Talk:Airfoil

Missing Re on the last diagram

The "Lift and drag curves for a typical airfoil" is a meaningless chart since it does not mention the Reynolds Number. It also looks dramatically wrong - real lift and drag coefficients never look like that. Someone should insert a real chart in place. — Preceding unsigned comment added by 2001:8000:1BED:6D00:C2D:7209:A7E7:3834 (talk) 10:51, 24 May 2020 (UTC)

The chart is an aid to understanding the twin concepts of lift coefficient and drag coefficient; and their relationship with the angle of attack. It is supplied to help readers who are new to the subject. The chart is not intended to be a scientific document to be used by aerodynamicists and aircraft engineers in the design of an actual aircraft. If the chart showed Reynolds Number and Mach Number, as well as angle of attack, it would not be appropriate in this article so we would have to find an alternative, more appropriate diagram. This principle is well explained at Wikipedia:Make technical articles understandable. Dolphin (t) 11:56, 24 May 2020 (UTC)

Why does laminar flow airfoil redirect here?

Airfoil doesn't seem to discuss laminar flow airfoils. Will (Talk - contribs) 10:39, 5 June 2020 (UTC)

Potential resource

I accidentally stumbled onto a resource that might have some value as an encyclopedic resource of airfoil information from the past.

It mentions another resource as well that may be worth a look.

I do not plan to edit here but leave this as a drive by link drop for idle hands to do what they want.

https://m-selig.ae.illinois.edu/ads/aircraft.html

Idyllic press (talk) 19:44, 21 March 2021 (UTC)

center of lift?

excuse the question of a lay person: is there no such notion as center of lift of an airfoil? meanining: where would I attach a load under a wing (in its ideal position) without changing the angle of attack? or: where must a rotatable axis to hold a wing in an airflow must sit for it to remain stable i.e. not to rotate when a load is attached there? if that notion is identical with "aerodynamic center" why then are these simple functional explanations not used? thanx! HilmarHansWerner (talk) 11:31, 22 June 2021 (UTC)

The lift on an airfoil is merely one component of the aerodynamic force; the other component is the drag. The aerodynamic force can be considered as a line of action and the point at which this line intersects the airfoil's chord line is called the center of pressure. We say both the lift and the drag act through the center of pressure; we never talk about the center of lift or the center of drag because there is only one point and it is called the center of pressure. The center of pressure of a symmetrical airfoil is considered to be in a fixed position round about the 25% chord point.

Unfortunately on a cambered airfoil the center of pressure moves about as the angle of attack changes. With a cambered airfoil it isn't possible to attach something to one point and hope that the arrangement will remain stable at all angles of attack because the center of pressure moves about. The best you can do is attach something near the aerodynamic center and then keep the arrangement stable by installing a horizontal stabiliser (tailplane). Which is what most airplane designers do! Dolphin (t) 13:41, 22 June 2021 (UTC)

Definition of an airfoil

The current version of the article begins with a definition of an airfoil (or aerofoil), saying it is "...the cross-sectional shape of an object ..." By defining an airfoil as a cross-sectional shape, Wikipedia is saying it is a two-dimensional figure.

Paradoxically, elsewhere in the article is the clear implication that an airfoil is not simply a two-dimensional figure because it is stated that airfoils generate lift. (A two-dimensional figure has a planform area of zero because it has thickness of zero, and therefore regardless of the pressure difference on the two sides of the figure there will be no force generated.) Consider the following expressions taken from the lead:

"resulting in a force on the airfoil in the opposite direction to the deflection."

"An airfoil is a streamlined shape that is capable of generating significantly more lift than drag."

"The lift on an airfoil is primarily the result of its angle of attack."

"... cambered airfoils can generate lift at zero angle of attack.

These four examples show that Wikipedia considers airfoils as devices that can generate lift, and therefore are three-dimensional. The sole inconsistency is in the first sentence which defines an airfoil as a "cross-sectional shape".

I will attempt to eliminate this inconsistency by adjusting the definition of an airfoil to include all bodies that are capable of generating significantly more lift than drag. As a result, it will be reasonable for Wikipedia to state, or imply, that wings, propeller blades, rotor blades and stabilizers are all examples of airfoils.

Here is the definition I intend to insert into the first sentence of the lead:

"An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more lift than drag. Wings, sails and propeller blades are examples of airfoils."

The two-dimensional cross-sectional shape of typical airfoils should hereafter be described as "airfoil sections". Dolphin (t) 11:54, 28 April 2023 (UTC)

No history?

When were aerofoils applied to flight? Was there a development path from ancient times or was the principle discovered in the 20th century? Humphrey Tribble (talk) 13:14, 21 August 2023 (UTC)

If we accept the idea that the wings of birds and other flying creatures are aerofoils, we must accept that aerofoils have been operating almost since the beginning of geological time. However, one idea is that aerofoils are man-made devices that must qualify under fairly strict criteria, then the first aerofoil was likely seen in the wing of the glider designed and built by George Cayley in the first decade of the nineteenth century. See George Cayley#Flying machines. The strict criteria I am referring to include the requirement that an aerofoil must be able to develop significantly greater lift than drag. Dolphin (t) 13:45, 21 August 2023 (UTC)

Aerofoils don't actually use Bernoulli's principle

According to this paper from the University of Cambridge, aerofoils don't use Bernoulli's principle at all, but it is actually centripetal force that causes the lift. The tldr is that bernoulli's principle (a) only applies on a single streamline and has nothing to do with neighbouring streamlines and (b) doesn't apply because the air doesn't actually travel at different speeds above and below the foil. I just wanted to ask before I changed a large section of the article. AHumanIBelieve (talk) 21:31, 2 April 2026 (UTC)

I think you have misunderstood the reasoning in both the WP article and the IoP article you reference. The explanation correctly critiqued by the author is well understood by contributors to this page and is thankfully not the one in the WP article.

Bernouilli's principle does apply in the limited way as described, and the air does travel at different speeds above and below an aerofoil. Neither of these points are adressed in the quoted article. Please don't change the article based on your interpretation of one published paper aimed at helping educators teach the topic. Somewhen (talk) 21:55, 2 April 2026 (UTC)

If the principle only applied along a streamline it would be insignificant and of no use to people working in the field of aerodynamics. However, if the fluid is flowing from a region of uniform energy density, such as a reservoir of stationary fluid or a stationary atmosphere, the Bernoulli constant is the same along every streamline. This is the property that raises the principle from trivial to profoundly significant and immensely useful in science and engineering.

In the case of steady flow of an incompressible fluid, it is only the streamlines that experience viscous shear forces that have a unique Bernoulli constant. This concept is sourced from the reference “Fluid Mechanics” by Victor Streeter - see Reference No 6 at Bernoulli’s principle.

We say “Within a region of irrotational flow, all streamlines have the same value of the Bernoulli constant along their entire length.” Dolphin (t) 08:01, 3 April 2026 (UTC)

This topic has arisen before and been discussed in detail. AHumanIBelieve might like to read through:

1. Misunderstandings about the generation of lift
2. Oversimplification Dolphin (t) 12:16, 3 April 2026 (UTC)