Aero elasticity – Red Bulls front wing

 

A very public exposure of the front wing flexing on the Red Bull was made during the German GP, the analysis by journalist Stephane Samson and photographer Darren Heath, showed the tips of the Red Bull front wing running far closer to the ground than their rivals. While some of these pictures can be explained partly by different ride heights, roll positions or attitude changes, some pictures show the Red Bull front wing in a drooped (anhedral) attitude. This has been backed up by on board footage, where by the roll hoop camera is fixed rigidly to the car and any movement of other sprung parts of the car should remain immobile in relation to the camera. Yet still the RB6 has routinely exhibited excessive movement through out the car speed range.

Aero Elasticity
Since the nineties F1 teams have been exploiting a phenomenon called “aero elasticity”, this is where the bodywork of the car, mainly the wings, flex to alter their aerodynamic characteristics. At first this was largely created by the entire rear wing assembly bending it backwards, then more specific parts of the rear wing and as exposed this season, the front wing of the Red Bull has been visibly flexing.

This flexibility can be for three different benefits, either reduced drag, improved balance or greater downforce. With a rear wing limiting top speed, most attention has been paid to reducing its drag. As mentioned this was first tackled by the top rear wing and endplates being angled backwards by the beam wing twisting. A few pre-season failures leading to big accidents saw the FIA introduce the first bodywork flexibility rules. In order to enforce the rules, the FIA designed the first deflection test, a rig pulls the wing backwards by the endplates and the deflection was measured. While this test stopped this practice, it also set a standard to which the cars had to meet in order to be deemed legal. Thus if the car passed the scrutineers deflection test, it was approved to race. However if the car could flex its wings and still meet the test, then they had an advantage that couldn’t be immediately penalised.
Soon teams sought to reduce the angle of attack of the rear wing via flexing the flap or main plane. Then as the FIA introduced additional deflection tests to circumvent these workarounds, the teams flexed the wings to reduce the slot gap and stall the rear wing (Much like a passive F-duct), again deflection tests and latterly the slot gap separator effectively stopped this practice.

Front wing flex

 
Exploiting aero-elasticity with the front wing has not been to reduce drag for greater straight-line speed, as the front wing produces very little drag. At the end of the nineties teams were using front wings that drooped into an anhedral shape (i.e the tips drooping downwards creating an inverted “V” shape). This placed the wing and its endplates closer to the ground, both of which gained more downforce. Firstly the wing was closer to the ground which increased the ground effect. Up to a point the lower a wing is to the ground the more downforce it generates. Then the endplates role in sealing the high pressure above the wing from the low pressure below it, is improved if the endplate can run closer to the ground. Effectively make it act like an Eighties wing-cars skirt. To prevent this the FIA produced another deflection test; a 50kg (500n) load is applied to the wings endplate, should not produce more than 10mm of movement. Again this had largely stopped the practice of excessive deflection for front wings.

However there were still benefits to be had from flexing the front wing flap that was not affected by this test. Instead the wing has been flexed to main a stable centre of pressures position, flexing the flap downward at speed to reduce the wings angle of attack reduced downforce and moves the centre of pressure backwards, reducing the cars tendency to be oversteery at high speed. There is now a deflection tests to prevent this practice.

Red Bulls RB6 front wing

At some races last year and evident through out this year is the front wing of the Red Bull RB6 flexing at speed. Visible from the on board camera above the drivers head, the front wing tips can be seen to slowly run closer to the ground as the car accelerates. As this is a low frequency movement, the effect can be seen in reverse as the cars brakes from high speed. The wings endplates springing up as the car rapidly loses speed and the aero load applied to the wing diminishes. This was clearly visible from the early season races and as early as the Chinese GP I emailed the FIA about this practice and whether it was deemed legal. They reiterated the standard 500n – 10mm deflection test and suggested the car was legal, not directly countering the point that the wing is seen flexing. While most teams wings will flex at high speed, whereby some movement is often seen as the car brakes from high speed. The amount of movement and the low speed at which it starts to occur are startling with the Red Bull wing. The point made by the FIA to me back in April and again after the German GP in late July was that the car met the deflection test, thus was legal to race.

This flex was seen back in China 2010, not simply Germany

Front wing Load cases
An F1 car makes its own weight in downforce at just 70mph, that’s ~600kg of load on the car, half of this load is from the wings and half from the diffuser, thus the wings create some 300Kg of load at this speed. With the cars centre of pressure being some where near 45% forward biased, this means the front wing is creating something like 140Kg of load, split between the left and right wing each wing is producing 70Kg of load at just 70Mph. this is the speed of the slowest turn at the Hungaroring this weekend and only slightly faster than the hairpin at Monaco! Thus the FIA limit of 50kg is vastly under specified for the actual load an F1 car sees at even the slowest circuits. Its not surprising a team can created a wing to beat the 50Kg-10mm deflection test and yet achieve far greater deflections, suggested to be as much as 25mm, at much faster corners.

How’s this done – is it legal?
An F1 front wing is a complex moulding of carbon fibre bonded to metal sections. Although the flaps and endplate are detachable, from a structural point of view a front wing is a single piece. Mounted at its centre section by pylons affixed under the nose cone, itself stoutly fastened to the front of the chassis. In the eyes of the rules and with the exception of the driver adjustable front flap, the front wing should meet the regulation 3.16 regarding aerodynamic influence:

-must be rigidly secured to the entirely sprung part of the car (rigidly secured means not having any degree of freedom);

- must remain immobile in relation to the sprung part of the car.

Therefore the entire assembly can not be allowed to move in relation to the rest of the car. However no car can be 100% rigid and F1 cars are subjected to huge aerodynamic loads, hence the reason for the FIA to set the deflection test. If the wing can meet the test and still deflect above the test load, then the FIA deem it legal and the car can race. This could be achieved by accident or by design. Its possible that the carbon fibre lay up creating the wing will continue to deflect in a linear way all the way from zero load to 50kg and then for loads of 50kg upwards. It’s reasonable to assume most teams wing respond this way. However it’s possible to alter the layup of the carbon fibre or add some from of mechanical system (i.e. hinges or springs) to allow a non-linear repsonse to create the 10mm of movement at a 50Kg load, then create greater deflections above 50Kg. Thus the engineers could create wing that meets the deflection test, but would then deflect down to a desired ride height at a specified maximum speed.

While this is against the “spirit of the rules” which prohibit flexible bodywork they meet the test as defined by the FIA for flexible bodywork, thus the Red Bull and the Ferrari front wings are free to race in the eyes of the FIA.

I have again emailed the FIA to ask about additional deflection tests and have yet to receive a response.