How do symmetrical airfoils work

In general, the lift is a very complex function of the shape. Aerodynamicists model the shape effect by a lift coefficient which is normally determined through wind tunnel testing. For some simple shapes, we can develop mathematical equations to determine the lift coefficient. The simplest model, the two dimensional Kutta-Joukowski airfoil, is studied by undergraduate students. The FoilSim computer program provides the results of this analysis in a form readily usable by students.

A result of the analysis shows that the greater the flow turning, the greater the lift generated by an airfoil. This slide shows the flow fields for two different airfoils. The airfoil on the left is a symmetric airfoil; the shapes above and below the white centerline are the same. The airfoil on the right is curved near the trailing edge.

The yellow lines on each figure show the streamlines of flow from left to right. The left figure shows no net turning of the flow and produces no lift; the right figure shows a large amount of turning and generates a large amount of lift.

The front portions of both airfoils are nearly identical. The aft portion of the right airfoil creates the higher turning. The example shown above explains why the aft portion of wings have hinged sections to control and maneuver an aircraft. How do symmetrical airfoils do this when the top and lower surfaces are the same shape and length? By using a non-zero angle of attack. When the trailing edge is pointed downwards, and assuming the airstream leaves the trailing edge smoothly, the exiting airstream is deflected downwards.

This causes lift via conservation of momentum. Increasing the angle of attack will increase your lift until such time as the airstream over the trailing edge becomes non-smooth. When this happens, you're close to stalling. Image from this page , which unfortunately appears to be down. As you can see from the above graph, a symmetric airfoil at zero angle of attack generates no lift; see this site from NASA as well as the above Wikipedia page.

Sign up to join this community. The best answers are voted up and rise to the top. Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Learn more. But these aircraft look so much like a basic trainer, such as the Midwest Aero Star, that you could be wondering what the differences would be.

All of them look the same. Not really. The three aerobatic trainers have one major difference. Their wings possess a nearly symmetrical airfoil shape. The airfoil is the curving shape of the wing when viewed from the side. This makes all the difference in the world. To understand why, we need a very brief course on what makes an aircraft fly. Promise, it will be a very brief course. This happens because the air on the top must cover a greater distance in the same time frame than does the air on the bottom.

Since the air on the bottom of a flat-bottom wing is moving more slowly, it must have a greater pressure than the air on the top and tends to push the wing upwards against the lower top pressure. The wing develops lift. Great story. It is even somewhat true.

But most all full-size and model aircraft have wings that are nearly symmetrical called semi-symmetrical or truly symmetrical. The wing shapes are the same top and bottom. How then do these aircraft fly?