Steam turbine 1 Steam turbine A rotor of a modern steam turbine, used in a power plant A steam turbine is a device that extracts thermal energy from pressurized steam and uses it to do mechanical work on a rotating output shaft. Its modern manifestation was invented by Sir Charles Parsons in 1884. [1] Because the turbine generates rotary motion, it is particularly suited to be used to drive an electrical generator – about 90% of all electricity generation in the United States (1996) is by use of steam turbines. [2] The steam turbine is a form of heat engine that derives much of its improvement in thermodynamic efficiency through the use of multiple stages in the expansion of the steam, which results in a closer approach to the most efficient reversible process. History 2000 KW Curtis steam turbine circa 1905. The first device that may be classified as a reaction steam turbine was little more than a toy, the classic Aeolipile, described in the 1st century by Greek mathematician Hero of Alexandria in Roman Egypt. [3][4][5] In 1551, Taqi al-Din in Ottoman Egypt described a steam turbine with the practical application of rotating a spit. Steam turbines were also described by the Italian Giovanni Branca (1629) [6] and John Wilkins in England (1648). [7] The devices described by al-Din and Wilkins are today known as steam jacks.
Transcript
Steam turbine 1
Steam turbine
A rotor of a modern steam turbine, used in a power plant
A steam turbine is a device thatextracts thermal energy frompressurized steam and uses it to domechanical work on a rotating outputshaft. Its modern manifestation wasinvented by Sir Charles Parsons in1884.[1]
Because the turbine generates rotarymotion, it is particularly suited to beused to drive an electrical generator –about 90% of all electricity generationin the United States (1996) is by use ofsteam turbines.[2] The steam turbine isa form of heat engine that derivesmuch of its improvement inthermodynamic efficiency through theuse of multiple stages in the expansionof the steam, which results in a closerapproach to the most efficientreversible process.
History
2000 KW Curtis steam turbine circa 1905.
The first device that may be classified as a reaction steam turbine waslittle more than a toy, the classic Aeolipile, described in the 1st centuryby Greek mathematician Hero of Alexandria in Roman Egypt.[3][4][5]
In 1551, Taqi al-Din in Ottoman Egypt described a steam turbine withthe practical application of rotating a spit. Steam turbines were alsodescribed by the Italian Giovanni Branca (1629)[6] and John Wilkins inEngland (1648).[7] The devices described by al-Din and Wilkins aretoday known as steam jacks.
Parsons turbine from the Polish destroyer ORP Wicher.
The modern steam turbine was invented in 1884 by SirCharles Parsons, whose first model was connected to adynamo that generated 7.5 kW (10 hp) of electricity.[8]
The invention of Parson's steam turbine made cheapand plentiful electricity possible and revolutionisedmarine transport and naval warfare.[9] His patent waslicensed and the turbine scaled-up shortly after by anAmerican, George Westinghouse. The Parsons turbinealso turned out to be easy to scale up. Parsons had the
satisfaction of seeing his invention adopted for all major world power stations, and the size of generators hadincreased from his first 7.5 kW set up to units of 50,000 kW capacity. Within Parson's lifetime, the generatingcapacity of a unit was scaled up by about 10,000 times,[10] and the total output from turbo-generators constructed byhis firm C. A. Parsons and Company and by their licensees, for land purposes alone, had exceeded thirty millionhorse-power.[8]
A number of other variations of turbines have been developed that work effectively with steam. The de Laval turbine(invented by Gustaf de Laval) accelerated the steam to full speed before running it against a turbine blade. DeLaval's impulse turbine is simpler, less expensive and does not need to be pressure-proof. It can operate with anypressure of steam, but is considerably less efficient.
Cut away of an AEG marine steam turbine circa 1905
One of the founders of the modern theory of steam andgas turbines was also Aurel Stodola, a Slovak physicistand engineer and professor at Swiss PolytechnicalInstitute (now ETH) in Zurich. His mature work wasDie Dampfturbinen und ihre Aussichten alsWärmekraftmaschinen (English: The Steam Turbineand its perspective as a Heat Energy Machine) whichwas published in Berlin in 1903. In 1922, in Berlin,was published another important book Dampf undGas-Turbinen (English: Steam and Gas Turbines).
The Brown-Curtis turbine which had been originallydeveloped and patented by the U.S. company
International Curtis Marine Turbine Company was developed in the 1900s in conjunction with John Brown &Company. It was used in John Brown's merchant ships and warships, including liners and Royal Navy warships.
Schematic operation of a steam turbine generator system
Steam turbines are made in a variety ofsizes ranging from small <0.75 kW(1< hp) units (rare) used as mechanicaldrives for pumps, compressors andother shaft driven equipment, to1,500,000 kW (2,000,000 hp) turbinesused to generate electricity. There areseveral classifications for modernsteam turbines.
Steam supply and exhaustconditions
These types include condensing,non-condensing, reheat, extraction andinduction.Condensing turbines are mostcommonly found in electrical powerplants. These turbines exhaust steam ina partially condensed state, typically ofa quality near 90%, at a pressure well below atmospheric to a condenser.
Non-condensing or back pressure turbines are most widely used for process steam applications. The exhaust pressureis controlled by a regulating valve to suit the needs of the process steam pressure. These are commonly found atrefineries, district heating units, pulp and paper plants, and desalination facilities where large amounts of lowpressure process steam are available.
Reheat turbines are also used almost exclusively in electrical power plants. In a reheat turbine, steam flow exits froma high pressure section of the turbine and is returned to the boiler where additional superheat is added. The steamthen goes back into an intermediate pressure section of the turbine and continues its expansion.Extracting type turbines are common in all applications. In an extracting type turbine, steam is released from variousstages of the turbine, and used for industrial process needs or sent to boiler feed water heaters to improve overallcycle efficiency. Extraction flows may be controlled with a valve, or left uncontrolled.Induction turbines introduce low pressure steam at an intermediate stage to produce additional power.
These arrangements include single casing, tandemcompound and cross compound turbines. Single casingunits are the most basic style where a single casing andshaft are coupled to a generator. Tandem compound areused where two or more casings are directly coupledtogether to drive a single generator. A cross compoundturbine arrangement features two or more shafts not inline driving two or more generators that often operateat different speeds. A cross compound turbine istypically used for many large applications.
Two-flow rotors
A two-flow turbine rotor. The steam enters in themiddle of the shaft, and exits at each end,
balancing the axial force.
The moving steam imparts both a tangential and axial thrust on theturbine shaft, but the axial thrust in a simple turbine is unopposed. Tomaintain the correct rotor position and balancing, this force must becounteracted by an opposing force. Either thrust bearings can be usedfor the shaft bearings, or the rotor can be designed so that the steamenters in the middle of the shaft and exits at both ends. The blades ineach half face opposite ways, so that the axial forces negate each otherbut the tangential forces act together. This design of rotor is calledtwo-flow or double-exhaust. This arrangement is common inlow-pressure casings of a compound turbine.[11]
Principle of operation and designAn ideal steam turbine is considered to be an isentropic process, or constant entropy process, in which the entropy ofthe steam entering the turbine is equal to the entropy of the steam leaving the turbine. No steam turbine is trulyisentropic, however, with typical isentropic efficiencies ranging from 20–90% based on the application of theturbine. The interior of a turbine comprises several sets of blades, or buckets as they are more commonly referred to.One set of stationary blades is connected to the casing and one set of rotating blades is connected to the shaft. Thesets intermesh with certain minimum clearances, with the size and configuration of sets varying to efficiently exploitthe expansion of steam at each stage.
Schematic diagram outlining the difference between an impulse anda 50% reaction turbine
To maximize turbine efficiency the steam is expanded,doing work, in a number of stages. These stages arecharacterized by how the energy is extracted from themand are known as either impulse or reaction turbines.Most steam turbines use a mixture of the reaction andimpulse designs: each stage behaves as either one orthe other, but the overall turbine uses both. Typically,higher pressure sections are reaction type and lowerpressure stages are impulse type.
Impulse turbines
An impulse turbine has fixed nozzles that orient thesteam flow into high speed jets. These jets containsignificant kinetic energy, which is converted into shaftrotation by the bucket-like shaped rotor blades, as thesteam jet changes direction. A pressure drop occursacross only the stationary blades, with a net increase insteam velocity across the stage. As the steam flowsthrough the nozzle its pressure falls from inlet pressureto the exit pressure (atmospheric pressure, or moreusually, the condenser vacuum). Due to this high ratioof expansion of steam, the steam leaves the nozzle with a very high velocity. The steam leaving the moving bladeshas a large portion of the maximum velocity of the steam when leaving the nozzle. The loss of energy due to thishigher exit velocity is commonly called the carry over velocity or leaving loss.
A selection of turbine blades
The law of moment of momentum states that the sum of the momentsof external forces acting on a fluid which is temporarily occupying thecontrol volume is equal to the net time change of angular momentumflux through the control volume.
The swirling fluid enters the control volume at radius withtangential velocity and leaves at radius with tangentialvelocity .
A velocity triangle paves the way for a better understanding of therelationship between the various velocities. In the adjacent figure wehave:
and are the absolute velocities at the inlet and outletrespectively.
and are the flow velocities at the inlet and outletrespectively.
and are the swirl velocities at the inlet andoutlet respectively.
and are the relative velocities at the inlet and outletrespectively.
and are the velocities of the blade at the inlet and outletrespectively.
is the guide vane angle and is the blade angle.Then by the law of moment of momentum, the torque on the fluid isgiven by:
For an impulse steam turbine: .Therefore, the tangential force on the blades is
.
Work done per unit time or power developed: When ω is the angular velocity of the turbine, then the blade speed is .
Work done per unit time or power developed = .Blade efficiency
Blade efficiency ( ) can be defined as the ratio of the work done on the blades to kinetic energy supplied to thefluid, and is given by
= =
Stage efficiency
A stage of an impulse turbine consists of a nozzle set and a moving wheel. The stage efficiency defines a relationshipbetween enthalpy drop in the nozzle and work done in the stage.
Therefore the maximum value of stageefficiency is obtained by putting the
value of = in the
expression of
We get:
For equiangular blades = ,therefore Putting we get
If the friction due to the blade surface is neglected then
And Conclusions on maximum efficiency
1. For a given steam velocity work done per kg of steam would be maximum when or .2. As increases, the work done on the blades reduces, but at the same time surface area of the blade reduces,therefore there are less frictional losses.
Reaction turbines
In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. This type of turbinemakes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. Steam isdirected onto the rotor by the fixed vanes of the stator. It leaves the stator as a jet that fills the entire circumference ofthe rotor. The steam then changes direction and increases its speed relative to the speed of the blades. A pressuredrop occurs across both the stator and the rotor, with steam accelerating through the stator and decelerating throughthe rotor, with no net change in steam velocity across the stage but with a decrease in both pressure and temperature,reflecting the work performed in the driving of the rotor.Blade efficiency
Energy input to the blades in a stage:is equal to the kinetic energy supplied to the fixed blades (f) + the kinetic energy supplied to the moving
blades (m).
Or, = enthalpy drop over the fixed blades, + enthalpy drop over the moving blades, .The effect of expansion of steam over the moving blades is to increase the relative velocity at the exit. Therefore therelative velocity at the exit is always greater than the relative velocity at the inlet .In terms of velocities, the enthalpy drop over the moving blades is given by:
=
(it contributes to a change in static pressure)The enthalpy drop in the fixed blades, with the assumption that the velocity of steam entering the fixed blades isequal to the velocity of steam leaving the previously moving blades is given by:
inlet velocity of steam in the nozzleis very small and hence can be
neglected
Therefore, =
+
A very widely used design has halfdegree of reaction or 50% reaction andthis is known as Parson’s turbine.This consists of symmetrical rotor andstator blades. For this turbine the velocity triangle is similar and we have:
= , = = , =
Assuming Parson’s turbine and obtaining all the expressions we get
From the inlet velocity triangle we have
=
=
Work done (for unit mass flow per second): = = Therefore the blade efficiency is given by
Comparing Efficiencies of Impulse and Reaction turbines
If = , then
=
For maximum efficiency ,
we get
and this finally gives
Therefore is found byputting the value of = in theexpression of blade efficiency
=
=
Operation and maintenanceWhen warming up a steam turbine for use, the main steam stop valves (after the boiler) have a bypass line to allowsuperheated steam to slowly bypass the valve and proceed to heat up the lines in the system along with the steamturbine. Also, a turning gear is engaged when there is no steam to the turbine to slowly rotate the turbine to ensureeven heating to prevent uneven expansion. After first rotating the turbine by the turning gear, allowing time for therotor to assume a straight plane (no bowing), then the turning gear is disengaged and steam is admitted to the turbine,first to the astern blades then to the ahead blades slowly rotating the turbine at 10–15 RPM (0.17–0.25 Hz) to slowlywarm the turbine.
A modern steam turbine generator installation
Any imbalance of the rotor can lead to vibration, whichin extreme cases can lead to a blade breaking awayfrom the rotor at high velocity and being ejecteddirectly through the casing. To minimize risk it isessential that the turbine be very well balanced andturned with dry steam - that is, superheated steam witha minimal liquid water content. If water gets into thesteam and is blasted onto the blades (moisture carryover), rapid impingement and erosion of the blades canoccur leading to imbalance and catastrophic failure.Also, water entering the blades will result in thedestruction of the thrust bearing for the turbine shaft.To prevent this, along with controls and baffles in the
boilers to ensure high quality steam, condensate drains are installed in the steam piping leading to the turbine.Modern designs are sufficiently refined that problems with turbines are rare and maintenance requirements arerelatively small.
Speed regulationThe control of a turbine with a governor is essential, as turbines need to be run up slowly to prevent damage andsome applications (such as the generation of alternating current electricity) require precise speed control.[12]
Uncontrolled acceleration of the turbine rotor can lead to an overspeed trip, which causes the nozzle valves thatcontrol the flow of steam to the turbine to close. If this fails then the turbine may continue accelerating until it breaksapart, often catastrophically. Turbines are expensive to make, requiring precision manufacture and special qualitymaterials.During normal operation in synchronization with the electricity network, power plants are governed with a fivepercent droop speed control. This means the full load speed is 100% and the no-load speed is 105%. This is requiredfor the stable operation of the network without hunting and drop-outs of power plants. Normally the changes inspeed are minor. Adjustments in power output are made by slowly raising the droop curve by increasing the springpressure on a centrifugal governor. Generally this is a basic system requirement for all power plants because theolder and newer plants have to be compatible in response to the instantaneous changes in frequency withoutdepending on outside communication.[13]
Thermodynamics of steam turbines
Rankine cycle with superheat Process 1-2: The working fluid is pumped from lowto high pressure. Process 2-3: The high pressure liquid enters a boiler where it isheated at constant pressure by an external heat source to become a dry saturatedvapor. Process 3-3': The vapour is superheated. Process 3-4 and 3'-4': The dry
saturated vapor expands through a turbine, generating power. This decreases thetemperature and pressure of the vapor, and some condensation may occur. Process
4-1: The wet vapor then enters a condenser where it is condensed at a constantpressure to become a saturated liquid.
The steam turbine operates on basicprinciples of thermodynamics using the partof the Rankine cycle. Superheated vapor (ordry saturated vapor, depending onapplication) enters the turbine, after ithaving exited the boiler, at high temperatureand high pressure. The high heat/pressuresteam is converted into kinetic energy usinga nozzle (a fixed nozzle in an impulse typeturbine or the fixed blades in a reaction typeturbine). Once the steam has exited thenozzle it is moving at high velocity and issent to the blades of the turbine. A force iscreated on the blades due to the pressure ofthe vapor on the blades causing them tomove. A generator or other such device canbe placed on the shaft, and the energy thatwas in the vapor can now be stored andused. The gas exits the turbine as a saturatedvapor (or liquid-vapor mix depending onapplication) at a lower temperature andpressure than it entered with and is sent tothe condenser to be cooled.[14] If we look atthe first law we can find an equationcomparing the rate at which work isdeveloped per unit mass. Assuming there isno heat transfer to the surroundingenvironment and that the change in kineticand potential energy is negligible when compared to the change in specific enthalpy we come up with the followingequation
where• Ẇ is the rate at which work is developed per unit time• ṁ is the rate of mass flow through the turbine
Isentropic turbine efficiency
To measure how well a turbine is performing we can look at its isentropic efficiency. This compares the actualperformance of the turbine with the performance that would be achieved by an ideal, isentropic, turbine.[15] Whencalculating this efficiency, heat lost to the surroundings is assumed to be zero. The starting pressure and temperatureis the same for both the actual and the ideal turbines, but at turbine exit the energy content ('specific enthalpy') forthe actual turbine is greater than that for the ideal turbine because of irreversibility in the actual turbine. The specificenthalpy is evaluated at the same pressure for the actual and ideal turbines in order to give a good comparisonbetween the two.The isentropic efficiency is found by dividing the actual work by the ideal work.[15]
where• h
1 is the specific enthalpy at state one
• h2
is the specific enthalpy at state two for the actual turbine• h
2s is the specific enthalpy at state two for the isentropic turbine
Direct drive
A small industrial steam turbine (right) directly linked to a generator (left). Thisturbine generator set of 1910 produced 250 kW of electrical power.
Electrical power stations use large steamturbines driving electric generators toproduce most (about 80%) of the world'selectricity. The advent of large steamturbines made central-station electricitygeneration practical, since reciprocatingsteam engines of large rating became verybulky, and operated at slow speeds. Mostcentral stations are fossil fuel power plantsand nuclear power plants; some installationsuse geothermal steam, or use concentrated solar power (CSP) to create the steam. Steam turbines can also be useddirectly to drive large centrifugal pumps, such as feedwater pumps at a thermal power plant.
The turbines used for electric power generation are most often directly coupled to their generators. As the generatorsmust rotate at constant synchronous speeds according to the frequency of the electric power system, the mostcommon speeds are 3,000 RPM for 50 Hz systems, and 3,600 RPM for 60 Hz systems. Since nuclear reactors havelower temperature limits than fossil-fired plants, with lower steam quality, the turbine generator sets may bearranged to operate at half these speeds, but with four-pole generators, to reduce erosion of turbine blades.[16]
The Turbinia, 1894, the first steam turbine-powered ship
In ships, compelling advantages of steam turbines overreciprocating engines are smaller size, lowermaintenance, lighter weight, and lower vibration. Asteam turbine is only efficient when operating in thethousands of RPM, while the most effective propellerdesigns are for speeds less than 100 RPM;consequently, precise (thus expensive) reduction gearsare usually required, although several ships, such asTurbinia, had direct drive from the steam turbine to thepropeller shafts. Another alternative is turbo-electricdrive, where an electrical generator run by thehigh-speed turbine is used to run one or moreslow-speed electric motors connected to the propellershafts; precision gear cutting may be a production bottleneck during wartime. The purchase cost is offset by muchlower fuel and maintenance requirements and the small size of a turbine when compared to a reciprocating enginehaving an equivalent power. However, diesel engines are capable of higher efficiencies: propulsion steam turbinecycle efficiencies have yet to break 50%, yet diesel engines routinely exceed 50%, especially in marineapplications.[17][18][19]
Nuclear-powered ships and submarines use a nuclear reactor to create steam. Nuclear power is often chosen wherediesel power would be impractical (as in submarine applications) or the logistics of refuelling pose significantproblems (for example, icebreakers). It has been estimated that the reactor fuel for the Royal Navy's Vanguard classsubmarine is sufficient to last 40 circumnavigations of the globe – potentially sufficient for the vessel's entire servicelife. Nuclear propulsion has only been applied to a very few commercial vessels due to the expense of maintenanceand the regulatory controls required on nuclear fuel cycles.
LocomotivesA steam turbine locomotive engine is a steam locomotive driven by a steam turbine.The main advantages of a steam turbine locomotive are better rotational balance and reduced hammer blow on thetrack. However, a disadvantage is less flexible power output power so that turbine locomotives were best suited forlong-haul operations at a constant output power.[20]
The first steam turbine rail locomotive was built in 1908 for the Officine Meccaniche Miani Silvestri Grodona Comi,Milan, Italy. In 1924 Krupp built the steam turbine locomotive T18 001, operational in 1929, for DeutscheReichsbahn.
TestingBritish, German, other national and international test codes are used to standardize the procedures and definitions used to test steam turbines. Selection of the test code to be used is an agreement between the purchaser and the manufacturer, and has some significance to the design of the turbine and associated systems. In the United States, ASME has produced several performance test codes on steam turbines. These include ASME PTC 6-2004, Steam Turbines, ASME PTC 6.2-2011, Steam Turbines in Combined Cycles, PTC 6S-1988, Procedures for Routine Performance Test of Steam Turbines. These ASME performance test codes have gained international recognition and acceptance for testing steam turbines. The single most important and differentiating characteristic of ASME performance test codes, including PTC 6, is that the test uncertainty of the measurement indicates the quality of the
[3][3] turbine. Encyclopædia Britannica Online[4][4] A new look at Heron's 'steam engine'" (1992-06-25). Archive for History of Exact Sciences 44 (2): 107-124.[5][5] O'Connor, J. J.; E. E. Roberston (1999). Heron of Alexandria. MacTutor[6] " Power plant engineering (http:/ / books. google. com/ books?id=Cv9LH4ckuEwC& pg=PA432& dq& hl=en#v=onepage& q=& f=false)".
P. K. Nag (2002). Tata McGraw-Hill. p.432. ISBN 978-0-07-043599-5[7] Taqi al-Din and the First Steam Turbine, 1551 A.D. (http:/ / www. history-science-technology. com/ Notes/ Notes 1. htm), web page,
accessed on line October 23, 2009; this web page refers to Ahmad Y Hassan (1976), Taqi al-Din and Arabic Mechanical Engineering, pp.34-5, Institute for the History of Arabic Science, University of Aleppo.
Turbines. pdf). PhD Engineer. . Retrieved 2011-09-22.[12] Whitaker, Jerry C. (2006). AC power systems handbook. Boca Raton, FL: Taylor and Francis. p. 35. ISBN 978-0-8493-4034-5.[13][13] Speed Droop and Power Generation. Application Note 01302. 2. Woodward. Speed[14] Roymech, http:/ / www. roymech. co. uk/ Related/ Thermos/ Thermos_Steam_Turbine. html[15][15] "Fundamentals of Engineering Thermodynamics" Moran and Shapiro, Published by Wiley[16] Leyzerovich, Alexander (2005). Wet-steam Turbines for Nuclear Power Plants. Tulsa OK: PennWell Books. p. 111.
ISBN 978-1-59370-032-4.[17] "MCC CFXUpdate23 LO A/W.qxd" (http:/ / ansys. com/ assets/ testimonials/ siemens. pdf) (PDF). . Retrieved 2010-09-12.[18] "New Benchmarks for Steam Turbine Efficiency - Power Engineering" (http:/ / pepei. pennnet. com/ display_article/ 152601/ 6/ ARTCL/
none/ none/ 1/ New-Benchmarks-for-Steam-Turbine-Efficiency/ ). Pepei.pennnet.com. Archived (http:/ / www. webcitation. org/5uL1DFU6x) from the original on 2010-11-18. . Retrieved 2010-09-12.
[19] https:/ / www. mhi. co. jp/ technology/ review/ pdf/ e451/ e451021. pdf[20] Streeter, Tony: 'Testing the Limit' (Steam Railway Magazine: 2007, 336), pp. 85[21] William P. Sanders (ed), Turbine Steam Path Mechanical Design and Manufacture, Volume Iiia (PennWell Books, 2004) ISBN
1-59370-009-1 page 292
Further reading• Cotton, K.C. (1998). Evaluating and Improving Steam Turbine Performance.• Parsons, Charles A. (1911). The Steam Turbine. University Press, Cambridge.• Traupel, W. (1977) (in German). Thermische Turbomaschinen.• Thurston, R. H. (1878). A History of the Growth of the Steam Engine. D. Appleton and Co..
External links• Steam Turbines: A Book of Instruction for the Adjustment and Operation of the Principal Types of this Class of
Prime Movers by Hubert E. Collins• Tutorial: "Superheated Steam" (http:/ / www. spiraxsarco. com/ resources/ steam-engineering-tutorials/
steam-engineering-principles-and-heat-transfer/ superheated-steam. asp)• Flow Phenomenon in Steam Turbine Disk-Stator Cavities Channeled by Balance Holes (http:/ / www. softinway.
com/ news/ articles/ Steam-turbine-disk-stator-cavities-1. asp)• Extreme Steam- Unusual Variations on The Steam Locomotive (http:/ / www. aqpl43. dsl. pipex. com/
• Interactive Simulation (http:/ / www. powerplant. vissim. com) of 350MW Steam Turbine with Boiler developedby The University of Queensland, in Brisbane Australia
• "Super-Steam...An Amazing Story of Achievement" (http:/ / books. google. com/ books?id=79oDAAAAMBAJ&pg=PA236& dq=Popular+ Science+ 1933+ plane+ "Popular+ Mechanics"& hl=en&ei=RasMTuyGFYifsQLC3sGzCg& sa=X& oi=book_result& ct=result& resnum=2&ved=0CC0Q6AEwATgK#v=onepage& q& f=true) Popular Mechanics, August 1937
