tIANci

I need some help guys ... I got a Balsa USA Enforcer ready and want to go EP. From what I know the plane is decently fast with something like a TT61Pro turning a 11x7 pusher at about 13,000 RPM. My estimated AUW is about 5.5 lbs.

Need to know the following:

i. What power plant can I get to achieve such a performance? Might an AXI 4130 be able to do it?

ii. Am I better off with an Inrunner with gearbox or Outrunner?

ii. Does an EP prop produce as much thrust as a glow prop of the same diameter and pitch?

Thanks for your input!

Wazmo

At 5.5 pounds, you probably want to target around 500-600 Watts. An AXI 4130 should handle that well, perhaps even a 4120.

An inrunner with a gearbox will generally be more efficient (longer run-time) but high-power gearboxes have to be set up just right and you must check them regularly. There's always the risk of a pinion slipping or spur teeth grinding down, which will lead to a sudden loss of power. An outrunner will tend to be less efficient, but it's much simpler and easier to maintain.

Electric works a bit differently than glow. Generally, glow needs a prop that allows the engine to run at higher RPM where the it develops the most power, while EP has a much flatter power curve and spins a larger diameter prop at lower RPM more efficiently with pitch speeds closer to the airplane's flying speed. If you target 100 Watts per pound and prop the motor to match the target current, you'll get the thrust you need. As always, prop pitch can be chosen to balance thrust versus top speed.

tIANci

Wazmo - thanks for the info. What sort of a prop would give me good speed to start out with? A 14x10? The 14x10 will give about almost 35-40A on a 6S.

Wazmo

It depends on which motor and how many winds (KV). The higher KV (fewer winds, IIRC) will tend to draw more current with the same load and are used more for higher RPM, lower load applications. Lower KV motors (more winds) will draw less current with the same load and are used where lower RPM is required, with a higher load. If you're looking for speed, choose a higher KV motor with a smaller diameter, higher pitch prop. Keep in mind that more speed will cost you static thrust and therefore low-end acceleration. I'll try to run some numbers on MotoCalc tonight to get you something to work from.

40 amps on 6S (21V loaded) is about 840 Watts, which would be quite a bit more than you need, about 150 Watts/pound, which is approaching 3D/hover power levels. Keep in mind that excess power also means excess weight, which reduces effective power and efficiency.

Wazmo

Not sure how fast you want to go, but these are the highest pitch speeds I could find within reason. These configs are probably pushing the motor hard, so proper cooling would be critical. I used LiPo since they'll have longer run-time and much better power-to-weight, but NiMH would be a lot cheaper. These numbers are just an estimate and your mileage may vary.

Motor: Model Motors AXI AC4120/14; 650rpm/V; 2A no-load; 0.041 Ohms.
Battery: Thunder Power TP2100; 7 series x 2 parallel cells; 2100mAh @ 3.7V; 0.022 Ohms/cell.
Drive System: Generic 9x7in Prop; 9x7 (Pconst=1.25; Tconst=0.956) direct drive.
Airframe: Balsa USA Enforcer; 1080sq.in; 97.1oz; 12.9oz/sq.ft; Cd=0.049; Cl=0.29; Clopt=0.43; Clmax=1.11.
Filter: 45A max; 0.8 max pitch:diam.
Stats: 114 W/lb in; 99 W/lb out; 18mph stall; 29mph opt @ 54% (42:02, 121°F); 35mph level @ 59% (34:23, 126°F); 1050ft/min @ 24.4°; -381ft/min @ -8.6°.
Conditions: 400ft above Sea Level, 29.92inHg, 80°F

AirSpd (mph) = 56.0
Drag (oz) = 42.0
Lift (oz) = 250.3
Batt Amps = 28.8
Motor Amps = 28.8
Motor Volts = 23.5
Input (W) = 677.4
Loss (W) = 89.2
MGbOut (W) = 588.2
MotGb Ef(%) = 86.8
Shaft Ef(%) = 78.8
Prop RPM = 14258
Thrust (oz) = 42.4
PSpd (mph) = 38.5
Prop Ef(%) = 50.1
Total Ef(%) = 39.5
Time (m:s) = 8:45

[hr]
Motor: Model Motors AXI AC4130/16; 390rpm/V; 1.8A no-load; 0.063 Ohms.
Battery: Thunder Power TP2100; 10 series x 2 parallel cells; 2100mAh @ 3.7V; 0.022 Ohms/cell.
Drive System: Generic 10x8in Prop; 10x8 (Pconst=1.25; Tconst=0.956) direct drive.
Airframe: Balsa USA Enforcer; 1080sq.in; 109oz; 14.5oz/sq.ft; Cd=0.049; Cl=0.29; Clopt=0.43; Clmax=1.11.
Filter: 0.8 max pitch:diam.
Stats: 122 W/lb in; 106 W/lb out; 19mph stall; 31mph opt @ 53% (54:23, 122°F); 37mph level @ 58% (44:40, 127°F); 1294ft/min @ 28.7°; -404ft/min @ -8.6°.
Conditions: 400ft above Sea Level, 29.92inHg, 80°F

AirSpd (mph) = 61.0
Drag (oz) = 49.8
Lift (oz) = 297.0
Batt Amps = 23.3
Motor Amps = 23.3
Motor Volts = 34.3
Input (W) = 799.0
Loss (W) = 103.6
MGbOut (W) = 695.4
MotGb Ef(%) = 87.0
Shaft Ef(%) = 80.7
Prop RPM = 12617
Thrust (oz) = 50.6
PSpd (mph) = 34.6
Prop Ef(%) = 55.0
Total Ef(%) = 44.4
Time (m:s) = 10:49

Wazmo

A 5320 makes things a bit more exciting (72 MPH and 3091 ft/min climb).

Motor: Model Motors AXI AC5320/28; 249rpm/V; 1.5A no-load; 0.057 Ohms.
Battery: Thunder Power TP2100; 10 series x 2 parallel cells; 2100mAh @ 3.7V; 0.022 Ohms/cell.
Drive System: Generic Prop; 15x12 (Pconst=1.25; Tconst=0.956) direct drive.
Airframe: Balsa USA Enforcer; 1080sq.in; 112oz; 14.9oz/sq.ft; Cd=0.049; Cl=0.29; Clopt=0.43; Clmax=1.11.
Filter: 0.8 max pitch:diam.
Stats: 179 W/lb in; 155 W/lb out; 19mph stall; 31mph opt @ 42% (72:51, 105°F); 37mph level @ 48% (55:24, 109°F); 3091ft/min @ 90°; -409ft/min @ -8.6°.
Conditions: 400ft above Sea Level, 29.92inHg, 80°F

AirSpd (mph) = 72.0
Drag (oz) = 69.4
Lift (oz) = 413.8
Batt Amps = 31.3
Motor Amps = 31.3
Motor Volts = 33.4
Input (W) = 1045.4
Loss (W) = 121.2
MGbOut (W) = 924.2
MotGb Ef(%) = 88.4
Shaft Ef(%) = 79.7
Prop RPM = 7714
Thrust (oz) = 72.7
PSpd (mph) = 15.7
Prop Ef(%) = 70.3
Total Ef(%) = 56.0
Time (m:s) = 8:03

tIANci

Wazmo ... thanks a lot. I think I will probably go with the 4130/16 on 6S2P 4,500 mAh turning a 12" prop. Also the Hyperion motors are very tempting, may wanna try that.

91zulu

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What about an electric jet set up for the Enforcer.. what will I need to convert?