Dihedral is the upward angle of an aircraft's wings when viewed from the front. It matters because it helps many airplanes naturally resist unwanted rolling after a gust, turn, or sideslip. This self-righting tendency is called roll stability, and it makes an aircraft easier to control.
The idea is central to trainer aircraft, gliders, airliners, and many stable designs.
Key Facts
- Dihedral angle is the upward wing angle measured from the horizontal when viewed from the front.
- Positive dihedral produces a stabilizing rolling moment during sideslip.
- Anhedral is a downward wing angle and usually reduces roll stability or adds roll responsiveness.
- Rolling moment can be written as L_roll = q S b Cl, where q is dynamic pressure, S is wing area, b is wingspan, and Cl is rolling moment coefficient.
- Dynamic pressure is q = 1/2 rho v^2.
- A stable dihedral effect means a sideslip creates a rolling moment that tends to level the wings.
Vocabulary
- Dihedral
- Dihedral is the upward angle of the wings from the horizontal when an aircraft is viewed from the front.
- Anhedral
- Anhedral is the downward angle of the wings from the horizontal when an aircraft is viewed from the front.
- Sideslip
- Sideslip is motion where the aircraft has sideways airflow across it because its nose is not aligned with its path through the air.
- Rolling moment
- A rolling moment is a torque about the aircraft's nose-to-tail axis that tends to rotate one wing up and the other wing down.
- Roll stability
- Roll stability is the tendency of an aircraft to resist or correct unwanted banking motion.
Common Mistakes to Avoid
- Thinking dihedral lifts both wings equally, which is wrong because its stabilizing effect appears mainly during sideslip when the two wings meet the airflow differently.
- Confusing roll stability with turning ability, which is wrong because a stable aircraft can resist unwanted roll but still needs control inputs to turn.
- Assuming more dihedral is always better, which is wrong because too much dihedral can make the aircraft overly stable and less responsive to pilot control.
- Ignoring the vertical tail during sideslip, which is wrong because the tail helps create and control sideslip angles that interact with the wing dihedral effect.
Practice Questions
- 1 An aircraft wing has a dihedral angle of 6 degrees on each side. What is the included angle between the left and right wing panels when viewed from the front?
- 2 A small aircraft flies at 50 m/s in air with density 1.2 kg/m^3. Calculate the dynamic pressure using q = 1/2 rho v^2.
- 3 During a sideslip, the right wing is lower and the sideways airflow comes from the right. Explain why positive dihedral tends to roll the aircraft back toward level flight.