Flying-wing aircraft are a type of aircraft consisting only of a single horizontal lifting surface. These airframes are used for aircraft ranging from small UAVs to large manned bombers. Flying wings require significant sweep in order to make the aircraft stable in pitch. However, man-made flying wings look significantly different than natural flyers (birds). This is partly due to the fact that most birds use their tail as a major source of pitch stability. However, one noticeable difference between our man-made wings is that birds do not use positive sweep near their centerline. We think we’ve found one reason why, as explained in this video.
We are now looking at developing crescent wings, which look more like bird wings. Initial simulations show that crescent wings can decrease induced drag by 10-15% compared to their straight, flying-wing counterparts. This is significant for long-range airframes, as an increase in aerodynamic efficiency can equate directly to an increase in range and endurance.
Locus of Aerodynamic Centers of Swept Wings
Student: Jackson Reid
The aerodynamic center of a two dimensional airfoil is defined as the point about which the moment created by the airfoil is independent of the angle of attack. For most airfoils, the aerodynamic center is located near the quarter-chord.
The locus of aerodynamic centers for a finite wing can change as a function of span. For a straight wing, this locus lies very near the quarter-chord. However, for a swept wing, the local aerodynamic center at each spanwise location can vary significantly from the quarter-chord as shown in the figure [Phillips, Hunsaker, and Niewoehner]. Accurately predicting the locus of aerodynamic centers on swept wings will make low-order aerodynamic models more accurate. The goal of this research is to describe the shape and location of the aerodynamic center for swept wings throughout the range of operating conditions.
Numerical Lifting-Line Algorithm for Swept Wings
Student: Jeff Taylor
Prandtl’s lifting line theory provides analytical predictions for the lift and induced drag on a single lifting surface with no sweep or dihedral. However, the numerical lifting line model developed by Phillips and Snyder, which is based on the foundation of Prandtl’s lifting line theory, is capable of predicting the lift and induced drag on multiple lifting surfaces, which may have sweep and dihedral.
This method has been shown to produce accurate results for wings without sweep, but for wings with sweep, it fails to grid resolve [Hunsaker]. The purpose of this project is to investigate the numerical lifting line method and develop any necessary modifications to increase its stability and accuracy for wings with sweep and dihedral.
Propulsion of Flapping Airfoils
The plunging motion of an airfoil in flapping flight rotates the lift vector forward and creates thrust. This is the method of propulsion used by almost all natural flyers and swimmers such as fish and birds. The pitching and plunging motion of an airfoil creates an oscillating wake of vorticity behind the airfoil. This wake alters the pressure distribution on the airfoil compared to that which would be experienced in a steady condition.
A model was developed, which shows that the time-dependent aerodynamic forces are related to two Fourier coefficients evaluated here from computational results. Correlation equations for these Fourier coefficients were obtained from a large number of grid- and time-step-resolved inviscid computational fluid dynamics solutions, conducted over a range of flapping frequencies. The correlation results can be used to predict the thrust, required power, and propulsive efficiency for airfoils in forward flight with sinusoidal pitching and plunging motion. Within the range of parameters typically encountered in the efficient forward flight of birds, results obtained from the correlation equations match the computational fluid dynamics results more closely than do those obtained from the classical Theodorsen model.
Flapping is the method nature has chosen to propel almost all living creatures through fluids. Birds, bats, fish, rays, insects, and snakes each use a form of flapping to propel themselves through fluids.
Flapping flight has been an area of research for several years and continues to be studied by several organizations including AFRL, NRL, DARPA, and NSF. Many underlying relationships of aerodynamics, fluid/structure interaction, twist distributions, and flapping frequencies are still not well understood. If these fundamental principles could be understood and replicated, more efficient unmanned air and water craft could be developed and open up a host of extraordinary missions. The ability to maneuver swiftly, quietly, and efficiently as well the ability to perch and observe would benefit future defense and research organizations.
Initial work has focused on modeling the propulsion of 2D airfoils [Hunsaker and Phillips, 2015] and twist optimization for low-frequency flapping [Phillips, Miller, and Hunsaker, 2013]. Further funding would allow these results to be incorporated into a 3D analytical model that could be used to study twist distributions and flapping frequencies. The following presentations show a viable path forward to developing efficient flapping systems:
- Optimization of Flapping Flight using Numerical Lifting-Line Analysis
- A Fundamental Approach to Modeling Flapping Flight
The flapping simulations shown to the left were produced using MachUp, and open-source geometry creation and analysis tool.
A wing flying near the ground experiences a phenomenon called ground effect. The ground hinders the influence of shed vorticity, and thus decreases the downwash across the wing. This in turn reduces the induced drag produced by the wing.
This phenomenon can be predicted using potential flow models such as the modern lifting-line algorithm. This phenomenon is modeled by producing a mirror-image of the aircraft reflected across the symmetry plane of the ground. This symmetry plane requires that no flow can cross the boundary, and thus models the suppression of the shed vorticity.
MachUp was used to evaluate the induced-drag ground-effect influence ratio as well as the lift ground-effect influence ratio as a function of height above ground for a wide range of wings of varying aspect ratio and taper ratio. Empirical relations were fit through the results to provide closed-form predictions for the change in lift and induced drag in ground effect [Phillips and Hunsaker, 2013].