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Mercury RC Model

Old 08-27-2015, 06:06 AM
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Location: Colorado
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Default Mercury RC Model

I finished building this model just last week and have put two flights on it so far. I built it from cad drawings I found online. It was drawn as a free flight glow powered model but I made it an Electric RC model. I also changed the all wood sheet body to stringers on the rear of the fuselage. This is a 90" span model that weights 8 pounds. The wing is built in three sections and the entire tail assembly comes off with one screw. The landing gear wires are held in place with rubber bands.

I have a question I was hoping someone may be able to help me with. The model has a RimFire 60 motor and is powered with a 5-cell 5Ah pack. First flight was with a APC 12x6 electric prop. It took off in about 20 feet but it would not climb very well. For the second flight I installed the only other prop I had with me which was an APC 15x8 electric Prop. Now the plane jumps into the air and climbs great. I mainly flew the plane as a powered glider in that I climb to altitude, shut the motor down and glide around. I also cruise it around at about 1/3 throttle but gliding is most likely what I will be doing with the model. When I landed after the second flight my 20C battery was warm, but the motor was at room temperature. Of course most of this flight was gliding so the motor was being run much. I decided to see how many amps the motor was drawing and I was shocked. The 15X8 prop draws 98 Amps and the 12X6 prop draws 48 Amps. I can live with the 48 Amps if the performance was a little better. So I guess my question is does a 48 Amp draw on a APC 12x8 electric prop seam about right? Anybody out there run a RimFire 60 and what cell count and prop do you use.

John Boren
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Old 08-27-2015, 09:37 AM
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You need to check the battery temp a few times during one flight.
A LiPo gets hot much faster as you get nearer to hitting LVC. It might be running very cool 80% of the charge and then heat rapidly. This would be an indication that you should reset the LVC slightly higher to protect the battery.

There is a VERY wide selection of props between that 15X8 and the 12X6.

I commonly have pulled 100+ amps from 4000 to 5000 mah 20C packs. Its really not a big problem for the packs if you watch their temperature.

I don't have the Rimfire 60... and no idea what ESC you are using. But I have a few "60 class" e-conversions and typically I run them on 6S made up of 2 X 3S packs in series. The reason I use 2 3S instead of one 6S is the middle cells of a 6S can't cool so they fail early, especially when pulling high amps.

Heat is the enemy of LiPo. If the packs are over 120 F when you pull them you need to do something different if you want long pack life. While they have been proven to deliver best current at 120F... pre-heating before flight to maximize performance is a good way to have the packs last less than 50 cycles. (maybe less than 5) Keeping to the rule of avoiding 120F or higher has the potential for well over 500 cycles and over 3 years of use.
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Old 08-27-2015, 12:22 PM
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48A sounds about right on the 12x6. The motor is rated for 64A continuous and 85A burst so I'd expect that a 14x7 prop should put you in a good position of ample power and within the motors rating. That's assuming you have an ESC rated for 80A or so.
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Old 08-27-2015, 02:58 PM
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Location: Colorado
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I'm not too worried about the batteries, they were only a little warm; it was the high amp draw that worried me the most. I only have a 70 Amp Turnigy ESC in the thing and I am really surprised I didn't burn it up drawing 98 Amps during my 30 second motor runs. If 48 Amps sounds about right for the RimFire 60 then I'll simply re-prop with something in the 13 to 14 inch range to get a middle of the road in performance. Until then I'll simply use the 12x6 I have since we have a Giant Bird Fly In this weekend that I want to fly in. Thanks for your help.

John Boren
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Old 05-08-2016, 08:56 PM
Paul Paolozzi
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Very good job on the plane.... I enjoyed it very much
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