Where Should an RC Airplane Center of Gravity be?

by FliteTest | January 3, 2019 | (6) Posted in Tips

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 

- Find this article & others like it at www.FliteTest.com -

COMMENTS

Ran D. St. Clair on January 3, 2019
The standard and safe CG position for a typical wing in front, tail in back plane is 33% of the wing chord. A large wide nose can move it forward. When dealing with unusual configurations, like canards, flying wings, etc. it is often helpful to make a small flat plate glider. 1/8" (3mm) foam works great for a 10" span model. Be sure to include the flat plate area of the fuselage, tail, etc. Add just a touch of up elevator and then add nose weight for a stable glide. You want it to pull out of a slight dive and achieve a steady down slope glide. If you throw it slightly nose up, it should round over and go into a steady down slope glide. There is no need to throw it really hard, but if you do, it will probably climb a bit. If it does a swoop-stall-swoop-stall... then it is tail heavy. Without proper dihedral these gliders will often be spirally unstable, which means they will go into a turn and gradually go into a spiral dive. You can ignore this when setting the longitudinal CG. Just warp the wing slightly to get it to fly straight for the short distance it will fly with a gentle toss. If you toss it with the wing tilted left or right, it will turn left or right, so just try again and throw it wings level, nose level at about gliding speed or slightly faster. If it dives and won't pull out, reduce the nose weight or add a touch more up elevator. Throw it lots of times and play with it a bit. You can learn a lot this way. If it won't fly at all, you might have more serious problems, like a lack of vertical stabilizer area. It is often useful to include a profile fuselage as well. That will help you to see if the chosen CG location works for both pitch stability as well as lateral stability. It takes just 10 minutes to build one of these, and is way faster than building and crashing and rebuilding a full RC plane.
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shoe1938 on January 4, 2019
Well now that's finally clear,not
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AERODROME on January 4, 2019
So as I analyze all of this it seems that if I build my plane with my tail 25 percent of the wing area and the moment at least 50 percent of the wing length I should be okay with a 25 percent COG at the MAC. Someone recently showed me how to get the MAC very easy when dealing with a tapered and sweeped wing.. Cut out half of the wing shape from a piece of heavy cardboard and balance the cardboard tip to root and mark the MAC at the point the cardboard balances. Then measure across the MAC divide by 4 and mark the spot and punch a hole and then lay the cardboard on the wing and mark the COG on the wing thru the hole with a felt pen. Flip the cardboard over and do the other side of the wing. Then glue on two thumbtacks at those points . This makes it easy to feel the balance point.
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Where Should an RC Airplane Center of Gravity be?