Leading edge cuff

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A drooped leading edge cuff installed on an American Aviation AA-1 Yankee as part of a NASA experiment DroopedLeadingEdgeCuff01.JPG
A drooped leading edge cuff installed on an American Aviation AA-1 Yankee as part of a NASA experiment

A leading edge cuff is a fixed aerodynamic wing device employed on fixed-wing aircraft to improve the stall and spin characteristics. Cuffs may be either factory-designed or an after-market add-on modification. [1]

Aerodynamics branch of dynamics concerned with studying the motion of air

Aerodynamics, from Greek ἀήρ aer (air) + δυναμική (dynamics), is the study of motion of air, particularly as interaction with a solid object, such as an airplane wing. It is a sub-field of fluid dynamics and gas dynamics, and many aspects of aerodynamics theory are common to these fields. The term aerodynamics is often used synonymously with gas dynamics, the difference being that "gas dynamics" applies to the study of the motion of all gases, and is not limited to air. The formal study of aerodynamics began in the modern sense in the eighteenth century, although observations of fundamental concepts such as aerodynamic drag were recorded much earlier. Most of the early efforts in aerodynamics were directed toward achieving heavier-than-air flight, which was first demonstrated by Otto Lilienthal in 1891. Since then, the use of aerodynamics through mathematical analysis, empirical approximations, wind tunnel experimentation, and computer simulations has formed a rational basis for the development of heavier-than-air flight and a number of other technologies. Recent work in aerodynamics has focused on issues related to compressible flow, turbulence, and boundary layers and has become increasingly computational in nature.

Fixed-wing aircraft Heavier-than-air aircraft with fixed wings generating aerodynamic lift in the airflow caused by forward airspeed

A fixed-wing aircraft is a flying machine, such as an airplane or aeroplane, 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 distinct from rotary-wing aircraft, and ornithopters. The wings of a fixed-wing aircraft are not necessarily rigid; kites, hang gliders, variable-sweep wing aircraft and aeroplanes that use wing morphing are all examples of fixed-wing aircraft.

Contents

A leading edge cuff is a wing leading edge modification, usually a lightly drooped outboard leading-edge extension. 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. [2]

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 and delay the stall. A dog tooth can also improve airflow and reduce drag at higher speeds.

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 [1] [3] 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 control at low speed.

Aileron Aircraft control surface used to induce roll

An aileron 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, which normally results in a change in flight path due to the tilting of the lift vector. Movement around this axis is called 'rolling' or 'banking'.

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. [4] In these reports and others NASA reports on the same object, [5] "leading edge cuff" expression was not used.

NASA space-related agency of the United States government

The National Aeronautics and Space Administration is an independent agency of the United States Federal Government responsible for the civilian space program, as well as aeronautics and aerospace research.

Other authors use simply "cuff" or "wing cuff". [6]

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. [7]

The effect of a central notch at mid-span on the wing maximum lift was demonstrated in 1976. [8] 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 [9] 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. [10]

Some much older reports gave some similar results. A 1932 NACA report [11] 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), [12] 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. [13]

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. [14]

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." [15]

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. [16] Wings of more than 8 or 9 aspect ratio features other devices to complete the cuff effect, [17] for example stall strips (as used on the Cirrus SR22 and Cessna 400), "Rao slots" (as used on the Questair Venture), vortex generators or segmented droop (as used on a NASA modified Cessna 210). In the case of the high aspect ratio Cessna 210 wing (AR =11:1), roll damping at stall was not as efficient. [18]

The case of high-wing configuration wing was different. Full scale testing of a modified Cessna 172 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. [19]

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". [20] 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. [21] Impact on cruise speed of the Piper PA-28 RX (modified T-tail) was not measurable. [22] For the Questair Venture, "In carefully controlled performance tests, the penalty in cruise performance was found to be imperceptible (1 kt)". [23]

Applications

The first use of outboard cuffs, other than on NASA research airplanes, was on the Rutan VariEze in 1978. They were wind tunnel tested in 1982, and later (1984) replaced by vortilons. [24]

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

Leading edge cuffs are used on 1900s high-performance light aircraft like the Cirrus SR20 and Columbia 350, which both gained FAA-certification with the device. [32] [33]

Several after-market suppliers of STOL kits make use of leading-edge cuffs, in some cases in conjunction with such other aerodynamic devices as wing fences and drooping ailerons. [34]

See also

Related Research Articles

Wing Surface used for flight, for example by insects, birds, bats and airplanes

A wing is a type of fin that produces lift, while moving through air or some other fluid. As such, wings have streamlined cross-sections that are subject to aerodynamic forces and act as an airfoils. A wing's aerodynamic efficiency is expressed as its lift-to-drag ratio. The lift a wing generates at a given speed and angle of attack can be one to two orders of magnitude greater than the total drag on the wing. A high lift-to-drag ratio requires a significantly smaller thrust to propel the wings through the air at sufficient lift.

Stall (fluid dynamics) abrupt reduction in lift due to flow separation

In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded. The critical angle of attack is typically about 15 degrees, but it may vary significantly depending on the fluid, foil, and Reynolds number.

Delta wing wing shaped in the form of a triangle

The delta wing is a wing shaped in the form of a triangle. It is named for its similarity in shape to the Greek uppercase letter delta (Δ).

STOL A class of airplanes that are designed to takeoff and land in a short distance

STOL is an acronym for a short takeoff and landing aircraft, which have short runway requirements for takeoff and landing. Many STOL-designed aircraft also feature various arrangements for use on runways with harsh conditions. STOL aircraft, including those used in scheduled passenger airline operations, have also been operated from STOLport airfields which feature short runways.

Angle of attack angle between the chord of the wing and the undisturbed airflow

In fluid dynamics, angle of attack is the angle between a reference line on a body and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body's reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or airfoil moving through air.

Vortex generator

A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface or a rotor blade of a wind turbine. VGs may also be attached to some part of an aerodynamic vehicle such as an aircraft fuselage or a car. When the airfoil or the body is in motion relative to the air, the VG creates a vortex, which, by removing some part of the slow-moving boundary layer in contact with the airfoil surface, delays local flow separation and aerodynamic stalling, thereby improving the effectiveness of wings and control surfaces, such as flaps, elevators, ailerons, and rudders.

Airfoil

An airfoil or aerofoil is the cross-sectional shape of a wing, blade, or sail.

Cessna 152 Two-seat tricycle gear general aviation airplane

The Cessna 152 is an American two-seat, fixed tricycle gear, general aviation airplane, used primarily for flight training and personal use. It was based on the earlier Cessna 150, including a number of minor design changes and a slightly more powerful engine running on 100LL aviation gasoline.

A spin is a special category of stall resulting in autorotation about the vertical axis and a shallow, rotating, downward path. Spins can be entered intentionally or unintentionally, from any flight attitude if the aircraft has sufficient yaw while at the stall point. In a normal spin, the wing on the inside of the turn stalls while the outside wing remains flying. It is possible for both wings to stall, but the angle of attack of each wing, and consequently its lift and drag, are different. Either situation causes the aircraft to autorotate toward the stalled wing due to its higher drag and loss of lift. Spins are characterized by high angle of attack, an airspeed below the stall on at least one wing and a shallow descent. Recovery and avoiding a crash may require a specific and counter-intuitive set of actions.

Flap (aeronautics) aircraft wing device used to increase lift by extending the trailing edge of the wing

Flaps are a kind of high-lift device used to increase the lift of an aircraft wing at a given airspeed. Flaps are usually mounted on the wing trailing edges of a fixed-wing aircraft. Flaps are used for extra lift on takeoff. Flaps also cause an increase in drag in mid-flight, so they are retracted when not needed.

Grumman American AA-1 aircraft

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

Leading-edge slot

A leading-edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading-edge slot is a spanwise gap in each wing, allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of attack and thus reduce the stall speed.

Stall strips

A stall strip is a fixed device employed on the leading edge of fixed-wing aircraft to modify the aerodynamic characteristics of the airfoil. Stall strips are used to initiate flow separation at chosen locations on the wing during high-angle of attack flight, so as to improve the controllability of the aircraft when it enters stall. They are typically employed in pairs, symmetrically on both wings. On aircraft where wing airflow is affected by asymmetrical propeller wash, a strip may be used on a single wing to reduce risk of entering a spin.

Washout (aeronautics)

Washout is a characteristic of aircraft wing design which deliberately reduces the lift distribution across the span of an aircraft’s wing. The wing is designed so that the angle of incidence is greater at the wing roots and decreases across the span, becoming lowest at the wing tip. This is usually to ensure that at stall speed the wing root stalls before the wing tips, providing the aircraft with continued aileron control and some resistance to spinning. Washout may also be used to modify the spanwise lift distribution to reduce lift-induced drag.

Wing configuration Describes the general shape and layout of an aircraft wing

The wing configuration of a fixed-wing aircraft is its arrangement of lifting and related surfaces.

Leading-edge slat aerodynamic surface on the leading edge of the wings of fixed-wing aircraft

Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are usually used while landing or performing maneuvers which take the aircraft close to the stall, but are usually retracted in normal flight to minimize drag. They decrease stall speed.

Strake (aeronautics)

In aviation, a strake is an aerodynamic surface generally mounted on the fuselage of an aircraft to improve the flight characteristics either by controlling the airflow or by simple stabilising effect.

Vortilon

Vortilons are fixed aerodynamic devices on aircraft wings used to improve handling at low speeds.

Leading edge

The leading edge is the part of the wing that first contacts the air; alternatively it is the foremost edge of an airfoil section. The first is an aerodynamic definition, the second a structural one. As an example of the distinction, during a tailslide, from an aerodynamic point of view, the trailing edge becomes the leading edge and vice versa but from a structural point of view the leading edge remains unchanged.

References

  1. 1 2 Crane, Dale: Dictionary of Aeronautical Terms, third edition, page 144. Aviation Supplies & Academics, 1997. ISBN   1-56027-287-2
  2. ↑ Location referred to half-span : Beech C23 0.54, Piper PA-28 0.55, Yankee AA-1 0.57, Cirrus SR20 0.61, Lancair 300 0.66, Questair Venture 0.70, Cessna 172 0.71 - according to SAE TP 2000-01-1691, page 14
  3. ↑ Cox, Jack (November 1988). "Questair Venture, Part Two" . Retrieved 2009-08-08.
  4. ↑ Stough, DiCarlo Spin Resistance Development for Small Airplanes - A Retrospective, SAE TP 2000-01-1691 or "Nasa Stall Spin Paper from 1970s, or .
  5. ↑ Nasa TP 2011 (Yankee AA-1), Nasa TP 2772 (Cessna 210)
  6. ↑ Burt Rutan, Canard Pusher n°19 (1979), "Wing cuff improves VariEze stalls" or more recent Wing Cuff Design for Cessna CJ1
  7. ↑ H. Paul Stough III and Daniel J. DiCarlo, Spin Resistance Development for Small Airplanes - A Retrospective, SAE TP series 2000-01-1691
  8. ↑ Kroeger, R. A.; and Feistel, T, Reduction of stall-spin Entry Tendencies Through Wing Aerodynamic Design, SAE paper 760481
  9. ↑ NASA TP 1589, Wind-Tunnel Investigation of a Full-Scale General Aviation Airplane Equipped With an Advanced Natural Laminar Flow Wing
  10. ↑ NASA TP 1589 : "The mechanism by which the outer-panel lift is maintained to such improved stall/spin characteristics has been unclear".
  11. ↑ NACA TN 423, Weick, Fred E. Investigation of lateral control near the stall flight investigation with a light high-wing monoplane tested with various amounts of washout and various lengths of leading-edge slot.
  12. ↑ Hoerner, Fluid Dynamic lift, 12-24
  13. ↑ Zimmerman, NACA TN 539, 1935 , "Aerodynamic characteristics of several airfoils of low aspect ratio". "The preservation of unturbuled flow to very high angles of attack ... is apparently due to the action of the tip vortices in removing the boundary layer that ends to built up near the trailing edge of the upper surface of the airfoil".
  14. ↑ Addition of a fairing ... to eliminate the discontinuity reintroduced abrupt tip stall (SAE TP 2000-01-1691)
  15. ↑ Summary of results for spin attempts for four NASA research aircraft.,
  16. ↑ Barnaby Wainfan, KitPlanes July 1998, Wind Tunnel, Foiling stalls is the month's topic : "It has been found that the single-droop cuff configuration described in NASA TP 1589 is not sufficient to prevent spins on high ratio wings."
  17. ↑ Murri, Jordan, Nasa TP 2772, Wind-Tunnel Investigation of a Full-Scale General Aviation Airplane Equipped With an Advanced Natural Laminar Flow Wing (Cessna 210), Leading-Edge Modifications, p.9, "The data for the outboard-droop configuration show significantly enhanced roll damping characteristics at the stall; however, unstable roll damping characteristics are not completely eliminated with the outboard droop alone."
  18. ↑ NASA TP 2722, "... an unsteady stalling and reattaching behavior occurring inboard on the wing upper surface as wing stall progressed."
  19. ↑ Investigations of modifications to improve the spin resistance of a high-wing, single engine, light airplane, SAE Paper 891039 (1989)
  20. ↑ H. Holmes, Nasa's general aviation stall/spin program, Sport Aviation, January 1989
  21. ↑ Effects of Wing-Leading-Edge Modifications on a Full-Scale, Low-Wing General Aviation Airplane, Nasa TP 2011, Drag characteristics, p. 13
  22. ↑ Nasa TP 2691, Flight Investigation of the Effects of an Outboard Wing-Leading-Edge Modification on Stall/Spin Characteristics of a Low-Wing, Single-Engine, T-Tail Light Airplane : "within the measurement accuracy, no difference was found in airplane drag for lift coefficients typical of cruising flight."
  23. ↑ "Spin Resistance" (PDF). whycirrus.com.
  24. ↑ Rutan VariEze, NASA TP 2382 (1985) et NASA TP 2623 (1986)
  25. ↑ NASA TP 1589, Nasa TP 2011
  26. ↑ NASA CT 3636, NASA TP 2691
  27. ↑ SAE paper 891039
  28. ↑ AIAA 86-2596
  29. ↑ Sport Aviation Nov. 88. Meyer et Yip, AIAA 89-2237-CP report.
  30. ↑ NASA TP 2772
  31. ↑ DOT/FAA/CT-92/17, AIAA/FAA Joint symposium on GA
  32. ↑ "Data". grumman.net.
  33. ↑ Cessna (2009). "This beauty is more than skin deep". Archived from the original on 2009-07-26. Retrieved 2009-08-08.
  34. ↑ Horton Inc (n.d.). "Description of the Horton STOL Kit" . Retrieved 2009-08-08.
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