As of the most flight critical aspects of RC planes, you’ve just got to get the CG right.
In the RC hobby, understanding the center of gravity (CG) is simpler than in full-scale aviation, but the principles and theories are the same. Knowing these theories can help you balance your models more effectively. Let's go over some of this information to improve your understanding.
The Center of Lift
The basic principle of the center of gravity (CG) is that it should align with the aircraft's center of lift, which represents the average location of lift on the aircraft. This point is also known as the 'aerodynamic center.'
If you've built and attempted to fly a model aircraft before, you may already know that the centre of gravity (CG) is very, very sensitive! But, although it is sensitive, it's helpful to think of CG placement as falling within a range rather than having a singular point. This range is known as the stability margin (S.M). This is the area where the airplane remains controllable. If the CG falls outside this range, the airplane will either be too nose-heavy or too tail-heavy, making it unstable.
25%
Most airfoils have their aerodynamic center around 25% to 33% of the mean aerodynamic chord (MAC) from the leading edge, so using 25% as a reference point is a common practice for models. Placing the CG at 25% is a safe choice for a maiden flight. Keep in mind that a nose-heavy plane is preferable to a tail-heavy one.
To determine where 25% of your wing is, the MAC of a rectangular wing is simply the total chord, as it has no sweep or taper. For a tapered wing, you will need to perform some calculations to find the mean aerodynamic chord.
Understanding Moments and Neutral Points
Okay this is where things get a bit more complicated...
The key question is: what sets the range of the stability margin (S.M) and how can it be expanded? The main factor is the size of the horizontal tail. A larger horizontal stabilizer allows for a wider safe range of forward and aft CG. Additionally, extending the horizontal tail moment arm (Lh) can also help increase the range.
Here’s a helpful formula for calculating what the volume of your tail should be.
Vh = Sh x Lh / Sw x c
Vh is tail volume coefficient
Sh is horizontal stabilizer area
Lh is length 1/4 wing MAC to 1/4 horizontal MAC
Sw is wing area
c is wing MAC length
This demonstrates how important the size of the tail is in stabilizing an aircraft’s flight.
The key points
CG is fairly simple but absolutely critical to a successful flight. It’s essential to get right or else your plane will not fly.
Hopefully, some of this more in-depth information on the subject of the CG will set you up for success with your next RC flights. If in doubt, use that 25% rule and go from there - a nose heavy plane is better than a tail heavy plane!