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Battery Configuration

Old 01-02-2021, 04:25 PM
  #1  
RKTLaunch
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Cool Battery Configuration

I am going to be challenged to keep my model to 6.5 to 7 lbs.

I know little about RC electrics, but have been reading and searching, but there's nothing like the experience that many of you have. I'm going to run twin JP Hobby 70mm EDFs (purchased) and 3 servos....receiver and 2x Talon 90a w/20a BECs (purchased). I do have the Futaba 6J and receiver, but no servos yet.

The options I'm looking at are:

1. One large battery for everything, which would be the lightest, but I'm not sure that's possible.

2. One large battery for the 2x EDFs, and one smaller battery for receiver and 3 servos....seems like a decent idea, but wouldn't that be wasting the BECs?

3. Two medium sized batteries for everything. This might be the heaviest option.

All opinions and advice are appreciated!
Thanks
.
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Old 01-03-2021, 06:22 PM
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Wildflyer
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All of those ideas are possible. EDFs are power hungry things and 2 of them are twice as bad.
Your ESCs will cut power the the fans but save some for the servos and a little booster punch if you need it for landing. In most cases unclip the red wire from one of your ESCs so that they don't fight each other. The Talon instructions should tell what they want or give CC a phone call.

Final choice may be controlled by the internal design. I have a few planes that run paralleled batteries or in series to fit my needs. That way I don't have to buy so many batteries.

Good luck
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Old 01-03-2021, 10:00 PM
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quorneng
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One or two batteries really comes down to the best configuration to fit in the plane.
About the only consideration is that if you are using two batteries they really should be identical, not just the same cell count and capacity but ideally from the same manufacturer.
This applies regardless as to whether the pair are in series or parallel. e.g.2x 3000 mAh 3s in parallel to give the equivalent of 6000 mAh 3s or in series to give 3000 mAh 6s.
With a 20a BEC it seems a pity not to use it, particularly if you are only using 3 servos, but you can only use one BEC so as Wildflyer suggests disconnect the red wire on one ESC so its BEC provides no input.
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Old 01-04-2021, 01:29 AM
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RKTLaunch
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Originally Posted by quorneng View Post
One or two batteries really comes down to the best configuration to fit in the plane.
About the only consideration is that if you are using two batteries they really should be identical, not just the same cell count and capacity but ideally from the same manufacturer.
This applies regardless as to whether the pair are in series or parallel. e.g.2x 3000 mAh 3s in parallel to give the equivalent of 6000 mAh 3s or in series to give 3000 mAh 6s.
With a 20a BEC it seems a pity not to use it, particularly if you are only using 3 servos, but you can only use one BEC so as Wildflyer suggests disconnect the red wire on one ESC so its BEC provides no input.
I would like to run one battery...
I've been looking at a SMC 6S 8000/20C, or the SMC 6S 8000/120C. They are 2 Lbs (either one) but the difference in weight isn't a lot... going down to a 6000. I'm on my smartphone at the moment tho, so I can't look it up. SMCs have very good testing and consumer ratings, but in 5000 and up, they don't carry a 40C or any other mid rated battery. They're all 100-120C. The two above are their only choices in 6S. Of course on their website, they say their 20C is like everybody else's 40C....for what that's worth.

I wonder if running the power (red wire) off one BEC has any chance of effecting"it's" EDF? As indicated I'll use both BECs, but only one red wire on one of them....or will they naturally balance thru the other BEC wires? I haven't drawn any schematics yet....to early for that. I'm still detailing the wings. Tapered wings are fun to design, but very time consuming.

...but aren't the servos powered thru the receiver? If they are then all the servos are on one BEC?

Thanks, appreciate the experience y'all have.
I'm trying to edit type-Os. I hate the spell check on smartphones. I get a lot of things I didn't type.

Last edited by RKTLaunch; 01-04-2021 at 02:03 AM.
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Old 01-04-2021, 10:55 AM
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quorneng
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RKTLaunch
Yes the connection from the ESC to the receiver not only controls the ESC (via the outer wires) but powers the receiver itself and all the servos through red wire coming from the BEC built inside the ESC. The ESC itself is powered by the direct connection to the battery.
If the BEC is 20 A it will have more than enough power for 3 or 4 servos. A 6A BEC would likely be quite adequate. The receiver itself take very little power.
You would be using just one of the BECs but they are very reliable, usually more so then the ESC itself, so control is retained even through the EDF has stopped. In a twin such a situation is likely to end in a crash.
Don't be too concerned at the reliability of the electronics. It is a great many times better than the risk of the plane damaging itself from hitting the ground accidentally.

What the specified full power amps is for the 70mm EDF on 6s? Double that figure for 2 EDFs represents the maximum amps the battery will have to provide.
My guess is you are looking a possible 160A and that would represent 20C from a 8000 mAh battery. It is unwise to run a LiPo at its recommended limit as the figure tends to more 'marketing' that realistic.
EDFs are power hungry and work the battery hard.
Remember even a 20C discharge rate will exhaust a battery in less than 3 minutes and doing so will shorten its life.

Also such big heavy batteries are going to make your relatively modest size plane 'heavy' and thus it will have to fly fast just to keep in the air.
To avoid damage it is likely you would have to perfect a vertical take off and landing from its first flight. Not a very likely scenario.
A serious crash could prove to be expensive.







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Old 01-04-2021, 04:43 PM
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.
Originally Posted by quorneng View Post
RKTLaunch
Yes the connection from the ESC to the receiver not only controls the ESC (via the outer wires) but powers the receiver itself and all the servos through red wire coming from the BEC built inside the ESC. The ESC itself is powered by the direct connection to the battery.
If the BEC is 20 A it will have more than enough power for 3 or 4 servos. A 6A BEC would likely be quite adequate. The receiver itself take very little power.
You would be using just one of the BECs but they are very reliable, usually more so then the ESC itself, so control is retained even through the EDF has stopped. In a twin such a situation is likely to end in a crash.
Don't be too concerned at the reliability of the electronics. It is a great many times better than the risk of the plane damaging itself from hitting the ground accidentally.

What the specified full power amps is for the 70mm EDF on 6s? Double that figure for 2 EDFs represents the maximum amps the battery will have to provide.
My guess is you are looking a possible 160A and that would represent 20C from a 8000 mAh battery. It is unwise to run a LiPo at its recommended limit as the figure tends to more 'marketing' that realistic.
EDFs are power hungry and work the battery hard.
Remember even a 20C discharge rate will exhaust a battery in less than 3 minutes and doing so will shorten its life.

Also such big heavy batteries are going to make your relatively modest size plane 'heavy' and thus it will have to fly fast just to keep in the air.
To avoid damage it is likely you would have to perfect a vertical take off and landing from its first flight. Not a very likely scenario.
A serious crash could prove to be expensive.
My plan is to slingshot the takeoff.....at about 35 to 45 degrees. Although many people do it level....haven't made that decision yet. The wings at 30" are about half length delta, so I'll have quite a bit of wing area for the 30" span.

The SMC 8000/120C is 929g=32.7oz

Their 5600/60C
797gr is only 4.6 oz lighter. Not a big difference.

Not until I go to their 5200/120C; 627gr 10.6 oz lighter,
is there a big difference.

5200mAh doesn't seem like it'll keep the 2 EDFs running very long. Would save me a substantial amount of weight.....but not much running time.

From the JP Hobby website. I don't know if they're talking about one or both:
For Twin EDF Jet - Counter Rotative CW version
Combo 70mm / 12 blades / 2250Kv (outrunner) / 4-7S / 225g
> 6S (22.2V) / 76.6A / 2.35kg / 1700W
.

Last edited by RKTLaunch; 01-04-2021 at 07:42 PM.
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Old 01-04-2021, 07:44 PM
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Do you know the wing area and a guess at the likely all up weight?

I have a suspicion the wing loading (oz/sg ft) is going to end up at an realistic figure for the size of plane.
With full 'drone' stability you might be able to get it to hover if the control surfaces are big and responsive enough but the transition to forward wing flight with a high wing loading is difficult to achieve and then back to the hover is even harder.
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Old 01-04-2021, 07:51 PM
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Originally Posted by quorneng View Post
Do you know the wing area and a guess at the likely all up weight?

I have a suspicion the wing loading (oz/sg ft) is going to end up at an realistic figure for the size of plane.
With full 'drone' stability you might be able to get it to hover if the control surfaces are big and responsive enough but the transition to forward wing flight with a high wing loading is difficult to achieve and then back to the hover is even harder.
Hover? Your pulling my leg, right? Yes, high-wing anhedral.

I was going "T" tail, but looked at the approach angles and put the elevator below the main wing at mid fuselage. No potential buffeting from the main wing, that way.... during high angle approach.

I was aiming for 6.5 to 7 lbs with a bigger battery. With one single 5200/120C, maybe 6 lbs.

130.32 x 2 = 260.64 sq" Total wing area…..from the 2nd rib out.
.

Last edited by RKTLaunch; 01-04-2021 at 09:27 PM.
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Old 01-04-2021, 10:51 PM
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Using your best figures, 6lbs and 260sq" would be 53oz/sq' or 3.33333lbs per sq ft.
That's really high for a model, hope it works.

The ESC does not need the red wire to function properly. it gets it's information from the signal wire giving it positive pulses and the negative wire. The red wire is just supplying power to the receiver and the rest of the system. Two BECs in the ESCs hooked together is not a good thing in most cases. It can be done through special circuits, but you will have plenty of power with using one Talon.

I would suggest mounting your EDFs on a test mount and trying your whole power system before mounting in the plane.
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Old 01-05-2021, 02:49 AM
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Too heavy.

Hopefully I'm over estimating the weight of the model. It's been some time since I built one. My original electrical weight also included 5 servos and two batteties. I'll go thru it again to if I'm off on my list.

Originally;
3 Lbs model, 5 lbs electrics. I'll check again.

Yeah, my original estimate was with 2x 5000/100c for the engines and a receiver battery too.

I'll go back thru it all.

Yes, 2x EDFs, 1 Bat, 2x ESCs, 1 receiver, 3 servos;
46.97 oz = 2 lbs 15oz.

Model weight....just a big guess until I get all the materials and start cutting it all out.

Last edited by RKTLaunch; 01-05-2021 at 04:23 AM.
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Old 01-06-2021, 01:59 AM
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If necessary, I can make the wings bigger. All the pieces cam be "grouped", then I make the overall size anything I want. I just have to ungroup them, put the width of each piece where it belongs and record the new lengths...much easier then starting from scratch.

I won't do that though until I get all the materials and weigh them. I also have another design I started years ago that has more wing. With the input of some of you (thanks), I'll have a good chance of a thrust to weight that's more suitable for EDFs.

Ok, I get it.

(6*16)/(260/144)=53.** oz/sq'

Last edited by RKTLaunch; 01-06-2021 at 05:52 AM.
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Old 01-06-2021, 10:28 AM
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RKTLaunch
For a vertical take off the total weight is not too much of a problem, it just has to less than the available thrust! However 'balancing' a plane on thrust is a lot more difficult.
If the plane is intended to 'fly' using its wings then the resulting wing loading does have an impact on the speed it has to move through the air.
This wing loading 'formula' roughly calculates the minimum flying speed.
"Awing
's stall speedin mph is approximately equal to 4 times the square root of the wing loading in ounces per square foot"
With a wing loading in the order of 53 oz/sqft the minimum speed comes out at about 29 mph.
Remember this is only an indication, the actual minimum speed depends on a number of parameters including it physical size. For a given wing loading the bigger the plane the better.
In addition this would be the speed that effective aerodynamic control would be lost so the minimum practical speed would be something like 25% higher.

At the moment your predicted wing loading is pretty high for the size of plane you propose.


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Old 01-06-2021, 01:54 PM
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Originally Posted by quorneng View Post
RKTLaunch
For a vertical take off the total weight is not too much of a problem, it just has to less than the available thrust! However 'balancing' a plane on thrust is a lot more difficult.
If the plane is intended to 'fly' using its wings then the resulting wing loading does have an impact on the speed it has to move through the air.
This wing loading 'formula' roughly calculates the minimum flying speed.
"Awing
's stall speedin mph is approximately equal to 4 times the square root of the wing loading in ounces per square foot"
With a wing loading in the order of 53 oz/sqft the minimum speed comes out at about 29 mph.
Remember this is only an indication, the actual minimum speed depends on a number of parameters including it physical size. For a given wing loading the bigger the plane the better.
In addition this would be the speed that effective aerodynamic control would be lost so the minimum practical speed would be something like 25% higher.

At the moment your predicted wing loading is pretty high for the size of plane you propose.

I had originally considered rocket launching this model. I have a lot of experience at that. Then I looked at the cost of the rocket motors that would be appropriate. Nix on that. I think a sling-shot will do the job, but landing would be pretty difficult without slats and flaps, which of course....there's no way I'd build that into a model. I think what I have to do is weigh all the materials, before I make any final decision. I may play around with making the model bigger and adjusting the wing area. The added balsa and birch is small compared to the electrical. Again, I won't make those decisions until I order and weigh the materials.

In the '90s I designed and built a push-pull with twin queen-bees, only about 30 x 30". There was so much power, I just barely had to toss it and it had monster power. Free-flight and I did not damage to it. It landed flat in a soccer field, but I went on to testing wing design with rocket launching instead. Putting a small model 300ft in the air in a second and a half was a good way to test my experiments. I was successful, but I have some real adjusting for the EDFs....batteries, etc.

Last edited by RKTLaunch; 01-06-2021 at 02:11 PM.
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