To understand the concept let us understand a few basics:

Every aircraft, whether an airplane, helicopter or rocket, has four forces acting on it:

  1. Thrust: Thrust is produced by an aircraft’s propulsion system or engine. The direction of the thrust dictates the direction in which the aircraft will move.
  2. Lift: Lift is generated by the motion of air passing over the aircraft’s wings. The direction of lift is always perpendicular to the flight direction. 
  3. Drag: Drag is simply resistance of the aircraft against the air. Drag acts in the opposite direction to thrust.
  4. Weight: The magnitude of the weight is the sum of all the airplane parts, plus the fuel, passengers, crew and cargo. While the weight is distributed throughout the entire airplane, its effect is on a single point called the center of gravity.

Actual Lift Vector

Actual Lift Vector is a vector quantity responsible for effective lift of the aircraft which is actually less than the original lift.

Controlling the Motion of Flight:

The following are the flight controls to control the forces acting on the aircraft.

  1. Elevator for Pitch control
  2. Ailerons for Roll Control
  3. Rudder for Yaw Control

Now to understand why aircraft wings are tilted we need to understand roll control.

The ailerons that are located at the trailing edge of the wing, one on each side are used to control roll of the aircraft. They are located opposite to each other, i.e. when one is raised, the other is lowered, thus increasing the lift on one wing, while reducing the lift on the other. By doing this, they roll the aircraft sideways, which allows the aircraft to turn.

Different kinds of wing locations (with no slope):

High Wings: Wings placed the above the fuselage of the aircraft

Mid Wings: Wings placed by the side of the fuselage of the aircraft

Low Wings: Wings placed below the fuselage of the aircraft

Now what is Dihedral?

Dihedral is where the wing slopes in a positive (upward) degree in relation to the wing base.

So why is dihedral required?

If a plane is banked, the lift vector acting upwards (normal vector, in case of wings with no lilt) also tilts with the wing thus the aircraft starts slipping in the direction the aircraft is tilted. So straight wing will provide no effective force to bring the aircraft back in position which is undesirable for passenger jets. Thus Dihedral comes into play.

As in the diagram, the aircraft is banked towards the left and due to dihedral configuration off the wing the area of the wing that comes in direct contact with the wing is more for the left wing than that for the right wing. Thus the left wing produces more lift than the right wing, thus restoring back the position of the aircraft.

So, by giving an upward tilt to the aircraft wings (Dihedral Angle), the aircraft roll stability increases and this helps the aircraft to gain back normal position or lateral stability quickly without much efforts from the pilots.

And why Anhedral wings are required?

As we all have seen Antonov 225, and Lockheed galaxy C17, or some of the fighter jets example AV- 8B harrier II all these aircraft need high roll performance. So this is achieved by tilting the wings downwards from the horizontal plane (Anhedral Angle). This makes the aircraft slightly unstable for normal cruising thus increasing the roll performance and giving great roll maneuverability. This configuration is basically seen in aircrafts like An 225-heavy, Harrier II-fighter jet.

In a Nutshell:

Both type of wings whether Dihedral or Anhedral are introduced in the aircraft wing structure for better maneuvering performance during “roll”. But they come at a cost of reduced vertical and lateral stability (in an acceptable range).

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