How to get the C of G right with heavy Foam Board

by rick_harriss | April 17, 2015 | (9) Posted in Tips

As do many builders in Europe I find the foam board I can easily get is somewhat heavier than the DTFB seems to be. One result of this is I find it very hard to achieve the required Centre of gravity without adding  more weight, Often as much as 20 - 30 gms!.

To combat this I have come up with a simple idea.

I have built several of the FT designs and have had this balance problem with all of them. I see from the Forums this isn't uncommon. Perhaps I am heavy handed with the hot melt glue or it's the foam board I use but whatever the reason I always end up tail heavy, not, as Josh says, a good position to be in.

After some thought I have devised a simple way to combat this which should work with all of the swappable designs which use the standard power pod and should be adaptable for other designs.

My choices were to add weight to the nose to make it heavier, shorten the tail section or make it lighter but all these are less than desirable because of the added weight, change in the control action and because foam board is hard to lighten without the loss of a lot of the inherent strength.

The alternative then is to allow the motor and battery to balance the aircraft by lengthening the nose, but by how much?

With this idea in mind I built a sliding nose which contained the motor, the electronics and the battery. This slides inside the normal full sized power pod and allows me to extend the nose until I get a satisfactory balance point.

I hot glue the power pod to the wings or the fuselage to fix it in place and insert the sliding fit extra pod with all the electronics and battery fitted inside the now fixed power pod and adjust until the C of G is correct. I mark the sliding pod with a pencil so I can easily reset the position if required.

I then fix the position of the sliding pod with a couple of skewers through the entire fuselage. I find I sometimes have to extend the power pod by up to 2 inches (50 mm) to get a good C of G as per the plans.

 

COMMENTS

1959cutter on April 17, 2015
i just solved my CofG problem on my 200% blunt nose versawing, by building a balsa powerpod 18" long and installed at the front to slide the 3S battery forward or back, depending on what camera gear I have.
before I would need 300g to balance with!
all my dads gassers had a ton of melted lead tire weights in the front of his planes,and when they ran out of gas,it would drop like a lawn dart.
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rick_harriss on April 18, 2015
Yep, that's going to be a problem! I used to add weight all the time but with relatively low powered and light aircraft I don't think you can really. Even putting a bigger battery on it will degrade the performance.

I am guessing this is a particularly hard issue for Newbies to sort out.
The sliding nose at least allows me to find out how long it needs to be before I make a permanent final job out of it.
Actually the foam aircraft don't generally last so long I bother.
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1959cutter on April 18, 2015
my dad used to put pennies on the wing tips for ballance of the wings,but when it came to CofG he never considered the weight penalty for lead in the front of his planes.
i guess the mad power of the gassers made up for it!

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rick_harriss on April 19, 2015
I still use coins if necessary as I have plenty! However lead has a higher weight concentration. I just don't want to add a lot of excess weight even in the form of a heavy battery so this was a good solution for me.
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pintokitkat on April 18, 2015
Genius!
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rick_harriss on April 19, 2015
Thanks (modestly)
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johanjonker on April 17, 2015
I just add a bigger motor, I just upsize the motor till it tilts a little to the nose then I just add a little weight to the tail.. ;) I dont think I have ever build a FT design that balance as nicely as the DT version.. since I cant find DT foam, I guess CG will always be a little harder
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dharkless on April 19, 2015
Great idea! In models that have the power pod inserting into the fuselage you could extend the front of the fuselage a couple of inches and just let the whole pod slide forward. Trim the fuselage, add the motor mount skewers and install 2 or three skewers to secure the pod after balance is established. That way larger batteries would still fit.
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rick_harriss on April 19, 2015
Indeed you can. I also find this useful when making scratch builds without plans. This site http://adamone.rchomepage.com/cg_calc.htm tells you where the theoretical C of G should be but getting it there without making several nose cones or fuselages is difficult. The adjustable model makes things much easier.
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SnapPunchRobert on April 21, 2015
Really great idea. Great.

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rick_harriss on April 22, 2015
Thanks.
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squishy on April 20, 2015
I have a pet peeve, I've carried it a long time, and your title has kicked the rock over...The center of gravity on an rc craft is always there, it is not some elusive thing you must find. Balance the aircraft on your fingers and it will balance where the CG currently is. It's always there, never lost, so "finding the CG" or "how to get the CG right" are kind of funny statements huh? The proper statement is: "Find and balance of the proper CG point for lateral stability in reference to the center of pressure". I hear guys all the time "Well, did you find the CG?" "Did you get the CG right?" "Did you find the CG?" lol, It all makes me laugh when I hear it now...I picture the CG, like a masked man, running down the street and people trying to find him, make him right, lol...when really the craft is sitting over there back on the table and cannot exist without a inherent CG...you just need to put it at the right spot so that the center of pressure (lift) balances the whole craft for level flight...so go out there and find your CG guys...lol
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rick_harriss on April 21, 2015
I have to sympathize with you to some degree. Although in my defense The title is a limited number of characters and you can only say so much and be catchy. I also don't refer to "finding " the C of G, only to getting it in the place you want it to be.

http://adamone.rchomepage.com/cg_calc.htm

Has all the necessary calculations to locate you theoretical C of G BUT reality always seems to shift it elsewhere. This is just an easy and quick way to make sure it is where you want it to be without adding too much extra weight to the aircraft.
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makinger on April 17, 2015
Good tip!

Another very successful method for me has been to cut out servo placement last in the build. My FT Spitfire is built using heavier than DT foam. I ended up moving the tail servos a good 40mm forward. Found placement by tying a knot of the two servo wires and taped the pushrods to the servos to get the final weight in a single unit. Hung the whole thing over the fuselage and and started sliding it back and forth until I found the correct CG. I then just cut servo placements where the servos ended up.

Might be a good solution to when you cannot change the position of the power pod.
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raphino on April 21, 2015
Hey Rick, same old story on my side. Its horrible in Switzerland to find Foam Board anyway. I sticked to 3mm Kapa Plates very long, because they were little bit lighter, but still heavier than DTF. Than 2 weeks ago I stumbled over this Foamboard, which is produced in France and new avalaible. I ordered them and they are lighter than the Kapa Plates. I even tested them out on the FT 3D. No issues at all with tail heavines. Check out for Transotype 5mm Foam Board with 125g/m2 paperlike here under amazon:
http://www.amazon.co.uk/transotype-Foam-Boards-500-Weiss/dp/B0076RA7C6
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rick_harriss on April 22, 2015
Will have a look at that - thank you for the tip.
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rick_harriss on April 22, 2015
This product shows on amazon as being out of stock and no date for restocking - You must have bought it all!
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How to get the C of G right with heavy Foam Board