Material Design Analysis of a Trunnion
The landing gear of an aircraft has a number of demands placed upon it. It needs to be able to absorb shock energy from landing, allow the aircraft to be steered, and ultimately support the aircraft when it is on the ground. With the technological knowledge that is available, most aircraft now have retractable undercarriages to reduce the parasitic drag that the aircraft experiences - another technical specification for the landing gear.
A particular part of the landing gear system that this report will investigate is the trunnion. This is a fixed structural support that is a part of (or attached to) the upper strut cylinder of a landing gear strut. It contains bearing surfaces, which permit the rotation of the entire gear assembly for steering.
This report will explore the objectives and functions of the trunnion and investigate the most suitable materials that could be used for the component. Specifically, the report will consider the trunnion used in the port mainwheel undercarriage of the F-35 Joint Strike Fighter, since the Department of Defence recommended fixes to the F-35C landing gear in January 2017 (Mizokami, 2017). From there a case study of the materials and the manufacturing methods that could be used can be put together, and then compare this with what is currently used and what academic papers and journals suggest.
Functions of a Trunnion
According to Dale Crane in Volume 1 of the Aviation Maintenance Technician book series, trunnions are "projections from the cylinder of a retractable landing gear strut about which the strut pivots to retract" (Crane, 2006). The purpose of a trunnion is to structurally support the landing gear in bearing the weight of the aircraft when it is not flying. This includes during takeoff, landing, and taxiing, as well as when static, and so the landing gear is subjected to several different stresses and strains in different directions to variable extents, depending on the task at hand and the type of environment it is subjected to. By environment, it is important to consider different situations such as the type of landing surface - whether it be tarmac, snow, grass, etc; the altitude from sea level; and the pilot's inputs such as sharp or gradual turns and braking, rough or gentle landing, etc. There is a considerably wide variety of variables that can affect the forces subjected to the undercarriage of the aircraft differently.
Another major thing that factors into the stresses and strains that the undercarriage is subjected to is the design of the undercarriage. There are several types of undercarriage, with the most popular now being the tricycle - one nose wheel and two main wheels - due to its good ground handling characteristics and stability. Retractable undercarriages are also popular in modern designs. Early aeroplanes had fixed undercarriages and cruised at slow airspeed, so parasite drag was not a major consideration in the design. However, when speed became of great importance for aircraft, the retractable landing gear was introduced to reduce this parasite drag and create a more streamlined shape when flying.
The F-35 has a retractable tricycle undercarriage, with Oleo shock struts to dampen vibrations and absorb shock energy from the impact of landing. However the trunnions in landing gear are still subjected to some shear stresses from the impact of landing and the weight of the aircraft when static and during taxiing. The trunnion in the leading nose wheel would also endure torque from steering, and when the undercarriage is retracted and lowered.
Constraints and Objectives
The material of a trunnion would need to be able to withstand a number of different tensile and shear stresses and strains throughout its lifetime in service, and therefore have a high modulus of elasticity and modulus of rigidity. It would also have to be considerably hard in order to reduce the amount of wear and fatigue from the continuous cycle of retracting and lowering the undercarriage, since this is the main pivotal point for the retractable undercarriage. This will also come into play during steering.
Another highly important constraint is corrosion resistance. On the ground the trunnion will be exposed to the elements. Corrosion can grow and jeopardise the functionality and the airworthiness of the aircraft, leading to expensive repairs and premature replacement of components.
The trunnion would also need to be able to endure some shock energy from takeoff and landing that the damping system of the undercarriage may have been unable to absorb, and would therefore need to be relatively tough. This is the main reason that the landing gear of the Royal Navy's F-35C's need redesigning. During takeoff and landing tests, it was realised that the aircraft made "a sudden jarring motion" on takeoff, which is "not only uncomfortable but the Helmet-Mounted Display (HMD) and oxygen mask push up and down against the pilot's jaw" (Mizokami, 2017).
The material used to manufacture the trunnion would also need to be as lightweight as possible without jeopardising the tensile and shear strength requirements. Since the cost of the F-35 has been a huge issue throughout its production, the material would want to be as cost effective as possible. In terms of machining costs, since the part would probably be produced through CNC milling, the material would be preferably easy to machine in order to reduce the hours that each part is on the machine, however again this would be second to the tensile and shear strength requirements of the material.
Free variables for the design of the trunnion would be limited. The colour of the material would be a free variable since the part would later be painted.
Using the Ashby plots of materials (see Appendix 1), a selection of potentially suitable materials for the trunnion of the F-35C nose landing gear was put together. The best choices are the following:
- Titanium alloys
- Steel alloys
- Carbon fibre reinforced polymer (CFRP)
Carbon fibre reinforced polymer contains carbon fibres bound with a polymer, giving the material a considerably high strength-to-weight ratio and a high resistance to corrosion. The main problem with using CFRP for the undercarriage is the cost of the material and manufacturing. CFRP is still a very expensive material to make, and due to its high strength it requires especially made tools to machine it. Tungsten tools with a diamond coating are the best choice for machining CFRP (Kennametal), however the tool is required to be replaced every few components, and the lead time for machining would be considerably longer than for other metals.
Steel and titanium alloys are very commonly used for the undercarriage of aircraft, as they are very strong, can withstand a large amount of stresses and strains, and are easier to machine than composites. They require a non-corrosive coating to protect them from the elements, however they are still widely used for the landing gear on aircraft.
Validation of Materials
There is a restriction of information available to the public regarding the exact materials currently being used for the undercarriage of the F-35. However, it is clear that at least some of it is made from composite materials.
Fokker Technologies - a division of GKN Aerospace - signed a contract with Lockheed Martin to supply parts of the landing gear for the F-35C (Fokker Technologies, 2015). The Dutch company, founded in 2011, specialise in landing gear for helicopters and aircraft, and received the contract for the design and development of a drag brace polymer matrix composite (PMC) for the F-35 landing gear in 2015 (Fokker Technologies, 2015). Since the production of composite aircraft parts is still a relatively new and highly considered technology, it is understandable that specific information is so scarce.
In previous fighter and bomber aircraft, such as the Tornado and the Harrier, different grades of steel and titanium alloys were used for the undercarriage due to their strength and machinability. The material properties could then be further improved to suit the purpose of the part with the help of heat treatment and coatings. However these parts are expensive to produce in terms of material, labour, machinery, and waste material costs. A number of technological advances have been made since the initial production of these aircraft 40+ years ago, and the aerospace industry is now leaning closer towards the use of composites.
Fokker have provided a statement on the benefits that their PMC technology offers compared to traditional metal alloys:
- "Increased aircraft performance due to weight reductions of up to 30%
- Increased durability and robustness of landing gear
- Elimination of metal corrosion and cracking" (The Shot Peener, 2011)
Although they are considerably lighter than metal alloys, composites are extremely expensive, mainly due to the need for a mould to create a sufficient product. Making a mould is not always easy and often requires the help of a specialist to produce, which increases the production time and cost considerably.
However it appears that composites are now used widely in the landing gear of aircraft. UTC Aerospace Systems - formally Goodrich - are also involved in supplying parts for the landing gear for the F-35, made from composite materials as well (UTC Aerospace Systems, © 2017).
Investigating the suitability of different materials for a structural support such as a trunnion offered a small range of both traditional metal alloys and the more innovative composites. Since trunnions are subjected to a number of different linear and angular stresses and strains, materials with a high modulus of elasticity and rigidity were considered, while maintaining as low a density as possible. This resulted in a choice between a number of metal alloys such as titanium and aluminium, and the more intricate composites such as carbon fibre reinforced polymer (CFRP) and polymer matrix composites (PMC). Each has its advantages and disadvantages: metal alloy parts are easier and cheaper to produce, however composites have a greater strength-to-weight ratio.
Although there was a limitation to the amount of information available due to the F-35 being a currently used aircraft in the military, Fokker Technologies have confirmed that they provide PMC parts for the undercarriage. PMC has a considerably higher strength-to-weight ratio, which is ideal for a multirole STOVL aircraft. Future research could explore previous aircraft designs and the materials used for retractable undercarriages throughout the years for the validation of each of the suggested materials, and investigate different composites in more detail.
Crane, D. (2006). Chapter 6 Aircraft Landing Gear Systems. In D. Crane, Airframe Volume 1: Structures (p. 427). Newcastle: Aviation Supplies & Academics.
Fokker Technologies. (2015, November 19). Fokker Technologies celebrates continuation of the F-35 Landing Gear Composite Drag Brace Program. Retrieved from Fokker: http://www.fokker.com/Fokker-celebrates-continuation-of-the-F-35-Landing-Gear-Composite-Drag-Brace-Program
Fokker Technologies. (2015). Programs. Retrieved 2017, from Fokker Technologies: http://www.fokker.com/Landing_Gear_Programs
Kennametal. (n.d.). Machining Carbon-Fiber Reinforced Polymer (CFRP). Retrieved April 2017, from Kennametal: https://www.kennametal.com/en/industry-solutions/aerospace/machining-cfrp-composites.html
Mizokami, K. (2017, January 6). The Navy's F-35 May Need New Landing Gear. Retrieved from Popular Mechanics: http://www.popularmechanics.com/military/aviation/a24633/navy-f35-landing-gear/
The Shot Peener. (2011, Fall). New Landing Gear. The Shot Peener , p. 6.
UTC Aerospace Systems. (© 2017). Programs - Military. Retrieved from UTC Aerospace Systems: http://utcaerospacesystems.com/cap/programs/Pages/military-aircraft.aspx
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© 2018 Claire Miller