Gurney flap
Encyclopedia
Trailing edge
The trailing edge of an aerodynamic surface such as a wing is its rear edge, where the airflow separated by the leading edge rejoins. Essential control surfaces are attached here to redirect the air flow and exert a controlling force by changing its momentum...
of a wing. Typically it is set at a right angle
Right angle
In geometry and trigonometry, a right angle is an angle that bisects the angle formed by two halves of a straight line. More precisely, if a ray is placed so that its endpoint is on a line and the adjacent angles are equal, then they are right angles...
to the pressure side surface of the airfoil
Airfoil
An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....
,
and projects 1% to 2% of the wing chord
Chord (aircraft)
In aeronautics, chord refers to the imaginary straight line joining the trailing edge and the center of curvature of the leading edge of the cross-section of an airfoil...
.
This trailing edge device can improve the performance of a simple airfoil to nearly the same level as a complex high-performance design.
The device operates by increasing pressure on the pressure side, decreasing pressure on the suction side, and helping the boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
flow stay attached all the way to the trailing edge on the suction side of the airfoil.
Common applications occur in auto racing
Auto racing
Auto racing is a motorsport involving the racing of cars for competition. It is one of the world's most watched televised sports.-The beginning of racing:...
, helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
horizontal stabilizers, and aircraft where high lift
Lift (force)
A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...
is essential, such as banner-towing airplanes.
History
Dan Gurney
Daniel Sexton Gurney is an American racing driver, race car constructor, and team owner.The son of a Metropolitan Opera star, he was born in Port Jefferson, New York, but moved to California as a teenager...
, was a right-angle piece of sheet metal, rigidly fixed to the top trailing edge of the rear wing on his open wheel racing cars of the early 1970s. The device was installed pointing upwards to increase downforce
Downforce
Downforce is a downwards thrust created by the aerodynamic characteristics of a car. The purpose of downforce is to allow a car to travel faster through a corner by increasing the vertical force on the tires, thus creating more grip....
generated by the wing, improving traction
Traction (engineering)
Traction refers to the maximum frictional force that can be produced between surfaces without slipping.The units of traction are those of force, or if expressed as a coefficient of traction a ratio.-Traction:...
.
He field tested it and found it allowed a car to negotiate turns at higher speed, while also achieving higher speed in the straight sections of the track.
The first application of the flap was in 1971, after Gurney retired from driving and began managing his own racing team full-time.
His driver, Bobby Unser
Bobby Unser
Robert William "Bobby" Unser is a retired U.S. automobile racer. He is the brother of Al Unser, Jerry Unser and Louie Unser, the father of Robby Unser, and the uncle of Al Unser, Jr. and Johnny Unser...
, had been testing a new Gurney designed car at Phoenix International Raceway
Phoenix International Raceway
Phoenix International Raceway, also known as PIR, is a one-mile, low-banked tri-oval race track located in Avondale, Arizona. The track opened in 1964 and currently hosts two NASCAR race weekends annually. PIR has also hosted the Indycar Series, CART, USAC and the Rolex Sports Car Series...
, and was unhappy with the car's performance on the track. Gurney needed to do something to restore his driver's confidence before the race, and recalled experiments conducted in the 1950s by certain racing teams with "spoilers" affixed to the rear of the bodywork to cancel lift. (At that level of development, the spoilers were not thought of as potential performance enhancers—merely devices to cancel out destabilizing and potentially deadly aerodynamic lift.) Gurney decided to try adding a "spoiler" to the trailing edge of the rear wing. The device was fabricated and fitted in under an hour, but Unser's test laps with the modified wing turned in equally poor times. When Unser was able to speak to Gurney in confidence, he disclosed that the lap times with the new wing were slowed because it was now producing so much downforce
Downforce
Downforce is a downwards thrust created by the aerodynamic characteristics of a car. The purpose of downforce is to allow a car to travel faster through a corner by increasing the vertical force on the tires, thus creating more grip....
that the car was understeer
Understeer
Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...
ing. All that was needed was to balance this by adding additional downforce in front.
Unser realized the value of this breakthrough immediately and wanted to conceal it from the competition, including his brother Al
Al Unser
Alfred "Al" Unser is a former American automobile racing driver, the younger brother of fellow racing drivers Jerry and Bobby Unser, and father of Al Unser, Jr....
. Not wanting to call attention to the devices, Gurney left them out in the open. To conceal his true intent, Gurney deceived inquisitive competitors by telling them the blunted trailing edge was intended to prevent injury and damage when pushing the car by hand. Some copied the design, and some of them even “improved” it by pointing the flap downwards, which actually hurt performance.
Gurney was able to use the device in racing for several years before its true purpose became known. Later, he discussed his ideas with aerodynamicist and wing designer Bob Liebeck of Douglas Aircraft Company
Douglas Aircraft Company
The Douglas Aircraft Company was an American aerospace manufacturer, based in Long Beach, California. It was founded in 1921 by Donald Wills Douglas, Sr. and later merged with McDonnell Aircraft in 1967 to form McDonnell Douglas...
. Liebeck tested the device, which he later named the “Gurney flap,” and confirmed Gurney’s field test results using a 1.25% chord flap on a Newman symmetric airfoil.
His 1976 AIAA paper (76-406) “On the design of subsonic airfoils for high lift” introduced the concept to the aerodynamics community.
The Gurney flap is the first aerodynamic development made in automobile racing that has been successfully transferred to aircraft engineering.
Gurney assigned his patent rights to Douglas Aircraft, but the device was not patentable, since it was substantially similar to a movable microflap patented by E.F. Zaparka in 1931, ten days before Gurney was born. Similar devices were also tested by Gruschwitz and Schrenk
and presented in Berlin in 1932.
The Gurney flap has also been implemented on the rear spoiler of the 2010-2011 Shelby Mustang GT500.
Theory of operation
The Gurney flap increases the maximum lift coefficientLift coefficient
The lift coefficient is a dimensionless coefficient that relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body...
(CL,max), decreases the angle of attack
Angle of attack
Angle of attack is a term used in fluid dynamics to describe the angle between a reference line on a lifting body and the vector representing the relative motion between the lifting body and the fluid through which it is moving...
for zero lift (α0), and increases the nosedown pitching moment
Pitching moment
In aerodynamics, the pitching moment on an airfoil is the moment produced by the aerodynamic force on the airfoil if that aerodynamic force is considered to be applied, not at the center of pressure, but at the aerodynamic center of the airfoil...
(CM), which is consistent with an increase in camber
Camber (aerodynamics)
Camber, in aeronautics and aeronautical engineering, is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil...
of the airfoil. It also typically increases the drag coefficient
Drag coefficient
In fluid dynamics, the drag coefficient is a dimensionless quantity that is used to quantify the drag or resistance of an object in a fluid environment such as air or water. It is used in the drag equation, where a lower drag coefficient indicates the object will have less aerodynamic or...
(Cd),
especially at low angles of attack,
although for thick airfoils, a reduction in drag has been reported.
A net benefit in overall lift to drag ratio is possible if the flap is sized appropriately based on the boundary layer
Boundary layer
In physics and fluid mechanics, a boundary layer is that layer of fluid in the immediate vicinity of a bounding surface where effects of viscosity of the fluid are considered in detail. In the Earth's atmosphere, the planetary boundary layer is the air layer near the ground affected by diurnal...
thickness.
The Gurney flap increases lift by altering the Kutta condition
Kutta condition
The Kutta condition is a principle in steady flow fluid dynamics, especially aerodynamics, that is applicable to solid bodies which have sharp corners such as the trailing edges of airfoils...
at the trailing edge. The wake behind the flap is a pair of counter-rotating vortices that are alternately shed in a von Kármán vortex street
Von Kármán vortex street
A Kármán vortex street is a term in fluid dynamics for a repeating pattern of swirling vortices caused by the unsteady separation of flow of a fluid over bluff bodies...
.
In addition to these spanwise vortices shed behind the flap, chordwise vortices shed from in front of the flap become important at high angles of attack.
The increased pressure on the lower surface ahead of the flap means the upper surface suction can be reduced while producing the same lift.
Helicopter applications
Autorotation
In aviation, autorotation refers to processes in both fixed-wing and rotary-wing aircraft. The term means significantly different things in each context....
, it may be +15°. As a result, at least half of all modern helicopters built in the West have them in one form or another.
The Gurney flap was first applied to the Sikorsky S-76
Sikorsky S-76
The Sikorsky S-76 is an American medium-size commercial utility helicopter. The S-76 is powered by two turboshaft engines, which drive both the main and tail rotors, each with four blades...
B variant, when flight testing revealed the horizontal stabilizer from the original S-76 did not provide sufficient lift. Engineers fitted a Gurney flap to the NACA 2412 inverted airfoil to resolve the problem without redesigning the stabilizer from scratch. A Gurney flap was also fitted to the Bell JetRanger to correct an angle of incidence
Angle of incidence
Angle of incidence is a measure of deviation of something from "straight on", for example:* in the approach of a ray to a surface, or* the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.-Optics:In geometric...
problem in the design that was too difficult to correct directly.
The Eurocopter AS355
Eurocopter AS355
The Eurocopter AS355 Ecureuil 2 is a twin-engine light helicopter originally manufactured by Aérospatiale...
TwinStar helicopter uses a double Gurney flap that projects from both surfaces of the vertical stabilizer
Vertical stabilizer
The vertical stabilizers, vertical stabilisers, or fins, of aircraft, missiles or bombs are typically found on the aft end of the fuselage or body, and are intended to reduce aerodynamic side slip. It is analogical to a skeg on boats and ships.On aircraft, vertical stabilizers generally point upwards...
. This is used to correct a problem with lift reversal in thick airfoil sections at low angles of attack. The double gurney flap reduces the control input required to transition from hover to forward flight.