Why is lift perpendicular to the wing?

[FactChecker]

Well, if you look at the drawing where freestream velocity is along the plane of the drawing, then drag is exactly along the freestream velocity, because drag is the only force that does work.

Are you assume straight and level flight? Otherwise, lift is doing work.

Lift is simply the remaining component which means that it has to be at 90 degrees to freestream velocity.

So you are assuming the lift and drag are orthogonal. That is how the convention defines it, but it is just a convention.

 

[russ_watters]

well, perpendicular to the relative wind is not necessarily upward.

No, but straight and level flight is the most basic and common flight situation. Every other situation is an added complexity.

I suspected it might be as simple as an arbitrary definition. But why define it that way? What is the advantage of defining it that way?
For example, it could have just as easily been defined as 10 degrees aft of perpendicular. So why use 90 degrees. Why not 90 degrees to the chord rather than relative wind?

Well think about how it is actually used. Here's a basic question: how much lift is required for an airplane to fly straight and level? With the current definition, the answer is simply W: the weight of the plane. If you define "lift" to be perpendicular to the chord line, then the answer depends on angle of attack and to calculate it, you still need the vector opposite the weight vector to use in the calculation. Since that's a useful vector it should probably have a name of its own...

Now take, for example, load factor in a level turn. The graph of load factor is showing the additional lift required to keep the plane level at increasing angles of bank. It is based solely on the angle of bank. It doesn't vary with airspeed or angle of attack. But if you define lift to be perpendicular to the chord line, then in addition to the load factor calculation itself, you also have a similar lift vs angle of attack calculation to do.

Defining the lift vector a different way makes for more work in many common situations. Can you think of a common aviation situation where a different definition such as perpendicular to the chord line can be used "just as easily" (or easier) than the current one?

 

[A.T.]

Knowing the Physics of the things we use everyday often doesn’t affect how well we use them.

Yeah, I have to keep telling me that, every time I see alleged long time pilots arguing for things like the downwind turn myth (basically against Galilean invariance).

 

[russ_watters]

Yeah, I have to keep telling me that, every time I see alleged long time pilots arguing for things like the downwind turn myth (basically against Galilean invariance).

[googles] wow, never heard of that one. That could actually be dangerous if extrapolated to the other legs.

I'm learning to fly right now and my first CFI (mid-20s, recent grad of a major college flight program) invoked the "equal transit time" myth when explaining how airplanes fly. That was disappointing. He's an airline pilot now. Great CFI.

 

[rcgldr]

Well, if you look at the drawing where freestream velocity is along the plane of the drawing, then drag is exactly along the freestream velocity, because drag is the only force that does work. Lift is simply the remaining component which means that it has to be at 90 degrees to freestream velocity. Anywhere along the plane perpendicular to freestream velocity - including out of drawing plane.

Using the wing as a frame of reference with the x-axis parallel to the freestream velocity, the freestream velocity is diverted downwards, and during the interaction related to lift, the wing exerts a downwards force onto the air (from both above and below), with a component of that force in the same direction as the diverted stream, so the lift force is also doing work. Drag could be considered to be doing negative work, since it decreases the freestream velocity (wrt wing). The net work done would be related to the net change in freestream velocity (wrt 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.