A short take-off and landing (STOL) aircraft must be able to fly at low controlled speeds, yet it must also offer acceptable cross-country (cruise) performance. The challenge is to design a wing with a high lift coefficient, so that the wing area can be as small as possible while still allow very low take-off and landing speeds. Short wings make the aircraft easier to taxi, especially when operating in an off-airport environment with obstructions. They also allow for better visibility and require less space for hangaring, while also being easier to build and stronger (less weight and wing span to support). Designing a wing with a very high lift coefficient makes each square foot of wing area do more lifting, requiring less wing overall.
Zenith STOL designs like the STOL CH 750 Super Duty use a combination of several different features to achieve very high lift, low stall speeds, and high strength. A thick wing cross section, full-length leading-edge slats, and full-length trailing edge junker-type flaperons develop a maximum wing lift coefficient of 3.10, with a short wingspan for maximum strength and ground maneuverability.
Wings stall when they are at their highest lift coefficient, when the airflow can no longer go around the leading edge and separates from the upper wing surface. Conventional trailing-edge wing flaps help delay the stall to a higher lift coefficient, but only with limited effectiveness. However, by combining the use of trailing-edge flaps with leading-edge slats, the wing’s lift coefficient can be effectively doubled if both devices are used on the full span of the wing.
High-Lift Devices: How They Work
Leading edge slats allow the aircraft to fly at a higher angle of attack (and thus a lower speed) by both effectively changing the airfoil profile of the wing and by keeping the airflow attached to the top of the wing at lower speeds. At low angles of attack the air “sees” the slats as part of the wing airfoil and they have no effect. As angle of attack increases, air starts flowing through the slot between the slat and the wing. Now, the air “sees” the airfoil without the slat, which is a higher lift design. The slot between the slat and the wing is shaped like a funnel, bigger at the bottom than the top. This air accelerates through this slot in a venturi effect. This effectively “pulls” the air around the leading edge, thus keeping airflow attached to the wing at very low true airspeeds.
The leading edge slats allow for steep climb angles of up to 30-degrees. For maximum reliability and to keep construction simple, they are engineered to remain in a fixed position in all flight attitudes, and do not retract. In level flight, the fixed leading edge slats have minimal effect on cruise.
The disadvantage of leading-edge slats is that accelerating air through the slot requires energy and creates additional drag. While many STOL designs utilize retractable slat systems, these systems add weight, complexity, and cost, as well as being less reliable. For a simple light aircraft like the Super Duty, keeping the slats fixed is a reasonable tradeoff.
The full-length flaperons act as both full-span ailerons and full-span flaps. They have their own airfoil separate from the wing, and are hung below the wing trailing edge to supply them with fresh undisturbed air for maximum control effectiveness even at low speeds.
At the wing tip, the STOL design utilizes Hoerner tips to maximize effective lift area and minimize wing tips vortices. Hoerner wing tips provide the largest effective span for a given geometric span and wing weight. The wings are braced by dual steel wing struts, and are bolted to the fuselage at the cabin frame with two bolts each for easy wing attachment and removal.
Wing Design for Maximum Visibility
The aircraft wings are positioned above the cabin and fuselage, which gives excellent horizontal visibility, as the wings are located above the occupant’s horizontal line of sight. In addition, the wings taper at the wing root, giving a very wide viewing angle upward, a feature especially useful in steep turns. As well as providing great visibility, the above-cab wing design minimizes the frontal area of the aircraft to reduce drag, while also allowing the airflow to travel undisturbed from the propeller to the tail sections, thus further maximizing slow flight control of the aircraft.
Click here for a detailed schematic of the STOL CH 701 Wing Assembly, (very similar to the Super Duty).
Click here for a detailed schematic of the Slats and Flaperon Assemblies.