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How to CNC machine a wind-tunnel model

The Forze H2: a zero emission racing car, built by students

The German Nürburgring is a world famous motorsports racing complex, located about 70 km south of Cologne. In fact two 'rings' are present: for GP racing the 5 km Südschleife (South Loop) is used, for many other events the far more challenging 21 km Nordschleife (North Loop) - once nicknamed by Jackie Stewart as "The Green Hell".

One of the newer records of the Nordschleife is the lap record for hydrogen electric cars. Current holder of this record is a a Nissan X-Trail: 11 minutes, 58 seconds. Which is quite fast, considering the 300 m elevation in the track, and it's 154 turns.

The complete Force team with their racing car
The Forze team, showing their 2013 racing car.

The Forze-team, a group of about 75 students at Delft University of Technology (the Netherlands) promotes the use of hydrogen as an alternative fuel, by developing and racing zero emission racing cars.
Their aim for the 2014 racing car is to beat this Nordschleife lap record.

The new car is powered by a hydrogen fuel cell, generating power for the two electromotors that drive the rear wheels. The car's only exhaust product is water: zero emission !
Based on this alternative fuel system, the car's specifications are superb: its peak power of 190 kW makes the car accelerate from 0 to 100 in four seconds! Top speed of the car is 220 km/h.

SolidWorks screenshot
The design of the bodywork, shown as SolidWorks screenshot.

In previous years, the team designed a new car every year. After five years the team decided to scale up to a circuit racing, which became a multiple year project.

This car design needs to be complete: frame, bodywork, suspension, drive train, electronics, fuel cell etc. Each part is designed by the team, and every year the design will be improved further.
Most parts were designed using the SolidWorks CAD software.

Calculating the toolpaths in DeskProto
The milling machine in action
DeskProto being used; machining the top part of the car.

DeskProto was used to create a wind-tunnel model: making the drag (air resistance) just a bit lower can make the difference between beating the lap record or not. The bodywork was divided into a number of parts, to be separately machined. DeskProto makes it very easy to create the toolpaths for each part. The above picture (right) shows the top of the car, with the space where the driver will be located.

The Forze team is located in the D:DREAM hall, together with a few more student DreamTeams, like the Nuon Solar Team, the SolarBoat team and the Formula Student Team Delft. The D:DREAM hall offers various facilities, including a large size router made by Komplot Mechanics, which can handle the required size without any problem. Material used for this model is SikaBlock tooling board.

Two foam parts immediately after machining
Manually finishing some parts
Two parts still on the machine; finishing the parts.

As foam blocks were used of a limited height, higher parts needed to be machined in several layers.
The left picture above shows two such layers for the back side of the car, the picture on the right shows gluing two such layers together to form one part. Finally all these parts were mounted on a rectangular block of foam to form the complete car model (scale 1:3.75).
Extra parts that needed to be added for the wind tunnel tests were the driver, the wheels, the steel roll-cage (for the model in PVC) and the radiator (with real fins).

Manually combining parts
The parts combined to form a car model
The complete model being assembled.

The tests were done in the University's Subsonic Low Turbulence Tunnel. This wind tunnel is able to reach a maximum speed of 120 m/s (430 km/h) and has a test section of 1.8 x 1.25 m.
Tests have been done at different speeds and for different sideslip angles. In addition a few design variations of the roll-cage have been tested.

The Forze model in the wind tunnel
The final model, being tested in the wind tunnel (Upside-down).

The resulting model could be used in the wind tunnel, to optimize the air flow and minimize the drag. Martin Hartvelt, the Chief Vehicle Dynamics, comments:
"With the improved front panel, the drag was reduced by more than 10%. Moreover, it was found that adding airfoil profiles to the roll-cage can further reduce the drag by about 10%.
Also some visual checks were made on the air flow attachment/separation on the model. We found that the flow was surprisingly well attached in the ducts, which is good news for our cooling performance."

Read more about this project on the team's website,
or follow the team on Facebook: