Experimental Investigations of Airplane Stability and Control Characteristics in Stall
Half of the unintentional stall accidents in light general aviation airplanes likely cause fatalities. At stall, airplanes typically experience a vast rolling moment due to asymmetric lift distribution caused by uneven stall between the two wing laves. This dangerous rolling moment (known as wing drop) and a yaw moment caused by the same uneven stall cause lateral-directional instability at stall. The developed spin becomes more difficult to avoid as the roll control authority and handling qualities are significantly degraded at stall. In the first part of this dissertation, we focus on studying the airplane stability and control authority characteristics near stall. We designed a wind tunnel experiment to investigate both the wing drop and roll control authority in the pre-stall, stall, and post-stall regimes of a wide range of angles of attack (from -9 to +62 degrees). We designed and built five airplane models of different wing planform configurations: rectangular, tapered, hybrid (rectangular-tapered), cropped delta, and cropped elliptic to study the effect of the planform on the wing drop direction and magnitude. Simple flow visualization using tufts was used to study the effect of the wing planform on the stall progression. We developed a novel multi-axis load cell capable of withstanding the violent dynamic loads associated with stall. We measured lift, drag, rolling, and pitching moments vs. aileron and elevator deflections to assess the effectiveness of the control surfaces in the whole range of the angle of attack.
In the second part of this dissertation, we focus on the dynamic stall of two-dimensional wings. A wind tunnel experiment is designed for a continuously pitching wing over full rotation. Lift enhancement and stall delay are observed due to pitching rotation. The effect of angular velocity, Reynolds number, and airfoil type on this lift enhancement is investigated.