By Boris Marovic and Thierry Olbrechts, Siemens Digital Industries Software
Aircraft that carry crew and/or passengers must pass one or more icing-related standards for certification. The Federal Aviation Association (FAA) is concerned with commercial and private aircraft icing because it can cause the engine to stop working. For light aircraft, ice on the wings and tail can impact the weight and balance for continued flight, leading to possible fatality. For manufacturers, designing, optimizing, and testing these systems is a significant engineering challenge. Icing damages the aerodynamic shape, increases aircraft drag, and reduces its controllability, which impacts flight performance and safety. Ice protection systems and components play a crucial role in safe aircraft operation. Such systems are usually installed in wings, nacelle intakes, pitot tubes, stabilizers, and propeller and helicopter rotor blades. These safety-critical systems follow a certification requirement per Code of Federal Regulations (CFR) part 23, 25, 27, 29 and others, for the various types of aircraft and rotorcraft as well as engines [1].
To avoid the build-up of ice in cold weather flight conditions while maintaining energy efficiency, from components to systems integration, ice protection systems must be developed early to avoid icing related certification and operational problems. For aircraft manufacturers, simulation and testing play an important role to find the right design alternatives and assure flight safety, even before the first component is built.
System simulation helps optimize ice protection systems development
Most aircraft use bleed air (hot air extracted from the engines) to heat the aircraft interior solid skin and raise the surface temperature to melt the ice and prevent its accretion. Other systems use piezo-electric actuators to break thin sheets of ice that might have built-up. Yet another system uses inflatable leading edges, again to break the ice and evacuate it with the flow before it has the chance to build up. Before choosing the type of ice protection and/or de-icing system, top-level tradeoff studies are required. System simulation provides a key advantage in early engineering design by coupling system simulation with 2D ice accretion code. This allows the system’s power requirements to be refined for various operating and icing conditions during program development.
With pneumatic de-icing, engineers can apply system simulation to evaluate geometries and pre-size some of its individual components, such as the piccolo tubes or piccolo exhaust holes. The entire ducting and the impact of these geometries on the leading-edge nacelle temperature can be evaluated with simulation. This enables a smooth integration of the resulting ice protection system with other pneumatic consumers and their coupling with the engine bleed air system. System simulation also allows engineers to perform “what-if” analyses such as air leaking from a bleed pipe and its impact on the overall system.
Ice protection system component analysis with 3D CFD simulation
After the system type is chosen and the component and performance sizing are completed during system simulation, the components require a more detailed 3D computational fluid dynamics (CFD) simulation for individual performance and optimization.
The most common anti-icing type is the piccolo tube. In the advent of the carbon-neutral aviation industry, development efforts are focused on hybrid electric aircraft which will less or no bleed-air available. Hence heater foils or mats (Fig. 1) are the best choice. These foils attached to critical zones, such as the leading edges of wings and engine nacelle intakes, will be electrically heated. The initial concepts of induction heating coils to heat up the metal leading edge or embedded metal mesh in the composite leading edges are in development, targeted for future aircraft.
Fig.1: Heater foil and thermal simulation to showing temperature distribution and detailed analysis. (Source: ELiNTER AG)
In the early design of such devices, engineers must conduct CFD simulations to identify the areas that will be prone to ice accretion. Simulation gives insight in how much heat energy is needed to avoid accretion in the worst possible environmental conditions. Secondly, at component level, simulation assists the system designers to detail the design and assure the system will work according to the requirements. In the case of heater foils, the general foil design, meander distribution, equal distributed heating performance and other parameters, such as analyzing the limits of the component, can be conducted early.
The next stage is the foil integration onto a wing leading edge and simulating the heat distribution over the airfoil under icing conditions to evaluate that sufficient heat is provided to prevent or remove ice buildup. Parameters such as temperature and heat transfer coefficient distribution over the wing’s leading edge are important for component performance evaluation. Component performance can then be fed back or coupled with a system simulation model with more accurate parameters to increase system simulation accuracy.
Aerospace manufacturers can address numerous icing-related engineering challenges by using advanced simulation software such as Simcenter solutions from Siemens. First off, droplet collection efficiency can be calculated for complex 3D configurations experiencing icing conditions. In addition to collection efficiency, ice accretion can also be predicted on full 3D geometries. The prediction is based on a first-principles simulation of all the relevant physics, which allows the ice shape to be captured in a time-dependent fashion. The impact of the accumulating ice on lift, drag, or any other engineering quantity of interest can also be tracked to make sure the system maintains the needed aerodynamic performance throughout the flight envelope under icing conditions (Fig. 2).
Fig. 2: Using simulation software to predict ice accretion on an airplane’s wing under various climate conditions, featuring prominent horns and scalloping. The simulation shows the influence of the ice on the lift and drag of the wing. (Simulation by Scott Wilensky.)
Using simulation software, an optimization study can be performed to determine the best placement and orientation of the holes on a piccolo tube anti-icing system. Hot air is blown through these holes to warm the skin at the leading edge of the wing. The study’s objective is to maximize the average temperature of the respective wing surface. Due to the modular multi-physics approach, a high level of fidelity can be achieved by activating an appropriate subset of the models implemented. Thus, droplets can be modeled as a dispersed phase (Eulerian physics) or as individual particles (Lagrangian physics), as needed. Similarly, impinging droplets can be set to all stick to the surface upon impact or to have some bounce back into the air based on local conditions. Water on the surface is incorporated into a film layer, which may freeze immediately or run back over the heated surface before freezing. Additional models allow the prediction of phenomena such as droplets being stripped off the liquid film by the air blowing past.
Wing anti-icing systems testing
Wing anti-icing systems are commonly fitted in the internal slat structure and run throughout its span in between the ribs. A critical function, this system must pass a qualification test dictated by international standards which prescribe several dynamic tests, including random, shock and sine excitation tests, to study their effect on the parts composing the anti-icing system. Target vibration levels are defined at the attachment locations of the system to the wings’ ribs. However, one issue specific to the anti-icing system is its typical dimension. The system runs through the wingspan, which makes it a very slender body (length is much larger than cross-section area). Such a flexible structure is cumbersome to excite with a traditional single shaker setup. An even bigger challenge is to establish a uniform and representative excitation level at the attachment points.
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Fig.3: Multi-exciter setup for piccolo tube vibration and performance testing.
Multiple-output, multiple-input (MIMO) technology can help overcome these difficulties and make sure each excitation point is simultaneously excited with the appropriate loading2. Figure 3 shows up to five shakers mounted at the locations where the loads are transmitted to the structure. The amplitude levels are maintained at the prescribed levels using state-of-the-art MIMO control algorithm from Simcenter testing software and hardware. The system can be exposed to the right level of vibration at each location, drastically reducing the uncertainties related to its operational exposure to ordinary (ground-air-ground) and extra-ordinary (fan blade out) vibratory loads. Using this state-of-the-art technique enables test engineers to increase efficiency in the whole vibration qualification process.
Conclusion
Design and evaluation of ice protection systems, from the component level to systems integration, should be considered early in the development stage to reduce development cost and time, as well as to ensure certification. Whether it is pneumatic boots in older airplane models or induction-heated leading edges in the new all-electric or hybrid aircraft generations, simulation and test play a major role in the development and certification process for safe flights.
Because ice protection systems are mission-critical systems, their design, development, verification, and certification have been prioritized as part of the Siemens Xcelerator portfolio’s design, engineering, and verification management digital threads, providing performance engineering tools and the data needed for proof of compliance based on virtual and physical tests, specifically for aircraft manufacturers. To find out more, read this white paper: https://resources.sw.siemens.com/en-US/white-paper-aerospace-defense-aircraft-ice-protection-systems-design
References
1. Code of Federal Regulations (CFR), Title 14 Chapter I Subchapter C, available at: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-C (accessed: 03 August 2022).
2. R. Van Der Vorst, S. Vandenberk, A. Carrella, J-L. Magerman, B. Bernay. “Qualification test of an aircraft piccolo tube using Multiple Input Multiple Output control technology”, SAE International, 2012.
About the Authors
Thierry Olbrechts, Director of Simcenter Aerospace Industries Solutions, Siemens Digital Industries Software
Thierry Olbrechts is the Director of Simcenter Aerospace Industries Solutions, Siemens Digital Industries Software. In 1996, he joined Siemens Digital Industries Software. Since 2000, Thierry has been responsible for Siemens simulation and test business development and go-to-market strategies for the aviation, space and defense industry segments.
Boris Marovic, Business Development Manager, Aerospace & Defense, Siemens Digital Industries Software
Boris Marovic is the Business Development Manager for Aerospace & Defense at Siemens Digital Industry Software. He has over 15 years of experience as an application engineer, industry manager, technical manager, and in business development, and has been working with SMBs and large enterprises of various industries worldwide. Boris holds a MBA from the Frankfurt School of Finance and Management, and an aerospace engineering degree from the University of Stuttgart.