Go Back  RCU Forums > RC Airplanes > RC Gliders, Sailplanes and Slope Soaring
Reload this Page >

Advice needed on electric power for 2m and 100" sailplanes

RC Gliders, Sailplanes and Slope Soaring Discuss rc gliders,rc sailplanes and slope soaring in this forum. Thermaling techniques, airfoils, tips, etc

Advice needed on electric power for 2m and 100" sailplanes

Reply

Old 01-29-2016, 12:01 PM
  #1  
USSBB57
Junior Member
Thread Starter
 
Join Date: Jan 2016
Posts: 8
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default Advice needed on electric power for 2m and 100" sailplanes

I left the hobby for about 15 years and things have changed a bit.

I started at about age 12 with an Ace single channel .020 power glider, lived through the conversion to narrow band radios and got out about the time PCM was cutting edge.

Since then:
- radios have changed A LOT; and
- electric motor, battery and ESC technology has greatly improved.

Which leaves me with some questions.

1) I've figured out the difference between "in runners" and "out runners", as well as brushed and brushless motors.

I suspect that the out runner design would have a bit more torque and operate more efficiently at a lower RPM and accomplish a lot of what used to be done with a gear box (larger prop at a lower, more efficient RPM) but without all the gear box issues and expense. Is that more or less correct?

2) I have not figured out how motors are sized, rated, etc. That totally eludes me and is making motor selection difficult.

3) I'm a mode 2 kind of guy at heart and will stick with that, so I'm planning on a 6 channel 2.4 radio system.
Any suggestions?
I've noted that you can buy a lot more radio with $200-$300 than you used to be able to get for that kind of money. I've also noted TX/RX systems in the $50 range. How much does that actually buy and how much are you giving up? ($300 is do-able, I just need a reason.)

4) longer term I have a 70" 1-26D scale kit in the closet as well as an 80" 1-26E short kit. I plan to complete both, probably with a power pod, spoilers and 6 channels total, but I want to knock the rust off my thumbs first.

In the short term I have as Astroflight Challenger kit as well as a Sig Riser 100 that I never started before I dropped out. The Challenger was in response to the horrible performance I got with an Electra. It climbed (sort of), was sluggish and over weight, and didn't thermal worth a damn. My plan for the Challenger was a geared motor, but that never happened.

Now it seems like it might be a great project with a modern lightweight out runner motor, although with a short nose, I'm not sure lighter motor weight makes sense if you're making it up with even more balls behind the motor.

So of course large motor + lightweight battery = potential rocket ship with a 10 second climb to altitude comes to mind.

Please educate me on the pros and cons of both approaches.

The Sig Riser has a longer nose and I have the same questions - what makes the most sense, a small motor in the long nose or a large motor with the nose shortened up a bit - and about 4 degrees down thrust built in either way.

5) Finally, I'm confused by motor ratings, Kv, watts per pound, and related shaft size and folding propeller sections and options.

Your advice and assistance is greatly appreciated as I try to transition to modern electric sailplanes - or at least electric powering of old school sailplanes.
USSBB57 is offline  
Reply With Quote
Old 01-29-2016, 02:37 PM
  #2  
BMatthews
 
BMatthews's Avatar
 
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
Posts: 12,337
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

I'm not up on the latest radios so I won't even go there.

On the motors you've got it right that outrunners tend to have lower RPM and more torque. And in fact they are sort of like having a built in gear box.

The value of Kv for the motors is an indicator of the "gear ratio" of this added torque and lower RPM. This is a poor analogy but it gives you the sort of idea for your "old brushed motors with gear box" ideas of yesteryear... which was only about 15 years ago or so....

In reality what the Kv gives you is actually first of all a value for how many thousands of RPM per volt the motor will try to produce and also an indicator of how much you can load down the motor with a bigger prop without exceeding the motor's maximum power and letting the smoke out.

The other key spec is the maximum power you can run. This is typically given in watts for continuous running. There may also be a short term burst amperage or watts which is OK for the amount of time indicated. But if run at that level for longer you risk burning something out.

For sport flying I prefer to plan on running the motor at around 75 to 80% of what it states for max continuous power or max current. That SHOULD lead to the motor running a little cooler provided we allow for proper airflow. And cooler means a longer usable life span.

How much power is enough? Lots of folks these days say that we NEED 100 watts per lb or more. But it just ain't so. 100 watts per lb with the right prop is going to produce a possible climb angle of something around 80 degrees to vertical. Not everyone needs that. If we lower the power rating down to more like 70 to 80 watt/lb for a glider with a large fine pitch "climbing prop" expect to see around a 60 to 70 degree climb angle. Or 45 to 50 degrees with a good head of speed. Dropping down to around 50 to 60 watts/lb of model weight will generate a climb angle of something more like 30 to 40 degrees with a fair speed that climbs up to around 500 feet in a bit over a minute of run time. These figures all assume that you spend a bit of time matching the prop to the motor and model. For each motor and model there's going to be a couple of good diameter and pitch combinations. Blade shapes in terms of outline and airfoil shape and twist from different brands of prop can also play a part in this.

All in all it's pretty simple these days to find a motor and battery that will supply 70 to 80 watts/lb that produces a fairly light package for the model size. So there's really not much point in going with less unless you're really wanting to keep the wing loading down in the "super floater" range.

But if you want to go with keeping the model looking fairly original in the case of the Riser there's nothing at all wrong with aiming at a motor which will be lighter and using a smaller pack and running at more like 60 to 70 watts/lb and be willing to try some different props to fine a good match which produces the best climb for the least power used.

A Riser is typically going to come in at around 32oz/2 lbs. So in this case you're looking at needing between 120 and 140 watts. Let's say 140. Now that means I'd want to find a motor that can handle a max of around 180 watts continuous so it is running well under the maximum. But if you find a nice looking motor in the 160 to 170 watt max range that would be OK too. Especially if you work at holding the power used down to more around the 120 to 130 watts range by using a slightly smaller prop.

You see, the motors are very efficient at converting the power to use in the prop. And they will pull power in direct relationship to the prop used. Our job is to prop them to control their power use since they don't know they have a limit. Prop them wrong and they'll produce power way out of their range quite happily... up to where the smoke leaks out.

Back to the RIser. The good news is that a 180 watt max power motor is crazy small and light compared to the old "600" can brushed motors. Don't be surprised if you find yourself looking at them and thinking "... there's just no way.....". But there is and it will.

For a glider at this motor size you're going to want to look for Kv values of around 1000 to 1200. This should allow you to run a 9 to 10 inch prop with around a 6 inch pitch.

The shafts of the motors at this size are pretty thin and weak. So try them out with a fixed prop and when you find something that works either modify it to be a folding prop or shop for a folder. To avoid damage during the testing use the "motor saver" O ring banded on nub ends which let the prop pivot to avoid damage to either the prop or motor. These would work for regular use as well of course. But the drag of that big prop would suck the glider down faster. That's why I'm suggesting trying the model with the "motor saver" and cheaper fixed blade props first then find a folding prop option that is a close match.

For your other projects just use the values per lb to find out what will work for the other models. And note that as the motors get larger the Kv range tends to get lower to deal with the bigger props used by those motors. So for a 100 to 120 inch glider you'll be looking at motors more in the 900 to 1000 Kv range.

That should at least get you started out.
BMatthews is offline  
Reply With Quote
Old 01-31-2016, 01:17 PM
  #3  
USSBB57
Junior Member
Thread Starter
 
Join Date: Jan 2016
Posts: 8
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Thanks, that is very helpful.

I'll have to decide which to tackle fist, but this should help me get it properly powered with minimal fuss.
USSBB57 is offline  
Reply With Quote
Old 02-03-2016, 07:18 AM
  #4  
Aquila1954
Senior Member
My Feedback: (1)
 
Aquila1954's Avatar
 
Join Date: Oct 2004
Location: La Vista, NE
Posts: 238
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Hello;

I have a Riser 2 meter on the board now and I am installing this motor in it. http://www.hobbyking.com/hobbyking/s...ess_Motor.html and a folding prop of 10x6
ESC: http://www.hobbyking.com/hobbyking/s...ontroller.html
I also have a Riser 100 that I will be building also: And the motor and ESC for this will be. Motor: http://www.hobbyking.com/hobbyking/s...3_5_960kv.html and the folding prop for this will be a 12x6. The Esc: http://www.hobbyking.com/hobbyking/s...ontroller.html
You want to get into electric, it is a good thing to have a couple of other ideas too. Like a watt meter: http://www.hobbyking.com/hobbyking/s...strSearch=watt
This also has a battery checker built in too. and as I use Turingy esc's I bought this programming card so I can put a brake on my motor, so my folding prop will fold. http://www.hobbyking.com/hobbyking/s...ming_Card.html better than trying to listen at the beep codes when trying to program your esc. And since I am on a set income and can not afford the high end radio I bought a 6 channel 2.4 Mode 2 radio called Fly Sky FS-I6. But Turnigy has the same radio under their name http://www.hobbyking.com/hobbyking/s...r_Mode_2_.html it come with a park flyer rx. But they came out with a full range rx which has telemetry built in it http://www.hobbyking.com/hobbyking/s..._Receiver.html
My FS-i6 I used all last year with no problem on the park flyer rx. But I am going to by the I6B full range receiver for the 2 meter and the 100.
Aquila1954 is offline  
Reply With Quote
Old 02-03-2016, 11:05 AM
  #5  
USSBB57
Junior Member
Thread Starter
 
Join Date: Jan 2016
Posts: 8
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Thanks, that's very helpful and confirms my plans to date.

I selected the same motor for the Riser 100, but it'll be the second project.

I've decided to build the Astroflight Challenger first and have the required stuff on the way.

I'm using this motor in the Challenger:

https://www.hobbyking.com/hobbyking/...dProduct=69389

Spec.
Battery: 2~4 Cell /7.4~14.8V
RPM: 1100kv
Max current: 22A
No load current: 1.6A
Max power: 315W
Internal resistance: 0.077 ohm
Weight: 73g (including connectors)
Diameter of shaft: 5mm
Dimensions: 35x30m
Prop size: 7.4V/12x6 14.8V/8x4
Max thrust: 1100g

Along with a 30A ESC and 2200mAh 3S 30C battery packs.

I'm using a 40mm alloy folding prop spinner and I'm going to try both 10-6 and 11-6 propellers with it.

I've selected the same transmitter and receiver package, along with HXT900 servos. I might decide I want more bells and whistles later, but this is leaps and bounds ahead of what I had when I stepped out of the hobby 15 years ago.

I ordered a balanced charger as well. I have a watts meter I use to monitor the solar power system in my observatory, I've also used Eagle Tree data loggers with the optional power panel for several years on DPVs, which should address the same needs in term so determining current in static tests, as well as overall current use. Weight wise, I should be able to fly the data logger with minimal impact on overall weight until I decide what if anything I want to do with telemetry.
USSBB57 is offline  
Reply With Quote
Old 02-03-2016, 07:58 PM
  #6  
Aquila1954
Senior Member
My Feedback: (1)
 
Aquila1954's Avatar
 
Join Date: Oct 2004
Location: La Vista, NE
Posts: 238
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

I came back to the hobby last year after a 27 yr nap. Boy, did I have to learn a lot. I started pouring over this forum and my favorite forum RCGroups.com. That is where I reside mostly under the name alienmenacet54.
Aquila1954 is offline  
Reply With Quote
Old 03-05-2016, 12:49 PM
  #7  
Small Block Mike
Junior Member
 
Join Date: Jun 2015
Posts: 14
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default

Very helpful

Just the info I need to add electric power to an old Oly !!.
Small Block Mike is offline  
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service