Application of a Hinge-Moment-Based Envelope Protection System on a Wing

Abstract

S TALL warning on a conventional fixed-wing aircraft is commonly provided through a simple angle-of-attack system, composed of a pivoting vane element and supporting equipment. This type of system is commonly used across a broad spectrum of aircraft classes, including general aviation vehicles, utility aircraft, and transport-class aircraft. For such systems, the angle of attack can be measured and a stall warning is provided when a preprogrammed threshold in α is reached. However, stall of an aircraft can be unwittingly approached in the presence of adverse environmental conditions (e.g., heavy rain, icing conditions), which can reduce the baseline clean stall α or insufficient situational awareness of the flight crew to the state of the aircraft. Under such circumstances, a stall warning or envelope protection systemmust be capable of adapting to changing conditions. Busch et al. [1] identified that the effects of a horn-ice shape on a NACA 23012 airfoil included a reduction in Cl;max of 55%, a reduction of αstall by 50%, and increases in Cd of approximately 400%. Similarly, Wickens and Nguyen [2] identified that the effects of a leading-edge ice shape on a NACA 4415 threedimensional (3-D) wing model included a reduction in wing CL;max of 30–50%, along with reductions in αstall by 4–6 deg. Traditional angle-of-attack systems, like those mentioned previously, compensate for the reductions in maximum lift and stall angle of attack associated with icing by reducing the angle-of-attack threshold of a stall warningwhen the icing protection system is turned on.While the angle-of-attack reduction is highly aircraft dependent, conventional angle-of-attack systems on business jets are programmed to reduce the angle-of-attack threshold for a stall warning by approximately 3–5 deg when the icing protection system is engaged. In the case of a severe ice shape, such as the previous examples from Busch et al. [1] and Wickens and Nguyen [2], this reduction in stall warning angle of attackmaynot fully compensate for the ice-induced reductions inαstall. Reductions in the stall angle of attack from a clean aircraft configuration are also not limited to icing conditions. Premature stall can occur due to other environmental contaminants, such as rain, frost, or distributed surface roughness. For example, Luers and Haines [3] identified a reduction in maximum lift by upward of 30% for an aircraft in heavy rain conditions. Broeren and Bragg [4] also identified significant reductions in maximum lift for various airfoils with distributed leading-edge roughness. In an effort to improve the current state of stall prediction and envelope protection for aircraft under adverse conditions, Gurbacki and Bragg [5,6] proposed a stall-prediction system for iced airfoils based on unsteady flap hinge-moment measurements. This stallprediction method was also proposed to be used as a part of smart icing systems [7]. This method was further developed by Ansell et al. [8] and Kerho et al. [9], who developed a stall-prediction system for airfoils in clean or contaminated configurations by processing hingemoment measurements using three unique “detector functions.” As the airfoil begins to stall, the amount of unsteadiness present in the airfoil flowfield increases. The detector functions are used to operate on the unsteady hinge-moment signal in order to evaluate the extent of the flowfield unsteadiness, which can then be correlated to a prescribed angle-of-attack boundary to stall. The resulting detector function system for these hinge-moment measurements provided airfoil stall predictions within 0.7 deg in angle of attack across most simulated contamination configurations. With the effective implementation of the hinge-moment-based stall-prediction concept on flapped two-dimensional (2-D) airfoil sections, it is desirable to understand how such a system could be applied to an aircraft. For this reason, this Note documents the extension of the hinge-moment-based stall-prediction system to a straight, tapered 3-D wing system with multiple flaps.