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Help power my big 42% gasser

Old 12-28-2023, 01:17 PM
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ucme2fly
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Question Help power my big 42% gasser

I built a big gas powered plane with 10 digital servos back in the 80's. Now I am getting back into flying, and need a radio system and batteries. All my old stuff is FM, and or totally obsolete, so I'm getting all new. I need 8 channels to drive everything. New to 2.4 ghz technology! I was thinking about (2) 5000 mah LIFe packs and a new Spectrum 10 channel transmitter and receiver? Very concerned about too much voltage from those packs. Servos are Futaba 9451 digitals , and show 6V max. Defibrillator, and voltage regulators needed? It looks like the 10 channel Spectrum would eliminate the need for a powerbox device from what I infer. Ignition is a seperate battery.
Old 12-29-2023, 08:30 AM
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BarracudaHockey
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Defibrillator? You plan on it having a heart attack?
Old 12-29-2023, 08:44 AM
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Those are car servos btw but that aside a 6V rating is a data point, not a max. That comes from the old days of 4-cell and 5-cell Nickel batteries where a 4-cell was 4.8v nominal and a 5-cell 6v nominal, they were higher when charged.

Any servo with a 6v rating is fine with a Life/A123 battery chemistry.

Run a TechAero IBEC and don't worry about having a separate ignition battery and that gives you a transmitter activated kill switch as well.
Old 12-29-2023, 12:51 PM
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Thanks Andy, I will do just that. One question though... The IBEC is ONLY for the (Desert Aircraft) ignition correct? Not run between the batt and the receiver...?
Old 12-29-2023, 12:55 PM
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The IBEC plugs into a channel in the receiver. It takes power from the receiver battery or batteries and provides power to the ignition, also that channel is switched by the transmitter to kill the ignition. Its optically isolated between the receiver channel and the actual ignition so there's no interference back to the radio. A high percentage of guys flying giant scale gas the preference is dual receiver packs and a tech-aero IBEC to power the ignition. Its simple and pretty bulletproof
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Old 12-29-2023, 01:30 PM
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Thanks. That is very helpful. Makes good sense now. I will do exactly that. Final question for you... With ten digital servos how would you power the 42% with A123 batteries? What mah. size would you think needed for 3D/ Pattern/ sport?
Old 12-30-2023, 07:07 PM
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You are setting up a large 42% scale gasser and the servo current draw for 10 digital servos is quite significant. You will need a power expander board to be able to deliver the power needed to each of the servos. Typical JR/Futuba/Hitec battery connectors are only rated for 5 amps, and you will probably be looking at 15 amps or more depending on how aggressively you fly. Just one stalled out large digital servo can exceed 5 amps. Check out the SMART FLY power expander board. It uses two Deans connectors for power input from two batteries. It has battery-safe circuitry to draw from both batteries and switch to the remaining good battery in case one battery fails. It also supplies buffered 5 volt power to the receiver to prevent brownouts while delivering the full battery voltage to the servos. It has multiple (2 to 4) separate servo connectors for each channel so no cable splitters are needed and no one connection has to carry the full load of all the servos on one channel. Be sure to use heavy duty switches and battery leads. As for batteries, Lithium Iron Phosphate (LiFe) batteries are fine to use on 6 volt systems. The five cell nickel metal hydride "6 volt" batteries normally read about 7 volts when fully charged. 2S LiFe batteries come off the charger at up to 7.2 volts but quickly settle to about 6.7 to 6.8 volts within just 2 or 3 minutes with no load. When load is applied, the voltage quickly drops to about 6.65 volts. The discharge curve is very flat and drops to about 6.5 volts with 30% capacity remaining. Any system for which you could use 5 cell NiMH batteries is safe to use with LiFe batteries. Just don't try to determine the remaining battery capacity by reading the voltage. The difference between 90% charged and 30% charged is only 0.15 volts on a 2S LiFe battery. Your uncalibrated field tester is probably not that accurate. As for ignition batteries, check to see what voltage your ignition system requires. It can vary a lot depending on the age and brand of the ignition system. Older CH ignition systems were limited to 4.8 volts. Most modern systems will state 6-14 volts or something like that. The instructions that came with my latest ignition system state the voltage must be not less than 7.2 volts under load in order to not overstress the ignition system and shorten its life, even though it may seem to work fine at 6.6 volts. A 2S LiFe will not provide 7.2 volts. It also said not to use switch mode battery eliminator circuits for the ignition system since the output voltage is not pure DC and could produce erratic ignition. I suspect a linear BEC should work OK though since it is straight resistance to drop the voltage. It also said to keep the ignition battery separated from the receiver since some RF noise could feed back through the battery wiring. The optical kill switch will prevent a direct RF feed back into the receiver from the ignition module. I have a 53cc gasser with a Smart Fly power expander, two flight batteries, a Spektrum 10 channel telemetry receiver with 2 satellite receivers, an optical kill switch and a separate ignition battery. With this setup, I normally get 15 minute flights with less than 4 frame losses... rock solid RF link.

Last edited by LLRCFlyer; 12-30-2023 at 07:35 PM. Reason: additional info
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