2m glider

hello183

Active member
I added the side longerons and this fuselage is looking real now.
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Next I need to figure out what I'm going to do to the firewall before I glue that, then I need to cut out the notches for the bottom longerons
 

hello183

Active member
using water to bend the top longerons. The fuselage is mostly done.
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I think for the wing joiners, it would be better if the smaller tube was in the fuselage and the wings had the bigger tubes and slid on. They also have to fit into each other with no play. Is there some sort of rule as to how big or small the wing joiners should be? Would it be fine if it passed through two wing ribs?
 
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hello183

Active member
I want to use a laptop cell for the battery, but it only has like 6 amps. So instead of connecting it to an esc and all of that, I was thinking of making some sort of timer that could turn it off after a few minutes of flight. Can anyone show me where to buy those geared brushed motors?
 

quorneng

Master member
I would strongly suggest that once you find a suitable brushed motor (40W?) and prop (it will have to be small diameter and fine pitch = inefficient unless it is geared) you bench test it. You might find you only get a couple of minutes run anyway with a Laptop battery. They are not designed for high current output. If it appears to work well enough only then worry about a timer.

Brushed motors are really heavy compared to brushless, at least 3 times the weight for the same power.
 

hello183

Active member
True. But I found this motor and it says it spins at 3 volts and 165 mA:
click this link
It's also geared 1:6 so it could spin a normal prop, maybe. It probably weighs more than brushless but it also means I can avoid using an esc.
And I'm not looking for the most power possible. All I need is enough power to get the plane in the air without having to spend money on a brushless or an esc. Also, if I could design a timer that would be cool.

As for the battery, I think it's good for 6 amps and its capacity is 3000 mah, so theoretically it should last about 20 hours powering a 165 ma motor, right? That seems a bit too long though. Is there something I'm missing?
 

quorneng

Master member
165 mA at 3 volts equals half a Watt! You are likely to need something like 40 times that to fly.
That sort of geared motor is designed to produce a high torque but at low speed, far too low for a practical prop.

What is the voltage of your laptop battery? Battery volts times its max amps gives the max power (Watts) available from the battery.
If you actually need the full 6A to fly then the duration from 3000mAh will be no more than 30 minutes and that's assuming the battery can deliver its full capacity at the max amp rate. Most do not!

The likely weight of your plane will give an indication of the power required to fly. 40W/pound (0.1W/g) should be adequate.
Do you have any idea of what its likely final all up weight? An intelligent guess might be all you can do at this stage.
That will then give an indication of the motor power required. You then have to find a motor that can deliver that power at the battery voltage you have.

Electric propulsion is heavy for a plane so you do have get the motor/prop/battery all running at their optimum level to be successful.
I hope this helps.
 

hello183

Active member
Thanks @quorneng. The battery is li-ion, so I think its 3.7 volts. Using a laptop battery is also one of the design challenges for this airplane, so I don't want to change that.
I think the weight might be about a pound. if the battery can produce 22 watts, then it could only power half of a pound, but I'll just build the airplane and see. If it's too heavy, I'll solder two of them together.
I only need 5 minutes of power then I can steer into thermals and do normal glider things to stay up. small servos won't overload a laptop cell, right?
 

quorneng

Master member
Servos only take power when they moving. Small servo have less power so take less mA, typically less than 500mA. Do ensure the servo linkage and the control surface itself are free running and mechanically efficient so all the servo power available is used to move the surface against the airflow. Done well you can safely use a small low power servo.

You might get away with as little as 22W/lb. It will depend on how aerodynamically efficient the plane is, particularly its wings, but that power level will only maintain height. Climbing requires virtually double the 'maintain height' power and even then that only allow it to climb as fast as it sinks with the power off. For an efficient glider that is quite slow. :(

Good luck hunting thermals.
It is not that easy even for pilot in a full size glider when he has instruments, much harder when piloting remotely from the ground.
I've done both. ;)
 

hello183

Active member
pull pull is the least friction, so if I combine that with counterbalanced surfaces then the battery will have a lot more power for the motor?
I didn't know you were a real pilot. If you're looking for a thermal, can you rob off the ones that birds are in or are they not big enough for full scale?
 

quorneng

Master member
I can assure you that birds are so much better than any human pilot in finding and using any thermal, full size or model. They can however provide a good marker for models. Note they can use the weakest thermal so don't assume you or your model will be able to as well.
Also remember they are not controlled by a person on the ground so continuing to thermal a long way down wind is no problem to them but it is to you, if you want your plane back!
Also be aware that birds of prey may have a nest nearby. They then may see your plane as an 'intruder' and attack it, usually very successfully! If one or more birds starts to buzz your glider they are not doing it for fun, simply fly away asap. Loosing the thermal and having to land is preferable to loosing the plane.

It is not unknown for several full size gliders to use the same thermal. The rule is everybody must go round the same way, set by the person who gets to it first so good observation is required to avoid a possible 'head on' collision.
 

LitterBug

Troll Spammer
FYI, ArduPilot has a mode which can automatically look for thermals... It would add electronic complexity to your build.


The more manual way is to use a reciever with a barometer built in. Using telemetry you can set up a variometer in your transmitter that uses a tone to indicate if the plane is rising or falling in altitude. Gives better feedback than line of site alone. Full scale glider pilots use this same indicator.
 
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danskis

Master member
Very nice work and a huge step up from your first effort. Congratulations!!! I like the challenge of using the 18650 laptop batteries. As you know they are heavy compared to LiPo. As a benchmark I can only compare this project to my 2 meter motorized glider in hopes that will give you some ideas to work with. My plane is a 2 meter, 15 ounce balsa pod and CF boom glider similar to yours. I use a 3 cell 450MaH lipo with an 1806 drone motor (2300kv) connected to a 7X4 folding prop. This will give the plane a very steep angle of climb, around 60 degrees. A ten second motor run will get me plenty of height to catch thermals. I get 4 to 5 climbs from this power combination. I had a 5 minute flite on a 8 second motor run yesterday as the thermals were booming. I could have easily stayed up longer. I could also run this on a 2S battery which of course will give me less power, and a lower height for the same motor run. If your plane comes in around 16 ounces (even 18 or 19) it will be very capable of thermaling even from a very low altitude (but that also depends on the pilot). A guy I fly with regularly catches thermals at 100 feet. I like the idea of using 2 batteries in series. I would consider putting one closer to the nose to move around for ballast and the other closer to the CG. Since 18650 batteries are fairly heavy I would think that you would not need a long nose moment on this plane.

Keep in mind that one of the keys to higher thermaling performance on a motor glider is a folding prop. In my experience a folding prop will increase glide performance anywhere from 25-40 percent. If I were designing a glider from scratch I'd start with the folding prop and work backwards on the power train by finding a motor that works with the prop and meets design requirements.

One of the challenges of gliders is finding thermals without instrumentation. Variometers are not allowed in model glider competitions. A motor glider is the perfect plane to learn how to find thermals. Start doing some reading on how to find thermals and after some practice you'll find a great deal of satisfaction in watching your plane going up with the motor off.

This could be a great platform to experiment with different power options.

I've often thought about using old electric toothbrush motors to power a plane.

Thanks for posting.
 
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Piotrsko

Master member
Thermals start at ground level but you cant turn tight enough to stay in one that low. Somewhere between 50 ft and 100 ft agl they are getting big enough to stay inside of with a two meter wingspan. Expansion is related to cooling rate.
dust devils are your best friend because they are visible thermals

Your gear motor is way too heavy and underpowered for what you are doing, and 10,000rpm isn't too fast for modern props. Correct series of assumptions on your selection , however.
 
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hello183

Active member
So I was able to find a tiny motor from a thing I got a long time ago. Plus, it's a brushless and it has a prop and everything. It says it's size is 1510. Now that I found a motor for free it would be cheaper to just buy an esc than anything else. But what esc should it take? The prop says it's a 7035, but it came with the motor so they should work together. @danskis, I'll consider buying a folding version of that prop but how do I keep it from damaging the fuselage as it folds in?
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I like the idea of experimenting with motors though. Maybe I'll build another freeflight later and make a timed motor thingy.
 

danskis

Master member
My first assumption is that this is an RC plane because you mentioned servos.

The prop folds by itself when the motor stops and the air pressure of the plane moving through the air forces the prop to fold so there is no danger of destroying the plane. The nose of the plane should be about the same diameter as the motor so the prop has room to fold against the fuselage.

I made the assumption this could be your motor. The specs are on the website.


For a folding prop you'll have to determine if you've got a 2 or 3 mm shaft on that motor. You can buy folding props on Amazon. This prop goes on a 2mm shaft and is recommended to be used with a 2 cell battery/motor combination.



If you use two 18650 batteries in series to power your BL 1510 motor than you've got a fairly conventional set-up. The 18650 batteries are anywhere from 2000 to 3000 mah which will give you plenty of flight time for a 1 to 1.5 pound model. You'll need a 12 to 15 amp speed controller.

If you can hit the target weight than this should be a great plane to fly on low wind days and to learn how to find thermals.
 

hello183

Active member
I used a Dremel to drill holes in the firewall for screws and the motor wires
 

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hello183

Active member
I have been doing some sanding on the fuselage formers to get the sheeting to fit. I'm doing the bottom side now and I'm doing it in four pieces because It's too difficult to cut one piece so accurately. Hopefully this will add a lot of strength to the fuselage.
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