Detail of bearing fitted to wheel
In order to perform testing with the models on a floor, the fixed wheels on the models
had small (7mm OD. By 3mm Wide) ball bearings fitted on the bottom to allow the car
to move freely in the air stream. The model was tethered by thin nylon cord to the wind
tunnel measuring arm.
Testing was carried out exactly as before. The results obtained follow.
Air Speed Drag readings Newtons Coefficient for simulator * Cd
M/sec Stretched Airship Block Stretched Airship Block Stretched Airship Block
8.4 0.0792 0.2073 0.0045 0.0118 0.2622 0.7033
11.3 0.1341 0.3536 0.0042 0.0111 0.2454 0.6629
16.94 0.2632 0.6842 0.0037 0.0095 0.2195 0.6976
Note: The drag figures shown are the actual drag in Newtons on the model.
The coefficient for the simulator is obtained by scaling the drag up to the expected
drag for a full size car ie. multiply by 4 then dividing by the air speed squared. The
Simulator * is the one written by Ross Perry & Ian Gardner.
These results show a slight increase in drag when the airship shape car is tested on a
floor as opposed to on a “stick”. This is in line with expectations.
However the lower drag recorded on the Block model was unexpected, it is possibly
due to the bluff front close to the ground acting like a deflector skirt (as seen on
most racing cars) limiting the airflow under the model as well as modifying the flow
over the top.