Choosing an aerofoil for a fixed-wing UAV

A design team has shortlisted three aerofoil sections for a long-endurance UAV wing. Wind-tunnel time is limited, so the question for simulation is which section to take forward, and why.

The question

Endurance depends on the lift-to-drag ratio at cruise, but the wing also has to climb, loiter and recover from gusts. The study has to answer three things: which section has the best lift-to-drag ratio at the cruise lift coefficient, how each one behaves as it approaches stall, and how sensitive the ranking is to Reynolds number across the speed range.

The approach

Each section is meshed with a body-fitted grid that resolves the boundary layer down to the wall (y⁺ ≈ 1), because the drag difference between candidates is small and wall functions would blur it. A sweep of angles of attack is run for each section at cruise and loiter Reynolds numbers.

Before any section is compared, one of them is run on three successively finer meshes. Only when lift and drag change by less than the difference between candidates is the mesh accepted. Our article on mesh independence explains the check.

What the study delivers

• Lift, drag and pitching-moment polars for each section at each Reynolds number
• Surface pressure and skin-friction distributions, showing where and how separation begins
• A ranking at the cruise condition with the numerical uncertainty stated alongside it
• A note on stall character: gradual trailing-edge stall or abrupt leading-edge stall

Why it matters

The section with the lowest cruise drag is not always the right choice. A study like this shows the trade between cruise efficiency and stall behaviour before a wing is built, and narrows the wind-tunnel programme to confirming one design instead of exploring three.