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.

Specs:

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

(9A-14.5A)

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

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/

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 14oz.........my 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 mph.....is 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????

TIA

Mike

- posted 17 years ago