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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. 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 //////////////////////////////////////////////////////////////////////////// / 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

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

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Reply to
KreAture

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

Reply to
MK

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