Aerodynamic investigation of a novel aircraft configuration

2016-09-29Commercial aviation has seen no gross configuration changes since its inception with the Douglas DC-3 in 1935. Aviation industry leaders project major increases in commercial aircraft demands over the next two decades, where production is expected to more than double. Such demands may require large aerodynamic improvement in aircraft performance such that the aircraft configuration changes from the dominant "tube-and-wing configuration." ❧ Ideal flight of a fixed span object consists of a constant spanwise downwash distribution, resulting in a state of minimum induced drag. Aviation serves a practical purpose however, where a central body used for carrying a payload disrupts the constant spanwise downwash distribution. This is a problem the tube-and-wing configuration does not solve. In 2012, Huyssen et al. proposed placing a horizontal control surface closer to the wing trailing edge, such that deflection of this horizontal surface alters the circulation about the new (wings + body + tail) system thereby restoring downwash lost due to the presence of the central, payload carrying body. Placing a horizontal control surface closer to the wing trailing edge requires a lower fineness ratio body than the tube-and-wing configuration currently employs. This may be more efficient, due to decreases in skin-friction drag and packing efficiency, compared to the long, cylindrical fuselage of the tube-and-wing. A shorter body leads to the loss of the tube-and-wing trademark rear empennage, used for purposes of pitch and yaw control, but other methods for maintaining a positive static margin and achieving yaw control are possible. As a result, a novel aircraft configuration called the Natural Dominant Configuration (NDC) has been proposed, and was shown by Huyssen et al. to restore downwash as predicted. ❧ Aerodynamic performance improvements due to a (short body + wing + deflected tail) combination seem possible, but parametric variation was limited and these results come from the pre-WWII era of wind tunnel testing. Thus, explanations based on measurements of the boundary layer interactions and resulting changes in profile drag over an aircraft aft-body and horizontal tail combination are missing. Contemporary wind tunnel tests on a (low fineness ratio body + wings + horizontal tail) system are also incomplete, primarily because they lack a complete airplane model or direct force measurements. Direct lift and drag measurements as well as a drag decomposition are missing from the work of Huyssen et al. These measurements combined with measurements of the boundary layer interaction over a deflected horizontal tail will quantify the aerodynamic performance of the Natural Dominant Configuration (NDC) aircraft, and, most importantly, determine the sources and fluid dynamic mechanisms for lift and drag variation. ❧ Lift and drag forces were measured with a force balance on a new NDC model aircraft inside a low turbulence wind tunnel at a chord-based Reynolds number of 1.04×10⁵. The ideas behind the NDC are essentially inviscid, or Reynolds number independent, so wind tunnel tests must be carefully justified and controlled. Boundary layer trips are used to help avoid Re-dependent variations. If benefits of the NDC are measurable at a Reynolds number of 1.04×10⁵, they should be larger, or even amplified at larger Re. A drag decomposition was conducted by combining total drag measurements with 3D profile drag estimates from the momentum defect of the NDC model's wake using Particle Image Velocimetry (PIV) techniques. These forces were measured as a function of tail presence and deflection angle. PIV investigations along the aft-body and tail of the model were also made to determine if differences in velocity profile positions, boundary layer thicknesses and separation locations to determine the tail’s effects on pressure (form) drag. ❧ The combination of force balance and PIV measurement techniques show that the tail can affect both the outer and boundary layer flows, improving aerodynamic performance by increasing lift while decreasing drag. Results show a deflected tail can improve the lift to drag ratio by 10%. Induced drag reductions are inferred from the improved uniformity of the spanwise downwash distribution caused by a moderately deflected tail. The tail also decreases interaction between boundary layers of the various airplane components, which can decrease pressure drag by delaying separation. Measurements here show that the Natural Dominant Configuration possesses better aerodynamic characteristics than the tube-and-wing configuration by synthesizing airplane components into a system