Your question has a lot of open-ended variables in it, so it is tough to give you an exact answer. I will make a few assumptions here based on my experience with the Cobra products. Typically, the Cobra 3525/12, 650 Kv motor would be run in an aircraft that was designed for a .45 size Glow engine. It is most commonly run from a 5-cell Li-Po battery with a prop such as 12x8 or 13x6.5. The Propeller Data Chart for this motor can be found on our website at the following link.
If you look in the middle of this chart, in the 18.5 volts section, you can find the data for several props running from a 5-cell battery pack. As an example, we can look at the data for the APC 13x6.5-E prop. From the chart we can read the following full throttle performance numbers.
Volts – 18.5
Amps – 47.01
Watts – 869.7
RPM – 9,012
Thrust – 120.99 ounces
Pitch Speed – 55.5 MPH
When you calculate flying time, you need to know the motor current, and the capacity of the battery. Typically, for a motor like this, a 5-cell battery with a capacity of between 4000 and 5000mah is used. For the sake of this discussion, we will assume that a 5-cell 5000mah battery is being used.
When calculating flight time, you need to express the current draw in terms of the C-rate of discharge. By definition, if you pull a 1C discharge rate from a battery it will be fully used up in 1 hour, or 60 minutes. If you have a 5000mah battery, it can also be called a 5 Amp-Hour battery because there are 1000mah per Amp-Hour. Therefore, a 5 amp discharge from a 5000mah battery will take 1 hour to completely drain the pack.
In the case shown above, at full throttle with an APC 13x6.5-E prop, the Cobra 3525/12 motor pulls 47.10 amps. If we divide this by the size of the battery, the C-rate of discharge is equal to 47.01 ÷ 5, which equals 9.4C. To get the actual flight time, you take 60 minutes and divide it by the C-rate, so for this case 60 ÷ 9.4 equals 6.38 minutes. This would be assuming that you are flying at full throttle the entire time. The other thing to consider is that you NEVER want to fully discharge Li-Po batteries, since this will quickly ruin them. For the best service life, you only want to use up 80% of the true capacity during each flight. This means that instead of 6.38 minutes, we can really only get 5.1 minutes of flying time so we only use 80% of the battery energy.
In a real flight, unless you are pylon racing, you never fly at 100% throttle the entire flight. Typically, you will reduce the power to ½ to 2/3 throttle most of the time, with occasional bursts to full throttle for high speed fly-bys or while doing a big loop. The current draw of a motor drops dramatically as the throttle is lowered, and for most motor is approximately equal to the throttle percentage squared times the full throttle voltage.
For example, if you are running at 80% throttle this is 0.8 of full throttle, and if you square this number, taking 0.8 x 0.8 you get 0.64, so at 80% throttle you only pull about 64% of the full throttle current. At 70% throttle you only pull 49% of full throttle and at 50% throttle you only pull 25% of the current that you would at full throttle. This plays an important role in determining how long you fly.
As we calculated earlier, if you flew the entire flight at full throttle, you can really only fly about 5 minutes per charge. At 70% throttle this is extended to 10 minutes, and at 50% throttle you can fly about 20 minutes per charge. All of these variables must be taken into account when determining how long you can fly a plane per charge.
With all of that being said, one Cobra 3525/12 motor spinning a 13x6.5-E prop will fly for 20 minutes from a 5-cell 5000mah battery pack, and leave 20% of the battery capacity in the pack at the end of the flight. If you had 2 of these motors, then each one would need a 5-cell 5000mah battery to fly for 20 minutes at 50% throttle. If you wanted to fly for 30 minutes, then you would need to have 3 of these batteries connected in parallel to produce a 5-cell 15,000mah battery, and together these batteries would power a pair of the motors for 30 minutes, once again assuming that you flew at 50% throttle for the entire flight.
If you did need to fly at a different power setting, then you can go back and re-calculate the current draw for the new condition and re-run the numbers to determine how much battery you would need.
Hopefully this gives you the information that you need to determine the power system for your project.