The animation shows the forces acting on an airfoil in an airstream. In the upper left area of the animation, an aircraft with a so-called Clark-Y airfoil is shown.
The magnitudes of the lift force and drag force are displayed using vector arrows. The angle of attack of the wing can be changed. The lengths of the vector arrows adjust accordingly.
Instructions for Use
As with all animations, you can enlarge or reduce the windows by clicking on them.
After starting the application, you can watch the animation in full-screen mode. To do so, click on “View” and then on “Full Screen”:
To exit full-screen mode, press the Esc key.
Description of the Animation
When clicking on the large content window of the animation, additional layers become visible. The polar diagram shows how the lift and drag coefficients of the wing relate to each other. As an alternative to the polar diagram, an analytical diagram is also displayed.
The magnitude of the lift force can be calculated using the following formula:
\[ F_L = C_L \cdot \frac{\rho}{2} \cdot v^2 \cdot A \]
- CL: Lift coefficient. The lift coefficient depends on the shape of the wing. The value is determined through measurements in a wind tunnel or by simulations.
- ρ (Rho): Density of the air through which the wing moves. It is given in kilograms per cubic meter (kg/m³) and can vary depending on altitude, temperature, and humidity.
- v: Speed of the object through the air in meters per second (m/s).
- A: Reference area. This is the area on which the air pressure acts to generate lift. For wings, this is typically the planform area (the wing area as seen from above).
The drag force is calculated analogously:
\[ F_D = C_D \cdot \frac{\rho}{2} \cdot v^2 \cdot A \]
- CD: Drag coefficient – likewise profile-dependent and determined through wind tunnel measurements or simulations.
The ratio of lift to drag is called the lift-to-drag ratio (or glide ratio):
\[ E = \frac{C_L}{C_D} = \frac{F_L}{F_D} \]
A high lift-to-drag ratio means the profile generates a lot of lift with little drag – essential for efficient gliding flight.
Even outside aviation, the principle of dynamic lift plays an important role: The rotor blades of modern wind turbines generate aerodynamic forces similar to aircraft wings to rotate efficiently. Even race cars use “inverted airfoils” to generate downforce – a force that presses the vehicle onto the road at high speed. In nature, birds use finely tuned wing geometries to glide for hours with minimal energy expenditure.
The Clark-Y Airfoil in Detail
The Clark-Y airfoil was developed by Colonel Virginius E. Clark in the early 1920s. The “Y” refers to one of several variants he designed — and it became by far the most successful. What made this profile special was its remarkably simple geometry: a flat lower surface combined with a cambered upper surface.
Why a Flat Bottom?
The flat underside of the Clark-Y profile offers significant practical advantages. It simplifies manufacturing because the lower wing skin can be made from a flat sheet of material — no complex shaping required. This was especially important in the early days of aviation, when many aircraft were built in small workshops. Additionally, a flat surface makes it easier to measure and verify the angle of attack, and it simplifies mounting the wing to the fuselage.
Clark-Y vs. Symmetric Profiles
A symmetric airfoil such as the NACA 0012 generates zero lift at zero angle of attack — the upper and lower surfaces are mirror images. The Clark-Y, by contrast, produces lift even at an angle of attack of 0° because of its cambered upper surface. This makes it well-suited for applications where consistent lift is needed at modest speeds, such as general aviation and model aircraft.
The trade-off is that symmetric profiles are preferred for aerobatic aircraft and helicopter rotor blades, where the aircraft must generate lift equally well in both “upright” and “inverted” orientations.
Clark-Y Today
Although modern computational methods have produced more refined profiles (such as the NASA NLF series or Eppler designs), the Clark-Y remains popular in model aviation, drone design, and education. Its well-documented performance data makes it an ideal teaching tool — which is exactly why it was chosen for this animation.
Stall — What Happens at High Angles of Attack?
As you increase the angle of attack in the animation, the lift coefficient CL rises — at first almost linearly. But there is a critical limit. For the Clark-Y profile, this critical angle of attack is approximately 15°.
What Happens Physically?
At small angles, the air flows smoothly along both surfaces of the wing. As the angle increases, the airflow on the upper surface must travel an increasingly longer path and accelerate more, creating lower pressure that generates lift.
Beyond the critical angle, the airflow can no longer follow the contour of the upper surface. It separates from the wing — this is called flow separation. The result is a turbulent wake behind the wing. Lift drops abruptly while drag increases sharply. This phenomenon is known as stall.
Why Does This Matter?
Stall is one of the most critical phenomena in aviation safety. Many aircraft accidents — especially during takeoff and landing, when speeds are low and angles of attack are high — are caused by unintentional stalls. Pilots are extensively trained to recognize and recover from stall situations.
Try it in the animation: Increase the angle of attack beyond 15° and observe how the lift vector changes. The polar diagram shows the stall point clearly — it is the peak of the CL curve.
Practical Examples with Real Numbers
The formulas above become much more tangible with concrete values. Below are two examples that demonstrate how the lift equation is applied in practice.
Example 1: A Glider in Cruising Flight
A glider has a wing area of A = 15 m², flies at v = 100 km/h (≈ 27.8 m/s) at an altitude where the air density is ρ = 1.225 kg/m³ (sea level, standard conditions). The Clark-Y profile at an angle of attack of about 5° has a lift coefficient of approximately CL = 0.8.
\[ F_L = 0.8 \cdot \frac{1.225}{2} \cdot 27.8^2 \cdot 15 \approx 5{,}680 \text{ N} \]
This corresponds to roughly 579 kg — enough to keep a glider with pilot comfortably airborne.
Example 2: A Small Cessna at Takeoff
A Cessna 172 has a wing area of approximately A = 16.2 m² and a takeoff speed of about v = 100 km/h (≈ 27.8 m/s). At a higher angle of attack during takeoff (CL ≈ 1.2):
\[ F_L = 1.2 \cdot \frac{1.225}{2} \cdot 27.8^2 \cdot 16.2 \approx 9{,}197 \text{ N} \]
This is about 937 kg of lift — enough for the aircraft’s maximum takeoff weight of around 1,111 kg.
How to Read a Polar Diagram
The animation includes a polar diagram — one of the most important tools in aerodynamics. Understanding how to read it unlocks a wealth of information about the performance of an airfoil.
The Axes
In a polar diagram, the drag coefficient CD is plotted on the horizontal axis, and the lift coefficient CL on the vertical axis. Each point on the curve corresponds to a specific angle of attack. As you change the angle in the animation, you can see the operating point move along the polar curve.
Finding the Best Glide Ratio
The optimal glide ratio can be found graphically: draw a straight line from the origin (0, 0) that is tangent to the polar curve. The point where this line touches the curve is the angle of attack with the highest ratio of CL to CD — in other words, the best lift-to-drag ratio.
For a glider pilot, this is the angle of attack that maximizes range — the aircraft covers the greatest horizontal distance for each meter of altitude lost.
What the Shape of the Curve Tells You
- A narrow, tall curve indicates a profile with excellent lift and low drag — typical of high-performance glider profiles.
- A wide, flat curve suggests high drag across all angles — typical of thick or poorly shaped profiles.
- A sharp peak at the top means stall occurs abruptly — a safety concern.
- A rounded peak means the profile stalls gently, giving the pilot more time to react.
Try it in the animation: Open the polar diagram and vary the angle of attack. Observe how the operating point moves along the curve. Can you find the angle with the best glide ratio?
Comparison: Airfoil Shapes and Their Characteristics
The Clark-Y is just one of thousands of documented airfoil profiles. Different applications demand different shapes. The following table compares four common profile types:
| Profile | Shape | Typical Use | Key Characteristic |
|---|---|---|---|
| Clark-Y | Flat bottom, cambered top | General aviation, model aircraft, drones | Good lift at low speeds; easy to manufacture |
| NACA 0012 | Symmetric | Aerobatics, helicopter rotors, tail surfaces | Zero lift at zero angle of attack; equal performance inverted |
| NACA 23012 | Cambered, max. thickness at 30 % | Transport aircraft, Cessna-type planes | High maximum lift; good for short takeoff distances |
| Supercritical (e.g. NASA SC(2)-0612) | Flat top, curved bottom | Transonic jet airliners | Delays shock wave formation at high speeds |
Each of these profiles would produce a distinctly different polar diagram. A symmetric profile like the NACA 0012 has its polar curve centered on CL = 0, while the Clark-Y’s curve is shifted upward because of its built-in camber.
The choice of airfoil is always a trade-off. There is no single “best” profile — the right choice depends on the intended speed range, manufacturing constraints, and mission requirements. The Clark-Y’s enduring popularity is a testament to its excellent balance between simplicity, performance, and predictable behavior.
Overview and Download
| Title | Aerodynamic Lift on an Airfoil |
| Target Audience | Teachers and Lecturers |
| Platforms | Microsoft® Windows® Apple® Macintosh® (version-dependent) |
| Features | Full-screen mode Lossless scaling Large screens and projectors supported |
| License | Freeware |
| Download | Contact |
Background Information
The animation shows a real Clark-Y airfoil. Actual measured coefficients for lift and drag are used.
Note 1: The drag coefficients were multiplied by a factor of 10 so that the polar diagram and the force vectors are easier to see.
Note 2: The mass of the aircraft influences how it behaves in reality. This behavior is not realistically represented in the animation because many additional aspects would need to be considered. You can also think of the wing as being mounted in a wind tunnel. The moving clouds indicate the movement of the wind.
The Clark-Y airfoil was one of the first systematically measured wing profiles in the 1920s. It became so popular that it was used in over 20 aircraft types — from gliders to light motor aircraft. The straight underside simplifies both construction and assembly, while the curved upper surface generates stable lift. Even today, the profile is still used in drones and model aircraft.
Contributors
C. Hein, S. Rikowski
Sources
- Idea and first concept: Tamara Riehle
- Clark-Y airfoil: http://www.ae.illinois.edu/m-selig/ads/coord_database.html
- Authoring tool: Adobe Animate CC
Es wird einem nicht verraten, mit welcher Applikation man die .exe öffnen soll…?? android will das aber wissen. oder kann man auf android nicht öffnen?
Guten Tag, leider kann man die exe-Dateien nur auf Windows-Systemen öffnen. Eine apk-Datei für Android gibt es derzeit nicht.