thestoryonthegurneyflap.

The story on the Gurney flap.

How Dan Gurney was thinking outside of the box well before the term was coined.

First, a quick definition:

A Gurney flap is a short lip that protrudes upward from the upper trailing edge of a racing car’s wing. It’s used to enhance and/or fine tune the wing’s downforce characteristics.

A discussion about wings:

It should be mentioned that wings on aircraft are present to provide aerodynamic lift. Any wing must have at least a certain velocity of air moving across it in order to accomplish its intended function. Most aircraft use what are known as fixed or rotary wings to "pull" them away from the Earth's surface, with the exception of lighter-than-air or experimental lifting-body aircraft (where the shape of the aircraft itself creates lift as it moves through the atmosphere).

Wings used on racing cars, conversely, have the job of providing aerodynamic downforce to literally push the vehicle to the track as it travels forward (hopefully). The idea with that is to help press the car’s tires to the road surface in order to increase their grip (traction) through any turns or corners that might be present. This then increases the car’s speed potential in these areas. A race car’s wing manages air to do its job much as an aircraft’s, only in the former application the whole operation is essentially turned upside down.

Added April, 2008: This page's writer has recently become aware that, opposed to the effect illustrated above, many aerodynamicists assert the reaction of air deflected from a wing's surface is the predominant contributor to that wing's production of lift or downforce. Please see the note at the end of this piece for details. *

So who is Dan Gurney?

Daniel Sexton Gurney is a retired racing driver, team manager and highly innovative race car builder. He was born in Port Jefferson, Long Island on April 13, 1931. He and his family moved to Riverside, California after his completion of high school.

It was in Riverside that Gurney got his knack for fast driving when he took to speeding through the orange groves there at the time. He started his racing career in 1952 driving a Triumph TR2 sports car.

Spanning a time from then to 1970, the year Gurney retired as an active racing driver, he raced in 303 events in 20 different countries with cars of 25 different make. He won 48 races and finished on the podium (2nd or 3rd place) an additional 41 times.

Dan Gurney raced with success in Grand Prix (Formula 1), Indy Car, NASCAR and sports car events. He is the only United States citizen to have won a Formula 1 Grand Prix in a car of his own construction, that being the 1967 Grand Prix of Belgium.

Cars of his own manufacture range from his Grand Prix car to cars that have won in Indy car racing to a successful sports prototype racer built for the Toyota Racing Development (TRD) organization. He has also fielded winning racing teams of cars built by manufacturers other than himself.

How did the Gurney flap originate?

Prior to the start of the 1971 Indy car season Gurney's driver Bobby Unser was with the team at Phoenix International Raceway testing their car. The car was noticeably slow so Unser eventually challenged Dan Gurney to come up with something to bring its speed in line with the competition’s racers.

At that time Gurney happened to be mindful of the successful experimentation of another racing driver, Richie Ginther, with aerodynamic spoilers attached to the rear of Ferrari sports racing cars. (A spoiler is basically a slat of a certain height set more or less perpendicular to the flow of air used to interrupt or modify that air flow over a surface.)

By 1971 the use of downforce producing wings located at the front and rear of an Indy car had become established. After being challenged by Bobby Unser to do something to increase his car’s performance Gurney came up with the idea of applying the spoiler technology with which Ginther was experimenting directly to his car’s wing, hoping it would seem to increase the wing’s downforce capacity while not dramatically increasing its aerodynamic drag.

It should be noted here that increasing any given wing’s lift or downforce capacity usually means introducing more of its surface to the air stream moving over it. This is because, up to a point, increasing the angle of a wing incidental to the air moving over it increases the air’s effect on the wing. The problem with that is it also increases the friction imparted by the wing to the air stream thus increasingly producing what is known as drag. It can take considerable engine power to overcome aerodynamic drag. It’s been found that inserting just the right amount of spoiler height into the air stream can induce added downforce with little penalty in added aerodynamic drag.

So, as mentioned above, Dan Gurney came up with the idea of attaching what amounts to a small spoiler to the upper trailing edge of his car’s rear wing. That spoiler took the form of a strip of sheet aluminum cut to the width of the rear wing with a small lip bent ninety degrees in the upward direction. That was then attached (presumably with rivets) to the wing. The whole operation from concept to implementation is said to have taken about 45 minutes.

Bobby Unser went back out on the Phoenix International Raceway track and turned some laps with Gurney’s aluminum-strip-with-the-90-degree-bend in place on his car’s rear wing. The car didn't really go any faster. Bobby came in the pits to speak with Gurney.

The folks on the team just assumed Gurney’s aluminum strip, what’s come to be known as the Gurney flap, had no effect on the car’s performance. Unser made sure anybody who shouldn't hear it was not around to listen to his conversation, then explained to Gurney that the car didn't go any faster because the rear wing had begun to work so well with the Gurney flap installed that the front wing wasn't doing enough work to keep the front-to-back aerodynamic balance of the car in a workable range.

An adjustment of the front wing returned said aerodynamic balance and Unser went back out on the track and ended up showing some impressive speed.

A seasoned racing veteran, Gurney had the folks on his team keep the purpose of the new Gurney flap under their hats. Any competitors who noticed the flap on Gurney’s car and asked about it were told it was nothing more than a structural element to stiffen the wing’s trailing edge.

It’s reported that the ruse went on for a good part of the following season until others finally began to catch-on and copied the idea. It appears that no one, not even Dan Gurney himself, knew exactly why his flap idea worked. It simply turned out that the flap provided an effective way of increasing a wing’s downforce production without very much corresponding increase in aerodynamic drag.

In the years after its initial concept Gurney and others had a major aerospace contractor test the flap in its wind tunnel to learn just how and why it worked.

The Gurney flap has been refined through the many years its been in use. It’s used as a key element in race car aerodynamic design to this day. In many race car designs, rather than being bolted or riveted to it there are now slots built into the wing that allow for a Gurney flap to be quickly and easily slid in and out. The flaps for this type wing are no longer strips of aluminum with a right angle bend but simply straight strips. These strips are produced in a wide variety of heights in order to enable the team to quickly change the amount of Gurney flap exposed to the air stream. This allows the team to tune and tweak the flap’s effectiveness and in turn tune the wing's performance.

Other uses for the Gurney flap:

Gurney flaps are said to have found their way on to helicopter tail planes (the stubby stabilizer wings found sprouting out of a helicopter's tail boom).

Wrap-up:

While Dan Gurney applied for a US patent on his flap one was not actually issued. Still, Gurney retains the honor of having this particular race car component named after him.

The official story of the Gurney flap can be found on Dan Gurney's All American Racers On line site at the following link: http://www.allamericanracers.com

To access the story "Gurney Flap" by Keith Howard, reprinted from England's Motorsport Magazine, September 2000 edition, featured on the All American Racers On line site click-on "BITS & PIECES" on the homepage then scroll down through the index of featured stories.

Much of the information contained in the above article was found on Gurney's Website from either the Keith Howard piece or Gurney's bio.

* This writer has long been under the impression that the differential in pressure between a wing's upper and lower surfaces, caused by air moving over said surfaces at differing velocities with the air moving at the higher velocity imparting less pressure perpendicular to it than the air moving at the lower velocity and widely known as the Bernoulli effect, is the main factor in that wing's generation of lift or downforce (depending its implementation). It has come to light, though, that for some time there has been a school of thought among a considerable number of aerodynamicists that the reaction of air being deflected from the surface of a wing predominantly presented to the airflow is the main source of force acting on that wing This is a Newtonian action-reaction type effect. It would also appear to be a basis for this old saying in the aviation community: "With enough power you can make a barn door fly."

It appears there are those in the aerodynamics field who agree that a combination of both of the effects mentioned above contribute to a conventional wing's production of lift or downforce. It would seem likely that for a given wing's profile (i. e. cross section) the degree of influence of Bernoulli vs. action-reaction effects on its performance varies depending on its attitude in relation to the oncoming air, commonly known angle of attack.

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The diagram below illustrates a theoretical example of a wing profile that mainly utilizes the reaction of oncoming air in generating downforce rather than pressure differential between its upper and lower surfaces. This wing profile would best be thought of as a diverter vane rather than a classic airfoil section in that it's meant to completely change the direction of the airflow presented to it. By turning the flow of air perpendicular and upward to the wing's direction of travel (action) the air in turn pushes downward on the wing (reaction).

The wing profile in the above drawing would present considerable aerodynamic drag and thus require significant engine power to fully utilize its downforce capabilities. It's shown to help illustrate the action/reaction principle discussed in the above addendum.

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Added June, 2008: As a final note regarding the discussion of Bernoulli's principle vs. the action - reaction effect on the function of a wing, Peter Garrison's "Technicalities" column in the June, 2008 edition of FLYING magazine, entitled "The Bernoullli Brigade", provides an excellent insight in to essentially the same subject.

Added June, 2012: Okay, one more note, to shed even more light on the subject one may wish to read Peter Garrison's "Technicalities" column in the June, 2012 issue of FLYING entitled "You Will Never Understand Lift". I did say doing so would shed more light on the subject, did I not?

Text and illustrations by DFH July, 2006. Updated June, 2008 and June, 2012.

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