Professor David Crolla
The key issue in the dynamic behaviour of BLOODHOUND SSC is the directional stability over its entire operating speed range from 0 to 1000 mile/h. The vehicle must also be able to steer and turn like a conventional wheeled vehicle, but its prime objective is extremely high speed, straight running rather than cornering as is the case, for example, with high speed racing vehicles.
The term ‘stability’ is used in many vehicle design contexts, but it is important to define ‘directional stability’ from the outset: when the vehicle is perturbed from its equilibrium condition, e.g. straight running, it is directionally stable if the force and moment system causes it automatically to return to the original equilibrium condition.
Despite the enormous design challenges with this type of vehicle, there are still only two fundamental sources of external forces and moments which contribute to its directional stability:
The wheels are assumed to be able to generate lateral forces in response to slip angles in rather the same way as we know tyres behave. The mechanics in our case are however, completely different; tyres generate forces at the contact region due to friction between the rubber tread and the ground surface. For solid wheels operating on a deformable medium, e.g. some type of salt or silt desert surface, the force generation mechanism is controlled by the wheel/soil friction and the internal soil friction when it deforms. One of the most serious shortcomings in attempting to predict the dynamic behaviour of BLOODHOUND SSC is the lack of data or understanding of this wheel/soil interaction.
There are three mechanisms by which the aerodynamic forces and moments have major influences on stability.
The first mechanism is in directly influencing the directional stability through the substantial aerodynamic side forces and yaw moments on the vehicle body; these forces and moments arise when the vehicle body acts at a small angle of attack relative to the straight ahead position. The net resulting aerodynamic side force and yaw moment are sometimes combined and referred to as a single force acting at the aerodynamic centre of pressure in side view. Directional stability is associated with this centre of pressure being aft of the vehicle mass centre – often referred to as the aerodynamic static margin.
The second mechanism is associated with the wheels. For the non-steered wheels which may be directly in the airstream or faired in, their aerodynamics simply add on to the body terms and are therefore included directly. However, for the steered wheels, these are capable of generating additional lateral forces by operating at an angle to the vehicle body, and hence, a different angle of attack relative to the airstream. They behave in a similar way to the wheel/ground system in generating a lateral force in response to a slip angle. The effectiveness of this mechanism depends very much on how much the steered wheels are in the airstream rather than hidden in the body.
The third mechanism is an indirect route; as the speed varies the aerodynamic downforce and pitch moment vary, and these in turn control the wheel loads, which in turn then influence their ability to generate lateral forces at the wheel ground contact region. On BLOODHOUND SSC it is proposed to manage these wheel loads throughout an entire run using programmable winglets over the front and rear axles.
The vehicle – rather obviously – has an enormous speed range and it is important to remember that it must retain controllable (and preferably stable) behaviour right throughout this range, rather than focussing attention in its unique extremely high speed range. Broadly speaking, it is fair to assume that its low speed behaviour (up to 200 mile/h) will be dominated by the wheel forces, whereas at higher speeds it will be dominated by the aerodynamic behaviour, with some contributions from the wheel forces.
Throughout the speed range, therefore we need data or estimates of how the following parameters change both with speed and acceleration/deceleration in order to estimate a complete stability profile for the vehicle:
Mass centre position
Aerodynamic forces and moments
One particular challenge will be to ensure that the steering system maintains an acceptable level of control and consistency over the vehicle trim throughout the speed range.
The directional stability of BLOODHOUND SSC has been analysed using a linear dynamics model using the MATLAB/Simulink package (Fig 1).
Fig 1 Plan view of vehicle model showing the wheel/ground and aerodynamic restoring forces and moments
The data requirements for this model pose some problems; the vehicle layout, mass and inertia properties are relatively straightforward outputs from the overall CAD model. The aerodynamic force and moment data are estimated from the CFD prediction work described elsewhere. However, the estimated data for the properties of the wheel/ground forces are one of the least certain areas. There is very little data in the off road literature on the force properties of rigid wheels on deformable surfaces, and none at all above low speeds, i.e. several mile/h, so we have been forced to estimate these properties based on collecting together off road experiences from a range of sources including THRUST SSC and DIESELMAX.
The outputs from these modelling results have been used to in two ways:
1. To predict the directional stability of the vehicle, and hence propose requirements for managing the airflow to achieve acceptable stability.
2. To understand the sensitivity of the predictions to assumptions in difficult areas, e.g. the wheel/ground contact forces.
Some of the design guidelines emerging from this study are:
1. Maintain the vehicle mass centre ahead of the mid-wheelbase position.
2. Maintain the aerodynamic centre of pressure aft of the mass centre.
3. Maintain the axle loadings close to their static values with a bias towards slightly more at the rear.