Welcome to Aerospace Engineering DESIGN-CENTERED INTRODUCTION TO AEROSPACE ENGINEERING Notes 5 Topics 1. Course Organization 2. Today's Dreams in Various Speed Ranges 3. Designing a Flight Vehicle: Route Map of Disciplines 4. Mission Specification & Take Off Weight 5. Force Balance during flight 6. Earth's Atmosphere 7. Aerodynamics 8. Propulsion 9.Performance, Stability & Control 10. Structures and Materials 11. High Speed Flight 12. Space Flight
SELECTING ENGINES: INTRODUCTION TO PROPULSION ge.ecomagination.com g http://www.geocities.com/ronliz.geo/ge90.jpg As a thumb rule, takeoff maximum thrust available must be around 30% of the gross weight. For airliners, takeoff is the most demanding thrust condition, so it drives the selection of the engines. Airliners must be able to take off with one engine out. Modern airliners use 2, 3 or 4 engines, and the trend appears to be towards using just 2 engines if possible. Some large commercial aircraft engines are: Pratt & Whitney 4084: Rated thrust of 77,200 lbs. Pratt & Whitney 4090: 90,000 lbs. General Electric GE90: 90,000 lbs. 110,000 lbs Rolls-Royce Trent 890: 90,000 lbs. Jet Engine Animation http://www.rendermedia.co.uk Created for the History Channel UK Documentary Whittle the Jet Pioneer by 3D Animation Stuidio Rendermedia
HOW JET ENGINES WORK Jet engines work using the Gas Turbine Cycle, a process which consists of the following 4 steps: 1) Compression: Air enters the engine at pressure P 1, and is compressed to a very high pressure P 2. This is done by doing work on the air using the rotating blades of a fan and a compressor. The temperature also rises as the air is compressed. For the GT2010, we will assume that this Overall Pressure Ratio is 50.
, where, so that 2) Heat addition: Heat is added to high-pressure air by burning fuel. Pressure remains constant, temperature rises to highest in the engine, called "Turbine Inlet Temperature". Controlled by varying the amount of fuel added. Modern military engines, this temperature > 2000K.. 3) Expansion and Work Extraction Hot, high pressure air allowed to blow out through a turbine, and then a nozzle. The turbine is forced by the air to spin at high speeds, driving the compressor and fan. This takes work out of the air, lowering its pressure and temperature. The air then blows out as a jet, with a velocity u E and static pressure P E = P A. 4) Cooling: ) g To complete the cycle, we must consider the cooling of air (constanttemperature heat release) in the atmosphere before the next jet aircraft comes along and gulps it in. However, since we don't directly pay for this step inside the engine, we don't consider it much.
Thrust Thrust is calculated using Newton's Second Law of motion. http://www.pratt-whitney.com/presskit/images/f135_3_low.jpg Let's say that the speed of air entering the engine) is U a The engine gulps in kg/s, moving at u a with respect to the engine. It adds kg fuel per second. All of this mass blows out of the exhaust at u e m/s.
Rate of change of momentum of the fluid through the engine per second is: Define fuel-to-air ratio Rate of change of momentum = This must be equal to the thrust. The function of the jet engine is to increase the momentum of fluid passing through it.
Engine with Bypass Flow All the air need not pass through the "core" of the engine where the fuel is burned. Enough air must go through the core to add heat and drive the turbine, to drive the compressor, and the fan or propeller. The air going through the fan or propeller also gets accelerated; this is the Bypass air. Bypass Ratio = Cold Air Flow Rate / Hot (Core) Air Flow Rate Thrust is: where H and C refer to the hot and cold flows respectively. We can get the same thrust in 2 ways: 1) Accelerate a small amount of air through a large velocity difference. 2) Accelerate a large amount of air through a small velocity difference. Different types of engines use different bypass ratio.
Turbojets, Turbofans, and Turboprops Turboprop http://www.fas.org Low Bypass Engine for F-35 Joint Strike Fighter The aircraft above has Turboprop engines, where the majority of the work extracted from the flow by the turbine is fed to a gearbox (to reduce the rotational speed down to propeller speeds), and then used to run a propeller.
Rockets Space Shuttle Main Engine: LOX/LH 2 Pressure thrust" in addition to "momentum thrust". The exhaust of a rocket comes out highly supersonic, and expands down to the atmospheric pressure, which may be very low or even zero if the rocket is at a high altitude or in outer space. When the exhaust is supersonic, exhaust jet will not adjust to outside pressure before it comes out of the nozzle exit. Pressure thrust is force due to pressure difference between the exhaust plane and the outside, acting on the exhaust area. Also, for rockets, incoming mass flow rate is zero. We can group these two sources of thrust together, divide the total by the mass flow rate of gas exiting the nozzle, and the result has units of velocity. We call this the "effective exhaust velocity", c E. = m E c E http://www.seas.upenn.edu/courses/meam203/class/ssme.jpg http://www.boeing.com/defense-space/space/rdyne/sightsns/images/ssmetest.gif
TSFC Thrust-Specific Fuel Consumption of an engine f g 3600 fg 1 f u u m g 3600 Thrust e Also known as Specific fuel consumption (SFC) and given as the weight of fuel consumed per unit time per unit thrust. Usually expressed in units of (per hour). Typical values: 0.55 per hour (turbofan, business jet engine) 0.4 efficient high-bypass turbofan 0.65 low-bypass fighter engine without afterburner ( dry thrust ) 2.0 low-bypass fighter engine with afterburner ( wet thrust ) Specific Fuel Consumption changes mainly with speed for a given jet engine. Much milder Specific Fuel Consumption changes mainly with speed for a given jet engine. Much milder increase in SFC (which is bad) with altitude
Empirical Relations: Thrust Lapse Rate As we go up in the atmosphere, the density decreases. So, for a given flight speed, thrust decreases as altitude increases. This rate of decrease is called the thrust lapse rate If you have no other data, assume that thrust is proportional to density of air. From data on a high-bypass engine, the following empirical expressions can be developed for large turbofan engines of the type envisaged for modern commercial transport aircraft: Thrust at altitude = (Static Thrust at sea-level) *(0.45-0.17*10-4*(altitude-5000)), 0 4*(altitude 5000)) altitude in meters. Below 5000 m, assume that thrust varies linearly, or scales with density. The variation of thrust with Mach number is milder than the variation with altitude. The expression is not monotonic (i.e., it does not just keep increasing, or keep decreasing, with increasing Mach number), so it is not attempted here. Specific Fuel Consumption vs, Mach Number for High-Bypass Engine Specific Fuel Consumption (sfc) = 0.55 + (0.65-0.4)/0.35*(M-0.3) for 0.3< M <0.85 This expression is obtained using data for sfc vs. bypass ratio (which is based on 1970s technology) and then reducing the sfc by 10% to anticipate technology advances.
Hot Air mass flow rate = 100 kg/s Flight speed = 200 m/s Fuel/air ratio = 0.015 Hot exhaust velocity = 800 m/s Fan exhaust velocity = 250m/s Bypass ratio = 6 Example: Thrust calculation Thrust = 100* ( 6*(250-200) + 1.02*800 200) ) = 100*(300 + 616) = 91600 N Specific fuel consumption = fuel wt. flow rate*3600 per second/ Thrust =100*0.015*9.8 * 3600/91600 = 0.5775 per hour Some implications. Assume Thrust to weight ratio of the engine ~ 4.5 Weight = 20,335 N Assume engine is installed using the 30% of TOW guideline. TOW = 305,333 N Engine weight fraction = 20,335/305,333 = 6.65% If L/D 12 Th t d d t fli ht ltit d i l 25 444 N If L/D = 12, Thrust needed at flight altitude is only 25,444 N. Can fly at an altitude where density is about 0.28 of sea-level.
Design Step 4 15. Select engines based on takeoff thrust and 1-engine-out criteria 15. Select engines based on takeoff thrust and 1 engine out criteria 16. Find thrust-specific fuel consumption 17. Estimate thrust variation with altitude
Ramjet
Some Notes on Engine Design 1. Thermodynamics shows that the efficiency in converting heat to work is highest when the heat is added at the highest possible pressure. Modern engines have high Overall Pressure Ratio P B/P A. Even when this ratio is 40, note that the thermal efficiency, as computed above, is only 65%. 2. The Propulsive Efficiency measures the efficiency in converting the kinetic energy of the fluid (air) to thrust. This is maximized by driving the exhaust velocity as close as possible to the flight speed u. This is done using high bypass fans, or turboprops.