How to Design an Airfoil for Long-Endurance UAVs

What endurance asks of a section

A long-endurance fixed-wing UAV spends most of its energy in loiter. For a propeller-driven aircraft the classical endurance parameter is CL^3/2/CD, evaluated at the lift coefficient the wing can actually hold for hours. Range cares more about CL/CD. Electric propulsion does not repeal induced drag: the section parameter is still only the profile part of an airplane sum. Design the section for the loiter CL, then check that the wing’s aspect ratio and weight make that CL reachable without a silly angle of attack.

The Reynolds number is often low. A 30 cm chord at 16 m/s near sea level is around Re = 3×10⁵. At that Re a laminar separation bubble can sit on the upper surface, add pressure drag, and make CL max sensitive to the leading-edge shape. Sections copied from light aircraft at Re = 3×10⁶ are a starting catalogue, not a result.

Shape choices that usually help

• Moderate thickness, often near 12% or a bit more at the root, so the spar is deep and the stall is not a thin-airfoil cliff. The tip can be thinner if the structure and the tip Reynolds number allow it.
• Camber matched to loiter CL, so the best section efficiency sits near the hours-long condition rather than at a dash CL you use for five minutes.
• A leading edge that still works with a trip. Long-endurance airframes collect insects, tape edges and rain erosion. A design that loses 20% of its endurance when Ncrit drops is a fair-weather design.
• A tame moment. Hours of flight with a large tail download is hours of extra induced drag. Put CM in the objective or in the constraints.
• A rounded stall. Autopilots and automatic landings prefer a lift curve that bends before it breaks. An abrupt leading-edge stall at low Re is a handling problem, not a detail.

A design loop at low Re

1. Fix loiter speed, chord and altitude. Compute Re. Compute a target section CL from the wing loading you believe, and write down the uncertainty in that loading.
2. Generate a small family: two thicknesses, two cambers, one reflexed candidate if the aircraft is a flying wing.
3. Run free-transition and tripped polars at the loiter Re, and at a lower Re that represents a slower search or a smaller tip chord.
4. Rank on the endurance parameter at the target CL, then discard anything whose tripped polar collapses or whose CM the tail cannot trim.
5. Look at the pressure distribution of the survivor. A very sharp suction peak followed by a long bubble is a fragile way to buy low drag.
6. Only then talk about optimization. Optimize at the loiter Re with a trip constraint or a second objective, not only at a clean Ncrit.

Trades you should expect

The section with the lowest clean cruise drag is often too thin, too lightly cambered, or too dependent on laminar flow for a 12-hour flight. The section with the highest CL max is often draggier in loiter. Long endurance sits between them, closer to the loiter CL than newcomers expect, and closer to “still works when dirty” than a single clean polar suggests. When you publish the choice, publish both polars.

---

Where this sits in the series

In simple words: what this page is for

Endurance is a high-lift efficiency problem at modest Reynolds number. The section has to hold that efficiency after the leading edge gets dirty.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. How to Design an Airfoil for Long-Endurance UAVs helps you keep a clean trail from shape to polar to choice.

Why the topic matters

Engineers lose time when a title sounds clear but the page only sketches the idea. This article expands How to Design an Airfoil for Long-Endurance UAVs so you can answer three questions without jargon: what is the idea, when do you use it, and how do you check it in a HENDOI product.

Read the sections above for the core definitions. Use the steps below to move from reading into a real project trail you can share with a teammate.

How to apply this in HENDOI

In practice: create a HENDOI account or log in, open HENFOIL, bring a section in (create, import DAT, or digitize), pass the geometry gate, climb the analysis ladder only as far as the decision needs, then compare and archive. Every calculated number should keep its method label.

When the section choice is ready for the aircraft, continue in HENSYNTH for mission, compliance and trades. Same account. Same plans.

What this page will not claim: a teaching polar or geometry note is not a flight-Re certificate. HENFOIL labels methods so you know what you bought. Tunnel data and professional judgement still sit outside any single software run.

Checklist before you leave this page

• Can you state the idea of How to Design an Airfoil for Long-Endurance UAVs in one sentence?
• Do you know which product owns the next action — HENFOIL or HENSYNTH?
• Will every number you keep carry a method label or a stored-run source?
• If you are new here, have you opened Register or Login?

Related reading on this site

• How Boundary Layer Thickness Develops Along an Airfoil Surface
• How Airfoil Pitching Moment Influences Aircraft Longitudinal Stability
• All HENDOI blog articles
• HENFOIL product page
• HENSYNTH product page
• Pricing — Student, Professional & Team
• FAQ
• Login · Register now

• Previous: Airfoil optimization explained
• Next: Airfoil coordinates: DAT, CSV and CAD
• Index: all HENFOIL airfoil lessons

These pages teach the ideas. They are not wind-tunnel data and not a certification analysis. When a number matters, compute it at a stated Reynolds number, Mach number and transition setting, and keep the method label on the result. In HENFOIL that production polar is a VIC polar — see the analysis ladder and the polar field guide.