Lazair Electric Conversion Project


New member
Hi Gang.

I'm needing to tap into this brain trust a bit, hoping you guys can help me out.

I've built a few SeaDucks, and have a great feeling for how twin electric motor flight feels at that scale.

Also I just purchased myself a Lazair series II ultralight this week, with the aim of swapping out it's twin 2 stroke
9.5 hp engines, with 2x 10KW brushless motors and as much LiPo as allowable, which I've calculated to be (to meet the regulation of 5 gal limit by volume, so 12x Turnigy 20,000mAh 6s, operating at 24s, 88.8V.

So given those constraints, I'm wondering if folks have any suggestions about what manufacturer I should be talking to in Shenzhen. If there is a North American dealer in agricultural drone motors. Is there a problem with my math?

So What are the reasons for this conversion besides the fun and challenge of a build?

1. Just Cool. Will sound like a starship.

2. More Green. These 2 strokes are tuned to run forever (lots of lube), which means they belch a lot of fuel from the naked carbs. Spooling up from idle to full power seems to take forever as super rich carbs bog etc. Soooo much power wasted in noise and vibration.

4. The engines are Rotax 185's which are no longer built and no available parts. The Airframe still has decades of life left, but these engines are obsolete. To keep it flying have to look at systems that use the latest technology.

3. 2x 2stroke pull-start engines are dangerous to re-start in the air. The Lazair has so much wing that it can soar with engines off, but re-starting the pull-start engines while in flight is dangerous, strenuous and prevent many from flying this plane. My aim would be to make this plane flyable by a Senior citizen, smaller person who lacks the arm strength who wants to soar quietly with geese, but have power on demand.

5. Yaw Stability. This plane has 2 factors that could be assisted by a gyro. 1 The Lazair uses an inverted V tail. 2 It's a tail dragger that is difficult to control at taxi speeds less than 10 knots as the 2 gas engines often run up to speed differently and the tail wheels can get caught and cause a sudden turn in early takeoff. So to smooth out these issues and make those takeoffs feel like you're on rails I want a gyro.

6. The designer of this airplane has already converted his personal Lazair to Electric almost a decade ago, and used it to make the first ever electric water landing. Really more than any air frame, this one is begging to be electrified.

Thanks for reading this post and please add any suggestions.



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2 suggestions/comments:
- run your power system at 48v 12s--much more efficient for large scale power systems (check out T-motor U12 motor specs)
- the rotors are way too far inward to have much effect on yaw authority, so a gyro wouldn't do much for you. Especially at this scale, many factors come into play such as motor torque required to spool up to generate the desired control moment or simply the speed of onset of the disturbance. Not much bandwidth to work with there


Build cheap, crash cheap
How did you figure the battery capacity you'd be aiming for? Available weight while staying within ultralight weight limits?
5gal gasoline: ~14.8kg/32.6lb
12 of those batteries: ~31.6kg/69.5lb
16.8kg/36.9lb difference, pretty substantial at this scale. I figure the motors will weigh less than the Rotax 185s, which I could not find a weight for. If you can get, say, 3.8KW/Kg out of your motor (2.6kg/5.8lb each) and the ESCs will probably weigh 1kg/2.2lbs each, that brings you to 7.3kg/16lbs sans props or wiring. If the difference between the 185s and the electric motors is 37lbs or more, or you have weight to spare, you're good I guess. It would really help to have a weight for those Rotax 185s to figure that out.

I would not trust any hobby grade parts on any manned aircraft at any rate. Especially not the, to quote Dave Jones; "Wun Hung Lo" electronics we're all so used to in this hobby. However, I wouldn't put a hobby gyro on the plane no matter who makes it, since tuning it could be physically dangerous if it oscillates, feedback loops or over-corrects. Simplicity is king here, the less electronics to fail the better, and the fewer single points of failure the better. To the point that I'd have the ESCs and throttles themselves wired completely separately so you'd at least have something if there's a failure of any single component. The glide ratio on this bird seems to be just a little better than your typical GA plane, so I'd be no more willing to glide it. The risk taker in me rides a motorcycle, but I'm also an engineer and a (dormant) pilot, and that part of me wins out in all things aviation. I'll build all sorts of crazy experiments that fly without me in them.


New member
How did you figure the battery capacity you'd be aiming for? Available weight while staying within ultralight weight limits?

This conversion has been done a few times, and to make the weight work, firstly the E-motors will weigh a good deal less than the Rotax (don't know their weight yet but I think it's 11-12 lbs), so I lose a bit of weight there. And when the aircraft's original designer built his 9 years ago, he reasoned that the "Fuel" is a substance that gives off energy, measured by volume, not weight. So just doing the calculations based on published measurements, I end up with the ability to cary 12x 20,000mah 6s Turnigy. Just under 5 gallons by volume.

I'm looking at these motors as they are man-rated for paragliding.

As for the motor separation, actually I was thinking for the first rounds of testing I will probably leave the gas motors on and mount the electrics farther out on the wings, and cary far fewer cels, but at the right voltage, just to test the system. In that configuration the yaw diff may help, especially when ground handling, but I hear you about the spool up time and using hobby grade electronics. Paraglider props that I want to use will be more sturdy for higher power.

RCJETFLYER: As for voltage, it was my understanding as to why 6s has become the norm for long range fpv over 4s because all things electromotive become more efficient at higher voltage.

Before I decide and order, I will do some tests in flight with the gas engines, looking at what the power output is for cruising speed, and do the math and maybe use a load cell while running on the ground to figure out how much power those rpms are in watts, lbs of thrust I need to actually sustain over a 60 minute flight. Previous eLazair's have reportedly gotten 2 hour flight times. With Gas engines they are just tuned and run very wastefully. If I try to really economize and match props with motors with desired cruising airspeed, and we fly on a summer day, it could actually stay up for hours with thermals, give some endurance fatigue for smaller pilots.
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Master member
Going by volume for batteries is suspect. Gas is lighter than water. A LiPo most definitely is not. Your '5 gallons' of LiPo will weigh significantly more.
About 69 lbs for 12 x 20,000 mAh LiPo against 30 lbs for 5 gallons of gas.


New member
Going by volume for batteries is suspect. Gas is lighter than water. A LiPo most definitely is not. Your '5 gallons' of LiPo will weigh significantly more.
About 69 lbs for 12 x 20,000 mAh LiPo against 30 lbs for 5 gallons of gas.
For the first iteration, I'm probably going Li-ion. Starting with 2x 24s 21700 5000 mAh 20A ($5 a pop in bulk order.) To run in tandem with the gas engines for the first round. So that will put me well inside the fuel weight issue.


New member
The first Lazair flew marginally on (2) 5.5hp engines = 11hp, then was upgraded to (2) 9.4hp engines = 18.8hp, some people still weren't happy with its Performance, so was upgraded to (2) Solo 210 15hp engines = 30hp. If you use the 1kw for 10kg Rule. MTOW of 450 lbs = 204.1166 kg / 10 kg = 20.41166 kw needed to Fly Well = 27.37249 hp / 2 = 13.686245 hp per Engine.

These 4 Stroke Honda/Clone GX200/212 Engines can make 15hp@5000rpm! 15hp x .5 = 7.5 lbs per hr. 5 Gallons = 30 lbs/7.5 = 4hr/2= 2hrs

Those Rotax 185's can be upgraded to make 15hp each easily. It's better to use a Belt Reduction Drive so you can run Bigger Props. They are still made today and used on Watertrax Water Pumps! The Stock 185s burned 2.0 gph. 5 Gallons /2.0 = 2.5 hrs or 2hrs with a Reserve.

You can add just a Good Tuned Pipe to easily make your Target 15 hp each at the same 5000rpm. Or go the conventional way. A +/- 1.0cr = 1hp on Avg. Going from 8.0cr to 11.5cr = 3.5hp. 9.4hp + 3.5hp = 12.9hp. Cylinder & Case Porting can Gain you 10-15% more hp. 12.9hp + 10% = 14.19hp. A +2mm Bigger Carb can Gain you 10% more hp = 15.6hp! A New Mikuni Flat Side Carb for less than $40 on eBay. Your Heads Milled for a Higher CR for about $80 each. 184cc at 5000rpm making 15hp = 23cfm / 1.5 = 15.3hp made.

As you said the Designer tried to go Electric with (2) 11hp Motors = 22hp and Loaded up with Batteries at Part 103 Weight of 254lbs and with his Weight I think he said he was 175 lbs he only got 35-40minutes of Flight. With his Weight and loaded with Batteries to the MTOW as a Light Sport he got about 1hr and 20min. Or you would say 1hr with a 20min Reserve.

Now you want to use 2 x10kw (13.41022 hp) brushless motors, so your Flight Times will probably be even less.