Convert 2M glider to electric - rx battery necessary?
I have flown glow and petrol powered planes for 15 years, but never anything electric. I have finally decided to convert my Blue Phoenix to electric, and a bit of advice would be welcome. :)
The Blue Phoenix is a two meter polyhedral balsa glider, very similar to the Gentle Lady. I want to add a motor system to get it up to thermal altitude. Nothing fancy, just enough to make it climb gently.
How do people manage battery capacity during thermal flights? Should I use a separate receiver battery and use the main battery just for the motor? The worst case scenario is that the battery is nearly empty after a climb, and there won't be enough capacity left for the rest of the flight. In good conditions, this plane can easily stay aloft for half an hour, and I don't want the receiver to shut down in the air.
As you probably know by now, I am a beginner when it comes to electrics, so any tips and advice are welcome.
A separate RX battery should not be needed but you still can opt to use one if desired.
It is much easier to estimate maximum RX operating time with the separate RX battery. The only indication you will have of time remaining in the power system battery is the low voltage cutoff if your radio system does not include telemetry downlink of the battery voltage.
When using just the power system battery with BEC, the radio will function long after the low voltage cutoff prevents the motor from operating. But continuing to fly for very long after the cutoff will lead to damaging the battery. Damaging the battery is more of a concern than losing control of the model.
This is a place where your flying style will be very important in the choice.
If you opt for a separate RX pack you could go with a LiFe pack which would save weight.... or if you need the nose weight anyway the older NiCd/NiMh is still fine.
We used to fly models similar to the Gentle Lady with brushed power systems and NiCd packs made of 6 sub-C cells (old RC Car packs) and the separate NiCd RX battery. The sailplane models this size are capable of passable performance even with that much weight on board. The brushless power systems and modern mini/micro servos allow the electric powered versions to now be lighter than the pure sailplane versions used to be... So don't be too concerned about the weight of the separate RX battery.
I'd go for the external uBEC (Switching Power Supply Battery Elimination Circuit)
A lot of them are available. I've got 9 or 10 of the Castle Creations 10 Amp uBEC in my various models. They've all been flawless, and two of them are in 2500 watt giant scale models.
If you're flying with LiPo batteries, just be careful not to use the motor power for over 50% of the Lipo's capacity, and you'll be just fine. That 50% rule for the receiver power is one I've been following for near 50 years of flying RC models. That includes a whole bunch of sailplanes with electric power in the 1980's.
IMHO, electric launch of those sailplanes is a natural match. That gets rid of the rubber band launch parachute landing in the trees, or trying to find a big enough field for a winch launcher. (Did both of them) A properly setup electric motor launch will get your sailplane far higher in the air than either the bungee or winch launch.
One caution with electric launch though. If you can fit them in, spoilers are a wise addition to your model(s). Problem is, with those very high launch altitudes with electric launch, it can be an issue trying to get your model back on the ground in strong thermal activity. Spoilers really help in this regard.
fhhuber and kyleservicetech,
thank you for the explanation!
Using only one battery seems to be the easiest route if I can keep track of the battery capacity. Would this be the correct way to find the 50% discharge point: Charge the battery. Run for one minute at full throttle. Recharge and read how many mAh was put into the battery. Divide full battery capacity by this number to find how many minutes the battery will last at full throttle, and never run the motor for more than half of that time. Does this make sense, or have I misunderstood it?:)
If stuck in a thermal (has happened WITH SPOILERS)
roll to a 90 deg bank and apply full elevator to hold the plane in a tight circle. Adjust with rudder if needed.
Speed will not build up excessively. The plane will come down if it can come down.
If that does not work you can try inverted but speed can build rapidly and rip the wings off.
It happened to me with this plane a few years ago! :) I was up in the mountains flying off a small slope, searching for thermals in the surrounding area. Suddenly the plane hit a monster thermal and threatened to disappear in the sky. I held full rudder and elevator and managed to spin it down, but it was a scary moment. It doesn't take much speed for the wing to flutter. It looks like stalling and/or spiraling works best. I have never tried to sink inverted, but that could be fun. :)
You can do this with that "C" factor, or, just do it the old fashioned way.
Lets assume you've got a 2000 Mah LiPo battery. That would be 2.00 Ampere HOURS. Or, 2 times 60 minutes per hour equals 120 Ampere MINUTES.
Now, lets assume you took 650 Mah (0.65 Amp Hours) out of that battery in one minute. That would be 0.65 times 60 or 39 Ampere Minutes.
So, that 39 Amp Minutes is 39/120 or 32% of the battery. Divide that one minute running time by 32% and multiply times 50% and you'd get 1.56 Minutes, or just over 1 1/2 minutes.
Just a caution here, it might not be a good idea to run your motor wide open on the ground. With little airflow through the motor and model, something might overheat.
First, if you are new to electric flight this might be helpful.
Everything you wanted to know about electric powered flight.
There is a chapter on estimating battery time.
I do not use a separate receiver pack, I use the BEC in the ESC to power the servos and receiver. In short, here is how I look at it.
To power your 2.4 GHz receiver and servos will run between 150 and 350 mah/hour depending how aggressively you fly that thermal duration glider. So if we use 250 as an average number, a 1500 mah pack can give you 6 hours of soaring time.
Let's say your motor pulls 15 amps at full throttle. That would be 60 times more than it takes to run your receiver and servos. So you calculate the maximium motor run time and almost ignore the servos and receiver.
If you have a 1500 mah pack that means it can put out 1.5 amps for one hour. At 15 amps (your motor's pull) it can sustain that for 1/10 of an hour or about 6 minutes or 360 seconds.
Say you don't want to run full throttle for more than 4 minutes of climbs. Figure 30 seconds per full climb and 20 seconds for each reclimb. So that is 2 full climbs and 9 reclimbs for 4 minutes of full power climbs.
If you do 4 minutes of full throttle climbs then you have used 4/6 or about 2/3 of the battery to run the motor. That leaves 1/3 of the capacity of that 1500 mah pack to run the receiver and servos. At 250 mah/hour (estimated) you have about 2 hours of receiver/servo time for soaring.
If you hit a boomer on the first 30 second climb you have used 30 seconds of 360 seconds of capacity (6 minutes of available full power run time) or about 20 % of the battery capacity to get that first climb. 80% is remaining.
1500 mah *.8 = 1200 mah remaining for the servos and receiver.
1200/250 per hour = 4.8 hours of soaring time.
See how it works?
All rough estimates but close enough to start, then you adjust over time based on observed charge put back into the battery.
My rule of thumb, if the LVC kicks in on a full power run I have used up about 80% of the capacity of my pack. Using the example above, I have about 60 minutes of servo/receiver time left as the LVC kicks in before you use up all the capacity in the battery. I would come down in 30 minutes or less.
Was that helpful?
thank you very much for your detailed explanation! It was very helpful. (In fact, I can't think of any more questions or problems at the moment, I'm just looking forward to converting the plane and flying it)
Be sure to look at the ebook at the link I gave you. You will find a lot of good stuff there.
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