I've been spending a lot of time over the last few weeks trying to figure out the optimum battery, motor and prop configurations in my skyfun to provide the maximum range and endurance options. Operating on a limited budget, I've been trying to avoid trial and error and make the right choices based on specs.
This is not easy but I've settled on a lower kv motor than many have chosen, running a larger prop with a 3300Mah 3s battery at only 20C (lower C batteries weigh less). The motor I've chosen is a Scorpion II SII-2208, 1100kv, 130W, 45grams, 12Amps, 10 x 4.5 prop.
I'm still mulling over ESC choice.
The final choice was also based somewhat on easy availability in New Zealand (Turnigy is not widely stocked).
There are a lot of suggestions on this and other forums about motor and prop choice which is helpful but what is missing is a semi technical discussion about endurance, range and speed and how to maximize these on any given platform. In the commercial UAS and military worlds, range, endurance and speed can mean the difference between a useful platform and something that is useless. In general, many of the systems people are building here do not seem to be optimized and are achieving quite poor endurance and range.
In commercial aviation, the optimum cruise speed is not based on endurance or range but rather to minimise time. Airliners need to get to their destination in the minimum time while using the least amount of fuel possible so the optimum cruise speed is the speed that flies the fastest possible speed using the least possible fuel per hour.
UAS's are totally different, in general our missions are to fly to a destination, loiter over a target for as long as possible and then return. In this scenario, we need to maximise range for the first part of the journey and get there as efficiently as possible (minimum watts per mile). Once at the target we need to switch to an endurance mode and fly at the minimum speed possible to maximise the amount of time over the target. We then need to return home at a speed that maximises range again (this might alter if a payload is dropped).
Alternatively we might need three modes, in an emergency you might need to get to a target quickly (optimal cruise speed as per airliner), loiter for as long as possible (maximum endurance) and then return as efficiently as possible (maximum range).
I'm not much of a maths guru but using logic I figure the first and easiest step is determining the maximum endurance of an airframe which would be done by finding the minimum amount of thrust required to maintain level flight. At this point, power consumption is minimized and the endurance is maximised (while speed and range are sacrificed), this would have to assume that motor and ESC efficiency are linear (from what I have read brushless motors are more efficient in the medium power bands anyway).
At the other end of the spectrum is the maximum range which I'm a little less sure on how we calculate on a model. For this I figure we need to know about the drag characteristics of each airframe across the full range of speeds.
I also figure that to calculate the optimal cruise speed (max speed with minimal power as per airliner) we would also need to know about drag at different speeds.
Is there anyway the APM logs could be used to figure out useful drag measurements to calculate these speeds for our airframes?
Or, are these speeds the APM could actually calculate itself by performing an 'in flight calibration'?
You could define in the flight planner which portions of the journey should be for calibration and which for maximimum range, endurance or efficient speed.
The APM could then slowly adjust the crafts speed downward in the calibration phase to determine the minimum amount of power required to maintain altitude and store this as the loiter speed. It could then slowly increase the power and measure which power setting delivers the maximum range for input power and then the maximum speed that uses the least fuel per hour.
I realise that there is also windspeed which could affect these numbers, which might mean that the APM might always need to be in 'calibration mode' and once put into one of these modes, constantly optimize the power so as to deliver maximum range, endurance or minimum time depending on what you want.
Once my new motor arrives (my stock motor now barely maintains flight) I will do some testing and add to this post, but would be interested in others thoughts on this topic as there seems to be a severe lack of discussion on such a relevant topic to UAS.