A leading-edge cuff is a fixed

aerodynamic Aerodynamics, from grc, ἀήρ ''aero'' (air) + grc, δυναμική (dynamics), is the study of the motion of air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dyn ...

wing device employed on

fixed-wing aircraft A fixed-wing aircraft is a heavier-than-air flying machine, such as an airplane, which is capable of flight using wings that generate lift caused by the aircraft's forward airspeed and the shape of the wings. Fixed-wing aircraft are distinc ...

to improve the stall and

spin Spin or spinning most often refers to: * Spinning (textiles), the creation of yarn or thread by twisting fibers together, traditionally by hand spinning * Spin, the rotation of an object around a central axis * Spin (propaganda), an intentionally b ...

characteristics. Cuffs may be either factory-designed or an after-market add-on modification.Crane, Dale: ''Dictionary of Aeronautical Terms, third edition'', page 144. Aviation Supplies & Academics, 1997. A leading-edge cuff is a wing leading-edge modification, usually a lightly drooped outboard

leading-edge extension A leading-edge extension (LEX) is a small extension to an aircraft wing surface, forward of the leading edge. The primary reason for adding an extension is to improve the airflow at high angles of attack and low airspeeds, to improve handling an ...

. In most cases of outboard leading-edge modification, the wing cuff starts about 50–70% half-span and spans the outer leading edge of the wing. The main goal is to produce a more gradual and gentler stall onset, without any spin departure tendency, particularly where the original wing has a sharp/asymmetric stall behaviour with a passive, non-moving, low-cost device that would have a minimal impact on performance. A further benefit is to lowering stall speed, with lower approach speeds and shorter landing distances. They may also, depending on cuff location, improve

aileron An aileron (French for "little wing" or "fin") is a hinged flight control surface usually forming part of the trailing edge of each wing of a fixed-wing aircraft. Ailerons are used in pairs to control the aircraft in roll (or movement around ...

control at low speed.

Terminology

Leading-edge cuffs were called droop concept or drooped leading edge (DLE), or modified outboard leading edge in technical reports on stall/spin resistance. In these reports and others

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...

reports on the same object, "leading-edge cuff" expression was not used. Other authors use simply "cuff" or "wing cuff".

History

NASA led a general aviation stall/spin research program during the 1970s and 1980s, using model and full-scale experiments, seeking an effective means to improve stall/spin characteristics of general aviation airplanes. The effect of a central notch at mid-span on the wing maximum lift was demonstrated in 1976. Following the testing of different leading-edge modifications on models and full-sized aircraft NASA eventually selected the semi-span ''drooped leading edge'' (DLE) that was tested first on an American Aviation AA-1 Yankee (1978). A 1979 NASA report explains that at high angles of attack the cuff discontinuity generates a vortex that acts as a fence, preventing the separated flow from progressing outboard. The lift slope has a flatter top and the stall angle is delayed to a higher angle. To reach high angles of attack, the outboard airfoil has to be drooped, some experiments investigating "exaggerated" drooped leading edges. The physical reason for the cuff effect was not clearly explained. Some much older reports gave some similar results. A 1932

NACA The National Advisory Committee for Aeronautics (NACA) was a United States federal agency founded on March 3, 1915, to undertake, promote, and institutionalize aeronautical research. On October 1, 1958, the agency was dissolved and its assets ...

report about the effect of leading-edge slots of various lengths said, "this is an indication that the slotted portion on each tip of the wing operates to some extent as a separate wing". Getting higher lift coefficients as a result of boundary layer removal is well known on propellers (centrifugal force causing an outward displacement of the boundary layer), or wings (boundary-layer suction). The leading-edge cuff inboard vortex and wing tip vortex act both to remove the boundary layer of the wing's outer section, helping this low-aspect-ratio virtual wing to achieve a higher stall angle. An important point is that the wing seems to be aerodynamically split in two parts, the inner stalled part and the outer part that behaves as an isolated low-aspect-ratio wing, able to reach a high angle of attack. The sharp discontinuity of the cuff is a key factor; all attempts by gradual fairing to suppress the vortex and the positive effects of the modification reintroduced an abrupt tip stall.

Stall/spin results

According to a NASA stall/spin report, "The basic airplanes: AA-1 (Yankee), C-23 (Sundowner), PA-28 (Arrow), C-172 (Skyhawk) entered spins in 59 to 98 percent of the intentional spin-entry attempts, whereas the modified aircraft entered spins in only 5 percent of the attempts and required prolonged, aggravated control inputs or out-of-limit loadings to promote spin entry."

Wing aspect ratio and location effects

The most successful NASA experimental results were obtained on a quite low 6:1 aspect ratio wing (Grumman Yankee AA-1), with a DLE placed at 57% of the semi-span. As the vortices (inboard cuff and wing tip) are efficient on a limited span length (about 1.5 times the local chord), a DLE alone is unable to preserve enough outboard lift to keep the roll control in case of high aspect ratio wing. Wings of more than 8 or 9 aspect ratio features other devices to complete the cuff effect, for example stall strips (as used on the

Cirrus SR22 The Cirrus SR22 is a single-engine four- or five-seat composite aircraft built from 2001 by Cirrus Aircraft of Duluth, Minnesota. It is a development of the Cirrus SR20, with a larger wing, higher fuel capacity, and a more powerful, 310-horsep ...

and

Cessna 400 The Cessna 400, marketed as the Cessna TTx, is a single-engine, fixed-gear, low-wing general aviation aircraft built from composite materials by Cessna Aircraft. The Cessna 400 was originally built by Columbia Aircraft as the Columbia 400 un ...

), "Rao slots" (as used on the Questair Venture),

vortex generator A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface (or airfoil, such as an aircraft wing) or a rotor blade of a wind turbine.

s or segmented droop (as used on a NASA modified

Cessna 210 The Cessna 210 Centurion is a six-seat, high-performance, retractable-gear, single-engined, high-wing general aviation, general-aviation aircraft. First flown in January 1957, it was produced by Cessna until 1986. Design and development ...

). In the case of the high aspect ratio Cessna 210 wing (AR =11:1), roll damping at stall was not as efficient. The case of high-wing configuration wing was different. Full scale testing of a modified

Cessna 172 The Cessna 172 Skyhawk is an American four-seat, single-engine, high wing, fixed-wing aircraft made by the Cessna Aircraft Company.

showed that the outboard leading-edge cuff alone was not sufficient to prevent a spin departure, the aircraft lacking directional stability at high angles of attack. With a ventral fin added, the aircraft entered a controlled spiral in lieu of a spin.

Drag penalty

Depending on the cuff length and shape, the leading-edge cuff can exert an aerodynamic penalty for the stall/spin resistance speed obtained, resulting in some loss of cruise airspeed, although sometimes too small "to be detected with production instruments". In the case of the best wing modification of the AA-1 Yankee, the loss of cruise speed amounted to 2 mph or 2% and there was no speed loss in climb. Impact on cruise speed of the Piper PA-28 RX (modified T-tail) was not measurable. For the Questair Venture, "In carefully controlled performance tests, the penalty in cruise performance was found to be imperceptible (1 kt)".

Applications

The first use of outboard cuffs, other than on NASA research airplanes, was on the

Rutan VariEze The Rutan VariEze is a composite, canard aircraft designed by Burt Rutan. It is a high-performance homebuilt aircraft, hundreds of which have been constructed. The design later evolved into the Long-EZ and other, larger cabin canard aircraf ...

in 1978. They were wind tunnel tested in 1982, and later (1984) replaced by

vortilon Vortilons are fixed aerodynamic devices on aircraft wings used to improve handling at low speeds. The vortilon was invented by aerodynamicists working at Douglas Aircraft who had previously developed the engine pylons for the Douglas DC-8. The or ...

s. Following aircraft were modified for experiments with the addition of an outboard leading-edge cuff as a result of NASA stall/spin research program : *

Grumman American AA-1 The Grumman American AA-1 series is a family of light, two-seat aircraft. The family includes the original American Aviation AA-1 Yankee and AA-1A Trainer, the Grumman American AA-1B Trainer and TR-2, plus the Gulfstream American AA-1C ...

X (1978) * Beechcraft C-23X (1980) * Piper PA28 RX modified (T-tail) (1981) *

Cessna 172 The Cessna 172 Skyhawk is an American four-seat, single-engine, high wing, fixed-wing aircraft made by the Cessna Aircraft Company.

X (1983), * Verilite Sunbird, high-wing pusher (1986) * Questair Venture *

(1987), high wing aspect ratio, * Smith Trainer (1992) Leading-edge cuffs are used on 1900s high-performance light aircraft like the

Cirrus SR20 The Cirrus SR20 is an American piston-engined, four- or five-seat composite material, composite monoplane built since 1999 by Cirrus Aircraft of Duluth, Minnesota. The aircraft is the company's earliest type certificate, type-certified model, earni ...

and

Columbia 350 __NOTOC__ Year 350 ( CCCL) was a common year starting on Monday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Sergius and Nigrinianus (or, less frequently, year 1103 '' ...

, which both gained

FAA The Federal Aviation Administration (FAA) is the largest transportation agency of the U.S. government and regulates all aspects of civil aviation in the country as well as over surrounding international waters. Its powers include air traffic m ...

certification Certification is the provision by an independent body of written assurance (a certificate) that the product, service or system in question meets specific requirements. It is the formal attestation or confirmation of certain characteristics of a ...

with the device. Several after-market suppliers of

STOL kits A short takeoff and landing (STOL) aircraft is a conventional fixed-wing aircraft that has short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on airstrips with harsh condition ...

make use of leading-edge cuffs, in some cases in conjunction with such other aerodynamic devices as

wing fence Wing fences, also known as boundary layer fences and potential fences are fixed aerodynamic devices attached to aircraft wings. Often seen on swept-wing aircraft, wing fences are flat plates fixed to the upper surfaces parallel to the wing ch ...

s and drooping ailerons.

References

External links

* Wing Vortex Device

{{Aircraft components Aircraft wing components Aircraft wing design