What is involved regulation wise adding an electric motor to a glider?

On Saturday, February 6, 2021 at 7:05:37 PM UTC-7, 2G wrote:
On Saturday, February 6, 2021 at 5:18:16 PM UTC-8, kinsell wrote:
On 2/6/21 8:05 AM, Mark Mocho wrote:
Sorry, but I disagree that electrics will turn "pure" sailplanes into a fringe activity. I personally like pure sailplanes over motorgliders and sustainers. Probably comes from 28 years of hang gliding and 20 years of gliding (45 years total). I played around with powered ultralights, but never really wanted one. I have a self launch endorsement in gliders (as well as two turbojet Type Ratings in jet powered two-seat gliders), but I prefer unpowered flight for the challenge, as well as lower complexity and lower insurance costs. Sure, I am forced to depend on tows, but you can pay for a LOT of tows with the differential in the price of a motorglider. As far as propulsion type, I don't particularly care for the limited battery capacity due to poor energy density compared to fuel. And don't kid yourself that batteries are completely safe. Any energy storage system has risks of 'dumping' that energy in an enthusiastic manner. Everybody screeching about getting rid of oil & gas had better do some research on what it takes to produce a wind turbine. (Hint: They don't grow from magic beans.) And you might not have noticed, but there are NO solar-powered solar cell factories. And the Tesla S 100 kWh battery (1,375 lbs.) stores the energy equivalent of 2.1 gallons of AvGas.

Perfect solution would be a battery-powered winch. Hook a bunch of
batteries up to a motor, hook the motor up to a drum. How hard could
that be? No folding props to deal with, no certification, no annual
inspections, no insurance, no skilled labor required to run it. Plus
you don't have to carry a load of undumpable ballast around with you all
the time.
I ran into a guy (who's name I can't remember) at the 2016 Reno Convention who's club was in the process of designing an electric winch using direct DC drive. It is a non-trivial task.

Tom
https://www.hdelectriclaunch.com/

On Friday, February 5, 2021 at 9:52:57 AM UTC-8, kinsell wrote:
On 2/2/21 6:09 PM, David Scott wrote:
On Tuesday, February 2, 2021 at 1:26:10 PM UTC-8, David Scott wrote:
I DON'T mean to stir up trouble on this forum with this question, especially being new, but have been wondering about this for some time. I am wondering how feasible it would be to do this with either a homebuilt or experimental glider here in the US?

I figure this has been asked but didn't find any threads on it.

Thank you for your responses. To be clear I don't have a sailplane but would like to get into the sport and the answers to this question would possibly affect what glider I would get. I am smart enough to get all my ducks in a row before doing anything, and this is the first I have talked about it. From an engineering standpoint, it doesn't look too difficult, navigating the regulations is where I expect the most trouble.


Putting together a motorglider is a strange path towards getting
involved with the sport. Might be better to take lessons, get the
rating, and have some time under your belt before taking on a project
like this.

In my local club, I see people going solo and maybe getting their
rating, and immediately thinking about buying a glider. This is with a
reasonable selection of under utilized club ships sitting around. I
encourage them to wait a couple years first.

If you're all set on owning an electric motorglider, there's a
reasonable selection of Silent 2 Electro's on W&W. Do yourself and the
owners a big favor and pick up one of those.

-Dave

In response to the first paragraph, I am more curious as to what my options would be IF my local club dissolves and tows are no longer available. I absolutely will get flying before doing anything else! The single place club gliders are 2 1-26s and a LET L-33, @$40hr. It sounds like none let you reach much of the areas soaring because of their low performance from club YouTube videos. Since my whole interest would be cross country, or the wave window south of Mt Hood, I will need to get access to a better glider than the club offers.

The Grasshopper has some very interesting aspects. It is the first articulated pylon I have seen, a design I have thought about quite a lot to get it tucked into the fuselage with the smallest opening possible. Designing it to be as compact as possible to fit as many gliders would be an obvious design criteria.

I do not want to reinvent the wheel, just make as few modifications as necessary for a different application. With this in mind electric paramotors are quite interesting. 110-120 lbs of thrust with 4ah batteries would be a good starting point, perhaps. I am not saying the cheapest versions are acceptable but the higher volume will bring the prices down. It looks like $5-$6k would buy the parts needed for a glider, plus the pylon assembly. A complete paramotor weighs around 65lbs so I figure this is close to what it would add to a glider, if done right. The biggest problem to an electric propulsion system is the batteries, and those are going to get vast improvements in the near future.


This is all just food for thought. To add more food I have a few questions. Let us use a 500lb 15 meter glider as the reference.

Any idea on how much thrust is need to sustain altitude?
How much thrust is needed to climb at 200 fpm? 300 fpm?

On Sunday, February 7, 2021 at 2:46:31 PM UTC-5, David Scott wrote:
On Friday, February 5, 2021 at 9:52:57 AM UTC-8, kinsell wrote:
On 2/2/21 6:09 PM, David Scott wrote:
On Tuesday, February 2, 2021 at 1:26:10 PM UTC-8, David Scott wrote:
I DON'T mean to stir up trouble on this forum with this question, especially being new, but have been wondering about this for some time. I am wondering how feasible it would be to do this with either a homebuilt or experimental glider here in the US?

I figure this has been asked but didn't find any threads on it.

Thank you for your responses. To be clear I don't have a sailplane but would like to get into the sport and the answers to this question would possibly affect what glider I would get. I am smart enough to get all my ducks in a row before doing anything, and this is the first I have talked about it. From an engineering standpoint, it doesn't look too difficult, navigating the regulations is where I expect the most trouble.


Putting together a motorglider is a strange path towards getting
involved with the sport. Might be better to take lessons, get the
rating, and have some time under your belt before taking on a project
like this.

In my local club, I see people going solo and maybe getting their
rating, and immediately thinking about buying a glider. This is with a
reasonable selection of under utilized club ships sitting around. I
encourage them to wait a couple years first.

If you're all set on owning an electric motorglider, there's a
reasonable selection of Silent 2 Electro's on W&W. Do yourself and the
owners a big favor and pick up one of those.

-Dave
In response to the first paragraph, I am more curious as to what my options would be IF my local club dissolves and tows are no longer available. I absolutely will get flying before doing anything else! The single place club gliders are 2 1-26s and a LET L-33, @$40hr. It sounds like none let you reach much of the areas soaring because of their low performance from club YouTube videos. Since my whole interest would be cross country, or the wave window south of Mt Hood, I will need to get access to a better glider than the club offers.

The Grasshopper has some very interesting aspects. It is the first articulated pylon I have seen, a design I have thought about quite a lot to get it tucked into the fuselage with the smallest opening possible. Designing it to be as compact as possible to fit as many gliders would be an obvious design criteria.

I do not want to reinvent the wheel, just make as few modifications as necessary for a different application. With this in mind electric paramotors are quite interesting. 110-120 lbs of thrust with 4ah batteries would be a good starting point, perhaps. I am not saying the cheapest versions are acceptable but the higher volume will bring the prices down. It looks like $5-$6k would buy the parts needed for a glider, plus the pylon assembly. A complete paramotor weighs around 65lbs so I figure this is close to what it would add to a glider, if done right. The biggest problem to an electric propulsion system is the batteries, and those are going to get vast improvements in the near future.


This is all just food for thought. To add more food I have a few questions. Let us use a 500lb 15 meter glider as the reference.

Any idea on how much thrust is need to sustain altitude?
How much thrust is needed to climb at 200 fpm? 300 fpm?

Some data from first hand experience:
ASW-24E converted to electric from 2 cycle Rotax gas.
Power system including all items is right at 100 lb added to pure sailplane airframe weight.
This is a pylon mounted retractable system.
Battery is 120 volt,4.9 kwh lithium ion weighing 60 lb.
Climb rate at 160 amps is 300 ft/minute. Actual power delivered is about 16kw.
Climb rate at 230 amps is 500 ft/minute. Actual power delivered is about 23kw at this time
Your cost estimate is a bit less than1/2 what it would require for parts, not including items required to do the airframe conversion and assuming the person doing this can fabricate required items, engineer and wire the system, design and construct the prop, etc.
This assumes perfect efficiency and nothing destroyed or scrapped going through the learning process. Of those I am aware of that have done ,or are doing this, nobody has had that good fortune.
FWIW
UH

On Sunday, February 7, 2021 at 12:30:26 PM UTC-8, Hank Nixon wrote:
On Sunday, February 7, 2021 at 2:46:31 PM UTC-5, David Scott wrote:
On Friday, February 5, 2021 at 9:52:57 AM UTC-8, kinsell wrote:
On 2/2/21 6:09 PM, David Scott wrote:
On Tuesday, February 2, 2021 at 1:26:10 PM UTC-8, David Scott wrote:
I DON'T mean to stir up trouble on this forum with this question, especially being new, but have been wondering about this for some time. I am wondering how feasible it would be to do this with either a homebuilt or experimental glider here in the US?

I figure this has been asked but didn't find any threads on it.

Thank you for your responses. To be clear I don't have a sailplane but would like to get into the sport and the answers to this question would possibly affect what glider I would get. I am smart enough to get all my ducks in a row before doing anything, and this is the first I have talked about it. From an engineering standpoint, it doesn't look too difficult, navigating the regulations is where I expect the most trouble.


Putting together a motorglider is a strange path towards getting
involved with the sport. Might be better to take lessons, get the
rating, and have some time under your belt before taking on a project
like this.

In my local club, I see people going solo and maybe getting their
rating, and immediately thinking about buying a glider. This is with a
reasonable selection of under utilized club ships sitting around. I
encourage them to wait a couple years first.

If you're all set on owning an electric motorglider, there's a
reasonable selection of Silent 2 Electro's on W&W. Do yourself and the
owners a big favor and pick up one of those.

-Dave
In response to the first paragraph, I am more curious as to what my options would be IF my local club dissolves and tows are no longer available. I absolutely will get flying before doing anything else! The single place club gliders are 2 1-26s and a LET L-33, @$40hr. It sounds like none let you reach much of the areas soaring because of their low performance from club YouTube videos. Since my whole interest would be cross country, or the wave window south of Mt Hood, I will need to get access to a better glider than the club offers.

The Grasshopper has some very interesting aspects. It is the first articulated pylon I have seen, a design I have thought about quite a lot to get it tucked into the fuselage with the smallest opening possible. Designing it to be as compact as possible to fit as many gliders would be an obvious design criteria.

I do not want to reinvent the wheel, just make as few modifications as necessary for a different application. With this in mind electric paramotors are quite interesting. 110-120 lbs of thrust with 4ah batteries would be a good starting point, perhaps. I am not saying the cheapest versions are acceptable but the higher volume will bring the prices down. It looks like $5-$6k would buy the parts needed for a glider, plus the pylon assembly. A complete paramotor weighs around 65lbs so I figure this is close to what it would add to a glider, if done right. The biggest problem to an electric propulsion system is the batteries, and those are going to get vast improvements in the near future.


This is all just food for thought. To add more food I have a few questions. Let us use a 500lb 15 meter glider as the reference.

Any idea on how much thrust is need to sustain altitude?
How much thrust is needed to climb at 200 fpm? 300 fpm?

Some data from first hand experience:
ASW-24E converted to electric from 2 cycle Rotax gas.
Power system including all items is right at 100 lb added to pure sailplane airframe weight.
This is a pylon mounted retractable system.
Battery is 120 volt,4.9 kwh lithium ion weighing 60 lb.
Climb rate at 160 amps is 300 ft/minute. Actual power delivered is about 16kw.
Climb rate at 230 amps is 500 ft/minute. Actual power delivered is about 23kw at this time
Your cost estimate is a bit less than1/2 what it would require for parts, not including items required to do the airframe conversion and assuming the person doing this can fabricate required items, engineer and wire the system, design and construct the prop, etc.
This assumes perfect efficiency and nothing destroyed or scrapped going through the learning process. Of those I am aware of that have done ,or are doing this, nobody has had that good fortune.
FWIW
UH
That is good for a start but without knowing the efficiency it doesn't tell much. The devil is in the details, of which there are a lot.

What propeller do you use? Any photos of it extended?

On Sunday, 7 February 2021 at 21:00:17 UTC, David Scott wrote:
This is all just food for thought. ... I have a few questions.
Let us use a 500lb 15 meter glider as the reference.
(a) Any idea on how much thrust is need to sustain altitude?
(b) How much thrust is needed to climb at 200 fpm? 300 fpm?

(a) 11lbs (just the drag) and
(b) 28lbs for 200fpm, or 36lbs for 300fpm (drag + gravity)

Why? Working with sensible accuracy: glide ratio of 45:1 at 60kt:
Drag = 11 lbs at 60kt (500/45)
sin(gradient) = 2/60 for 200fpm, or 3/60 for 300fpm, so...
Force to overcome gravity @ 200fpm: 16.66lbs (500/30)
Force to overcome gravity @ 300fpm: 25lbs (500/20)
J.
(This was posted elsewhere, but has not shown up. Apologies in advance should this end up in a duplication!)

David,

He already gave you the power consumption from the electric side. That is already taking the efficiency in count.
Rule of thumb, 50% overall efficiency.
So, if you are modifying something similar to the 24, you will need component rates for that power needs (always over rate).
The batteries won't modify all the others component. If in the future there are better cells, you can change the packs only and save weight.

Regards

Emir

David--

Here's a little bit of math for helping you wrap your head around this. The post is long, I apologize, but in my experience it's best to have the right numbers in hand before squaring off against the laws of physics.

Efficiency: for a well chosen system it's going to be in the 60-70% range. For example, 94% motor * 98% ESC * 75% propeller = 70%. Note that this doesn't include the battery, since defining efficiency for the battery is somewhat dependent on charging cycles and that's not really important in the air.

Heat: heat is the hardest part of this conversion. Gas engines have it easy, they're big and can safely get very hot. Motors really shouldn't go past 80C for continuous operation, and that's hard when you're in Texas and the ground OAT is 35C. From experience with high-power motors in aviation, dumping heat is so much harder than anyone thinks, and there are very few strategies to radically improve heat transfer when it's not doing very well. Most solutions are bandaids in wait of a white-sheet redesign. DON'T NEGLECT MOTOR COOLING.

Power: it's always wise to specify which power is being referred to, as there are 3 notably different definitions of power in an electric aircraft: Electrical power, shaft power, and propeller output power.

- Propeller output is the easiest to calculate, it's simply force * airspeed (to make sure you don't make unit conversion mistakes and wind up crashing into Mars, do everything in metric. Trust me on this one.)
- Shaft power is the propeller power divided by the props's efficiency at that RPM and airspeed. You calculate this by torque * rotational speed (again, metric for everything). You know RPM, and you get torque from the propeller manufacturer.
- Electrical input power is the shaft power divided by the motor's efficiency. This is the figure you'll see when you're shopping for motors, so be *very* careful not to confuse it with shaft power, which for some motors can be 10-20% less.

Drag: an earlier poster is exactly right, drag is simply the aircraft's weight divided by the L/D. However, don't neglect to consider the drag you care about is when the motor is operational. On my AC-5M, which is around 35:1 with the engine retracted, it is only 20:1 with the engine deployed. (The engine really does almost double the airplane's drag!) So in my case, at 660lbs MTOW, that's 33lbs. Converting to standard units, we get 20kg * 9.8 = 196N. At 25m/s (~49kts), that's 25*196 = 4.9kW of drag for level flight.

Climb power: Climb power is NOT level flight plus climb rate. This is because an inclined plane requires less lift (imagine an aerobatic plane doing a prop hang. The wings are producing 0 lift in this case). Formally-- I can provide the analysis to show this upon request--, it's F = W*(sin(theta)+alpha*cos(theta)), where theta is your climb angle. For a glider climbing at 2m/s (~400fpm) it's around 5 degrees. In my case, that means 380N of force and 9.5kW of propeller output power. For that, I will need a motor between 13kW and 18kW, depending on system efficiency.

Hope this helps!

==========================

Hank-- The ASW-24E would be a poor choice, IMHO. It's paper empty weight is 275kg, a full 88kg more than my AC-5M's true weight. 50% heavier means 50% more everything, and costly mistakes are all too easy to make at these power levels. (Everything else about it is a great plane, though.)

I would especially like to generally caution against considering 120V systems. Unless you know what you are doing and are extremely comfortable around deadly voltages, don't go higher than 48V. We ran 1kV at our drone company, from experience I can say it takes a lot more than a backyard engineering project to do anything 60V safely. Even 60V is dicey, but at least it's not very likely to kill you, only somewhat likely.

The three advantages to 100V voltage are cheaper motor controllers, lower cabling weight, and reduced radio interference. For a one-off prototype the motor controller cost is high no matter what you do. The actual cable weight in a glider is negligible, and can be halved by moving to aluminum, so there are better, safer ways to keep resistive losses low. Lastly, we don't need to worry about radio interference when working on experimental, non-consumer, NORAD aircraft.

Please note that motor efficiency is, surprisingly, not dependent on motor voltage. I can go into this on a separate post, but I've already gone on really long as it is...

On Sunday, February 7, 2021 at 6:30:16 PM UTC-5, wrote:
David,

He already gave you the power consumption from the electric side. That is already taking the efficiency in count.
Rule of thumb, 50% overall efficiency.
So, if you are modifying something similar to the 24, you will need component rates for that power needs (always over rate).
The batteries won't modify all the others component. If in the future there are better cells, you can change the packs only and save weight.

Regards

Emir

On Monday, 8 February 2021 at 00:16:06 UTC, Kenn Sebesta wrote:
Climb power: Climb power is NOT level flight plus climb rate. This is because an inclined plane requires less lift (imagine an aerobatic plane doing a prop hang. The wings are producing 0 lift in this case).
Of course. But the difference in the case which the OP was asking about is about 1.25% of the drag. And I did mention "reasonable accuracy". ;^o)
J.

On Sunday, February 7, 2021 at 4:32:11 PM UTC-8, wrote:
On Monday, 8 February 2021 at 00:16:06 UTC, Kenn Sebesta wrote:
Climb power: Climb power is NOT level flight plus climb rate. This is because an inclined plane requires less lift (imagine an aerobatic plane doing a prop hang. The wings are producing 0 lift in this case).
Of course. But the difference in the case which the OP was asking about is about 1.25% of the drag. And I did mention "reasonable accuracy". ;^o)
J.


Something I haven’t seen mentioned is the Maximum Weight of Non-Lifting Surfaces. I doubt that most 15 meter birds can stand adding 100 pounds in the fuselage without exceeding thIs max allowed weight? The LS-3a for example shows an empty weight of 551 + pilot & parachute (200#) + 100# motor, prop, spinner, control unit, wiring and batteries = 851, minus wing weight at 140 X2 = 571..........the maximum allowable weight of non lifting surfaces is 507!
If the max weight of non lifting surfaces is substantially exceeded, the structural strength of ship will is reduced according!
Something else to think about while your test flying your home made electric machine?
JJ

David Scott
I don't know what your budget is.. But If you want to work on a electric glider a good choice might be to get a Antares.
From what I've read their very high performance, very nice to fly, but need ALOT of hands on work to keep them running.
Building something from scratch sounds very daunting to me.
JJ's post above is a eye opener.
Hanks posts are too. He's a full time professional in the Glider Biz, and look at the hurtles and challenges he had to overcome.
It seems to me you could easily spend a bunch of money, get in way over your head and end up with a very expensive, unflyable, unsafe, un airworthy, piece of junk.
Good Luck!
Nick
T