I want to design an octocopter that will respond to incidences of crime and terrorist attacks. Survey and gather intel and come back to HQ
I want this Octocopter to carry radioactive/chemical testing equipment and have a flight time of 45 mins
How likely am i to achieve 45min flight times?
There must be a way of optimizing flight time. I know i can double the amount of batteries but this may make the craft unstable I may need to scale up on motor size to make payload less of an issue..
The larger the craft the more dangerous and costly the craft. So i'm really interested in optimizing it the best i can.
I know there are hydrogen fuel cells, maybe some other sort of alternative battery.
Please help thanks
Replies
John Fitter said:
I have attached a document I wrote last year which should answer the questions you have. It is a pdf version of a ppt presentation so it doesn't have all of the commentary. Essentially a modelling program was written which designs an octocopter based on standard engineering principles. Mechanical design accounts for arm resonance and other structural effects. The result is a practical airworthy copter for each particular input data set. The program has been run for a sequence of combinations of batteries, motors, payloads, etc. For every data point a new, optimized copter design is created. The document shows the results of the modelling in graphical form in addition to discussing a whole lot of other stuff including power sources and their practicality.
The presentation was prepared for a technical audience who know very little about the subject so it should be quite self explanatory. Subsequent to this presentation the research has moved away from the general topic of the paper, copter endurance, and is now much more focused on getting the radar working. We will return to the endurance problem when the radar development is progressed enough to do practical field surveys. It's a curly problem and looks like it's going to take quite a while.
In answer to your questions, look at the comments associated with each of the graphs. Bottom line: endurance is energy storage technology limited. The best motors and lightest airframes and no payload will be limited simply by the energy source you have. In fact you can model the copter for zero airframe weight, zero motor weight, and zero payload and you get a finite limit to your endurance. In this case the only item having any weight is the energy source.
Note here: Do not believe anything published by commercial vendors about endurance or the value of very expensive motors. Better motors are more reliable but the performance gains are marginal (see the graphs). Published endurance figures are designed to sell copters and are generally not realizable. Hydrogen fuel cells are not ready yet for VTOL aircraft. Hybrids (PMFC+LiIon) are being used on aeroplanes successfully but practical VTOL survey vehicles are, at present, a dream. Battery technology (that you and I can get our hands on) is currently limited to 350Wh/kg and that is your tech limit to endurance. If you want more you will have to wait until it is invented.
To put this in perspective, a gasoline engine uses an energy source of around 12,000Wh/kg, and even accounting for it's enormous inefficiency, you are still way ahead of the game.
I hope this helps.
Cheers, JohnF
RadarCopter document
Nothing hard about it DJI products are not suited for this type of work! DJI has nothing that will carry a payload & fly for 45min DJI radio communications are not secure and susceptible to harking! professional solution 90min flight duration VOLT all electric 46k USD.
Diven Parker said:
45 min flight time is too hard, I haven't flight octocopter, but I know DJI S1000 with Tattu 16000mAh 6S Lipo battery no load only can flight about 20mins, but you said you want this Octocopter to carry radioactive/chemical testing equipment and have a flight time of 45 mins, I think it is toohard.
actually VOLT air carat offer the best endurance but your looking at 50k minimum for commercial model !
1, You want a helicopter, 45 min is easy on that platform.
2. If you are stuck using a multicopter, reduce weight. This is by far the biggest factor for flight times.
@ Kurt
Believe it or not, we are using turnigy multistar batteries in series and parallel to achieve 48Ah @ 8S voltage (x6 16000 mah). AUW is 54lbs to stay within Part 107. The craft hovers at approx. 90 amps, and the current draw seems to stay the same as long as the flight speed stays under 10 mph (~5m/s). The batteries are used with a fairly low amp draw, ~1.8C, I would like to make a liion pack for this vehicle to really stretch the flight times.
And yes, we are using a standard Pixhawk with a custom vibration mount (I had to make a tuned mass damper due to the high amplitude, low frequency vibrations that were effecting the flight controller, even though the logs showed that vibrations were within acceptable levels).
Your flight times sound similar to what I was seeing on the first octocopter I built, a tarot T18 kit. I built this beast to address the shortcomings that the tarot had. Please feel free to drop me a line if you have any more questions, I would be happy to help.
@Subatomic
I looked at the Tmotor U8 when in the design stage of this bird, and decided to go KDE for a couple of reasons. The main reason was I feel that the KDE motors are better built. The Tmotor units are very nice, but KDE has a better bearing design, and cleaner windings and potting in my opinion. I also like the fact that KDE serializes and tracks their parts. Coming from an engineering background, I appreciate parts traceability.
As for the efficiency, I believe that the Tmotors are slightly more efficient. However, the KDE units are very good as well, and the KDE motors have more torque than the U8s, which is important on a multirotor that is constantly changing rotor speed to maintain control of the vehicle. The ability to change speed rapidly was more important to me than absolute efficiency on this unit, seeing that we are swinging 30" props.
Lastly, I have flown this vehicle with a small camera, gimbal, and video downlink payload and i have been getting flight times right around 43 minutes. So if Rafi's payload is relativly light (~2lbs) then I think this platform could carry his payload for 45 minutes. Also, Rafi could simply add more batteries as well, the vehicle only hovers at 42% throttle at 55lb AUW.
You can see some videos of the machine in flight below
https://www.youtube.com/watch?v=pXaGcBko-nI&t=41s
https://www.youtube.com/watch?v=UOmEpnU0YPA
Thanks
Brian, did you ever compare the T-Motor U series to the KDE motors in terms of efficiency during your testing/development? Rumor has it that the KDE test results on their website are over-rated - just wondering what real world testing shows.
Also, if Rafi's payload is no where near 20lbs, then Brian's setup might just reach 45 mins, no?
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