A theoretical X8 copter question

For hobbyists who have experience building electric copters in an X4 or X8 configurations, I would have a question for you.

I assume that most hobbyists want to build their UAV as 1) powerful, 2) lighter and 3) cheaper as possible. But let’s say that money is not an issue and you are building your X8 copter with commercial parts; is it possible to build an x8 copter of about 35 kg (including the payload) with a flight time of about 90 minutes (with lots of batteries I assume)?

Some of you might say that, if money is not an issue, everything is possible. But there is a point where, despite batteries of high performance and powerful motors, the weight is too much to make it fly with a good performance.

In your gut’s feelings, is it a feasible project? 

Thank you very much for your help.

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Replies

  • Another way to do something like this would be to use gas engines. There is a racing quad that uses one gas engine and four variable pitch props. That is probably something that would scale up nicely. The problem with scaling up a normal quad or x8 is the momentum of the props makes larger props hard to vary, so the craft becomes really slow to respond. At this point you are talking about bolting four helicopter main rotors together, so you. Ought as well just make one big helicopter, or rather, one tiny full scale helicopter.

    I think the point of quads and hexes gets lost when people start building them to carry any more than a basic SLR. Helicopters and airplanes become more effective at doing most tasks of any size above that.
  • Not currently possible, I think.  The world record set last month for a quad was 97 minutes with no payload.  They are hoping for 90minutes with a GoPro as the payload in the near future.  That's a machine that probably weighs a lot less than 3kg. 

  • Thank you for your input. I think that an APU could be a nice solution to my power problem. I have looked on the web and there is some work about “Micro Gas Turbine Generator”. Some are a work in progress but they are light, small (less than 5 sq. cm) and capable to produce around 1 kw. Do you know a manufacturer for that kind of device? 

  • Moderator

    And in my gut feeling, all I can tell you is that I do not know what power source could drive this, for two minutes, let alone 90 minutes, and I don't know the motors or props to use. If faced with a project like this, I would look wide and far for materials that I do not currently know about, and I would really be looking hard at other designs, like variable pitch, and gas engines. And I would also look at helicopters, since I think the parts are more readily had for them in this scale of aircraft than for an X8, and because the power loss due to inefficiencies is a lot, and inefficiencies make getting to 90 minutes so much harder...

  • Moderator

    A theoretical answer for a theoretical question. I know you wanted more experience, but I have only built five much smaller multicopters, so I can only offer the math, I have never built anything of this size. 

    My math suggests that for an X8 to meet this requirement, it would need to have a motor/prop combo that generates thrust between min 6Kg and max 13Kg for each motor. The more efficiently the motor is at 6Kg the better battery you get for hover,  but if you will be flying at speed, you'll want to push the most efficient point up towards the 13Kg or somewhere in the middle.

    I don't know that market of prop/motors well, but I suspect that electric motors in that range need a lot of power, and a good sized prop. I could speculate more, but it would only be guessing. For example, I think perhaps a 13x6.5-8 prop driven at 12000 RPM (if you could make one that did not break at these speeds/angle) might provide the minimum thrust to hover and move around a bit. Bigger is better. But I am only speculating, I have no experience at building at this size.

  • Some things must then be designed a bit different. Most likely a X4 or Tri concept is more realistic. I guess - that this size will require already pitch control instead of variable speed. I'm not sure if APM is able to handle that size, but I've no experience here.

    I personally head for an tri/winged solution.

    -/Tobias   

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