What's a sure fire method of finding the C of G???

[Masterpiece]

Hi ya all,

I have seen many ways to find the C of G, but none that I have been happy with. So what is a sure fire way to work out the C of G, with a scratch built thermal sailplane, .... namely mine :D ? Websites would be good too, as well as personal suggestions. The airfoil is the SD7080. Does anybody know of this airfoil's sweet spot?

Cheers,
Glen

[Ollie]

http://www.djaerotech.com/dj_askjd/dj_questions/hlgcg.html

[Jose E Bruzual]

Masterpiece,

I am sure you have seen this, "CG by Dr. Drela", I am not sure what you are looking for, formulas are a starting point, so short of flying your ship you, there isn't much you can do on the ground except get a good estimate and make sure your ship is ready. Before finding your CG make sure all your surfaces are true and that your angle of incidence is "properly set".

Try this link, and maybe post questions based on this method since it works for many people.

http://www.charlesriverrc.org/articles/supergee/CGMarkDrela.htm

Click on the image to see a larger version.

Jose

[Masterpiece]

Many thanks,

Just reading your link at the moment, Ollie. Jeb thanks for yours too, I have a print out of this from before and I intend to use it in flight too. I was thinking of starting at 35% back from the LE and see what happens. I used to balance my planes at 32%, but lately people have started to really put the C of G back which made me think too much.

Cheers again,
Glen

[nuevo]

I would suggest you start at a fixed percentage of MAC (Mean Aerodynamic Chord), not percentage of the root chord. MAC size and location are entirely determined by your planform.

Depending on the planform, 35% back from the LE on the root chord might be about 22% MAC.

[dustn]

the dive method has always worked well for me... its a great indicator of what your horiz stab is doing.

[SoCalGliderFlyr]

I keep removing nose weight till the plane flies backwards then add a half ounce.

Did you know that "CG" does not mean center of gravity? It's actually an honorarium for Carl Goldberg! So every kit and plans has his initials. The point in question is actually the BP or Balance Point.

;)

[thelocust]

One of the guys in the club told me, while I was trimming my DLG: "Fly it fast". And that's how he trims -- flying fast will expose any trim and later, CG issues. The dive test is part of that, yes, but so is flying inverted, etc. If everything is in trim, you shouldn't need a whole lot of "down" to keep the plane "up" when inverted. It has worked well for me!

Essentially, fly the plane beyond the speed range it would normally fly at, and then after trimming and possible shimming, then play with the CG.

It's really a matter of personal preference -- one thing that will tell you how well you and your plane is flying is flying the plane of someone better than you :) All of these things work well for me.

[Masterpiece]

I would suggest you start at a fixed percentage of MAC (Mean Aerodynamic Chord), not percentage of the root chord. MAC size and location are entirely determined by your planform.

Depending on the planform, 35% back from the LE on the root chord might be about 22% MAC.

Cheers. Can you remind me how to calculate the MAC? Or pass me onto a website that deals with it.

Thanks again,
Glen

[Phil Barnes]

Before finding your CG make sure all your surfaces are true and that your angle of incidence is "properly set".

The angle of incidence on an RC model varies in flight with every movement of the elevator control. The decalage or angle of the fixed portion of the stabilizer relative to the wing is set during construction of the model. The proper decalage is just that angle which results in the model flying at the desired trim speed with zero elevator deflection and this is dependent on the model's CG.

If you have enough aerodynamic knowledge (I don't) then you can decide/calculate a desired CG position and then calculate the required decalage angle based on that, all before building the model . If you don't have that level of aerodynamic knowledge then you can just build the model, set the decalage close based on rule of thumb guestimates, then fly the model. You then set the model's CG based on the desired flying qualities. The correct CG is just a matter of personal preference and others have already posted some ways of determining a CG location (dive test, inverted flying, etc.). Once the CG is set then you set the decalage such that the model flies at the desired trim speed with zero elevator deflection. The decalage is not a variable and is not something that can be optimized separate from CG.

[nuevo]

Cheers. Can you remind me how to calculate the MAC? Or pass me onto a website that deals with it.


Here's a small program you can download that computes it for you.
http://perso.wanadoo.fr/scherrer/matthieu/english/mce.html

The picture below shows a sample input using the planform of Mark Drela's Aegea 130" wing.

After you enter the wing dimensions, you enter the %MAC you want to balance the plane at. It's called "Center Position %". The M.A.C Distance is the number you want. That is the distance from the LE to balance your plane.

Note, if you change any dimension, the MAC will change. Exactly what you want to know.

[Masterpiece]

After you enter the wing dimensions, you enter the %MAC you want to balance the plane at. It's called "Center Position %". The M.A.C Distance is the number you want. That is the distance from the LE to balance your plane.



Just giving the program a try now. Is the "Centre Position %" the desired balance point I'm wishing for (such as I quoted above, I was wishing to balance at about 35% of the root chord origanally), but I believe that in this case it will be a 35% of the MAC not the root chord. Is this right?

Cheers,
Glen

[Sparky Paul]

The c.g. in Jean-Claude's program tells you both where it is on the m.a.c., and where it will then be at the root, but it's the m.a.c. position that counts.