Drag in Aviation – Induced Drag

Induced drag is a by product of  lift and is directly related to the angle of attack.But before we understand induced drag, we need to know that lift is produced by lower static pressure on the upper surface compared to surrounding atmosphere. So, there is a low static pressure on the upper surface of the airfoil as compared to the lower surface of the airfoil. The higher pressure on the lower surface tries to equalise the low pressure on the upper surface causing a span wise flow outwards or air towards the wingtip on the lower surface. The opposite flow takes place on the upper surface, which means the air moves towards the body of the aircraft.

At the trailing edge of the airfoil both flows meet, and forms vortices. Sheets of these vortices are formed along the trailing edge of the airfoil, both flows are twisting and moving rearwards. These vortices cause drag which is termed induced drag. Now, the strongest vortices are the wing-tip vortices. Like what happens at the trailing edge of the airfoil happens at the tip of the wing. The airflow underneath the wing spills around the wing-tip and forms a large, twisting vortex.

When the airfoil is producing high values of coefficient of lift ( which means a non dimensional measure of the lifting efficiency of a wing, lift co-efficient varies with angle of attack of camber. In this case the aircraft is in manoeuvres or at a low speed with high angles of attack), the pressure difference between the upper and lower surfaces of the wing is greatest. In this scenario very strong vortices are formed. In moist air, the pressure drop in the core of these vortices will cause condensation, this cause the vortices to be seen as visible vapour. These vortices are different from the high altitude phenomenon, those are vapour trails caused by condensation in the jet engine exhaust.

Its important to remember that induced drag reduces with increase in airspeed. Note down that we say it reduces, induced drag will be present at all times of flight except that its strongest and sometimes visible during landings and take-offs (nothing but low speed with high angles of attack ). These vortices have a tendency to move downwards or towards the ground, once at ground level they tend to move apart from each other and are influenced by the wind.

To reduce induced drag designers can take various steps:
1. High Aspect Ratio Wings – Aspect ratio is the measure of the wing span to the wing chord. It was discovered that long and narrow wings had less induced drag, compared to short and stubby wings , a long and narrow wing had smaller wing tips, thus weaker wingtip vortices, less induced down wash and therefore less induced drag. Unfortunately long and narrow wings are harder to build.
2. Washout - Is a wing with an inbuilt twist, where the angle of incidence reduces from wing root to wing tip. This design comes with benefits -
a.less induced drag
b.lower bending load on the wing root as most lift is generated by the inboard section and also allows the wing to be             tapered, thus reducing weight.
c.greater control and more docile stall.In the stall, the inboard section will be stalled while the outboard section                  and ailerons will function normally.

3. Tapered Wings – A tapered wing has less induced drag because of its smaller wing tip. The aspect ratio is the most important factor.
4. Wing Tip Design – Designs such as wing let, wing fence , modified wing tip , wing tip tanks, etc all reduce induced drag.

Lets look at conditions where induced drag is increased :
High Angles of Attack – A high angle of attack causes greater pressure difference over the upper and lower surface of the wing, therefore causing strong vortices formed.The greater pressure changes also leads to more down wash over the trailing edge of the wing. It can be said that induced drag varies as a function of angle of attack. Any time the aircraft is at high angles of attack, high induced drag is created.

High Lift Production – A heavy plane requires more lift to fly straight and level , a manoeuvring plane requires greater lift. A high lift condition relates to greater pressure difference and therefore increasing induced drag.
Ground Effect – We know the basics of  lift generation, which is basically the pressure difference on the upper and lower surface of the aerofoil.The higher pressure is on the lower surface of the wing while the lower pressure is on the upper surface. Now when close proximity to the ground ( around 1 to 1.5 times the length of the wingspan ), the air in this region provides whats called the cushion or the ram effect. Basically the wings  an aircraft close to the ground they produce a cushion of air, which helps support the aircraft.This is just the basic understanding required for now. We will describe in detail what ground effect. For now, how does it effect induced drag? It decreases induced drag