How to Select an Airfoil for High-Altitude Long-Endurance UAVs

Altitude changes the Reynolds number first

High-altitude long-endurance aircraft, from solar prototypes to operational HALE platforms, cruise where the density is a small fraction of sea level. Near 20 km the standard-atmosphere density is on the order of a tenth of the sea-level value, and the temperature is low and fairly flat through the lower stratosphere. Viscosity depends on temperature, not on density, so it does not fall with ρ. Reynolds number Re = ρ V c / μ therefore falls almost in proportion to density if true speed and chord stay fixed. A chord that would have been “high Re” at sea level can be a few hundred thousand at altitude. The section problem is a low-Re problem that also has to carry a very large aspect ratio and a very small weight.

What the section is being asked to do

• High section L/D at the cruise CL. Induced drag is already attacked with aspect ratio. The remaining bill is profile drag, hour after hour, paid from a limited energy store. The design CL comes from a low wing loading and a dynamic pressure that is small because ρ is small, even when true speed is not tiny.
• A wide, robust low-drag bucket. Laminar flow is attractive and fragile. Bugs are less of an issue in the stratosphere than on a low-altitude loiter, but surface waviness, tape and the trip from a joint still are. Check a tripped polar anyway.
• Gentle stall and a real margin. Gusts at low dynamic pressure produce large angle-of-attack excursions. A section that cliffs at CLmax is a poor partner for a flexible, high-aspect-ratio wing.
• Thickness you can build light. The spar of a huge, light wing wants depth. Thickness that helps the spar and thickness that preserves a laminar bucket have to be chosen together, not by the polar alone.
• A manageable moment. A highly cambered section can demand a tail download the aircraft, which has almost no spare lift, should not pay.

A selection sequence

1. Compute ρ, μ and the speed of sound at the cruise altitude. Compute Re and Mach with the true speed and the local chord. Mach is often still moderate; do not ignore it if the true speed is high.
2. Compute the cruise CL from weight, area and q = ½ ρ V². This CL is often higher than a sea-level intuition suggests, because q collapsed with density.
3. Analyse candidates at that Re and CL, clean and tripped, and at a lower Re that represents a slower search or a smaller tip chord.
4. Prefer the section whose L/D survives the trip and whose stall is rounded, over the section with the single best clean bucket.
5. Re-check at a climb or descent condition. HALE missions are not one point, even when they loiter for a day.

The low-Re analysis lesson is the method. This page only adds the atmospheric fact that makes HALE a low-Re airplane in disguise.

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Where this sits in the series

In simple words: what this page is for

At altitude the air is thin, so the same chord and true speed give a much lower Reynolds number. A HALE section has to be efficient there, light, and willing to stall gently.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. How to Select an Airfoil for High-Altitude 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 Select an Airfoil for High-Altitude 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 Select an Airfoil for High-Altitude 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

• Active Flow Control on Airfoils: Suction, Blowing and Synthetic Jets
• How Adverse Pressure Gradients Affect Airfoil Boundary Layer Separation
• What Is the Aerodynamic Center of an Airfoil? Location, Meaning and Applications
• What Is Airfoil Aerodynamic Efficiency Across a Range of Flight Conditions?
• All HENDOI blog articles
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• HENSYNTH product page
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• 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.