Thread: Build Log 100% scale, Electric Lazair
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Old Jun 23, 2011, 01:56 PM
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Originally Posted by jlorenco View Post
Maybe someone can help me out here because I don't see how you can get close to 2 hours duration with 96 4S batteries. Here is my understanding:

Weight of the plane = 475 lb loaded
L/D on the order of 12-14 to 1 (I used to own a Lazair and recall this to be the l/d - am I correct?)
So the thrust required is on the order of 475/12 ~ 40ish lbs

Looking at the data for the Turnigy motor this equates to about 90 amps for cruise flight or 90 Ah per hour. Using 80% battery capacity means you need 90/0.8 = 112.5 Ah of battery capacity per hour of flight time. Two motors producing only 20 lbs each looks like a little less amps, but in the same range.

If there are 5 Ah packs, then 112.5/5 ~ 22 packs * 4 batteries/pack ~ 90 batteries for a single hour of flight time not considering take-off power requirements.

What am I missing? My expertise in this is somewhat limited, so I'd like to know where my calculations are wrong because I'm working on a project to electrify a paramotor and these Joby motors look very promising.

Thanks for your help.
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 (

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 ups 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.

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Last edited by KiloOne; Jun 23, 2011 at 03:42 PM.
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