Mechanical Drag Reduction System (DRS) - Design

Typically drag reduction systems (DRS), as used in F1 and some Formula Student vehicles, are electronic and lift towards the end of the chord of the flaps, this is done when there is a reduced requirement for down force and an increased desire to reduce drag.

The reason for these being electronically controlled are to allow certain limits on the activation of the system, such as yaw rate, steer angle, throttle demand etc. However this can be difficult to control, measure and implement so there is potentially room for a mechanical system.

The design initially has flaws as any method that mechanically, and automatically, prevents the flap from dropping also mean that there is an release and a return threshold. The return threshold velocity (in terms of a moment on the flap) is lower than the release threshold, meaning that going into a long sweeping corner then the flap may not return and thus you have reduced the down force when it is required.

Even with the known flaws with the core design principal it is still seen as a good technical exercise to explore the possibilities of another potential system rather than following the path most common.

The insert used is seen in Figure 1, this is used in place of a thinner aluminium flap insert design but has essentially the same external profile.

Figure 1 - Insert Design

In order to have a release threshold a bales catch has been used, this can be seen dissembled in Figure 2 but is just a sprung ball bearing, in a drilled hole in the plate (which could be an end plate) it is attached to, this provides the necessary resistance for a release threshold.

In principal this value is calculable and the equation that I have derived is seen below (Equation 1), it is a simple threshold calculation (F_release if the force required acting perpendicular to spring force direction, F_spring is spring preload force, r_ball is the ball bearing diameter and y_insertion is the depth below the surface that the bottom of the ball bearings sits). However as this bales catch was purchased from a popular hardware retailer the calculations have not been performed.

Figure 2 - Bales Catch exploded view

Equation 1 - Release Force Calculation

In order to allow for the return, a linear pull spring has been added. Whilst the system does not currently have secondary position in which the flap can rest but this would be an easy addition. The lack of this addition would mean that the amount of flutter (oscillation of the flap position) would be higher.

In Figure 3 two images can be seen, the first being the resting position of the flap insert and the second being an extended flap position, as would be expected once the moment on the flap was greater than the threshold. Whilst a torsional spring would be better (and more integrated), that could be designed in at a later date, there wasn't one suitable to hand.

Figure 3 - Compressed and Extended Operation

Figure 4 - Video of operation/return

In Figure 4 there is a video of the general operation of the system. It can be seen to work in principal (note also the return position is aided by the full compression of the spring), however the practice and implementation into a full size system is both impractical and counter productive as the time spent refining this system would be better spent refining an electrical system.