Airfoil Hysteresis and Unsteady Aerodynamics: Understanding Dynamic Stall

Steady polars assume the flow has forgotten the path

A standard CL(α) curve is single-valued: each angle has one lift. That is true while the flow is attached and the angle is changing slowly. It stops being true in two related ways. Static hysteresis means that even a slow sweep upward and a slow sweep downward disagree, usually because a laminar separation bubble or a stalled region bursts at one angle and reattaches only at a lower angle. Dynamic stall means the section is pitching, plunging or yawing fast enough that time itself is in the problem. Helicopters, wind turbines, maneuvering wings and some UAV gust encounters live here. A cruise polar does not.

What a hysteresis loop looks like

Plot CL against α while α increases through stall and then decreases. The upstroke and the downstroke form a loop. Inside the loop, lift at a given α depends on where you came from. Drag and moment loop as well. If your measured or computed data show a loop and you publish only one branch, say which branch. Averaging them invents a flow that did not occur. Viscous panel codes in their ordinary steady mode will not produce this loop. Absence of a loop in an XFOIL-class polar is not evidence that the airfoil is hysteresis-free. It is evidence that you did not ask an unsteady question.

Dynamic stall, in outline

When a section pitches up rapidly past the static stall angle, the boundary layer still separates, but the separated shear layer rolls up into a leading-edge vortex. While that vortex sits over the upper surface it deepens the suction. Lift can overshoot the static CL max. The moment may first pitch nose-down or nose-up depending on where the vortex is. When the vortex convects off the trailing edge, the suction collapses and a strong nose-down moment appears — the moment stall, which is often the load that damages a blade or saturates a control system, more than the lift spike is. As the angle falls, reattachment lags. The loads on the downstroke do not retrace the upstroke.

The nondimensional pitch rate and the reduced frequency (how fast the motion is compared with chord and airspeed) decide how large the overshoot is. A slow maneuver is almost static. A helicopter retreating blade or a turbine blade in yaw is not.

What to do with this as a designer

• Use steady polars to choose a cruise section and to estimate the static stall boundary. Label them steady.
• If the vehicle pitches through stall — rotors, turbines, aggressive UAV maneuvers, sailplane gusts at high reduced frequency — budget a dynamic-stall method or a test. Do not stretch CL max from a VIC polar upward because you read that dynamic stall “can” overshoot. It can also produce a moment you have not designed the spar for.
• Leading-edge shape and a trip change the static stall type, and they change dynamic stall too. A device that makes static stall gentle does not automatically delete the vortex.
• Report loops as loops. A single CL max taken from the upstroke of a dynamic test is not the static CL max, and it is not a number to drop into a steady performance deck.

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Where this sits in the series

In simple words: what this page is for

Past a point, the forces on the way up in angle of attack are not the forces on the way down. Dynamic stall adds a leading-edge vortex that steady polars never show.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Airfoil Hysteresis and Unsteady Aerodynamics: Understanding Dynamic Stall 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 Airfoil Hysteresis and Unsteady Aerodynamics: Understanding Dynamic Stall 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 Airfoil Hysteresis and Unsteady Aerodynamics: Understanding Dynamic Stall 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
• What Is the Aerodynamic Center of an Airfoil? Location, Meaning and Applications
• How to Identify Numerical Convergence Problems in Airfoil Analysis
• How to Build an Airfoil Analysis Database with Consistent Simulation Conditions
• All HENDOI blog articles
• HENFOIL product page
• HENSYNTH product page
• Pricing — Student, Professional & Team
• FAQ
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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.