Dihedral & Anhedral Aircraft Wings

Image from: - Wikimedia

In a case of low wing Aircraft, such as Boeing 747, wings are angled upward while in a case of high wing aircraft, such as Antonov An225, wings are angled downwards.

But why are wings angled?

Upward angled wings of low wing Aircraft are known as dihedral wings & Downward angled wings of high wing aircraft are known as anhedral wings.

Need of Dihedral wings

Suppose wings of low wing aircraft are straight without any angle. So when Aircraft is cruising, Lift force will act perpendicular to the relative upcoming airflow. So if Aircraft rolls, the direction of lift force on each wing will change with aircraft. Suppose aircraft rolls to left ( Anti-Clockwise), So the direction of lift force on each wing will also tilt to the left at some angle.
So now there are two components lift force. One in vertical direction & other in a horizontal left direction. The magnitude of components of lift force will be same on each wing of the aircraft.
So Due to the horizontal component of lift force, aircraft will slip towards left. And Aircraft will become unstable.

How do dihedral wings work?

Image from: -Wikimedia

In a case of dihedral wings, at the time of cruising direction of lift force on each wing is already tilted towards aircraft as wings are angled in the upward direction.
Now suppose again if aircraft rolls to left(Anti-Clockwise direction), lift force on each wing will tilt to the left at some angle. Here, the left wing of aircraft is lower wing & right wing is the upper wing. If we find components of lift force on each wing, the magnitude of the vertical component of lift on left wing will be higher than the magnitude of the vertical component of lift force on the right wing as the lower left wing has a higher angle of attack than the higher right wing.

Image from: - Wikimedia

This difference in lift force on aircraft wings will produce a moment in opposite direction. This prevents Aircraft from slipping & stabilizes aircraft.

Anhedral Wings

Image from: - Wikimedia

In a case of the high wing aircraft, The Anhedral angle is given to move the center of gravity of an aircraft to the bottom. So that such aircraft have good rolling stability.

Image From: - Wikimedia

Remember Center of gravity doll (Roly-poly toy)? Even such dolls have a center of gravity at the bottom. Same principle as of this doll is the reason for the better stability of high wing aircraft.