As of the most flight critical aspects of RC, you’ve just got to get the CG right.
Like with most things in the RC airplane hobby, the subject of the centre of gravity (CG) can be less complicated in models when compared to full-scale aviation. That being said, it’s still based on exactly the same principles and uses the same theory. Learning some of this theory can help you to balance your models better, so let’s cover some of this now.
The Center of Lift
The basic principle of the CG is that it should effectively line up with the Center of Lift which is, as you might imagine, the average location of the aircraft’s lift. We can also call this the ‘aerodynamic center’.
More info: To learn more about wings and lift, check out this article from our Flite Test Aerodynamics Simplified series.
As you may have experienced before, the CG is quite a sensitive thing. It might be helpful to imagine the CG placement to be inside a range. We can call this the stability margin (S.M). This is the envelope where the airplane is controllable. If the CG is placed outside of this, the airplane will be either too nose heavy or too tail heavy to be stable.
25%
Most airfoils have an aerodynamic center located around 25% - 33% of the mean aerodynamic chord (MAC) from the leading edge, so 25% is often used as a reference point on our models. Locating the CG at 25% is a safe bet for a maiden flight. Remember, it’s always better to have a nose heavy plane than a tail heavy one.
To work out where 25% of your wing is, the MAC of a rectangular wing will always be simply the total chord. This is because it has no sweep or taper. For a tapered wing shape, you’ll need to do some simple calculations to find the mean average.
More info: Here’s an article that will help find the mean aerodynamic chord of a wing.
Understanding Moments and Neutral Points
So, the question is, what determines the range of this stability margin (S.M) we talked about earlier and how can it be increased? Well, it mostly comes down to the horizontal tail size. The larger the area of the horizontal stabilizer, the more forward and aft CG at which you can be safe. In addition to this, you can increase the horizontal tail moment arm (Lh).
Here’s a formula to help you calculate 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.
To sum up, the CG fairly simple to understand but difficult to master. It’s really quite critical to your overall flight experience so it’s super important to get right - as everyone who’s experienced the terror of a tail heavy RC plane flight will agree. 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.
Article by James Whomsley
Editor of FliteTest.com
Contact: james@flitetest.com
YouTube Channel: www.youtube.com/projectairaviation
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