Wing loading is the single number that best predicts how an RC plane will feel in the air: floaty and forgiving, or fast and demanding. This calculator turns three easy measurements into wing loading, the size-independent cubic loading index, and a rough stall-speed estimate.
The three formulas behind the result
Wing area is span times mean chord, and loading is weight over that area:
area (sq ft) = (span_in × chord_in) / 144
loading = weight_oz / area_sqft (oz / sq ft)
cubic load = weight_oz / area_sqft^1.5 (size-independent index)
stall ~ k × sqrt(loading) (k from a typical Cl_max)Cubic wing loading is the fairer cross-size comparison because dividing by area to the 1.5 power cancels the way larger models naturally carry more weight per unit area while still flying gently.
Worked example
A 48-inch span, 9-inch chord sport model weighing 48 oz has:
- Wing area: (48 × 9) / 144 = 3.0 sq ft
- Wing loading: 48 / 3.0 = 16.0 oz/sq ft — solid mid-sport
- Cubic wing loading: 48 / 3.0^1.5 = 48 / 5.196 ≈ 9.24 — right at the floaty-to-sport crossover
Now add a heavier 6S battery pack and bump all-up weight to 64 oz. Wing loading rises to 21.3 oz/sq ft — the plane now lands noticeably hotter and requires more runway. The stall speed estimate scales with the square root of loading, so the heavier version stalls roughly 15% faster than the lighter one.
What the numbers mean in practice
| Wing loading (oz/sq ft) | Character | Typical type |
|---|---|---|
| Below 10 | Ultra-floaty, very slow stall | Gliders, foamies |
| 10 – 16 | Gentle trainer feel | Trainers, slow-fly |
| 16 – 24 | Agile sport | Sport, aerobatic |
| 24 – 30 | Fast, requires a runway | Hot sport, pylon |
| Above 30 | Very demanding | Jets, scale warbirds |
Cubic wing loading below 9 is a floater; above 13 is a quick sport model. The two indices together give a more complete picture than either alone.
The physics behind the stall-speed estimate
Stall speed comes from the full-size lift equation — lift equals
½ × ρ × V² × S × Cl — which applies to a park flyer exactly as it does to an
airliner (NASA’s Glenn Research Center publishes an accessible derivation in its
Beginner’s Guide to Aeronautics).
At the stall, the wing is flying at its maximum lift coefficient Cl_max, so
solving for velocity gives:
V_stall = sqrt( 2 × W / (ρ × S × Cl_max) )Since W / S is wing loading, stall speed grows with the square root of
loading — double the loading and stall speed rises by roughly 41%, not 100%.
The calculator assumes a Cl_max typical of the flat-bottomed and semi-symmetrical
airfoils used on most RC models. Real values vary with airfoil, Reynolds number
(small, slow wings generate lift less efficiently than full-size ones), and flap
deployment, which is why the output is a comparative guide rather than a
flight-test number.
Two practical consequences of the square-root law:
- Weight creep is sneaky but forgiving at first. Adding 10% weight raises stall speed only ~5% — one reason overweight models often still fly. But the landing energy grows with V², so that same 10% makes arrivals ~10% harder on the airframe.
- Ballistic descent is not a stall fix. If a heavy model feels mushy on approach, the answer is more airspeed, not less. High-loading models must be flown onto the runway; trying to float them on like a trainer produces the classic tip-stall crash.
Tips and practical notes
- Always weigh the model ready to fly — battery, motor, prop, receiver, and all servos installed. Paper weights underestimate real mass.
- For a tapered wing, use the mean aerodynamic chord (average of root and tip) to keep the area calculation accurate.
- To soften a plane that lands too fast: add wing area, drop all-up weight by choosing a lighter battery, or use a larger, slower-turning prop to increase low-speed thrust.
- A high cubic loading can still be manageable if the model has generous flap authority and good low-speed stability — numbers are a guide, not a verdict.
- Weight matters legally as well as aerodynamically: in the United States, any model aircraft over 250 g (0.55 lb) must be registered with the FAA before outdoor flight, and club fields typically follow the Academy of Model Aeronautics safety code, which sets additional requirements for larger and heavier models. Check both before the maiden flight of a new heavy build.