Tailerons are actuated aerodynamic surfaces used for roll control of flexible wings, which have several benefits relative to traditional aileron control. These include elimination of torsional divergence and favorable changes to control reversal constraints. Both reduce the need for wing torsional stiffness, which can lead to significant weight savings. This work extends classical 2D aeroelastic bending-torsion section theory to include the effect of tailerons and derives the aeroelastic constraints needed to design wings with tailerons. A point design study based on the wing of the Dawn One solar aircraft is then conducted to illustrate the benefits of tailerons over traditional aileron control in such an aircraft. Here, replacing ailerons with tailerons reduces the required wing torsional stiffness by a factor of 4. An aerostructural ASWING analysis for the entire aircraft is used to validate key findings of the 2D theory, and to evaluate operationally-relevant limits of taileron-based control. We then generalize these results for aircraft designers by estimating the relative net weight benefit of tailerons vs. ailerons purely as a function of high-level conceptual aircraft design variables. In some cases, mass savings in excess of 6% of aircraft gross mass are found.

This two-part paper describes a new approach to determine the effect of surface waviness, arising from manufacture of composite fan blades, on transition onset location movement and hence fan profile losses. The approach includes analysis and computations of unsteady disturbances in boundary layers over a wavy surface, assessed and supported by wind tunnel measurements of these disturbances and the transition location. An integrated framework is developed for analysis of surface waviness effects on natural transition. The framework, referred to as the extended e(N) method, traces the evolution of disturbance energy transfer in flow over a wavy surface, from external acoustic noise through exponential growth of Tollmien-Schlichting (TS) waves, to the start and end of the transition process. The computational results show that surface waviness affects the transition onset location due to the interaction between the surface waviness and the TS boundary layer instability, and that the interaction is strongest when the geometric and TS wavelengths match. The condition at which this occurs, and the initial amplitude of the boundary layer disturbances that grow to create the transition onset is maximized, is called receptivity amplification. The results provide first-of-a-kind descriptions of the mechanism for the changes in transition onset location as well as quantitative calculations for the effects of surface waviness on fan performance due to changes in surface wavelength, surface wave amplitude, and the location at which the waviness is initiated on the fan blade.

A 16-foot (4.88m) diameter wind tunnel fan is designed to maximally exploit Boundary Layer Ingestion (BLI). Its features include a highly non-uniform design radial load distribution to cancel the large inlet BL stagnation-pressure defect at the blade tip, and a twist distribution which accommodates the inlet BL axial-velocity defect. The design compensates for the upstream streamsurface radial redistribution resulting from the nonuniform radial loading. The blade airfoils are designed to work over a very large Reynolds number range to enable stable flow over a wide tunnel speed range of 10--230 mph. The resulting fan blade planform has a distinctive reverse-taper chord and forward sweep towards the tip. Design validations include 3D RANS simulations of the entire wind tunnel flowpath circuit with the fan blades resolved, and pitot rake surveys of the constructed fan's inlet and exit flows. The calculations and measured performance indicate that the BLI-exploiting fan gives nearly the ideal uniform exit stagnation pressure as intended. This improves the performance of the downstream diffuser, and also gives a 17.5\% reduction in wind tunnel drive power compared to a more traditional fan designed for uniform radial loading.

Distributed electric propulsion presents new opportunities to design aircraft which take advantage of the deliberate close interaction between propellers, wings, and flaps. Significant forces and moments arise which would not be well-captured by modeling the components in isolation. The ability to accurately predict these forces is important for a number of new vehicle designs, including electric short takeoff and landing (eSTOL) aircraft which use the interaction between distributed large propellers and wing/flaps to generate very high wing lift coefficients, if normalized by the flight speed. The flow field at the propellers is strongly influenced by the presence of the wing, and vice versa. This paper presents CFD analyses relevant to an eSTOL wing in a high-lift configuration, comparing four different approaches to modeling the propellers - an actuator disk, a steady and an unsteady blade element model, and a blade-resolved unsteady simulation. This is done in the Flow360 Navier-Stokes solver developed by Flexcompute. We begin with an isolated rotor, continue with a model problem that comprises a single rotor with behind it a section of wing and flap, and conclude with a full 3D configuration. The actuator disk is advantageous from a computational time point of view, but introduces error into the solutions because it cannot adapt to strongly varying local inflow conditions. The time-resolved simulations are expected to give the most accurate solutions, but are computationally expensive. An adaptive blade-element model gives a good compromise between accuracy and performance.

The integral boundary layer (IBL) method is computationally efficient and effective for viscous aerodynamic analysis, thus particularly suitable for the preliminary design of aircraft and wind turbines. Although two-dimensional (2D) IBL methods and alike have been well established, off-the-shelf three-dimensional (3D) IBL methods remain to be desired, mainly due to lack of suitable and reliable closure modeling. To address this gap, the current work develops 3D IBL closure models to enable practical applications of the 3D IBL method for aerodynamic analysis involving laminar and turbulent boundary layers. Extensive 3D boundary layer data are collected from numerical solution of boundary layer flow. Closure models are then constructed using regression methods based on feed-forward neural networks. Variable selection is carried out using a data-driven sparse-input regularization approach and exact embedding of symmetry and anti-symmetry physical constraints is achieved by customizing the function representation. Also, the turbulence model for 3D IBL is calibrated using field inversion and neural-network-based regression. Results of the 3D IBL method equipped with new closure models demonstrate the effectiveness of the overall method as compared to higher-fidelity boundary layer data.

Stochastic gradient-based optimization is used to overcome the combinatorial scaling problem of deterministic multi-point optimization over potentially many design parameters. The specific application is transonic airfoil design to minimize the mission fuel burn of a transport aircraft flying over some ranges of the operating (design) parameters, specifically the Mach number, Reynolds number, and lift coefficient. At each optimization descent step, mini-batch samples are used to approximate the objective function and its design gradient over the design variables, which here are Chebyshev-mode geometric design variables and the angle of attack. Robustness of optimal solution to initial seed airfoil and on the chosen operating ranges is examined. Adaptive variation of the Mach range is implemented to address the fact that the appropriate range is not known a priori, and also to suppress problems arising from solution non-convergence at unrealistically high sampled Mach numbers. The transonic airfoil analysis code MSES is used as the flow solver, with flow parameters chosen in each design step by mini-batch random sampling. The stochastic present optimization method improves performance over the multi-dimensional parameter space, and prevents the appearance of irregular geometries frequently seen in single-point or sparse-sampled deterministic airfoil optimization.

Electric short takeoff and landing (eSTOL) aircraft and electric vertical takeoff and landing (eVTOL) aircraft are being developed for missions where availability of ground infrastructure is a critical design driver. Because eSTOL aircraft can generate high effective lift coefficients through the interaction of the wing, flaps, and distributed propellers they can achieve takeoff and landing distances comparable with the ground footprint proposed for eVTOL facilities. eSTOL aircraft require smaller propulsion systems and less energy for takeoff and landing than eVTOL aircraft, which in turn translates to reduced vehicle weight or increased payload, range, and/or speed. This paper compares the performance difference between eSTOL and eVTOL aircraft, for both hybrid- and battery-electric propulsion architectures. Both tilt-duct and tilt-rotor eVTOL configurations are examined. For aircraft with an equivalent weight and span to proposed eVTOLs, eSTOL aircraft are able to carry 1.8-2.6x the payload at the same speed and range, depending on the eVTOL type and propulsion system architecture. This number is sensitive to eVTOL disk loading, design mission, and modeling of blown wing performance. The benefit of eSTOL arises primarily from reduced propulsion system weights and reduced energy consumption in the takeoff and landing phases. This benefit varies significantly with design ground footprint and payload; and less so with range and speed.

This paper presents first-of-a-kind measurements, and complementary computations, of the flow through the propulsion system of a boundary layer ingesting, twin-engine advanced civil transport aircraft configuration. The experiments were carried out in the NASA Langley 14- by 22-foot Subsonic Tunnel, using a 1:11 scale model of the D8 “double-bubble” aircraft with electric ducted fans providing propulsive power. Overall force and moment measurements and flow field surveys at the inlet and nozzle exit planes were obtained. The computations were carried out with the NASA OVERFLOW code. The measurements and computations were conducted for a range of aircraft angles of attack and propulsor powers representing operating points during the aircraft mission. Velocity and pressure distributions at the propulsor inlet and exit, and integral inlet distortion metrics, are presented to quantify the flow non-uniformity due to boundary layer ingestion. The distorted inflow exhibits qualitative and quantitative changes over the mission, from a unidirectional stratified stagnation pressure at cruise to a streamwise vortex structure at climb conditions. The computations capture these flow features and reveal the interactions between airframe and propulsor that create these three-dimensional flow variations.

Part II describes the experimental assessment and the application of the ideas in Part I concerning the mechanisms that determine the role of blade surface waviness on laminar-turbulent transition and their consequent effect on civil aircraft fan performance. A natural transition wind tunnel was designed and constructed to characterize the impact of surface waviness on transition, using both hotwire anemometry and infrared thermography. The experimental results support the new hypothesis presented in Part I, concerning the way in which blade surface waviness affects fan performance through motion of the transition onset location due to interaction between surface waviness and Tollmien-Schlichting (TS) boundary layer instability. In particular, the theoretical amplification of the TS waves, and the corresponding transition onset location movement due to surface waviness, was borne out over a range of variations in Reynolds number, non-dimensional surface wavelength, non-dimensional surface wave height, and location of surface wave initiation, relevant to composite fan blade parameters. Further, the increase of receptivity coefficient, and thus the initial amplitude of disturbances due to geometric resonance between surface wavelength and TS wavelength, was also confirmed by the experiments. Surface waviness was estimated, in some cases, to result in a nearly 1% decrease in fan efficiency compared to a non-wavy blade. Suggestions are given for mitigation of the effects of waviness, including the idea of blade curvature rescheduling as a method to delay transition and thus decrease loss.

The concept of regional and intra-city mobility using small aircraft, commonly known as Urban Air Mobility (UAM), has rapidly become one of the primary interests in commercial aviation; the idea focuses on passenger travel using small, electric aircraft able to operate in constrained take-off and landing environments of urban and suburban areas. While many of the aircraft developed for this concept have focused on vertical take-off and landing (VTOL) capabilities, it has been demonstrated that fixed-wing aircraft using distributed electric powered lift systems might be able to operate under similarly-constrained environments as short take-off and landing (STOL) aircraft. While powered lift is not a new concept, design and analysis tools remain underdeveloped for this purpose because the data needed to validate such methods does not exist. In this paper, a series of wind tunnel surveys were conducted on several configurations of a quasi-2D blown-flap airfoil to enhance understanding of their performance characteristics and generate the validation data needed for future computational analysis.

View Video Presentation: https://doi.org/10.2514/6.2021-2552.vid The aerodynamic forces acting on an aircraft are examined, along with their decomposition into components commonly used in aerodynamic design and analysis, and aircraft engineering in general. Distinctions are made between rigorous force definitions, force approximations, and correlation-based force estimates. The attributes of exactness, consistency, and uniqueness, are also presented as a means of evaluating and categorizing possible alternative force decomposition approaches. The feasibility of force component extraction from CFD or experimental data is also examined. A major goal of the paper is to serve as a roadmap for comparisons and for further development of theoretical and practical formulations of aerodynamic force decomposition.

The paper presents the screened expanding corner vane concept, which can turn a channel flow by $$90^\circ$$, while simultaneously increasing the flow area by at least a factor of two and thus halving the mean velocity, all without incurring any significant flow separation. The concept is demonstrated experimentally, and investigated with analytical and computational models. One target application is an ultra-compact closed-circuit wind tunnel, whose overall length is roughly half that of a conventional wind tunnel with the same test section.

This paper presents the results from initial flight tests of a 30\% scale demonstrator of a blown-wing SuperSTOL concept aircraft, intended for operation from extremely short runways of 100 ft or less. The subscale demonstrator is aimed at investigating the maximum achievable in-flight lift coefficients with the blown wing, as well as the control and handling qualities with a mostly conventional aircraft configuration with unblown control surfaces. With a relatively modest amount of blowing power - a static thrust/weight of 0.45 - the flight tests show that the blown wing SuperSTOL concept can generate high lift coefficients greater than 10 in flight. It was observed that reducing the size of the propeller enabled larger $C_L$ values to be achieved. In high-$C_L$ flight the roll control authority of conventional ailerons was found to be marginal, partly due to the low fight dynamic pressure and partly due to the local stall over the unblown part of the aileron. In the configuration tested most of the elevator deflection was consumed to obtain pitch trim at low speed. A finite rotation rate to takeoff attitude was found to significantly contribute to the ground roll distance.

Viscous analysis is crucial for understanding aerodynamic performance metrics such as profile drag. For three-dimensional (3D) viscous analysis, Reynolds-averaged Navier-Stokes (RANS) solvers are often the fastest available tool in practice but the required computational efforts preclude its extensive use for preliminary design. In contrast, the integral boundary layer (IBL) method offers a computationally more efficient alternative with comparable effectiveness when applicable. However, existing IBL methods mostly rely on two-dimensional (2D) or quasi-2D assumptions and thus remain to be extended to a fully 3D formulation for general configurations. To this end, we continue the development of an IBL method with discontinuous Galerkin (DG) finite element discretization and strong viscous-inviscid coupling. The current work proposes a captured laminar-to-turbulent flow transition treatment for the IBL method that can be more conveniently extended to the 3D case compared to a previously examined fitted transition approach. The current captured transition treatment also leverages a more robust nonlinear solution method and achieves accurate solution of transitional flows. Moreover, correction to the standard DG discretization is introduced for well-behaved numerical solution. Numerical results of the proposed method in a 2D implementation compares well with XFOIL and demonstrate its capability for practical aerodynamic analysis with free transition.

We present a novel closed-circuit ultra-compact wind tunnel with an 8:1 contraction ratio and high flow quality. Its overall footprint area is less than half that of a conventional tunnel with the same test section size and same contraction ratio, enabling significantly smaller material and construction costs. The tunnel’s key features which enable the small footprint include a two-dimensional main diffuser, a minimum-length contraction, and expanding turning vanes with a 1.167:1 ratio in corner two and an aggressive 1.875:1 ratio in corner four. Separation in the latter is prevented using a screen and honeycomb integrated into each vane passage—the first time this has been used in a wind tunnel. The tunnel exhibits excellent flow quality with less than ± 1

Transonic aeroelasticity is an important consideration in the conceptual design of next-generation aircraft configurations. This paper develops a low-order physics-based flutter model for swept high-aspect-ratio wings. The approach builds upon a previously developed flutter model that uses the flowfield's lowest moments of vorticity and volume-source density perturbations as its states. The contribution of this paper is a new formulation of the model for swept high-aspect-ratio wings. The aerodynamic model is calibrated using offline two-dimensional unsteady transonic computational-fluid-dynamics simulations. Combining that aerodynamic model with a beam model results in a low-dimensional overall aeroelastic system. The low computational cost of the model permits its incorporation in a conceptual design tool for next-generation transport aircraft. The model's capabilities are demonstrated by finding transonic flutter boundaries for different clamped-wing configurations and investigating the influence of transonic flutter on the planform design of next-generation transport aircraft.

The paper presents the HSM finite element formulation which extends common existing C-continuous large-deflection shell methods by the introduction of spherical interpolation of unit basis vectors, and higher-order representations of a virtual C continuous shell surface and its covariant basis vectors. These modifications greatly improve accuracy for elements which are highly curved, either in the undeformed case or after deformation, with the result that HSM can tolerate very coarse grids, especially in bending-dominated problems. This makes the method particularly well suited for intermediate-fidelity aeroelastic modeling, since coarse surface grids on aerodynamic bodies naturally results in highlycurved elements. Various structural phenomena common in nonlinear aeroelasticity, such as large deformations and buckling, can be thus be predicted with modest cost.