Density on its own decides nothing. What matters is strength per unit weight and stiffness per unit weight, because you are never choosing between equal volumes — you are choosing between two ways of carrying the same load. The last two columns below are the ones to read, and they say something the first three do not.
Structural materials
These eight are the ones Sheet 10 sizes a spar cap from, and this page reads them straight out of that sheet so the two cannot disagree. Strength is the bending figure that governs compression, because a cap fails on the compression side long before the tension side lets go.
| Material | Strength MPa | Stiffness GPa |
Density kg/m³ | Strength / weight kN·m/kg |
Stiffness / weight MN·m/kg |
|---|---|---|---|---|---|
| Balsa, medium | 19 | 3.4 | 160 | 119 | 21.2 |
| Balsa, hard | 27 | 5.0 | 240 | 112 | 20.8 |
| Basswood | 60 | 9.0 | 420 | 143 | 21.4 |
| Spruce | 70 | 10.0 | 450 | 156 | 22.2 |
| Pine | 50 | 8.5 | 520 | 96 | 16.3 |
| Birch ply | 60 | 8.0 | 680 | 88 | 11.8 |
| Glass uni | 450 | 40.0 | 1900 | 237 | 21.1 |
| Carbon uni | 600 | 130.0 | 1550 | 387 | 83.9 |
On stiffness per unit weight, five of these eight are the same material. Medium balsa 21.2, hard balsa 20.8, basswood 21.4, spruce 22.2, glass 21.1 — a spread of seven per cent across a range of densities from 160 to 1,900 kg/m³. If your spar is limited by how much the wing bends rather than by whether it breaks, glass cloth buys you nothing at all over spruce, and balsa is as good as either.
Only carbon breaks away, at 83.9 — 3.8 times spruce. That single number is why carbon spar caps exist, and it is a stiffness argument rather than a strength one. On strength per unit weight carbon is only 2.5 times spruce, and glass 1.5 times.
Two caveats on reading the table that way. These are the right figures of merit for a spar cap at a depth the aerofoil has already fixed, which is the usual case on a model. If the section depth is free, thicker and lighter wins and the ranking changes. And unidirectional glass and carbon are only these numbers along the fibre: at 45° they are a fraction of it, which is what the sheet's warning about laying cloth straight is about.
Foams
Cores, and whole aeroplanes. Densities are compiled ranges because the same trade name covers a wide span, and the figure on the packet is the one to believe over the figure here.
| Foam | Density kg/m³ | Where it is used |
|---|---|---|
| EPS, white bead foam | 15–30 | Cheap cores and free-flight models. Crushes rather than springs back. |
| XPS, extruded — Depron, Selitac | 30–40 | Flat-plate and profile models. Stiffer and smoother than EPS, and it does not absorb glue. |
| EPP, moulded | 20–40 | Slope and combat models. It comes back from an impact, which is the entire point of it. |
| EPO, moulded | 25–35 | Most ready-to-fly foamies. Denser and stiffer than EPP, and it breaks rather than bouncing. |
| Polyurethane, rigid | 30–80 | Cores that will be sheeted. Denser and stronger than the polystyrenes, and it sands cleanly. |
Coverings and cloth
Quoted per square metre of material, and per square decimetre because that is the unit the weight budget works in. A covering goes on both surfaces, so a wing of area S takes about 2S of it, plus a tenth for overlap and waste.
| Covering | g/m² | g/dm² |
|---|---|---|
| Iron-on film — Oracover, Solarfilm and the like | 55–75 | 0.55–0.75 |
| Lightweight iron-on film | 35–50 | 0.35–0.50 |
| Tissue and dope | 25–40 | 0.25–0.40 |
| Glass cloth, light | 25 | 0.25 |
| Glass cloth, medium | 50 | 0.50 |
| Glass cloth, heavy | 80 | 0.80 |
| Carbon cloth, light | 80 | 0.80 |
| Carbon cloth, structural | 200 | 2.00 |
That gives a useful cross-check on the weight budget. Film at 0.6 g/dm² over both surfaces of a wing is about 1.2 g/dm² of wing area, against the 14 g/dm² a built-up balsa wing comes out at — so the covering is roughly a twelfth of the finished wing, and switching to a lightweight film saves about a fortieth of it. Worth doing; not worth expecting much from.
Finished structures
What a completed wing actually weighs per unit area, including spar, ribs or core, covering, hinges and horns, with servos counted separately. These are the figures Sheet 05 uses, read from that sheet.
| Build method | g/dm² | oz/ft² |
|---|---|---|
| Moulded composite — carbon or glass sandwich | 12 | 3.9 |
| Built-up balsa, film covered | 14 | 4.6 |
| Balsa-sheeted foam core, film covered | 20 | 6.6 |
| Foam board or Depron, flat plate | 16 | 5.2 |
| Moulded foam — EPP, EPO | 24 | 7.9 |
| Plywood and balsa box, film covered | 22 | 7.2 |
Note the ordering, which surprises people: a moulded composite wing is the lightest and moulded foam the heaviest, and built-up balsa sits nearer the light end than the middle. Foam is cheap, tough and quick, and none of those is the same as light.
Every number here is compiled
Not one figure on this page is a measurement of anything you will build with. They are assembled from published wood and composite data and from what materials are sold as, and the sheets that use them say so in those words.
The spread is the reason this matters rather than a technicality. Two pieces of balsa off the same shelf differ by forty per cent in density and in stiffness with it — a spread wider than the difference between a careful design and a careless one. Weigh your own wood. A figure you measured beats every number in these tables, and Sheet 05 will take a wing you have actually weighed in place of its table.
Foam densities and covering weights on this page are compiled from what the materials are sold as. The structural figures come from Sheet 10 and the finished-structure figures from Sheet 05, so a correction to either sheet corrects this page too.