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Hi all,

Looking for advice regarding picking a motor for an E-Help powered harness for a hang glider. From what I know so-far the formula for RPM is motor kv * motor voltage * losses .8 = RPM. Early E-Help systems use small propellers at very high RPM. More recent higher performing systems are using bigger propellers with geared reduction 1:285. All these designs seem to be using the Rotomax15cc 15kv motor at around 52 volts, which would yield 150 * 52 * .8 = 6240 RPM. After this motor RPM is geared down 6240 / 2.85 = 2190 RPM.

I would like to spec a motor that has high torque at 2000 – 3000 RPM whereby I do not need to add mechanical gear reduction to the design. E-Power Hobby has an 80kv motor that would produce 80kv * 52v * .8 = 3,328 RPM, which puts me within the direct drive range. This motor can actually take 70v, but I would have to add gear reduction. Plus I have had an electric airplane manufacturer mention that they can only operate their motors at 45%. If I run the E-Hobby motor at 52 volts then I am dropping the current by 20% and if I do not operate at full power, just enough power to achieve around 2300 RPM then I generally will not be running the motor at full capacity.

E-Power has a 50kv motor at twice the price and power (45hp). They publish the usage of this motor to be for para gliders. 50Kv * 70v * .8 = 2800 RPM. This may be a good motor to use direct drive if I could get one made at 18000Wt.

I do not know how to factor in propeller load as applied to RPM and motor load.

Any advice is appreciated.
Thanks Tom

User avatar
By TjW
If that 45 hp value is correct (seems awfuly high), you'll need to supply around 500 Amperes at 70 volts.
Four and a half hp would be about a tenth of that, of course.
By tommy42
I just mentioned the E-Power 25000W (actually it is 33hp not 45hp, 25000W/750W) because it was a 50kv motor, the RPM I’m thinking I’d like to be at for direct drive. However afterwards I was watching a video that had some formulas and curves that suggested that you just have to keep the tip speeds subsonic. I’ll have to look at that some more to figure out how fast I can turn a 21 inch prop without losing efficiency. I had a private airplane pilot tell me that light airplane RPM is usually 2500 – 3500 RPM, so I was thinking that I would have to be within that range. However, a 21 inch prop on a hang glider may be much smaller than your typical airplane prop and I may be able to go faster.
User avatar
By DMarley
I did a BUNCH of research into this very topic last year, then did some soul searching and found that this kind of power assist would not really be to my liking, so I'm working up the calcs on hydraulic / electric static tow winch systems, as no one is currently making/producing anything like that now that will tow a HP HG or a tandem up with step towing.

But let me give you a few clues.
A Mosquito NRG and the Wasp with the Radne Raket make approx 110 lbf of static thrust with the same prop (52" x 21"). The wasp is published with slightly less power @ 10.4 kW (14 hp) @2639 prop rpm max, 3.54:1 reduction, and the Mossi has published 11.2 kW (15 hp) @ 2486 prop rpm max, 3.6:1 reduction. Those were the numbers that I would need to achieve to make this electric propulsion project a success. Any smaller disk area and the efficiency would drop dramatically, not to mention the substantial loss of low-speed thrust. Nothing I could find that was reasonably economical (compared to say a used mossi) could match the performance and total power that I wanted. I'm no lightweight, and with the added weight of an E-power harness, I would be afraid of getting into slightly rowdy thermally conditions, just the stuff you need to stay up with a heavy e-harness system. The higher the wing loading on a pitch-sensitive, short-chord, low Cm craft, the easier it is to fall over the falls. ':shock:'

I also found that a higher reving motor is quite a bit more efficient than a slow reving motor, and the higher reving motors are much less mass. The reduction drive is actually on par if not more efficient than a larger diameter, slower reving motor. But for simplicity in build, perhaps a large diameter pancake motor would be easier. I was looking into E-bike hub motors and I found a unit that would possibly work, but I cannot find the source now. But I would start down that avenue.

The gentleman that actually invented LiPo battery technology but that history forgot recently indicated that he and his team were hoping to increase the capacity of current LiPo technology by a whopping 60%. He indicated that would put electric driven vehicles within the realm of ICE vehicles. LOL. I think only 60% more capacity is still just a drop in the bucket when it comes to doing real work. Think XC.
Anyway, hope this helps in some small way.
By tommy42
Thanks Dmarley. These are good data points to take into account.

I live in San Diego where there is abundant daily thermal lift. My property is half way up a canyon mountain, so I'll only need a little electric/thermal assist to get me up to mountaintop level that will enable me to soar around the neighborhood. I think this will add a whole new energy management dimension to the sport. I have limited battery to work into my thermalling, so it will still very much be a thermalling sport/activity for me. The E-Help seems to have been started in the Northern European countries to get from thermal to thermal in the winter time. They are using small props and direct drive at very high RPMs. Now there are many Youtube videos with flatlanders getting up a few thousand feet using bigger props and geared reduction.[/url]
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By tommy42
PS: They won't let me insert any links here, but here are the specs of the motor I am looking at. 18000Wt/750Wt(1hp)=24hp.

Product Description

12090 Brushless Motor

Brushless motor 12090 KV80
MOTOR: 12090
KV: 80
MAX POWER: 15000W-18000W
ESC: 250/300A
SIZE: 120 x 90 ( without shaft )
Stator: 40mm
WEIGHT (g): 2.835
SHAFT: 10mm Accessory pack: Yes
User avatar
By DMarley
Hey Tommy,
That is an 'Alien Power Systems' motor. Very substantial... kinda heavy at 6.25 lbs and at 15 - 18kW it's overkill for a direct drive using a prop of approx 34"x22". Reidar uses that sized prop on his 6 kW delta-connection drive system that turns the prop at ~5000 rpm and beats the air into submission (very high tip speeds) If you are not using a reduction gear with this powerful motor, you will be wasting power trying to push the prop way past efficient thrust speeds. That motor would be still overkill (if it meets it's published specs) with a 2.04 reduction drive and a 52"x21" Bolle prop, but that's not what you want to hear. For a direct drive I'd look for a smaller motor (11-12 kW) that would have max RPM's less than 5000 rpm and use a larger prop than 34" if the tip speeds aren't too high.
Just my thoughts after a few quicky calculations.

By the way, that motor is over-spec'd. If max E=70v and max I =200 amps, then the true power output would be something less than 14 kW. So, in a pinch, if the input spec's are correct (hmmm), then that motor would still be overkill. Be damned sure about the specs and find a power chart showing rpms/torque/ I / V / Efficiency / etc. Even these charts can be very misleading if not read correctly, or could be intentionally derived to be wrong. The best way to be sure is to find other projects that have the same motor that you are proposing with published inputs/outputs. The small electric motor industry is full of bad information and scams. Most of these motors are rebranded from china. So you understand that published specs can be very misleading. The best motors are manufactured and designed in Germany and the USA. Try Hacker (higher priced and RPM's), Plettenberg (very excellent, expensive,good rpm's), Geiger engineering (very expensive, perfect rpm's), etc. Study their specs and power charts for each motor to get a feeling for what you need to know. Also as I indicated before, look at e-bike hub motors. These are rated at lower rpm's, higher torque, but perhaps slightly heavy because of the larger diameter / size required for proper lower rpm's.
Also, the prop is probably the most important part of the equation, so find some good reading material about efficiencies at low air-speeds and proper pitch and blade-loadings for the flying you *think* you want to design to.

Another edit: Ha! Perhaps at 52v, that motor would be fine. Sorry. If specs are close to being correct, that would put you at approximately 3300 rpm (good!) for a 40" diameter disk (prop) at approximately 10 kW at 200 amps. Be sure you get the power curve chart for that motor! It looks close. But you have to ensure that the torque curve has enough oomph at higher rpm's so it will swing the selected prop with enough torque to accelerate it to it's proper thrust band. That means ya gotta calc the prop curves too. Awesome fun!
Last edited by DMarley on Tue May 02, 2017 9:21 am, edited 5 times in total.
User avatar
By DMarley
I seem to recall that Alien Power Systems (I think he is a small company in the UK) can wind you a custom motor to your power specs from one of his motor shells. He can probably give you some good info if you get directly in touch with him.
Many times the torque curve on BLDC motors, and many others, are at a peak from near-zero RPM's to about 10% - 15% of the max RPM. After that, going up in rpm's, the torque falls off in a quadratic curve such that at max rpm's the torque is essentially zero Nm.
You may find that at E=70v and I=~150 amps will provide an optimal side of the torque curve to swing a proper-sized prop at an appropriate rpm. This will also possibly reduce the cost of the speed controller as well as the current is not as strong, and you can use smaller gauge wiring to boot. Lighter and less expensive and more power. Whoo hoo! At least it sounds like you're on the right track. Get those motor torque curve charts for each voltage range that you believe you may use, based upon the battery pack potentials you can assemble ! You can never know what is happening unless you can trust those charts, and/or have faith and purchase the thing, then test it and make your own chart, which you will have to do anyway just to verify the motor's output. May the FORCE be with you, young Luke! :P
User avatar
By RobinANew
i made one with 6000w motor wich is enough to fly beach
i have the alien power sytem one 18000w wich is enough to do power loops
ha ha just kidding but maybe not.

my perspective the weight is in the battery's
3lb 6000w 6lb 18000w
who cares about the 3lb the glider doe not

and the bigger motor is about the same price and its going threw less stress
same with the esc get a big one everything will b happier

big prop more efficient but more problems find a balance
i like 3 to 4 blades 26 to 32 inch 10 to 12 pitch

i will b only using the bigger one most of the time
why use some power ?
because birds flap sometimes ha ha ha

hope this helps
" math good i can't write worth a crap"
User avatar
By DMarley
Ha, almost forgot. You may want to get up with Wagner on the prop calcs. He has (I suspect) the theory backed up by prop building/using experience, so he would be a good start on your quest for the proper prop.
May the E-FORCE be with you!
User avatar
By DMarley
Yes, a smaller disk diameter is a lot easier to manage on an HG, but it will not be as much thrust as a larger disk, especially if you can swing it. Meaning, you will most definitely require a good breeze or a good downhill slope to become airborne with a smaller disk area because you cannot over-pressurize the disk. The larger the disk area, the greater the 'punch', and the more sure you will be to climb when you desperately need to, especially when you have rocks and boulders to contend with rather than nice, soft sand. As well as fickle, shifting winds rather than nice, laminar sea breezes.
You can relate disk area to say, a hydraulic cylinder. With the same, limited fluid pressure, a large cylinder area will provide more 'thrust' than will a small cylinder area.
Of course with the larger disk you may need to trim your keel / stinger back to the aft wires. It would be prudent with any sized blade back there, though. :)

Robin, could you provide more info on your system, such as ESC, battery pack voltage/amperage, rpm's, climb rate, endurance time, level flight airspeed, glider model, hook-in weight, etc? Thanks!
By tommy42
Wow, thank you so much guys. This is great information. It seems that my theories about motor torque, low RPM, and direct drive are all wrong. I also got the following input from an engineer airplane pilot friend:

“Another thing to consider is the torque needed to turn the prop at a certain speed.  The higher the RPM the more torque would be required.  Also, the gear reduction reduces the RPM but increases the torque.  For example, a motor with 14 HP at 6,000 will produce a shaft torque of 12.25 flbs.  Now if you feed that through a gear box with a ratio of 1:2.83, the output RPM would be 2,120 and the torque output would be 34.7 flbs.â€￾

So in light of all of this I will be going for a higher RPM motor and employing reduction. I have also found the Alien Power Systems web page and have think this motor looks like a really nicely built high quality product:

12090/S Sensored Outrunner brushless motor 130KV 18000W
£ 339.00 £ 309.00
MOTOR: 12090
KV: 130
ESC: 250/300A
SIZE: 120 x 90 ( without shaft )
Stator: 40mm
WEIGHT (g): 2.835
SHAFT: 12mm with 3mm keyway
Accessory pack: Yes
Internal PCB with 120 degree hall effect sensors
I could have spent $1200 in experimentation to find this out. Most likely I would have purchased the wrong motor, it would not have worked well, and I would have given up without funding more experimentation.

BTW, I have a mosquito harness that I can convert and use the props and gear reduction that came with it. I should actually just go out and fly the gas version. I'm just kind of enamored with the E-Thermal assist with respects to launching and landing on my property. Electric will be lighter and more statically balanced, and sporty, I'll still be working the thermals as much as possible with a little E-Help. I'm thinking I will save the gas stuff for when I ever get the time to go hang out with the motor heads down in La Salina Baja Mexico. An easy Google search to find the La Salina to Rosarito run that they do on the coast.

Again, thank you so much,
By tommy42
I like this one too:

Alien Power System



MOTOR: 12090
KV: 130
ESC: 250/300A
SIZE: 120 x 90 ( without shaft )
Stator: 40mm
WEIGHT (g): 2.835
SHAFT: 10mm with 3mm KEYWAY
Accessory pack: Yes
User avatar
By DMarley
I really wish that businesses like Alien and any other manufacturer / dealer of electric motors would publish not only some specs of the motors they sell, but also easily readable rpm/torque/I/E charts so that an engineer or anyone else attempting to use the motor in a new implementation could choose the best motor to fit the application. Otherwise the maximum motor specs don't mean much at all. Companies like Hacker and Plettenberg most always provide charts so that intelligent people can choose wisely. All the others seem to think we are a bunch of chumps. Industrial motors ALWAYS are chosen by charts that are verified to be accurate. Why should it be any different for smaller motors? Even most small gasoline engines have torque/rpm charts so that they can be properly chosen not just for peak power, but for the torque along a given range of rpm's.
End of rant.
User avatar
By Jim brindley
I checked out the epower hobby site and immediately found a motor suitable for a Ppg set up that will swing a 38" prop direct drive....its expensive and heavy, but other than that it seems to be made for this flphg application....or am I missing something? Jim

15070 S model...7.1 kg....38" prop....18S battery....1162.50$http://epowerhobby.com/wp-content/uploa ... motor1.jpg
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By tommy42
A friend sent this. The tables did not paste so good.

Composite Propeller Specifications
Competition Aircraft: Composite Propellers for Ultralight Aircraft Since 1983
Propeller Selection Guide
Please see Chart A and Chart B when referring to the following specifications.

A detailed list of Competition Aircraft's Recommended Ultra-Prop Combinations is also available.

Propeller selection assistance is available from your local sales representative or by contacting us.
General Information

Normally, the selection of the optimum propeller for your ultralight - and predicting propeller performance - would be a difficult and sometimes exasperating experience. The Ultra-Prop makes this task easy.

The data presented on the two charts were obtained on a dynamometer with props operating under very similar conditions to those you experience. That is, the ambient temperatures were 80-90 degrees F and the dynamometer presented a forebody similar to most ultralights.

The horsepower (HP) levels on the charts are Propeller Shaft HP. Not Rated Engine HP. Temperature, engine condition, reduction unit, intake and exhaust system differences can make your installed HP more or less than the engine manufacturers' rated HP. Don't be surprised if your static thrust is different from predicted.

Note that we use the term "pitch blocks" or "pitch angle" as opposed to the conventional pitch or advance in inches. This series of Ultra-Props has no twist in the blade, so our designation is appropriate.

Ultra-Props are carefully engineered and tested for good performance and high structural integrity. Your particular ultralight, however, may have different vibration characteristics than those we have experienced - that is, your engine, reduction unit, extension shafts and inflow may present different vibration inputs to the propeller. When you install your Ultra-Prop, assure yourself that the installation is at least as smooth as that you have had with a balanced wooden prop. In most cases the Ultra-Prop will be significantly smoother. If not, carefully check your power system installation. If nothing is amiss, contact Competition Aircraft for engineering assistance.

If a loud "buzzing" noise is noted at high RPM with high-pitch blocks, it is because the tips are stalling. Do not operate the propeller in this range. Either a lower pitch or smaller dia. prop must be used.

Tests have shown that at and below 52-in. dia., no blade tip stalling will occur with 18-degree pitch blocks below the rated 3100 RPM.

Between 52" dia. and the full 59" dia., the approximate tip stalling RPM or onset of buzzing is shown in the table below for various pitches.

18 deg pitch

16 deg pitch

14 deg pitch

1900 RPM

2400 RPM

2700 RPM






















Sample Calculation Using Charts

Use these charts to find the number of Ultra-Prop blades and their angle of pitch to match your specific ultralight.

To begin, you must know your propeller RPM, your engine HP and the diameter of your present prop or diameter that will match your airframe structural limit.

The following typical data is used in our calculation example:

Engine: Rotax 277, 25 H.P. @ 6200 RPM
Reduction Unit: Integral Rotax gear box 2.58:1
Prop RPM: 6200/2.58 = 2400 RPM
Prop dia.: 56 in. (assumed airframe structural limit)

First, we solve for a 59" dia. Ultra-Prop (See Chart A). We find that the 2400 RPM line intersects with a 25 HP curve in the 2-blade, 3-blade, and 4-blade envelopes. Reading the locations of these 3 points (circled) on the pitch degree lines identifies three 59" dia. Ultra-Prop options as 14 degree 2-blade, 11 degree 3-blade, or 9 degree 4-blade. To solve for a 52" dia. Ultra-Prop (See Chart B) we find that the 2400 RPM and 25 HP intersections identify only two different options: 18 degree 3-blade, or 13 degree 4-blade.

Since our example requires solving for a 56" dia., we must interpolate between the 52" and the 59 " dia. data we read from the two charts. A table such as the following is helpful:




Prop Dia. (")

52 56 59

52 56 59

52 56 59

Pitch (degrees)


18 (14) 11

13 (11) 9

Thrust (lb.)


163 (179) 182

170 (180) 188

To complete the table for 56" dia. (that is, to insert the numbers in parentheses), we note that this diameter is 56-52/59-52 = 4/7 = .57 of the way up from 52" dia. The pitch angle for 56" dia. can be approximated as follows:

For the 3-blade prop:

Incremental pitch angle = .57 (18 deg. - 11 deg.)

= .57 x 7 deg. =4 deg.

Pitch at 52" = 18 deg. minus incremental pitch decrease of 4 degrees equals 14 degrees.

We must subtract the 4 degrees from 18 degrees since we are going up 4/7 to 59". The 4-blade prop pitch for 56" dia. is computed the same way.

Completing the table for a 2-blade prop is not practical, since all required information is not available from the charts. We know that the pitch required for a 56" dia., 2-blade prop would be higher than the 14 deg. we saw for a 59" dia. 2-blade prop. Looking at the table for tip stalling, we see that at 16 deg., 59" dia. and 2400 RPM we would already be at critical stall angle.

To complete the table for estimated static thrust, the 3-blade calculations are as follows:

Incremental thrust

Increase = .57 x (182 lb. - 163 lb.)

= .57 x 29 lb.

= 16 lb.

Thrust at 52" = 163 lb. minus the Increm. Increase = +16 lb.

Thrust at 56" = 179 lb.

Estimated thrust for the 4-blade prop is calculated the same way.

Which prop should we choose? The 3-blade prop should be used since it is the lightest. Also, its odd number of blades makes it the smoothest running. If the 4-blade is selected, the thrust difference in this case is insignificant.
Competition Aircraft, Inc.
10925 Shire Court
Grass Valley, CA 95949
Toll Free: 888-634-9839
Voice: 530-268-3048 Fax: 530-268-2321

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User avatar
By AlaskanNewb
I have an ehelp I will sell.

Here is pic
e help.jpg
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By tommy42
Hi All,

Well I was all ready to buy the Alien Power 12090/S Sensored Outrunner brushless motor 130KV 18000W
and use reduction, then I saw the Alien Power 15070/S Sensored Outrunner brushless motor 27KV 30000W Paramotor and contacted the Alien Power dealer about it. He said yes, this motor has plenty of torque for a low RPM motor and can be used direct drive, and he said the 130KV 18000W motor was good for glider usage with reduction. Then I saw this video E-HELP LIKE BUT MORE POWERFULL (home made) ELECTRIC MOSKITO 2
where the guy makes these claims:
“In this EXPERIMENTAL PROJECT we use a 6 lbs motor DIRECT SHAFT that could delivery 25 kw at 100 VOLTS , but we use only 17 kw at 66 volt so the motor produce less heat . we also use 6 or 9 LITHIUM POLYMER BATTERY 16 AMP 22 VOLTS . 3 IN SERIES to got 66 VOLTS x 2 OR 3 in PARALLEL to obtain 32 AMP OR 48 AMP , autonomy is aproximatively 1 hrs 10 min . “
So I went back to Alien Power and saw this motor 120100/S Sensored Outrunner brushless motor 50KV 25000W $ 606.04 thinking it would be a simular motor used in the EXPERIMENTAL PROJECT video. Then I started reading the comments in the EXPERIMENTAL PROJECT video where someone asked him where he got his motor and he provided a link to it. I seems that he has found a direct China Factory outlet. I found a motor ESC combo 2017 Hot Sale MP120100 / KV50 Outrunner Brushless Motor With 22 S 380 A ESC for a very good price Dongguan freerchobby (dang they will not let new users add all these links here)

When you run the motor at 66V like the guy is on the EXPERIMENTAL PROJECT video that puts you in at 3300RPM. At top voltage it is about a 33hp motor, but as the guy in the EXPERIMENTAL PROJECT video says, running it at lower power, 66V rather than 100V yields about 17hp and helps keep the motor running cool.

Take care,
User avatar
By DMarley
Hey Garrett,
Tell us more about your e-help.

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