Sources of error

Wind tunnel experiments on models, even at subsonic speeds, are liable to three main sources of error when used to forecast full-scale results. These are –

1. Scale Effect. As mentioned earlier, laws of resistance can be framed which apply well to bodies whose sizes are not very different, but these laws become less accurate when there is a great difference in size between the model and the full scale. A similar effect is noticed when the velocity of the model test differs appreciably from the full-scale velocity.

Corrections can be applied which allow for this ‘scale effect’ and enable more accurate forecasts to be made. Readers who are interested will find an explanation of scale effect, and of the advantages of the compressed air tunnel, if they refer to Appendix 2. They will also be introduced to the important term Reynolds Number.

2. Interference from Wind Tunnel Walls (Fig. 2.19). The second error is due to the fact that in the wind tunnel the air stream is confined to the limits of the tunnel, whereas in free flight the air round the aeroplane is, for all practical purposes, unlimited in extent. In this case too corrections can be applied which considerably reduce the error.

3.

Errors in Model. The smaller the scale of the model, the more difficult does it become to reproduce every detail of the full-scale body, and since very slight changes of contour may considerably affect the airflow, there will always be errors due to the discrepancies between the model and the full-scale body.