Leading edge cuff
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.
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.
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 control at low speed.
"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 reports on the same object, "leading edge cuff" expression was not used.
Other authors use simply "cuff" or "wing cuff".
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 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.
According 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 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.
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.
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)".
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-1X (1978)
- Beechcraft C-23X (1980)
- Piper PA28 RX modified (T-tail) (1981)
- Cessna 172X (1983),
- Devore Sunbird, high-wing pusher (1986)
- Questair Venture
- Cessna 210 (1987), high wing aspect ratio,
- Smith Trainer (1992)
- Wing Vortex Devices 
- Crane, Dale: Dictionary of Aeronautical Terms, third edition, page 144. Aviation Supplies & Academics, 1997. ISBN 1-56027-287-2
- 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
- Cox, Jack (November 1988). "Questair Venture, Part Two". Retrieved 2009-08-08.
- Stough, DiCarlo Spin Resistance Development for Small Airplanes - A Retrospective, SAE TP 2000-01-1691 or "Nasa Stall Spin Paper from 1970s, or .
- Nasa TP 2011 (Yankee AA-1), Nasa TP 2772 (Cessna 210)
- Burt Rutan, Canard Pusher n°19 (1979), "Wing cuff improves VariEze stalls" or more recent Wing Cuff Design for Cessna CJ1 
- H. Paul Stough III and Daniel J. DiCarlo, Spin Resistance Development for Small Airplanes - A Retrospective, SAE TP series 2000-01-1691
- Kroeger, R. A.; and Feistel, T, Reduction of stall-spin Entry Tendencies Through Wing Aerodynamic Design, SAE paper 760481
- NASA TP 1589, Wind-Tunnel Investigation of a Full-Scale General Aviation Airplane Equipped With an Advanced Natural Laminar Flow Wing
- NASA TP 1589 : "The mechanism by which the outer-panel lift is maintained to such improved stall/spin characteristics has been unclear".
- 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. 
- Hoerner, Fluid Dynamic lift, 12-24
- 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".
- Addition of a fairing ... to eliminate the discontinuity reintroduced abrupt tip stall (SAE TP 2000-01-1691)
- Summary of results for spin attempts for four NASA research aircraft., 
- 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."
- 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."
- NASA TP 2722, "... an unsteady stalling and reattaching behavior occurring inboard on the wing upper surface as wing stall progressed."
- Investigations of modifications to improve the spin resistance of a high-wing, single engine, light airplane, SAE Paper 891039 (1989)
- H. Holmes, Nasa's general aviation stall/spin program, Sport Aviation, january 1989
- Effects of Wing-Leading-Edge Modifications on a Full-Scale, Low-Wing General Aviation Airplane, Nasa TP 2011, Drag characteristics, p. 13
- 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."
- Rutan VariEze, NASA TP 2382 (1985) et NASA TP 2623 (1986)
- NASA TP 1589, Nasa TP 2011
- NASA CT 3636, NASA TP 2691
- SAE paper 891039
- AIAA 86-2596
- Sport Aviation Nov. 88. Meyer et Yip, AIAA 89-2237-CP report.
- NASA TP 2772
- DOT/FAA/CT-92/17, AIAA/FAA Joint symposium on GA
- Cessna (2009). "This beauty is more than skin deep.". Retrieved 2009-08-08.
- Horton Inc (n.d.). "Description of the Horton STOL Kit". Retrieved 2009-08-08.