At this point the boundary layer fails to reattach to the airfoil, causing very rapid full chord airflow separation. In smoke wind-tunnel tests the bubble appears to burst. This bubble remains attached to the airfoil until the AOA is increased to a critical value, whereupon it suddenly breaks down for reasons not presently understood. This separation causes the boundary layer to transition from laminar to turbulent, and at this point the airflow would form a small rotating “bubble” that helped to reattach the boundary layer to the airfoil. As the AOA is increased, an adverse pressure gradient increases to the point where a small amount of separation occurs near the leading edge. Earlier NASA studies had discovered that airfoils with this design exhibited a smooth laminar flow over the leading edge at low AOAs. One of its first challenges was to better understand these abrupt stall characteristics. In 1982, the Learjet flight-test team was tasked with trying to make improvements to the stall characteristics of its earlier aircraft. Suspected light ice accumulations on the aerodynamic surfaces may have contributed to a stall and loss of control.įlight Testing Uncovers the Leading-Edge Stall The ensuing loss of control resulted from inappropriate pilot techniques during the attempt to regain control of the aircraft. The NTSB determined the probable cause of the accident to be an encounter with strong, gusting crosswinds during the landing attempt, which caused the aircraft to roll abruptly and unexpectedly. Sadly, his wife of 26 years was one of the accident’s victims. One of the survivors of the violent impact was then Senate Minority Whip Ted Stevens, who had flown C-46s over the infamous “Hump” route in Asia during World War II and was a recipient of the Distinguished Flying Cross. Tragically, five of the seven occupants were fatally injured. The Learjet rolled to the right until its right wing struck the ground. The flight path was normal almost to touchdown, when the aircraft suddenly pitched up and began to bank steeply from side to side. along with turbulence and gusting winds in the vicinity of the airport. Light to moderate icing was forecast in clouds below 12,000 ft. 4, 1978, to a Learjet 25C that was on approach to Anchorage International Airport (PANC) with seven persons on board. The rapid roll-off and abrupt stall with no aerodynamic warning caught pilots by surprise, adversely impacting recognition or recovery. It was also noted that the stall characteristics deteriorated markedly with small accumulations of ice on the wing leading edges. Furthermore, there was little or no aerodynamic stall warning buffet. The airflow over one wing would suddenly separate entirely, causing a rapid and large roll-off, often beyond 90 deg. This impacted the airflow over the aileron. In complete contrast to the traditional trailing-edge stall that is a characteristic of slower aircraft, flight tests by Lear observed that as the angle of attack increased, the flow around the tip tank caused a high AOA at the junction of the tip tank and the wing leading edge, which caused a triangular shaped area of separated airflow. As a result, they were ill-equipped to properly diagnose, prevent and recover from phenomena such as Mach tuck, deep stalls, aileron snatch and unforgiving abrupt stalls with uncontrollable wing roll-off. While the potential cruise speeds of these jets were attractive, accidents and serious incidents revealed pilots’ lack of understanding of the different aerodynamics of airfoils designed for high-speed flight. It was designed with a 9% thick airfoil section, no wing twist, and the characteristic tip tanks. 7, 1963, signified the beginning of a revolution in transport aircraft to cruise at relatively fast transonic Mach numbers. The Learjet 23 was a product of this aeronautical concept. Thus, with the advent of the jet era, aeronautical engineers began to utilize thin airfoils as a design method to lessen the development of Mach waves for aircraft that would cruise at transonic speeds. One inherent disadvantage of thicker airfoils is the propensity to develop a strong Mach wave due to the acceleration of air over the bulbous leading edge during transonic flight. In Part 1 of this article, we discussed airfoil design and trailing-edge stall.Įven though you have been inundated throughout your flying career about the trailing-edge stall behavior of an airfoil, what is the likelihood that your high-performance aircraft utilizes an airfoil of this older design? Practically nil.
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