual flight-demonstration
aircraft in a simulator. This paper describes the flight test and results.

|1. Introduction
Aircraft flight control systems are designed to meet stability and control requirements byplacing top priority on aircraft flight safety. Aircraft operation ultimately depends, however, on
pilots and air traffic controllers (ATC).
MHI has advanced the study of cooperative aircraft swarm control, which treats a number ofaircraft as a swarm by utilizing a mutual information network formed among the aircraft. Thecompany has already implemented and introduced an automatic formation flight test for two
aircraft based on a swarm control algorithm1. In this study, we developed a swarm control
algorithm with better general versatility and lower calculation loads for implementation comparedwith our previous work. This new algorithm allows us to conduct flight tests with more than twoaircraft. We conducted an automatic formation flight test using three aircraft, including one small
actual flight-demonstration aircraft and two small virtual flight-demonstration aircraft, to verify thisalgorithm. The sections that follow introduce the developed flight control system and describe theflight test results.

|2. Flight control system
2.1

System configuration

The small flight-demonstration aircraft is based on a commercially available wireless control
system in the market and equipped with a flight control system consisting of a flight control
computer, inertial sensor, air data sensor, GPS receiver, surface angle sensor, data communication
unit, and a power source. This aircraft is approximately 2,500 mm long with a 2,700 mm wing
span, weighed 14 kg, and has a wing area of 1.2 m2, as illustrated in Figure 1.1
The ground station consists of a computer for monitoring and controlling the flight
conditions, a data receiver/transmitter for data communication, an aircraft motion simulator for
simulating the small virtual flight-demonstration aircraft, and a computer for processing the swarm
control algorithm, as illustrated in Figure 2.
The aircraft motion simulator runs the simulation for the two small virtual
flight-demonstration aircraft. It simulates the aerodynamic characteristics of the aircraft as well as
their sensor and computer characteristics to calculate exactly the same algorithm following theflight-control laws as used for the actual flight-demonstration aircraft.

*1

Engineering Manager, Defense Aircraft Division, Aerospace Systems*2

Defense Aircraft Division, Aerospace Systems

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
8Figure 1 Appearance of small
flight-demonstration aircraftIn addition to the equipment shown in the figure, thisaircraft also contains a flight control computer.

2.2

Figure 2

Appearance of the ground station

Monitoring of flight conditions, simulation of small
virtual flight-demonstration aircraft, and processingof the swarm control algorithm are conducted inthis ground station.

Flight control laws

The flight control laws consists of a stabilizing control law to maintain the aircraft flying
quality and stability and a swarm control law to provide acceleration commands to the stabilizing
flight control law (Figure 3).

Figure 3

Outline of the aircraft control law configuration

The control law consists of a stabilizing flight control law and a swarm control law,and it controls the aircraft motion.

2.2.1 Stabilizing flight control law
The stabilizing controller outputs the actuator command signal to the elevator, aileron,and rudder actuator to stabilize the aircraft and maintain the flight characteristics regardlessof the flight condition. The control law is designed by applying the H-infinity theory. Thecontrol mode from the ground station can be selected between RC mode (do not use thestabilizing control law) and FBW mode (do use the stabilizing control law) by the ground
operator. However, the flight test is cancelled if the flight control computer detects any
failures when the FBW mode is selected. In this case, the control mode is to be automaticallyswitched to RC mode to ensure flight safety, and this information is to be transmitted to theground station.
2.2.2 Swarm control law
Cooperative aircraft swarm control aims to control an aircraft swarm in an optimal way
while satisfying certain constraint conditions (altitude, speed, obstructions, etc.) imposed on
individual aircraft. Although MHI has already conducted an automatic formation flight testwith two aircraft1, the realization of controlling a swarm with more than two aircraft using
the same method is difficult because the logic becomes much more complicated.
To solve this problem, we applied the concept of model-based predictive theory to theswarm control law. In our newly developed swarm control law, the aircraft predicts the path
to be taken by obtaining information about obstructions, the position and speed of otheraircraft, and self-information and also by adopting the constraint conditions of the individual
aircraft such as its own turning ability and acceleration performance. Then, the swarm

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
9control law outputs acceleration commands, which are based on the predicted path, on to thestabilizing flight control law. The application of this predictive control method enables
matrix-form expression of the constraint conditions. Moreover, the expansion of thisconstraint-condition matrix allows us to easily increase the number of aircraft in the swarm.
The model-based predictive control theory itself, however, has difficulty enhancing thecomputer processing speed because the computation load to calculate the solution increases
with the number of constraint conditions. Nevertheless, because aircraft flight control
requires real-time processing, the processing load on the algorithm needs to be mitigated.
Additionally, depending on evaluation function settings and constraint conditions, thisalgorithm has a problem with infeasible solutions during flight. Thus, solving these problems
is mandatory to ensure flight safety. MHI decided not to simply expand the evaluation
function and constraint-condition matrix but to make them a practical form for aircraft
motion. This allowed us to develop an algorithm that mitigated the processing load to an
extent that meant it could be processed by current airborne computers and prevented
infeasible solutions. The flight-demonstration aircraft that MHI owns are small and cannot
carry the usual airborne computers used in operational fighters. Thus, the swarm control law
algorithm is processed by a dedicated computer at the ground station, and the calculated
results are transmitted to the small flight-demonstration aircraft via the data communication
unit. The data communication unit also provides various information required by the swarm
control algorithm from the small flight-demonstration aircraft.

|3. Flight test results
We conducted the flight test by dividing it into various steps while ensuring the flight safety.
The two cases below describe the automatic formation test that we conducted.
(a) Fly in a figure 8 while maintaining triangle formation, as shown in Figure 4(a).
(b) Introduce virtual obstructions in the path described in (a), and avoid them while maintaining
the formation to the maximum extent possible, as shown in Figure 4(b).

Figure 4

Flight test case

Both cases (a) and (b) were conducted in triangle formation; (a) was without obstructions, and (b) was withobstructions.

The flight test results are shown in Figure 5.
Although the small actual flight-demonstration aircraft experienced a reduction in formation
control because of the wind effect during flight tests (a) and (b), it did not violate the constraint
condition of minimum distance between aircraft, which was incorporated into the design of theswarm control law, and no collision occurred between aircraft. The flight test was completed
without any breakdown in flight control processing due to the increased computation load or theabsence of a solution
In flight test (b), virtual obstructions were placed on the route by a command from theground station and were incorporated into the swarm control law in real time. As a result, wedemonstrated that the aircraft could avoid virtual obstructions automatically with maintaining
formation. In the future, the aircraft will collect airspace information such as obstructions fromradar or other sources and will be able to avoid them at their discretion.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
10Figure 5

Flight test results

Each aircraft could successfully keep flying without collision in both cases (a) and (b).
The swarm could avoid collisions with the obstructions in (b).

|4. Conclusion
MHI conducted an automatic formation flight test using three aircraft (one small actual
flight-demonstration aircraft and two small virtual flight-demonstration aircraft in a simulator) witha swarm control algorithm as an example of cooperative aircraft swarm control technology, andobtained desirable results. We are advancing this technology to control more aircraft while
improving the control capacity toward practical applications. Improving aircraft-to-aircraft
information network reliability is vital to commercializing this research.
In consideration of the “Future Fighter R & D Vision2” recently announced by Japan
Ministry of Defense, the cooperative aircraft swarm control technology that MHI is developing will
become an integral part of an aircraft flight control system. Furthermore, cooperative aircraft
swarm control technology is expected to ensure safer, more efficient operation of aircraft. Linking
information among mobile objects on the ground, on the sea, in the air and in the space is expected
to improve overall safety during vehicle operation.

References(1)(2)

Masuko, Y., Toward Cooperative Aircraft Swarm Control, Mitsubishi Heavy Industries Technical Review
Vol. 42 No. 1（2005） p.20
The website of Japan Ministry of Defense (http://www.mod.go.jp/j/press/news/2010/08/25a_02.pdf)