Dynamic Stall on Airfoils: Understanding Unsteady Lift and Hysteresis

Fast motion makes the path matter

The hysteresis lesson introduced the loop. This page is the unsteady lift itself. Define a reduced frequency

k = ω c / (2 V)

with ω the pitching frequency in radians per second, c the chord and V the freestream speed. Small k means the flow has time to look like a succession of steady states. When k is not small, and the angle passes through the static stall angle, the boundary layer separates into a shear layer that rolls up as a leading-edge vortex. While that vortex is over the upper surface, suction is deeper than the steady stall allows. Lift overshoots the static CLmax. The moment follows the vortex: the dangerous part is often the nose-down break as the vortex passes the trailing edge, called moment stall, and it can arrive at a different time from the lift peak.

What the loop contains

• Upstroke. Lift stays attached-looking past the static stall angle, then rises further with the vortex, then collapses.
• Downstroke. Reattachment lags. At the same angle, lift and moment are not the upstroke values. The area inside the loop is energy per cycle, which matters for flutter and for rotor loads.
• Pitch rate. A faster pitch toward higher angle generally increases the overshoot, until the motion is so fast that the vortex story itself changes. There is no universal multiplier to apply to static CLmax.

Where you must not use a steady polar

Keep the steady polar for cruise and for the static boundary. Commission a dynamic-stall model or a test when the reduced frequency and the angle range say the vortex will exist. Do not scale static CLmax upward by a favourite factor and call it unsteady lift. The overshoot and the moment spike are a pair. Taking only the flattering half is how a blade gets the wrong load.

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Where this sits in the series

In simple words: what this page is for

When angle of attack changes quickly, lift can overshoot the static maximum and the return path is a different curve. Reduced frequency says whether the motion is fast enough to matter.

This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Dynamic Stall on Airfoils: Understanding Unsteady Lift and Hysteresis 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 Dynamic Stall on Airfoils: Understanding Unsteady Lift and Hysteresis 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 Dynamic Stall on Airfoils: Understanding Unsteady Lift and Hysteresis 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

• What Is the Aerodynamic Center of an Airfoil? Location, Meaning and Applications
• What Is Airfoil Aerodynamic Efficiency Across a Range of Flight Conditions?
• Airfoil Hysteresis and Unsteady Aerodynamics: Understanding Dynamic Stall
• All HENDOI blog articles
• HENFOIL product page
• HENSYNTH product page
• Pricing — Student, Professional & Team
• FAQ
• Login · Register now

• Previous: Vortex generators on airfoils
• Next: Detect duplicate and invalid coordinate points
• 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.