Why is lift perpendicular to the wing?

[hazza4257]

I think a source of some confusion is what exactly "lift" means. Lift is only one component of the force exerted by the aerofoil - the one that acts perpendicular to relative airflow. Drag is the other component, and acts parallel to (and in the same direction as) relative airflow. The combination of these two components will obviously be a line that is pointing up but angled back towards the leading edge, to be roughly perpendicular to the wing surface itself.

tldr: Lift and drag are just theoretical forces and are defined with comparison to relative airflow. They sum to make the actual force exerted by the wing.

 

[sophiecentaur]

Lift and drag are just theoretical forces

They are just components of the force on the wing. We nearly always resolve a force into two arbitrary orthogonal directions and choose the directions to be convenient for understanding a mechanical problem. 'Up and Back' are very handy directions to calculate the forces on an aircraft.

 

[snorkack]

They are just components of the force on the wing. We nearly always resolve a force into two arbitrary orthogonal directions and choose the directions to be convenient for understanding a mechanical problem. 'Up and Back' are very handy directions to calculate the forces on an aircraft.

And space has 3 dimensions. Which means that a force cannot be resolved into two components of given and orthogonal directions. You can resolve a force into a component in a given direction and a free direction orthogonal to the first.

 

[sophiecentaur]

And space has 3 dimensions. Which means that a force cannot be resolved into two components of given and orthogonal directions. You can resolve a force into a component in a given direction and a free direction orthogonal to the first.

That's a bit nitpicky. Basic theory considers just two dimensions. Most planes are symmetrical about a central plane and the y dimension effects will cancel out - at least at the level of this thread, in which the basic principle of resolving vectors is where we're at. The only aircraft I know of that aren't symmetrical are the model ones they used to fly on a control line.

 

[snorkack]

That's a bit nitpicky. Basic theory considers just two dimensions. Most planes are symmetrical about a central plane and the y dimension effects will cancel out - at least at the level of this thread, in which the basic principle of resolving vectors is where we're at. The only aircraft I know of that aren't symmetrical are the model ones they used to fly on a control line.

An airplane is symmetrical when the rudder is central, but stops being symmetrical when the rudder is deflected. So when a plane turns with wings rolled out of horizontal, is "lift" defined as the component of aerodynamic force that follows the direction opposite gravity, or as the component of aerodynamic force that tilts with the central plane of the plane, perpendicular to the wing?

 

[hutchphd]

When I use a word, Humpty Dumpty said in rather a scornful tone, ‘it means just what I choose it to mean — neither more nor less.’

’The question is,’ said Alice, ‘whether you can make words mean so many different things.’

’The question is,’ said Humpty Dumpty, ‘which is to be master — that’s all.”

Lewis Carroll

 

[Lnewqban]

Lift and drag are just theoretical forces and are defined with comparison to relative airflow. They sum to make the actual force exerted by the wing.

Welcome, @hazza4257 !

We could also state that that actual force exerted by the wing is the summation of many little forces distributed very differently over the surface of the wing, according to the velocity and pressure distribution created by certain conditions, especially AOA.

Please, see:
https://www.mh-aerotools.de/airfoils/velocitydistributions.htm

 

[sophiecentaur]

An airplane is symmetrical when the rudder is central, but stops being symmetrical when the rudder is deflected. So when a plane turns with wings rolled out of horizontal, is "lift" defined as the component of aerodynamic force that follows the direction opposite gravity, or as the component of aerodynamic force that tilts with the central plane of the plane, perpendicular to the wing?

True but we have to learn to walk before we can walk (or fly). Even 42 PF posts aren't sufficient to learn all of aerodynamics. Softly softly catchee monkey.

 

[A.T.]

Lift and drag are just theoretical forces and are defined with comparison to relative airflow.

Exactly, it's just a useful decomposition for analyzing flight characteristics. For other purposes, like designing the loaded structure of the wing, other decompositions might be more useful, like normal (N) and axial (A) forces.

 

[A.T.]

We could also state that that actual force exerted by the wing is the summation of many little forces distributed very differently over the surface of the wing, according to the velocity and pressure distribution created by certain conditions, especially AOA.

Note that this image of relative pressures can be misleading in the context of talking about "many little forces". The actual forces by the outside air are towards the wing surface everywhere (positive absolute pressure).

 

[FactChecker]

There is one force vector that can be decomposed into components in a variety of axis systems. People in aerodynamics often like to work in the wind axis system and their coefficient of lift is up in that axis. That certainly makes wind tunnel measurements easier to convert to coefficients. People in structures would work with the forces in the body axis and lift would probably be in that axis. People in stability and control would work in the locally-level Earth axis and could prefer to have lift as up in that axis.
The aerodynamics people are the source of the aerodynamics coefficients and anyone who wants to work in another axis system would need to convert to that system.