Level 7 Post Graduate Diploma in Mechanical Engineering Aerospace engineering

Transcription

1 Level 7 Post Graduate Diploma in Mechanical Engineering Aerospace engineering You should have the following for this examination one answer book non-programmable calculator pen, pencil, drawing instruments No additional data is attached General instructions This paper consists of nine questions. Answer any five questions. A non-programmable electronic calculator may be used but candidates must show sufficient steps to justify their answers. Drawings should be clear, in good proportion and in pencil. Do not use red ink. All questions carry equal marks. The maximum marks for each section within a question are shown. The City and Guilds of London Institute 2014 ST H1 PO

2 1 a) An aerofoil (NACA 4412) is mounted on an aircraft flying at an altitude of 3000 m. The characteristics of the aerofoil are shown in Figure Q1. Chord length of the aerofoil is 1.2 m. Determine i) the Lift per Unit Span, if the aircraft is flying at a speed of 50 m/s and the effective angle of attack is 4 degrees, ii) the minimum speed at which the aerofoil can produce lift per unit span of 3000 N/m. b) Compare and contrast incompressible flow from compressible flow. c) Explain the following terms. i) Critical Mach Number. (2 marks) ii) Aerodynamic Centre. (2 marks) d) Explain why a tapered wing is more likely to stall at the tip than a rectangular wing and describe why tip stall is a serious problem for an aircraft with an un-swept wing. Figure Q1 2

3 2 a) An elliptical orbit of a satellite is illustrated below in Figure Q2. Figure Q2 i) Redraw the Figure Q2 and identify the following parameters. 1. Semi-major axis 2. Radius of perigee 3. Radius of apogee 4. Major-axis ii) Calculate the eccentricity and Semi-Major axis of an orbit with a radius of perigee of 6600 km and a radius of apogee of km. b) During a typical mission, any spacecraft would encounter three environments. i) Name the three environments. (2 marks) ii) Selecting one of the three environments, explain the factors that the designer of the spacecraft must take into consideration to ensure the survival of the spacecraft. iii) A satellite is required to transfer from a lower earth orbit (LEO) with a particular inclination to a higher orbit with a different inclination. Explain a Hohmann Transfer Orbit and Inclination change manoeuvre that can be performed to achieve the new orbit. 3 a) Define Specific Impulse, I sp. Explain why it is desirable to use propellants with high I sp. b) A three stage rocket is used to send a 1000 kg payload into an orbit. Description about each stage is given in Table Q3. Stage Structure Mass (kg) Propellant Mass (kg) Propellant Type Specific Impulse (seconds) 1 6,000 60,000 LO 2 / RP I sp1 2 4,000 40,000 LO 2 / LK2 310 I sp2 3 2,000 20,000 N 1 O 4 / MMH 320 I sp3 Table Q3 The ideal burnout velocity is given by, m V BO = gi sp1 ln 1 m + gi sp2 ln 3 m + gi sp3 ln 5 m 2 m 4 m 6 where g gravitational force (9.81 ms 2 ) m 1, m 2 initial mass and final mass of stage 1 m 3, m 4 initial mass and final mass of stage 2 m 5, m 6 initial mass and final mass of stage 3 Find the ideal burnout velocity of the final stage. (10 marks) c) Outline the advantages and disadvantages of rockets with i) liquid propellant ii) solid propellant. 3 See next page

4 4 A four engine turbofan aircraft has the following characteristics: Mass kg Thrust 120 kn (From four engines) Wing Area 77 m 2 2 Drag Polar C L µ c l.ground roll 0.6 C L.max 2.0 a) Determine the initial acceleration and the acceleration when the aircraft is moving at 40 m/s. b) Estimate the distance required to reach a speed of 40 m/s. (If you could not solve part a), assume an acceleration of 3 m/s 2 for this part). c) Assume that an engine fails at a speed of 40 m/s. If the pilot takes 2 seconds to react and then brakes with a deceleration of 0.5g, determine the stopping distance. You may neglect acceleration during the reaction time. d) Briefly explain why aircraft use maximum flap setting during landing and not during takeoff. (5 marks) (5 marks) 5 a) i) Explain the formation of vortices with respect to a finite wing and describe how this affects lift and drag properties. ii) How do aspect ratio and elliptical lift distribution negate the effects of vortices? b) Describe the use of swept-back wings to increase the critical Mach number. (8 marks) 6 a) There are two types of aircraft navigation sensors, which are used in Inertial Navigation Systems (INS) and Global Navigation Satellite Systems (GNNS). Accelerometers and Gyros are the main INS sensors. Write a technical note on Accelerometers and Gyros among aircraft navigation sensing systems. (8 marks) b) Briefly explain the purpose and operation of i) Instrument Landing System (ILS) (3 marks) ii) Marker Beacon System. (3 marks) c) Name three other navigation aids available in the cockpit and explain how each helps in flight navigation. 7 a) Write the design objectives of a supersonic intake. b) There are essentially two primary types of options available for designing a supersonic intake, Normal Shock Diffusers and Oblique Shock Diffusers. Sketch them and discuss the advantages and disadvantages. c) A turboprop engine is selected for a general aviation aircraft. The drag when the aircraft is cruising at 70 m/s at an altitude of 2000 m is expected to be 950 N. i) Determine the required engine power if propeller efficiency is 85%. (2 marks) ii) Determine the fuel flow, if the Specific Fuel Consumption (SFC) is 60 µg/j. iii) The power is roughly proportional to the atmospheric density. Determine the sea level power, if the power required in part c) i) is at 2500 m altitude. 4

5 8 a) Explain the criterion for longitudinal static stability with the use of pitching moment variation with angle of incidence. b) i) Define the control fixed static stability condition. (2 marks) ii) With the information given in the graph below in Figure Q8 determine the control fixed neutral point. C L lift coefficient, η elevator angle, h centre of gravity. (2 marks) Figure Q8 c) i) Explain the two longitudinal dynamic stability modes and two of the three stability modes with the use of sketches. ii) Out of the following forces and moments identify the ones that describe lateral and directional dynamics. X Axial Force, Y Lateral Force, Z Normal Force, L Rolling Moment, M Pitching Moment, N Yawing Moment. (8 marks) 9 a) Explain how a pitot static tube is used to measure Equivalent Air Speed (EAS) and state why this speed measurement is called the EAS. b) Explain the primary and secondary effects of Aileron and Rudder. c) Explain why propeller engines are hardly used for large transport aircraft. (5 marks) d) Describe helicopter autorotation manoeuvre. (5 marks) 5