Landing gear shimmy. Detect, understand and fix shimmy problems early in the design using simulation. Igo Besselink.

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1 LMS Conference Europe, March 22/ Landing gear shimmy Detect, understand and fix shimmy problems early in the design using simulation TNO Automotive Igo Besselink 23 March

2 Contents introduction: what is shimmy? analytical stability results using a simple model solutions to solve shimmy stability problems detailed shimmy analysis -modelling of the landing gear -tyre modelling conclusions 23 March

3 Introduction Shimmy is an unstable lateral/yaw vibration 2 examples: helicopter NLG measurement on aircraft MLG 23 March

4 Tyre marks on the runway 23 March

5 Some notes: frequency typically in the range of 10 to 30 Hz degree of instability may vary from annoying vibrations up to structural damage or even a collapse of the landing gear shimmy can occur on both nose and main landing gears Twin wheeled cantilevered main landing gears as seen on many commercial aircraft may experience shimmy stability problems. Bogie landing gears and levered suspension configurations are generally not sensitive to shimmy 23 March

6 Landing gear configurations danger of shimmy! 23 March

7 The importance of simulation models full scale shimmy testing on an aircraft may be very risky and dangerous (also indoor testing on a drum may be not be successful) no room to experiment or create various prototypes: the design has to be the first time right! shimmy is a complex phenomenon: testing only does not directly lead to a solution: trial and error, unclear solutions to solve the problem shimmy stability should be considered in the initial design phase of an aircraft when no hardware is available 23 March

8 The cause of shimmy (1) interaction between the dynamic behaviour of the landing gear structure and tyre simplified representation tyre lateral force (Fy) results in a side slip angle of the landing gear due to the side slip angle the tyre develops a lateral force 23 March

9 The cause of shimmy (2) the feedback loop may become unstable! both the dynamics of the tyre (e.g. relaxation behaviour) and dynamics of the structure (e.g. eigenfrequencies/modeshapes are important) some other views on shimmy: energy is extracted from the forward motion of the aircraft and converted into a lateral/yaw vibration for a rolling tyre a combination of lateral and yaw input exists where self-excitation occurs 23 March

10 Analytical solutions trailing wheel system with lateral flexibility (topview) 23 March

11 Analytical solutions (q=0 and zero damping) also unstable, but generally less problematic problem area 23 March

12 What can we learn from the analytical results? shimmy problems can be solved by changing stiffness, geometry and/or inertia just increasing the (yaw) stiffness is no guarantee to solve the shimmy instability effectively two feasible solutions to avoid instability: small negative trail yaw stiffness low lateral stiffness high large yaw inertia big positive trail yaw stiffness high lateral stiffness low small yaw inertia note: tyre characteristics will also affect shimmy stability, but are difficult to change in practice 23 March

13 One step closer to a real landing gear simple multi-body model consisting of: three rigid bodies -strut -trail body -wheelaxle two wheels two tyres (TNO MF-Swift) bodies are connected by means of revolute joints simple, but representative 23 March

14 Animation (baseline) 23 March

15 Simulation of a landing event velocity: 75 m/s (=270 km/h) asymmetrical spin-up of the wheels on touch down tyre vertical force tyre longitudinal force 23 March

16 Some more simulation results tyre side slip angle tyre lateral force time scale: 2 sec. 23 March

17 Design study Yaw Stiffness Variation red: nominal (4e5 Nm/rad) green: increased yaw stiffness (5e5 Nm/rad) blue further increase (6e5 Nm/rad) 23 March

18 Animation (increased yaw stiffness) 23 March

19 modifying the geometry (negative trail) 23 March

20 Animation (modified geometry) 23 March

21 Modelling in an engineering environment so far: (highly) simplified models not suitable for designing a real landing gear but definitely important to develop some feeling for the problem! For an accurate, predictive shimmy analysis both landing gear and tyre have to be modeled accurately! Several component tests may be necessary to validate certain aspects of the model, e.g. stiffness tests, modal testing, tyre testing, 23 March

22 Landing gear structure 23 March

23 Stiffness modelling stiffness is dependent on the shock absorber closure due to changes in overlap and torque link geometry (stiffness may change by a factor 2 or more) also important: flexibility of the fuselage/wing 23 March

24 Modelling of the vertical air spring/damper characteristics 23 March

25 Non-linear behaviour free-play: reduces the effective stiffness of a landing gear. Older, worn landing gears may have a reduced stability margin. A simulation study may be necessary to determine the maximum allowable free-play limits friction: cantilevered landing gear designs may exhibit quite a bit of friction. To get a good agreement with tests on the aircraft and component tests it has to be included in the model. Friction may also hide possible shimmy instability problems 23 March

26 Tyre modelling traditionally in aircraft shimmy analysis the tyre models of Von Schlippe, Smiley or Moreland are used. Theory of these models dates back to the 1940 s and 1950 s More up to date models are available Over 20 years of research at the TU Delft and TNO Automotive under the guidance of professor Pacejka has resulted in tyre models called MF-Tyre and MF-Swift 23 March

27 Experimental validation (MF-Swift) yaw oscillation test stand frequency response functions of side force and aligning torque to yaw angle up to 60 Hz 10 6 amplitude ratio F ψ y 10 6 amplitude ratio M ψ z Fr equency [Hz] Frequency [Hz] Exper iment Simulat ion V [km/h] 23 March

28 MF-Tyre/MF-Swift originally developed for passenger car tyres validated extensively by numerous experiments most recent version: MF-Tyre/MF-Swift 6.0 in comparison to the older MF-Tyre 5.2 and Swift 1.2 much more useable for aircraft simulation studies: very significantly reduced data requirements build in parameter estimation procedures includes turn slip, important for shimmy 23 March

29 Advantages over traditional shimmy tyre models includes non-linear behaviour with side slip angle, both forces and dynamic behaviour (e.g. reduction of the relaxation length) combined slip (braking and steering at the same time) includes gyroscopic effects of the rotating tyre belt can drive over short wavelength obstacles includes bottoming of the tyre includes turnslip for steering/twisting moments at standstill one model which can cover all possible applications! 23 March

30 Concluding remarks Simulation allows to identify landing gear shimmy problems very early in the design stage, where you still have the chance to change the design for the better Modern tools as LMS Virtual.Lab Motion and TNO tyre model MF-tyre/MF-Swift 6.0 can be applied successfully for the analysis of aircraft shimmy stability Despite a long history, shimmy is still a very relevant design issue for aircraft landing gears! 23 March

31 Contact TNO Automotive - Helmond, the Netherlands Igo Besselink (Igo.Besselink@tno.nl) PhD thesis: Shimmy of Aircraft Main Landing Gears TU Delft, March