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PostPosted: Thu Feb 23, 2012 8:47 am 
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Joined: Tue Jan 15, 2008 8:50 pm
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Hello,

Just wanted to take a minute to write out some information on how to power your next airplane with electric power. If you follow these steps you should be able to get the performance you are looking for while minimizing experimentation.

1. First we need to know how much your model weighs. It's pretty simple, most models show a flying weight. Assume the highest value then add 10% because sometimes electrics can weigh more depending on the flight times desired. So for a great planes super sportster 40 ARF, it shows 4.75-5lbs. So 5.5lbs.

2. Once you have that you need to decide how much performance you want. General rule of thumb is 100 watts per pound for trainers and other slow flying aircraft, 125 watts per pound for well performing sport aircraft, 150 watts per pound for unlimited vertical, and 175+ watts per pound for 3d airplanes. So for this airplane lets go with 125 watts per pound. So 5.5 x 125 = 687.5 watts.

3. Now you need to decide what kind of rpm/prop you want to run. If you want 46 2-stroke like performance then you want a small prop turning 12000 rpm. If you want 52 or 70 4-stroke performance then you want a larger prop turning 10000 rpm. Generally speaking the larger the prop the more efficient so I would choose the larger prop at 10k rpm, but you might not be able to do that based on ground clearance or if you want speed. For our example I'm going to choose 10k rpm.

4. Now lets think about how many amps we are going to expect from the ESC and battery. Generally speaking, take the weight of the model and multiply it by 10 and that will give you a rough idea what kind of current you are looking at. So a 2lbs model would be 20amps, 3lbs 30amps, 10lbs 100 amps, etc. So in our example we have a 5.5lb airplane so 55amps.

5. Now we know a lot of things, we have a 5.5lb airplane, we want 687watts, we are looking for 10k rpm, and we want to keep it around 55 amps. So lets do a little math and figure out what works the best: Watts = amps x volts. So in order to figure out the volts we do volts = watts/amps. So 687/55 = 12.5 volts. It lands between a 3 cell battery (11.1v) and a 4 cell battery (14.8v). So we could run a 3 battery at more than 55 amps or a 4s battery at less than 55 amps. Well less amps is better so lets go with a 4 cell battery. Now lets calculate the actual amps: 687watts / 14.8v = 46.4 amps. Note: If you already have a stack of 5 cell batteries then you might want to change your calculations to use those so that you don't need to buy more batteries.

6. Now we know that we have a 5.5lb airplane, we want 687 watts, looking for 10k rpm, and want to run a 4s battery at around 46 amps. So now we figure out the kv of the motor. The KV is how many rpm the motor will try to turn per volt. So if we want 10k rpm, we take 10000 / 14.8 volts = 675kv. Now we can go a little higher or lower and fine tune with the prop, but that is generally where we want to be.

7. Ok now we are looking for a 687watt motor that will have a kv of around 675. Generally speaking most quality motors can do 3watts per gram. This is because of the how they shed heat. So 687watts / 3 = 229 gram motor. So we are looking for a 229gram motor that has a kv around 675, that can handle 46 amps.

So here are a number of motors that are close to our specs:
http://www.aero-model.com/Hacker-Brushless-A40-12S.aspx (210g, 610kv, 40-50 amps)
http://www.innov8tivedesigns.com/produc ... cts_id=859 (290g, 640kv, 62 amps)
http://www.innov8tivedesigns.com/produc ... cts_id=888 (216g, 700kv, 46 amps)
http://www.subsonicplanes.com/Motrolfly_Motors.html (DM3620-750) (230g, 750kv, 1200 watts total)

As you can see there are a number of options. The smaller cobra motor and hacker are lighter but they would be run at the edge of their performance envelope which is great for a light weigh 3d airplane. The Motrolfly is only slightly heaver and can do 1200 watts, so it could power a larger airplane later or double the performance of this airplane with just a different prop.

8. Now that we know which motor to run, we pick a speed control that has an amp rating higher than the expected amp draw. I really like the ZTW controllers from quantumhobby.com.

9. Now we need to figure out the size of the battery. Generally speaking, the larger capacity the battery the longer flight times, but also the heavier the model is. So here is how you figure that out. If we figure we want a 10minute flight time, assuming 70% average throttle, then we do this: 46amps/hour / 60 = .766ah per minute or 766mah per minute. Now multiply that by 70% because we aren't going to be full throttle all of the time. 766mah x .7 = 536.2mah per minute. Now take 536.2mah and multiply by 10 minutes: 536.2 = 5362mah. Keep in mind we never want to discharge a lipo over 80% so you might not get all 10 minutes, but then again it depends on throttle usage, since you will get less than 10 minutes anyway if you are using full throttle the entire flight.

10. Now that we have an idea of what size battery to run we need to make sure we aren't exceeding the C rating. The C rating is the multiple of how many times it's capacity the pack can deliver at any one time. So if we have a 20c 5000 battery then it's maximum discharge rate is 5amps x 20 = 100 amps. In our case a 20c battery is plenty, mostly because we want to have a long run time. If we wanted to keep things as light as possible and only run 3 minutes, then we would be looking at a 1600mah pack, which at 30c is only 48.3 amps (1.6 x 30 = 48.3). So this would work, and the battery much lighter, but we would be working it very hard. Generally, you don't want to have less than 6 minutes of flight time because otherwise you are working your packs too hard, and flight times are very short.

11. Figure out which prop to run. This is usually figured out by calculation, motor data sheet, or experimentation. I like to use this online calculator: http://www.ecalc.ch/motorcalc_e.htm" onclick="return urlWarning(); Put in 0 feet, 70f, 4000mah 20/30c lipo, 4s battery, 50 amp controller, hacker A40-12s motor, APC E prop, and then start experimenting with prop diameter and pitch until you find something that will load the motor to create the 687 watts. I put in a 14x7 prop and got 661 watts and a 14x8 and got 728 watts. So you know you are probably going to run a 14" prop which is consistent with the 4 stroke like power we want.

I hope that helps. If you have questions post them below.

schu


Last edited by schu on Thu Feb 23, 2012 11:35 am, edited 2 times in total.
Added battery size and C rating


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PostPosted: Thu Feb 23, 2012 11:34 am 
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Joined: Tue Jan 15, 2008 8:50 pm
Posts: 186
In summary, I would look for 750watts on 4 cell, with a 4000mah pack, or 5 cell on a 3300mah, depending on if I already have 5s or 4s packs already. That should give 6-8 minutes and very good performance.

If I was planning on building a large airplane and I already had 4s4000 packs, I would look at what kv I need to use them in series for 8s4000 which is why my 70" airplane and 600 size heli are 8s instead of 6s, and it's also why my 40 size stuff is 4s instead of 5s. I have many different sizes of aircraft, but only 3 different sizes of batteries.

schu


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