Hypersonic flight control is a challenging field due to the extreme speeds and conditions involved. Here are some of the concepts and technologies that were being explored:

Aerodynamic Control Surfaces: Traditional control surfaces like flaps, ailerons, and elevators have limited effectiveness at hypersonic speeds due to the high temperatures and pressures. Researchers were investigating novel aerodynamic control surfaces designed specifically for hypersonic flight.
Active Flow Control: Active flow control systems use small actuators, such as plasma actuators or pulsed jets, to manipulate the airflow over a vehicle's surface. These systems can be used to enhance control and stability at hypersonic speeds.
Thermal Protection Systems (TPS): TPS is crucial for hypersonic vehicles to survive the extreme temperatures generated by air friction. Improved TPS materials and designs are continually being researched to enhance both safety and control.
Artificial Intelligence (AI) and Autonomous Control: AI and autonomous systems were being explored to handle the complex and rapid decision-making required for hypersonic flight control. Machine learning algorithms can adapt and optimize control inputs in real-time based on sensor data.
Fluidic Thrust Vectoring: Fluidic thrust vectoring involves using the propulsion system itself to provide control. This can include manipulating the direction of exhaust gases to change the vehicle's orientation.
Active Cooling Systems: Hypersonic vehicles generate enormous heat during flight. Active cooling systems, such as regenerative cooling or transpiration cooling, can help manage these high temperatures and potentially be used for control purposes.
Reaction Control Systems (RCS): RCS systems are often used for fine control and maneuvering in space, and they can also be adapted for hypersonic flight to provide precise control.
Adaptive Control Algorithms: These algorithms continuously adjust control inputs based on real-time feedback from sensors, allowing the vehicle to adapt to changing conditions and disturbances.

NASA’s MHD patch technology consists of two electrodes positioned a prescribed distance apart on the surface of the TPS of an aircraft or spacecraft and an electromagnetic coil placed directly below the electrodes with the magnetic field protruding out of the surface. During hypersonic flight, the conductive ionizing atmospheric flow over the surface enables current to flow between the two electrodes. This current is harnessed to power the electromagnet which in turn generates strong Lorentz forces that augment lift and drag forces for guidance, navigation, and control of the craft. Alternatively, the current can be used to charge a battery.

Changing the size of the MHD patch (e.g., the length or distance between the electrodes), the strength of the electromagnet, or the direction of the magnetic field enables tuning of generated forces for a given craft design. Multiple MHD patches can be leveraged on a single craft. A 1m² MHD patch exerts forces up to 200 kN under simulated Neptune atmosphere entry, significantly increasing the lift/drag (L/D) ratio for the aeroshell investigated. Magnetohydrodynamic (MHD) flow has been developed by the Russians for aerodynamic control of hypersonic aircraft in project AJAX.

State of the art for hypersonic control is an active area of research and is critical for maneuverable aircraft.

Breaking the Sound Barrier

It may be worthwhile looking back on what it took to break the sound barrier. The Bell X-1 was a groundbreaking aircraft that became the first to officially break the sound barrier in level flight on October 14, 1947, piloted by Chuck Yeager. One of the most critical challenges in breaking the sound barrier was controlling the shock waves and aerodynamic forces that occur as an aircraft approaches Mach 1. These forces can cause severe buffeting and instability. The Bell X-1's stabilizer design, along with careful attention to other aerodynamic factors, helped it overcome these challenges. To achieve this historic milestone, the X-1 incorporated several critical flight control systems and mechanisms:

Rudder and Ailerons: The X-1 featured traditional control surfaces such as ailerons (on the wings) and a rudder (on the tail). These control surfaces allowed the pilot to control the roll and yaw of the aircraft, essential for maintaining stability and control during high-speed flight.
Elevators: The elevators on the horizontal tail provided pitch control, allowing the pilot to control the aircraft's nose-up and nose-down attitude. This was crucial for maintaining level flight and controlling the aircraft's altitude.
Trim Tabs: Trim tabs are small, adjustable surfaces on the control surfaces (ailerons, elevators, rudder) that allow the pilot to fine-tune the aircraft's control inputs. Trim tabs were used extensively in the X-1 to maintain stability and minimize control forces.
Horizontal Stabilizer: The horizontal stabilizer on the tail helped stabilize the aircraft in pitch, ensuring it maintained the desired attitude during the high-speed flight. Development of the stabilator enabled solution of the Mach tuch problem which had caused severe stability problems.
Control Locks: Special locks were employed to secure the control surfaces during takeoff and landing, preventing excessive movement and ensuring the aircraft remained stable during these critical phases of flight.
Control Systems Redundancy: The X-1 was designed with redundant control systems to enhance safety. If one system failed, the pilot could switch to another, allowing for continued control of the aircraft.
Reaction Control System (RCS): The X-1 also featured a reaction control system, which was used for fine control during high-speed flight and for adjustments during the critical moments leading up to and after breaking the sound barrier. This system used small rockets to make precise adjustments in roll, pitch, and yaw.
Hydraulic Systems: Hydraulic systems powered the control surfaces, ensuring that they could respond quickly and effectively to the pilot's input, especially during the high-speed portions of the flight.
Transonic Flight Considerations: The X-1's design took into account the unique challenges of transonic flight (speeds approaching the speed of sound). At transonic speeds, airflow over the wings can become turbulent, affecting control surfaces' effectiveness. Special aerodynamic features and careful attention to the aircraft's design helped mitigate these challenges.

The Bell X-1's success in breaking the sound barrier paved the way for supersonic flight and contributed to our understanding of the aerodynamics and engineering required for high-speed flight. Its flight control systems and mechanisms played a crucial role in achieving this historic milestone.

MAGMA UAV Demonstrates Innovative Flow Control Technologies | Unmanned Systems Technology

BAE Systems has announced that its MAGMA unmanned aerial vehicle (UAV) has demonstrated two innovative flow control technologies in a series of flight trials that took place in north-west Wales. For the first time in aviation history, an aircraft was manoeuvred in flight using supersonically blown air, removing the need for complex movable flight control surfaces.

MAGMA, designed and developed by researchers at The University of Manchester in collaboration with engineers from BAE Systems, successfully trialled the two ‘flap-free’ technologies at the Llanbedr Airfield.

The technologies demonstrated in the trials were:

Wing Circulation Control: Taking air from the aircraft engine and blowing it supersonically through narrow slots around a specially shaped wing tailing edge in order to control the aircraft. This concept has been around since at least 1981 and the Chinese are actively investigating

Fluidic Thrust Vectoring: Controlling the aircraft by blowing air jets inside the nozzle to deflect the exhaust jet and generate a control force. see state of the art review.

The trials form part of a long-term collaboration between BAE Systems, academia and the UK government to explore and develop flap-free flight technologies, and the data will be used to inform future research programmes. Other technologies to improve the aircraft performance are being explored in collaboration with NATO Science and Technology Organisation.

Flight control of a flying wing aircraft based on circulation control using synthetic jet actuators - ScienceDirect

1. Introduction

Flying wing aircraft, eliminating horizontal tails and vertical tails, markedly cut down adverse aerodynamic interference and radar cross sections, which have great significance for the future aircraft design.¹ Tailless layouts, however, display insufficient heading stabilities and poor longitudinal control abilities, presenting extremely high design demands for the flight control system based on mechanical rudders.² Rudders, with disadvantages of complex mechanical structures, large volume and weight, and insufficient control abilities at low speed, could also destroy the nice stealth performances of Flying Wing Aircrafts (FWAs).³ Active flow control actuators may replace mechanical rudders to become the next-generation flight control effectors. Active Flow Control (AFC), with advantages of no need for rudders, high efficiency, adjustable momentum and being easily integrated, has been applied in Circulation Control (CC)4, 5 and thrust vectoring control,6, 7 exhibiting dramatic engineering significance for the next-generation aircraft.

Circulation control, an extremely critical flight control technology, is considered as one of the most promising flight control methods.⁸ Until now, flight control technology realized by CC has been verified on DEMON,⁹ MEGMA,¹⁰ FWAs without rudders,¹¹ aircraft controlled by zero-mass-flux dual synthetic jet actuators¹² and so on. Traditional CC, however, generally runs by inducing a large amount of gas from engines and carrying an air bottle or fans, which discount the net thrust and payloads and consume a quantity of energy, such as ICE-04, the perfect profile, where the weight and volume of AFC module are 176 kg and 121.5 dm³, respectively.¹³ Significantly, nozzles, ducts and valves account for a large proportion, and enhance the complexity of system integration. Therefore, there is an urgent need for a new-generation circulation control actuator with characteristics of light weight, compact structures, low energy consumption, ease of integration, and convenience to adjust.

Synthetic Jet Actuators (SJAs),¹⁴ with the advantages of fast response, low energy consumption, compact structure, lightness, no need of gas sources or pipelines, and easily being integrated, have shown great application potentials in flight control.15, 16 Zhang et al.¹⁷ suggested that $Δ$ C_L/C_μ of Synthetic Jets (SJs) is one order of magnitude higher than that of steady blowing, where $Δ$ means difference, C_L is lift coefficient, C_μ is momentum coefficient. The blowing and sucking phases are both capable of delaying the Trailing-Edge (TE) separation, and improving the circulation and the lift. The lift pulsation, however, will be enlarged using low-frequency actuations (F⁺∼O(1)), which is not conductive to the flight control system. Meanwhile, Glezer et al.¹⁸ present that high-frequency actuations (F⁺∼O(10)) do not rely on coupling to a global instability, but could form a small quasi-steady flow interaction domain adjacent to the surface that displaces the local streamlines sufficiently to modify the local pressure distribution, which have emerged higher control abilities than low-frequency actuations and decoupled aerodynamic force from time. Moreover, SJs could modulate the vortex structures of “dead zones” near the trailing edge, change the Kutta condition and the pressure distribution, and then generate stable control force and moment for aircraft.¹⁹ Luo et al. invented a dual synthetic jet actuator,²⁰ which improves the energy efficiency and solves the problem of vibrating diaphragm failures, and integrated it into an Unmanned Aerial Vehicle (UAV), showing the nice three-axis attitude control ability.12, 21

To explore the application potential of SJAs in circulation control of FWAs, SJAs are integrated into a small-sweep FWA. Aerodynamic control characteristics and mechanism are analyzed by numerical simulations. Furthermore, SJAs are installed in an in-house FWA with a small sweep angle to demonstrate the flight control features.

2. Physical model and numerical method

An in-house half FWA, with a span b₁ of 1.816 m, an aerodynamic chord c₁ of 0.2946 m, a sweep angle of 26° and the wing area of 0.535 m², is chosen as the control objective, exhibited in Fig. 1(a). Thirty-two SJAs are located along the trailing edge, half of which are near the upper surface, while the others are close to the lower surface. The first SJA, with the distance from the symmetry surface of 34.7% b₁, is placed in the wing section, and the others are evenly arranged in the spanwise direction, which have the identical spanwise lengths of 2.6% b₁ and the same distance of 1.2% b₁ between each other, as displayed in Fig. 1(b), where red lines represent SJAs. The SJAs’ exits, perpendicular to the trailing edge, are tangent to the Coanda surface. The height of exits h₁ and the radius of rounded surfaces r₁ are chosen as 0.247% c and 2.29% c, respectively, where c means the chord of a local section, as shown in Fig. 1(c). In numerical calculations, Mach number is set as 0.1, and Reynolds number based on c₁ is 674119.

The finite volume method is applied to discretize the three-dimensional compressible unsteady Reynolds-averaged Navier-Stokes equations, and a density-based solver is used to solve it. Shear stress transport k-ω model is selected as the turbulent model. Roe flux differential splitting scheme is used to discretize the spatial term, the convection term is a second-order upwind scheme, and the time-discrete scheme is a first-order implicit scheme. Moreover, the convergence criterion, the residual error is less than 10⁻⁵, is set. In unsteady calculations, the time step is selected as 1/80 of the SJA driving cycle, the maximum number of iterations per time step is set as 40, and a total of 200 flow control cycles are calculated to ensure the convergence of results. In addition, time-average aerodynamic force and moment, considering the comprehensive effects of SJ reaction force and force on the solid surface,⁵ are calculated based on 10 flow control cycles.

The three-dimensional structured O-H grid is chosen for calculations, and computational domains and surface grids are shown in Fig. 2. Grids are encrypted on the wing surface, outlets of SJAs, leading edges, and trailing edges, respectively. The height of the first layer y⁺ is approximately equal to 1. In addition, the upstream and downstream streamwise lengths of computational domains are both 40c₁. The upward and downward normal lengths of computational domains are both 30c₁. The spanwise length is set as 30c₁. No-slip wall conditions are applied to the surfaces of aircraft and SJAs. Outer boundary is set as the pressure far-field condition. An array of SJAs is placed along the trailing edge, and the local mesh is shown in Fig. 2(a). For convenience, the internal structures of SJAs are omitted in calculations, and only SJA inlets, shown in Fig. 1(c) (red lines), are kept to simulate the SJA, and then a periodically fluctuating pressure inlet condition is utilized on the inlet of SJAs to generate SJs. It is noteworthy that SJAs close to the trailing edge of the suction surface will only be actuated for exploring its flow control mechanism.

Grid independence verifications before and after control at classical Attack of Angles (AOAs) are carried out, as shown in Table 1, which support that the lift coefficient C_L and drag coefficient C_D will keep constant when the grid number exceeds 10809704. Therefore, the final grid number is set as 10809704. Moreover, numerical verifications are implemented. An in-house wing with a sweep angle of 5° at Re = 250000 is selected and relative comparison is shown in Fig. 3(a). Meanwhile, a classical circulation airfoil CC-E0020EJ with momentum corfficient C_μ = 0.047, freestream Mach number Ma_∞ = 0.10049, and Re = 488000²² is also tested and pressure distributions are shown in Fig. 3(b), where C_p is pressure coefficient. Table 1 and Fig. 3 demonstrate that this method could predict the flow fields of FWAs with or without control of TE SJAs well.

Table 1. Grid independence verifications.

AOA (°)	Grid number	No control		Control
AOA (°)	Grid number	C_L	C_D	C_L	C_D
4	7598812	0.3791	0.0191	0.4226	0.0216
	10809704	0.3810	0.0201	0.4335	0.0226
	19523244	0.3811	0.0201	0.4337	0.0227
14	7598812	0.5465	0.1349	0.5541	0.1355
	10809704	0.5518	0.1359	0.5631	0.1398
	19523v244	0.5509	0.1357	0.5631	0.1399

The key control parameters contain dimensionless frequency F⁺ and momentum coefficient C_μ, which are described as(1) $F^{+} = f c / U_{\infty}$ (2) $C_{μ} = \frac{\sum_{i = 1}^{K} A_{i} U_{m a x}^{2}}{S U_{\infty}^{2}}$ where U_∞, U_max, f, A_i, K, S are the freestream velocity, the peak velocity of SJs, the driving frequency of SJAs, the area of SJA’s outlet, the number of SJAs and the wing area, respectively. F⁺ and C_μ are separately chosen as 4.24 and 0.0019. It is worthy to note that the difference of peak velocities at each SJA outlet is less than 1.45 m/s. In fact, these parameters are all chosen based on practical engineering experience and real design parameters of SJAs. In addition, our previous studies have shown the driving frequency of SJAs has little effects on pressure distributions and aerodynamic characteristics, so F⁺ is chosen merely based on our engineering experience.

3. Aerodynamic force characteristics

After control, C_L increases obviously before stalling, while C_L increment decreases gradually with the augmentation of AOAs. Meanwhile, C_D exhibits a non-linear increasing trend, with the maximum C_D increment occurring at AOA of 14°. Fig. 4(c) indicates that the pitching moment will be attenuated after control, namely the nose-down tendency could be generated. Moreover, C_m decrement is great before stalling, while decreases with the growth of AOAs, similar to the trend of C_L. Fig. 4(d) supports that the changing process of L/D emerges three stages under the control of SJAs. In detail, L/D will be improved from 0° to 4°, while slightly decreases between 6° and 12°, and then basically keeps constant after 14°. Based on the analysis of Fig. 4, TE SJAs can be qualified to control the roll and pitch attitudes at small AOAs for FWAs with a small sweep angle.

4. Flow control mechanism

The control mechanism will be analyzed based on flow fields and pressure distributions before and after control. For this aerodynamic layout, flow fields over the wing surface after stalling are dominated by three-dimension spanwise separation.²³ Pressure distributions and velocity evolution over the suction surface at AOAs of 2°, 8° and 18° are carefully analyzed to illustrate the flow control mechanism of TE SJAs.

At self-trimming AOA of 2°, pressure distributions over the upper surface and TE flow field evolution at section z = –1.2 m before and after control are shown in Fig. 5, where T is the driving period of SJs. The wing is dominated by attaching flow fields and separation appears from the beginning of the Coanda surface without control. After control, there is no significant change of pressures along the fuselage section, while an obvious low-pressure zone occurs at the wing section. This low-pressure area forms close to the trailing edge in the accelerating blow phase (1/4 T), evolves towards the leading edge in the decelerating blow phase (2/4 T) and the accelerating suction phase (3/4 T), and finally moves to the vicinity of the leading edge in the decelerating suction phase (4/4 T). The evolution of low-pressure zones also indicates effective AOAs gradually diminish from 1/4 T to 4/4 T. Moreover, the closer the low-pressure area is to the wing tip, the stronger this low-pressure zone is, and the better control effects could be achieved. This is because the section chord close to the wing tip is smaller, and the relative momentum coefficient is higher than that at the control position near the wing root with the same jet velocity. From the view of TE flow evolution, the area of “dead zone” is alleviated and the TE separation is significantly delayed. Meanwhile, the separation point emerges periodic fluctuations and reaches a peak of 53° in the accelerating blow phase (1/4 T), while the whole control process is still limited in the boundary layer control regime.¹⁰ What is more, the flow velocity adjacent to the trailing edge is greatly enhanced, indicating the pressure dips, and hence the lift and nose-down moment are improved. It is worthy to note that periodic evolution of the vortex and separation positions also indicates periodic fluctuations of aerodynamic force and moment.¹⁷ Fig. 6 shows the sectional average pressure distributions at z = –1.2 m before and after control, revealing that Leading-Edge (LE) and TE suction peaks are greatly improved, the LE stagnation point has also been slightly put downwards, and then the negative pressure on the suction side and the positive pressure on the pressure side both increase, which means the pressure envelope will be effectively enlarged, and hence a rise of C_L could appear.

At AOA of 8°, pressure distributions over the upper surface and TE flow field evolution at section z = –1.2 m before and after control are shown in Fig. 7. There is no obvious change of pressures and evolution of low-pressure areas over the upper surface under the control of SJAs. Pressure fluctuations also occur near the trailing edge but their influencing area is limited. Above pressure features generate the lower ΔC_L than that at AOA of 2°, with the reason that when the AOA increases, momentum of boundary layer augments, and then the valid momentum ratio reduces, weakening the CC efficiency.²² TE flow evolution process resembles that at AOA of 2°. Moreover, locations of separation point and the maximum separation angle do not show significant changes compared with those at AOA of 2°. Sectional average pressure distributions at z = –1.2 m before and after control are shown in Fig. 8, which also emerge the increase of LE suction and negative pressures close to the trailing edge, meaning the improvement of C_L. The variation of pressure envelope, however, is lower than that at AOA of 2°, causing the lower C_L increment.

When AOA is larger than 10°, flow at the wing section is dominated by large-area separation and cases at AOA of 18° will be taken as an example to illustrate the control mechanism. At AOA of 18°, TE flow evolution at z = –1.2 m before and after control are displayed in Fig. 9. TE separation point could be pushed to a farther position under the combined control of synthetic jets and separation over the upper surface compared to that at AOAs of 2° and 8°. The flow velocity is improved obviously along the Coanda surface, while does not rise ahead of the SJA outlet by entrainments. Sectional average pressure distributions at z = –1.2 m before and after control are shown in Fig. 10, indicating that the pressure near the trailing edge is changed, but the pressure in other areas basically remains unchanged with the control of TE SJs. Therefore, CC efficiency and C_L increment will dip significantly when the large-area separation appears over the suction surface.

5. Flight control tests

To demonstrate the roll and pitch control ability of synthetic jets, SJAs are integrated into an in-house FWA and flight tests without the deflection of rudders during cruising are carried out.

5.1. Novel CC effectors

The traditional SJA is optimized to get a higher energy level and stronger control abilities. The new generation of SJA, composed of four support plates, two shells, two exits (Exit 1 and Exit 2) and four piezoelectric diaphragms, is designed as a novel roll effector, which is shown in Fig. 11(a). In fact, the CC actuator, installed along the trailing edge, is an integration of two SJAs (SJA 1 and SJA 2) with the classical structure having two PZT diaphragms, one cavity and one jet exit. Every exit is controlled by two PZT diaphragms with the opposite vibrating directions. The CC effector has two operating modes. In detail, when SJA 1 (close to the wing upper surface) is actuated, this mode is called Positive Circulation Control (PCC), and when SJA 2 (close to the wing lower surface) is actuated, the mode is called Negative Circulation Control (NCC), as displayed in Fig. 11(b). Four PZT diaphragms, with a diameter of 50 mm, are the same. Moreover, the length and width of CC effectors are 57 mm and 55 mm, respectively. The height of CC effectors is variable for keeping the same dimensionless parameters r_f/c_f and h_f/c_f, where r_f, h_f, and c_f mean Coanda radius, height of SJA’s exits and chord of the local section, respectively. r_f/c_f and h_f/c_f are set as 0.0242 and 0.00226, respectively. Furthermore, the average weight of CC actuators is only 49 g, indicating great easiness to realize the integrated design. In addition, the spanwise widths of two exits are both 40 mm. In flight tests, driving frequency and alternating voltage of CC effectors are set as 710 Hz and ± 225 V, respectively. Every PZT diaphragm has the same actuated parameters, and the total power of a CC effector is 14.7 W, much less than that of traditional active flow control actuators. In addition, the peak jet velocities at 1 mm from the center of Exit 1 and Exit 2 measured by the hot wire anemometer are 80.5 m/s and 79.9 m/s, respectively, which are time-averaged values over 150 driving cycles. It is worth noting that the difference of peak velocity at exits of every CC effector is less than 1.15 m/s.

5.2. Flight platform and method

An in-house FWA platform, with characteristics of no horizontal tails, is chosen and detailed information is shown in Fig. 12(a) and Table 2. Fourteen CC effectors, distributed on both sides of the wings, are installed along the trailing edge. In detail, seven effectors are installed in the left-side wing, and the others are integrated into the right-side wing. In two symmetrical wings, two CC effectors are placed close to the wing tips, and five actuators are located between ailerons and elevators, as shown in Fig. 12(b), where the red parts represent CC effectors. Moreover, GoPro camera is installed in the middle of the fuselage. Aerodynamic force and moment along the wing could be modulated by the work of CC effectors, which realize the control of lateral and heading attitudes. Fig. 12(c) shows the FWA body coordinates. In this experiment, roll around the O_x_t is mainly focused.

Table 2. Detailed parameters of FWA platform.

Parameter	Value
Total weight (kg)	27.3
Wing span (mm)	3200
Wing area (m²)	1.33
Aerodynamic chord (mm)	416
Leading-edge sweep angle (°)	28
Length of actuators at a single side (mm)	296 + 107
Chord of wing root (mm)	825
Chord of wing tip (mm)	200
Flight speed (m/s)	35
Re based on aerodynamic chord	960143

The flight route is shown in Fig. 13. At Point A, FWA recovers from a condition of right turn and turns into level flight before reaching Point B with the help of mechanical rudders. When reaching Point B, FWA maintains level flight and all rudders will be turned off. At Point C, CC effectors will be actuated to control the attitudes of FWA. At Point D, CC effectors will be turned off and rudders are turned on to control FWA to resume the normal flight route. The dynamic response of FWA attitudes between Point C and Point D will be focused.

5.3. Results and analyses

For verifying the roll and pitch control ability of SJAs during cursing, CC effectors installed in the right wing and the left wing are separately operated in different modes to generate asymmetrical distributions of aerodynamic force. Dimensionless F⁺ and C_μ are kept as 8.44 and 0.0013, respectively.

When CC effectors of the right wing are operated in PCC modes, and CC effectors installed in the left wing are operated in NCC modes, the comparison of FWA status before and after control is displayed in Fig. 14, suggesting that the FWA rolls to the left under the control of CC effectors. Lift and drag of the right-side wing increase. Meanwhile, the lift and drag of the left-side wing decrease and increase, respectively. Changes of these aerodynamic force form a leftward rolling moment, achieving the control of lateral attitudes. In addition, the attitude parameters are shown in Fig. 15. At Point C, SJAs start to control FWA without rudders until Point D, showing that leftward roll angular velocity with a maximum of 12.64 (°)/s and leftward yaw angular velocity could be generated and keep increasing because of the leftward rolling moment and coupling of lateral and heading control. The roll angle also has an increasing trend. Moreover, there is almost no delay in the change of roll and yaw angular velocity, but a delay of 0.25 s in the change of roll angle appears. Similarly, when CC effectors of the right wing are operated in NCC modes and CC effectors of the left wing are operated in PCC modes, the comparison of FWA status before and after control is displayed in Fig. 16, suggesting that the FWA rolls to the right under the control of CC effectors. The attitude parameters are shown in Fig. 17, showing that the similar changing process appears and the rightward roll angular velocity could reach 10.05 (°)/s.

When CC effectors of the right and left wings are simultaneously operated in NCC modes, the comparison of FWA status before and after control is displayed in Fig. 18, suggesting that nose-up trend is generated under the control of CC effectors. CC effectors could produce nose-up moment, and hence achieve the control of longitudinal attitudes. In addition, the attitude parameters are shown in Fig. 19. At Point C, SJAs start to control FWA without rudders until Point D, showing that nose-up angular velocity could reach 8.51 (°)/s. The pitch angle also has an increasing trend. Similarly, when all CC effectors are working in PCC modes, the comparison of FWA status before and after control is displayed in Fig. 20, suggesting that the FWA emerges a nose-down trend. The attitude parameters are shown in Fig. 21, indicating that a similar changing process appears and the nose-down angular velocity could reach 8.36 (°)/s.

6. Conclusions

CC effectors are integrated into the trailing ledge of a small-sweep FWA, and numerical simulations are carried out to explore the aerodynamic control characteristics and control mechanism of TE circulation control using SJs. Finally, flight tests are implemented to verify the roll and pitch control ability of CC effectors. Detailed results are exhibited as follows.

CC based on synthetic jets could improve the lift, drag and nose-down moment. The variations of lift and nose-down moment decrease with the growth of AOAs, which means CC only has the ability of controlling roll and pitch attitudes at small AOAs. Synthetic jets could narrow the dead zone area, improve the flow velocity adjacent to the trailing edge, and then push the TE separation point and the LE stagnation point downwards, which make the effective AOA increase, thereby enhancing the pressure envelope area and the lift. The increase of AOAs improves the reverse pressure gradients, which augments the momentum thickness of the boundary layer, weakens the circulation control efficiency, so the lift increment decreases with the growth of AOAs.

Flight tests show that TE SJs could achieve roll and pitch attitude control during curing with the maximum realized roll angular velocity of 12.64 (°)/s and pitch angular velocity of 8.51 (°)/s. In the future, detailed control parameters, such as driving frequency and voltage, will be explored based on wind tunnel tests to establish an aerodynamic control model based on TE SJAs. Moreover, flight tests will be further carried out using the automatic flight control system based on TE SJAs.

Evolution of flight control systems

Aircraft control systems have evolved significantly, incrementally over the last 100 years. The first generation of aircraft designers, led by the Wrights and Curtiss used pioneering technologies that initiated the pace towards control systems that are now characterized by increasing sophistication and ingenuity.

The rapid evolution of aircraft design during the great War, interwar, and World War II put new demands for speed, stability, and maneuverability. Especially during the cold war era following achieving the dream of breaking the sound barrier placed a considerable pressure on aircraft designers to come up with more robust and better responsive aircraft flight control methods.

Although the basic principle of pulleys and rods to control flight surfaces survived throughout the second generation of aircrafts in World War II as well as continuing with some of the third generation of aircraft, the disadvantage of inefficient system weight marked its end, by the fourth generation, with the development of fly-by-wire systems. Hydro-mechanical actuation was incorporated in the second and third generation of aircraft, although this also was later replaced in the fourth generation by electrical hydro-mechanical control.

Following the fly-by-wire age that dominated the aerospace industry for the last thirty years, the current fifth generation of aircraft are moving more towards fibre controlled optical systems with more pure electrical actuation, replacing the heavier copper of the previous system as well as reducing the harm of hydro fluids to the environment whilst also reducing the total weight of the aircraft.

This review covers the journey of aircraft control systems as they have evolved through these generations and looks towards the future of flight control surfaces in the hypersonic era beyond Mach 5 examining the most recent research papers which anticipate a future aircraft achieving comparable if not improved efficiency but with no flight surfaces.

Early Flight Control from the Wright Brothers through WW1

The development of early aerodynamic flight control systems from the Wright Brothers through World War I marked a significant period in the history of aviation and set up the foundation for future developments. See CITIZENS OF THE AIR: PERCEPTIONS OF SAFETY IN THE SOCIAL IMAGINARY OF FLIGHT by Julia O'Grady for an interesting read about society in this era. During this time, aviation pioneers experimented with various methods to control aircraft in three dimensions and 6 degrees of freedom: velocity and attitude: roll, pitch, and yaw. Here's an overview of the key developments in early aerodynamic flight control systems during this period:

Wright Brothers (1903-1909):
- The Wright Brothers, bicycle mechanics from Dayton Ohio, made history with their first powered, controlled, and sustained flight in 1903.
- They used a system of wing warping to control the roll of their aircraft. By warping or twisting the wings, they could bank the aircraft and turn it.
- The Aircraft - The aircraft the Wrights designed and built between 1900 and 1905 were truly revolutionary, incorporating creative solutions to many of the technical problem standing in the way of mechanical flight.
- Aeronautical Engineering - The Wrights developed basic techniques still used by all modern aeronautical engineers, such as their pioneering use of the wind tunnel.
- Flight Testing - Systematic flight testing was critical to the Wrights success. Their method of evaluating data gathered in these tests, then refining their design based on the results, remains an important tool in aerospace research and development.
- Publications
  The Wright brothers did not publish any scientific papers on their work in aeronautics. However, they did write a number of articles for popular magazines, including:
  - "Some Aeronautical Experiments" (Western Electrician, 1903)
  - "The Art of Flying" (Century Magazine, 1908)
  - "The Wright Brothers' Aeroplane" (Scientific American Supplement, 1908)
  - "The Wright Brothers' Story" (Scribner's Magazine, 1913)
  Patents
  The Wright brothers held over 100 patents worldwide for their inventions related to aviation. Some of their most important patents include:
  - US Patent No. 821,393 (1906): This patent covers the Wright brothers' invention of the three-axis control system, which is still used in airplanes today.
  - US Patent No. 862,023 (1907): This patent covers the Wright brothers' invention of the wing warping system, which allowed them to control the roll of their airplane.
  - US Patent No. 980,675 (1911): This patent covers the Wright brothers' invention of the propeller.
  The Wright brothers' patents were essential to their success in developing and commercializing their airplane. They also helped to protect the Wrights' intellectual property and prevent others from copying their inventions.
  Wright brothers patent controversy
  The Wright brothers' patents were the subject of a number of legal challenges in the early years of aviation. Some have called them patent trolls. However, the Wrights were ultimately successful in defending their patents, which helped to establish their priority as the inventors of the airplane.
  The most significant patent challenge came from Glenn Curtiss, another early aviation pioneer. Curtiss had built a number of airplanes that were similar to the Wright brothers' design, but he had not obtained a license from the Wrights. In 1909, the Wrights sued Curtiss for patent infringement. The case went to trial in 1910, and the Wrights were ultimately victorious. The court ruled that Curtiss had infringed on the Wrights' patents and ordered him to stop building and selling his airplanes.
  The Wright brothers' victory in the Curtiss case helped to establish their dominance in the early aviation industry. It also sent a strong message to other potential competitors that the Wrights would vigorously defend their intellectual property rights.
Ailerons (1908-1910):
- As aviation continued to evolve, inventors like Glenn Curtiss and others began experimenting with ailerons, small hinged surfaces on the trailing edges of the wings.
- Ailerons provided a more efficient means of roll control compared to wing warping and became a standard feature in many subsequent aircraft designs.
- Curtiss was the target of a much-publicized patent suit brought by the Wright brothers prior to World War I. The issue was ultimately resolved by the U.S. government, and it had little impact on the growth and prosperity of the Curtiss Company.
Elevators and Pitch Control:
- Elevators, located on the horizontal tail surfaces, allowed for pitch control. By moving the elevator up or down, pilots could control the aircraft's nose-up and nose-down attitudes.
- The invention of aeronautical elevator and pitch control is attributed to the Wright brothers, Orville and Wilbur Wright.The Wright brothers' aircraft, known as the Wright Flyer, had a unique system for controlling pitch and roll. They used a combination of a rear horizontal stabilizer and an elevator (a hinged surface on the horizontal stabilizer) to control pitch, and they incorporated wing warping (warping the wings to control roll) for lateral control. These innovations allowed them to effectively control the aircraft's attitude and achieve controlled flight.
  
  Their pioneering work in aeronautics laid the foundation for modern aviation, and their concepts for pitch and roll control are still fundamental principles in aircraft design today.
Rudder and Yaw Control:
- Early aircraft used a rudder mounted on the vertical tail surfaces for yaw control. The pilot could move the rudder left or right to control the aircraft's yawing motion.
Monoplanes and Biplanes:
- Aircraft during this period came in both monoplane and biplane configurations.
- Biplanes, with their stacked wings and greater wing area, provided more stability and control authority but also had higher drag.
Warping vs. Ailerons:
- The debate between wing warping and ailerons continued into the World War I era. Some aircraft, especially in Europe, still used wing warping, while others adopted ailerons.
- Ailerons ultimately became more prevalent due to their effectiveness.
Innovations During WWI:
- World War I accelerated advancements in aerodynamic flight control systems.
- Aircraft designs became more specialized for combat, and improvements were made in control surfaces and stability.
- Pilots gained experience and developed dogfighting tactics, necessitating better control systems.

Overall, the period from the Wright Brothers' first flight in 1903 through World War I saw significant experimentation and innovation in aerodynamic flight control systems. The transition from wing warping to ailerons, along with the development of effective elevators and rudders, laid the foundation for modern aircraft control systems and contributed to the rapid evolution of aviation during this era.

Other Contenders for First Flight

All the technology required for flight was coming together at the turn of the century, but it was two bicycle mechanics from Dayton who used engineering method to put together for first flight. While the Wright brothers, Orville and Wilbur Wright, are widely credited with achieving the first powered, controlled, sustained flight on December 17, 1903, in Kitty Hawk, North Carolina, there were several other inventors and aviation pioneers who made significant contributions and came close to achieving powered flight around the same time. Some of these contenders include:

Samuel Langley: Samuel Langley was the leading establishment scientist competing for first flight. He was secretary to the Smithsonian Institution from 1887 until the year of his death in 1906. During this period, and in due course supported by the United States War Department, he conducted aeronautical experiments, culminating in his manned Aerodrome A. Under Langley's instruction Charles M. Manly attempted to fly the craft from a catapult on the roof of a houseboat in 1903. Two attempts, on October 7 and December 8, both failed with Manly receiving a soaking each time.

Some ten years later in 1914 Glenn Curtiss modified the Aerodrome and flew it a few hundred feet, as part of his attempt to fight a patent owned by the Wright brothers, and as an effort by the Smithsonian to rescue Langley's aeronautical reputation. The Curtiss flights emboldened the Smithsonian to display the Aerodrome in its museum as "the first man-carrying aeroplane in the history of the world capable of sustained free flight" a strait out lie.

The Smithsonian's action triggered a decades-long feud with the surviving Wright brother, Orville. It was not until 1942 that the Smithsonian finally relented, publishing the Aerodrome modifications made by Curtiss and recanting misleading statements it had made about the 1914 tests.
Gustave Whitehead: a German immigrant to the United States, is claimed by some to have made powered flight in a heavier-than-air aircraft in 1901, two years before the Wright brothers. However, the evidence supporting Whitehead's claims is disputed, and it remains a subject of controversy among aviation historians.
Octave Chanute: Octave Chanute was an aviation pioneer who made significant contributions to the field of aviation through his research and collaboration with other inventors, including the Wright brothers. While he did not achieve powered flight himself, his work in glider design and aeronautics laid the groundwork for future aviation developments.
Alberto Santos-Dumont: A Brazilian aviation pioneer, Alberto Santos-Dumont made significant advancements in aviation during the early 1900s. He is known for making the first public flight of an aircraft in Europe in 1906, but his work focused more on dirigibles (airships) and smaller, powered aircraft than on sustained, controlled flight.
Richard Pearse: Richard Pearse was a New Zealand farmer and inventor who is said to have made a powered flight attempt in a home-built aircraft in 1903, before the Wright brothers. Like Gustave Whitehead, the evidence supporting Pearse's claims is subject to debate.

Between the Wars

The period between World War I (1919) and World War II (1936) saw significant advancements in aerodynamic flight control systems technology. During this time, aviation was rapidly evolving, and engineers and scientists were experimenting with various control mechanisms to improve the stability and maneuverability of aircraft. Here are some key developments during this era:

Ailerons and Elevators: Ailerons, which control roll by changing the angle of the wings, and elevators, which control pitch by changing the angle of the horizontal tail surfaces, became standard features on most aircraft. These control surfaces were manually operated by the pilot initially but later became hydraulically or electrically assisted.
Trim Tabs: Trim tabs, small auxiliary control surfaces on the main control surfaces (ailerons, elevators, and rudders), were developed to reduce the control forces required by the pilot to maintain steady flight. Trim tabs allowed pilots to adjust the aircraft's control surfaces for various flight conditions.
Automatic Stability Systems: Engineers began to experiment with automatic stability systems that could assist the pilot in maintaining level flight and stability. One notable example was the gyroscopic stabilizer, which used gyroscopes to help keep the aircraft stable.
Rudder and Yaw Control: The development of more powerful engines and faster aircraft highlighted the importance of yaw control. Rudder systems were improved to provide better control over the aircraft's yawing motion. This was particularly important for maintaining stability during turns.
Wing Flaps: Adjustable wing flaps started to appear on aircraft during this period. These flaps could be extended to increase lift and reduce stall speed during takeoff and landing. They also helped improve the aircraft's maneuverability at low speeds.
Variable-Pitch Propellers: Variable-pitch propellers, which allowed the pilot to adjust the angle of the propeller blades in flight, became more common. This technology improved the aircraft's performance by optimizing thrust for different flight conditions.
Innovative Control Concepts: Engineers and designers experimented with various unconventional control concepts, such as wing warping and canard configurations. These ideas helped pave the way for future innovations in aircraft design.
Research and Testing: Research institutions, such as the National Advisory Committee for Aeronautics (NACA) in the United States, played a crucial role in advancing aerodynamic research and flight control technology during this era. Wind tunnel testing and empirical studies were used to gather data for design improvements.
Advancements in Materials: Improvements in materials, particularly the use of lightweight aluminum alloys, contributed to the development of more efficient and responsive control surfaces.
Military Aviation: The military's interest in aircraft technology led to significant advancements in flight control systems. Fighters and bombers were equipped with more sophisticated control systems to enhance their combat capabilities.

These developments laid the foundation for the modern flight control systems that we see in contemporary aircraft. The lessons learned during this period greatly influenced the design and operation of aircraft during World War II and beyond, contributing to the rapid evolution of aviation technology.

World War II Pressure Cooker

Along with the devastation and suffering caused across the globe in a struggle to determine the future of mankind, the Second World War had a lasting impact on major industries. The Second World War aviation developments paved the way for modern aviation. During World War II, aviation became a crucial weapon of modern warfare. From the Battle of Britain to dropping atomic bombs on Japan, much of WWII was fought in the skies. Investment in aircraft technology during this time drove the aviation industry in general forward in leaps and bounds, paving the way for the modern aircraft used in passenger operations today.

During World War II, significant advancements were made by both sides in aerodynamic flight control systems as part of the ongoing development of military aircraft in an air battle that was decisive. Here are some key aerodynamic flight control system technologies that were developed or improved during this period:

Flaps and Slats: The use of flaps and slats on the wings became more widespread during WW2. These devices allowed pilots to adjust the shape and area of the wing, providing better control over lift and drag, especially during takeoff and landing. They improved the aircraft's maneuverability and slow-speed performance.
Trim Tabs: Trim tabs, small adjustable surfaces on control surfaces like ailerons, elevators, and rudders, were refined and integrated into aircraft to help pilots maintain stable flight and reduce control forces.
Aileron Differential: Differential aileron control became more common during WW2. This design allowed for smoother and more responsive roll control by increasing the downward deflection of the aileron on one wing while reducing it on the other.
Power-Boosted Controls: Hydraulic and pneumatic systems were developed and used to assist pilots in controlling larger and faster aircraft. These systems reduced the physical effort required to manipulate the control surfaces, allowing for more precise control.
Variable-Pitch Propellers: Aircraft like the P-51 Mustang and the Supermarine Spitfire were equipped with variable-pitch propellers that allowed pilots to optimize engine performance for different flight conditions, such as combat, cruising, or climbing.
Gyroscopic Instruments: Gyroscopes were used in flight instruments, such as attitude indicators and turn-and-bank indicators, to provide stable and reliable reference points for pilots, even in turbulent conditions.
Engine Controls: Engine management systems were improved to provide better control over power output and fuel efficiency. This allowed pilots to optimize engine performance based on the aircraft's needs.
Aerodynamic Research: Extensive research into aerodynamics and wind tunnel testing during WW2 contributed to the understanding of airflow around aircraft and the development of more efficient wing shapes and control surface designs.

These advancements in aerodynamic flight control systems significantly improved the performance, stability, and controllability of aircraft during World War II. Many of these technologies continued to evolve and shape the design of post-war civilian and military aircraft.

Several designers and engineers played significant roles in the development of aerodynamic flight control system technologies. These individuals contributed to advancements in aircraft design and control systems that were critical to the success of various military aircraft during the war. Some notable designers and engineers and team leaders in this field include:

Jack Northrop: Jack Northrop was an American aircraft designer known for his work on innovative flying wing aircraft. The Northrop YB-35 and YB-49 were experimental flying wing aircraft that featured advanced aerodynamic control systems. His innovations continue to fly in todays air foce as the B1 and B2.
Alexander Kartveli: A Georgian-American aircraft designer, Alexander Kartveli played a key role in the development of aircraft like the Republic P-47 Thunderbolt. His contributions to aerodynamics and control systems helped create effective fighter aircraft during WWII.
Ed Heinemann: Ed Heinemann was an American aircraft designer who worked for Douglas Aircraft Company. He was responsible for the design of several successful aircraft during WWII, including the Douglas SBD Dauntless dive bomber, which featured advanced control systems for precision bombing.
Horten Brothers (Walter and Reimar Horten): German aircraft designers Walter and Reimar Horten were known for their work on flying wing aircraft, including the Horten Ho 229. Their designs incorporated advanced aerodynamics and control systems, although the Ho 229 didn't see operational use during WWII.
Robert H. Widmer: An American engineer, Robert H. Widmer, worked on the development of the Bell X-1 aircraft, which became the first aircraft to break the sound barrier in 1947. The X-1 featured advanced aerodynamic and control systems for high-speed flight.
Roy Chadwick: British aircraft designer Roy Chadwick is renowned for designing the Avro Lancaster bomber, which featured advanced control systems and aerodynamics for its time. The Lancaster played a critical role in the Allied bombing campaign over Europe during WWII.
Theodore von Kármán: Hungarian-American engineer and physicist Theodore von Kármán made significant contributions to the understanding of aerodynamics and flight control systems. He founded the Jet Propulsion Laboratory (JPL) and the Aerojet Engineering Corporation, both of which played roles in advancing aerospace technology during WWII.
Werner von Braun: While Werner von Braun is more famous for his contributions to rocketry, he also made contributions to the development of guided missile technology during WWII, which included advancements in aerodynamic control systems.
Robert H. Goddard: Robert H. Goddard, an American physicist and engineer, made pioneering contributions to rocketry and aerodynamics during WWII. His research laid the foundation for the development of rocket-propelled missiles and space exploration.
Charles Stark Draper: Charles Stark Draper, an American engineer, played a crucial role in the development of gyroscopic guidance and control systems for aircraft and missiles during WWII. His work paved the way for precision-guided munitions.
Robert Stanley: Stanley was a British aircraft designer and inventor who is best known for his work on the Hawker Hurricane and Typhoon fighters. He was also responsible for developing the first practical aileron tab system, which improved the handling of aircraft at high speeds.
Barnes Wallis: Wallis was another British aircraft designer who made significant contributions to the development of aerodynamic flight control systems. He invented the geodetic airframe structure, which was used in the famous Avro Lancaster bomber. He also developed the "Wallis flap," a type of high-lift device that was used on many Allied aircraft during the war.
Frederick Handley Page: Handley Page was a British aviation pioneer who is credited with designing the first successful four-engined bomber, the Handley Page O/400. He was also a pioneer in the development of automatic flight control systems.
William Boeing: Boeing was an American aircraft designer and manufacturer who founded the Boeing Company. He was responsible for developing some of the most iconic aircraft of World War II, including the B-17 Flying Fortress and the B-29 Superfortress. Boeing also made significant contributions to the development of aerodynamic flight control systems.
Clarence "Kelly" Johnson: Johnson was an American aeronautical engineer who is best known for his work on the Lockheed P-38 Lightning fighter and the U-2 spyplane. He was also a pioneer in the development of electronic flight control systems. He led the Lockheed Skunkworks which developed some of the most advanced cold war aircraft.
Robert J. Woods: Woods was an American aeronautical engineer who is best known for his work on the North American P-51 Mustang fighter. He was also responsible for developing the first practical boosted aileron system, which improved the controllability of aircraft at high speeds.
R. J. Mitchell: Although he passed away in 1937, Mitchell's work on the Supermarine Spitfire, which entered service during WWII, had a significant impact on aerodynamic and flight control technology. The Spitfire was renowned for its agility and maneuverability.

These designers and engineers, among others, made substantial contributions to the development of aerodynamic flight control systems and aircraft technology during World War II. Their work had a profound impact on the evolution of aviation and aerospace engineering in the years that followed.

Aerospace Electronic and Defense Systems

Monday, September 25, 2023

Innovative Flight Control Technologies from the Wright brothers to breaking the sound barrier into the Hypersonic Realm

Breaking the Sound Barrier

1. Introduction

2. Physical model and numerical method

3. Aerodynamic force characteristics

4. Flow control mechanism

5. Flight control tests

5.1. Novel CC effectors

5.2. Flight platform and method

5.3. Results and analyses

6. Conclusions

Evolution of flight control systems

Early Flight Control from the Wright Brothers through WW1

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Monday, September 25, 2023

Innovative Flight Control Technologies from the Wright brothers to breaking the sound barrier into the Hypersonic Realm

Breaking the Sound Barrier

1. Introduction

2. Physical model and numerical method

3. Aerodynamic force characteristics

4. Flow control mechanism

5. Flight control tests

5.1. Novel CC effectors

5.2. Flight platform and method

5.3. Results and analyses

6. Conclusions

Evolution of flight control systems

Early Flight Control from the Wright Brothers through WW1

Other Contenders for First Flight

Between the Wars

World War II Pressure Cooker

References:

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Post a Comment

When RAND Made Magic + Jason Matheny Response