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  1. #1
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    Tutorial on design considerations and effects?

    Completely new here, and with zero experience with RC or electronics. I'll be building a quad with my son and really need tips and advice from you all. I've bought the FC and the frame, but don't know how to choose the rest. My first questions are related to selecting appropriate components (motors, escs, props, battery)

    There must be some guiding philosophy that helps people choose. I assume the total quad weight and max flying time are the two big constraints. Unfortunately I do not know how all the other factors come into play and how they are related to each other. I am quite surprised that I have not yet found a clear tutorial or resource explaining the philosophy behind choosing electrical components based on how they affect or are affected by weight and endurance.

    For example, with all other things being unchanged, what are the effects (and knock-on effects) of each of the following?

    - adding more motors
    - using more powerful motors
    - using larger props
    - using props with steeper pitch
    - using several batteries in parallel
    - etc

    I would dearly love to see a simple tutorial that mentions the basic physics and logic behind these things. I really want to learn, but not by buying the wrong thing and crashing or realizing too late that I get 3 minutes in the air because I chose the wrong components. My son is 9 and I'm afraid he'll lose interest if this process becomes an agonizing series of trial and error. We want to learn!

    Thanks everyone.

  2. #2
    Senior Pilot
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    A lot of this stuff _is_ trial and error, but you can get a few pretty good starting points from reading around. A good starting point for a quad is;


    • 20-24" motor-to-motor distance (measured across the center of the quad)
    • 10" propellers. 6" pitch will work (that's what I use), but 4.5" pitch is better.
    • 1050kv or so motors. When you read about motors, they will recommend a propeller size. Get motors that are suitable for 10" props. Turnigy 2217-16's are suitable (and what I use).
    • A 3S 4000mAh 20C battery. Higher C values will increase weight, but provide higher peak current draws. You need high C ratings because the current drain of the motors can be very high (~20A each). Note that the battery is 3 cells in series. You need to pick motors and ESC's that are suitable for 3S batteries.
    • Suitable ESCs. Standard hobby ESCs (Turnigy Plush etc) are acceptable. Make sure you pick one that can supply enough current + some overhead for what the motor may draw. I used Turnigy Plush 25A ESCs.
    • An airframe. You can make an airframe from some aluminium tube. Make it robust.
    • Some really hefty copper cabling to make the main power trunk. I used 12 gauge cable.


    That covers the airframe and power systems. You can find more info on this website regarding flight control systems.

    As for your questions, wider props can provide more thrust, but have more angular momentum and thus can't change their thrust as quickly as smaller props. Props with more pitch generate more thrust, but because their thrust to rotation rate ratio is higher, they can fly more unstably. Adding more motors can lead to a more stable quad and also higher lift potential. More powerful motors can lead to more thrust, assuming that they are suitably mated to your props. More powerful motors will usually require bigger props.

    Oh, one thing. Keep in mind a running quad is actually a quite dangerous piece of equipment. It's a flying brushcutter, and can leave quite serious injuries on an adult. On a child with small fingers, I'd be worried about amputations. Just keep that in mind.

  3. #3
    Senior Pilot
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    Addendum: As long as you aim for a thrust:weight ratio of around 2:1 or slightly less, you'll wind up with a quad that can take off. You need slightly more than 1:1 so it can a) ascend, and b) maneuver. Keep in mind that in forward flight, some of the thrust is used to generate the horizontal component of the movement as well as the vertical component, so if you had say, 1.1:1 thrust ratio, you would only be able to commit a small amount of thrust to actually maneuvering and most of it would go to holding the thing up.

    By the same token, if you go too hard on thrust (say, 4:1), you'll wind up with a quad that is very hard to control when you're beginning. As long as you have enough thrust to lift it and move it, I'd argue for a new pilot it's more important to have controllability than thrust. In that vein, go for a larger quad (20-24" M2M) with larger slow-flyer props (10x4.5). You'll have enough thrust to lift off and fly around, and it'll be controllable.

  4. #4
    Senior Pilot wooden's Avatar
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    Darkwolf covered pretty much everything but I think it's important to reinforce the dangerous nature of quads. These things are essentially four rapidly rotating blunt knives and can cause some serious damage. There are threads on other websites of deep gashes requiring stitches, and we can't understate how vital it is to be very cautious around quads. That being said, I'm sure you'll take the necessary precautions to protect yourself and your son.

    Anyways, single motor max thrust * 4 >= ready to fly weight of quad * 2. As Darkwolf said, going too high in this ratio will make throttle control touchy and difficult to control for a beginner. Keeping combined max thrust to weight*2 ensures you'll hover around 50% throttle, which leaves plenty of room for maneuvering.

    ESC amperage > max current draw of motor by an amp or two at least

    Using more powerful motors will give you more thrust but they'll probably also require a heavier battery (higher discharge rate or more cells in series for more voltage).

    Using more motors will give you more thrust but the more motors you add, the more inefficient the system becomes as each motor is a source of lost energy through heat, etc. In this sense, adding motors is only necessary if you need the added thrust for carrying a heavy camera and gimbal.

    Using several batteries in parallel will not add thrust, it will simply increase your total capacity. If you do use batteries in parallel, make sure they're of all the same ratings and are at similar charge levels. Adding cells in series will increase thrust (a motor has a given kV, or RPM per volt, so more volts = more RPM) but keep things within the defined limits of your motor. Most motors are designed for 2-4 cells in series.

    If you're trying to decide on a setup, an easy way is to just decide on a prop size. I've been using 8" on a 20" quad and it works great. 10" worked too but I'm looking for more maneuverability than stability, and 8" props have less inertia. Once you pick a size of prop, search for motors that give a lot of thrust at a low current, and then find ESCs to fit those. Finally, choose a battery that can supply enough current. To calculate this, multiply the C rating by the capacity in amps, not mA. (4000mAh 20C battery would be 4*20 = 80A). The example battery would be suitable for four motors that draw a maximum of 20A each, or 6 that draw 13.33A each, etc.
    Last edited by wooden; 03-28-2012 at 06:54 AM.

  5. #5
    Flight Cadet
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    Sorry about the long delay ... Thanks guys for the feedback. I've done a lot more reading based on your tips and have definitely seen that desired thrust is the key here. All other factors can be derived once that one is determined.

    Happy flying (and/or building)!


 

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