Forward Flight Mode Research Articles

Performance improvement of small-scale rotors by passive blade twist control

A passive twist control is proposed as an adaptive way to maximize the overall efficiency of the small-scale rotor blade for multifunctional aircrafts. Incorporated into a database of airfoil characteristics, Blade Element Momentum Theory is implemented to obtain the blade optimum twist rates for hover and forward flight. In order to realize the required torsion of blade between hover and forward flight, glass/epoxy laminate blade is proposed based on Centrifugal Force Induced Twist concept. Tip mass is used to improve the nose-down torsion and the stabilization of rotating flexible blade. The laminate blades are tested in hover and forward flight modes, with deformations measured by Laser Displacement Sensor. Two Laser Displacement Sensors are driven by the tracking systems to scan the rotating blade from root to tip. The distance from blade surface to a reference plane can be recorded section by section. Then, a polynomial surface fitting is applied to reconstruct the shape of rotating blade, including the analysis of measurement precision based on the Kline–McClintock method. The results from deformation testings show that nose-down torsion is generated in each flight mode. The data from a Fluid Structure Interaction model agrees well with experimental results at an acceptable level in terms of the trend predictions.

Antiadhesion elastomer seal coatings for UV and atomic oxygen protection

ABSTRACTRadiation blocking sunscreen coatings have been developed for the protection of elastomer seals used in low‐Earth‐orbit (LEO). The coatings protect the seals from ultraviolet (UV) radiation and atomic oxygen (AO) damage. The coatings were developed for use on NASA docking seals. Docking seal damage from the UV and AO present in LEO can constrain mission time‐line, flight mode options, and increases risk. A low level of adhesion is also required for docking seals so undocking push‐off forces can be low. The coatings presented also mitigate this unwanted adhesion. Greases with low collected volatile condensable materials (CVCM) and low total mass loss (TML) were mixed with slippery and/or UV blocking powders to create the protective coatings. Coatings were applied at rates up to 2 mg/cm2. Coated seals were exposed to AO and UV in the NUV (near‐UV) and UV‐C wavelength ranges (300 to 400 nm and 254 nm, respectively). Ground based ashers were used to simulate the AO of space. The Sun's UV energy was mimicked assuming a nose forward flight mode, resulting in an exposure rate of 2.5 MJ/m2 day. Exposures between 0 and 147 MJ/m2 (UV‐C) and 245 MJ/m2 (NUV) were accomplished. The protective coatings were durable, providing protection from UV after a simulated docking and undocking cycle. The level of protection begins to decline at coverage rates less than 0.9 mg/cm2. The leakage of seals coated with Braycote + 20%Z‐cote ZnO sunscreen increased by a factor of 40 after moderate AO exposure; indicating that this coating might not be suitable due to AO intolerance. Seals coated with DC‐7–16.4%Z‐cote ZnO sunscreen were not significantly affected by combined doses of 2 × 1021 atoms/cm2 AO with 73 MJ/m2 UV‐C. Unprotected seals were significantly damaged at UV‐C exposures of 0.3 MJ/m2 and DC‐7–16.4%Z‐cote coated seals were undamaged at all exposures up to the limits tested thus far which were 147 MJ/m2 UV‐C and 245 MJ/m2 NUV. The coatings decreased adhesion sufficiently for docking seals at temperatures equal to or greater than −8°C thus offer a simple and inexpensive way to mitigate adhesion. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41662.

Fluid Dynamics of Flapping Insect Wing in Ground Effect

The fluid dynamics of flapping insect wing in ground effect is investigated numerically in this study. To model the insect wing cross-section in forward-flight mode, the laminar flow over a NACA0012 airfoil animated by a combination of harmonic plunge and pitch rotation is considered. To implement the simulation, the proposed immersed boundary-lattice Boltzmann method is employed. By fixing the Reynolds number and the amplitude of motion, we systematically examine the influences of the distance between the foil and the ground and the flapping frequency on the flow behaviors. As compared to the situation out of ground effect, the forces for foil placed in close proximity to the ground show some differences. The mean drag coefficient is increased at low frequency and decreased at high frequency. Meanwhile, the mean lift coefficient is increased at both low and high frequencies and decreased at middle frequency. Moreover, an interesting phenomenon with oblate vortices due to vortex interaction with the ground is observed.

비행조종성능을 위한 헬리콥터 FBW 비행제어법칙 설계

This paper is regarding the helicopter flight control law design for the handling quality performance. MIL-F-83300 and ADS-33E specification is used of the helicopter flight handling quality and to meet these requirements, ACAH type controller is required. This paper described the ACAH type controller design and performance evaluations. Helicopter dynamics first developed as nonlinear dynamics including rotor dynamics and then linear model was extracted from hovering to forward flight mode using trim condition. Control law used the model following to meet the handling qualities, the simple inverse model as feed forward gain, decoupling logic and phase model to decouple the axes, and linear model to calculate the coefficients. Handling quality evaluation used the matlab based Conduit tool and verified that Level 1 requirement is satisfied.

Modeling and Experimental Characterization of SMA Torsional Actuators

This article presents the material modeling and experimental characterization of SMA rod and tube actuators undergoing pure torsional deformations. The investigation of the torsional characteristics of SMAs is carried out in order to gain a fundamental understanding of the behavior of SMA torsional actuators. The proposed application is to alter the twist distribution of a tiltrotor blade from hover to forward flight modes, providing improved flight efficiency in both modes of flight. To describe the behavior of the actuator, a torsional model involving the extension of the one-dimensional (uniaxial) formulation of SMA phenomenology is presented. As an example, Brinson's model is chosen as the representative uniaxial model; however, the approach used here to extend the uniaxial model into the torsional domain is applicable to nearly any uniaxial SMA model. The parameters for the uniaxial model are derived from extensional testing of an SMA rod or tube, and are then used to predict the torsional characteristics of the same material. A major advantage of this method compared to prior art in torsional modeling is that the simplicity of deriving and implementing the parameters from uniaxial testing is carried over into the torsional domain. This simplicity derives from the fact that the measured properties (forces and displacements) in tensile loading are more readily suited to determining the model parameters than the measured properties in torsional loading which have been used by previous researchers (angles and torques which are, in fact, integrated quantities from strains and stresses). This article also presents a comprehensive experimental investigation and model correlation with both tension and torsional behavior of SMA rods and tubes. The model is compared with experimental data including torsional actuation tests against torsional springs. It is shown that the theoretical model demonstrates good agreement with the experimental data over a wide range of thermomechanical conditions. In addition, experimental phenomena associated with the torsional behavior of the SMA actuator such as the effects of heat treatment, twist rate, and loading pattern are examined.

A Study on Transient Performance Characteristics of the Canard Rotor Wing Type Unmanned Aerial Vehicle Propulsion System During Flight Mode Transition

A propulsion system of the CRW (Canard rotor wing) type UAV (unmanned aerial vehicle) was composed of the turbojet engine, exhaust nozzles (including some tip jet nozzles and a main nozzle), and the duct system (including straight ducts, curved ducts, and master valve). The CRW-type UAV has three different flight modes, such as the rotary wing mode for takeoff and landing, the high-speed forward flight mode with the fixed wing, and the transition flight mode between the previously mentioned two flight modes. In order to evaluate transient performance characteristics of the CRW-type UAV propulsion system during flight mode transition, the propulsion system was modeled using SIMULINK®, which is a user-friendly graphical-user-interface-(GUI) type dynamic analysis tool provided by MATLAB, in this study. The transition flight mode between the rotary wing mode and the fixed wing mode was simulated by considering area variation of the master valve and the main exhaust nozzle. In order to verify acceptability of the main turbojet engine model, performance simulation results using SIMULINK were compared to results using the commercial program GSP. Through this simulation, proper operation of the master valve and the variable area main nozzle can be found for safe flight transition. Therefore, performance characteristics were investigated depending on various angle positions of the master valve.

Tilt Duct Vertical Takeoff and Landing Uninhabited Aerial Vehicle Concept Design Study

¨A new autonomously controlled tilt-duct vertical takeoff and landing uninhabited aerial vehicle concept is proposed. This design combines the vertical flight capability of a helicopter and forward flight performance of a fixed-wing conventional aircraft. The two main engines and propellers are located inside the tilting ducts attached to the wing tips. There is a third engine‐propeller combination located inside the aft fuselage for pitch and yaw control during hover and transition. The advantages and disadvantages of the ducted propellers are discussed. A conceptual design study is performed including airfoil and geometry selection, initial sizing calculations, estimation of stability and control parameters, etc. Drawings of the aircraft in hover, transition and forward flight modes are presented.

Control of a thrust‐vectored flying wing: a receding horizon—LPV approach

AbstractThis paper deals with the application of receding horizon methods to hover and forward flight models of an experimental tethered flying wing developed at Caltech. The dynamics of the system are representative of a vertical landing and take off aircraft, such as a Harrier around hover, or a thrust‐vectored aircraft such as F18‐HARV or X‐31 in forward flight. The adopted control methodology is a hybrid of receding horizon techniques and control Lyapunov function (CLF)‐based ideas.First, a CLF is generated using quasi‐LPV methods and then, by using the CLF as the terminal cost in the receding horizon optimization, stability is guaranteed. The main advantage of this approach is that stability can be guaranteed without imposing constraints in the on‐line optimization, allowing the problem to be solved in a more efficient manner. Models of the experimental set‐up are obtained for the hover and forward flight modes. Numerical simulations for different time horizons are presented to illustrate the effectiveness of the discussed methods. Specifically, it is shown that a mere upper bound on the cost‐to‐go is not an appropriate choice for a terminal cost, when the horizon length is short. Simulation results are presented using experimentally verified model parameters. Copyright © 2002 John Wiley & Sons, Ltd.

Inverse method in tilt-rotor optimization

Tilt-rotor aircraft performance in hover and forward flight modes is optimized using the stiffnesses of the rotor blade as design variables. The performance in both flight modes is maximized simultaneously using a combined objective function. Aeroelastic analysis is based on mixed variational formulation for dynamics of moving beams along with finite-state dynamic inflow theory. Stiffnesses of an extension-torsion coupled and a fully coupled box beam are chosen as design parameters. Constraints are imposed on non-rotating natural frequencies of the beam, angle of attack and material failure. Based on the optimized stiffnesses, different composite materials for the rotor are studied to obtain a maximum performance in both flight regimes. Results show that significant improvements in the tilt-rotor performance are possible using optimally stiffened beam. The key stiffnesses affecting the performance of the rotor are identified.

Design of mode-to-mode fuzzy controllers

The mode-to-mode transition problem involves taking initial states in the start mode to the equilibrium point of the goal mode, where each mode of operation corresponds to an operating regime about an equilibrium point. Like the problem of dynamic transitions between various equilibria, there is no consistent theory that deals with the mode-to-mode transition problem. This paper presents a method of designing mode-to-mode controllers by blending the start and goal mode controllers. The blending gains are determined by the phase portrait assignment algorithm. The phase portrait assignment algorithm is a systematic technique that uses dynamic programming and center-point cell mapping to design fuzzy logic controllers. A hover mode to forward flight mode controller for a small-scale helicopter is synthesized to illustrate the design methodology. Simulation results show that the controller is able to transition stably from hover to forward flight. Finally, sensitivity analysis of the hover to forward flight controller is performed for small parameter perturbations. © 2000 John Wiley & Sons, Inc.