Performance reading
Airfoil Hysteresis Loops: Understanding Differences Between Increasing and Decreasing Angle of Attack
HENDOI · 26 October 2026
Past an abrupt stall, the lift on the way down in angle is often not the lift on the way up. The loop is a history effect. A single sweep, or a steady solver marched in one direction, can draw only one branch.
The boundary layer remembers
In attached flow, a slow change of angle usually gives one CL and one CD at each angle. Near an abrupt stall it often does not. A leading-edge bubble that bursts, or a short laminar separation that will not re-form, can leave the upper surface separated until the angle has fallen well below the angle where the burst happened. Lift on the decreasing branch stays lower, and drag stays higher, over a range of angles. Plotted against angle, the two sweeps form a loop. That loop is hysteresis. The dynamic-stall lesson is the related case when the motion is fast. Hysteresis here can appear even in a slow sweep. It is still history, not a sensor glitch, until a repeat of the same sweep fails to reproduce it.
How to read a loop without inventing one
| Observation | Usual meaning |
|---|---|
| Gentle trailing-edge stall, curves overlie | Little hysteresis. One branch is an honest polar. The post-stall lesson still applies past the peak. |
| Abrupt nose stall, decreasing-α branch inside the increasing-α branch | A real loop is plausible. Its width is a property of that section, that Reynolds number and that sweep rate. Do not copy a width from another test. |
| A steady panel or VIC march that stops at the last converged angle | One branch at most, and often neither. Non-convergence is not a measured hysteresis loop. |
| A loop that appears only in one of two repeats | Treat it as scatter until the second sweep confirms it. Fans, walls and a moving transition point can imitate a loop. |
If the application pitches up and then recovers, the recovery angle is the decreasing branch, and it can be several degrees below the angle where lift was lost. Record which sweep produced each point. A handbook CLmax with no sweep direction is the increasing branch by custom, and it is the wrong number for the recovery.
Where this sits in the series
In simple words: what this page is for
Past an abrupt stall, the lift on the way down in angle is often not the lift on the way up. The loop is a history effect. A single sweep, or a steady solver marched in one direction, can draw only one branch.
This lesson exists because section decisions fail when geometry, method labels and selection reasons are scattered. Airfoil Hysteresis Loops: Understanding Differences Between Increasing and Decreasing Angle of Attack 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 Loops: Understanding Differences Between Increasing and Decreasing Angle of Attack 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 Loops: Understanding Differences Between Increasing and Decreasing Angle of Attack 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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- Previous: Reading lift, drag and moment together
- Next: Performance trade-offs on a polar
- 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.
Both products live
Use this lesson in the HENDOI workspace
This page sits in Performance reading. HENFOIL and HENSYNTH are both live on one account. These articles are screening lessons — not certification reports.
Live · section
HENFOIL
Turn this lesson into a section you can draw, check, analyse and compare. Run the polar at the Reynolds number of the vehicle. The method name stays on the result.
To Know MoreLive · aircraft
HENSYNTH
After the section is chosen, set geometry and mission requirements, run screening, read compliance, synthesize candidates and download or mail the report.
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