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Jun 28, 2018, 02:50 PM
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mdickey's Avatar
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Picking out a motor


How do you guys pick out motors for sailplanes? I have yet to find any good information on this topic. Building a Paragon right now and going to put a motor in the nose.
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Jun 28, 2018, 03:32 PM
Sagitta Fanboy
I like a strong climb to altitude, so I go for the 100-125W/lb recommendation for ALES competition airplanes (which are expected to climb to 200m in less than 30 seconds).
Jun 28, 2018, 05:04 PM
Registered User
There's no mathematicaly correct formula to calculate the right motor. As mawz points out, 100 - 125 watts/pound for an ALES climb should do it. If your just flying for fun, then 70 + watts/pound should do it.
As far as efficiency goes, start with somewhere around 800 - 1000 Kv. That should reasonably fly it. After that, is up to you whether you want to spend $$$$$, not to mention hours, experimenting with different motor, prop, esc, and battery combinations.
Jun 28, 2018, 05:57 PM
If it flies - I want one!
Petem's Avatar
A recent similar query with some detailed advice, and one of the posters gave a link to his blog with more:
https://www.rcgroups.com/forums/show...n#post39787401
Cheers,
PeteM
Jun 28, 2018, 06:18 PM
Registered User
craigrrr's Avatar
Quote:
Originally Posted by mawz
I like a strong climb to altitude, so I go for the 100-125W/lb recommendation for ALES competition airplanes (which are expected to climb to 200m in less than 30 seconds).
So, the Paragon will weigh about 48 oz (3 lbs) .

As Mawz suggests, assuming the motor should develop about 100 watts per pound x 3 pounds = approximately a 300 watt motor would be required.
I have several planes in this weight range and use an outrunner motor with a folding prop that will give about 350 watts with the appropriate prop.
I run motors either full power or no power. Full power gets the plane at altitude quickly and efficiently; I just use an on/off toggle switch, not a stick to control the motor.

A 3 cell (3s) PiPo battery with a good C rating (of C=30 or higher), size of 1800 to 2200 milliamp hours would work.
You will have to buy a battery charger that can charge LiPo (lithium poly) batteries.

Power (watts) = volts x amps. A 3s (3 cell) LiPo battery delivers about 12 volts. Amps = watts / volts; 350 watts / 12 volts = 29.2 amps.
So, you need a speed controller that can easily take this current. I suggest a Turnigy Plush 40 amp speed controller ($24).

"Outrunner"brushless motors can be inexpensive. But they have a body that spins. if you put it inside of the fuselage (behind the fire wall, as almost everyone does), the diameter of the outrunner must clear of the fuselage interior and must clear the three motor wires that go from the motor to the speed controller.

It is possible to get an outrunner motor that mounts in front of the firewall. And if the fuselage is wide enough, you can build a simple solid balsa cowling to surround the motor for better looks. This is my preference, but I am in the minority.

For skinny fuselages, most people use an "Inrunner" brushless motor. These motors turn at high RPM so they are connected to a gear box that reduces the RPM. These motors are more expensive than Outrunner motors.

So, for your relatively wide bodied Paragon I would advise looking on Hobbyking.com for an outrunner motor.

I like the Turnigy LD3727A outrunner that is designed to be in front of the firewall (not inside the fuselage).
It is rated at 450 watts, 42 amps, 1300 rpm/volt, weighs 86 grams. For an outrunner it is good to have low rpm/volts, maybe a range of 700 to 1300 Kv.
I usually use a 9-5 folding prop which will give about 350 watts. A 9-6 prop will give over 400 watts.
It costs a mere $14 and is dependable and high quality.
But, like most people you will probably decide on an outrunner that mounts behind the firewall (inside the fuse).

If you are going to continue using electric motors in this hobby, the single most important thing you should buy is a WATT METER.
You plug it in between the battery and the speed controller; it will give you lots of info (min and max current, volts, watts).
It will tell you exactly what is going on and make sure you do not fry an undersized speed controller or motor.
HobbyKing sells one (Turnigy 180 Amp Watt Meter & Power Analyzer, $25).

I wish you success. I have heard that the Paragon is a great old soaring machine.

Craig
Jun 28, 2018, 07:46 PM
Sagitta Fanboy
Note for conversions like this that there are some 'Outrunner in a Can' motors, where the outer casing doesn't turn and the ESC connectors are at the rear. The Turnigy GliderDrives are like this and AXi makes some too.

These make installation much easier as there are no clearance problems and they mount easily behind the firewall.
Jun 28, 2018, 08:01 PM
Registered User
mdickey's Avatar
Quote:
Originally Posted by craigrrr
So, the Paragon will weigh about 48 oz (3 lbs) .

As Mawz suggests, assuming the motor should develop about 100 watts per pound x 3 pounds = approximately a 300 watt motor would be required.
I have several planes in this weight range and use an outrunner motor with a folding prop that will give about 350 watts with the appropriate prop.
I run motors either full power or no power. Full power gets the plane at altitude quickly and efficiently; I just use an on/off toggle switch, not a stick to control the motor.

A 3 cell (3s) PiPo battery with a good C rating (of C=30 or higher), size of 1800 to 2200 milliamp hours would work.
You will have to buy a battery charger that can charge LiPo (lithium poly) batteries.

Power (watts) = volts x amps. A 3s (3 cell) LiPo battery delivers about 12 volts. Amps = watts / volts; 350 watts / 12 volts = 29.2 amps.
So, you need a speed controller that can easily take this current. I suggest a Turnigy Plush 40 amp speed controller ($24).

"Outrunner"brushless motors can be inexpensive. But they have a body that spins. if you put it inside of the fuselage (behind the fire wall, as almost everyone does), the diameter of the outrunner must clear of the fuselage interior and must clear the three motor wires that go from the motor to the speed controller.

It is possible to get an outrunner motor that mounts in front of the firewall. And if the fuselage is wide enough, you can build a simple solid balsa cowling to surround the motor for better looks. This is my preference, but I am in the minority.

For skinny fuselages, most people use an "Inrunner" brushless motor. These motors turn at high RPM so they are connected to a gear box that reduces the RPM. These motors are more expensive than Outrunner motors.

So, for your relatively wide bodied Paragon I would advise looking on Hobbyking.com for an outrunner motor.

I like the Turnigy LD3727A outrunner that is designed to be in front of the firewall (not inside the fuselage).
It is rated at 450 watts, 42 amps, 1300 rpm/volt, weighs 86 grams. For an outrunner it is good to have low rpm/volts, maybe a range of 700 to 1300 Kv.
I usually use a 9-5 folding prop which will give about 350 watts. A 9-6 prop will give over 400 watts.
It costs a mere $14 and is dependable and high quality.
But, like most people you will probably decide on an outrunner that mounts behind the firewall (inside the fuse).

If you are going to continue using electric motors in this hobby, the single most important thing you should buy is a WATT METER.
You plug it in between the battery and the speed controller; it will give you lots of info (min and max current, volts, watts).
It will tell you exactly what is going on and make sure you do not fry an undersized speed controller or motor.
HobbyKing sells one (Turnigy 180 Amp Watt Meter & Power Analyzer, $25).

I wish you success. I have heard that the Paragon is a great old soaring machine.

Craig

Craig,

Is it possible to put the firewall former a few inches inside the fuselage and use the sides as the basis for a cowl? It would still get plenty of air flow. I like the idea of a $14 motor to pull the plane up.
Jun 28, 2018, 08:03 PM
Registered User
mdickey's Avatar
Quote:
Originally Posted by mawz
Note for conversions like this that there are some 'Outrunner in a Can' motors, where the outer casing doesn't turn and the ESC connectors are at the rear. The Turnigy GliderDrives are like this and AXi makes some too.

These make installation much easier as there are no clearance problems and they mount easily behind the firewall.
The glider drive series seems like a decent option here, but I'm afraid the first former is a bit small to fit one. My current idea is to rear mount the motor a bit further inside the fuse and then use the fuse sides to protect the can itself.
Jun 28, 2018, 09:09 PM
Registered User
craigrrr's Avatar
Quote:
Originally Posted by mdickey
Craig,

Is it possible to put the firewall former a few inches inside the fuselage and use the sides as the basis for a cowl? It would still get plenty of air flow. I like the idea of a $14 motor to pull the plane up.
This motor has a rather large diameter of 37 mm; I do not know the width of the Paragon fuse.

Also, the selection and placement of a motor depends a lot about achieving the proper center of gravity of the plane.
The motor I mentioned only weighs 86 grams; if you move it back, you might end up with a tail heavy plane.
So, you probably need to do a trial positioning of all your radio stuff in a nearly completed fuse.

The Glider Drive may make more sense to achieve the proper CG because it weighs about twice what my motor weighs.

So, consider the CG and the weight required as part of the motor selection process.
And you do not want to add lead weight anywhere.

Craig
Jun 30, 2018, 10:58 AM
Registered User
Avaldes's Avatar
Might try one of the online performance calculators. Here is one that is free on the Hacker USA website:

https://ecalc.ch/motorcalc.php?usahacker
Jun 30, 2018, 07:01 PM
Registered User
If you access ecalc from Castle Creation website, you get much more motor data.

I prefer planetary geared inrunners for sailplanes.
Latest blog entry: Lt25
Jul 01, 2018, 08:54 PM
Registered User

Motor mounted on front of firewall


I built my Oly II similar to the set up you were asking about a few posts ago. Firewall a few inches behind front of fuselage. I built a permanent "cowl" around the motor which then matched up with the spinner.

BTW the Oly weights 55 oz. I put a O.S. 3810 motor (1050 kV), 3S battery and an 11x7 prop. eCalc on the Castle website computes 303 watts and a climb rate of 5.6 m/s. So in theory I should only get to 168 meters in 30 seconds. On the bench, the watt meter showed about 10% more power (with a full battery) - so about 97 watts/pound). At the field it easily tops 200 meters in the 30 sec time limit. So, eCalc is a good tool to play around with motor, ESC, and prop combinations. But the performance results may be bit pessimistic for sailplanes.

- Mike
Jul 02, 2018, 01:35 PM
Build, Fly, Crash...Repeat!
Mike,

That's a beautiful build!
Jul 07, 2018, 06:45 PM
Registered User
mdickey's Avatar
Thanks for all the tips, I will have to check parameters on the website mentioned above. Decided to put a glider drive into the paragon, as it has a rather wide fuselage and will accept the motor. More updates soon!
Jul 09, 2018, 03:57 AM
myday.. myday
Quote:
Originally Posted by mdickey
How do you guys pick out motors for sailplanes? I have yet to find any good information on this topic. Building a Paragon right now and going to put a motor in the nose.
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