In the combined workshop/lab at the Division of Fluid and Mechatronic Systems, Flumes, at LiU, a test rig of the Saab 2000 aircraft has been standing more or less forgotten for more than 20 years. Saab donated the rig to LiU when their development of the plane was completed. For various reasons, the test rig, with its associated systems, was just left standing.
Such a test rig is known in the industry as an “iron bird”, and is a full-scale copy of the control system to be used for testing before it is taken into use. The control of the tail rudders and wing flaps can be tested on the "iron bird".
Photo credit Thor BalkhedThe rig consists of a steel skeleton in which various solutions and concepts can be tested to investigate the interaction between different systems and how they influence each other.
A research group at LiU is now to use Saab’s old test rig to explore the aeronautical engineering we can expect in the future. Given that the rig had been standing there unused for 20 years, getting it to work again was a rather nervous operation. Christopher Reichenwallner is a doctoral student in the Department of Management and Engineering (IEI) at LiU:
“After we had put the test rig together and connected up the computers and systems that Saab used 20 years ago, we pressed ‘Start’. Everything worked perfectly. It was a great relief, and now the challenge is to integrate the new technology with the old”, he says.
Trust in new systems
The first control systems in aircraft consisted of wires connected to the tail rudders and wing flaps. During steep climbs and sharp turns, it required a great deal of strength to direct the plane’s motion. And the larger and faster the aircraft, the more strength required. In such cases, hydraulic systems with servo control are needed to move the rudders and flaps. The hydraulic system has a central hydraulic pump, powered by the aircraft engines. The pump provides enough power to the actuator to turn a jumbo jet, for example.
In most commercial aircraft, the controls in the cockpit are connected to the actuator both electronically and mechanically. The mechanical connection between control and actuator is retained as a safety measure, in case the electronic system fails. This type of solution is still the most common today, even though it has been in use since the end of the 1960s. One of the main challenges with the project is integrating the old and new technology. Photo credit Thor BalkhedThe mechanical control system has been omitted in some more modern aircraft. These use a system known as “fly by wire”, in which the pilot movements control a computer, which in turn controls the aircraft. But basically, the same technology is used as previously, with a central hydraulic pump.
“It’s about whether to trust the new systems: that’s why it takes a long time before the industry dares to change to more modern alternatives. And the proven technology is often retained as back-up. Safety always comes first in the air industry”, says Felix Larsson, industry-based doctoral student at IEI.
More electric aircraft
The aircraft industry talks a lot about the concept of “more electric aircraft”. It describes the situation where as much as possible of the aircraft’s function, apart from the actual propulsion, is to be powered by electricity and controlled by electronics. The current combination of hydraulics, pneumatics, mechanics and electrical systems for different functions make aircraft unnecessarily heavy and complicated, in many cases, which gives high fuel consumption. Advances in components in recent years now make it possible to increase the degree of electrification.
The technology that the researchers at Flumes are now working with has the potential to change control system fundamentally, which will in turn affect the complete design of the aircraft. The LiU researchers, however, are clear over that several solutions are available, and they do not at the moment know which is best.The research group have access to the latest technology partly thanks to the collaboration with Saab. Photo credit Thor Balkhed It is equally possible that a combination of existing and new solutions will prove to be the way ahead. Alessandro Dell’Amico is head of research and senior lecturer IEI, employed part-time at Saab:
“We are contributing to creating the next generation of control systems in aircraft by electrifying the complete platform and decentralising the architecture that controls rudders and flaps. The goals are to increase energy efficiency, save weight, simplify maintenance and increase safety.”
The current plan is that the project will run until 2023, and the research group at the moment is focussing on creating a clear and clearly limited image of the project. Alessandro Dell’Amico continues:
"We have just installed an electromechanical actuator on our iron bird. There are two variants available, where the first is an electromechanical system in which an electric motor controls a ball screw, and the second is an electrohydrostatic system in which an electric motor controls a small hydraulic pump and cylinder. We plan to evaluate both types of actuator.”
Since the choice of control system influences other architecture of the aircraft, it’s not possible to test everything in the rig. This means that the research group is working with simulations to a large extent. Christopher Reichenwallner controls the test rig. Photo credit Thor BalkhedIt is using a digital twin of the test rig to evaluate larger systems, with a mixture of hardware and virtual models. In the long term, realistic flight situations will also be simulated.
“We need to simulate aerodynamic forces and study possible fail situations, to see how the new control systems react”, says Christopher Reichenwallner.
Felix Larsson adds:
“It’s important to understand the complete picture when working in aeronautical development. If you change one system, it will affect others. That’s why it’s incredibly valuable to be able to work on the iron bird. We can use it to validate our calculations against reality.”
The project is a collaboration with Saab, one of the LiU strategic partners. The collaboration agreement with the company focusses on the long-term supply of expertise, research and innovation. This is noticeable also in this project, where students have the opportunity to work on control system development in their master’s projects, something that Alessandro Dell’Amico is clearly proud about:
“It’s just great to be able to offer our students the chance to work with this technology, which truly is ‘state of the species’. Not many get this chance.”
Footnote: The project has been partially financed by the National Aeronautical Research Program (NFFP), whose most important role is to participate in the creation of well-functioning research environments in aeronautical engineering for industry, institutes, universities and university colleges, and to promote collaboration between these bodies.
Translated by George Farrants