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Electric Stunt => Gettin all AMP'ed up! => Topic started by: Uolevi heinonen on June 17, 2009, 02:13:46 AM
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I have been flying electric R/C for more than ten years, so I know the basics of electrics. I have also flown C/L stunt with glow engines about the same time. Now I'd like to start electric C/L with 15. and .20 size stunters. OS 15FP turns an 8x4 prop at about 12-13,000 rpm and OS 20FP the same with a 9x4. What kind of electric setups would be equivalent in power? I'm from Finland, Europe and would really appreciate if somebody could give advice about e.g. suitable AXI or Hacker motor sizes and also usable battery packs. Very little has been written about smaller electric C/L stunters in this forum so far.
T. Uolevi Heinonen
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What is the wing area and weight of this size model? Look in "List your setup" and you'll see several listings of smaller airplanes. Like a Barnstormer 470 sq in, a Super clown 355 sq in., a Shark 402 402 sq in.
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There is no reason that the smaller planes cannot be done. I have flown a "stock" 1/2a Baby Clown Arf with a small outrunner motor and a 2s740mAHr ThunderpowerLite 740mAHr battery that basically was only slightly heavier than the Cox Blackwidow setup.
In the above case I was forced to run a maximum 6" diameter prop for the particular contest (you need someway to define what "1/2 A" is in the absence of a "real" way to measure displacement in an electric system).
Anyway that doesn't matter to you!
So to give you a baseline, I fly a SuperClown Electric Arf that weighs in at 28 oz ready for takeoff. That included the stock Brodak motor (basically the weight of an AXI2814 motor) and a 3s2100mAHr battery, a 10-7 APC electric prop . I give more details in the above list. The reason I mention this is that I also have a "full" size FLiteStreak Arf with a Fox15 turning a 7-4 prop at some reasonably high rpm that flies quite nicely on 52 foot lines. I don't recall the weight of that setup, but it also is pretty light (why I went with the Fox15BB engine).
So I would argue is to get away from spinning a small prop at high rpm.
Here would be a good setup I think for a "true" 15 sized airframe, like a Sig Akromaster, which I have flown with an FP10. For motor I would choose either the Scorpion 3008, or AXI equivalent (2808?), or even consider dropping down to the smaller diameter 22xx series (probably the longest one of that series). As to battery, I would consider a 2s2100mAHr pack (~3.5oz) or any other pack with a total capacity of 4200mAHr divided into as many cells as you like (e.g. a 3s1400 should work too). Obviously adjust the motor kV to give you an unloaded rpm in the 12-14 krpm range. I might go for a 9 inch prop to provide the thrust at a lower rpm than you would have with a higher revving IC engine. I believe a larger slower turning prop is more efficient, and helps keep the battery weight down.
Just my preference!
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Thank you very much for your prompt replies, gentlemen. It makes things much easier when you can give me some starting point from where I can begin experimenting. My .15 powered stunters have a wing area of approx. 280 sq. inches and .20 planes about 350.
T. Uolevi Heinonen
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The Sig Akromaster Profile is a 34" wingspan plane---I am not positive what the area is (it wasn't given). It is claimed to be ok for 15-19 sized engines, but I think those are "old style" baffled engines, not the newer Schneurle types. Like I said, I flew mine with an FP10 and had plenty of power.
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Hei,
Uolevi, the rule of thumb is that an airplane on the full length allowed in F2B consumes approximately 0.7 Watt-hours of energy per ounce of airframe weight in order to fly the F2B schedule. The equivalent figure is 0.025 W-h per gram of weight.
This is of course proportional to ready-to-fly airframe weight, and roughly proportional to line length. As a result, if you are going to fly your .15 sized plane on 16 or 17 meters of line (55') then the consumption may be 20% less.
Now what size battery? assuming an all-up weight of 30 ounces (850 gms) then you will consume 21 Watt-hours. Then I will assume a 3-cell LiPo that delivers 11 volts under load. Dividing the two we see that you will consume 1.9 Amp-hours. Approximately 33% more battery capacity is recommended in order to prevent over-discharge of the battery, so you would fly a 30-ounce plane on a 3S x 2500 mAh pack. A 4S x 2000 will of course do as well.
I must run right now, but there is more to come.
Dean P.
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what do you think of this motor
Turnigy C3530-1400
Input Voltage : 7.4V~11.1V (2~3S Li-po)
KV : 1400 rpm/V within 10%
Nb. of turns : 22 Delta
Max. efficiency current : 10~15A
Current capacity : 22A/60s
Internal Resistance : 76 mohm
No Load current @ 7V: 1.19A
Dimensions : 35mm X 30mm / 1.38in X 1.18in
Shaft diameter : 4mm / 0.16in
Weight : 77g / 2.72oz
Number of poles/magnets : 14
Recommended model weight : 600~1300g / 21~45oz
Power equivalent : .12,2stroke
http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=2141&Product_Name=TR_35-30B_1400kv_Brushless_Outrunner_(Eq_AXi_2808
I am building a sig akromaster just picked up the kit today as its raining. I am also unsure of how I want to mount the motor and battery. The wing area is 250 sq in.
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Sounds like it would be fine--especially if you had a 2s lipo turning in the 8-9k rpm range. For >10k rpm, then I agree the 3s might be a better way to go.