U Gary Shaw SINCE WROTE about BTA autopilot November December 1994 columns Ive received several cards letters calls individuals have found difficult get touch company following address telephone number should solve problem BTA Automatic Piloting Systems Ltd 2422 Richmond Way Costa Mesa CA 92626 714 979-8948 Most mail regarding autopilot has folks needing flight assistance due varying degrees physical disability Its nice know theres something market promises open horizons otherwise disadvantaged unique somewhat opposing viewpoint letter sent Tom Cooperider Cameron Missouri recommended go slow approach promoting its use Tom feels use product modelers might have strong tendency make lazy pilots Students [sic] learn fly autopilot may never learn fly autopilot has good applications lets try sell cure-all Although certainly supported cure-all its ability save aircraft affected signal loss and/or provide flying assistance requiring itin capacity continues make worthwhile investment opinion PET Power September 1994 column wrote about perils using Polyethylene Tetrahyde PET containers compressed-air vessels driving air-powered retract systemsTo help illustrate concerns included photos sent modeler Bob Rodgers Orchard Park New York depicted scattered remains 1/4-scale B-25 sad condition otherwise proud aircraft What happened nutshell overpressurized PET container two-liter plastic soft-drink bottle used power its retract system erupted violent way leaving behind balsa junk heap read column sent cards letters describing own concerns and/or offering different viewpoints Olin McDaniel florence South Carolina retired Du Pont worker set record straight clarifying what PET really stands Its quite Polyethylene Tetrahyde actually name really fails identify chemical structure PET really stands Polyethylene Terephthalate polymeric chain repeating links link containing ethylene unit larger uait really modified benzene ring modified benzene ring derivative terephthalic acid correct definition out way bust another piece correspondence Franldin Vassallo Lancaster New York arrived via AMA recently some very interesting PET container pressure-testing results Using information gathered during trials Franklin feels applications PET containers could safely used safety being first consideration past several years have employing PET bottles air source use RC compressed air models Because am flying models about pound total weight using upwards 500 PET containers wrapped Keviar reduce explosion hazards increases useable pressures recommended r WEAR EARLUGS i U April 1995 21 Franklin Vassallos PET-powered Baromaster offers new horizons desiring light flight PRODUCTS roe yaJe Complete Line Exhaust Systems * Tuned Pipes led Tuned Pipes Headers uri Mufflers Expansion Mufflers Helicopter Ball Mufflers ialized Exhaust Systems Unsurpassed Wojkma.shio 8 Performance square inches wing area quantity air needed its pressure level requires use combined two-liter PET bottles enclosed photo shows compressed air RC models Baromaster search appropriate bottles use compressed air application have found two-liter bottles manufactured various configurationssome lend themselves use pressure vessels some Among bottle variations have observed size neck flange Small size preferred shape upper bottle ranges nearly hemispherical nearly conical Surprisingly have found conical types perform best used strapping shape base having square base molded bottle unacceptable Only having glued-on cup base should used cup removed better types have thickened section plastic sprue visible tests various bottles have found few two-liter bottles can pressurized beyond 130 psig significant damage bottle Usually residual deformation indicated whitish stress lines neck area Rupture bottle appears initiated neck region although some sidewall failures have observed 120 psig pressure level somebut muchsafety margin Because bottles vary tolerance imperative bottle tested least desired level pressure Pressure should maintained accurate pressure gauge preferably same gauge will used during later use tank bottle should inspected after use signs residual deformation particularly near neck Bottles exhibit bulging sidewalls should discarded creep failure may issue resulting slow dilation bottle under extended strain imposed long term pressurization have induced several long-term failures bottles observed have slight circumferential budging one-liter bottles also can found number shapes similar two-liter types Same rules apply Because smaller diameter can withstand significantly higher pressureusually over 150 psig Again bottle must tested prior use size restricted combined one-liter bottles can provide same volume gas two-liter bottle safety margin Three-liter bottles best variety rarely can taken beyond 100 psig risk failure would need wrapped applications require higher pressure Both aluminum plastic caps have found capable containing pressures above 175 psig prefer plastic caps seem show less deflection under pressure provide better seal have found over-wrapping bottles using braided nylon fishing line lacing cord can significantly increase bottles tolerance failure two-liter bottles approximately 70 axial strands 15-lb test nylon wound tank additional 24 hoop strands its midsection can extend its burst pressure about 150 psig Thus using tank level 120 psig gives about 30 psi safety margin See enclosed photo nylon lacing cord wrapped tank offer describe wrapping technique believe interest recent experiments 70-lb test Kevlar fishing line using same wrap configuration burst pressure has extended beyond 150 psig Again stress tank must tested level use desired wrapped varieties tests may performed considerably above expected use enclosed photo shows two-liter tank combination wrapped Kevlar has tested above 150 psig later oversprayed yellow paint better appearance flight above testing performed enclosure bottle rupture posed no safety concern Pressures measured using 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pressures 120 psig desired Over past several years have flown literally hundreds compressed-air RC flights using prepared PET bottles pressures up beyond 120 psig no burst failures during filling flight use pressure vessels can very safe attention given tank preparation filling examination proper use safety equipment Ill have agree Franklin regarding comment bursting exposed tank probably much less hazardous bursts inside structure model hence destroyed remains model depicted original column Youll also get no argument regarding need use standard safety equipment flightline What believe matters unless properly designed such uses modelers would much less likely follow independentlappropriate safety precautions ensure best possible environment Most just want models fly convenience mind ie no wrapping hassles stress marks contend Unless tend experimentminded assume safety already designed product being used Although Im sure will continue use PET containers pressurized vessels until 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