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Help with spar location and C of G

Old 08-26-2005, 05:38 AM
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Default Help with spar location and C of G

Hi all,
I need some help in finding the location to place upper and lower spars and C of G for an airfoil. The airfoil was from a small .020 gas freeflight model, probally from the 70's. What is interesting about it is that it resembles a slow flyer airfoil ( flat bottom like on the Great Planes Yard Stick ) which is what I was looking for. I scaled it up for my needs on a e-plane project I am working on. The original plans did not have spars but a sheeted wing. The demensions of the scaled up rib are:

length ( cord ? ) measured on bottom is 7 1/2" when I cut it with a 3/16" sq. balse LE sanded to LE profile, 7 5/8" total also add to that there will be a 1 1/2 strip ailerons behind the TE.

Height of rib is 1 inch with 1/16" sheeting/ cap strips total height is 1 1/8"

Length of wing will be 30" with tips bringing it up to 34"
Length of model 25". Its going to look similar ( a bit smaller ) to GWS Slow Stick F ( the one with the fusalage ) or like the "ugly stick/ stick" glow planes.
Model AUW weight 10oz.

Now about C of G. Seems like most most of my models C of G fall around the spar....give or what would be a good location to start for a mildy areobatic, tame "fun fly" plane???

If hope the above is helpfull. If not, I did scan the airfoil with my doodolings on it, BUT please ignore the dark cooper/black like streaks. My one kid put a penny down into the guts of the scanner so if you want to look at it please let me know.
The scan is in a jpg format about 236kb


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Old 08-26-2005, 12:08 PM
Sky Sharkster
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Default C.G., Spar Location

Hi Mike, welcome to E-Flying! I'll try to help you out with the questions...the fact that the C.G. and spar locations are near each other (most planes) is more coincidence than anything else. Most model designers put the main spar at or near the thickest (deepest) part of the airfoil. Most room, easy to see and cut slots. On typical flat-bottom, semi-symmetrical and undercambered 'foils this is around 33% or further rearward which also happens to be a good "guesstimate" location for C.G. on a new design. Problem with using a Free Flight C.G. location for R/C is most FF designs have a lifting stabilizer so C.G. is more rearward. R/C designs usually move C.G. forward to (about) 28 % to remove download forces on stab, we have elevator control and (usually) a non-lifting stab airfoil so the self-stabilizing effect isn't needed, glide duration and stall recovery is more important on FF models. In fact it's a drawback for R/C trainer and sport models-although it IS used (rearward) on 3D and aerobatic models, the reason there is 3D's WANT to remove stability, non-lifting (fully symmetrical wing and stab) so they can hover and spin easily. So I'd put the spar at or near the deepest part of the chord and the C.G. at 28% to start. This C.G. will be very stable, maybe a little nose-heavy for optimum performance and after you trim the model you'll likely move it back for more control response. Remember to measure the ENTIRE wing chord (L.E., T.E., flaps or ailerons) when determining C.G. Also, this is a greatly simplified guide, factors like tail moment arm, stab size, nose length, tail volume coefficent, exact airfoil lift/drag coefficents etc. all play a part. This is just a starting point but should be a safe one. Good Luck! Sky Sharkster
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Old 08-26-2005, 10:36 PM
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Thanks for the info Sky.

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