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-   -   Sizing LiPo and ESC for Electric Sailplane (https://www.wattflyer.com/forums/showthread.php?t=82544)

svenyonson 06-20-2022 08:23 PM

Sizing LiPo and ESC for Electric Sailplane
I am finishing a sailplane that my Dad started around 30 years ago. So, I am replacing all of the electronics with current technology. I recently bought an NX6/AR6610T combo as well as a 3S 2200mAh 30C 11.1v smart battery and a 30 amp smart ESC. For the motor, I needed an inrunner to fit the existing engine mount, and I found an 1880kv from an old ParkZone F-27B. I added a folding prop and it works great on the bench.

I am thinking that the battery and ESC may be overkill for this plane, since I will only be using the motor for 3-5 minutes at launch. I'd like to reduce the weight a bit but keep a reasonable margin of safety for run time and current capability. I just measured the DC current draw on the battery with a loop ammeter, and at full throttle I am only using 5 amps.

So, it seems like I could probably go down to even a 15 amp ESC and a 1300mAh smart battery?

The electric sailplane, with servos, but no receiver, battery, ESC, or motor - is 428 grams.
Wingspan: 66"
Length: 36"
Chord: 7.5"
Stab length: 18"

I am brand new at this, so I'd be interested to hear from some experienced electric folks.

UPDATE: I just realized that my current reading of 5.0 amps was incorrect. My ammeter is meant for AC current, not DC current. Reading the telemetry data, sure enough, I can get the current up to around 22 amps. So, I'll leave my question posted, mainly about sizing the battery.

quorneng 06-21-2022 05:26 PM

My first question is have you any idea what the sailplane was intended to weigh with the original gear. It would be advisable to keep within that weight, particularly if it originally used a brushed motor and NiCd batteries as it may be close to the wing's structural limit.
Even keeping to the same weight using brushless and LiPo should give you at least 1.5 times the power and duration of the original installation .
You also need to know what is the intended maximum amps for the motor and the size of prop required to keep within that limit when using a 3s battery. It is the amps drawn that gives you an idea of the full power duration for a given size of battery.
For example 22A draw would give you an absolute maximum of 6 minutes (1/10 of an hour) from a 2200 mAh battery.
If your motor and prop are well matched I suspect you won't need anything like a 3-5 minutes duration at launch. I would expect it would climb at full power to an "useful" 400' altitude in no more than 30 seconds allowing you to repeat such a climb several times in the course of a flight..
Unless you are using a 'known' motor and prop combination where the figures are likely to be available you will be in for a bit of 'trial and error' to get the best performance.
Remember with a LiPo battery if you get it wrong by much you can easily terminally damage everything and all at the same time. A suitable DC Wattmeter is a wise investment. They are not expensive.

svenyonson 06-21-2022 06:46 PM

Thanks for the excellent information and suggestions. I don't know which sailplane it is, only that the kit is from the late '80s. This was my Dad's first R/C project, so I can't say whether he got things right for the parts he had installed. I can say that so far, everything that was present and since replaced has been replaced with something lighter/smaller. Even the 2200mAh LiPo battery that I have is lighter than the NiCad that he had in it. (168/205 gms). The F-27B from which this motor came also came with a 3S 2200mAh 30C 11.1v, but since I'm flying a sailplane and not a stunt plane, it seems like I am safe going to the 1300mAh version, still 30C. The motor/prop "seem" well matched - it generates an incredible amount of thrust at full tilt, and seems I wouldn't need even close to that on launch. (which brings me to a side question: For getting the sailplane to initial glide altitude, is is better to go slower or full throttle, in terms of battery runtime/health?)

I don't need huge flight times, so not sure how many times I will need to repeat climbs during the flight. And my NX6 setup has telemetry for the battery status, so I can decide whether it is safe for another climb. I look into a DC Wattmeter.


(and yes.. I am prepared for lots of trial and error!)

ron_van_sommeren 06-21-2022 07:30 PM

Originally Posted by svenyonson (Post 1033873)
... and yes.. I am prepared for lots of trial and error! ...

Some info to reduce your trial and error process a bit.

Current and power drawn are proportional to velocity_konstant_Kv³ :eek::eek::eek:
That is (constitutes???) a very disproportional effect.
Some silly, some expensive and some realistic risky examples.
Three straightforward methods for determining unknown Kv

Without a watt/volt/current/multi-meter you are in the dark.
Until something starts to glow :( ;)

svenyonson 06-21-2022 08:10 PM

Wow, so much to consider - I had no idea the science behind it all was so complicated! I tried to use a few of the online motor/prop calculators, but surprisingly the databases don't seem to have any of the Spektrum gear, which is 100% of what I am using, except for the motor and the prop. I guess I will rely heavily on the watt-meter when it arrives.

I know this is a bit of guestimation, but I am thinking that my setup will be the very similar to the off the shelf ParkZone F-27B (motor, battery identical if I use the 2200mAh). The differences are in the ESC (ParkZone 25A vs the Spectrum 30A w/telemetry) and the prop. I am using a 9x5 folding prop (APC09050F), the F-27B uses a 5x3 prop. The props are significantly different, and so maybe this could be an issue?

I ran the power train on a bench setup to discharge the LiPos to storage level, at about 1/2 throttle for quite a long time, 8-10 minutes maybe?. Battery stayed cool, no indications if any issues looking at the telemetry data. I will run again with the watt-meter and see what I get.

[UPDATE: I just ordered APC 7x4 and 6x3 folding props so I can measure and see the difference in current draw.]

ron_van_sommeren 06-21-2022 10:18 PM

Originally Posted by svenyonson (Post 1033875)
Wow, so much to consider - I had no idea the science behind it all was so complicated!

It's only exponentiation :)
That's where e-calculators come in handy.
But, as with all software, garbage in, garbage out.

Originally Posted by svenyonson (Post 1033875)
... I am using a 9x5 folding prop (APC09050F), the F-27B uses a 5x3 prop. The props are significantly different, and so maybe this could be an issue? ....

Yes, two very different props.
E-motors try to keep same constant rpm, as set by voltage and Kv, no matter the load. The ESC and motor will pass on the current the system needs/wants/draws even if it would lead to their fiery death.

Current and power drawn are proportional to pitch and proportional to diameter⁴.
The 9"×5" prop will draw a lot more current and power, by factor
(pitch2/pitch1) × (diam2/diam1)⁴

I'm a motor man, know precious little about optimal plane/prop/rpm combinations. Usually the larger the prop-diameter, the better w.r.t. effectivity.

I always mention the disproportionalities to give some insight, and as a warning. Like you I cannot calculate absolute current and power drawn either.
Rpm and voltage are proportional.
Torque and current are proportional.

Current and power drawn are proportional to pitch and proportional to diameter⁴.
In your second case, going from 3" to 4" pitch current and power drawn want/try to increase by factor (4/3)=1.3.
Going from 6" to 7" diameter would give an additional increase by factor 1.8.
Resulting a combined increase by factor 1.3×1.8=2.4 :eek:
This is worst case, ignoring extra voltage losses in battery, wiring, controller and motor caused by higher current.

svenyonson 06-22-2022 03:05 AM

Thanks so much, great information.

I happened to scroll down to see "EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT". Man, what a great read, I wish I had stumbled upon that earlier. One of the things I had wrong was that I assumed a 2200mAh 30C pack and a 1300mAh 30C pack could deliver the same power to the motor. Now I see it's not just the "C" value, but the battery capacity as well.

Lots and lots to learn. I'm an engineer and thought this would be all be easy peasy, but there are lots of subtleties and easy to overlook some aspects of putting together a power system.

Very thankful that I found this forum. Thank you everyone.

quorneng 06-22-2022 07:03 PM

I would be very concerned that if the same motor was working effectively on a 5x3 then with a 9x5 the motor will be considerably over loaded.
As Ron points out a brushless motor will try to run at a rpm figure set by its kV and the voltage applied. In other words as you are using the same voltage the motor will try to turn the 9x5 at the same rpm as it did with the 5x3. My 'educated' guess it will be drawing at least twice as many amps to attempt to do that. It doubt it will be able to do that for long before it overheats and just as likely the speed controller burns out.

Just remember a battery will try to deliver the power the motor is asking for. The capacity of the battery determines how long it will be able to do it. The C rating of the battery just tells how easily the battery will be able to deliver the power.
C times the battery Amp Hours gives the max Amp limit for the battery. 30x2.2 = 60A. Be aware most manufacturers considerable over rate the C rating of their batteries for sales & marketing purposes. The C rating should be considered a burst figure of no more than 10 seconds duration.

As far as gliding is concerned if you are looking for thermals then it is better to climb as fast as possible so you have time to search. A glider under power will tend to mask the effect of any thermal lift.
Having said if the object is to learn to fly the plane the longer you fly the better so gentle cruising will be both aerodynamically & electrically more efficient thus giving the longest time in the air.

I hope this helps.
Any chance of a picture of your glider?

ron_van_sommeren 06-22-2022 09:55 PM

Originally Posted by svenyonson (Post 1033878)
... I happened to scroll down to see "EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT". Man, what a great read ...

Some well-structured reading for rainy/windy days, including e-flight_101 & 102, and some handy e-tools. Don't try to read, take it in, all at once, as long as you know/remember were to find the relevant info.
Will save you, and us :D a lot of questions. Notably the 'what went wrong?' kind of questions :(
Will also prevent you from burning up several controllers and/or motors and/or battery:
E-flight primer and tools

ron_van_sommeren 06-22-2022 09:58 PM

Originally Posted by svenyonson (Post 1033878)
... Now I see it's not just the "C" value, but the battery capacity as well. ...

Best to take advertised C-ratings with a grain of salt, some are downright silly, if not risky. I would half them at least by factor two.

About C-ratings, stickertuning and other blatant lies. Using uniform tests, getting published/referred in opening posts:
Battery Load Test Comparisons - RCG

From the link posted by JohnCT, this well help, some excellent battery info
LiPo best care practices - post#8346 - RCG
  • Maximizing Cycle Life and Performance
  • Storage Facts
  • Fun Facts
  • Nerd Facts a.k.a Solid-Electrolyte Interphase (SEI) layer
  • Thermal Runaway
  • Results, Comparisons, and Lies exposed in opening posts

And there's this post#198 by Frank Siegert, about (yet) another advantage of heating a battery to body temperature, through and through, for about one hour. Before using and before charging. Not only voltage is higher and battery wear lower, but also ...
Short flight time is killing me - post#198

Want more background info?
Lithium Polymer Battery Technology - RCG
An introduction, with special consideration of RC model lithium batteries.
By dipl.-ing. Frank Siegert, a.k.a. RCG user bzfrank.


ron_van_sommeren 06-23-2022 06:26 PM

A much faster method
Find same/similar plane (type, mass, span) and copy the successful power system. That way you can fly and rummage/read through the engineering stuff (I know those techo-geek-speak types all too well :D).

The paid version of eCalc has a lot more motors in it's database, at the cost of one broken prop per year.
Well worth the money.
Help/support, announcement, suggestions thread by eCalc developer Markus Müller (Airbus/Boeing driver)
eCalc: Q&A for propCalc, fanCalc, heliCalc, xcopterCalc - RCG

By Louis Fourdan, retired radar engineer
Scorpion Calculator - RCG

By a.o. Helmut Schenk, one of the German e-flight fathers.
Motor database gets updated on a regular basis.
Available in English and German, for Windows, Mac and Linux.

More calculators and calculation methods
e-flight calculators (compilation) - RCG

ron_van_sommeren 06-23-2022 07:14 PM

Below an excellent quote about motor selection and powersystem misconceptions
brushless motors Kv?.

Originally Posted by scirocco (Post 27290157)
While an absolutely critical part of the system ...
... Kv is actually the item one should choose last.
  1. Decide your peak power requirement based on the weight of the model and how you want to fly it:
    Magic numbers for modellers.
  2. Pick a preferred cell count (voltage) and pack capacity for how to deliver the power.
  3. Pick a prop that will a) fit on the model and b) fly the model how you want - often as big as will fit is a good choice, but if high speed is the goal, a smaller diameter higher pitch prop will be more appropriate.
  4. Look for a size class of motors that will handle the peak power - a very conservative guide is to allow 1 gram motor weight for every 3 watts peak power.
  5. Then, look for a motor in that weight range that has the Kv to achieve the power desired with the props you can use - a calculator such as eCalc allows very quick trial and error zooming in on a decent choice. For a desired power and prop, you'd need higher Kv if using a 3 cell pack compared to a 4 cell pack. Or for a desired power and cell count, you'd need higher Kv if driving a smaller diameter high speed prop compared to a larger prop for a slow model.
The reason I suggest picking Kv last, is that prop choices have bounds - the diameter that will physically fit and the minimum size that can absorb the power you want. On the other hand, combinations of voltage and Kv are much less constrained - at least before you purchase the components.

So Kv is not a figure of merit, in that higher or lower is better, it is simply a motor characteristic that you exploit to make your power system do what you want, within the constraints you have, e.g. limited prop diameter, if it's a pusher configuration, or if you already have a bunch of 3S packs and don't want to buy more, and so on.

Minor lay-out changes by RvS

svenyonson 06-24-2022 02:22 AM

Thanks everyone - all good advice. I just got my Watt-Meter today. Smaller folding props will come tomorrow, and I will purchase a license for eCalc. We have a wedding to set up tomorrow, but next week I'll be on it!

One additional thought about props - While a 9/5 is much larger than a 5/3, The 9/5 also is a folding prop, where the actual tip to tip measurement is actually only 8.25" and of that length, 1.75" of it is the hub. So actual length that can generate thrust is only 6.5", whereas a fixed prob can generate thrust for most of its length. Does this matter at all?

I will post a pic soon.

svenyonson 06-24-2022 02:26 AM

1 Attachment(s)
Here is a photo of the sailplane. I have no clue which kit it was built from.

quorneng 06-24-2022 10:08 AM

That looks to be a nice well made plane which should fly well. As a balsa/tissue construction glider I would be careful not to add too much weight. Do you know its likely total weight will be?

What absorbs the power is the speed each part of the prop is travelling through the air. Clearly the tip of a 8.25" prop at the same rpm is travelling faster than the tip of a 5" by a factor of 1.65. The problem is the power to push the prop through the air rises as the square of the speed so the 8,25" will require at least 2.7 times the power made worse by the fact it also has has a greater pitch (the angle of the blade to the air) which adds still further to the power requirements. No brushless motor that is running at anyway near is maximum power with a 5x3 can run for long with a prop that requires 3 times the energy to turn it. This is turn means 3 times the amps and 3 times the heat created. For a brushless motor getting rid of the heat generated within the motor tends to a limiting factor to the power it can produce.

The only way to be sure with a non standard prop/motor set up is to measure the amps the motor is taking at full power and compare it to the motor's design specification. This of course assumes you can find out what is was. You do know you speed controller has a limit of 30 A so that is a factor particularly as the electronics inside it do not tolerate going over their limit. Remember the max amp limit is set by the components attached to the battery not by the battery itself.

. .

svenyonson 06-24-2022 02:54 PM

It seems like I have been going about this backwards - finding a motor and then trying to find a prop/ESC/battery that will work with it. I'm thinking I should start with determining how much thrust I need, finding a prop/rpm that will deliver that thrust, find a suitable motor, and then ensuring the ESC/battery can deliver the power required. When I was shopping for new motors, almost every motor I looked at was an outrunner. I can possibly change the motor mount and build a custom cowl for an outrunner.

svenyonson 06-24-2022 03:52 PM

Another approach. I found this plane today, which has the same empty weight and basic configuration. My plane has a larger wing area, so the wing loading is about 75%. Likely more drag though. This motor has a max current of 30a using an 8/4 folding prop, so it seems like electrically this is something that could work, if it has enough power to sufficiently lift the plane.

ron_van_sommeren 06-24-2022 05:44 PM

Originally Posted by quorneng (Post 1033894)
... This is turn means 3 times the amps and 3 times the heat created. ...

You typed a bit too fast there, since resistive losses are proportional to current squared (Ploss=I²⋅Rcopper), power losses would go up by factor 3²=9.

Originally Posted by quorneng (Post 1033894)
... No brushless motor that is running at anyway near is maximum power with a 5x3 can run for long with a prop that requires 3 times the energy to turn it. ...

Yes, it would go up in smoke in a 1-2 seconds.

Originally Posted by svenyonson (Post 1033904)
... I'm thinking I should start with determining how much thrust I need, finding a prop/rpm that will deliver that thrust, find a suitable motor, and then ensuring the ESC/battery can deliver the power required. ...

Much better, the previous method was a bit like putting the horse behind the cart.

svenyonson 06-27-2022 02:10 AM

Hi Everyone,

Having fun learning about all of this. I found an old thread from 2007 about the Stryker F-27c, from which this motor came. After reading several hundred posts, I found one that stated the measured current draw of the stock setup: 23 amps. Yesterday my smaller props arrived, as well as the watt-meter. I soldered IC3 connectors to the watt-meter, with the third wire (sense? control?) as a pass-thru from battery to ESC. As expected, a short run with the 9/5, not near full throttle, registered over 35 amps and I could smell the heat from the tape holding the motor into the mount. I shut that down pretty quickly. Then I mounted the 6/3. Ran very cool, as expected, low 20s current draw. The 7/4 seems to be the sweet spot, between 23-24 amps, and around 150 degrees motor temperature.

All tests seemed to produce plenty of thrust, the 7/4 or 6/3 should work (I think)..

But... it's very loud.

I have been searching (googling) a LOT looking for a better motor match. I'm thinking I'd like a much lower Kv motor that can swing a larger prop to achieve the same thrust, but at the same time be much quieter. If I want to use the installed engine mount (very much preferred), I need an inrunner with a diameter of ~28mm, and a length up to 55mm. Most of the inrunners I find are HUGE, I think intended for crawlers.

Any recommendations on this type of motor - 28mm diameter and low-ish Kv, around 25 amps?

quorneng 06-27-2022 11:01 AM

2 Attachment(s)
Unfortunately by their design in runners tend to have a higher kV for a given motor diameter. In addition as out runners are used by virtually all quads the market is much larger hence there is a wide range of sizes available and they are cheaper too!
Further as the rotating part is 'external' an out runner is to a degree self cooling allowing them to be run that bit harder for a given motor weight.

Rather then burying the motor inside the nose you can use the over-hung nature of an out runner to advantage.
Attachment 188775
As a home build it was simple to shape the nose of the fuselage to exactly match the 28 mm diameter of the 1200 kV out runner..
Attachment 188776
It runs on an 850 mAh 3s with a 7x4 prop. Stuck out in the air stream motor cooling is not an issue.
The plane is so light it doesn't really need a folding prop but one could be fitted.
It is also quiet in fact the motor 'whine' is louder than the prop noise.

svenyonson 06-27-2022 03:52 PM

Wow, that is a great idea! I guess I was fixated on using the existing cowl and motor mount, but yes, I could 3D print a new cowl with an integrated motor mount. Thanks!

As it turns out, coming full circle, when I bought the 9/5 prop and 30A ESC from my local hobby shop, he also sold me this outrunner, which I am thinking may work perfect if I print a new cowl/mount:


ron_van_sommeren 06-27-2022 10:07 PM

Originally Posted by svenyonson (Post 1033916)
... But... it's very loud. ...

Propblades moving close to a post, table top, wing leading/trailing edge?
High rpm?
Prop not balanced yet?

Originally Posted by svenyonson (Post 1033916)
... I could smell the heat from the tape holding the motor into the mount. ...

Let's hope your motor did not get too hot. Feeling/measuring the can/bell of an outrunner doesn't tell much because the heat is produced in the statorcoils.
  1. Overheating magnets causes de-magnetizing (irreversible! :(), which in turn results in higher velocity konstant Kv. Motor wants to run faster, thus drawing more current and more power.
    Motorcurrent and -power drawn are proportional to velocity konstant Kv³ :eek::eek::eek: , resistance losses in the copperwire are proportional to current² :eek::eek:
    Therefore resistance losses are proportional to Kv⁶ :eek::eek::eek::eek::eek::eek:
  2. Higher current → hotter motor → weaker magnets → higher Kv → higher current → hotter motor → weaker magnets → higher Kv → higher current ⟲⟲⟲ etcetera etcetera.
    Thermal runaway, major conflagration :(
  3. Determining whether magnets still have original strength, by determining whether velocity konstant Kv (in rpm/volt) is still the same, three simple straightforward methods.
    www.bavaria-direct.co.za → motor constants
  4. Generator test, only using a power drill and a voltmeter
    (Re)winding and building motors - RCG (sticky) → opening post → #40 Generator test.
  5. If you don't trust your ESC: use another brushless motor as a generator:
    Three Phase Alternator - Three Phase Motor? - RCG

ron_van_sommeren 06-27-2022 10:23 PM

Originally Posted by svenyonson (Post 1033916)
... with the third wire (sense? control?) ...

Two of the three motorwires consecutively take turns (very fast) for providing motorpower, the third is used for measuring rotor rpm and position.
It's the motor that tells the controller when to switch/commutate. A brushless controller is an electronic commutator/switch, instead of the mechanical commutator in brushed motors.

3-phase power voltages to brushless motors in pictures:

edit: ha, you where talking about other watt-meter wires, not about motorwires.

quorneng 06-30-2022 10:21 AM

That 970 kV motor should be ideal for a 9x5 on a 3s.
Note the motor wires exit from the front of that motor so you have to be careful how you run them back into the fuselage. There must be no possibility of the wires rubbing on the rotating exterior of the motor.

ron_van_sommeren 06-30-2022 10:07 PM

Or use an outrunner disguised as an inrunner.

AXI Cyclone series

More/bigger pictures of the inside, the shaft is a piece of art:
Outrunners forHotliners p now with video!!!/page6 - RCG

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