ORIGINAL: Ima RcFlyer
Ok I'm new to rc but not to model rocketry where I've dabbled a bit in the mathmatics of CP (Center of Pressure) and CG calculation. In model rocketry the placement of CG and CP are quintessential to the stability of your model. Is this not the case with model airplanes? I understand that there may be different areas of CP versus CG (E.g. Wing, fusalage, total airplane) yet I see no topics about placement of these. Maybe they're termed differently than in model rocketry so don't flame me if it is. [link=http://www.apogeerockets.com/education/newsletter18.asp]Here[/link] is a link to a good explanation to how CP and CG are in relationship to each other along with Barrowman's Stability equation.
Ps. If I can get a hold of the .net software runtimes that are supposed to be comming I'm going to code a program to take the guesswork out of cp calculation. much more acurate that that pesky cardboard method. PM me if your interested.
For conventional designs (with a main wing and a horizontal stab) the CG location
range is usually between 28% and 33% from the leading edge of the main
wing's MAC, which means between about 5% and 15% ahead of the aircraft's
Neutral Point NP.
This is called the static margin, which is expressed as a percentage of the MAC.
When the static margin is zero (CG coincident with NP) the aircraft is considered
However, for conventional designs the static margin should be between 5% and
15% of the MAC ahead of the NP.
The CG location as described above gets close to the main wing's Aerodynamic
Center AC because the lift due to the horizontal stab has only a slightly effect on
the conventional R/C models.
However, those figures may vary with other designs, as the NP location depends
on the size of the main wing vs. the stab size and the distance between the main
wing's AC and the stab's AC.
The simplest way of locating the aircraft's NP is by using the areas of the two
horizontal lifting surfaces (main wing and stab) and locate the NP proportionately
along the distance between the main wing's AC point and the stab's AC point.
For example, the NP distance to the main wing's AC point is:
D = L Â· (stab area) / (wing area + stab area) as shown on the picture below: