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How hard would it be to convert a glider into an powered electric aircraft? I'm thinking something that looks like this: enter image description here

Would it be as simple as mounting two electric motors and props to the wings and plugging it into power the electronics and a battery system? Any thoughts?

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    $\begingroup$ How hard is it to convert a carriage to a automobile? $\endgroup$
    – user3528438
    Jun 1 '19 at 18:16
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    $\begingroup$ i think that might actually be harder because you would have to make a linkage between the engine and wheels. With a glider its a direct motor to air transmission. $\endgroup$
    – Daniel Caoili
    Jun 1 '19 at 18:23
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    $\begingroup$ I guess ill just have to try and find out ;) $\endgroup$
    – Daniel Caoili
    Jun 1 '19 at 22:15
  • $\begingroup$ @user3528438 How hard is it to convert a scooter to an electric scooter, or a bike to an electric bike? Also with gliders there is an intermediate waypoint of the sustaining motor glider, whose engine is too small for self-takeoff. $\endgroup$
    – Harper - Reinstate Monica
    Jun 2 '19 at 17:10
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    $\begingroup$ Re "How hard is it to convert a carriage to a automobile?", Quite easy. People add motors to skateboard, coolers, picnic tables and a lot of weird things all the time. Is that the point you were trying to make? $\endgroup$
    – ikegami
    Jun 2 '19 at 18:57
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"Hard" is a matter of opinion. But installing an electric motor on a glider is quite doable. You don't want to put motors on the outside, though. Would make more sense to do what most manufacturers do, put the motor on a retractable pylon behind the cockpit, with doors that open and close as needed.

Taurus Electro 2 seat electric-powered glider

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    $\begingroup$ Otherwise known as a motor glider. $\endgroup$
    – user
    Jun 1 '19 at 19:45
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    $\begingroup$ Sometimes. It can also be just a sustainer engine. $\endgroup$
    – Juan Jimenez
    Jun 1 '19 at 20:51
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    $\begingroup$ And you especially don't want to put the propellors on the wings, unless you also install a taller undercarriage :-) $\endgroup$
    – jamesqf
    Jun 2 '19 at 5:03
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    $\begingroup$ Where I come from these are called gliders fitted with a noise generator. $\endgroup$
    – Pavel
    Jun 3 '19 at 7:30
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    $\begingroup$ @jamesqf: above-wing engines are a thing. $\endgroup$
    – RedGrittyBrick
    Jun 3 '19 at 13:29
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It has been done!

In 2008 an Edgley EA-9 Optimist glider was "converted" in very much this manner for a university project, by strapping eight pylons to the wings, each containing a battery pack and two electric motors - one pushing, one pulling - for a total of 16 motors. This might make it the aircraft with the most engines in history!

It was test flown in this configuration by legendary glider pilot Derek Piggott, then 86, which might make him history's oldest test pilot as well.

Note however that the aircraft was not self-launching - the objective was to see if the electric propulsion system could sustain the glider in flight after an initial aerotow launch.

As a further bit of interesting background, this aircraft is the only example of this type of glider, designed by John Edgley, who is better known for designing the unusual Optica.

Edgley EA-9 Optimist with electric propulsion attached

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    $\begingroup$ Wow, that's even more engines than Helios's 14. $\endgroup$
    – T.J. Crowder
    Jun 2 '19 at 11:27
  • $\begingroup$ This is awesome. It looks like i might not have to 'reinvent the wheel' if i ever decide to try this. They way the motors are mounted is interesting. I think using smaller-ish (or bigger-ish RC) motors is the way to go since the economy of scale advantage. $\endgroup$
    – Daniel Caoili
    Jun 2 '19 at 17:34
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    $\begingroup$ How does it not destroy the outboard propellors when landing? $\endgroup$
    – jamesqf
    Jun 2 '19 at 19:36
  • $\begingroup$ @jamesqf It's a high wing aircraft and the propellers aren't large, it looks like even the outboard ones would have ground clearance on a flat surface. Not so much in the long grass shown, but when landing a glider you keep the wings level until you've come to a complete stop balanced on the mainwheel, before letting one wing gently down. So at worst you'd be lowering a blade into the top of the grass with the prop stopped. $\endgroup$
    – Martin L
    Jun 2 '19 at 20:15
  • $\begingroup$ Depending on your definition of "aircraft" (the first stage of a rocket does fly in the lower atmosphere, after all) the first stage of the Soviet N1 rocket sees your 16 puny electric motors and raises you 30 NK-15 rocket motors. Granted, it will also probably explode. $\endgroup$
    – reirab
    Jun 3 '19 at 6:57
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Yes it is very doable and is a project I've been thinking of doing.

The path I would go would be to purchase an amateur built sailplane with good performance like an HP-11 or HP-14. As an amateur built you can modify it easily paperwork wise.

A glider like an HP-11 that weighs 650lbs and has an L/D of 37 makes 17lbs of drag when flying at max L/D, and you get somewhere around 4-5lbs of thrust from each horsepower with a prop, so the power required to fly level is somewhere around 5 hp. For decent acceleration and climb performance however, you want several times that, at least 100-150lbs, so you need an electric motor that makes at least 20hp, maybe 25 or 30. These sorts of motors are readily available nowadays for electric paramotors.

You want simple simple simple, with the minimum performance penalty. This means NO crazy motor folding gizmos, or exotic mounting. You simply mount the motor on a fixed pylon adapted to the structure around the wing center section, finely faired, with a trail-folding prop. With good streamlined fairing for the pylon and motor and the prop folded back, you might lose, at most, 10-15 points off the L/D. An electric motor glider with a 22 to 27:1 LD is still a pretty good proposition. Not as good as one of the newest high performance electric sailplanes, but you'd only have about 20% of the money into it.

A battery pack would be mounted above and/or below the wing center section in a stainless steel vented fire proof box, with the motor and batteries right on the CG. Based on the numbers I see for electric paramotor setups, 50lbs of batteries might give you around 30-45 min, enough for a launch and several climbs to altitude, or a fairly extended time cruising level with the motor only running at 10 hp (increased from 5 to make up for the drag of the pylon and the higher all up weight - you'd have to increase the gross or reduce your payload obviously).

The main downside is the downside suffered by just about all motor gliders, higher sink rates. It's like water ballast you can never get rid of, so you won't be able to thermal very well on really weak days, but the ability to self launch and operate independently more than makes up for that.

An alternative would be to mount two smaller 15 hp motors on the leading edge on each side in small faired nacelles, but the installation would be a lot more complicated and you would need to find a feathering propeller, which may be tougher to find than a folding one.

enter image description here

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  • $\begingroup$ Or you go directly with a PIK-20E or similar and get the crazy motor folding gizmo for free. Batteries go into the wings, where they replace water ballast. $\endgroup$
    – Peter Kämpf
    Jun 2 '19 at 5:24
  • $\begingroup$ Yes but it's a certified glider and you'd have to do it as an STC/STA and would be extremely costly. This is a cheapo(-ish) project for a hard core self tinkerer. Has to be a homebuilt. A 20-E sells for, what, 40k+ and a conversion you could legally install would cost another 20. An HP-11k you can get for under 10, and a power installation you develop yourself with off the shelf paramotor based bits another 10. $\endgroup$
    – John K
    Jun 2 '19 at 13:49
  • $\begingroup$ I don't really like those top rear mounted big propellers. It makes it more dangerous to stand on top the plane/glider in mid flight, lol. but also I think it'd cause too much pitch forward force. $\endgroup$
    – Daniel Caoili
    Jun 2 '19 at 17:46
  • $\begingroup$ You would have a nose down pitching moment with power, but it doesn't seem to hurt the other retractable engine motor gliders with 40hp 2stokes on pylons. It's a characteristic you'd have to work around. On a glider where it hasn't been done before, you would get towed the altitude the normal way to test the flight characteristics the first time. $\endgroup$
    – John K
    Jun 2 '19 at 20:20
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Was waiting for Peter K to jump in on this one, so I'll recite some of his knowledge.

Notice the sharp nose and leading edges on the OP question photograph compared with the more blunt nose of the glider picture provided by Juan J.

The OP glider is fully optimized for Vbg and may be a little twitchy in pitch under power. So, certainly you can power it, but to enjoy the wider speed ranges of powered flight (with a wider variety of AOA), you may wish to pick a different glider (you'll have a lot of buyers for that one). Or, if a little twitchy is OK, go for it, keeping CG and wing load limits in mind.

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  • $\begingroup$ The blunt nose mainly means it will collect more bugs. The blown T-tail can be arranged to compensate for the engine pitch by giving the elevator some more positive camber. I say more because a regular glider elevator already has some camber for proper stick forces over speed. $\endgroup$
    – Peter Kämpf
    Jun 1 '19 at 19:49
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Unless you can put the complete propulsion system in the back seat of a two-seat glider, you will have to redesign the whole aircraft to take account of the changed mass distribution.

But if "replacing almost everything" counts as "conversion" then … yes, you could do it.

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  • $\begingroup$ The proposed system is a battery-electric aircraft. Balancing one of those is no problem: put the battery and motor on opposite sides of the center of mass. $\endgroup$
    – Mark
    Jun 2 '19 at 20:25
  • $\begingroup$ @Mark So long as you've got some kind of gear linkage from the off-centre motor to the central prop. You don't want the prop off-centre as well, for obvious reasons. :) Motors are generally pretty light though compared to batteries, and a gear linkage is losses you don't need, so probably better to fill both sides with batteries and put the prop directly on the motor assembly. $\endgroup$
    – Graham
    Jun 3 '19 at 21:22

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