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A plane that is off on center of gravity (CG) flies like a shopping cart with one bad wheel. Too far aft and it pitches up and bites; too far forward and it needs a runway like an airliner and lands like a brick. The good news is that the fix is usually a battery slid a few millimeters or a pinch of nose weight.
One note on method first. This page is built from manufacturer manuals, magazine articles from the Academy of Model Aeronautics (AMA) and maker product pages, checked in October 2026. It is not based on our own bench work, so there are no personal bench anecdotes. Where sources disagree, the page says so.
Who Decides the CG (Hint: Not the Balancer)
A balancer measures where a plane balances right now. It does not know where it should balance. The AMA's Model Aviation magazine says it plainly in its coverage of the RCCG Machine: neither the stand nor the Xicoy system it pairs with determines the CG of any model. That number comes from the designer, on the plans or in the manual, or from your own calculation.
Manuals are the first stop, and they show how varied the references are:
| Model | CG per the maker | Reference point |
|---|---|---|
| E-flite Conscendo Evolution 1.5m | 2-1/4 to 2-3/4 in (57-71 mm) | Behind the leading edge of the wing |
| E-flite Carbon-Z Cessna 150T 2.1m | 3.75 to 4.10 in (95-105 mm) | Back from the leading edge, at the root |
| E-flite Carbon-Z Cub SS 2.1m | 4.1 to 4.7 in (105 to 120 mm) | From the leading edge, measured at the fuselage sides |
| Hangar 9 J-3 Cub 10cc | 3 to 3-3/4 in (76-95 mm), 3-3/8 in (85 mm) recommended | Behind the leading edge, against the fuselage |
| Hangar 9 P-51D Mustang 20cc | 6 9/32 in (160 mm) | Behind the leading edge of the wing |
| Extreme Flight 78" Extra 300 | 5.5 to 6.25 in (140-168 mm) | From the wing leading edge at the root |
| Dream-Flight Libelle | 5 mm behind the carbon wing spar | Marked under the wing on each side |
Read the reference point before you measure. "At the root", "against the fuselage" and "behind the spar" are not the same line, and a measurement taken from the wrong place is the quickest way to a bad maiden. Horizon's J-3 Cub manual adds that its range is "a guideline based on testing" and that it is fine to adjust progressively and cautiously after starting at the recommended point.
If the manual gives only a percentage of chord, the next section is for you.
Percent of MAC: What the Sources Say
CG is often given as a percentage of the mean aerodynamic chord (MAC), measured back from the leading edge of the MAC. For a rectangular wing the MAC is simply the chord.
Published rules of thumb vary:
- Per Model Aviation's Stan Burak (February 2014), most airfoils have an aerodynamic center at roughly 25% of the MAC. He suggests a CG of about 20% to 25% as a good range, says a forward CG makes flying easier and more stable, and warns that going aft makes an aircraft unstable and harder to fly. He also cautions that aft of 30% "could end your day of flying!"
- The RC Airplane World balancing article puts the typical CG at about one-quarter to one-third of the wing chord (25% to 30%), and says it can fall anywhere between roughly 25% and 50%.
- Burak also cites a Stinger 120 plan range of 28% to 33% and a Reno racer range of 8% to 25%, which shows how much the answer depends on the design.
The honest takeaway: ranges depend on the airfoil, tail size and purpose of the model. Burak notes that tail volume sets how far aft the CG can safely go. Use the designer's figure and treat a generic percentage as a sanity check, not a target.
Calculating the MAC on a Tapered Wing
On a tapered wing, the MAC is not the root chord, and it is not simply the average of root and tip either. The standard formula for a straight tapered wing, with taper ratio λ equal to tip chord divided by root chord, is:
MAC = (2/3) × root chord × (1 + λ + λ²) ÷ (1 + λ)
Worked example, with a 10 in root chord and a 6 in tip chord:
- λ = 6 ÷ 10 = 0.6, and λ² = 0.36.
- 1 + 0.6 + 0.36 = 1.96, and 1 + 0.6 = 1.6.
- MAC = (2/3) × 10 × 1.96 ÷ 1.6 = 8.17 in (rounded).
The plain average of the two chords would be 8 in, so for a mild taper the difference is small. Burak makes the same point, saying an average chord is accurate enough for modelers. The gap grows with heavy taper, and on swept or delta wings the position of the MAC along the span matters as well as its length. For those shapes, use the designer's CG figure or a flying-wing calculator rather than improvising.
If the CG is quoted as a percentage, multiply: a 25% CG on an 8.17 in MAC sits about 2.04 in back from the leading edge of that chord. Mark it on the wing at the span position of the MAC, not at the root.
Choosing a Balancer
The right tool depends on model weight and how often the CG changes. Manufacturers themselves name the two cheapest options: fingers or a commercial balancing stand.
Fingers
Horizon's J-3 Cub manual and the Dream-Flight Libelle manual both say to support the plane at the marked wing points with your fingers or a stand. The RC Airplane World article describes placing fingertips under each wing at the CG point, a couple of inches out from the fuselage, then lifting the plane clear of any surface and letting it rest. A correctly balanced plane sits level, or with the nose pointing slightly downward. This works for light foam models. It stops being practical once a model is heavy, awkward or covered in fragile film.
Cradle and v-notch stands
Stands hold the plane at two points, which frees your hands and gives a repeatable reading. The Great Planes C.G. Machine (GPMR2400) is described in patent literature as a cradle-type balancer, and a Southwest Systems EZ Balancer II is mentioned in Model Aviation as another option. No current maker page was found for either, so availability is not confirmed. Stands with adjustable support towers and a graduated rule are sold under various names; check the weight rating before buying.
Search for an RC airplane CG balancer on Amazon
Digital scale systems
Digital systems weigh each wheel and calculate the CG, which is the only practical way to handle a heavy model. Two are documented well enough to describe.
SkyRC CG Gauge (CGG-020). Three weighing modules controlled wirelessly over Bluetooth 5.0 with a free app, for fixed-wing aircraft. SkyRC lists a maximum load of 20 kg per module, a switchable gram and ounce display, a rechargeable lithium battery with USB Type C charging, a measuring accuracy of ±3 g and a display accuracy of 1 g. The page also says the arc-shaped top of each module centers the wheel, and that the app indicates how much weight to add or remove against an entered target. SkyRC's page contradicts itself with a "0.1g accuracy" line elsewhere, so the ±3 g and 1 g figures are the ones quoted here.
Search for the SkyRC CG Gauge on Amazon
Xicoy Digital Weight and Balance meter with the RCCG Machine. Model Aviation's May 2024 article describes three high-precision puck scales "sensitive to 1 gram", a color touchscreen and a battery pack. The pucks go under each wheel, whether the model has a nose gear or a tailwheel, and the screen shows total weight, the current CG position and the weight needed at a chosen point to reach the target. The RCCG Machine, designed by Michael Matteson, is a rail rig that holds the pucks; it accommodates a main gear separation of up to 48 inches, and an accessory kit extends that to 64 inches. The article gives no weight capacity, and the Xicoy maker site could not be loaded for this guide, so confirm capacity with the maker before buying for a heavy model.
The three-scale method
A cheaper route for heavy taildraggers is the method Model Aviation described in August 2012. Block the plane up level, put one scale under the mains and one under the tailwheel, and measure the distance D1 between those contact points. With W1 as the main-gear weight and W2 as the tailwheel weight:
DCG = D1 × W2 ÷ (W1 + W2)
DCG is measured aft of a line through the main contact spots. The article's example uses D1 = 90 in, W1 = 30 lb and W2 = 5 lb, giving about 12.86 in. This takes arithmetic and a level setup, but it scales to models that no tabletop stand will hold. For the models that need this, see the giant-scale planes guide and the ground handling equipment guide. Models over 55 pounds also fall under the AMA's Large Model Airplane Program per the AMA safety code.
A prop balancer is a different tool
A prop balancer does not set CG, but it removes one source of vibration that can mimic a handling problem. The Du-Bro Tru-Spin Prop Balancer (DUB499) is listed in stock at Horizon. Du-Bro describes friction-free aluminum wheels, a locking cone that centers props on a hardened, centerless-ground shaft, and use with airplane props, drone props, spinners and more. Du-Bro does not state a weight range for it.
Search for the Du-Bro Tru-Spin Prop Balancer on Amazon
Which Tool for Which Model
| Model type | Practical choice |
|---|---|
| Light foam park flyers and gliders | Fingertips, per the maker manuals |
| Sport and scale models, a few pounds | A cradle or v-notch stand, if the weight rating covers the model |
| Heavy or high-value models, gas and giant scale | A digital scale system or the three-scale method |
| Turbine and large jets | A digital scale system; see the turbine engine guide and best RC jets |
The RC plane kits for beginners guide is a good companion for first builds, where fingers and a pencil mark are all the tooling you need.
Setting Up the Balance Check
Balance the plane the way it will fly. The manuals agree on the details that matter:
- Flight-ready. Horizon's J-3 Cub manual says to assemble the model fully before balancing, and Dream-Flight says final balancing on the Libelle must occur with the canopy mounted.
- Flight battery installed, ESC not armed. Horizon's Cessna 150T manual says to install the battery but not arm the ESC while checking CG, since personal injury may result.
- Landing gear in its normal position. Horizon's Cub manual has the pilot use the radio to move retractable gear up before balancing.
- Marks on both sides. Mark the CG on the underside of each wing half, at the same distance from the leading edge, using the maker's reference point.
- Fuel tank empty on gas and glow models. RC Airplane World says to balance with the tank empty, because the plane will become tail heavy as the tank empties. Which way the CG moves depends on where the tank sits relative to the CG, so check this against the plane in front of you.
Then support the plane at the marks. It should hang level or with the nose pointing slightly down. RC Airplane World recommends starting slightly nose-heavy rather than tail-heavy.
Adjusting: Battery First, Weight Second
Horizon's Cessna 150T manual states that its CG was determined with the recommended battery placed almost all the way back in the compartment, and says to adjust the battery forward or aft as needed. That is the first move on most electrics, and it costs nothing. The LiPo battery guide covers pack choice, since a different pack changes both weight and balance.
When moving the battery is not enough, add weight. Dream-Flight's Libelle manual shows how: expect 3 to 5 pieces (15 to 30 grams) of balance weight in the nose, fitted in recesses, and secure it with painter's tape while fine-tuning. Horizon's Cub manual adds that nose weight may be needed with lighter engines or when floats are installed.
One caution runs the other way. Burak writes that the last thing to do is mount the batteries to achieve the CG you want. Sliding a battery within the maker's tray is what the manuals describe. A battery that has to travel far outside the tray to make the numbers work is a reason to recheck the CG figure and the reference point.
Other setup choices feed into balance, too: servo placement is covered in the servo guide, motor and prop changes in the motors guide, and retractable gear in the landing gear guide. Re-check the balance after any of them.
Confirming in the Air: The Dive Test
Dream-Flight's Libelle manual describes a flight test that works for any model with enough altitude. The steps, from the manual:
- Climb to a safe altitude and trim for straight and level flight.
- Put the model into a 45-degree dive and let go of the sticks.
- Watch what it does as speed builds.
| What happens | Meaning (per Dream-Flight) | Fix |
|---|---|---|
| Stays in a straight dive, and glides hands-off in a straight line | CG is right | None |
| Noses up as speed increases | Nose-heavy | Remove some nose weight and add down elevator trim |
| Dive steepens, "tucks under" | Tail-heavy | Add nose weight and up elevator trim |
The pattern surprises people, because the nose-heavy plane pulls out of the dive and the tail-heavy one tucks in. Dream-Flight also notes that elevator trim must be adjusted slightly each time weight is added or removed. The manual describes this test for a glider, so treat it as a reference for gliders and as a reason to follow the maker's own CG range on powered models.
Symptoms of Getting It Wrong
RC Airplane World summarizes the two failure modes. A nose-heavy plane dives, needs constant up-elevator and glides poorly, but is generally stable and survivable. A tail-heavy plane pitches up, has twitchy elevator and is prone to stalling and spinning. Its saying: "A nose-heavy plane flies poorly, but a tail-heavy plane flies once."
Common mistakes follow from the sources:
- Measuring from the wrong reference. Manuals use the root, the fuselage side or the spar. Check which.
- Using the wrong MAC. Using the root chord on a tapered wing moves every percentage.
- Balancing a bare airframe. The battery, canopy and gear position all count.
- Arming the ESC while handling the plane. Horizon warns against it explicitly.
- Trusting any rule of thumb over the designer. Ranges quoted above run from 8% to 50%.
- Skipping the re-check. A new battery, a repair to the nose or added floats changes the balance.
FAQ
Q: What percentage of MAC should the CG be at?
There is no single answer. Burak suggests about 20% to 25%, RC Airplane World describes about 25% to 30% as typical, and plan ranges cited by Burak run from 8% to 33%. The designer's figure beats all of them.
Q: Can fingers be enough?
For light foam models, yes. Manufacturer manuals from Horizon and Dream-Flight both name fingers or a stand. Heavier models are easier and safer on a stand or a scale system.
Q: Is a digital balancer worth it?
It is worth considering for heavy, expensive or frequently changed models, because the system reports weight and CG and calculates what to add. The SkyRC CG Gauge lists 20 kg per module. Xicoy's capacity was not confirmed.
Q: Should the tank be full or empty?
RC Airplane World says empty on IC models, because the plane becomes tail heavy as the tank empties. Check the tank position against the CG on your plane.
