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Thread: Drones

  1. #21
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    Quote Originally Posted by Draco View Post
    One problem i don't think any one mentioned about the fuel cells is there discharge curve and the lack of a high current boost like you can get with batteries, if memory serves.
    Ideally you would have a small battery for bursts of power. The fuel cells would power normal flight and provide a small charging current to the battery. When you need extra power (such as for takeoff or sudden obstacle-avoidance maneuvers) you would draw that from the battery.

    one thing with brush-less motors is they are usually two or three phase so you would need a few more diodes and i would use a switching regulator
    You need a speed controller for each motor anyway. If you decide to use a brushless motor you simply use a brushless speed controller (it has 3 motor leads instead of just 2). So this is a wash.

    I honestly would keep it more simple and use batteries and have spares ready for swapping out
    To get 1 hour or more of flight time is going to require several kilograms of batteries. At that point a fuel cell becomes very attractive.

    Granted, some of the higher-end stuff is probably more expensive than the average hobbyist can afford (no idea what the Aeropak-1 I linked to above actually sells for, but I'm sure it's not cheap), but you can definitely cobble something together using smaller fuel cells designed to re-charge tablets and I-pads for a very attractive price. A quick Google search shows that these guys have the Brunton unit for $77.25, although it's out of stock at the moment. That Brunton unit delivers 5 volts at 2 amps, and a single fuel cartridge is good for 9 amp hours.

    So for a reasonably-sized quad, 10 of these units stacked together would deliver 100 watts of continuous power for 4.5 hours while weighing in at just 2.4 kilos (total weight). That's half the performance of the Aeropak-1 for a cost of less than $800. Sounds pretty affordable to me - especially compared to the cost of some of the nicer quads these days.

    The real question is this: How much power does the quad need for level flight, and how much excess lift capacity does it have?

    Adam

    EDIT - PS: I think the reason we haven't seen more of this sort of thing done is that the thought of piloting a quad for an hour or more is a daunting task. I get tired after just a few batteries (average time per battery is around 7 minutes). Trying to concentrate on flying for 60, 90, or 120 minutes sounds really hard. And if you're talking about flying via first-person-video, that's even more challenging.

    2nd edit:
    Quote Originally Posted by planters View Post
    I love the idea of the fuel cell, but it is such an obvious choice that it must be prohibitively expensive, otherwise we would be seeing this occasionally.
    And yet no one else suggested it here in this thread. Perhaps it's not quite so obvious? I'd at least do the math on it before you dismiss the idea out of hand.
    Last edited by buffo; 04-14-2015 at 14:31.

  2. #22
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    I am going to need to order a real fuel cell to play with, i did not realize the price had fallen to below a 100 bucks for something that has usefull power.

    I did not know you could use the motor controller backwards like that, i know it works for cooling fans, with the medical stuff i did in the past people would use an air gun on fans hooked to product, i made a procedure banning that as they could generate a fairly high voltage, way more then there normal operating voltage.

    Thanks for the info!
    Remember Remember The 8th of November, When No One Stood, but Kneel, In Surrender
    In a popular government when the laws have ceased to be executed, as this can come only from the corruption of the republic, the state is already lost. Montesquieu

  3. #23
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    Draco:

    I don't think you can use a speed controller as a voltage regulator to turn the motor into a generator. I simply meant that if you plan to run brushless motors, they need to be run off a speed controller that is designed to power a brushless motor (and thus will have 3 leads instead of 2 going to the motor). The speed controller will still have a power in connection (to draw DC power from the battery) as well as a control input connection (to receive speed setting information from the radio).

    I'm actually considering ordering one of the Brunton units myself, when they come back in stock. I currently have a 4500 mAh cell phone battery pack charger that I use for trips, but even though it's rated for a full 2 amps, it will not re-charge my Nook tablet. Thinking that a fuel cell might be the ticket, and at twice the capacity for nearly the same price I paid for my charger, it looks like a good deal. Though I will probably need to buy the hydrolyser as well so I can re-fill the tanks, and it's around $220 by itself.

    Adam

  4. #24
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    Quote Originally Posted by Draco View Post
    I did not know you could use the motor controller backwards like that, i know it works for cooling fans, with the medical stuff i did in the past people would use an air gun on fans hooked to product, i made a procedure banning that as they could generate a fairly high voltage, way more then there normal operating voltage.
    You can't use an ESC backwards. It is an output device with power
    going in only one direction... to the motor.

    Quote Originally Posted by planters View Post
    What are these low loss active rectifiers? What do they do? Do they regulate voltage as well as polarity?!

    Adam the 4 stroke engine I'm talking about would be a small hobby RC motor like:

    http://www.hobbyking.com/hobbyking/s...ow_Engine.html

    There are certainly better choices depending on the power requirements and I bet I could increase the power to weight ratio but as I start adding up the numbers, I think an all up weight of under 1Kg is reasonable
    That HK engine's specs states a weight of 455 grams. It is a Glow engine
    and no doubt runs on nitro-methane Glow Fuel.

    I just weighed a 5000mAh 3S (11.1V) battery and it weighs 415 grams.
    I still think that adding more battery capacity would be the way to go.
    It's a lot less mechanically complicated and requires no additional electronics.
    Of course that would all depend on the size of the multirotor. Definitely not
    doable on a 200 size machine.

    Good food for though just the same.


    Jerry
    Last edited by lasersbee; 04-15-2015 at 03:10. Reason: added info
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  5. #25
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    The hydrogen fuel cell seems to be limited by a low power as in 2W for the Brunton units. I think it might be straight forward to hack the fuel modules to a substantially larger volume. But I suspect the catalyst area is the rate limiting bottleneck. As a rule of thumb I think an alternative power source needs to be in the hundreds of watts range to have a significant contribution during a typical flight time. I now see that the gas generator suffers from the opposite problem. It only becomes competitive in the several thousand watt range. Here 1HP/lb for the motor and 2HP/lb for the generator would give a module at around 5 lb for 3 HP which should generate a realistic 1,500 W. Even in a high rotor loading condition with 6gm/W that translates to 9Kg of lift and with 3 lb of fuel, this results in a net lift of 12 lb for 2 hours.

    Practically, if the gas motor was unregulated and just throttled to around 3/4 power would the varying output voltages, lets say 10-13V, be acceptable to the controller and allow it to maintain steady output to the electric motors?

  6. #26
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    One possible way to regulate the voltage is a tight control on the engine rpm, the control method on a car's alternator uses a second voltage to control the strength of the magnetic field in the alternator, you can but small 2 stroke gas generators for camping but there power is only a few hundred watts in the one unit i saw, it used an engine from a weed trimmer.

    Thanks for the clarification on the speed controllers, the brush-less versions I have seen are 3 phase.
    Remember Remember The 8th of November, When No One Stood, but Kneel, In Surrender
    In a popular government when the laws have ceased to be executed, as this can come only from the corruption of the republic, the state is already lost. Montesquieu

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