Aircraft tyres have to sustain very high loads and as a result of this there is significant change in the tyre cavity when the tyre is loaded compared to the inflation pressure when it is unloaded. This is quite different from the situation with car tyres where there is very little change in the cavity pressure between the unloaded and loaded states. In conducting finite element analysis of a tyre, the first step is to inflate the tyre followed by applying the pre-load. It is normally assumed that the inflation pressure does not change from step 1 to step 2. However, it has been observed in physical tests that in an aircraft tyre the tyre cavity pressure increases by as much as 20% between the unloaded state and the application of rated load. Therefore in order to ensure the accuracy of FE simulation of aircraft tyres, the change in tyre cavity pressure must be taken into account.

In this paper, the air in the tyre cavity is modelled as an ideal gas in order to represent the physical behaviour of the tyre under loading conditions more closely due to the change in tyre cavity volume and air compressibility. The modelling approach adopted to achieve a more realistic simulation of tyre behaviour is to represent the inflator gas inside the tyre as an ideal gas. The ideal gas equation at constant temperature is given by:

p = -K[(V(p,θ) - V_o(θ))/V_o(θ_I)] = -Kρ_R[ρ^-1(p,θ) - ρ₀^-1(θ)]

where:

K: Fluid bulk modulus

p: Fluid pressure

θ: Current temperature

θ_I: Initial temperature

V(p,θ): Current fluid volume

V₀(θ): Fluid volume at zero pressure and current temperature

V₀(θ_I): Fluid volume at zero pressure and initial temperature

ρ(p,θ): Current fluid density

ρ₀(θ): Density at zero pressure and current temperature

ρ_R: Reference fluid density

 For all the simulations carried out, including tyre inflation, hyper-elastic material property was assumed for modelling rubber and cord materials, using the Yeoh model. Rim-tyre interaction is considered in the simulation and it is assumed that the cords are embedded into the rubber region. A fully detailed model was developed taking into account the variety of material properties used in the tyre construction, where the rubber region is formed of four different major rubber components.

 The predicted behaviour of the tyre in terms of predicted cavity pressure is compared to values measured in a physical test. Tyre performance in terms of load-displacement, tyre cross-section sizing, tyre footprint profile, and contact pressure distribution are also compared to measured values. Comparison is also made to results obtained from simulation assuming no change in tyre cavity pressure due to application of load. The behaviour of tyre simulated with ‘air-inflated’ (ideal gas) model shows a closer agreement with physical test in comparison with ‘pressure-inflated’ model in most parameters. Finally, the effect of modelling the air inside the tyre cavity as an ideal gas on the tyre forces and moments generated during braking and cornering is investigated.