Study on Fluidic Thrust Vectoring Techniques for

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Study on Fluidic Thrust Vectoring Techniques for Application in V/STOL Aircrafts

2015-01-2423 Published 09/15/2015

Samarth Jain, Soumya Roy, Dhruv Gupta, Vasu Kumar, and Naveen Kumar Delhi Technological University

CITATION: Jain, S., Roy, S., Gupta, D., Kumar, V. et al., "Study on Fluidic Thrust Vectoring Techniques for Application in V/STOL Aircrafts," SAE Technical Paper 2015-01-2423, 2015, doi:10.4271/2015-01-2423. Copyright © 2015 SAE International

Abstract The art and science of thrust vectoring technology has seen a gradual shift towards fluidic thrust vectoring techniques owing to the potential they have to greatly influence the aircraft propulsion systems. The prime motive of developing a fluidic thrust vectoring system has been to reduce the weight of the mechanical thrust vectoring system and to further simplify the configuration. Aircrafts using vectored thrust rely to a lesser extent on aerodynamic control surfaces such as ailerons or elevator to perform various maneuvers and turns than conventional-engine aircrafts and thus have a greater advantage in combat situations. Fluidic thrust vectoring systems manipulate the primary exhaust flow with a secondary air stream which is typically bled from the engine compressor or fan. This causes the compressor operating curve to shift from the optimum condition, allowing the optimization of engine performance. These systems make both pitch and yaw vectoring possible. This paper elucidates the research efforts which have been made to develop multifunctional nozzles employing fluidic thrust vectoring techniques, such as, co-flow, counter-flow, shock vector control, throat skewing and synthetic jet actuators and also makes a comparison of the intrinsic features of each method. It also makes an overview of how fluidic thrust vectoring has been utilized in the development of V/STOL aircrafts over the years and how it can be integrated with the next generation of fighter and civilian aircraft platforms.

Introduction Thrust vectoring technique provides moments to rotate a flying vehicle, providing control of the altitude and flight path of aerial vehicles. It works upon the principle of deflecting the line of action of the thrust of an aircraft away from the centerline so that when resolved into its components, the deflected thrust can generate an additional component of thrust perpendicular to the centerline. Thrust vectoring can provide control effectiveness superior to conventional aerodynamic surfaces at some flight conditions, and it can extend the aircraft performance envelope by allowing operation in the post-stall regime. In addition, thrust vectoring can improve takeoff and landing performance on short or damaged runways and aircraft carrier decks. The use of thrust vectoring techniques allows the reduction, and

possibly even the elimination, of conventional aerodynamic control surfaces such as horizontal and vertical tails. This would reduce weight, drag, and radar cross section, all of which can extend an aircrafts range and capabilities. The components of a thrust vectoring system include a working fluid source which is mounted on the aircraft and a vectoring nozzle which forms a passage for the working fluid, thereby acting as an extended conduit [1]. The vectoring nozzle allows the working fluid to change its exit direction with respect to the inlet and correspondingly changes the thrust vector of the aircraft. There are two methods by which thrust vectoring can be achieved: Mechanical thrust vectoring systems and Fluidic thrust vectoring systems. Mechanical thrust vectoring systems make use of gimbaling the nozzle, moving hinged flaps into the path of the jet exhaustor making use of divergent flaps to deflect the jet exhaust flow[2].Even though the mechanical thrust vectoring system is effective, its complexity, weight, integration and aerodynamically inefficiency have proven to be major hindrances in its effective utilization [3]. Fluidic thrust vectoring systems have been developed in order to achieve thrust vectoring capabilities similar to mechanical ones while featuring a simpler and light weight system having fixed geometry and capable of being implemented with stealth capabilities[4, 5, 6, 7]. Extensive research on fluidic thrust vectoring systems has been carried out so that the secondary air stream can be introduced into the primary jet flow to create an off-axis deflection of the jet thrust in contrast to the mechanical systems wherein vectored thrust was achieved by the deflection of divergent flaps or vanes [8, 9, 10]. The fluidic thrust vectoring techniques which have been developed till date include co-flow, counter-flow, shock vector control, throat skewing and synthetic jet actuators. The vertical and/or short take-off and landing (V/STOL) systems provide an aircraft the capability to hover, take-off and land vertically. An aircraft having V/STOL capability can operate on various modes, such as, VTOL (vertical take-off and landing), STOL (short take-off and landing) and/or STOVL (short take-off and

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vertical landing) [11,12]. These systems evolved primarily because of the use of thrust vectoring control systems based on mechanical actuation, however, fluidic thrust vectoring systems hold the potential to augment V/STOL capability of an aircraft by reducing the weight and complexity of the thrust vectoring system, thus, enhancing the thrust efficiency and overall performance level of the aircraft.

considered to be saturated. The secondary flow which is required to achieve this condition is still considerable, with at least 5% of the flow being required to initiate the operation of the system.

Fluidic Thrust Vectoring Techniques Co-Flow Thrust Vectoring Co-flow thrust vectoring is achieved by utilizing the principle of Coanda effect, as illustrated in Figure 1, involves the attachment of a jet flow to a nearby surface, even when the surface curves away from the initial jet direction[13]. The Coanda effect can be increased by passing a thin layer of high velocity turbulent air tangentially to the surface.

Figure 1. Coanda effect resulting in jet impingement [14].

In co-flow fluidic thrust vectoring system, a secondary air stream is blown in the form of a momentum injection which follows the profile of the Coanda surface and entrains the primary flow into a curved path. Due to the presence of the Coanda surface, entrainment by the secondary jet is inhibited on the side nearest to the surface. This entrained air then produces a pressure gradient which is perpendicular to the primary jet centerline. As a result of this, a localized lowpressure region is formed leading to an increase in the entrainment rate on the side of the secondary jet which is unbounded by the Coanda surface. Thus, in conjunction with small changes in the local flow field of the primary jet flow, this effect leads to thrust vectoring. The co-flow thrust vectoring is also be facilitated by positioning the curved surfaces to the rear of the engine nozzle and then introducing a secondary stream of air flowing parallel to the Coanda surface [15].Figure 2 shows co-flow fluidic thrust vectoring where To is the resultant thrust, Fx is the actual thrust of the primary jet, Fz,tv represents the thrust vectoring force generated and ςtv represents the thrust vectoring angle [16]. The use of Coanda effect as a means of providing vectored thrust has been fraught with difficulties. Since variable Coanda surfaces were required to achieve linear control, it became very difficult to control attached jet flows and early systems were therefore, limited to vectoring angles of only about 9 deg. It was later concluded by Mason [17] that a maximum vectoring angle of about 12.5° can be achieved by increasing the secondary mass flow rate. However, at this condition, no further deflection is possible and the system is

Figure 2. Co-flow fluidic thrust vectoring technique [16].

Co-flow method is considered to be the most suitable fluidic thrust vectoring system for practical use as both the primary and secondary flows are parallel. However, since the co-flow method is based on Coanda effect, it can cause instability in certain operating ranges of fluidic thrust vectoring. Hysteresis effect, attendant losses and physical complexity in incorporating the surrounding structure with the nozzles pose a significant problem in implementing this method. Research on co-flow method has led to improvement in several critical design parameters of the nozzle geometry, which has potentially reduced the pressure losses arising from the ducting of the secondary flow [18]. A co-flow fluidic thrust vectoring system has been developed for use on low observable unmanned air vehicles operating in the subsonic flight regime. Known as the FLAVIIR Demon UAV[19], it utilizes co-flow systems consisting of a high aspect ratio nozzle with thin secondary slots and a turbocharger compressor wheel, constructed to generate the secondary air supply. Touted as the world's first flapless plane, it was designed, manufactured, assembled and ground tested at Cranfield University in partnership with BAE Systems.

Counterflow Thrust Vectoring The counterflow thrust vectoring (CFTV) concept was first proposed by Strykowski and Krothapali [20]. The principle of CFTV is based on the pulling of small amounts of secondary flow in a counterparallel direction to the primary flow of the jet which leads to a marked improvement in the turbulent mixing characteristics of the shear layer. The application of counterflow on only one side of the jet leads to more rapid mixing of the shear layer as compared to the opposing free shear layer causing an imbalance in entrainment, leading to a cross-jet pressure gradient. The jet is bent by the pressure gradient in the direction of the applied counterflow, resulting in a vectored primary flow. This has been illustrated in Figure 3. Without a collar surface only a small degree of jet turning is possible because of the entrainment differential, however the addition of an extended collar surface makes CFTV a viable technology [21, 22]. Fluidic counterflow thrust vectoring studies have shown that the collar is the most vital component to the CFTV system as it amplifies the entrainment differential effect in two ways. Firstly, it channelizes the secondary flow, thereby, effectively creating a countercurrent mixing layer over the entire length of the collar. Secondly, it intensifies the cross stream pressure gradient by restricting the natural entrainment of the jet.

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jet unattached, thrust-vectoring angle is 12 deg and the thrust coefficient is 0.945. A maximum thrust-vectoring angle achieved is 15 deg at NPR = 5, but the thrust coefficient is only 0.92 [16].

Figure 3. Counterflow fluidic thrust vectoring technique [23].

Even though the collar is essential to ensure optimal efficiency of the CFTV system, still the hysteresis and bistability problem generated by the collar disrupts the continuity of the operating surface. This bistability problem causes the jet to attach to the wall of the nozzle, referred to as Coanda Effect, and reach a stable equilibrium under certain conditions [24, 25]. The loss of control because of the collar and the reduction in control pressure beyond that which was required to cause the attachment of the jet with the nozzle wall makes the implementation of continuous vector-control system very difficult. This situation can however be avoided if the geometry of the collar is properly designed. A longer collar accordingly becomes more preferable as the pressure forces can then act on a larger surface area to achieve the same net surface force and this consequently requires a lower amount of counterflow to sustain the given vector angle. This clearly shows that the collar geometry has to be optimized in order to optimize the performance of a CFTV system [26]. The design of the collar geometry should incorporate three qualities: aerodynamic stability, vector efficiency and ability to operate continuously up to the maximum desired vector angle. The presence of a collar inhibits the free-entrainment process and the transverse pressure field generated consequently leads to further surface pressure reductions. This transient phenomenon continues until the jet reaches a stable equilibrium wherein the transverse pressure forces across the jet must balance with the centrifugal forces demanded by the jet's curvature. When a CFTV system is to be designed, the most important thing which has to be kept in is that the jet attachment with the wall can only occur if equilibrium can be sustained. This means that the entrainment mechanisms within the jet must be able to sustain the low pressure necessary to hold the jet attached to the wall. It was observed that the shorter the collar, the sharper the jet must turn in order to attach and thus a lower bubble pressure will be required. Furthermore, with a short collar, the shear layer has less contact with the bubble region. This makes it more difficult for the pumping mechanism within the jet to generate the low pressure required to hold itself to the wall. Consequently, if the collar is sufficiently short, attachment will not occur [27, 28]. The use of CFTV allows the generation of large thrust-vector angles with small secondary flow rate which in turn allows more air to be directed through the engine. At nozzle pressure ratio (NPR) = 8 with

The advantage with CFTV is that no moving parts are directly required to steer the jet. As a result of this, the reliability of the system is greatly enhanced and since no surface of the system is in direct contact with the high temperature and high velocity exhaust gases, it eliminates the need for expensive high temperature materials and cooling systems. A greater percentage of thrust is recovered as the jet steers itself and this helps in increasing the aircraft performance. Counter flow also leads to a reduction in jet noise and emissions from the nozzle as it causes a higher mixing flow [29]. Additionally, the CFTV system can be retrofitted into an existing aircraft's engine without significant alteration of the airframe. This results in a reliable and robust system with a relatively lower initial and maintenance cost [30]. However, CFTV suffers from limitations such as suction supply source, stability with a highly over-expanded nozzle, unstable equilibrium effects, thrust loss and airframe integration [31, 32]. Therefore, while designing a CFTV system, it has to be ensured that the suction collars and slots are of small size so as to have minimum impact on the aircraft weight and drag. This will help in developing CFTV as a more efficient and widely acceptable thrust vectoring technique. Table 1. Comparison between co-flow and counterflow fluidic thrust vectoring techniques.

Shock Vector Control Shock vector control (SVC) is based on the asymmetric injection of a secondary flow into the divergent section of a convergent-divergent nozzle. This injection causes a low pressure region to be developed at the downstream of the injection port, disturbing the flow, and leading to the development of a strong oblique shock wave. As the primary flow interacts with the oblique shock wave, there is a change in flow properties across it, causing the flow to change direction, thus, vectoring the flow away from the injection port, as shown in Figure 4 [33].

Figure 4. Shock vector control fluidic thrust vectoring technique [34].

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SVC has demonstrated thrust vectoring efficiencies of 0.9 - 4 degrees per percent of flow injection and thrust efficiencies in the range of 0.86 - 0.94 [35]. The best thrust-vectoring efficiency achieved is 4.4 deg per percent injection at NPR = 3, with the thrust coefficient to be only 0.891. The thrust coefficient can be improved to 0.935 at NPR = 6, however, thrust-vectoring efficiency is reduced to 2.2 deg per percent injection [36]. SVC is characterized by loss of thrust due to the passage of the primary flow through the oblique shock wave. The pressure differential at the nozzle exit is varied by the resulting free stream velocities, shifting the shock wave attachment point further into the nozzle and causing a reduction in the effectiveness of the nozzle. Still, this system has a greater efficiency as compared to equivalent mechanical systems having a secondary mass flow requirement of 6-10%[37].However, in spite of the increased efficiency, it must be ensured that the oblique shock wave should not become too strong. If this happens, the other end of the shock wave impinges on the opposite nozzle wall, leading to the generation of a reflected shock wave of reduced intensity which vectors the primary flow in the opposite direction, thus, degrading the thrust vectoring performance and efficiency. Also, the system faces over-expansion losses as it has to operate at highly over-expanded conditions in order to achieve large vectoring angles[38,39]. Since SVC is achieved by fluidic injection into the divergent portion of the nozzle, no mechanical hardware is required other than control valves and therefore the problem associated with moving flaps is eliminated. However, a major problem associated with the fluidic thrust vectoring system is that the secondary stream injected into the nozzle draws air from the primary air supply and this reduces the maximum possible thrust that can be achieved by the engine. And if the amount of secondary air drawn from the primary air supply is reduced in order to maximize the thrust of the engine, then the fluidic thrust vectoring will be minimized and the benefits of fluidic thrust vectoring will become negligible when compared to movable flaps [40]. In order to overcome this constraint, the concept of utilizing multiple streams for fluidic thrust vectoring was conceived to maximize the pitch thrust vector angle without increasing the secondary flow requirement. In an experimental study conducted at NASA Langley Research Centre, it was observed that when the number of injection ports was increased from one to two, the pitch thrust vectoring capability improved without any lowering of thrust performance at NPR of less than 4 and high SPR [41]. This was done by making use of location-controllable injection ports that can optimize vectoring over a wide range of pressure ratios. The multiple port fluidic injection works by creating two oblique shocks. The first shock takes place upstream of the first injection port, while the second shock is between the two injection ports. The two oblique shocks created by the fluidic injection cause an asymmetric pressure distribution in the nozzle, helping in optimizing the shock vector effect produced. It was also observed that increasing the SPR caused the upstream oblique shock to get stronger and move closer to the throat while the downstream shock got stronger and moved closer to the upstream injection port. This resulted in degradation of the thrust vectoring effect produced by shock vector and therefore, it is essential that for the SPR to be kept at less than 4 to aid thrust vectoring [42].

Fluidic Throat Skewing Fluidic throat skewing is based on the symmetric injection of a secondary flow in the throat of a supersonic convergent-divergent nozzle in order to create virtual aerodynamic surfaces for jet area control and asymmetric injection for skewing the sonic plane. Due to asymmetric injection at the throat, using injecting points located at different points along the nozzle axis, reorientation of the sonic plane takes place causing the primary flow from the nozzle to turn, as shown in Figure 5. The sonic plane is further skewed with help of supplemental injection ports located downstream of the throat injection ports. This helps to further increase the thrust vector angle. The vectoring angle is mainly dependent on the offset between the injection points and on other factors such as the expansion ratio. Vectoring can be accomplished at all throttled operating conditions by controlling the injection flow rate both at the throat of the nozzle and at the flap[43].

Figure 5. Fluidic throat skewing technique [44].

In fluidic throat skewing technique, the thrust losses due to vectoring are low as the vectoring takes place on the subsonic side of the nozzle throat shockwave at lower speeds. Also, this system does not require any additional mechanical means to control the area of the throat as compared to other fluidic nozzles(possible only at zero vectoring angles). Fluidic throat skewing has demonstrated high thrust ratios in the range of 0.94 to 0.98 and vector efficiencies up to 2.15 deg per percent injection [45].Using an aft deck configuration at certain conditions, throat skewing method can achieve high thrust efficiency of up to 3 deg per percent injection, making it one of the most promising forms of fluidic thrust vectoring[46]. Tests have been conducted at static free stream conditions for this method, with the nozzle operating condition in the range of NPR=1.4-4.0, and fluidic injection flow rate up to 15% of the primary flow. The results showed that, at NPR=2, the throat shifted 45 deg with as little as 2% injection, but the thrust vectoring angle is only 3.3 deg. Increasing injection flow rate to 15% increases thrust vectoring angle to 22 deg at NPR = 2. So, the thrust vectoring efficiency is only about 1.65 deg per percent injection [47]. Dual throat nozzle (DTN) fluidic thrust vectoring technique has been developed as an extension to fluidic throat skewing method. According to Flamm [48], it is capable of achieving higher thrustvectoring efficiencies without large thrust efficiency penalties for vectoring operation as compared to any other fluidic thrust vectoring technique reported in the literature [49, 50]. Developed at NASA Langley Research Center, it utilizes the establishment of vortices in

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variable recessed cavities in order to enable thrust vectoring. As shown in Figure 6, a separated cavity is located downstream of the nozzle throat in addition to the fluidic throat skewing components, which maximizes the pressure differentials in order to obtain increased thrust-vectoring efficiencies. A new virtual aerodynamic surface of minimum area is formed at the downstream of the geometric minimum due to the asymmetric injection of secondary flow in the minimum area of the upstream. This causes skewing of the sonic line, thus, vectoring the primary flow from the nozzle. It has been investigated that the thrust vector angle and thrust efficiency decreases with a decrease in the angle of divergence of the cavity and increases with an increase in the angle of convergence of the cavity. For injection rates less than 4 percent of the primary flow rate, injection slot generated larger thrust vector angles while injection holes were found to be more efficient for a range of injection rates [51].

segments of the jet shear layer, which in turn, lowers the local pressure between the jet and the wall of the diffuser. This causes the exhaust from the nozzle to be vectored [52]. The flow field analysis of a two-dimensional synthetic jet, formed normal to an orifice in a flat plate, was studied [53] and it was found that these jets are synthesized by a periodic formation and subsequent advection of a train of counter-rotating vortex pairs. These vortices are formed at the edge of the jet orifice by the time-periodic motion of a diaphragm bound to a sealed cavity underneath the orifice plate as shown in Figure 7. In particular, owing to the suction flow, the time-averaged static pressure near the exit plane of a synthetic jet is lower than the ambient pressure and as a result of this; both the streamwise and cross-stream velocity components of the jet reverse their direction during the actuation cycle. The entrainment of the primary jet fluid by the adjacent synthetic jet consequently leads to alteration of the static pressure near the flow boundary and results in deflection of the primary jet toward the synthetic jet leading to a thrust vectoring effect even in the absence of an extended control surface like a diffuser or collar [54, 55, 56]. In case such an extension was present in the nozzle, then it would restrict the suction flow on that side of the jet centreline and would cause an increase in the flow rate on the opposite side of the jet orifice, thereby leading to the formation of a stagnation point between the primary jet and the co-flowing synthetic jet. This will result in the degradation of the thrust vectoring effect [57, 59]. Therefore, if an extended control surface has to be used in the case of synthetic jet actuation, its geometry must be optimized accordingly.

Figure 6. Dual throat nozzle fluidic thrust vectoring technique [44].

Therefore, with the presence of the cavity regions along with fluidic throat skewing, the thrust vectoring is enhanced to a greater level. Table 2. Comparison between shock vector control and fluidic throat skewing thrust vectoring techniques.

Figure 7. Synthetic jet actuation thrust vectoring technique [58].

Synthetic Jet Actuation Synthetic jet actuation has evolved as a very promising fluidic thrust vectoring method because it allows for a simplified flow control by making use of zero net mass flux actuators. These actuators consist of an oscillating diaphragm which repeatedly draws in and ejects a small amount of the primary flow into a cavity via an orifice, thus resembling a constant jet of secondary flow. Since each diaphragm is driven by a piezoelectric driver, no mechanical or fluid connections are required for the actuators to operate. Synthetic jet actuators deflect the primary jet towards the wall by creating time-periodic disturbances because of an increase in entrainment into the forced

In the work of Smith and Glezer [59] the time variation of the vectoring angle was measured at primary jet centreline velocities ranging from 5 to 20 m/s and the plot of the corresponding characteristic vectoring time with the primary jet speed showed that the characteristic vectoring time of the primary jet decreases almost linearly with the primary jet speed. The maximum vectoring angle was found to be 30 deg when the vertical structure resembling the starting vortex was formed at the primary jet centerline and this range of vectoring angle is the maximum which can be achieved out of all the possible thrust vectoring techniques. The analysis of the pressure field [52] also showed that the vectoring force increases with the primary jet speed and, in most cases, reaches a maximum before it begins to decrease.

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The synthetic control jets employed in this fluidic thrust vectoring method are synthesized from the working fluid of the flow system in which they are finally deployed and thus, in contrast to conventional continuous jets or pulsed jets, synthetic jets are able to transfer linear momentum to the flow without net mass injection across the flow boundary [59]. It is because of this that synthetic jet actuators are attractive, in addition, the localized combination of alternating blowing and suction effects generated eliminate the need for an additional fluid source, unlike other fluidic methods, and extraneous pumping and piping requirements. A major drawback of jet vectoring using synthetic jets is that most of the studies on synthetic jet actuation have been carried out at low primary jet velocities and this has shown the vectoring angle to decrease with increase in the primary jet velocity. This gives an indication that this system would be unlikely to vector the high speed/pressure flows experienced within an actual exhaust nozzle, therefore efforts need to be made in order to determine the exact behavior of synthetic jets in high speed exhaust nozzles.

Application of Fluidic Thrust Vectoring Techniques in V/STOL Aircrafts Fluidic thrust vectoring systems hold the potential of bringing about a breakthrough in V/STOL aircraft systems by increasing their thrust efficiency and overall performance levels. The primary challenge in developing such a fluidic thrust vectoring system has been to control the stagnation zones and high secondary air flow requirements which can lead to a decrease in the engine performance and an increase in fuel consumption. Simultaneously, the fluidic thrust vectoring system so designed should be able to operate effectively at all flight conditions whilst satisfying the design constraints of low cost, low weight and minimal impact on the radar cross section signature. The number of research studies which have been carried out into fluidic thrust vectoring is very limited. NASA's Langley Research Centre has been at the forefront in conducting a number of studies on counterflow, co-flow and throat skewing fluidic thrust vectoring methods [60]. The only commercial fluidic thrust vectoring system which has been developed till to date is the Exact nozzle for use on the Cirrus aircraft. The FLAVIIR Project is another significant effort in developing a co-flow thrust vectoring system. Besides this, there have been a number of smaller research efforts such as the one in Cranfield University. The following section makes a review of all such efforts in developing a fluidic thrust vectoring system.

FLAVIIR Project BAE Systems in collaboration with Cranfield University and University of Manchester has developed the Demon UAV, the world's first flapless aerial vehicle. The Demon Unmanned Aerial Vehicle (UAV) features a turbojet engine and a total vehicle weight of 90 kg with dimensions of 3m in length and 2.5 m wingspan. The Demon is able to achieve controlled roll maneuvers without using any moving conventional control surfaces (such as flaps, ailerons and elevators) and is representative of a full-size aerial vehicle [61, 62]. In Demon UAV, the conventional straight jet nozzle has been replaced by a fluidic thrust vectoring nozzle and utilizes the Coanda effect based on the co-flow concept in order to achieve manoeuvering. The fluidic thrust vectoring system guides the air from a rectangular exhaust nozzle over upper and lower Coanda surfaces to establish pitch vectoring. In order to control roll, bleed air is blown over a Coanda

surface installed on the trailing edge of the wing. This provides a novel aerodynamic control system using the engine exhaust and bleed air to provide the aerodynamic forces which are usually provided conventional aerodynamic control surfaces. Thus, by controlling the boundary layer conditions, the fluidic control system can also provide greater lift or drag during take-off and landing, providing this system with STOL capability. However, the system incorporates a secondary engine with the only purpose of providing bleed air to meet high supply of air mass flow rate required for secondary jet. If the engine bleed would have been used to obtain the bleed, it would have greatly reduced the main engine performance [37]. Currently, research is being conducted to further integrate secondary Coanda jet thrust vectoring system and synthetic jet actuators into the FLAVIIR Project so as to augment lift and control of an aerial vehicle and achieve an optimal design and build for a real aircraft [63].

Cirrus Vision SF50 Cirrus has developed a single-engine, low wing, seven seater, high performance light aircraft, Vision SF50, having a unique engine installation configuration that employs non-moving components to create a thrust vectoring effect. This is a typical example of fluidic thrust vectoring system integrated into an air craft giving it STOL capability. It is based upon Williams FJ33-5A turbofan engine generating1,800lbf (8.0kN) thrust and utilizes the Williams ‘Exact Nozzle’ technology which uses non-moving geometry to create a thrust vectoring effect that essentially changes with altitude[64]. The Exact Nozzle is a proprietary and innovative design by Williams that uses the Coanda effect to result in a varying thrust with no moving parts. The FAA certification of such a passive system is less risky than other active designs making it more attractive for commercial applications. The thrust vectoring is at its greatest at lower altitudes where cruise efficiency is not as important, while at higher altitudes the thrust gets in line with the aircraft direction to provide better cruise performance. However, greater nozzle vectoring results in better flying qualities but reduces cruise speed[37]. Cirrus has reported that the prototype has completed about800hoursof flight testing and approximately 1000 hours of engine running tests till February 2014 and is planning to start production of the SF50 Vision jets, thereby fueling a strong demand to improve the V/STOL capability provided by fluidic thrust vector control in subsonic and supersonic jets[65].

Pilatus PC-24 The Pilatus PC-24 is a regional jet which has been developed to operate from short, paved and even unpaved surfaces, thereby providing it STOL capability to operate form runways as short as 820m.The PC-24 employs two Williams FJ44-4A engines with a maximum take-off thrust of 3,435 lbf per engine. Moreover, an additional 5 percent power is available via a new automatic thrust reserve feature, which increases the thrust up to 3,600 lbf[66]. These engines are based on the William's ‘Exact Nozzle’ technology capable of producing passive thrust vectoring using the Coanda effect. The nozzle vectors thrust to generate a nose-up pitching force as opposed to the nose-down pitching of the aircraft in a go-around motion or during sudden application of high power.

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An anti-ice inlet and an integral pre-cooler have also been incorporated by in the engine in order to condition the engine bleed air and reduce the drag losses. The PC-24 does not require an APU and provides ground power efficiently with little noise by activating the Williams' Quiet Power Mode. Pilatus has already begun building the prototype PC-24 in it's Switzerland headquarters and expects to roll out its first serial production aircraft by 2017[67].

Conclusions The future of air superiority will greatly depend on the development and production of manned and unmanned combat aircrafts which will be supermanoeuverable and will have overall improved vehicle performance. A wholly integrated and optimized exhaust system consisting of a multifunctional nozzle, employing fluidic thrust vectoring technology will lead to an increase in the survivability and installed performance of the aircraft. The integration of fluidic thrust vectoring technology into an aircraft's existing systems has led to a weight reduction of 28-40% by implementing fluidic throat area control; a 43-80% reduction in weight by implementing fluidic throat area and exit area control; a 7-12% improvement in engine thrust-to-weight ratio; and a 37-53% reduction in nozzle procurement and life cycle costs. The authors find that in spite of the increased benefits that a fluidic thrust vectoring system offers over the conventional mechanical thrust vectoring system, only a handful of such systems have been developed. As per the present literature, of all the fluidic thrust vectoring systems developed till date, the throat skewing method is theoretically the most efficient and desirable technique which can be employed in a multifunctional nozzle owing to its high thrust efficiency and relatively simple structure. The only major drawback that this method faces is that the vectoring angle which can be achieved is small and therefore it limits the maneuvering ability of an aircraft. The loss in this ability can however be mitigated by the use of dual throat nozzles(DTN). The shock vector control method also has a simple geometry and can achieve a large vectoring angle but this comes at the cost of the thrust ratio of the primary exhaust flow, hence, this technique has still not been embraced. The co-flow method was found to be the most widely used fluidic thrust vectoring owing to its recent applications in a number of projects including the FLAVIIR Demon UAV, Cirrus Vision SF50 and Pilatus PC-24. Both the co-flow and counterflow fluidic thrust vectoring techniques are practically attainable and can be easily retrofitted in the engine nozzle without major changes in the airframe. But since these methods are based on the Coanda effect, the resulting jet impingement makes the flow regime unstable in certain ranges of fluidic thrust vectoring. Moreover, these methods require additional suction of the secondary jet stream; thereby they increase the complexity and weight of the thrust vectoring system. In this regard, jet vectoring using synthetic jet actuation has grown to become very promising because the secondary jet stream required in this case is synthesized from the working fluid of the flow system and therefore, zero net mass flux is ensured across the primary flow. The authors find that since synthetic jet actuation has been studied only for low primary jet velocities, therefore this fluidic thrust vectoring system needs further investigation specifically for high primary jet velocities before being deployed in an exhaust nozzle.

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Definitions/Abbreviations V/STOL - Vertical and/or short take-off and landing STOL - Short take-off and landing STOVL - Short take-off and vertical landing FTV - Fluidic Thrust Vectoring CFTV - Counterflow thrust vectoring SVC - Shock Vector Control NPR - Nozzle Pressure Ratio SPR - Secondary Pressure Ratio DTN - Dual Throat Nozzle

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