hitcttd

My Feedback: (8)

Me friend and I are working on building a durable 3-D profile plane... construction from balsa, aluminum and choroplast... Anyway, since i'm drawing the plane from scratch, i was wondering how you determine where the CG of the plane will be set... Usually it's right on te spar, but i'm sure engineers calculated wing position so that that is the case... Our plan right now is to launch the plane vertical on the first flight let it climb to 4 mistakes high and straighten it out... is there any way we can at least get a ballpark estimation so there aren't any nasty suprises?

Thanks guys

pettit

My Feedback: (23)

What is the plan form of the wing? Rectangular? Tapered?

Figure out the Mean Aerodynamic Chord. It's basically the average wing chord.

You can look it up here on RCU maybe under "Aerodynamics" topoc???

I'd start with 25% of the MAC back from the leading edge and go from there.

BMatthews

The safe CG position is not determined by the thick part of the wing airfoil. It's just a pure coincidence that it sometimes ends up close to that.

The preferred CG is found by finding the neutral point of the design and then placing the CG at or ahead of the NP by a % of the wing chord based on how stable you want the model to be. This NP is determined through a couple of equations that consider the wing area, chord, tail area and tail moment arm. This provides you a Tail Volume Coefficient.

But all that is a big bunch of math. Thanks to some kind and sharing folks we have online neutral point, MAC and CG calculators. Check out....

http://www.geistware.com/rcmodeling/cg_super_calc.htm

If your design work follows a look that is largely the same as much of the current 3D style designs I think you'll find that the Neutral Point is back around the 40% position. Whereever the calculator tells you it is you want your first flights to have the CG about 15% of the wing chord ahead of that point. From there fine tune it back as far as you dare. The closer you get it to the NP the easier it is to hover. But the closer it is to the NP the less stable it is in pitch.

hitcttd

My Feedback: (8)

Thanks guys... The wing shape will be similar to that of the funtana... a fairly thick wing that tapers on both sides... I will look into all of the things you guys suggested.. thanks again