International Journal of Unmanned Aerodynamic Design of Delta UAV Systems Engineering (IJUSEng) Technical Note IJUSEng – 2014, Vol. 2, No. S2, 1-15 http://dx.doi.org/10.14323/ijuseng.2014.4 Conceptual Aerodynamic Design of Delta-type Tailless Unmanned Aircraft Alexander S. Goodman Marques Aviation Ltd, USA Abstract: Goodman AS. (2014). Conceptual aerodynamic design of delta-type tailless unmanned aircraft. International Journal of Unmanned Systems Engineering. 2(S2): 1-15. This paper describes the conceptual design of a delta-like tailless unmanned aircraft for intelligence, surveillance and reconnaissance missions, with possible strike capabilities. The delta configuration yields greater performance Keywords: at high angles of attack (α) and can produce lower drag than Aerodynamics Aeroelasticity conventional aircraft. The lack of inherent flight stability allows the Conceptual design aircraft to perform combat manoeuvres and the absence of vertical Delta planform control surfaces reduces radar signature. Aspects to take into Unmanned aircraft consideration in the conceptual aerodynamic design of a delta-like Stability tailless configuration include airfoil selection, aerodynamic modelling for steady state stability, wing planform shape, planform geometric alterations, control surfaces, and adaptive wing technology and aeroelastic concepts. This study proposes the use of the HS 522 airfoil for the inward part of the wing and the NACA 22112 airfoil for the outer wing. The HS 522 is an excellent fit for swept wings and low Reynolds numbers. The HS 522 produces only a small negative pitching moment and also performs well at high speeds. The NACA 22112 has reflex camber and helps promote longitudinal stability. When a fuselage is added to a wing, the aerodynamic centre (a.c.) of the wing-fuselage combination shifts forward. This is known as the Munk effect and must be considered. The dominant flow pattern over the proposed delta-like configuration will consist of leading edge vortex patterns at high α, containing a primary vortex and a secondary vortex. Wing sweep affects the a.c., lift curve slope, and zero-lift slope along with many other individual parameters, such as dihedral effect. Consequently, wing sweep angle is a critical parameter that should be carefully analyzed and tested in relation to aircraft desired performance. Several smaller elevons are proposed for small adjustments of flight attitude to achieve less drag, easily dampen aircraft oscillations, and lessen the wing decambering effect. Winglets help lower the strength of the tip vortices and can act as rudders for lateral/directional stability and control by taking advantage of a long moment arm. Multiple slotted reversed-flow flaps that mimic the natural flight of birds should be further investigated for low α flight attitudes to suppress the proliferation of detached flow, maintain high lift, and delay the stall. © Marques Engineering Ltd. 1 www.ijuseng.com IJUSEng - 2014, Vol. 2, No. S2, 1-15 International Journal of Unmanned Aerodynamic Design of Delta UAV Systems Engineering (IJUSEng) 1. INTRODUCTION This paper describes the conceptual aerodynamic design of a variant of a delta blended wing-body tailless mid-range unmanned aerial vehicle (UAV) with intelligence, surveillance and reconnaissance (ISR) and possible strike capabilities. The tailless configuration is desirable for a number of reasons. With no vertical control surfaces, or at least no large vertical surfaces, the aircraft has minimal radar signature.[1] This configuration can produce lower drag than conventional aircraft and greater performance at high angles of attack (α). Also, with the lack of inherent flight stability, the aircraft can perform combat manoeuvres.[2,3] With the limitations in technology and materials in the past, the tailless design layout was not as efficient as the conventional layout and was also very unstable. With advancements in both technology and materials a tailless delta configuration can be designed to yield greater performance than conventional-style aircraft.[4] The performance of tailless aircraft is expected to improve with further experimentation and analyses. This paper addresses the following topics of aerodynamic design: airfoil selection, basic aerodynamic modelling for steady state stability, planform shape, planform geometric alterations, control surfaces, adaptive wing technology, and aeroelasticy. 2. AIRFOIL SELECTION Traditional cambered airfoils produce a negative pitching moment (Cm), nose-down effect, on the airfoil. This is counteracted through the empennage by the horizontal stabilators. In a tailless delta-wing type aircraft, careful selection of the airfoils is essential, since Cm strongly [5] contributes to the aerodynamic longitudinal stability of the aircraft. The Cm is measured around the aerodynamic centre (a.c.). Both Cm and a.c. are discussed in further detail in the next section dealing with aerodynamic modelling. With no tail for longitudinal stability, the airfoils selected should have low or zero Cm. Instead of using a symmetric airfoil, which has [4] zero Cm at zero α, a suitable solution is to choose a reflexed airfoil. The following presents an analysis on reflex airfoils. The camber shape on an airfoil is closely related to Cm. The aft part of the camber line has a great influence on Cm. Because of this influence, the aft end of the camber line is reflexed. By increasing the degree of reflex, the Cm vs. α curve shifts upward (positive). The drag polar is also affected and shifts down. This produces an undesired reduction in the maximum lift coefficient, but can be fixed by creating more camber. However, greater caber increases the negative nose down Cm. In aerodynamics this is usually the case, improvement of one aspect negatively affects other aspects of the aerodynamic design. Another option is to change the location of the maximum camber. The position of the maximum camber has a small influence on the drag polar, but can greatly [6] impact the pitching moment. If the maximum camber position is shifted backwards, the Cm increases in the negative direction (nose-down). The maximum camber position should, therefore, be towards the leading edge (LE) of the airfoil for a delta planform aircraft. This is illustrated in Figs. 1 and 2, where three different airfoils are compared. Fig. 1: Geometry of three airfoils (adapted from Jiangtao et al.[5]) 2 www.ijuseng.com IJUSEng - 2014, Vol. 2, No. S2, 1-15 International Journal of Unmanned Aerodynamic Design of Delta UAV Systems Engineering (IJUSEng) Fig. 2: Aerodynamic coefficients for the three airfoils (adapted from Jiangtao et al.[5]) 2.1. Analysis of the HS 522 and NACA 22112 Airfoils Two different airfoils are used throughout the wing planform of the proposed tailless aircraft. The inward portion of the wing is an airfoil with a close to zero Cm. The outward portion of the wing is comprised of a reflexed airfoil to counteract the negative Cm created by the UAV configuration.[1] The airfoils are also able to operate at lower Reynolds numbers (Re) because the aircraft’s speed and chord length are small, which directly influences Re. The HS 522 airfoil was selected as the inward span airfoil (Fig. 3). This airfoil is an excellent fit for swept wings and low Re. This HS 522 airfoil can also perform well at high aircraft [7] speed. This airfoil does produce a nose down Cm, however the Cm is small compared to that of many other airfoils. Table A1 in the Appendix displays the results for three different low Res.[4] Fig. 3: HS 522 airfoil [7] A reflex camber airfoil is used for the outboard portion of the wing. The NACA 5 digit series airfoils are known to produce low Cm. Specifically, the NACA 22112 airfoil was selected (Fig. 4). The NACA 22112 airfoil was then compared to a similar airfoil, NACA 23112, which differs from the NACA 22112 airfoil only by the farther aft position of the maximum camber location.[5] These airfoils were compared at the two Re of 0.5 x 106 and 1 x 106. The results are shown in Table 1. Plots of airfoil aerodynamic coefficients vs. α can be found in Figs. A.1-A.4 in the Appendix. It can be seen from the Figures that as the maximum camber is pushed aft, the maximum lift decreases and the nose down Cm increases. The combination 3 www.ijuseng.com IJUSEng - 2014, Vol. 2, No. S2, 1-15 International Journal of Unmanned Aerodynamic Design of Delta UAV Systems Engineering (IJUSEng) of these two airfoils may balance each other, depending on the spanwise distance distribution, and could create a slight washout. Fig. 4: NACA 22122 profile with camber line [8] Table 1: NACA 5-digit series airfoil analysis Airfoil and Re NACA 22112, Re = 5 x 105 1.45 15.0° 0.005 0.010 NACA 22112, Re = 1 x 106 1.55 17.0° 0.005 0.005 NACA 23112, Re = 5 x 105 1.45 13.5° 0.007 0.008 NACA 23112, Re = 1 x 106 1.50 15.0° 0.005 0.005 3. AERODYNAMIC MODELLING One of the most important parameters of an aircraft is the a.c. The a.c. is the point on the wing mean aerodynamic chord where the variation of Cm with α is zero. Finding the a.c. on the wing planform is very important, however the influence of the fuselage must be considered. When a fuselage is added to a wing, the a.c. of the wing-fuselage combination shifts forward compared to that for a wing. This means that the fuselage will add a positive [9] Cm with each increase of α. This is called Munk effect. The design of the proposed aircraft, however, resembles that of a blended wing-body similar to that of a flying wing. Thus, the addition of the fuselage will not play as big a role on the Cm as a conventional aircraft, but still cannot be ignored.[4] In order to know how the aircraft will perform, modelling of the aerodynamic and thrust forces and moments must be determined for steady state and perturbed state.