Originally Posted by KiloOne
My duration estimate was based on my guess that 3 kw would keep the Lazair in the air. Unfortunately that was probably a low estimate based on Series I and II airplanes that were up to 100 lbs lighter than my Elite Electric Lazair and also when I weighed 80 lbs less than I do now.
Actually, two motors at 20 lbs each use substantially less amps than one at 40 lbs. With this setup it would be 3000 watts vs 4000 watts (http://www.rcgroups.com/forums/attac...mentid=3895621
Anyway, AmpHrs are not what keeps a plane in the air. Power is a more appropriate unit. If 40 lbs of thrust would keep the Lazair in the air at 30 mph, that is about 3.2 hp (1hp=550 ft-lbs/sec) or 2.4 kw (at 747 watts/hp).
Lets say 70% efficient prop ups that to 3.4 kw and 90% efficient motor up it again to 3.8 kw.
I have 7.2 kwhr of batteries so an endurance of about 1.9 hrs.
I would be happy with that but I think my endurance will be less than that. I think I will be lucky to get 60% prop efficiency and it is probably taking more than 40 lbs to keep me in the air. Time will tell.
Personally, I do not think endurance should be listed at 80% pack capacity.
Endurance in an aircraft uses all usable fuel, reserve amounts are left to the pilot to decide.
Thanks for the reply Dale. I was using the 80% not as fuel reserve, but because you cannot use 100% of the battery capacity without destroying the batteries. I use 80% discharge capacity for RC applications as this seems to be a commonly accepted amount to safely discharge Lipo batteries.
The whole kwh concept for batteries has always confused me because of the voltage component. You can use full pack voltage to calculate the capacity, but the voltage changes as the pack is discharged. That is why I usually default to amps. My logic being that if it takes say 80 amps to produce enough thrust to stay in the air, then it will take 80 Ah of battery capacity in one hour. Since battery capacity is rated as mah, this is to me a more direct method to obtain duration. This seems like it will actually over estimate the duration because as the battery discharges the voltage will drop and it will require more amps to get the same thrust (or power).
If I use this method for your application using the data for the Turnigy motor, I get the following:
20 lb thrust requires 40A or 80A for both motors
This is 80 Ah for each hour of flying at cruise
80 Ah/0.8 = 100 Ah (since you can only effectively use 80% of the pack)
100 Ah/5Ah per 4 battery pack = 20 packs, or 80 batteries
So, for 96 batteries this is about 1.2 hours at cruise and this does not account for the voltage drop in the batteries, so it optimistic. It seems like one would be doing good to get 1 hour out of this system.
If I've missed the boat here, please let me know. I'm not trying to shoot holes in your project, I think it is great and have been watching with great interest. I just want to fully understand all this so I can have a realistic prediction of flight duration for my project.