FM 1-506 Fundamentals of Aircraft Power Plants ... - Survival Books

WWW.SURVIVALEBOOKS.COMFM 1-506the raw materials and the great number of man-hoursinvolved, complex designs drive up the cost. The mostsatisfactory design is generally the simplest that will meetrequirements.High Thermal EfficiencyThermal efficiency is a measure of the losses sufferedin converting heat energy in the fuel into mechanicalwork; it is the ratio of the heat developed into useful workto the heating value of the fuel. High thermal effciency,therefore, means high fuel economy-something ofgreat importance in aircraft engines. The less fuel requiredfor a military mission, the greater the military loadthat can be carried and the lower the fuel cost.Freedom From VibrationA power plant that is free from vibration is importantin the light, somewhat flexible aircraft structure sincesevere engine vibration will in some cases reduce the lifeof certain structural parts. The need for freedom fromvibration is met usually by using a large number ofcylinders to offset the vibration torque delivered by theindividual cylinders. Counterweights are installed oncrankshafts to balance rotating masses. These are usuallyhinged to provide dynamic damping of vibration whichresults from power impulses and to counteract undesirabletorsional or twisting vibration. Also, flexibleengine mount isolators are used to permit certain movementsof the power plant that are harmful to aircraftstructures.Ease of MaintenanceThe requirement of ease of maintenance is especiallyimportant to military operations in the field.Simplicity of design and use of standard parts, whenpossible, assist in keeping maintenance at a low level.Operating FlexibilityFlexibility is the ability of a power plant to runsmoothly and provide the desired performance at allspeeds from idling to maximum power output. The widerange of operating requirements demanded of aircraftengines presents difficulties rarely encountered in otherpower plant fields. In addition to the requirement ofunfailing reliability, the engine must operate in widelyvarying positions, altitudes, and atmospheric conditions.FUELSRequirementsAn engine fuel must be tailored to an engine and viceversa since there must be enough quantities of fuel availableto the engine. Some significant properties of aviation fuelsare discussed below.Heat Energy Content or Net Heating Value. Theenergy content or heating value of a fuel is expressed inheat units (British thermal units [BTUs]). A fuel satisfactoryfor aircraft engines must have a high heat energycontent per unit weight. A high heat energy contentcauses the weight of fuel carried to be lower than a lowheat energy content. Then more of the load-carryingcapacity is available for the payload Aviation gasolineand JP fuels are very desirable from this standpoint. Theheat energy content for aviation gasoline is about 18,700BTUs/pound, and for JP fuels about 18,200BTUs/pound. The various alcohols, which have maximumenergy content of about 12,000 BTUs/pound, dopossess some other desirable characteristics as an internalcombustion engine fuel.Volatility. A volatile liquid is one capable of readilychanging from a liquid to a vapor when heated or whencontacting a gas into which it can evaporate. Since liquidfuels must be in a vaporous state to burn volatility is animportant property to consider when choosing a suitablefuel for an aircraft engine. Volatility determines the startingaccelerating vapor-locking and distribution characteristicsof the fuel. Gasoline and JP fuels are verysatisfactory because they can be blended during the refiningprocess to give the desired characteristics. Because ofthe nature of constant pressure combustion in gas turbineengines a highly volatile fuel is not necessary. JP fuels areof rather low volatility while aviation gasoline is highlyvolatile. Comparing a highly volatile fuel like aviationgasoline to a less volatile one like JP fuel the followingeffects become apparent. The highly volatile fuel –Starts easier in cold temperatures.Has a slightly better combustion efficiency.Leaves less deposit in the combustion chamberand on the turbine blades.Is a greater fire hazard.Creates a greater danger of vapor lock of thefuel system.Has high evaporation losses through thebreather of the fuel tank at high altitudes.NOTE: The last two difficulties are practicallynonexistent with fuels having lowvolatility.Stability. The fuels used in aircraft engines must bestable. Because aviation fuels are sometimes stored forlong periods, they must not deposit sediment. The gums1-2