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May 21, 2019, 07:42 PM
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Question

Too good to be true? ButterFly setup design with WebOcalc


Hey all,

I have a now-vintage ButterFly (this one: https://www.rcgroups.com/forums/show...raft-Butterfly). The build was never finished, so I'm planning to complete it and get it airborne.

I'm using WebOcalc (also vintage) to design the setup, and some of the theoretical results I'm getting seem too good to be true.

(Link to WebOcalc: https://web.archive.org/web/20161028..._imperial.html)

So I'll break it down here, and I'm hoping some of you will be able to glance over these numbers and let me know if I'm missing anything. Thanks in advance!

* Weight with NO electronics is 118g/4.16oz, and the target weight of the model is 12.5 oz, leaving ~8.3 oz for guts.
* Wingspan is 30 inches with a chord of 10 inches, which gives me a wing-loading of 5.76 oz/ft2.
* This wing-loading gives me an estimated stall speed of 11.7mph, so my target top speed is 35mph.
* I'd like vertical climbs, so I want 13+ oz of thrust.
* Assuming 2S LiPo, the calculator recommends a ~49+ watt ~1,000 KV brushless with a 9x6 prop to achieve target speed, thrust, efficiency, etc.

This leaves me with the last variable: what is the largest-capacity battery I can use without going overweight?

I've calculated the weight of every other component I need (motor, receiver, servos, ESC, prop, etc.), and it seems I can afford to carry a Turnigy 2S 2200 mAh pack, which weighs in at 120g (assuming it physically fits).

The calculator says this setup gives me unlimited vertical climbs with hovering AND a flight time of 28-46 minutes!

Does this seem realistic to you?

Maybe a the super-light covered balsa construction makes this possible? Perhaps I'm accustomed to the performance of heavier-built models. My other model is a massive 64-oz glow conversion...

The 1999 setup only achieved 50-degree climbs with a flight time of 10 minutes or so, but it was with a Zoom 400 motor and a 7-cell NiCad.

Looking forward to reading your thoughts, ideas, and suggestions.

Thanks for your time!

Sam
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May 21, 2019, 08:30 PM
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scirocco's Avatar
Looks about right modelling a few motors that size I have that are pretty close in ECalc.

if you're thinking about weight, then you could simply use a 2S 1000. Actually I'd trade off a few grams to a slightly heavier motor, around 60g, which is likely to give you better efficiency, and offset that with a smaller battery. You should still get long flights at max current around 10A.
May 22, 2019, 03:06 AM
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Skylar's Avatar
I have to concur with Scirocco on battery size. Keep it light, because there is no way you'll get even close to 35oz thrust with a Kv1000 motor and 9x6 prop on 2S. My favorite calculator predicts about 13oz.

A 2208 motor should be fine. They weigh around 45g, but my gut feel says you should look for Kv higher than 1000. I would aim for 1500 or more on 2S. That still won't give you 35oz thrust, though. To get vertical performance, probably means keeping weigh as low as possible, but that directly impacts battery capacity, which will cut into your duration. So, I'm afraid you'll have to choose between thrust and duration. You can't have both, in my opinion.
May 22, 2019, 11:59 AM
Registered User
Thread OP
Thank you both for your input! Really appreciate it.

Skylar, 13 oz. of thrust is actually about right (I think), since the all-up weight of the model will be around 12-13 oz. I've attached the full WebOcalc report below if you're interested.

I'll mess with the weight trade-off between the motor and the battery and see if I can find a sweet spot...

Thanks again for the advice!
May 23, 2019, 01:29 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
You could check with other calculator(s)
Sticky: e-flight calculators (compilation) - RCG


Vriendelijke groeten Ron


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