Flexible Wings
The principle of the 'flexi wing' is straightforward enough and it exploits the concept of aero elasticity. This is the phenomenon of an aerodynamic component bending under a certain load, this change of its form clearly changing its aerodynamic characteristics.
The concept of applying aero elasticity to F1 wings is not new, in fact it goes back over 30 years. Wings which flex at speed have appeared at various times over that period, when new technology allows the rules to be circumvented and new rules the performance gain is attractive enough.
The Formula One flexi wing has sufficient structural integrity to hold its static form under the download generated when at maximum cornering speed or during FIA scrutineering. But the increased loading that comes with higher speed on the straight deflects it - or part of it - so that it sheds downforce and with that drag.
The rear wing has two elements and some of the downforce (and with it drag) that it creates can be shed by opening or closing the slot gap in between the two elements. For example, it can be arranged that under a certain load (read speed) the tip of the flap moves closer to the main element, reducing the gap. The gain can be worth 10 km/h extra top speed.
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Wings on low speed, low load |
Wings on high speed, high load |
Clearly this aero-elastic approach is easier where there are low speed corners and long, thus fast straights than where the speed differential is less. But in 2006 it appeared that some Formula One teams could take advantage even on circuits with high speed corners.
Sophisticated computer-based modelling (CFD and FEA based models) was at the heart of this, that and advanced manufacturing techniques. All carbon fibre based bodywork elements have some degree of flexibility, however miniscule. The key is to exploit the phenomenon of aero elasticity so as to find a gain without transgressing prescribed FIA checks. It helps that advanced composite materials are highly tuneable in its characteristics throughout a part and that those characteristics can be specified through a combination of Finite Element Analysis (FEA) and Computational Fluid Dynamics (CFD) work.
At the same time the FIA checks are simple two dimensional static tests whereas the reality is of complex three dimensional dynamic behaviour. For example the FIA has a deflection test that assesses the flexibility of the uppermost rear wing element at three specified points along its span. We saw in 2006 that the prescribed static test doesn't mean that in the dynamic state wing elements cannot be designed that flex in a manner that beneficially alters the slot gap at a crucial speed.
Ferrari started the 2006 season in Bahrain with a rear wing that passed the specified FIA test but which rival teams considered in the light of what they saw on track to be unacceptably elastic. There was further controversy at the next race, in Malaysia, when on-board camera footage show a Ferrari front wing upper element element deflecting.
Eight rival teams threatened a formal protest on the basis of the ruling that states that any part of the car that influences its aerodynamic performance '…must be rigidly secured to the entirely sprung part of the car (rigidly secured means not having any degree of freedom)'.
In response the FIA closely inspected the Ferrari and the cars of two other teams, McLaren and BMW-Sauber. All were deemed legal but the FIA asked all of them for bodywork modifications prior to the next race in Melbourne so as to put those cars within the spirit as well as the letter of the regulations.
While modifications were duly made, that was not the end of the affair. Amid on-going concern, for the Canadian Grand Prix the FIA made mandatory a slot gap control bracket on the centreline of the car, holding the flap in respect to the main element of the rear wing.
Bracket requested by FIA
That still wasn't the end of the affair. In Canada and the USA, the BMW-Sauber continued to come in for criticism from rivals. The FIA asked it to make changes prior to the French Grand Prix and then it asked the team to make further modifications for the German Grand Prix. Both Midland (next year Force India) cars were disqualified from the results of that race for excessive rear wing flexibility, which was blamed on a manufacturing fault. Or was it a case that it wasn't just BMW-Sauber still playing the game!
Moreover flexible bodywork doesn't have to only revolve around the rear wing slot gap. In the past certain teams have had a all wing support pillar deflect sufficiently to beneficially alter the angle of attack of the entire rear wing. Current stiffness tests rule out that approach but there is apparently still scope to do something in the area of the rear wing endplates…
A flexing rear wing impacts upon the performance of the diffuser since the two strongly interact. The work of the wing encourages air to flow through the diffuser: If the wing stalls then the diffuser will likely stall, too.
Diffusers do deflect under load while bargeboards sink under load? Maybe, but the amount that they can be designed to do so is limited by current FIA regulations. But the aerodynamicists will continue to exploit aero elasticity wherever they can. It has even been suggested that air could be pumped into a wing element with increased speed of the car, so as to balloon it and thereby alter its external form!
Since the 2007 season-opener in Australia, there has been much talk of 'movable' floors on Formula One cars. Teams are developing a way to mount the floor in such a way that it will pass the standard deflection test in scrutineering, but then lift under greater aerodynamic loads at speed on the circuit, providing significant aerodynamic gains.
Ride heights are crucial in terms of improving the aero performance of a Formula One car. For this reason, several years ago teams developed flexible floors that would bend under load, thus allowing the car to run much closer to the ground. The FIA decided to police the situation by introducing a floor deflection test. This test uses the normal scrutineering platform. A hydraulic ram pushes the floor upwards from beneath the car and the amount of movement under a specified load is measured.
As with wings, if the part passes this deflection test it is deemed legal, even if the part may flex further under a greater load. This apparent anomaly is present because the FIA decided to provide a certain degree of freedom in the regulation to avoid damage because of to high stifnes of aero parts.
There are two points of view on front wing flex; one is that a rigid front wing will give you exactly the same results on the track as you get in the wind tunnel and in the Computational Fluid Dynamics programmes. The other is that the lower you can get with the wing tips to the ground, the more downforce you will generate and this will be faster. It was documented with photo and video shots during middle part of 2010 season, that Red Bull wings touch the ground in certain, fast, parts of the track.
A flexi wing can bring gains of 2/10ths of a second or more in the wing tips alone, but there are risks to this approach.
It is easy to end up with a wing which makes the car loose in high speed corners, which spooks the driver. It can upset the balance of the car with some strange results. The reason for this is that it is very hard to do wind tunnel tests and CFD programmes with deformed shapes, which replicate the full flexing of the wing with the car at various angles in cornering. It's just far too complex to model. So having a flexing front wing is a bit of an unknown.
Another problem is that by definition, if it is flexing and thus creating less downforce as you go fast down the straights, it is therefore also creating less drag. And then when the driver lifts off the throttle and the wing rises up it gain downforce and can make the car unstable in a slow corner entry.
However it is very good on medium and fast corners, such as are found in Sector 2 in Budapest, where the Red Bull was untouchable during race weekend 2010.
After the heated debate during Germany and Hungary GP 2010 race about the Red Bull and Ferrari front wing flexing to increase front downforce, a new more stringent test was introduced by the FIA. And Red Bull and Ferrari passed the test. Red Bull front wing was photographed during qualification and race with wings tuching the track.
The Red Bull wing at Spa featured fewer elements than the Hungary wing and observers say that it did not flex out on track as much as in Budapest. The team says that they have changed nothing in the wing apart from things they would normally do when moving from an ultra high downforce circuit like Hungary to a faster circuit like Spa. However senior composites technicians from the team's Milton Keynes base, who do not normally attend Grands Prix, were noticed in the paddock, which means that something out of the ordinary was taking place. The theory is that the wing flexes outwards due to a sophisticated layering process of the carbon composite material.
The new stricter test for 2011 involve double the load being placed on the wing, so now is 100kg.
The technical regulations state that a front wing must be no lower than 75mm above the reference plane, which is the lowest point of the car excluding the plank. To check compliance with this rule, 100kg loads are applied to the two ends of the front wing in scrutineering on the distance of 790mm from the car centre line. Movement of no more than 20mm is allowed. This year the FIA have brought into force a stricter test in which loads are applied either simultaneously or on one side at a time. Despite controversy about their 'flexible' front wing, Red Bull have passed this test, leaving their rivals striving to develop similar solutions.
As the severity of the new test is arbitrary, there has been a considerable amount of lobbying of the FIA technical people by Red Bull and Ferrari on the one hand and McLaren and Mercedes on the other.
The outcome from Spa was that McLaren and Mercedes were both privately unsatisfied that the test was stringent enough, while observing that the Red Bull wing flexed less than it had in Budapest, when out on track. The car was much closer to the performance of its rivals than it had been in Budapest, but there are several possible explanations for that, including the weather and the fact that the wing has significantly fewer flaps and thus is creating less downforce anyway.
Though Bernoulli's principle is a major source of lift or downforce in an aircraft or racing car wing, Coanda effect plays an even larger role in producing lift.To know more about interaction of Bernoulli principle and Coanda effect check my article here.
3.15 Aerodynamic influence :
With the exception of the cover described in Article 6.5.2 (when used in the pit lane), the driver adjustable bodywork described in Article 3.18 and the ducts described in Article 11.4, any specific part of the car influencing its aerodynamic performance :
- must comply with the rules relating to bodywork ;
- 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.
Any device or construction that is designed to bridge the gap between the sprung part of the car and the ground is prohibited under all circumstances.
No part having an aerodynamic influence and no part of the bodywork, with the exception of the skid block in 3.13 above, may under any circumstances be located below the reference plane.3.17 Bodywork flexibility :
3.17.1 Bodywork may deflect no more than 10mm vertically when a 500N load is applied vertically to it 800mm forward of the front wheel centre line and 795mm from the car centre line. The load will be applied in a downward direction using a 50mm diameter ram and an adapter 300mm long and 150mm wide. Teams must supply the latter when such a test is deemed necessary.
3.17.2 Bodywork may deflect no more than 10mm vertically when a 500N load is applied vertically to it 450mm forward of the rear wheel centre line and 650mm from the car centre line. The load will be applied in a downward direction using a 50mm diameter ram and an adapter of the same size. Teams must supply the latter when such a test is deemed necessary.
3.17.3 Bodywork may deflect by no more than one degree horizontally when a load of 1000N is applied simultaneously to its extremities in a rearward direction 925mm above the reference plane and 20mm forward of the Uforward edge of the rear wing endplateU.
3.17.4 Bodywork may deflect no more than 2mm vertically when a 500N load is applied simultaneously to each side of it 200mm behind the rear wheel centre line, 325mm from the car centre line and 970mm above the reference plane. The deflection will be measured at the outer extremities of the bodywork at a point 345mm behind the rear wheel centre line.
The load will be applied in a downward direction through pads measuring 200mm x 100mm which conform to the shape of the bodywork beneath them, and with their uppermost horizontal surface 970mm above the reference plane. The load will be applied to the centre of area of the pads. Teams must supply the latter when such a test is deemed necessary.
3.17.5 Bodywork may deflect no more than 5mm vertically when a 2000N load is applied vertically to it at a point which lies on the car centre line and 380mm rearward of the front wheel centre line. The load will be applied in an upward direction using a 50mm diameter ram. Stays or structures between the front of the bodywork lying on the reference plane and the survival cell may be present for this test, provided they are completely rigid and have no system or mechanism which allows non-linear deflection during any part of the test.
3.17.6 The uppermost aerofoil element lying behind the rear wheel centre line may deflect no more than 5mm horizontally when a 500N load is applied horizontally. The load will be applied 950mm above the reference plane at three separate points which lie on the car centre line and 190mm either side of it. The loads will be applied in a rearward direction using a suitable 25mm wide adapter which must be supplied by the relevant team.
3.17.7 The forward-most aerofoil element lying behind the rear wheel centre line and which lies more than 730mm above the reference plane may deflect no more than 2mm vertically when a 200N load is applied vertically. The load will be applied in line with the trailing edge of the element at any point across its width. The loads will be applied using a suitable adapter, supplied by the relevant team, which :
- may be no more than 50mm wide ;
- which extends no more than 10mm forward of the trailing edge ;
- incorporates an 8mm female thread in the underside.
3.17.8 In order to ensure that the requirements of Article 3.15 are respected, the FIA reserves the right to introduce further load/deflection tests on any part of the bodywork which appears to be (or is suspected of), moving whilst the car is in motion.3.18 Driver adjustable bodywork :
A single closed section situated each side of car centre line in the volume bounded by :
- lines 450mm and 800mm in front of the front wheel centre line ;
- a vertical plane which intersects these lines at a distance 250mm from the car centre line ;
- and the inboard face of the bodywork described in Article 3.7.5 ;
is allowed to change incidence while the vehicle is in motion within a maximum range of 6°, provided any such change maintains compliance with all of the bodywork dimensional regulations.
Alteration of the incidence of these sections must be made simultaneously and may only be commanded by direct driver input and controlled using the control electronics specified in Article 8.2. Except when the car is in the pit lane, a maximum of two adjustments may be made within any single lap of a circuit.
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