Help, i dont know where to put CG!

[snakeX2]

I am in the process of building a sp-400 pylon racer with a F5D airfoil section. I haven't a clue where the C of G will end up being located and if i can't find a C of G, what would be a good test flight postion to find it? Heres a picture of the model i intend to complete soon, and the foil i used, above it. Thanks anyone!


James,

[CloudyIFR]

25% Mean Aerodynamic Chord.

If you don't have the formulas to calculate this and have MS Excel send me an email and I'll send a file that will calculate it for you.

Curtis

[Sparky Paul]

There's 80 threads mentioning mean aerodynamic chord here.

[BMatthews]

I like to print out some of those little checkerboard circles and stick them on the fin.....:D




25% is overly conservative. 30% is very safe for most cases. And your stab and tail moment arm is sufficiently long enough to easily get away with the 30% CG. From there do some dive tests to figure out how much further back you can move it. You'll probably end up at around 35% but it's best to sneak up on it.

[vintage1]

I started a thread in the scale forum because this question was fully answered in last months flying scale models with an article by Gordon Whitehead. I suggest you look at that thread and apply it to your model.

http://www.rcgroups.com/forums/showthread.php?t=339401

Looking at your model I would estimate about 35% MAC, but its wise to do the calculations.

[snakeX2]

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[snakeX2]

Thanks for the input guys. I will find a thread with the MAC but the basic specs are

root chord = 6 "
tip chord = 4.5"
Mean Average Chord = 5.1" approx
Sweep = 0
Half span = 15"

This is what i found on the model, MAC=5.1'' it is! Or around 30% as said.

'I like to print out some of those little checkerboard circles and stick them on the fin.....' Good idea!



James,

[CloudyIFR]

Snake2,

You're MAC is actually 5.25" exactly. So if you wish to balance at 25% MAC then that would be 1.32" aft of the root leading edge, or aft of anywhere since you have no sweep. 30% MAC would be 1.58" aft.

Curtis
Montana

[adam_one]

I’ve been working on a calculator that uses the Tail Volume coefficient to find the NP and the CG location by choosing the Static Margin you want (typical between 5% and 15% of MAC).
Just measure the plane and type in the figures, no math needed at all.
You may check it here (http://adamone.rchomepage.com/cg_calc.htm)

[snakeX2]

Thanks guys, your great!! You have helped me get past this hard decision and i hope this means i won't be making balsa chips too soon! Pity, your link doesn't seem to work.

[adam_one]

Pity, your link doesn't seem to work.
Try the following:
http://adamone.rchomepage.com/cg_calc.htm

Hope it works.

[snakeX2]

cheers, it works now, and it looks brilliant! Thanks.