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Aug 19, 2006, 04:05 AM
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WebOCalc: Free calculator program helps design your foamy

Hi everyone, some of you may have run across WebOCalc before. It's a little Javascript program I wrote to help design ones own planes, and this forum seems like the right place to introduce the newest version.

WebOCalc was written from the start with three goals in mind: it had to be very easy to use, it had to work on your computer whether you use Linux, FreeBSD, Mac OS, or Windows, and it had to be Free as in freedom (Free software, like Linux or Firefox). It also works whether you live in the USA and are stuck with imperial units, or whether you live elsewhere in the world and use metric units.

Yes, there is a new version, WebOCalc 0.7.3. You can try it out or download it (free) from my RC Groups website, . Click on "Software", then "WebOCalc".

If you enter a few numbers into WebOCalc (things like wingspan and chord, the weight of your model, the battery voltage, and the desired current draw), the program will tell you how the model will fly, in simple English. It will also suggest some propellers that will work with the model, and appropriate gear ratios matching each propeller. For direct-drive, pick the propeller(s) with a gear ratio near 1.0.

All this means you will know before you build if your plane is going to be too heavy, too small, lacking in power, etc. You will know if you need a bigger battery or bigger motor, and you will know which propeller to use.

So what's new in the new version? Older versions of WebOCalc chose the propeller pitch speed for you, based on calculating the models stall speed and using a rule-of-thumb for pitch speed. The newest version allows you to decide how fast you want your plane to go, so you have more flexibility if you want to build something that flies unusually fast, or unusually slowly.

At the same time, if you want, WebOCalc can suggest a pitch speed for you. This is very handy for quickly getting a model in the air, or if you have no idea what a good pitch speed for the model might be.

I use WebOCalc all the time when I build a new plane, whether it be an ARF or a scratch-built design. It helps me choose motors, props, batteries, and to set a weight target that keeps the model flying the way I like it to.

I hope some of you download it and try it out, and I hope it helps you make your next DIY foamie.

I'm attaching a screenshot of the newest version of WebOCalc to show you how easy it is to use. Clicking the "Help" button might answer any questions you have, if not, I'll try and answer them in this thread.

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Aug 19, 2006, 06:32 AM
I'm not flying backwards!
Tony65x55's Avatar
Thansk Flieslikeabeagle, great contribution. I love your avatar
Aug 19, 2006, 11:08 AM
Victoria Super Mortem
UBILDIT's Avatar

Thank You!

I am conducting a test for Superfly and using your software online quickly told me I have a good setup for the purposes I am trying to achieve. I had to put wingspan and chord in as a monoplane even though it's a delta wing and wingspan and chord is all there is. Then the motor and prop deal was really cool. I put in "premium brushless inrunner" 3000 rpm/kv and 70 mph pitch speed. Its a Mega 16/15/3 with a 6x4 prop and it reccommended a 6X3 as being close to 1 to 1 since it is direct drive. Said it would fly "wild verticle" and also slowly, which is a characteristic of the regular superfly. Confirmation of your beliefs before flying has given me less anxiety about the maiden that's for sure.
Thanks Flieslikeabeagle... Awesome software..

Aug 19, 2006, 01:33 PM
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Tony, Bo, thanks for the feedback! So glad you found WebOCalc helpful, Bo.

I run most of my planes through WebOCalc now when I'm in the planning stages for the build. I even run the numbers for planes I'm interested in buying, but haven't bought yet - its steered me away from some overweight models.

WebOCalc multiplies the average wingspan and average chord to find the wing area, then uses that and the models weight to calculate the stall speed. If you select "Biplane", it doubles that wing area to allow for both wings.

For a delta wing, the average chord is equal to the wing chord halfway between root and tip. You can measure it with a ruler, or just add the root and tip chords and divide the result by two. And you are quite right, it is a monoplane (at least I've never seen a biplane delta-wing model, and certainly never a biplane superfly!).

Aug 20, 2006, 06:04 PM
“There’s no place like Foam”
gpw's Avatar
Prof , Glad to see such a usefull tool in this new DIY Foamie section ...!!!
Guys ... this works GREAT !!! Get it !!! Use it !!!! (saves alot of math ... )
Aug 20, 2006, 11:03 PM
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GPW, thanks for chiming in!

Guys, I forgot to mention that a couple of years ago GPW explained the wing loading and power loading business to me, here on RC Groups - he told me how he does the math to make his Blucor designs fly. That is what got me started thinking about writing a progam to make quick work of all that math. So GPW was the inspiration for what eventually turned into the version of WebOCalc discussed in this thread.

During those two years I've learned a few more things about our little RC modes, so current versions of WebOCalc go beyond wing loading and power loading. WebOCalc now knows about things like scaling laws (big models fly slower than small models with the same wing loading), and aspect ratio (planes with long skinny wings fly slower than planes with short stubby wings with the same wing loading).

But GPW was the guy who got this all started - so three cheers to GPW!

Last edited by flieslikeabeagl; Aug 22, 2006 at 04:24 PM.
Aug 21, 2006, 01:52 AM
Registered User
ToniArte's Avatar
Nice software!

One comment about the metric measurements, though. Even if everything else is metric, the propellers are still measured in inches (both the diameter and pitch). Also, it should be possible to force the calculation to use direct drive as most of the outrunners are used without a gearbox.
Aug 21, 2006, 12:57 PM
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Toni - thanks for your comments!

Every APC propeller I buy has its dimensions written on it in both inches and millimetres. So I assume some people are using the metric numbers rather than the imperial ones. You may also notice that WebOCalc finds metric props that have dimensions that are close to integer inch sizes - it might suggest a 205 mm x 150 mm prop, which is basically the same as an 8"x6" prop.

The outrunner question is a little more complex. Firstly, you can use the current version of WebOCalc to find props for outrunners. As I said in post #1 on this thread:
For direct-drive, pick the propeller(s) with a gear ratio near 1.0.
Now, I could certainly force the software to search only for propellers with a gear ratio of exactly 1.0, but if I did that, the software would often fail to find any propellers at all (because there is no guarantee that any off-the-shelf propeller exists that will simultaneously consume a specified amount of power, provide a specified pitch speed, and turn at a specified rpm).

In real life, given an outrunner and a model that it can fly, you will usually find a propeller that is in the ballpark for all these parameters, but not exactly - it may draw a bit less current than you wanted, for example, or perhaps provide a pitch speed that is somewhat higher than optimal.

Keep in mind also that WebOCalc cannot be 100% exact; it asks you for so little information that the answers it comes up with cannot possibly be perfectly accurate. For example, not all "premium inrunner" motors are equally efficient, and not all airfoils stall at the same lift coefficient.

So even if I forced the software to search for a propeller with exactly a 1.0 gear ratio, and even if such a propeller existed, it would not necessarily be any better in the real world than a propeller that came up with a 0.96:1 gear ratio.

I think there is an advantage to the program showing you more than one propeller that might work with your motor and plane in direct drive, letting you make the decision as to which one to pick. Computers are good at doing fast math calculations, but lousy when it comes to common sense; people are usually the opposite. So I think it works better to let the person using the program use their judgement to make the final cut as to which propeller to use.

If the software comes up with only one propeller with a gear ratio near 1.0, then your answer is clear; if the software comes up with one with a gear ratio of, say, 0.95 and another with a gear ratio of, say, 1.1, then you can choose between them. The one that should be geared 0.95:1 will end up pulling slightly less current than you planned if used direct drive (1:1 ratio instead of 0.95:1). Of course, the one that should be geared 1.1:1 will pull a bit more current than you planned if you use it direct drive.

Hope that helps!

Last edited by flieslikeabeagl; Aug 21, 2006 at 06:30 PM.
Aug 22, 2006, 04:12 AM
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ToniArte's Avatar
Originally Posted by flieslikeabeagl
Every APC propeller I buy has its dimensions written on it in both inches and millimetres. So I assume some people are using the metric numbers rather than the imperial ones. You may also notice that WebOCalc finds metric props that have dimensions that are close to integer inch sizes - it might suggest a 205 mm x 150 mm prop, which is basically the same as an 8"x6" prop.
The funny thing is that some things are still measured in inches even though everything else is in metric. Like TV's and computer screens, wheel diameter on tyres (width is in millimeters), bicycle wheel sizes, nails, garden hose diameter etc.

I have personally never seen anybody talking about metric propeller sizes, it's always on inches. Like the basic parkflyer prop, APC/GWS 10x4.7.
Aug 22, 2006, 10:47 AM
Lazy Builder
How do we calculate ducted fans?
Aug 22, 2006, 12:52 PM
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flieslikeabeagle's Avatar
Originally Posted by ToniArte
The funny thing is that some things are still measured in inches even though everything else is in metric.
Toni, you're shattering my illusions here!

I grew up in a country that used metric units, and the only people I knew who used inches for anything were people who were 30 - 40 years older than me - they grew up with imperial units. Of course, I wasn't involved in RC flight at the time, so I didn't realize those vile imperial units had snuck their way into prop sizes even in metric countries!

I can modify WebOCalc to spit out prop sizes in inches, even if you select metric units. It just seems so wrong, though!

Aug 22, 2006, 01:44 PM
If u wind it it will run!
usedhippo's Avatar
i love free software
Aug 22, 2006, 02:46 PM
Registered User
FlynAZ's Avatar
Great - Thanks for the nice tool.
Aug 22, 2006, 03:12 PM
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Usedhippo (your username makes me chuckle every time I see it!), I love Free software too! I feel quite deeply that information should be free, and denying people knowledge until they pony up money seems unethical to me. And yet that is what closed software does - you are not allowed to study the code to find out how it works, you are denied the ability to figure out why it breaks, and if you somehow find a way to fix the broken software, you are not allowed to fix it - by law! No wonder Windows 9X was so badly broken, despite the fact that millions of programmers around the world used it - there was no way for them to contribute fixes or improvements back.

Six years ago now I switched to Linux because I was sick of Windows and its problems, and even now I am still amazed at the wonderful wealth of free software I use every day. Everything I need is available free (and Free), from the operating system (Linux, FreeBSD, and a few others) to the usual everyday apps (browsers, email, music players, word-processors, spreadsheets), to the more exotic stuff: The Gimp is a free Photoshop substitute and meets all my needs for working with digital pictures, CRRCSIM is an RC flight simulator that lets me keep my flying skills alive when its too cold, dark, rainy, or hot to fly, RSIBreak warns me not to stay at the computer too long to avoid carpal tunnel and other RSI problems, and so on, and so on.

All this sounds a wee bit off topic, but actually I think there is a lot in common between the Free Software movement and this forum - the central idea in both cases is the same. Design and build something for the joy of it, share the fun for free with anyone who wants to try it, and everyone benefits from any good suggestions and feedback you get from all those other creative people who try the original idea. The only difference is the software guys are sharing code, while we're sharing plans for foam planes!

By the way, WebOCalc is Free in the same sense as Linux - the code is open, and if anyone else is a programmer and thinks of a way to improve my work, they can legally do just that. And all of us will benefit from that persons creativity.

Aug 22, 2006, 03:21 PM
Lazy Builder
So, do ducted fans apply in your program? Is there a way to make it work? I put in a 2 inch prop, (about the same size as the impeller) and it says its impossible because the prop is to small.

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