Structures & Spar Sheet

The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.

Sizing to the manoeuvre limit alone. The gust increment goes inversely with wing loading, and a model's wing loading is around a tenth of a light aircraft's. Gusts a full-size aeroplane rides through can exceed the manoeuvre limit on a model, which is why the design load factor here is the largest of the three cases rather than the one you picked.

Taking the spanwise load as uniform. A uniform load puts the centroid of the half-wing load at 0.5 of the semi-span. Schrenk's approximation, which this sheet uses, puts it between 0.379 and 0.462 depending on the taper, and a pure ellipse at 0.424. Assuming the easy answer overstates the root bending moment by eight to thirty per cent — which is the safe direction to be wrong in, and expensive. Assuming an ellipse under a constant-chord wing is the same mistake with the sign reversed and it costs nine per cent, which is not.

Reading a strong spar as a stiff one. Strength and stiffness are different requirements. The tip deflection reported here is for a spar sized exactly to strength, which is why it is often larger than a builder would accept — that is the sheet telling you stiffness is the binding case.

Trusting the wood. Balsa varies about 40% between two pieces off the same shelf, which is a wider spread than the difference between careful and careless design. Weigh the actual stick.

Thinking a strong wing is a safe wing. Flutter is a stiffness problem rather than a strength one, and it is what actually destroys fast models. Nothing on this sheet models it, and nor does the shear web, the root joint, the wing bolts or the torsion box.