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Race Tech 104: Gurney Flap
Whats a Gurney?
If youve watched Champ Car or IRL races on TV, youve
probably heard a pit reporter mention that a crew member made
a change to the Gurney during a pit stop. Ive
heard Gary Gerould, one of the best of the TV announcers, explain
that this Gurney is a device the teams use to adjust
the downforce generated by the wing.
Gerould is correct, of course, but in aerodynamic terms that
Gurney, also called a Gurney flap or
wickerbill, is a trailing edge flap. Airplanes use
flaps for increased lift during takeoff and landings. Dan Gurneys
Eagles were the first to use a trailing edge flap at the Indy
500 in the early 1970s and his name stuck to the device. A trailing
edge flap improves the performance of a wing at high angles of
attack. Here are some sketches to show whats going on.
The first sketch shows an upside down wing which generates
downforce or negative lift as it moves through the air. The air
has to accelerate to go around the rounded side of the wing and
loses pressure when it speeds up. As you remember, Bernoullis
Theorem says at any place in a fluid, you get the same value
when you add up the static and dynamic pressure (what you feel
when you put your hand out the window of a moving car). A still
fluid has no motion and no dynamic pressure. The slower air on
top is at a higher pressure and presses down on the wing surface.
The force a wing produces depends on the airfoil shape, the area
of the wing, and the square of its speed through the air.
The second sketch is a wing at a high angle of attack. At
high angles of attack, air is unable to follow the contour of
the lower wing surface and can detach, lowering the efficiency
of the wing. As a result, downforce goes down and drag goes up.
A small lip on the trailing edge, shown in the bottom sketch,
causes a lower pressure just behind it which sucks the lower
flow back up to the wing surface. The Gurney flap causes some
extra drag, but the wing can be run at a higher angle of attack
and produces more downforce.
You can only hang so much wing on a racecar because of rules
limiting the number and dimensions of wings or just by running
out of room. Those pesky radiators and tires get in the way and
they just cant be left out. A designer has to get all the
downforce possible out of the wing surfaces allowed by the rules.
A Gurney gets more downforce from the allowable wings because
you can run them at higher angles of attack.
A Gurney is also used as a quick way to fine tune the force
a wing generates in order to adjust the way a car handles. Varying
the height of the Gurney adjusts downforce (and drag, of course)
and race teams have devised ways of changing the Gurney quickly.
L-shaped aluminum extrusions are available in various sizes,
and the rear wings are made so these extrusions can be quickly
inserted from the side into a groove in the wing during a pit
stop. Some teams use lengths of 4 or 6 inch diameter white plastic
water pipe with caps on each end to store and transport their