Balancing RC Model Aircraft

Balancing RC Model Aircraft

Every RC airplane (and all other aircraft for that matter) has a specific CG position, it's the mean point where all gravitational forces act upon the plane, and the point where the plane balances fore-aft correctly. Technically this is called longitudinal balance. You can liken a plane's Centre of Gravity to the fulcrum of a see-saw, for example.  There is an old saying...a nose heavy plane flies poorly and a tail heavy plane flies once. 
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Nose heavy planes may exhibit some of the following symptoms

A nose heavy plane will tend to exhibit some or all of these symptoms...
Feels heavy on the sticks and may be slow to react.
In a banking turn the nose may dip
Tends to lands hot or fast.
Requires some "up" elevator to fly level...
Aircraft will dip into a steep dive in a power off glide.
Will may porpoise in flight (up then down, up then down)

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Tail heavy planes may exhibit some of the following symptoms


A tail heavy plane tends to feel overly sensitive on the sticks.
In a banking turn the nose may lift.
Glide is almost non-existent and may stall out or tail dive
Glide may fall off to one side.
Generally flying in a nose high altitude
Applying throttle may result in steep climb

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On a straight wing, the CG point is generally 1/3rd of the wing cord, however, it gets pretty tricky to determine the CG on a swept wing design. There are wing calculators on line, however none of this is required provided the manufacture has included the CG for you.  Manufacturers include the CG so that you don’t have to guess and you’re flight is very enjoyable. The CG is generally determined during the design phase of the of the aircraft so if you’re built from a kit & plan, the CG will most likely be clearly marked on the plan.  If you've bought an ARF or RTF plane then the instruction manual will likely give the CG position in terms of distance back from either the leading edge of the wing or from the nose. If you’re unsure then RC forums can be a wealth of information.

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Methods of balancing RC airplanes.

With Each method below ensure your plane is fully loaded or battery installed and ready to fly. If we were to balance a plane that was not fueled it would be completely different once we added fuel.  

High Wing

High wing planes are the easiest to balance and will take little time at all. If this happens to be your first plane and you’re in a position to have a second set of eyes look over your work, do it. An experienced set of eyes has done it all before and made many mistakes. They may just save you a plane and damage to your ego.
 
Referring to the manual find the CG and measure that on your aircraft. Apply masking tape to the model and using a marker, mark the position of CG. Do this on both sides of the wing. The reason for not drawing directly in the model is that we may have to adjust it and it would look unsightly with various markings. Later we can make a final mark once we’re happy. 
 
Place the battery into the model and then check the CG by placing your finger tips on the points you marked earlier. Now allow the model the balance and see what it does. If the nose points down you should move the battery more toward the tail. If the model nose points up then move the battery forward towards the nose.
 

Low Wing

A low wing ping plane is similar to a high wing plane, just inverted. So we can measure the CG in the same way except the plane will be upside down when we measure the CG.
 
Referring to the manual find the CG and measure that on your aircraft. Apply masking tape to the model and using a marker, mark the position of CG. Do this on both sides of the wing. The reason for not drawing directly in the model is that we may have to adjust it and it would look unsightly with various markings. Later we can make a final mark once we’re happy. 
 
Place the battery into the model and then check the CG by placing your finger tips on the points you marked earlier. Now allow the model the balance and see what it does. If the nose points down you should move the battery more toward the tail. If the model nose points up then move the battery forward towards the nose.
 

Balancing Rigs

A balancing rig can be handy as it will save you a lot of time in readjusting. You place the plane on the balancing rig and adjust the placement of parts as you go.  I’ve got a balancing rig that I bought from Hobbyking.com and I have no complaints. It was cheap enough that I didn’t bother fabricating one and it breaks down for easy storage.
 
You can also make a balancing tool - something as simple as a block of wood with several holes drilled into it might be enough. You can see how the plane is balanced upon the pencil rubbers. Simply drill holes in the block and push the dowels into them, ensuring a tight fit. 
So long as the plane can balance on two points, one under each wing, then you have a balancing tool.

https://hobbyking.com/en_us/hobbyking-center-of-gravity-machine-for-airplanes.html

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Gas or Nitro Fuel Tanks

A fuel tank may affect the CG if it’s placed at the extremities of a plane. For example, you could take off nose heavy but land tail heavy. For that reason it is advised to keep your fuel tank as close to CG as possible. This mean your plane will be balanced for the duration of the flight. Move the fuel tank if necessary as it will be worth the trouble.
 

Adding Weight 

If the balance does need to be adjusted to get the correct Centre of Gravity, the first thing to do is try moving things around.  By moving things around you may avoid adding weight. Adding weight is the last thing you want to do because a heavier plane is not as nice to fly as a light plane. Additional weight leads to a faster landing speed and higher stall speed, and reduce straight up performance. 
 
If you need to add weight perhaps there are smarter ways to achieve the same outcome. If you need to add nose weight, go for a larger motor and different prop. You might end up with a more powerful motor and prop combo. This is better than adding lead as its just dead weight. Once you're happy with the new balance, make sure that the battery pack is completely secure and won't move from its new position. If it moves during a flight, then the CG moves too - and that's really no fun!
 
If you need to add ballast to correct the CG, add as little as possible. Add the ballast as far forward or as far back as you possibly can on the model. By doing this, the ballast will have the most effect on the Centre of Gravity. Add only enough to make your plane balance correctly, nothing more. There are many suitable options for adding weight. for example. Thin sheet lead is perfect because you can cut it and bend it to shape and the pieces don't have to be big too adjust the CG balance. 
 
Do be sure that whatever ballast you use that it is fastened to the plane and won't drop off in flight. Should it fall off your CG is going to be affected in flight and you might have a very short flight.

https://hobbyking.com/en_us/fe-balance-weight-60g-x-2-bag.html

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Lateral Balance

Now that we have longitudinal balance out of the way we need to check lateral balance. An imbalance laterally will cause a bais to one side in flight. This is often overlooked because it isn’t as critical as longitudinal balance, but an rc plane that has one side heavier than the other will have a bias to roll and turn to the heavier side, making your life on the sticks a bit harder. Also, a heavier side will almost always result in that wing dropping when the plane stalls, potentially putting the plane in to a spin.
 
You cannot trim out lateral balance because although you may be able to trim for level flight at half throttle when at full throttle it generally results in the plane rolling to one side. The usual cause of a plane being out of balance laterally is unequal weights of the wings.
 
Checking lateral balance is fairly simple. We can use a loop of string around the motor shaft and the tail.  We are looking to see if one wing is heavier than the other. To correct this we add weight to the wing tip. Thin strip lead of steel is fine.  
For smaller rc planes, say about the 40" wingspan mark or less, it's quite possible to hold each piece of string in each hand:
 
Taping the ballast to the wingtip is an easy method, although you might want to take the trouble to set them into the wingtip and cover over them to hide them. Bear in mind that adding larger bits of tape will actually add weight in addition to the ballast.

For rc airplanes with foam wings, pushing a small gauge nail or panel pin into the foam is a great way of adding any necessary weight, and it can be easily hidden with suitable paint or marker pen.

Lateral Imbalance

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Correct Lateral Balance

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That’s about it guys.  Balancing CG is pretty simple to get right
 
 
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