The figure of Dan Gurney, who unfortunately left us this week, will be remembered both for his sporting successes and for his extensive contribution to the performance and safety in different motor racing disciplines. But the name of Dan Gurney will be forever tied to aerodynamics for being the designer of a simple and very efficient wing complement, known as wickerbill in the US, which has been extensively used not only in car racing but also in aeronautics.
A detailed history of his contribution can be found on this excellent website. What he discovered is the benefit of adding a flap, that is, a small plate mounted at large angles (close to 90º) onto a wing’s trailing edge, its height on the order of a few percents of the wing’s chord (according to the definition of J. Katz – Racecar Aerodynamics).
Gurney’s finding was so effective that it still can be found, for instance, in all of the rear wings of the 2018 WRC car generation. Citroen and Ford include a Gurney flap in the trailing edge of the top plate of their rear wings.
S.Al Qassimi/C.Patterson, Citroën C3 WRC, Rally Catalunya 2017, 17th
M.Østberg/T.Eriksen, Ford Fiesta WRC, Rally Catalunya 2017, 5th
picture by Markus Ippach
C.Breen/M.Scott, Citroën C3 WRC, Rally of Germany 2017, 5th
Toyota and Hyundai have included a Gurney flap also in the lower plate of their rear wing.
A.Mikkelsen/A.Jaeger, Hyundai i20 Coupé WRC, Test pre-Catalunya Rally, October 2017
E.Lappi/J.Ferm, Toyota Yaris WRC, Rally Catalunya 2017, ret.
Even the last design of the VW Polo WRC 2017 included a Gurney flap on the lower plate of the rear wing.
S.Ogier/J.Ingrassia, VW Polo WRC, Test in Catalunya, September 2016
The main benefit of using a Gurney flap is an increase in downforce generated by the wing. The main source of downforce comes from the fact that the presence of the flap allows the flow to remain attached for longer than without any flap; if separation happens later, the wing can be set at higher angles of attack (as risk of premature separation is avoided), and higher angle of attack means higher downforce.
On the other side, the presence of the flap generates drag, as it generates air recirculation in front of and behind the flap: recirculation in front of the flap causes the increase in drag, as pressure is higher there, while is lower behind the flap. At the same time, the presence of the flap allows the flow under the airfoil to remain attached for longer (separation is delayed) which results in a drag reduction. Such reduction can partially or even totally compensate the drag caused by the flap itself, although the most common situation is a net drag increase.
A deeper explanation can be found in this excellent video from GraysGarage.
Due to the benefits (downforce) and drawbacks (drag) of the Gurney flap, aerodynamicists spend a lot of time refining their design to get the best compromise. But Gurney’ solution continues to be widely used, as the 2018 WRC cars still prove.