Help in prop selection, stall speed/ prop speed and opinions on power loading

I am going to try lipo/brushless, and as a start I want to upgrade
my Great Planes EP Piper Cub. One reason is because the plane is over weight
with the stock setup......17.5oz balanced at recomended CG.......the
suggested ready to fly weight is 14oz.
Wingspan: 41"
Wing Area: 269 sq"
Weight: 14oz RTF
Wing Loading: 7.5 oz/sq ft
Length: 29.5"
Motor t-280 (maubuchi) with 3.5:1 gear box
Battery (supplied with plane) 8 cell 600mAh 9.6V NiMh pack
Prop: 10 by 7 APC
A review article on the Cub said it flew fine at the increase in
weight and higher wing load. Before giving this bird its maiden flight I
tested the motor and toasted it. Instead of replacing the motor for another
brushed 280 I decided to try out a brushless for efficiency and lipos to
bring the weight of the model down. To start selecting a new power plant I
figured I needed know what the stock one put out so I ran the calculation on
motor calac program on a r/c web site.
The results:
sp 280
3.5:1 gearing
10 by 7 prop
8 cell 2/3AA 600 mAh NiMh battery
Watts in: 36.65
Watts out: 21.09
Current: 3.8 A
Efficiency: 57.5%
Watts in at M.E.: 17.19
Watts out at M.E.: 11.79
Current at M.E.: 1.75
Maxx Efficiency: 68.6%
Prop RPM: 4132
Static Thrust: 11.7oz
In flight thrust: 6.5oz
Model RTF Weight: 17.5oz
Flight times (probably at 14 oz) 9 min
After a $200.00 shopping spree at the LHS for a lipo charger, lipo
battery, esc, and brushless motor ( this is like starting the hobby all
over) I installed the gear and jury rigged the motor in place I checked my
models weight...........13 1/2oz....balanced at recomended CG. I ran a new
calculation for my new motor.
Battery to be used: Thunder Power 2 cell lipo 900 mAh 10C cont/ 16C Burst
Specs for the motor: Brushless E-Flite Park 370 Outrunner
KV: 1080
IO: .7A
RI: .19 ohms
KT: 1.275
Continuous Current: 7A
Max Burst Current: 10A
Cells: 6-10 NiCd/NiMh or 2 to 3 cell lipos
Weight: 1.6 oz
Recomended Props: 9 by 4.7 to 10 by 4.7 Will try a 9 by 4.5 and used
it for the calculations
Watts in: 48.04
Watts out: 34.61
Current: 6.634 A
Efficiency: 72.044%
Watts in at M.E.: 38.14
Watts out at M.E.: 27.87
Current at M.E.: 5.2 A
Maxx Efficiency: 73.07%
Prop RPM: 6459
Static Thrust: 13.4oz
In flight thrust: 10.4oz
Model RTF Weight: 13 1/2oz
Flight times: 8 min
Big Question.....Why do some people suggest I run 3cell lipos ( some said
2 cells are fine) when the manufacture finds that 11.7 at the prop is good
to fly this model at setup, 27.87........a 3cell lipo pack
54 watts at the prop???? Do I REALLY need this much????
Next question: Props, 9 by 4.5 or a 8 by 5 or 8 by 6 ????? 9 by 4.5 the
prop speed is 27 mph the 8's at 40 there a rule of thumb or
formular for how much prop speed you need in relation to the models stall
speed?????? OBTW what is my models stall speed????? What would be a good
choice for this plane to maintain scale like flying????
Reply to
Mike R
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Going for 2.5 to 3x the models stallspeed has always served me well. Using about 50w/lb input power for minimum good flying and more to increase the fun.
As for 3-cell LiPo's the ansver is purely electrical. By adding 50% more weight to the pack, you can get 50% more power out at the same max load on pack. This would be done with higher voltage and thus almost no extra losses in the motor's copper. (Assuming you reduce the prop so you pull same current as before ofcource.) The higher rpm will cause more losses in iron and in gearbox etc, but theese losses are linear, the current losses are exponential.
The copper-losses in a motor is simply current squared (I^2) times resistance (R). (I^2 * R) The power in is current (I) times voltage (V). (I*V) Thus increasing I means losses increase based on I-squared, while increasing V means losses increase based on V.
The simple ansver: More voltage with same current can give you more power and run the motor at higher effichiency than if you used bigger cells (same voltage) and increased the current.
I would set that plane up for about 10 amps at full throttle (as that is what the motor is specced for on burst) and since you already have the 2s pack you are thus limited to around 74 watts. This should fly that model more than adequately no matter what you do to it. You should only use full throttle for short bursts or not at all, but this setup ensures that you have the full range of the motor available should you choose to utilize it.
It will provide a much more enjoyable flight experience when you have the entire range available like that.
Here's a video of a 8oz, 23" wingspan plane with a 60w brushless cdrom sized motor on:
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That one is a bit too intense hehe due to the small size and high flyingspeed, but the power available is lovely.
Kyrre Aalerud
Reply to
Here's a video of a 8oz, 23" wingspan plane with a 60w brushless cdrom sized
Pretty cool. All the trees made me nervous. mk
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