A sail is not a wing

Gliders normally have negative angle of incidence. The angle of attack is always neutral to positive glider or plane. Angle of attack in a fluid is useless when talking about a vehicle if you don't relate it to the vehicle. By your definition of aoa being related between free fluid flow and the horizon, a plane in still air has 0 angle of attack.

 

Do you measure drag parallel to the chord and lift perpendicular ?

 

No. Because air does not always move horizontal and neither do planes.

 

In that case, the foil is the frame of reference or the air?

 

The reference line is the chord . The chord is defined as the line between the leading and trailing edge. If the trailing edge is brought downwards the chord line points further up. This is exactly how flaps work. Increase the angle of attack which increases lift and drag whic moves the lift line aft of perpendicular to the centerline slowing the aircraft.

 

I fly gliders rc and full size shaped like kites.

 

No . Parallel and perpendicular to the airflow direction around the body.

 

Foil and air are the frame of reference for angle of attack. Air is the frame of reference for lift and drag which act on the foil.

 

Sorry, not sorry, really had to catch up to everyone.

 

If foil and air are moving with respect to each other, only one can be the frame of reference.

 

Lift and drag are arbitrary but useful decomposition vectors of the overall aerodynamic pressure acting on the body caused by fluid flow.

Angle of attack is just an angle. In the reference point of the chord the air is attacking at let's say 15 degrees. In the reference frame of the air the wing is attacking it at 15 degrees . But all we are doing is measuring the angle between them.

This is off point but I'll say it anyway. A thermal or ridge lift or anywhere there is wind the air has an angle of attack in relation to the ground or horizontal.

Excuse my hand writing.

 

I understand is arbitrary. However, if we don't use the same frame of reference the discussion will never make sense. In the diagram, lift is perpendicular to the chord, so it is not necessarily vertical. With that frame of reference, gravity changes angle depending on air movement and angle of attack. Without the force created by gravity, the foil would lift to infinity.

 

The boom and air flow are the frame of reference for the upper side of a boat. Even in the diagram lift isn't perpendicular to chord its perpendicular to airflow. Lift is only vertical if airflow relative to the body is horizontal like a powered plane in level flight or in rarer cases a glider flying at angle of incidence above horizontal which matches the normal gliding angle of attack. But this is unsustainable as there is no thrust , no foreward component of lift , drag now pulling horizontally backwards . Only momentum, for now.

 

And too with the same frame of reference on a boat. If you change your heading or sheeting the leeway angle changes with regards to the water. Gravity always pushes down. But not always the same direction related to the top and bottom of a plane which can have any roll pitch or yaw. Water always pushes on the hull and keel. But not always in the same direction related to the left and right of a boat which can have any heading ,heel or pitch.

 

The angle between a plane centerline and gravity is the angle of incidence plus 90degrees. The angle between plane centerline and horizontal is angle of incidence. Angle of gravity changes with respect to plane centerline when angle of incidence is changed. Down is pretty much always defined as perpendicular to horizontal, parallel and in the direction of gravity. Forwards is a little harder to define for a vehicle moving in 3d. But could be taken as horizontal or the centerline.