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Old Aug 31, 2012, 12:19 AM
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first time builders questions

Hello im new to building foam planes and have built 2 but they won't fly i was wondering how do i weigh my plane? And how can i tell if have good trust? thanks for any help :-)
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Old Aug 31, 2012, 12:49 AM
Fremont, CA
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Any pictures of your planes?
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Old Aug 31, 2012, 01:12 AM
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Ill post up my p40 and cub i built tomorrow. I love building them i just can't get them to fly but i can fly all my parkzone planes fine so i don't know what im building them wrong.
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Old Aug 31, 2012, 07:21 AM
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Yes... pics first, answers second
Don't forget to add some detail like building material and electrics used.
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Old Aug 31, 2012, 08:13 AM
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Southern Vermont
Joined Feb 2007
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You can weigh a plane even if you don't have a scale.

(To the inevitable commenters who would normally follow a post like this with "Or get a get a Harbor Freight digital scale for ten bucks." Beat you to it. Yes you can buy your way out of any lack of tool problem, more or less -- the following is about thinking your way out.)

Place the plane on one end of a ruler or yardstick with a fulcrum under it (a thick dowel or D-cell battery lying on a table for instance).

Temporarily attach a disposable thin plastic or paper cup to the ruler, past the fulcrum with, say, some dots of hot glue.

Gradually add water to the cup until the plane balances more or less at the other end. You may need to put the fulcrum either closer or farther from the plane to get things to work out.

When it balances, write down the distance from the plane to the fulcrum, and the cup to the fulcrum.

Pour the water into a measuring cup. Most are graduated in metric units (ml) these days on one scale, even in the US, and that makes it a little easier.

Write down how many ml the plane needed to balance.

Divide the distance from the fulcrum to the cup by the distance from the fulcrum to the plane..

Multiply that result times the number of ml in the cup.

That's what your plane weighs in grams.

If you want ounces, divide that figure by 28. (No need for decimals, that's close enough -- within about 1%).

Notes:

you may need different materials than a ruler and dowel, etc to make it work for the size plane you have -- but the principles are the same -- I bet you can figure out how to make it work, using what's around the house.

Measure distances to the middle of things -- middle of the plane where it rests on the ruler, middle of the cup where it rests, etc.

You may need to fasten the plane to the ruler temporarily. All depends on the size of things.

Good luck!
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Last edited by vtdiy; Aug 31, 2012 at 11:08 AM.
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Old Aug 31, 2012, 09:57 AM
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There are tons of great tools to use once you get a handle on the weight of the plane which will help you size your motor and propeller, like WebOCalc, at http://flbeagle.rchomepage.com. Go to software, webocalc button, and pick webocalc 1.7.6. You can download it and run it offline, or run it via the webpage.

Also have a look at the center of gravity calculator: http://adamone.rchomepage.com/cg_calc.htm

In addition to pictures, also describe the behavior of "it won't fly." The more specific you are in describing the problem, the quicker someone can help you in solving it. For instance: "I hand launch my planes at full throttle and they glide very well but they won't climb and eventually glide into the ground." Or, "I can't rotate off ground" or "I hand launch the plane and it pitches straight up and stalls, crashing back to the ground."

Finally - and I mean no disrespect at all to vtdiy, but to correct something in his very informative post, you'd need to divide gram weight by approx 28 to arrive at ounces, not multiply.
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Old Aug 31, 2012, 10:21 AM
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Vtdiy,

You just posted one of the most interesting comments I ever read on RCG. You always amaze me! So glad to have you always thinking outside the box!

Fred
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Old Aug 31, 2012, 10:43 AM
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vtdiy's Avatar
Southern Vermont
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bhoov128, thanks muchly! Corrected.

Will you be my copy editor for future posts? I tend to do that kind of thing often!
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Old Aug 31, 2012, 10:55 AM
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Thanks Freddie, as always you are super kind to forum users.

I had the same weighing without a scale problem once when was going to tow a big houseboat behind a Ryder truck for a thousand miles. I wanted 500 lbs of tongue weight to keep it controllable. How to measure that?

I took a piece of 4by4 as a lever, used a log cutoff as a fulcrum, and calculated I needed about 2.8 times the distance from me to the fulcrum, as I did for the trailer tongue to the fulcrum.

Set that up by measuring. Then just pushed down on the end of the lever with all my weight.

When I got the boat situated on the trailer so I could just manage to lift it, I knew I had 500 lbs tongue weight.

I also made a gram scale out of paper, push pin, paperclip and coins for weighing paper glider models, but that's another story.


ps. I wouldn't need to be as far from the fulcrum these days for that same tongue weight!
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Old Aug 31, 2012, 11:08 AM
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vtdiy

That is insane and I must test it!
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Old Aug 31, 2012, 11:08 AM
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Quote:
Originally Posted by vtdiy View Post

ps. I wouldn't need to be as far from the fulcrum these days for that same tongue weight!
lol...... :d

f
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Old Aug 31, 2012, 11:10 AM
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Quote:
Originally Posted by dedStik View Post
vtdiy

That is insane and I must test it!
Be sure to use the latest version -- it usually takes me three tries to explain things correctly.....
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Old Aug 31, 2012, 11:43 AM
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Southern Vermont
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If you guys like doing things without tools, you might get a kick out of measuring (and correcting) a foam wing attachment to within a couple thousandths of an inch without a dial indicator. The exchange starts at post #72 here:

http://www.rcgroups.com/forums/showt...1480285&page=5
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Old Aug 31, 2012, 04:47 PM
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hey guys thanks for all the replys! well here is my p40 that im having problems with its build out of $1 store foam and a little styrene and birch. For electronics im running blue wonder motor, thunderbird 18 esc, 3 hs-55 servos, spektrum ar6000 parkfler reciver, gws ep-7060 prop and a 300mah 7.4 electrifly 20c. And i planed on using the same stuff in the cub but the p40 wont fly so i dont want to break 2 planes in a row if im doing somthin wrong. the wing span on the p40 is the leanth is 30inch well thanks again this is the best rc foam plane form around!





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Old Aug 31, 2012, 05:12 PM
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Southern Vermont
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Okay which blue wonder, they come in different kVs.

Your batt is only a small capacity 2 cell and matching that against a low kv blue wonder and therefore a too small prop, won't get you much thrust.

So first of all, which exact "blue wonder?".

Your planes look alright, by the way.

Your prop is high pitch, and low diameter if your kV is low and batt voltage is only 7.4.

Probably you'll need a longer prop with less pitch to work with that batt, but I'll wait to hear what the motor is before giving a specific suggestion.

At 20C you have maybe 40 watts available out of the batt which should be sufficient to fly, but prop matching to that system will be key.

A 3 cell of the same capacity would give you 60 watts available, which would be plenty.
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