Free-flight Tests Research Articles

Design optimization of anti-spin parachute for fighter-class aircraft

Abstract In the absence of spin wind tunnel test and model free flight test support, the traditional empirical formulas had low accuracy and large dispersion, which made it difficult to effectively support the anti-spin parachute design of fighter planes. Based on the principle of spin motion, the empirical formula is modified from two aspects: aircraft aerodynamic layout and mass characteristics. At the same time, the relationship between the parachute area and the flight weight is summarized, which makes the estimated result closer to the actual value and more applicable. In addition, aiming at the problem of excessive load in the design of anti-spin parachutes, the causes of the problem are analyzed in detail. The design criterion of the anti-spin parachute is optimized. Under the premise of ensuring the recovery effect, the need to strengthen the aircraft body structure is effectively reduced. The results show that the engineering estimation methods and design criteria of the anti-spin parachute proposed in this paper are effective after simulation and actual application and can provide technical support for subsequent anti-spin parachute design of new fighter planes.

Aerodynamic interactions of blunt bodies free-flying in hypersonic flow

This paper takes a new look at how the aerodynamic interactions of multiple bodies in high-speed flow affect their motion behaviors. The influence of the body shape and orientation on aerodynamic and stability behavior in the case of shock–shock and wake–shock interactions is the focus of this publication. Experiments were performed in the hypersonic wind tunnel H2K at the German Aerospace Center (DLR) in Cologne. Free-flight tests with tandem arrangements of spheres and cubes were performed with a synchronized dropping of both objects at various initial conditions of relative streamwise and vertical distance as well as pitch angle. A high-speed stereo-tracking captured the model motions during free-flight, and high-speed schlieren videography provided documentation of the flow topology. Based on the measured 6-degrees-of-freedom (6DoF) motion data, aerodynamic coefficients were determined. As a result, the final lateral velocity of trailing cubes is found to be many times greater than that of spheres regarding shock-wave surfing. For rotating cubes, the results showed that stable shock-wave surfing can become possible over an increasingly wide range of initial positions. This study has identified that the trailing drag coefficient of two axially aligned objects varies strongly with their relative streamwise distance. Furthermore, it was shown that the wake is a region of stability for downstream objects.Graphical abstract

Frequency-dependent control for wind disturbance rejection of a fully actuated UAV

Abstract In this paper, an $\textrm{H}_{{\infty }}$ dynamic output feedback controller is experimentally implemented for the position regulation of a fully actuated tilted-rotor octocopter unmanned aerial vehicle (UAV) to improve wind disturbance rejection during station-keeping. To apply the lateral forces, besides the standard tilt-to-translate (attitude-thrust) movement, tilted-rotor UAVs can generate vectored (horizontal) thrust. Vectored-thrust is high-bandwidth but saturation-constrained, while attitude-thrust generates larger forces with lower bandwidth. For the first time, this paper emphasizes the frequency-dependent allocation of weighting matrices in $\textrm{H}_{{\infty }}$ control design based on the physical capabilities of the fully actuated UAV (vectored-thrust and attitude-thrust). A dynamic model of the tilted-rotor octocopter, including aerodynamic effects and rotor dynamics, is presented to design the controller. The proposed $\textrm{H}_{{\infty }}$ controller solves the frequency-dependent actuator allocation problem by augmenting the dynamic model with weighting transfer functions. This novel frequency-dependent allocation utilizes the attitude-thrust for low-frequency disturbances and vectored-thrust for high-frequency disturbances, which exploits the maximum potential of the fully actuated UAV. Several wind tunnel experiments are conducted to validate the model and wind disturbance rejection performance, and the results are compared to the baseline PX4 Autopilot controller on both the tilted-rotor and a planar octocopter. The $\textrm{H}_{{\infty }}$ controller is shown to reduce station-keeping error by up to 50% for an actuator usage 25% higher in free-flight tests.

Study of Resistance of Aedes aegypti Mosquitoes to Pyrethroids by Free-flight Method

Efforts to combat dengue hemorrhagic fever (DHF) in Bali are focused on controlling the vector of Aedes aegypti Mosquitoes. One method is fumigation (fogging) with insecticides. The use of insecticides for a long time and improper doses can trigger resistance. Many countries including Indonesia have reported incidents of insecticide resistance. Methods for determining the level of insecticide resistance are bioassay, biochemistry, and gene mutation. The method does not describe the actual environmental conditions of Mosquitoes. Queensland Institute of Medical Research (QIMR) Berghofer developed a free-flight test procedure to evaluate insecticide resistance in free-flying mosquito conditions. The study, which was conducted in June 2019-January 2020, aimed to measure the level of insecticide resistance with the free-flight method in the A. aegypti population in North Denpasar. A total of 20 female A. aegypti aged 5-10 days were released in the free-flight room for 24 hours with permethrin or cypermethrin insecticide boards. The level of resistance is determined based on the mosquito mortality rate using WHO standards. The results showed permethrin and cypermethrin resistance in the A. aegypti population of North Denpasar, with mortality of less than 80%.

Analysis of uncertainty factors in the prediction of missile pitch damping derivatives by unstructured CFD solver

Based on the transient planar pitching method, the dynamic derivative calculation function is implemented on an unstructured computational fluid (CFD) software NNW-FlowStar. Then the pitch-damping dynamic derivatives of the standard model of the ANF missile at zero angles of attack and different Mach numbers (0.5∼4.5) are calculated. The influence of various numerical and physical parameters on the calculation results of dynamic derivatives is studied in detail. It is found that the dynamic derivatives of pitch damping are independent of the values of amplitude (0.125° ≤ A ≤ 1°) and reduction frequency (0.025 ≤ k ≤ 0.2). When the number of calculation steps of the sine oscillation period is 200 and the number of inner iterations is 40, the prediction accuracy and computing efficiency can meet the engineering requirements simultaneously in this case. The influence of different grid resolutions on the dynamic derivative calculation results is studied. Results show that the grid requirements of dynamic derivative calculation are consistent with those of steady-state calculations. Finally, the calculation results are compared with the free flight test results, which verifies the reliability of the methods and parameters suggested in this paper.

Design, modelling, and experimental validation of a self-rotating flapping wing rotorcraft with motor–spring resonance actuation system

Compared with traditional flapping motion, the flapping wing rotor (FWR) allows rotating freedom by installing the two wings asymmetrically, which introduces rotary motion characteristics and enables the FWR to have higher lift and aerodynamic efficiency at low Reynolds number. However, most of the proposed FWRs contain linkage mechanical transmission structures, the fixed degrees of freedom of which prohibit the wings from achieving variable flapping trajectories, limiting further optimization and controller design of FWRs. In order to fundamentally address the above challenges of FWRs, this paper presents a new type of FWR with two mechanically decoupled wings, which are directly driven by two independent motor–spring resonance actuation systems. The proposed FWR has 12.4 g of system weight and 165–205 mm wingspan. In addition, a theoretical electromechanical model based on the DC motor model and quasi-steady aerodynamic forces is established, and a series of experiments are conducted in order to determine the ideal working point of the proposed FWR. It is notable that both our theoretical model and experiments exhibit uneven rotation of the FWR during flight, i.e. rotation speed dropping in the downstroke and increasing in the upstroke, which further tests the proposed theoretical model and uncovers the relationship between flapping and passive rotation in the FWR. To further validate the performance of the design, free flight tests are conducted, and the proposed FWR demonstrates stable liftoff at the designed working point.

Derivation and validation of a similarity law for free-flight wind tunnel tests of parallel stage separation

Aiming at the safety problem of the stage separation of parallel reusable high-speed air vehicles, this paper studies the unsteady test method and focuses on deriving a similarity law of parallel stage separation free-flight wind tunnel tests. The new similarity law considers the influences of aerodynamic force and gravity on the motions of the two stages, as well as the influence of aerodynamic interference between the two stages on each other’s motion. From the perspective of multi-angle physical equations, the conditions to ensure that the two-stage separation trajectory of a wind tunnel test is similar to that of a real air vehicle are derived innovatively, so as to ensure the authenticity and credibility of wind tunnel test results. The similarity law is verified by an HIFiRE-5 air vehicle, and the separation trajectories of wind tunnel tests and the real air vehicle are obtained by numerical simulation. The research shows that the similarity law derived in this paper can ensure that wind tunnel free-flight tests have the ability to predict the two-stage separation characteristics of real parallel vehicles. By analyzing the separation trajectory curve of the typical state, it is found that the new method can ensure that the trajectory error of a wind tunnel test does not exceed 1%, which indicates that this method is credible. The establishment of the new method lays the foundation for subsequent wind tunnel tests and provides support for research on the safety of the stage separation of parallel reusable air vehicles.

Laser-Induced Diaphragm Rupture for Improved Sequencing and Repeatability in a Hypersonic Facility

For hypersonic facilities where the flow conditions are established through the rupture of a diaphragm, such as in the University of Southern Queensland’s hypersonic wind-tunnel facility, the variability in the flow conditions is related to the uncertainty of the pressure at which the diaphragm ruptures. Variability in the diaphragm rupture pressure also results in uncertainty of the time at which the diaphragm will rupture. For experiments that require knowledge of when the test flow will be initiated, the sequencing of events relative to the flow onset is difficult when the flow is initiated using the natural rupture of a diaphragm. The challenge of experiment sequencing that arises due to rupture pressure variability is addressed by introducing a laser for rapid thermal weakening of the diaphragm. Event sequencing challenges are discussed in the context of free-flight testing, including model release strategies for such testing. The work proceeds through a review of Ludwieg tube flow initiation strategies and a discussion of the present context, which requires a reliable method for sequencing the retraction of the free-flight model holder. The natural variability of strength of the Mylar diaphragms in the present work is found to result in around uncertainty in rupture pressure. This rupture pressure variability is demonstrated to have a significant temperature dependence through empirical results and engineering models. Implementation of the laser-induced diaphragm rupture method is demonstrated to enhance repeatability in generating the flow conditions; the variability in rupture pressure was reduced to when the laser method was used. Based on the remaining sequencing uncertainties with the laser-induced rupture method and practical speeds for model platform retraction, uncertainty in the positioning of the free-flight models at the time of flow onset is shown to be .

Technique Used to Perform Rocket-Plane Free-Flight Tests

Abstract This paper includes description of the technique that was applied for free-flight (drop) tests of the rocket-plane scaled model. The main aim of the experiment was to validate the numerical approach to be used to simulate the gliding flight of the rocket-plane, especially the transition between high to low angles of attack and the rocket-plane response to control. The primary goal of this paper is to show what kind of challenges must be addressed when planning the flight test campaign. This paper includes description of how the rocket-plane model was scaled and built, the model preparation, experimental design and flight procedure. This paper shows an overview of how the experiment can be planned for different scenarios and the lessons learned during the deep stall free-flight tests.

Solving the Moment Amplification Factor of a Lateral Jet by the Unsteady Motion Experimental Method

In this paper, unsteady motion tests of a lateral jet adjusting an air vehicle’s attitude are carried out. Curves of pitch moment amplification factors (KM) for a lateral jet versus angle of attack (α) are obtained using a wind tunnel free-flight test technique with a jet and data processing method. This new method overcomes the disadvantage of previous experiments that can study only one unsteady characteristic. The free-flight test technique in the proposed method ensures that the test model can be coupled in real-time with multiple parameters (unsteady flow caused by the jet, unsteady air vehicle aerodynamic force, and unsteady air vehicle motion). This approach simulates an actual air vehicle’s complete jet test process and ensures more authentic and reliable test results. In the new data processing method, continuous data curves are fitted to discrete data points, making it easier to convert the angular displacement versus time curve into the pitch moment versus α curve to obtain KM. The results show that when the pressure of the micro high-pressure gas cylinder is 2.0 MPa, KM is below 1, indicating that the lateral jet does not significantly promote the pitching moment. When the gas cylinder pressure is 4.0 MPa and the angle of attack is 5° < |α| < 16°, KM is greater than 1, and the lateral jet promotes the pitching moment. When 16° < |α| < 20°, KM is less than 1, and the lateral jet does not significantly contribute to the pitching moment. It was further found that KM decreases slowly with increasing α. When |α| > 30°, the influence of the jet on the pitching moment nearly disappears.

Self-Learning MAV Under Safety-Guaranteed Flight Test Environment

This paper presents a novel methodology for designing the flight control system of a micro aerial vehicle (MAV); it allows the MAV to learn how to fly by itself, without the risk of damage. The proposed methodology uses a magnetic levitation–based safety-guaranteed flight test environment and deep reinforcement learning techniques to avoid the current problems in flight control system design procedures, such as the reality gap issue of simulations, and the safety issues inherent to real flight testing. The safety-guaranteed flight test environment was achieved using a developed magnetic suspension and balance system, which dynamically adjusts magnetic forces interacting with a magnetically levitated MAV. As a result, the MAV can perform in either a free-flight condition for reinforcement learning or can be constrained for safety. This learning environment was developed to permit safe reinforcement learning of MAV, since trial-and-error-based learning typically makes the MAV unstable. In this regard, the MAV can learn to fly by itself based on trial and error, by interacting with the emulated free-flight test environment. Notably, the entire learning process can be conducted without numerical models for both the MAV itself and the flight environment, and the safety of the MAV is guaranteed, even when attempting undesirable actions that might cause the model to become unstable. By avoiding the modeling errors typical of computational simulations and preventing the risk of damage in real flight tests, this approach could enhance the use of reinforcement learning in the development of advanced flight control systems.

Low-Interference Wind Tunnel Measurement Technique for Pitch Damping Coefficients at Transonic and Low Supersonic Mach Numbers

An experimental method for the determination of the pitch damping moment coefficient sum Cmq+Cmα˙ in a wind tunnel at transonic and low supersonic Mach numbers is developed. With support interference being a major issue for dynamic tests at these velocities, a minimum interference wire suspension approach is used. The motion of the wind tunnel model is restricted to a single-degree of freedom pitching oscillation through the geometry of the support system. A statistical evaluation procedure allows the simultaneous evaluation of multiple tests to increase confidence in the results. The influence of the wires as well as nonlinear effects are accounted for. The method is validated in an extensive test series at Mach numbers ranging from 0.6 to 2.0. Two reference missile models—the Basic Finner and the Army-Navy Spinner Rocket (ANSR)—are used. The results agree very well with CFD calculations throughout the transonic range. In comparison to free-flight tests the accuracy is significantly improved and result uncertainties are reduced by an order of magnitude.

A review of experimental techniques to predict aircraft spin and recovery characteristics

PurposeThis paper aims to provide a detailed review of the experimental research on the prediction of aircraft spin and recovery characteristics using dynamically scaled aircraft models.Design/methodology/approachThe paper organizes experimental techniques to predict aircraft spin and recovery characteristics into three broad categories: dynamic free-flight tests, dynamic force tests and a relatively novel technique called wind tunnel based virtual flight testing.FindingsAfter a thorough review, usefulness, limitations and open problems in the presented techniques are highlighted to provide a useful reference to researchers. The area of application of each technique within the research scope of aircraft spin is also presented.Originality/valuePrevious reviews on the prediction of aircraft spin and recovery characteristics were published many years ago and also have confined scope as they address particular spin technologies. This paper attempts to provide a comprehensive review on the subject and fill the information void regarding the state of the art aircraft spin technologies.

Use of a MEMS Differential Pressure Sensor to Detect Ground, Ceiling, and Walls on Small Quadrotors

Effective autonomous indoor flight requires an ability to sense and navigate ground, ceiling, and walls. While this can be accomplished in many ways including vision, one bio-inspired approach that is not well studied uses the detection of pressure changes when flying close to surfaces. As an example, an increase in pressure can be detected when a quadrotor flies close to the ground, a phenomenon known as ‘ground effect’. This work evaluates the viability of differential pressure measurements as a means of sensing ground, ceilings, and walls for small quadrotors. Extending existing models for thrust in ground effect, a model is derived to predict pressure changes due to ground effect in hover and tilt. The pressure response with ground effect is characterized across a range of distances, thrust levels, sensor locations, and quadrotor tilts using a Crazyflie 2.0 quadrotor and a small MEMS differential pressure sensor. Characterization experiments are also used to evaluate pressure measurements at varying distances from ceilings and walls, as well as the pressure behavior due to step changes in ground height. Manually controlled free flight tests are used to validate ground sensing in both nominal and visually challenging environments, as well as wall sensing in free flight.

Performance evaluation for scramjet based on ground direct-connected test: A method investigation

Scramjet has become one of the most efficient hypersonic vehicles because of its broad application potentiality. The ground direct-connected test is a relatively easy and economical way to test the performance of the scramjet under “real” condition. In this paper, an efficient performance evaluation method based on ground direct-connected test was developed. It can determine the performance parameters of the scramjet combustor (internal resistance, combustion efficiency, and total pressure recovery coefficient) and that (specific impulse and thrust) of the overall scramjet during the working time under the assumptions of adiabatic inlet and adiabatic-isentropic nozzle. Compared with the previous evaluation methods, using this method not only greatly reduces the time and money cost, but also provides more accurate boundary conditions for unsteady numerical simulation, and reference values for free jet and flight test furtherly. In the end, a case analysis was conducted and the real-time performance parameters were obtained. The results show that the internal resistance, combustion efficiency, and total pressure recovery of combustor are about 280 N, 0.69, and 0.21, respectively. The specific impulse and thrust of the overall scramjet reach about 6000 Nskg−1 and 370 Nskg−1, correspondingly.

In-flight Lift and Drag Estimation of an Unmanned Propeller-Driven Aircraft

The high-power density and good scaling properties of electric motors enable new propulsion arrangements and aircraft configurations. This results in distributed propulsion systems allowing to make use of aerodynamic interaction effects between individual propellers and the wing of the aircraft, improving flight performance and thus reducing in-flight emissions. In order to systematically analyze these effects, an unmanned research platform was designed and built at the University of Stuttgart. As the aircraft is being used as a testbed for various flight performance studies in the field of distributed electric propulsion, a methodology for precise identification of its performance characteristics is required. One of the main challenges is the determination of the total drag of the aircraft to be able to identify an exact drag and lift polar in flight. For this purpose, an on-board measurement system was developed which allows for precise determination of the thrust of the aircraft which equals the total aerodynamic drag in steady, horizontal flight. The system has been tested and validated in flight using the unmanned free-flight test platform. The article provides an overview of the measuring system installed, discusses its functionality and shows results of the flight tests carried out.

Optimization and verification of wind tunnel free-flight similarity law for separation of cluster munition

In view of the separation form of the separator from the back of the carrier upward and from the side of the carrier outward, separation-safety research is carried out by taking the separation of a cluster munition as an example. In previous wind tunnel free-flight tests, the similarity law of vertical, downward, moving submunition was used to design submunitions at different positions in different initial-velocity directions, which resulted in large discrepancies between wind tunnel test results and real flight. In a wind tunnel test, each submunition has an independent time-reduction ratio with respect to the dispenser. Even if the separation trajectory of a single submunition is accurate, there will be errors in the position of each submunition at a given time. Therefore, it is necessary to determine the time-reduction ratio between submunitions, and to modify the test results later. In order to ensure the accuracy of wind tunnel test results, the similarity law of a free-flight test in a wind tunnel is derived in this paper. The time-correction scheme to ensure motion similarity between submunitions is solved. Numerical simulation is used to simulate the separation of a wind tunnel test and real aircraft, and the motion parameters of different submunitions are solved. The results show that the new similarity laws derived for different types of submunitions can greatly reduce the errors caused by previous similarity laws. In addition to the case for the separation of a cluster munition, the similarity law can also be applied to the free-flight test design of wind tunnels for vertical separation and horizontal separation of other kinds of aircraft.

Prediction and evaluation of the stage-separation compatibility of an internally carried air-launch vehicle

This work mainly predicts and evaluates the stage-separation compatibility of an internally carried air-launch vehicle by a multi-body separation wind-tunnel free-flight (WTFF) test without restraining the motional freedom. The Mach number is Ma=1.5, and the initial-launch angle of attack is considered to be α0=0° and α0=3°. The result shows that WTFF test is a feasible prediction and evaluation method for the stage-separation compatibility of an ICAL vehicle when the combined vehicle exhibits static stability. The launch-vehicle can be separated safely from the carrier-aircraft and has a good pitch-angle for ignition when α0=0°. However, the launch-vehicle exhibits a large pitch-angle when it separates completely from the carrier-aircraft when α0=3°, which is unacceptable. Different separation velocities have little influence on the stage-separation compatibility of the ICAL vehicle when α0=3°. The stage-separation compatibility of the ICAL vehicle is first investigated by a free-flight technique in a wind-tunnel, which provides a new research idea for interdisciplinary studies of the kinematics and aerodynamics of multi-body systems.

Study on flexible flapping wings with three dimensional asymmetric passive deformation in a flapping cycle

In order to further improve the efficiency of flapping wings, inspired by bird flight, a new compliant joint structure is proposed, which is easy to manufacture. The joint has the capacity of passive bending, twisting and sweeping under the aerodynamic effect of the flapping wing. A multi-objective optimization program for the compliant joint is developed, which is combined with the measured joint load. The optimized compliant joint has the effect of asymmetric passive deformation in a flapping cycle. The model wings with compliant joints are studied in the windless flapping experiment, wind tunnel experiment and free flight test. The experimental results show that the model wings have a significant advantage of power consumption compared with the original wings, and also have an advantage in the aerodynamic performance at low flight velocity and low angle of attack.

Derivation and verification of a similarity law for wind-tunnel free-flight tests of heavy-store separation

Test-method research was carried out to consider the separation of a heavy store, and a similarity law for unsteady wind-tunnel free-flight tests of air-launch rockets was derived. The derivation of this similarity law considers the particular characteristics of a heavy store and aerodynamic interference with the carrier and focuses on solving the following problems: the separation of a heavy store causes a real carrier to have an acceleration and a velocity that cannot be ignored; the carrier in a wind-tunnel test is vertically fixed; and a wind-tunnel test cannot meet the Froude number (Fr) similarity condition. According to the special characteristics of heavy-store separation, a similarity law for wind-tunnel free-flight tests of heavy-store separation is derived. Computational fluid dynamics simulations are used to verify the new similarity law. The results show that the new similarity law can simulate the separation trajectory more realistically than existing methods, and the linear and angular displacement errors are decreased by an order of magnitude. The experimental accuracy of the new similarity law is even higher than that of a separation trajectory satisfying Fr matching. It is demonstrated that the new similarity law can be used to carry out unsteady experimental research on the separation of a heavy store such as an air-launch rocket, and this new law provides strong support for establishing the safety boundaries of heavy-store separation.