Folding prop for 2-meter thermal sailing - RC Groups
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Feb 03, 2013, 02:06 PM
ripacheco's Avatar

Folding prop for 2-meter thermal sailing

I'm new to electrics. Trying to make sense of all this. I am building a two meter glider and my intention is to put a prop on it for "powered launch". (about 30 sec powered launch)

In their "power packages" Espirit models suggests:
  • 1.5-2 m sailplane...Aeronaut folding 12.5x6
  • 2-2.5 m sailplane...Aeronaut folding 14x9

Which of the two props is reasonable for a glider like the 2 meter Spirit?

Where can I find what is the ideal RPM for these props?

If I know the ideal RPM it would simplify picking a motor/esc/battery

I have a good motor sim program for that.

The plan is to run the motor for 30 secs to climb.
Last edited by ripacheco; Feb 03, 2013 at 02:41 PM.
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Feb 03, 2013, 02:30 PM
ripacheco's Avatar
I found the specs for the Aeronaut props:

Max RPM for the props is listed as:
  • 12,000 for the 12.5x6
  • 11,000 for the 14x9U
My understanding is that beyond these RPMs the tips of the props will go faster than Mach 1 and that is terrible performance efficiency wise. Too little RPM you don't get the static/dynamic thrust you seek.

The plan is now ...
  • What power system (motor/ESC/batt) can handle some % of that max rpm for 30 secs...
  • Must keep in mind that as the plane accelerates so will the prop. Most not exceed the max rpm during the run...
  • Will have to figure out % of the max RPM so I can test the power system on the bench...

I'm right or I'm nuts?
Last edited by ripacheco; Feb 03, 2013 at 02:58 PM.
Feb 03, 2013, 02:54 PM
Registered User
I'll give it a shot! Electric power systems are complex due to the number of components (motor, prop, battery, ESC, gear box) and the aircraft they go into (wing area, weight, wing airfoil, etc.). An 11x6 prop on a 2m glider seems 'reasonable', however changing to a 13x6 with all other components remaining the same could very well destroy one or more components (usually motor, ESC or battery).

Once you select a motor the manufacturer will (usually) recommend a range of props to use for a given battery (2S, 3S, 4S) and the expected current draw. Current then helps you select an appropriate ESC (eg 35A, 45A). Go to the Model Motors web site (or AXI motors at Hobby Lobby) and look at the data for some of their products to see examples of the above.

A good software tool like MotoCalc lets you 'play' with changes to the package components without creating a bunch of smoke which can be quite expensive.

Most 2m gliders I have use a 9 or 10 inch folding prop for direct drive motors and a 12 or 13" if geared. Inrunner or outrunner will also affect prop size.

Good luck.

Feb 03, 2013, 02:55 PM
jackosmeister's Avatar
Its more likely the hubs will break past their rpm limits (which can be very ugly), most props turn to custard preformance wise at around .8 mach by the way.

Your looking at a couple of kW to spin that size at 95% of their rated speed which is getting pretty hot,

What glider are you refering too? A couple of kW might be fine in a good warmliner, but will destroy a thermal floater (and will weigh a lot)
Feb 03, 2013, 03:08 PM
Ascended Master
Sparky Paul's Avatar
I use 9 inch folders on 370-400 sized outrunners direct drive on my 2 meters.
A 12x9 to 13x10 on a Speed 600 brushed, 3:1 gear box.
The outrunners are way lighter.
Feb 03, 2013, 03:26 PM
ripacheco's Avatar
Originally Posted by jackosmeister
What glider are you refering too? A couple of kW might be fine in a good warmliner, but will destroy a thermal floater (and will weigh a lot)
I have a thermal 2-meter glider. GreatPlanes Spirit.

Wingspan: 78.5 in (2000 mm)
Wing Area: 676 inē (44 dmē)
Weight: 30 oz (850 g)
Wing Loading: 6.5 oz/ftē (20 g/dmē)
Fuselage Length: 39.25 in (1000 mm)

The weight is, of course, before adding the power system.
Last edited by ripacheco; Feb 03, 2013 at 03:41 PM.
Feb 03, 2013, 03:50 PM
jackosmeister's Avatar
Best first step is too work out what will balance, theres no point hauling around lead when it could be useful weight (like more motor/battery).

250-400W through an 11" + (bigger props work better fwiw) prop would be ideal for a spirit.
Feb 03, 2013, 09:24 PM
Registered User

Different gliders have different needs from a prop. "Hotliners" are propped like a racer but most sport gliders fly best when optimized for low speed thrust. The general rule of thumb is to use the biggest prop that doesn't overtax your motor and ESC. If you have a motor that's in the class that will pull your glider to height in around 10 seconds, which is how I like it, you will want to get within a few percent of the motor's maximum ratings. Here are a couple examples with 28mm motors running on 3 cell LiPos:

I have a Specter 1800 that weighs about 800 grams. I use a KDA 22-20L 1000kv. It weighs 80 grams and is rated for 25amps continuous with a 30amp burst. With a 10X6 folding prop I get around 8300 RPM, @250 watts, and 23amps for a calculated thrust of around 1700grams. It will climb vertically from my hand as high as my old eyes can see it in 8-15 seconds depending on light, wind, and just how full my batteries are for those first few seconds.

I also ran a cheap 11X6 that proved to be defective. I had made a static watt/amp check and it drew 27 amps for 280 watts but i decided I wouldn't bother with an RPM test at the time and went flying. One of the blades snapped about 3 seconds into my first flight. When I closely examined the break I noticed that the plastic had bubbles and voids in it and felt like Lava soap. That particular prop was unsafe at any speed.

This motor would provide excellent performance in any glider up to at least 1500 grams with anywhere from a 9inch to 11inch diameter and 4-8inch pitch. In fact, it could probably swing a high quality 12X5 folding prop because it has never even gotten warm with any of the props I've tried.

My other favorite motor is a HobbyKing 2834 1250kv that weighs 66grams with bullet connectors. I have only tested it with slo-fly props and my 875gram foam Yak54 3D but it's a real torque monster. HK claims a maximum capacity of 40amps and 580 watts with a 4 cell battery. I get 37 amps and 410watts with a 11X4.7 prop and a 3 cell 2200mah 25C battery. It gets pretty hot in ten seconds but the Yak will hover at around 1/3rd throttle because the static thrust is around 2.1 kg and I almost never use full throttle.

Either of these motors would give excellent performance with a 9 to 11 inch prop in any 2 meter glider. But I have a hunch that the 2834 with a Aeronaut 11X4 would make any 2 meter glider under about 1.8kg climb like a rocket.
Feb 03, 2013, 09:48 PM
Mesa AZ, it's a dry heat!
Here is a starting point: What is the weight of the model without the nose weight used for balance?
Add about 8 ounces to that number to give you a total flying weight.
Using that weight, and using 75Watts per pound or part of a pound will give you a power requirement for the power system.
NOW look at some motors which have a power rating in that range.
Get a battery which will give that amount of power (Volts X Amps = Watts) voltage wise, and will provide that current for about 3 minutes. The prop choice is the one which causes the motor to draw the current which then provides the Wattage from the battery voltage .

Model weighs 2lb without the balance weight,
add 8oz, and using 75W per lb gives 225W.
With a 3 cell battery supplying 11Volts the current will be 225/11=20A+
The ESC needs to be a 25A (for safety), and the motor will need to be capable of handling 20A also.
Follow the motor manufacturers recommendations as a starting point for your prop size, but bear in mind that you will most likely need to go to a larger diameter to get the power required. Prop blades are not cheap, but they are durable and the ones you buy first may not work the best, but may be ideal for another model/motor set-up later on.
Having said all that, a Wattmeter is you best friend when setting up a new installation. it will prevent releasing the magic smoke from ESCs motors and batteries.
You can experiment with some free software such as eCalc, or buy a program such as Motocalc, but they are only an estimate of performance, and some will only add to your confusion until you understand more of the technical terms.
Whatever you chose, remember it is a HOBBY, and is supposed to be FUN, so enjoy it!

Feb 03, 2013, 10:29 PM
Registered User
Take a look under the "Great Planes Spectra" thread in this section, the details of my updating a Spectra are in detail there and they may be a suitable reference as it's a 2 metre glider, just a powered Spirit really.
Despite my expecting to end up with a too rearward cg it was definitely not the case, there was considerable variance in where I could place the cg, from about an inch too far forward to more rearward than I would like, and any position in between.
Feb 03, 2013, 10:40 PM
Registered User
I agree on all points, lain. My only quibble is that I really, really, like excess power so I strive for closer to 150watts per pound. 30grams difference in a motor might be the difference between a shallow 30 second climb to a reasonable altitude and 10 seconds to climb vertically out of sight.
Feb 04, 2013, 01:27 AM
ripacheco's Avatar
Originally Posted by hohocc
... my updating a Spectra are in detail there and they may be a suitable reference as it's a 2 metre glider, just a powered Spirit really...
Your experience with the spectra is interesting to me because the model is very similar to the Spirit.

This is my first take:
  • Battery: 2100mAh, 3S, 147g
  • ESC: 15V, 35A, 50g (with packaging)
  • Motor: 885 Kv, 13 Amps/140 Watts (continuous), 58g/164g (with packaging)
These folding props can be used within the motor limits:
  • 9x5, 8,154 RPM, 51%, 92.1 Watts, 586 gf
  • 10x6, 7,777 RPM, 49%, 116.5 Watts, 678 gf

I think the setup can handle a bigger prop than the 10x6 but the props are not listed in the program's database. I'll have to figure out how to enter the parameters manually

But is food for thought for the time being.

I can jump up one motor size to get to drive bigger props but the weight penalty is large: 164g vs 233g (packaged)

May be worth it though to climb fast but then after prop folds is all dead weight.
Feb 04, 2013, 01:45 AM
ripacheco's Avatar
Originally Posted by jackosmeister
...Your looking at a couple of kW to spin that size at 95% of their rated speed which is getting pretty hot...
agree... those recommendations came from a vendor... Seems an overkill although some people like their gliders to launch like a rocket, but the weight penalties after the prop is folded are big...

I'll try to stay within 200w (I think)
I'm still plugging numbers on the motor sim and trying to understand the relationships...
Feb 04, 2013, 01:47 AM
ripacheco's Avatar
These folding props are highly recommended by several folks here on these forums:

For the sake of simplifying my "learning" and "understanding" I am going to stick to this particular kind. I wish they had all those props listed in the motor sim program I'm using.

I should have taken German for my second language requirements when I was in the University... Sigh..
Feb 04, 2013, 01:54 AM
jackosmeister's Avatar
This may help you with how it all works, wrote it a few years ago and its in need of a refresh but...

Aeronaut Cam carbons are prefectly fine for this sort of stuff btw, and arguably perform the best, Dont worry about other flavours till your dealing with a lot more power.

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