Help! Tips on battery ratings/selection?

[Monte.C]

I'm designing a foamboard mini WB-57D. Twin 1806 2280kV motors (I'm sure that's more than plenty for this), 26" wingspan, as close to scale as I can get it. The problem is that to fit many of the battery packs we might use it would make the fuse much too fat. I've got this thing to comfortably fit an Ovonic 1500mAh 2S 50C standing on its thin side (I've got two right now). One reason it fits is because it's thin at about 14mm.

I would normally design this with the option for a 3S as well, of course, but for thickness it's looking like the best I can find would only be maybe something around 650mAh, maybe 850mAh.

I expect an empty shell weight at around 90-95 grams. Light servos, light 20A ESCs. The objective isn't speed, but gentle graceful flight. I'll use two aggressive 4" 3-blade props (I know the motors would be happy with a lot more prop).

QUESTIONS:
1. Will the 2S pack give me enough push so it doesn't feel underpowered under any normal conditions? - Coming out of a reasonable dive, etc.
2. What kind of run time could I get out of a 650mAh 3S running these 2 motors?

This forum is the most amazing resource. Used to be all you had was what you could read in the magazines, and some people were local club members and could ask Larry or Bob over there to see what they say. I want to thank everyone here just for being here.

 

[Monte.C]

Let me rephrase the question:
I'm designing a very small FB plane with a lot of wing and a very narrow fuselage. Twin 1806 2280kV motors, two aggressive 4" 3-blade props. I have two Ovonic 1500mAh 2S 50C batteries and they fit because they're thin. I'm wanting a graceful flyer, not a racer. Will these 2-cells give me enough push?

For space/size reasons it's looking like for a 3S I might only be able to fit maybe up to an 850mAh 3S with the current design. What sort of runtime might I hope to achieve?

Thanks in advance!

 

[Merv]

Take a look at the data on your motor from your supplier. They should give you an idea of a recommended prop an thrust expected from a 2S setup. On 2S you may need a larger prop. Anything above a 1:1 thrust to weight ratio should work.

 

[Monte.C]

Take a look at the data on your motor from your supplier. They should give you an idea of a recommended prop an thrust expected from a 2S setup. On 2S you may need a larger prop. Anything above a 1:1 thrust to weight ratio should work.

I'll certainly be using these motors at less than their ideal ("recommended") thrust capacity; those figures are for maximum performance without burning anything. But on this very small plane only one A-size motor with a larger prop would be perfectly fine, and it never hurts to under-utilize one of these motors. The only issue is with the (significant) added weight of another motor & ESC, but doubling the thrust with the 2nd motor I think should more than compensate for it. I'll bet anything two 4" bullnose tri-blades with a really big bite will work great. (Until I prove myself wrong!)

Anyway I found a 3S 850mAh lipo that should fit great, and it's even lighter than the 2S. Two motors will draw from it an awful lot faster... We'll just have to see about flight time. I'll fly both. Moving forward with the present design. Thanks!

 

[Monte.C]

Take a look at the data on your motor from your supplier. They should give you an idea of a recommended prop an thrust expected from a 2S setup. On 2S you may need a larger prop. Anything above a 1:1 thrust to weight ratio should work.

Thanks @Merv, I took a look at their motor specs. Of course they're only listing specs for 5", 6" and 7" props, but it's helpful anyway. Based on what I'm seeing I should be fine with a 3S and might do well with a 2S. Can't wait to see this design in actual foam.

 

[Merv]

I’m guessing the 7 inch prop was on 2S.

 

[Monte.C]

Yeah sort of.
Based on this info I'll likely beat 1:1 even with a 2S with the props I described. Either way, I can run it on 3S. And it'll be fun with either battery.