"Iron Bird" test stand for verifying the flight control system
Ratio-Drive^® test stand, test actuators as hydraulic test cylinders

The demands on system reliability in commercial aircraft, especially in the area of the flight control system, are very high, as the failure of systems during flight can have catastrophic consequences. The DLR - German Aerospace Center - at the Institute of Flight Systems Technology is working to further increase the safety of systems and to ensure a safe termination of the flight even after unforeseen events.

Hänchen supplies the complete linear drive system for the "Iron Bird" test stand, designed for two elevators, one aileron, and one rudder. To thoroughly test the aircraft hydraulics, the hydropulse test stand simulates the in-flight aerodynamic loads on the control surface actuation systems. This enables the system's behavior to be examined in critical situations under realistic conditions.

Häko product configurator

What makes our
test cylinders special:

Very high response characteristics
Reliability
Stick-slip-free movements
Very high positioning and repeat accuracy

Image 2 The Hydopuls test rig includes an actuator test stand and a cockpit simulation environment coupled to it.

Image 3 The DLR uses a so-called "Iron Bird" as a test environment, which is a test stand for the holistic examination of modern fly-by-wire systems and their subsystems.

Test Stand Technology

Requirements for the "Iron Bird" test stand

Complete solution as a closed testing system
External setpoint specifications from the customer
Hydraulic actuator including sensor technology and control
Engineering and software from Hänchen
Coupling hardware interfaces of external controllers with electronics from Hänchen

Hydraulic test cylinder as test actuator in use

Dimensions 54 / 40 / 180
Suitable for fluid Skydrol
Mounting and accessories: Spherical rod eye, rod flange, protection tube, position transducer, force transducer, mounting plate, diaphragm accumulator, control valves, intermediate plate, flushing plate
Sensor technology: position transducer, force transducer
Seal combination Servofloat^®

Configure your test actuator now.With our HäKo, you can hydraulically design and fully configure a test actuator.

To the configurator

"[...] Ultimately, we were convinced by the good concept and the price-performance ratio of Hänchen products."

Dipl.-Ing. Holger Spangenberg, DLR

Iron Bird Test Environment - Actuator Test Stand and Cockpit Simulation

For researching new solutions, the DLR developed an "Iron Bird" as a test environment. This is a test stand for the holistic examination of modern fly-by-wire systems and their subsystems. In addition to an actuator test stand with hydropulscylinder, it also includes a coupled cockpit simulation environment. This combination enables the research of new flight control concepts and system solutions, as well as extensive testing of these systems at a very early stage of development. Malfunctions can occur, for example, if an aircraft hydraulic system for powering elevators, rudders, and ailerons fails. Therefore, tests in this area, among others, are among the main tasks of the Iron Bird.

The Iron Bird or the "System of Systems"

Since commercial aircraft typically operate at high altitudes and speeds, and malfunctions can lead to severe consequences, a large portion of new development systems are initially examined on the ground to minimize risks to human life. To efficiently conduct research in the area of these safety-critical systems, a test stand was developed that simulates flight conditions as realistically as possible. The Iron Bird enables the execution of flight control concept tests in a closed control circuit. The central objective of the test setup is to determine and evaluate the interactions of the various flight control components.

Interaction and Integration
This means that not only individual systems are tested, but also their interaction and the integration of new components into the system, up to and including the investigation of the influence of new system solutions on flight characteristics. Dipl.-Ing. Holger Spangenberg, responsible for the Iron Bird at DLR, calls this the "System of Systems." A continuous overall system must be present from the control input to the actuator deflection. This allows for more precise investigation of subsystems and the development of solutions for reconfiguring flight control systems. Interactions of aircraft systems are considered with the goal of increasing aircraft safety, even under extreme conditions. Early consideration of system interactions is particularly necessary for complex systems like flight control, as otherwise potential negative influences of various systems might only be recognized late, requiring a significant portion of the development to be redone. "This is also what makes our Iron Bird special: a simulation of the entire flight control chain, combining cockpit simulation and an actuator test stand, allows for a holistic evaluation of flight control systems, even by test pilots," emphasizes Spangenberg.

"From the control input to the actuator deflection, there must be a continuous overall system."

Dipl.-Ing. Holger Spangenberg

Testing options of the test stand

A test stand for many tasks

Modern test stand technology is capable of providing a test environment for a variety of research tasks: new subsystems such as actuators like hydropulscylinders, sensors, or computer systems can be integrated into the test stand. With their help, new algorithms for control or condition monitoring and prognosis are developed, and subsequently tested for their function – even under various fault conditions. Furthermore, the research of new data bus systems such as "Fly-by-light" or "Wireless Flight Control" is possible. Another area covered by the Iron Bird is the research of new flight control concepts for the entire flight control system. This allows for the investigation of the influence of and interaction with subsystems such as sensors, actuators, and computers.

Condition monitoring

For the research of condition monitoring methods for condition monitoring and prognosis of electro-mechanical or electro-hydraulic flight control actuators, the developed procedures are to be investigated and evaluated under conditions as realistic as possible. Here, Hänchen test actuators are used as hydraulic test cylinders to precisely simulate the aerodynamic loads that act on the rudders during flight. The setpoint values for controlling the hydropulscylinders are provided by a flight mechanical simulation. The test system continuously calculates new setpoint values depending on various parameters such as flight speed, flight altitude, and rudder deflection, and transfers these to the cylinder control for the simulation of aerodynamic loads.

Maximum flexibility: Linear drive system Ratio-Drive^® made by Hänchen

Hänchen Test Stand Construction supplies the entire linear drive system for two elevators, one aileron, and one rudder. The complete Ratio-Drive^® system forms a closed system that receives external setpoint specifications. The test stand operates with pressure cells integrated by Hänchen. This means that the complete actuator, including sensor technology, control system, and software, was realized by Hänchen as a complete solution, including engineering. The company was tasked with fulfilling the requirements for the hydraulic cylinders regarding dynamic key parameters, the maximum forces that must act on the system, the accuracy of the sensors, and the general conditions of the existing hydraulic system – specifically, the fluid, maximum pressure flow, and maximum pressure range. This implies extreme flexibility, even with external setpoint specifications. Additionally, interfaces had to be resolved at the hardware level: For example, to bypass Hänchen's control system and implement proprietary control concepts quickly and flexibly with a Rapid Control Prototyping System, it was necessary to couple external controllers with Hänchen's electronics. Since hardly any lateral forces occur during the tests, the DLR uses test actuators of Servofloat^® quality for its test stand.

The Servofloat^® floating gap seal

Hänchen recommended these test actuator for test stand construction, as the patented floating gap seal is the first choice, especially for applications with limited lateral forces with maximum dynamics and quality. The hydraulic cylinders in Servofloat^®-quality are practically friction-free. With extremely slow and very fast movements, they offer non-contact pressure reduction to the outside, low leakage, highest positioning and repeat accuracy and are stick-slip-free.

Equipment test actuator

These seal combinations work according to the following principle: When the system starts, a hydrodynamic oil flow is built up in the cylinder chambers in the floating gap seal, so that the floating gap seal moves metallically without contact in a "floating" and self-centring manner radially around the piston rod: The floating gap seal has a cylindrical shape with a defined clearance to the rod and lies eccentrically in a random position in the installed state. The applied pressure deforms a steel bushing into a pressure-dependent, non-contact sealing gap of a few 1/100 mm. However, this process only works with a production accuracy in the range of a few μm, as otherwise the leakage would lead to high hydraulic losses. Leaking oil is returned pressureless to the fluid circuit via a collecting port. The absence of a touching, pressurised seal minimises static and sliding friction in the hydraulic cylinder. Because no hydrodynamic lubricating film is necessary due to the relative movement of the sliding partners, stick-slip-free movement can also take place in the range of the smallest amplitudes and speeds. The floating gap seal only has a pressure-reducing function with minimal leakage. For this reason, the piston rod of the Hydropuls cylinder is guided by means of friction-optimised wear rings. Nevertheless, a test actuator with a floating gap seal can also absorb certain lateral forces.

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Video about the Servofloat^® design

In addition, this series has anti-seizure properties due to the bronze-coated piston and bearing surfaces, which additionally increase operational reliability. Another compelling argument for test actuator with floating gap seal is undoubtedly the price. This series only differs from test actuator with hydrostatically mounted piston rod guide in terms of its somewhat higher sensitivity to side loads, but offers a significant price advantage. Learn more about Hänchen hydraulic test cylinders as test actuator.

Concept and performance must be right

According to Spangenberg, the experts at DLR decided to use Hänchen cylinders because of "the good concept and the price-performance ratio of Hänchen's test stand technology. The quasi-hydrostatic behaviour with the same control quality and at a very favourable price ultimately convinced us." The recommendation of a colleague from Airbus, who has been working successfully with Hänchen for years, also played a role. Together with Hänchen, DLR developed the requirements for the system, with helpful tips from the hydraulic experts from Ostfildern being incorporated time and again. A current project is the investigation of new control concepts for dynamic load simulation. Once the control parameters and the concept have been finalised, DLR will incorporate the control structure into the mapping system. "Because simply entering parameters is, of course, more convenient than working with external controllers," explains Spangenberg. This makes Hänchen a manufacturer, service provider and system integrator in one for DLR.

Why Hänchen as a test stand manufacturer?

Our competence as a manufacturer of test stands and test machines

Our internal product development and research for the development of test stands, test fields and complex test machines laid the foundation for modern test stand construction. Our high quality standards for our own products gave this development a massive boost. Today, Hänchen has developed into a reliable partner for individual test stands and test machines. Every EoL test stand is custom-made to customer specifications. Hänchen test stands are Ratio-Drive^® projects, i.e. individually developed special machines with a consistent concept. You as the customer have a central contact person for every project. In this way, individual and energy-efficient test fields and complex test machines can be seamlessly integrated into your own systems through customer-defined interfaces.

Hänchen Ratio-Drive^® - The complete solution for drive systems & test stand construction

Complex in-house and customer projects make Hänchen a reliable quality provider of complete test machines today. This reflects our many years of competence development - starting with cylinder construction, expanded to include electronic components and complete drive systems. Hänchen now offers its customers entire machines, including the drive axis, frame and machine elements.

More than half a century of hydraulic and seal competence
Extensive experience in special machine construction of hydraulic and electrical special machines
Experience in the systematised individual construction of industrial machines and retrofits
Development of proprietary modular software, including graphical interface for control and regulation as well as communication at process level
Planning and implementation of test machines and test stands
Comprehensive safety concepts up to PLe in accordance with the Machinery Directive, CE conformity, safety software