**2. Working of load simulator system**

**1. Introduction**

78 Recent Progress in Some Aircraft Technologies

In a flight control system, aerodynamic loads are introduced during flight at the control surfaces or fins of flight vehicle as a function of air density, fin angle, etc. The fin control system must respond to these loads to maintain the accuracy of flight control system [1–3]. Since load simulators are used for the qualification of flight actuation system, the control performance of load simulators is vital [4, 5]. Before the invention of the motorized load simulators, the mechanical springs or torsion bars were utilized for the testing process. Conventional testing has several disadvantages, for example, the loading devices are not adaptable to varying loads. In modern ages, the testing and qualification of flight actuators are done through a hardware in the loop simulator system. In order to reduce cost and time, hardware in the loop simulators are utilized in which a loading motor is used as the aerodynamic loading device [6–9]. There are three types of load simulators depending on the type of applications and loading require‐ ments [2–6]. Electro-hydraulic load simulators (EHLS) are widely used as hardware-in-theloop-simulation (HILS) system in flight control applications that can simulate the air load executed on flight actuation systems. The EHLS can simulate rapid and large torque/force loads with big volume. Some advantages and disadvantages are listed. The hydraulic load simulators move fast and have rapid steering. The natural frequency of the electro-hydraulic servo valve is generally above 100 HZ, so the hydraulic load simulator's frequency response is very fast and smooth. The low-speed performance of electro-hydraulic actuator is good. The load simulators can be operated in wide speed range. On other hand, due to big size and volume, maintenance cost is high. Temperature effects can cause degradation of performance. At low temperature the friction loss is high, and at high temperatures the leakage phenomenon is significant. Leakage of hydraulic fluid or oil is a major drawback. It causes the environmental pollution. This can easily cause a fire. The oil contamination causes system blockage and system failures. Another type is the pneumatic load simulator that is a typical hardware in loop (HIL) system used to apply medium-range torque/force loads on the actuator under test. The pneumatic load simulator can track the fast deployed load with compressibility. Pneu‐ matic systems are often used instead because pneumatic pressure is usually cheaper to obtain, especially since most industrial facilities already have compressed air available. The pneumatic load simulators use air or any other compressed gas to transmit power. It is hard to obtain smooth performance due to large compressibility factor, especially at low speeds. Similarly, temperature changes can degrade the performance of pneumatic load simulators. The pneumatic load simulators are smaller in size and volume as compared to hydraulic load simulators, so the maintenance cost is lower. The electrical load simulator is an important ground-based hardware tester simulator used for qualification of flight actuation system, such as the aircraft control surfaces, ship steering system, robotics arm, undercarriage of the plane and high-speed elevator system. The electrical load simulators are used to apply medium and small loads on actuators under test. Some advantages of electrical load simulator are its small size and volume, low installation and maintenance cost, less sensitive to temperature changes as compared to hydraulic and pneumatic load simulators and high performance. There are no

environmental hazards associated with electrical load simulators.

Laboratory testing and qualification of flight actuators play vital role for ensuring the design of reliable flight control with cost-effective solutions. The hardware diagram given in **Figure 1** is the minimal setup that is used in the laboratory testing of the flight actuators. The flight actuator is directly connected to the loading motor through a stiff shaft. The direct mechanical connection allows the application of the loading torque generated at the output shaft of loading motor in a close loop system. The Autopilot controller is used to set the reference position trajectory of the flight actuator. An important block of the system is the aerodynamic load calculator system, and the inputs to this block are the reference position command of the actuator system, air speed, Mach number and angle of attack. The output of the load calculator block serves as reference command to the loading motor. Thus, the torque load loop is closed using the feedback signal from torque sensor. The advantage of using the proposed setup provides the designer a freedom for applying real-time loads simulated in the laboratory with minimum cost.

**Figure 1.** Block diagram of electrical load simulator.
