Nehru Institute of Technology Department of Aeronautical Engineering AE3301 Elements of Aeronautical Engineering Anna university Question Bank UNIT V PART A POWER PLANTS USED IN AIRPLANES UNIT V Basic ideas about piston turboprop and jet engines Use of propeller and jets for thrust production Principles of operation of rocket types of rockets 10 1 State the propellant burning configurations possible in solid propellant motors Ans A simple solid rocket motor consists of a casing nozzle grain propellant charge and igniter The grain behaves like a solid mass burning in a predictable fashion and producing exhaust gases The nozzle dimensions are calculated to maintain a design chamber pressure while producing thrust from the exhaust gases Once ignited a simple solid rocket motor cannot be shut off because it contains all the ingredients necessary for combustion within the chamber in which they are burned More advanced solid rocket motors can not only be throttled but also be extinguished and then re ignited by controlling the nozzle geometry or through the use of vent ports Also pulsed rocket motors that burn in segments and that can be ignited upon command are available Modern designs may also include a steerable nozzle for guidance avionics recovery hardware parachutes self destruct mechanisms APUs controllable tactical motors controllable divert and attitude control motors and thermal management materials NB read the answer understand and write the required one I have given this answer for 2 What are hypergolic propellants Give an example you to understand Ans A form of liquid propellant in which the fuel ignites spontaneously upon contact with an oxidizer thereby eliminating the need for an ignition system The easy start and restart capability of hypergolics make them ideal for spacecraft maneuvering systems Also since hypergolics remain liquid at normal they don t pose the storage problems of cryogenic propellants On the debit side hypergolics are highly toxic and must be handled with extreme care Hypergolic fuels commonly include hydrazine monomethyl hydrazine MMH and unsymmetrical dimethyl hydrazine UDMH The oxidizer is typically nitrogen tetroxide N2O4 or nitric acid HNO3 temperatures 3 Why is it that turbo fan engine is very widely used in commercial aircraft Ans Thrust developed is higher than turbo jet engine Weight per unit thrust is lower than turbo prop engine For the following reason the turbo fan engine is very widely used in commercial aircraft Less vibration and noise Better fuel economy High take off thrust 4 What are the limitations of a piston engines for aircraft propulsion Cannot be used in transport category aircraft as the weight to thrust ratio is less Ans systems including the induction ignition fuel oil cooling and exhaust systems The engine is operated at a fairly low speed Continuous operation of the engine depends on the simultaneous function of auxiliary 5 Define specific impulse of a rocket Ans Specific impulse is a comparative measure of the efficiency of different rocket engines and is defined as the thrust per unit weight flow at sea level It is denoted by Isp 6 What is jet thrust Ans The force which propels the aircraft towards at a given speed is called as jet thrust The thrust mainly depends on the velocity of gases at the exit of the nozzle 7 What are the requirements of a good power plant Ans Aircraft are one of the most demanding applications for an engine presenting multiple design requirements many of which conflict with each other An aircraft engine must be Reliable Aircraft engines operate at temperature pressure and speed extremes and Light weight as a heavy engine increases the empty weight of the aircraft and reduces Powerful therefore need to perform reliably and safely under all reasonable conditions its payload installed create too much drag relatively inexpensive and possible outside of specialized shops Fuel efficient to give the aircraft the range the design requires to overcome the weight and drag of the aircraft Small and easily streamlined Field repairable large engines with substantial surface area when to keep the cost of replacement down Minor repairs should be Capable of operating at sufficient altitude for the aircraft 8 Why does the blade angle of a propeller decrease from boss to tip Ans A propeller is a rotating airfoil and is subject to induced drag stalls and other aerodynamic principles that apply to any airfoil The propeller provides the necessary thrust to pull or in some cases push the airplane through the air This is accomplished by using engine power to rotate the propeller which in turn generates thrust in much the same way as a wing produces lift The propeller has an angle of attack which is the angle between the chord line of the propeller s airfoil and its relative wind airflow opposite to the motion of the blade A propeller blade is twisted The blade angle changes from the hub to the tip with the greatest angle of incidence or highest pitch at the hub and the smallest at the tip see figure The reason for the twist is to produce uniform lift from the hub to the tip As the blade rotates there is a difference in the actual speed of the various portions of the blade 9 What is Ramjet engine Ans Ramjet engine is the simplest type of air breathing engine The functions of supersonic and subsonic diffusers are to convert the kinetic energy of the entering air into pressure energy This energy transformation is called ram effect The air breathing engine which is used this energy transformation called ramjet engine 10 What are the advantages and the disadvantages of rocket propulsion Ans Advantages or so kinder on vehicle structure at high speed efficiency at very high speeds good cost thrust ratio Disadvantages Very few moving parts Mach 0 to Mach 25 efficient at very high speed Mach 5 0 Thrust weight ratio over 100 no complex air inlet high compression ratio very high speed hypersonic exhaust fairly easy to test works in a vacuum indeed works best exo atmospheric which is fairly small surface area to keep cool and no turbine in hot exhaust stream Very high temperature combustion and high expansion ratio nozzle gives very high Needs lots of propellant very low specific impulse typically 100 450 seconds Extreme thermal stresses of combustion chamber can make reuse harder Typically requires carrying oxidizer on board which increases risks Extraordinarily noisy 11 Write down the difference between reciprocating and Gas