abufletcher

As I approach completion of the largest model I've ever had (3m wing span, weight somewhere around 25lbs) I'm beginning to wonder how to balance it. The fingertips under the CG isn't going to cut it. In the past I've experimented with a Vanessa rig but don't feel entirely confident in that and the challenges of suspending such a large model is not inconsiderable.

So I'm wondering about the method used for full-scale aircraft, namely, placing scales underneath it. Could someone explain the method (and, yes, also the math) of doing this? Can it be done with a single scale? How much weight does the scale have to be able to handle? At present I have a small type of digital scale. To get a rough idea of the overall weight on my 1/6 scale WWI models, I've been hanging them on a cheap spring-type fishing scale.

Would a bathroom scale (with increments to 0.1 kg) do the job?

Lone Star Charles

The FAA put out this circular a few years back. It may be more info than you want or need, but if you can wade through it, it's a pretty good description.

http://www.faa.gov/regulations_polic...-h-8083-1a.pdf

The EAA also made this video:

http://www.eaavideo.org/video.aspx?v=2421149531001

OldScaleGuy

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Here is what i posted this week regarding balancing big planes. I used this on my 50% Smith Miniplane and 30% Extra. Worked fine for me.
http://www.rcuniverse.com/forum/rc-w...l#post11985830

TFF

I think you need to get into the gram scale. .1 kg is too much an error. You only need 1 scale, moved to each gear position with a fake scale spacer under the other gear. Check at all three gear positions and then do the math.

Boomerang1

Abu, I don't know why the Vanessa rig doesn't suit you,
the only bad thing about it is I didn't build one earlier!

I put the model on my bench, loop the balancer up then
slowly winch it up, it cannot fall far.

This is the first time I used it, off the floor.

John.

abufletcher

Thanks for all the replies. Good stuff. Let me address them in reverse order. John, I've used the Vanessa rig with smaller models but with a model this size and weight the rig has to be substantially stronger (metal rod and less "stretchy" rope) not to mention that you need a more secure way of hanging it from the ceiling (even if the model only has to be an inch or so off the ground). There is nowhere in my house that I could securely hang a fully assembled 3 meter wingspan model.

TFF, I do have a small gram scale, but don't know what its maximum weight is. I would imagine that it would have to be able to deal with something like a third of the total weight of the model. That's why I asked about using a bathroom scale.

Steve, I like the simplicity of just picking the model up by the wingtips. There's only one room in the house, namely my main workshop (aka, what used to be the living room) that is large enough do this inside.

LoneStar, that looks like a lot of good, solid information.

TFF

The small one would probably work for the tail. Your bathroom scale is about the error used on real airplanes.

TFF

It might be ok if you use the gram on the tail and bathroom on mains. .1kg error on the tail is too much.

acerc

Abu I am with you on the normal methods and as for the Vanessa, well, my latest build is 43lbs and the trailing edge is only 1/8"x1/2" balsa top and bottom. I definitely don't want that kind of weight hanging on such a lightweight T/E. So I bought a cheap digital scale and went the weight and math route. There is a good how to just for us modelers, give me a bit and I'll se if I can find it

abufletcher

I just checked my digital scale. When I put it under one of the front wheels, I get an error message. I assume that means the weight is over the limit. Does the weight and calculations method require THREE scales (as explained in LoneStar's EEA video)?

Rob2160

Hi,

I made you an Excel spreadsheet to help with calculations - you will need three scales accurate to the gram.

This is how to you do it.

Define a "datum" - Use the front of the aircraft. IE the tip of the spinner.

1. Measure the distance from datum to the nose gear (or tail gear)
2. Measure the distance from datum to main gear

3. Place the aircraft on the three scales - adjust the nose (or tail) scale position so the aircraft is in a normal flying attitude.

4. Record the values.
5. Enter the data into the spreadsheet - it will give you the calculated CG position (from the datum - IE the nose of the aircraft)

Here are two samples - one for a tricycle undercarriage and one for a tail dragger.

Always ensure the blue cells have an actual value or Zero (0) as per the example.

The Math simply adds the three moments for the three wheels and divides the total moment by the total weight to give CG Position.

Download the Excel sheet here - https://www.dropbox.com/s/201ngkyfwb...20CG.xlsx?dl=0 (Click open if a window pops up)

Rob2160

Yes you will need three scales to do this accurately.

abufletcher

Rob, very cool! I've downloaded that and will play around with it a bit. I assume that each of the scales would have to deal with something like 1/3 of the weight (or actually more on the main wheels). My little digital is only good up to 1000g.

abufletcher

Is it not possible to weigh the model, properly "blocked up" in a level position, one wheel at a time? For example, put the scale under the right main gear with the left main gear up on a block equal to the scale? I'm put off by the idea of having to buy three heavy duty scales accurate to 1g.

*****

The prospects for using the weighing method are not good. I just got back from the hardware store where I was looked at digital scales. There was one with a 5kg limit and an accuracy of 2g and one with a 10kg limit with an accuracy of only 50g. The price for either was around $110. Even getting one would an unwelcome expenditure. Getting three would be ridiculous.

It seems to me that the methods for balancing a model fall into two categories: 1) Actually visual demonstrations that the model is "balanced" and 2) Calculations that the model SHOULD be balanced. Examples of the former are the "fingertip" method, the use of some sort of balance stand, or even suspending a model from a string (which I have seen done and done myself). In all of these methods we can directly observe the position of the model and whether it appears to be balanced as some particular CG.

In contrast CG calculations don't visibly demonstrate that the model is in balance. One has to trust the engineering and the math. This can be hard. It's hard for me. The Vanessa rig also falls into this category since the only visual evidence is the position indicated by the plumb bob. The model itself is never actually "balancing" in the rig.

RBACONS

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There's absolutely no reason you need 3 scales. One scale suffices. You just need to block up the other 2 wheels such that the plane is sitting in its flying attitude and try to keep that attitude as you move the scale from wheel to wheel. And you should be able to order a scale on-line for a lot less that that. I got a Cen-Tech from Harbor Freight for $40. 70lb limit with accuracy to tenths of ounces and/or grams.

http://www.harborfreight.com/70-lb-3...ale-95069.html

abufletcher

That looks great! And I'm going to be in the US towards the end of March so if I order now, I can bring it back with me.

Actually, this one which weighs up to 5kg also looks good. I could pick up three of these for about $70 bucks. But, hmmm, I wonder whether 5kg would be enough for one of the main wheels. The 3 x 5kg (15kg/33lbs) is enough for any model I'm likely to build. My P-56 is supposed to weigh around 18lbs which seems extremely optimistic for a 3mm Cub-style model with a gas engine. To that weight, I've added the considerable weight of the UMS 770 radial, its metal mount, a metal cowl, a glow system and its battery, and dual Rx batteries. I don't even have a rough weight at the moment but let's say it's a lot more than 18lbs.

http://www.harborfreight.com/digital-scale-95364.html

But, hmm, I wonder whether 5kg would be enough for one of the main wheels. The 3 x 5kg total (15kg/33lbs) is enough for any model I'm likely to build. My P-56 is supposed to weigh around 18lbs which seems extremely optimistic for a 3mm Cub-style model with a gas engine. To that weight, I've added the considerable weight of the UMS 770 radial, its metal mount, a metal cowl, a glow system and its battery, and dual Rx batteries. I don't even have a rough weight at the moment but let's say it's a lot more than 18lbs.

rtstestpilot

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Hello all,
I've been doing a version of this for many years now on all my larger models and have made an excel spreadsheet for the other members of our Club to simplify it a bit. I can't take full credit for all of it because it was assembled from a number of other sources I had found on the internet and compiled into our version. You only need one scale (a postage scale - I use one from Harbor freight) that can weight the whole complete model. Do as the directions say for the set up - then the scale is used for the tail wheel or nose wheel only from then on. Believe me, it makes it really simple and very accurate without any major lifting or damage to the aircraft. A plus is that you now have a very accurate record of it and if you alter the aircraft, all the hard work is done when rebalancing it.

The full scale guys use 3 scales to get the complete weight of their aircraft for ease but they and we can use one - the trick being to use 2 spacers of the same thickness as the scale to hold the other 2 points of contact at the same level as the one with the scale under it, then just add up the 3 numbers.

Apparently the upload manager is giving me an invalid file problem. PM me and I'll be glad to send it to you.

Todd

OldScaleGuy

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I am all for new and better ways of doing things (well,,, new to me).

PM sent. Thank you!

rtstestpilot

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I emailed it to you OSG. Let me know if you have any questions or problems. Enjoy!

Todd

OldScaleGuy

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Thank you Todd. File received and opens fine and it is very nicely done.
Looking forward to giving this method a try.

abufletcher

From the EAA video above, I understood that the weighing should be done with the plane at a normal flying attitude, so for a tail-dragger that would be with the tail propped up. Is this right? Or does it not matter?

abufletcher

From the EAA video above, I understood that the weighing should be done with the plane at a normal flying attitude, so for a tail-dragger that would be with the tail propped up. Is this right? Or does it not matter?

drac1

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PM sent Todd.

abufletcher

I've place my order for the HF scale. I'm having it sent to my address in SoCal and will pick it up in March and bring it back to Japan. Actually I ordered two different scales, the large one and another 5kg scale that will be very useful for other purposes.

Joe Fisher

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This might give you some ideas.