Cranfield Group Design Project
A Grand Tour of a Small Island
Abstract:
This project covers the design of a vehicle that complies with the regulations of A Grand Tour of a Small Island Championship. The GR25 meets all the constraints imposed by the regulations and will compete on the various surfaces that the championship visits, as well as the various formats.
Powering the GR25 is a series arrangement of 4 units of the AIE Wankel engine, required by the regulations, coupled with a Helix CTSM-242 HP electric unit, powering all four wheels through a mechanical four-wheel drive arrangement using an RS 90 SAMSONAS transaxle gearbox. The power unit produces a peak torque of 1015 Nm and peak power of 644 kW (864 HP). The MGU contributes 250kW of continuous electrical power and a peak of 400kW. The battery pack is a custom design made from GREPOW Lithium-polymer cells. This 36kg design has an energy capacity of 6 kWh and a maximum charging power of 460kW to optimise recuperation through regenerative breaking.
Inspired by the design of a Radical car, the shape of the vehicle is optimised to meet the various track demands and meet the UK IVA regulations for on-road prototypes. To lower the overall embodied energy, the chassis is a tubular spaceframe built from steel, weighing 105 kg. The wheelbase, trackwidth, mass distribution and aerodynamic balance have been optimised to achieve a peak lateral acceleration of 3.5G and longitudinal deceleration of 4.9G. The ideal mechanical balance is 50% for circuit races, and 16% for rally races. This enables optimum vehicle balance and approaches neutral steer behaviour for a range of cornering scenarios.
The high downforce vehicle leverages its floor, front wing, and rear wing to achieve an overall CL of 2.36 and a CD of 0.71 in its circuit configuration, maintaining an aero balance of 44.6% front.
Overall, the GR25’s global performance numbers are a top speed of 293 km/h, and a peak lateral acceleration of 3.5G. Simulations using AVL VSM revealed qualifying lap times of 61.11 seconds at Anglesey Coastal, 84.9 seconds at Cadwell Park, 49.77 seconds around Lydden Hill, and a ¼ mile drag time of 10.9 seconds.
Aerodynamic Development
Model Validation and Mesh Convergence
Validating the CFD methodology was crucial to gain confidence in the process and its resultant data. Given the similarities in operating conditions between the baseline geometry and the MCR prototype, the use of existing wind tunnel data of the MCR was used to validate the parameters of the CFD studies.
MCR Prototype Used for Model Validation
A mesh independence study was conducted to ensure the validity of results, while maintaining reasonable run times. The initial geometry was meshed and solved, then a scale factor ranging from 0.6 to 1.6 was applied to the mesh parameters, and the resultant change in laptime was compared.
2D Aerofoil Selection
The first design decision revolved around choosing the correct aerofoil to use for both wings. The rear wing featured a triple element design to extract maximal downforce, the use of a DRS system ensuring a limited drag in the straights. The front wing employed a two-element design.
This was executed as a parametric 2D study, varying angle of attack across a variety of aerofoils (S1223, NACA 6421, BE60, E420, CH10, E423, FX74, GOE652, LA203, and M27). The first phase aimed to identify high performing aerofoils, which would be combined in the next phase. The second sweep tested the 4 best performing configurations, sweeping through angle of attack for all combinations of primary and secondary elements.
Developing the Vehicle
With the wing study providing baseline front and rear wing elements, the rest of the body was sculpted. The streamlined design featured a basic diffuser, shark fin and front scoop.
To extract more performance from the initial design, the new configuration featured a set of dive planes, diffuser strakes and front wing gurneys.
Looking at the rear of the car, a problematic low pressure zone was observed. This greatly reduces the potential performance of the body, as this zone tends to generate positive lift. To fix this issue, a gurney was added to the body at that location.
The gurney worked well to remove the low-pressure region. However, it also reduced the performance of the wing, due to the high pressure created by the gurney bleeding into the low-pressure region on the underside of the wing. This discovery required a new set of studies aimed at reaping the benefits of the gurney without compromising rear wing performance.
Three different configurations were tested to that end. FO12 (top right) featured a gurney on the tertiary element of the rear wing. This increased the high-pressure zone on top of the wing, without making much change to the underside, marginally increasing the performance of the wing. FO13 (bottom left) tested a new wing placement, without the use of the tertiary gurney. The new position moved the underflow of the wing outside the bounds of the high-pressure zone on the body, also increasing the downforce generated by the wing. FO14 combines the two modifications (higher rear wing and tertiary gurney)
To extract further performance, a series of rake angles were tested for the vehicle, as the benefit of ground effect generates high downforce with limited drag penalties. Looking at the images on the right, we can observe the presence of a large vortex being formed by the edge of the floor at high rake angles. This pushed the decision to set a static rake angle of 1.5 degrees, in order to benefit from the increased performance of 1.75 degrees under braking. Having 1.75 as a static rake angle would stall the floor under braking, causing instability in corner entry.
The focus was then shifted onto the rallycross event. The circuit geometry was maintained as a baseline; however, some key elements were altered for safety and performance. First, the static ride height was raised from 40mm to 100mm, to allow for further wheel travel. With this new ride height compromising floor performance and factoring in the risks associated with driving on gravel, the diffuser strakes were removed. Additionally, a set of strakes were added to the rear wing, to provide stability at the 8-degree yaw angles seen during this event.