Ducted Fan Propulsion System with Thrust Vectoring Capability - Research Paper Example

Analysis/ Design of Ducted Fan Propulsion System with Thrust Vectoring Capability Table of Contents 4 In modern era, researchers and scientists have come up with the state of the art, amazing innovations in the fields of aeronautics and aerodynamics. Ducted fan propulsion system is one of such amazing achievements by the scientists of 20th century. Now we hardly observe any airbus or fighter without ducted system as they provide fool proof safety to the passengers and are observed to provide least damage to the plane and passenger in case of emergency situations. Ducted fans have now found enormous applications in the field of avionics due to their low noise high efficient mechanism. This versatile system is capable of providing greater speeds with low fuel consumption. 4 2.Introduction 5 2.1Propulsion System 5 A propulsion system is a mechanical system designed specifically to provide enough force to the subjected body to move in a forward direction. Different types of mechanisms are used to provide propulsion force depending upon the nature of movement required and subjected body(Piolenc and Wright). In case of automobiles, engines are used to provide propulsion to the wheel and axle mechanism and in aerodynamics engines provide enough force to propellers and propulsive nozzles to move the plane against air currents. 5 2.2Ducted Fan Propulsion System 5 2.2.1Advantages and Disadvantages of Ducted Fan System 6 3.Literature Review 7 3.1Ducted Propulsion System with Thrust Vectoring 7 9 3.1.1Length and Angle of Diffuser 9 3.1.2Blade Tip Clearance 10 3.1.3Inlet Lip Radius 11 3.1.4Optimal Configuration of Ducted Fan for Thrust Vectoring 11 3.2Applications of Thrust Vectoring in Fluid Mechanics 12 3.2.1Turbo Fans 13 4.References 15 Abrego, Anita I., and Robert W. Bulaga. "Performance Study of a Ducted Fan System." American Helicopter Society Aerodynamics, Acoustics, and Test and Evaluation Technical Specialists Meeting. 2002. Print. 15 BROADWELL, JAMES E. "An Analysis of the Fluid Mechanics of Secondary Injection for Thrust Vector Control " AIAA (1962). Print. 15 Carter, Philip. "Humming Bird: Duct Physics". 2009. . 15 Pereira, and Chopra. "Effects of Shroud Design Variables on Hover Performance of a Shrouded-Rotor for Micro Air Vehicle Applications." AHS International Specialists’ Meeting on Unmanned Rotorcraft. 2005. Print. 15 Piolenc, Marc de, and George E. Wright. Ducted Fan Design. 2001. Print. 15 Preston, and Chee. "“Performance and Flowfield Measurements on a 10-Inch Ducted Rotor Vtol Uav." US Army Research, Development, and Engineering Command. 2004. Print. 15 Tian, Wah Keng. "Unmanned Air Vehicle (Uav) Ducted Fan Propulsion System Design and Manufacture." National University of Singapore, 2010. Print. 15 Weelden, Scott D. Van, and Dudley E. Smith. "Preliminary Design of a Ducted Fan Propulsion System for General Aviation Aircraft." SAE Technical Paper (1995). Print. 15 Table of Figures Figure 1: Design of a typical ducted fan system 6 Figure 2: Ducted fan system showing thrust vectoring 7 Figure 3: Five principal parameters affecting the performance of ducted propeller/fan system 9 Figure 4: Open propeller design showing reverse flow near blade tip 9 Figure 5: Ducted fan assembly showing minimal effect of reverse currents on propeller blade 10 Figure 6: Construction and working of turbofans making use of thrust vectoring 13 1. Abstract In modern era, researchers and scientists have come up with the state of the art, amazing innovations in the fields of aeronautics and aerodynamics. Ducted fan propulsion system is one of such amazing achievements by the scientists of 20th century. Now we hardly observe any airbus or fighter without ducted system as they provide fool proof safety to the passengers and are observed to provide least damage to the plane and passenger in case of emergency situations. Ducted fans have now found enormous applications in the field of avionics due to their low noise high efficient mechanism. This versatile system is capable of providing greater speeds with low fuel consumption. 2. Introduction 2.1 Propulsion System A propulsion system is a mechanical system designed specifically to provide enough force to the subjected body to move in a forward direction. Different types of mechanisms are used to provide propulsion force depending upon the nature of movement required and subjected body(Piolenc and Wright). In case of automobiles, engines are used to provide propulsion to the wheel and axle mechanism and in aerodynamics engines provide enough force to propellers and propulsive nozzles to move the plane against air currents. 2.2 Ducted Fan Propulsion System Ducted fan propulsion system is a specialized propulsion system in which propeller is designed in the shape of a fan and is mounted inside a cylindrical duct. This duct is specialized in reducing losses in air thrust that are common at the tip of propeller(Piolenc and Wright). Also the variable cross section makes the ducted propellers efficient enough to vary velocity and pressure of outgoing air according to Bernoulli’s principle. Figure 1: Design of a typical ducted fan system Ducted fan systems are extensively used in aircraft applications. Such systems are more efficient than un-shrouded propellers as they have limited speed due to sound barriers faced at lower speeds. 2.2.1 Advantages and Disadvantages of Ducted Fan System Advantages of implementing a ducted fan system are(Abrego and Bulaga; Piolenc and Wright): On ground enhanced safety. Thrust vectoring capability allows them to be used instead of tilt rotors. Produce less noise as blade noise is shielded. Ducted fans can perform better with smaller diameters and low speeds. Disadvantages of ducted fan system are(Piolenc and Wright): They are less efficient as compared to propellers at cruise. For greater efficiency, clearance between blade and tip needs to be reduced to minimum. Sensitive to vibration and rpm is to be kept higher for efficient flight. Complex design and weight issues. 3. Literature Review Ducted fan propellers are capable of producing more thrust as compared to a free propellers with same propeller diameter (Carter). This can be understood by having a look on the ducted propeller in static operating conditions. In such cases if the inlet area is larger as compared to the outlet area then the net pressure acting inside the duct will be added to the thrust as the flow is 3-dimensional. Therefore, for any propeller size duct shape is always kept bell shaped this will add thrust vectoring capability without excessive usage of fuel and hence improves performance of the air vehicle(Carter). Figure 2: Ducted fan system showing thrust vectoring 3.1 Ducted Propulsion System with Thrust Vectoring The most important benefit of ducted fan propulsion system is its thrust vectoring or augmenting capability. Augmentation of thrust allows the propulsion system to increase the engine power instantly either by injecting additional or water in the combustion chamber. In order to achieve this phenomenon, there is a need to analyze the design of duct geometry and its relationship with thrust augmentation. Ducted propellers were first introduced in mid of 20th century. At that time they were mostly used with large ducts and huge propellers. They were efficient enough to work under turbulent conditions i.e. high Reynolds Number and provide great thrust vectoring capabilities as well. But nowadays it can commonly be observed with smaller sized ducts and propellers. Preston and Pereira have also worked a lot in this field especially on smaller sized ducted fans to get a relationship between fan geometry and its thrust vectoring capability(Pereira and Chopra; Preston and Chee). The diameters used by both the researchers were 25cm and 16cm whereas the Reynolds Number used was lying within 11500 to 23000. According to Pereira five parameters are integral to thrust vectoring as they directly affect the efficiency of the system. These factors affecting the performance of ducted rotor are listed below(Pereira and Chopra): Clearance of blade tip, δtip Radius of inlet lip, Length of diffuser, Diffuser angle, Diameter of throat, Figure 3: Five principal parameters affecting the performance of ducted propeller/fan system 3.1.1 Length and Angle of Diffuser In order to know the significance of ducted fan system we need to first analyze the open propeller configuration. In an open propeller, the air streams passing by experience natural contractions near the blade tip. This results in the increased velocity of flow of turbulent air flow in reverse direction, hence increasing the power losses at the blade end. Figure 4: Open propeller design showing reverse flow near blade tip If contraction of air streamlines is reduced by improving the design of propeller, then performance can be increased to its maximum value. This can only be done by achieving best performance parameter values of diffuser angle and length which can be done in case of ducted fans. The duct protects the fan blades from the reverse air flow currents and hence, losses are reduced. In ducted fan assembly, performance is good only if the pressure gradient developing within the diffuser assembly is less than the inlet air(Weelden and Smith). Otherwise it will adversely affect the propeller performance. Figure 5: Ducted fan assembly showing minimal effect of reverse currents on propeller blade 3.1.2 Blade Tip Clearance Blade tip clearance refers to the very small gap between the duct’s internal surface and the blade tip of propeller. Generally it is good to keep this clearance minimum as it discourages vortex effect taking place near the tip. This vortex effect is generated as a result of turbulence the fans impart in the incoming air, and reducing blade tip clearance minimizes vortices formation, hence, resulting in least power loss. 3.1.3 Inlet Lip Radius The inlet lip radius of the ducted fan is another performance parameter which determines the amount of turbulence in the in stream air. If the inlet lip of the duct is large enough then it will allow the air to turn in with more ease and less turbulence, but diameter of inlet lip is specified in order to maintain weight and size issues. Much larger bodies also result in increased frictional drag. 3.1.4 Optimal Configuration of Ducted Fan for Thrust Vectoring Pereira defined the mathematical expression for all the principle parameters involved in the design of ducted fan which give maximum performance and best thrust vectoring capability during flight(Pereira and Chopra). Clearance of blade tip, Radius of inlet lip, Length of diffuser, Diffuser angle, By making use of the above mentioned relationships, Reynolds number for electric brushless motor (AXI 2814/12) was calculated(Tian). RPM for this motor were measured at full throttle and its value came out to be 15122 (radius = 0.0725m). Detailed calculations are listed below(Tian): Tip speed = Tip Chord = Density of air, ρ = 1.2kg/ Dynamic viscosity, µ = 1.7894× kg/ms R = 2.3× This value of Reynolds number falls in the category of laminar flow. This shows that even at full throttle the system is stable enough to provide laminar flow rather than turbulent. This also shows that the ducted fan designs fit better to the situations when high performance is required with least disturbing forces thus maintaining the fuel consumption. 3.2 Applications of Thrust Vectoring in Fluid Mechanics Fluid mechanics deals with the study of fluids including liquids, gases and plasma and also includes interaction of forces on these mediums. It can further be divided into two major categories: Fluid statics Fluid dynamics Aerodynamics is one of the major applications of fluid dynamics techniques. It deals with the dynamics of air especially when it is in contact with solid bodies like the aero foils and propellers. Ducted fan systems under thrust vectoring are basically an example of fluid dynamics. Thrust vectoring capability of ducted fans opened new doors to the development of aerodynamics by the addition of turbofans. These turbofans having thrust vectoring were then used to develop supersonic devices which led the human being to the invention of missiles, satellites and supersonic jet planes(BROADWELL). All these inventions were based on the development of fluid mechanics. 3.2.1 Turbo Fans Nowadays, turbofans are widely being used in almost types of passenger as well as fighter planes. These are not only used to provide high speed at low fuel consumption but are also stable enough to be added to meet different size and weight requirements. This is basically an upgraded version of gas turbine engine. There is a gas turbine based core engine which is surrounded by a air foil blade like assembly called compressor. The inlet air is captured in the compressor assembly where it is compressed before entering the combustion chamber. The turbo fan assembly is also composed of another turbine assembly at the rear end of the chamber just before the nozzle or outlet of the duct. The diagram below shows the cross sectional view of detailed design of turbo fan. Figure 6: Construction and working of turbofans making use of thrust vectoring Once the incoming air reaches the combustion chamber it is mixed with fuel and combusted. The hot gases are then passed through the core of gas turbine, other fan turbine units and then it is directed out of the duct assembly. Whereas, the leftover air passes around the engine, thus adding thrust vectoring to the original output thrust. Therefore, turbofans get dual thrusts, some from core and some from fans. 4. References Abrego, Anita I., and Robert W. Bulaga. "Performance Study of a Ducted Fan System." American Helicopter Society Aerodynamics, Acoustics, and Test and Evaluation Technical Specialists Meeting. 2002. Print. BROADWELL, JAMES E. "An Analysis of the Fluid Mechanics of Secondary Injection for Thrust Vector Control " AIAA (1962). Print. Carter, Philip. "Humming Bird: Duct Physics". 2009. . Pereira, and Chopra. "Effects of Shroud Design Variables on Hover Performance of a Shrouded-Rotor for Micro Air Vehicle Applications." AHS International Specialists’ Meeting on Unmanned Rotorcraft. 2005. Print. Piolenc, Marc de, and George E. Wright. Ducted Fan Design. 2001. Print. Preston, and Chee. "“Performance and Flowfield Measurements on a 10-Inch Ducted Rotor Vtol Uav." US Army Research, Development, and Engineering Command. 2004. Print. Tian, Wah Keng. "Unmanned Air Vehicle (Uav) Ducted Fan Propulsion System Design and Manufacture." National University of Singapore, 2010. Print. Weelden, Scott D. Van, and Dudley E. Smith. "Preliminary Design of a Ducted Fan Propulsion System for General Aviation Aircraft." SAE Technical Paper (1995). Print. Read More