Which electro motor to use for steering a kite? (guestimation marathon)

I'm doing a pet project in which I want to fly a kite from my computer. For this to happen my plan is to build a little box hanging under the kite which includes a raspberry pi (connected to my laptop by wifi), a battery, a motor controller and an electromotor to pull the cords. I am now in the quest of finding a suitable motor.

In reading around and asking previous questions here on stackexchange I found I would need a basic DC motor including a transmission because they have a high torque-to-weight ratio and are easy to control. Furthermore, I will attach a little wheel (diameter == 3cm) to the motor so that the rope can be hung around it and the motor its force can be translated to rope motion.

Some requirements:

== Minimum Revs/Minute ==

I want to be able to change the rope length about 40cm in 2 seconds. Since the wheel has a circumference of ≈ 9.4cm (π * 3), it has to be able to have at least 4.24 revs per 2s or 2.12 revs per second, or 127 RPM.

== Minimum Torque in Kg/cm ==

I haven't got the kite as of yet, but I want to fly a "very small kite-surf kite" in "moderate wind". This means that at full force it will be able to pull me off my position, but not lift me in the air. So I estimate the maximum rope pull at about 50Kilograms, divided by 4 ropes equals 12.5Kilogram/rope. Since I line the two steering ropes in a loop, pulling one will release the other. This means that the rope being released "helps" pulling the other rope in. Seeing that the rope pulling force will vary between the two ropes and I also have to overcome inertia I guestimate the power needed to adjust the rope lengths at 40% of the total force, or 5 kilograms. Since we're using a wheel with a radius of 1.5cm (diameter == 3cm) we take 5 kilograms times 1.5 which results in a minimum needed torque of 7.5Kg/cm.

So I searched around on websites and found the motors below. All sites are in Dutch, so I just translated the info below.

1. https://iprototype.nl/products/robotics/servo-motors/metal-gearmotor-50-1

• Free spinning: 200 RPM, 300mA, 12 kg-cm
• Average force: 1A
• Full force: 5A

• RPM on force 104 RPM
• Max. torque 9 kg/cm · permanent 3 kg/cm
• Free spinning at 1,5 V 120 RPM (why 1.5 volt if its a 12V motor?!?)
• Max. Current 2.1 A

• Max. torque 0.6 Nm (doesn't this equal 0.06 Kg/cm?)
• free spinning at 1,5 V 110 RPM
• Continuous RPM: 85 RPM
• free spinning current: 0,10 A

The problem I have now is that the information is so different (RPM in free spinning vs. with force attached to the motor) and varies so much (12 vs 0.06kg/cm?!?) that I'm kind of unsure of my whole train of thought.

So my question: does it make any sense what I'm writing here, or do I make mistake upon mistake upon mistake? What advice would you give me here? Is the first motor clearly the best, or am I overlooking things?

All tips are welcome!

• I've seen kites controlled by a hobby servo motor, pulling a yoke attached to the steering lines to left or right for steering. Not sure if using a regular DC motor would provide sufficient repeatability for return-to-center after steering either way. Commented Apr 3, 2013 at 11:26
• @kramer65 - there is a uk site for conrad: conrad-uk.com/ce/en/product/244163/150-type-20G-gearb-motor - this is the 3rd motor and it doesn't mention the 1,5V on free -spinning. Maybe it's not important? Maybe it's just the back-emf acting like a generator? Commented Apr 3, 2013 at 11:53
• @AnindoGhosh - I will indeed have to fully control the motor to keep it going to the center (if needed). I don't think a servo would suffice though, since I need to make quite a large movement (40cm) Commented Apr 3, 2013 at 12:01
• @Andyaka - Thanks for that link. I didn't know conrad was international. So that 1.5V could be an error as well. For the rest, do you think my calculations and reasonings are somewhat valid? Commented Apr 3, 2013 at 12:03
• @kramer65 The servo would not need to make a 40 cm movement - it would be at the fulcrum of a yoke, with the ends of the yoke moving the required 40 cm (or whatever) by translating the rotational motion of the servo arm. For an angular motion of +/- 15 degrees, for instance, the required linear motion at the ends of the yoke arm can be used to calculate the required yoke length. Commented Apr 3, 2013 at 12:05