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

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

On Sunday, February 7, 2021 at 9:41:30 PM UTC-8, Nicholas Kennedy wrote:
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
Nicholas, I am on the very low scale of budget now, which is why I am looking at the cheapest options, but expect the future will improve. To be clear, I will not cut up a good glider and not finish the project. Touching a glider would be the last part, after the power module is fully working and debugged on the bench. I fully understand it is not a small project. This really isn't a hard project there are many ways to accomplish it. I have embarked on bigger ones without knowing how to solve all of the problems involved, and failed at one. I have enough experience not to fail at a project like this.

John, this is something I have kept in mind. I am 165lbs dressed and can lose 10 with no problem, and should. I also think I could keep a power setup well under 100 lbs.

The FES distributor I found is indeed Blanik America.

Here is an interesting video showing the difference of a small prop vs a larger prop with gear reduction. Other than that the complete drive system is the same.
Small prop is 26kg of thrust @ 168 amps, large prop is 40kgs of thrust @ 138 amps. The video is https://www.youtube.com/watch?v=WgNMc35zqYo&t=154s..

Kenn, I PMed you about joining your Slack group, thank you. As for the drag of the pylon, yes, that is a big issue and one that going electric could really make a difference on. I think the unit that GP Gliders uses is the only one that tries to reduce the drag. The Grasshopper unit really fails in this regard, but wouldn't be hard to fix later on.

Thank you to J. and Kenn for the drag numbers, that is very informative. It looks like 110lbs of thrust would perhaps work pretty well. This is kind of the low end for electric paramotors and they don't operate on a streamlined pylon.

At the moment I am really liking the idea of a custom retractable pylon module and steal the rest from a paramotor to take advantage of their scale of economy and leverage what developments they make. This also means using a fixed 60" 2 blade propeller for improved efficiency. Or the closest I can get to match the motor used.

Hank, I already said I didn't think the FES system was viable for what I am looking for and fully understand why they would want nothing to do with it for the reasons you mention. As a machine shop I deal with it too. Thanks for the $$$$ info, looks like I was close but a little low.

Dan, seveal electric gliders store the batteries in the wings. I think the Antares does and know the GP Gliders do. I wonder how viable that would be for a retrofit? I also wonder how Antares and GP deal with thermal stability and fire control in the wings?

Your reduced pilot weight (165+ 15 parachute) only reduces the fuselage load by 20 # below my LS-3a example, and that still leaves you something like 50# over the max allowable fuselage load. I’d nail down all these little problems before committing to the project..........you will find plenty of unforeseen issues that you haven’t even thought about.
I agree that a pylon motor mount is far superior to a FES, because there is no big CG issues to deal with. The FES nose motor weight must be counterbalanced with an aft battery which is located as far behind Your CG, as the nose motor is in front of the CG!
A big problem in selecting a ship to modify is the area you wish to install a pylon is the area where the wing controls are located on most ships.........ailerons, flaps and dive breaks......... need to come up with a plan on how to redesign the control linkage? BTW, installing a ballistic parachute shares this same problem........the chute and rocket need to go in the area where the wing controls are located.........?
I would think batteries located inside a wings D tube should be attached to the vertical shear web and strong enough to withstand a 5G load, none of which is taken by the wing skin? I’d place the batteries as far out in the wing as they will fit and you will need to design a way to get cooling air to all the battery cells. Make sure to properly repair any holes you cut in the D tube as that section takes some of the bending load and all of the twisting load seen by the wing!
What have I missed? Oh, how about a fire extinguisher......Inside the wing would be nice?
Have fun,
JJ




O
On Monday, February 8, 2021 at 10:20:20 AM UTC-8, David Scott wrote:
On Sunday, February 7, 2021 at 9:41:30 PM UTC-8, Nicholas Kennedy wrote:
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
Nicholas, I am on the very low scale of budget now, which is why I am looking at the cheapest options, but expect the future will improve. To be clear, I will not cut up a good glider and not finish the project. Touching a glider would be the last part, after the power module is fully working and debugged on the bench. I fully understand it is not a small project. This really isn't a hard project there are many ways to accomplish it. I have embarked on bigger ones without knowing how to solve all of the problems involved, and failed at one. I have enough experience not to fail at a project like this.

John, this is something I have kept in mind. I am 165lbs dressed and can lose 10 with no problem, and should. I also think I could keep a power setup well under 100 lbs.

The FES distributor I found is indeed Blanik America.

Here is an interesting video showing the difference of a small prop vs a larger prop with gear reduction. Other than that the complete drive system is the same.
Small prop is 26kg of thrust @ 168 amps, large prop is 40kgs of thrust @ 138 amps. The video is https://www.youtube.com/watch?v=WgNMc35zqYo&t=154s.

Kenn, I PMed you about joining your Slack group, thank you. As for the drag of the pylon, yes, that is a big issue and one that going electric could really make a difference on. I think the unit that GP Gliders uses is the only one that tries to reduce the drag. The Grasshopper unit really fails in this regard, but wouldn't be hard to fix later on.

Thank you to J. and Kenn for the drag numbers, that is very informative. It looks like 110lbs of thrust would perhaps work pretty well. This is kind of the low end for electric paramotors and they don't operate on a streamlined pylon.

At the moment I am really liking the idea of a custom retractable pylon module and steal the rest from a paramotor to take advantage of their scale of economy and leverage what developments they make. This also means using a fixed 60" 2 blade propeller for improved efficiency. Or the closest I can get to match the motor used.

Hank, I already said I didn't think the FES system was viable for what I am looking for and fully understand why they would want nothing to do with it for the reasons you mention. As a machine shop I deal with it too. Thanks for the $$$$ info, looks like I was close but a little low.

Dan, seveal electric gliders store the batteries in the wings. I think the Antares does and know the GP Gliders do. I wonder how viable that would be for a retrofit? I also wonder how Antares and GP deal with thermal stability and fire control in the wings?

John Sinclair wrote on 2/8/2021 12:02 PM:
Your reduced pilot weight (165+ 15 parachute) only reduces the fuselage load by 20 # below my LS-3a example, and that still leaves you something like 50# over the max allowable fuselage load. I’d nail down all these little problems before committing to the project..........you will find plenty of unforeseen issues that you haven’t even thought about.
I agree that a pylon motor mount is far superior to a FES, because there is no big CG issues to deal with. The FES nose motor weight must be counterbalanced with an aft battery which is located as far behind Your CG, as the nose motor is in front of the CG!
A big problem in selecting a ship to modify is the area you wish to install a pylon is the area where the wing controls are located on most ships.........ailerons, flaps and dive breaks......... need to come up with a plan on how to redesign the control linkage? BTW, installing a ballistic parachute shares this same problem........the chute and rocket need to go in the area where the wing controls are located.........?
I would think batteries located inside a wings D tube should be attached to the vertical shear web and strong enough to withstand a 5G load, none of which is taken by the wing skin? I’d place the batteries as far out in the wing as they will fit and you will need to design a way to get cooling air to all the battery cells. Make sure to properly repair any holes you cut in the D tube as that section takes some of the bending load and all of the twisting load seen by the wing!
What have I missed? Oh, how about a fire extinguisher......Inside the wing would be nice?
Have fun,
JJ

It's not quite so bad, JJ. A 20kw motor might weigh only 7 kg; a 4.7kWh battery would weigh
about 22kg, so the battery can usually be mounted behind the wing. That also puts it clear of
the wing control mechanisms. The total added weight will easily be below 45kg/100lb. If that
still exceeds the non-lifting weight limit, the Vne, etc, can be reduced to keep the structure
safe.

There are many advantages to the fuselage mounted batteries. A major one is easy removal for
charging elsewhere. A BRS is desirable, but the only there aren't many that offer it. The GP15
and the Silent Electro are the only two I know of. A fire extinguishing system is also
desirable, but I only know of one glider that plans to offer one (GP15).

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to email me)
- "A Guide to Self-Launching Sailplane Operation"
https://sites.google.com/site/motorg...ad-the-guide-1

I spoke with Luka about 5 years ago about installing FES in my LAK-17a.
There are plenty of very competent folks at Moriarty to do the work, but
Luka was firm that only he and his people would do it. He said that for
roughly $25K each for a minimum of 4 gliders, plus travel and living
expenses, they'd come to the US and do the work. I got a Stemme, instead.

Dan
5J

On 2/8/21 11:20 AM, David Scott wrote:
On Sunday, February 7, 2021 at 9:41:30 PM UTC-8, Nicholas Kennedy wrote:
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
Nicholas, I am on the very low scale of budget now, which is why I am looking at the cheapest options, but expect the future will improve. To be clear, I will not cut up a good glider and not finish the project. Touching a glider would be the last part, after the power module is fully working and debugged on the bench. I fully understand it is not a small project. This really isn't a hard project there are many ways to accomplish it. I have embarked on bigger ones without knowing how to solve all of the problems involved, and failed at one. I have enough experience not to fail at a project like this.

John, this is something I have kept in mind. I am 165lbs dressed and can lose 10 with no problem, and should. I also think I could keep a power setup well under 100 lbs.

The FES distributor I found is indeed Blanik America.

Here is an interesting video showing the difference of a small prop vs a larger prop with gear reduction. Other than that the complete drive system is the same.
Small prop is 26kg of thrust @ 168 amps, large prop is 40kgs of thrust @ 138 amps. The video is https://www.youtube.com/watch?v=WgNMc35zqYo&t=154s.

Kenn, I PMed you about joining your Slack group, thank you. As for the drag of the pylon, yes, that is a big issue and one that going electric could really make a difference on. I think the unit that GP Gliders uses is the only one that tries to reduce the drag. The Grasshopper unit really fails in this regard, but wouldn't be hard to fix later on.

Thank you to J. and Kenn for the drag numbers, that is very informative. It looks like 110lbs of thrust would perhaps work pretty well. This is kind of the low end for electric paramotors and they don't operate on a streamlined pylon.

At the moment I am really liking the idea of a custom retractable pylon module and steal the rest from a paramotor to take advantage of their scale of economy and leverage what developments they make. This also means using a fixed 60" 2 blade propeller for improved efficiency. Or the closest I can get to match the motor used.

Hank, I already said I didn't think the FES system was viable for what I am looking for and fully understand why they would want nothing to do with it for the reasons you mention. As a machine shop I deal with it too. Thanks for the $$$$ info, looks like I was close but a little low.

Dan, seveal electric gliders store the batteries in the wings. I think the Antares does and know the GP Gliders do. I wonder how viable that would be for a retrofit? I also wonder how Antares and GP deal with thermal stability and fire control in the wings?