Aircraft Center of Gravity – Vital for Successful Flight
Understanding the importance of center of gravity for your RC airplane
Aircraft center of gravity is an odd term to apply to flying RC models when you first hear it. However, ensuring that your RC model airplane balances at the proper center of gravity location is an absolute requirement for safe flight. I can state without any hesitation that if a model attempts flight with its center of gravity out of limits, no amount of piloting skill can prevent a crash.
A simple method to locate the center of gravity
Understand Center of Gravity
The idea of an airplane’s center of gravity is simple to understand. The CG is the position on an airplane at which the entire structure would balance if you could suspend it at that point.
CG symbol
There are three balance points for any aircraft. The lateral axis relates to aircraft roll, the vertical to yaw movement, the longitudinal axis relates to pitch. For radio control model flight, we are mainly concerned about the proper longitudinal balance.
For any aircraft to fly safely, whether a Boeing 747 or your new ready to fly micro RC model, it must balance at the proper longitudinal center of gravity location. The location of the CG is determined by the aircraft designer, and is usually between 25 to 35 percent of the distance back from the wing leading edge on a plane with a straight constant chord wing. For a model airplane built from a kit or a set of construction plans, the CG is clearly marked as a circle divided into quadrants. On a computer airplane flight simulator, you can adjust the center of gravity to see how flight performance is affected.
Camera located under the wing will not affect CG location
Fly with correct CG location
If a model plane you have built attempts flight with its center of gravity out of the recommended range, the input of the control surfaces will not be powerful enough to overcome the pitching moment of the airplane. If the CG is too far forward (not usually the case), the nose will force itself down and controls will be sluggish. If the center of gravity is too far aft (the most common occurrence), the tail of the airplane will drop uncontrollably and force your model into a series of aerodynamic stalls with an ultimate loss of control. Bottom line: Ensure that your new RC model airplane balances at the correct center of gravity location.
Advanced center of gravity device
The detailed aerodynamic reasons on how an aircraft loses control with an out of limit center of gravity are complex, and relate to a variety of factors to include the wing’s center of lift, aerodynamic center and the effects of elevator control on nose pitch. The important point is that the pilot cannot overcome the instability resulting from an airplane’s center of gravity falling outside the proper limits. The only way to prevent an accident due to improper CG location is to ensure you never, ever try and take off with the center of gravity out of range.
If you build your RC models from a kit or plans, keep the balance point of the aircraft in mind as construction progresses. Use as light a grade of balsa possible for the tail area. Excess weight towards the rear of the model has a greater effect on the center of gravity due to its longer moment arm as compared to the nose.
CG location
Full scale aircraft can make large adjustments to the center or gravity location by distribution of the fuel load, passengers or cargo. There is not much that we can do to affect CG location with a model aircraft once they are complete. As you determine the finished airplane’s center of gravity, you may be able to shift some weight (such as a battery) to help fix the CG.
Effect of cargo loaded aft on an MD-11 aircraft
Most models come out slightly tail heavy which means weight will need to be added to the nose section. Your goal always is to minimize any excess weight added to an RC model. If balance weight must be added to remedy a tail heavy condition, the best method is to place nose weight as far forward as possible to minimize the total amount required. Due to the distance of the tail from the CG, a much smaller balance weight is required to cure a nose heavy model. See the EAA Bleriot restoration for an example of an airplane with a very short nose moment and a successful CG location.
It is far more preferable to fly an RC model airplane design with a slightly forward center of gravity as opposed to aft. The flight controls may be slight more sluggish when nose heavy, but the aircraft is fully controllable. An aft CG is much more of a concern. With an airplane that has an aft center of gravity, you will notice the tail of the model falling immediately after takeoff. The elevator will have little corrective effect. Pilots tend to reduce power is these situations, as the model is nose high and climbing. The power reduction results in a lower airspeed which compounds the challenge of maintaining controlled flight.
Ensuring that your RC model airplane balances at the correct center of gravity location is an absolute requirement for safe flight. Think of the CG location as you construct the model to avoid inadvertently building in a nose or tail heavy condition. If extra weight must be added to achieve the correct CG location, strive first to relocate internal components of the aircraft (such as the battery) to meet the balance point. If this cannot be done, add the minimum weight required to get the CG within limits.