How to get longest flight time on octocopter that has decent payload already on it?

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

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Post deleted due to entirety being in violation of the terms of service. Robust debate is fine but personal attacks are not accepted. Thanks.
JohnF post is bull & do not take in to account VOLT gass electc craft able to fly for 6-7 hours!

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

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