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Batteries & Chargers Discuss Li-P, Li-Ion, NiMh, Nicad battery technology and the chargers that juice 'em up!

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Old 01-28-2014, 04:49 PM   #1
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Default Need help choosing battery specs

Hi I am new to wattflyer and am looking for advice on my RC that I am building.
The maximum weight for this plane is 5kg and it is required to fly continuously for 40 minutes on a single charge. Based on these weight and flight time requirements what specifications for batteries should I choose?

Currently I am thinking of using Li-po batteries. 3S 30C 11.1V 3900mah. Will this be enough to meet the requirements? This is a ball park choice as I don't know how t calculate what specifications I will need.

I also need to choose an electric motor for the plane.

Any help will be greatly appreciated, thank you.
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Old 01-28-2014, 04:54 PM   #2
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Originally Posted by realnewbie1 View Post
Hi I am new to wattflyer and am looking for advice on my RC that I am building.
The maximum weight for this plane is 5kg and it is required to fly continuously for 40 minutes on a single charge. Based on these weight and flight time requirements what specifications for batteries should I choose?

Currently I am thinking of using Li-po batteries. 3S 30C 11.1V 3900mah. Will this be enough to meet the requirements? This is a ball park choice as I don't know how t calculate what specifications I will need.

I also need to choose an electric motor for the plane.

Any help will be greatly appreciated, thank you.


What kind of model airplane are you working on?

IMHO, a 40 minute flight for an electric model will be very difficult to accomplish. (Read as expensive) That would be more in the area of a glow or gasoline powered model.

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Old 01-28-2014, 05:04 PM   #3
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Originally Posted by kyleservicetech View Post


What kind of model airplane are you working on?

IMHO, a 40 minute flight for an electric model will be very difficult to accomplish. (Read as expensive) That would be more in the area of a glow or gasoline powered model.
Its a group project to design a plane that can take aerial pictures of a terrain. It will not be required to fly too quickly or perform any complex acrobatics just level constant speed flight for 40 minutes.
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Old 01-28-2014, 05:23 PM   #4
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where is the rest of the group?

everything is too dependent on other factors. are you using an edf? 3500kv with a 5x5 prop on a flying wing? 500kv motor for a 60 sized airframe? who knows. the same battery you asked about might be 3-4 minutes on the first two motors, and 10-20 minutes on the second. but I have no idea how fast you need the prop to spin to.be efficient, I Dontt know how slippery the airframe is.

powered glider? sure. .10 sized pylon racer... not so much.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 01-28-2014, 05:29 PM   #5
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Originally Posted by hayofstacks View Post
where is the rest of the group?

everything is too dependent on other factors. are you using an edf? 3500kv with a 5x5 prop on a flying wing? 500kv motor for a 60 sized airframe? who knows. the same battery you asked about might be 3-4 minutes on the first two motors, and 10-20 minutes on the second. but I have no idea how fast you need the prop to spin to.be efficient, I Dontt know how slippery the airframe is.

powered glider? sure. .10 sized pylon racer... not so much.
Sorry I am a total newbie I don't understand some of what you're saying for example whats an edf?
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Old 01-28-2014, 05:31 PM   #6
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Originally Posted by realnewbie1 View Post
Sorry I am a total newbie I don't understand some of what you're saying for example whats an edf?
EDF is an Electric Ducted Fan system. These power systems are generally used for very high performance, very fast models. These EDF's are not very efficient for the type of application you're interested in.

For the type of application you're interested in, you need a slow flying model with a big wingspan. Something on the order of a Piper Cub, or even a sailplane. I've taken a lot of pictures and videos with both types of planes. The Piper or the sailplane would provide a more stable platform for your camera. Plus, these larger models have the ability to place a smaller camera off to one side on the wing, so the prop isn't in the field of view. A larger sailplane with perhaps a 6 foot or more wingspan will allow power on for about 30 seconds, then fly for awhile with power off. And repeat until the battery is depleted.

Something that might help is below:
"C" and what it is
http://www.wattflyer.com/forums/showthread.php?t=65869

If you have no experience in RC, join a club nearby!!!! Or get help in flying. The usual flight time for a person that has never flown an RC model is often measured in seconds.

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Old 01-28-2014, 05:36 PM   #7
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Originally Posted by kyleservicetech View Post
EDF is an Electric Ducted Fan system. These power systems are generally used for very high performance, very fast models. These EDF's are not very efficient for the type of application you're interested in.
Okay thats understandable since the tasks are very simple. Level flight for 40 minutes. The design model looks very similar to the RQ-11 Raven to give you an idea of what we are going for
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Old 01-28-2014, 06:11 PM   #8
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It is not advised to cross-post in many different forums. Here is a link to what people have said in answer to the same question posted on RC Groups: http://www.rcgroups.com/forums/showt...2#post27333891
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Old 01-28-2014, 07:07 PM   #9
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Originally Posted by realnewbie1 View Post
Okay thats understandable since the tasks are very simple. Level flight for 40 minutes. The design model looks very similar to the RQ-11 Raven to give you an idea of what we are going for
IMHO, you are going to be needing a very large model with slow flying characteristics. If you ever watched cross country sailplane races, these models are very large, on the order of a 14 foot wingspan. These larger models can be more efficient in use of the power system on board.

Along with those large wingspans, comes the cost factor. They won't be cheap. The models we used back then cost thousands of dollars, because of the requirement to be able to fly fast, like over 100 MPH.

Back in the mid 1980's I built up a pair of Craftaire Viking sailplanes, with 10 foot wingspan, 1500 square inch wing, and a boat anchor geared brush type motor with a folding prop. Those models were powered by an 18 cell 2400 Mah Nicad battery pack. Typical flights were on the order of an half hour in dead air. Those models also had a number of two hour flights on them. Those models weighed in at 120 ounces, or 3.4 Kg

Nowdays, with the much more efficient brushless motors and the high powered LiPo batteries now available, it should be fairly easy to hit 40 minute flying times on these sailplanes. The motor would have to use a folding prop, and it would be off much of the time.

One thing became very apparent, when a camera was mounted on the model, The drag caused by that camera was more than the drag of the entire model. The air drag from that 35 mm camera cut the flying time in half.

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Old 01-28-2014, 07:18 PM   #10
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You would be doing pretty well to get a 5Kg airplane off the ground on a 3s 3900mAh battery, let alone keeping it in the air for 40 minutes.

Sounds to me like this is far too ambitious a project for you to tackle given your experience level.. Have you even built or flown an RC plane before?
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Old 01-28-2014, 07:27 PM   #11
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Originally Posted by JetPlaneFlyer View Post
You would be doing pretty well to get a 5Kg airplane off the ground on a 3s 3900mAh battery, let alone keeping it in the air for 40 minutes.

Sounds to me like this is far too ambitious a project for you to tackle given your experience level.. Have you even built or flown an RC plane before?
Agreed:
A decent 3900 Mah battery will put out 30 Amps or so with no issues. With a 3S LiPo, that would be 30 Amps at 3.5 VDC/cell times 3 cells, or about 300 watts. With a 5 Kg model, that would translate to 11 pounds. That makes it 300 watts/11 pounds, or 27 watts per pound of airplane.

If you decide to use a motor and power system that will run at 27 watts per pound, you will have a very decent model that won't fly. And, worse, that 30 Amps won't last more than about 7 minutes at full power. To make that battery last for 40 minutes, you'd need to drop the watts down to abut 50 watts. And that comes out to about 5 watts per pound. Just won't work. (Hope my math is correct!)

One thing that might help with your project is one of those computer programs for this purpose. One good one is www.motocalc.com. This program is free for 30 days, then it's $39.00. This program will back up our statement that your model won't fly on 27 watts per pound of airplane, much less 5 watts per pound.

As a very crude estimate, you'd want something like 1/3 or 1/2 of the total 5 kg model weight in batteries. Maybe even more. And, they won't be cheap.

Here is some real world information on electric power. These models are running about 130 Watts per pound of airplane. Yes they are acrobatic models, but hopefully these threads will provide some useful information.

Thread on 70 size glow engine conversion to electric
http://www.wattflyer.com/forums/showthread.php?t=45222

Hacker 6S2P A123 powered Models
http://www.wattflyer.com/forums/showthread.php?t=44686

Hangar 9 Kantana Model
http://www.wattflyer.com/forums/showthread.php?t=68844

Hanger 9 Twist 40 Model
http://www.wattflyer.com/forums/showthread.php?t=70548

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Old 01-28-2014, 10:27 PM   #12
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I've flown my slow stick for an hour before, but that was with a 1800mah battery and a 450 sized motor. this combo flew with under 1/4 throttle. full throttle or aggressive throttle would make it 15 - 20 minutes of fight on the same combo.

get a large sail plane for a baseline, then once you have your model picked out, build it to suit.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 01-28-2014, 10:52 PM   #13
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Originally Posted by kyleservicetech View Post


What kind of model airplane are you working on?

IMHO, a 40 minute flight for an electric model will be very difficult to accomplish. (Read as expensive) That would be more in the area of a glow or gasoline powered model.
The answer is in the aircraft design, not the battery choice.

What he needs is essentially a powered sailplane for the duration desired. An e-conversion of the Dynaflight Butterfly could easily exceed 2 hours at a takeoff weight under 5 kg. Not expensive and not hard to do at all.

http://www.wattflyer.com/forums/showthread.php?t=5031
See post #4
Old thread... there are newer and cheaper options now.
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Old 01-29-2014, 02:57 AM   #14
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Originally Posted by fhhuber View Post
The answer is in the aircraft design, not the battery choice.

What he needs is essentially a powered sailplane for the duration desired. An e-conversion of the Dynaflight Butterfly could easily exceed 2 hours at a takeoff weight under 5 kg. Not expensive and not hard to do at all.

http://www.wattflyer.com/forums/showthread.php?t=5031
See post #4
Old thread... there are newer and cheaper options now.
Yup
Read posting #6. That dynaflite model would be a very good model for this sort of application. The Dynaflite model is similar in design to those two Craftaire Viking sailplanes I built some 25 years ago. Here is access to a photo of one of these units.(From Ray to Pete) It is a very large model.
http://www.skybench.com/index.html?h....com/home.html

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Old 01-29-2014, 06:41 AM   #15
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Originally Posted by hayofstacks View Post
I've flown my slow stick for an hour before, but that was with a 1800mah battery and a 450 sized motor.
Yes, but your Slow Stick wasn't anywhere near 5Kg, was it?

Obviously its perfectly possible to fly for 40minutes, but that wasnt the quastion. the question was what bettery would you need to fly a 5Kg model for 40 minutes... The answer to that is a VERY large battery Something like an 8s, 5000mAh might give you a chance.
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Old 01-29-2014, 07:29 AM   #16
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Originally Posted by realnewbie1 View Post
The maximum weight for this plane is 5kg
Question
It just happens that cross country sailplane race I was involved in years ago was also limited to a 5 kg maximum weight, per competition requirements.

Question, are you also working on a national or international competition?

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Old 01-29-2014, 07:37 AM   #17
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Originally Posted by JetPlaneFlyer View Post
Yes, but your Slow Stick wasn't anywhere near 5Kg, was it?

Obviously its perfectly possible to fly for 40minutes, but that wasnt the quastion. the question was what bettery would you need to fly a 5Kg model for 40 minutes... The answer to that is a VERY large battery Something like an 8s, 5000mAh might give you a chance.
H'mmm
An 8S 5000 Mah battery would weigh in at something like a bit less than 3 pounds or so. With a wild guess of the battery being 1/3 of the models weight, that would be a 9 pound airplane.

If we can make another assumption that the model will stay in the air at 40 watts per pound, that would be 360 watts to keep it in the air. That would be pulling about 15 Amps out of the battery pack. That 5000 Mah battery will only last in the neighborhood of about 20 minutes.

So, 40 minutes might be doable, but it's going to take some pretty good tweaking along with a high efficiency motor to make it work. Those Hacker's and similar units can hit 90% efficiency, but only for the larger motors capable of one KW or so. Running a one KW motor at 300 watts will really hurt efficiency.

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Old 01-29-2014, 08:37 AM   #18
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If the model was a slow flying sailplane type then it should easily fly in level flight on 20W / lb. That's what I was basing my battery size guess on.

In the testing that Quarneng did recently he was able to maintain flight on much less than 20W /lb but his models are remarkably light weight. It all depends on the type of model used.
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Old 01-29-2014, 09:21 AM   #19
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the larger the prop, the more efficient it is. id get the largest prop I could, and base the motor and battery off of that a large motor pulls less amps below full throttle then a smaller one would at wot in my experience.

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 01-29-2014, 12:32 PM   #20
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The large prop thing is very true. It's the same principal that says that long wings are more efficient than short ones.
You will find most serious electric duration models use reducing gearboxes so that a very big prop can be used while the motor can also operate at it's peak efficiency.
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Old 01-29-2014, 04:57 PM   #21
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Originally Posted by realnewbie1 View Post
Its a group project to design a plane that can take aerial pictures of a terrain. It will not be required to fly too quickly or perform any complex acrobatics just level constant speed flight for 40 minutes.
If you haven't jelled on an aircraft yet you might want to take a look at a couple planes that were designed for photography. http://www.outerzone.co.uk has at least two, both in the 85" wingspan range, the Paypod and the Gas Camera Model (aka Westener).

Although older and not electric powered they could be easily converted.

just curious, but why the 40 minute required light time and do you have an experienced R/C pilot?

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Old 01-29-2014, 06:21 PM   #22
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Even if you have picked the airplane... we can't recommend a battery without knowing a lot of information about the airplane.

Have you picked a power system?

Each choice you make will influence all of the other choices. Any airplane is a flying set of compromises.
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Old 01-29-2014, 08:58 PM   #23
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Originally Posted by hayofstacks View Post
the larger the prop, the more efficient it is. id get the largest prop I could, and base the motor and battery off of that a large motor pulls less amps below full throttle then a smaller one would at wot in my experience.
Yeah
Prime example of this is those propellers used in the rubber band powered models. Or, take a look at any of the human body powered aircraft designs. Every one of them uses big diameter, low RPM propellers.

One reason these electric motors do such a decent job on our models. They can turn big diameter props that a similar horsepower glow engine could never turn.

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Old 01-30-2014, 01:25 AM   #24
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Exactly. Try a 10x10 on a .049

slow stock prop reversal. it flies! easily! 543 watt dual motor bipe slow stick. push-me-pull-you. 242 watt 3 channel slow stick. 365 watt mini ultra stick. 415 watt mini contender. 810 watt ultra stick .25e. 220 watt alpha 450 sport (retired).
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Old 01-30-2014, 02:14 AM   #25
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Originally Posted by JetPlaneFlyer View Post
If the model was a slow flying sailplane type then it should easily fly in level flight on 20W / lb. That's what I was basing my battery size guess on.

In the testing that Quarneng did recently he was able to maintain flight on much less than 20W /lb but his models are remarkably light weight. It all depends on the type of model used.
Twenty Watts per pound, on an 11 pound model would be about 220 watts. Dividing that by 746 is about 1/3 Hp. At 550 foot pounds (Yeah obsolete mathematics ) per second for one horsepower, that is about 160 foot pounds per second.

As a wild a***d guess, with an over all flying efficiency of 50% that would be about 80 foot pounds per second available to lift the airplane vertically.

Dividing by the 11 pounds, that would be some 7 feet per second, or 400 feet per minute or so. Yeah, it might even work at 20W per pound. But suspect the take off run would be really long!

A lot really depends on how close that 50% estimate is to the real world. And, as indicated, would really depend on the type of aircraft used for the project.

Just ran those numbers through www.motocalc.com on my giant Big Stick model, using 20 watts per pound by dialing back the watts input. Per motocalc, at 320 watts input on a 16 pound model, it will not maintain altitude. Having tried this during a flight, that is pretty much verified. The Castle Creations ICE ESC built in flight data recorder showed that about 600 watts is required to maintain level flight on that Big Stick. (Agreed, that Big Stick is no sailplane!)

Great Planes Giant Big Stick Electric Conversion
http://www.wattflyer.com/forums/showthread.php?t=65052

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