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 have used a 16000mAh 6S lipo battery with no lead, but only can flight 20mins. 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
Actually the constraints you set of using an Octorotor is already impeding the autonomy of the system. Indeed multi-rotor platforms are a trade-off between on one hand the ability to over and a simple dynamics of the system and on the other hand the flight efficiency and thus higher payload. My answer to your question
How likely am i to achieve 45min flight times?
is that I think it is very unlikely if you platform weights around 1.5kg and you want to carry around 1kg of payload.
To your question
How to get longest flight time on octocopter ?
lest first review your strategies:
I can double the amount of batteries but this may make the craft unstable
Double the battery is just embedding more energy on board (i.e. reducing the payload) but should not bring you platform to instability as long as your flight controller knows the mass and center of mass of the platform. Note that the more you increase the payload the closer you get to the actuation limits of you motor+propeller, what can happen is then that you use almost all your control authority for weight compensation and that nothing is left to allow attitude control, resulting in unstable behavior.
I may need to scale up on motor size to make payload less of an issue
Scaling up the motor alone is a partial solution. As the thrust is produced by the combination motor+propeller, you can increase the thrust by tunning your motor+propeller combination. As noted by @hauptmech, propeller shape can be optimized to improvement the performances, but this is not trivial due to the aerodynamic at play and the interferences between the different propeller, the relative wind and so on. Note that increasing motor "size" will most likely lead in an increase in current consumption and so you might need to change your ESCs, also it will drain more power so you will need more battery and we are back to the same issue as described above.
I know there are hydrogen fuel cells, maybe some other sort of alternative battery
Hydrogen fuel cells is very new for multi-rotor and seems to not be super mature technologies, so it comes with potentially high development costs. The idea fuel cells is that the power density of battery is rather low, another approach in the same direction is to have a gasoline motor and an alternator, but then even if you store more energy in the tank the weight of the full machinery doesn't scale for typical multi-rotor size.
Lastly my solution, from a system engineering point of view I would recommend you to start the design from the problem; monitoring which kind of area, is stationary flight required, typical payload weight, typical payload power consumption, desired autonomy, and so on and so fourth. And based on those info choose the more adapted hardware for both your payload and your flying platform.
Hello I have experience in building rigs for all sorts of uses. If you are from any rescue service I suggest you contact someone proffesionally. Hydrogen fuel cells I think are overkill. Try 20000mah batteryes and make a hexacopter or quadcopter with more powerfull motors to compensate wattage. I am guessing your best bet in flight time is about 30-40 mins since the record is just bellow an hour. I guess you are informed on long range systems and athaurized to fly in urban areas. You should set a budget since this builds get extreemly expensive. Good flight times are achived trying. And about the chemical stuff I don´t think you can fly that over an urban area
Since you want to keep the mass of the vehicle constant, I suggest you focus on the efficiency of the propellers.
Area 1 to look at is optimizing the propeller shape for the exact speed and thrust you need for your vehicle. (The motors you are using will have an optimal operating point that defines the ideal speed). There are electric helicopter enthusiasts that compete for long duration flights (Which can last more than 60 min for some configurations if I remember correctly); so that is probably a good place to start looking for information.
Area 2: Each propeller you remove from the system will reduce the overhead in mass and system losses. A helicopter is more efficient than a quad or octo copter. Quadcopters are popular because of their mechanical simplicity but you pay for this in efficiency (flight time).