Thursday, September 14, 2017

Aeronautical Terms: Drag

Drag is one of the four fundamental forces affecting aircraft in flight, with the other being lift, thrust and weight. It is the force that opposes the movement of a body through a fluid.

Drag is a force that has to be considered in all objects, flying or not. Even in sports, drag is an important consideration. However, this post will be focusing only on drag as experienced by aircraft.

Types of Drag

There are many types of drag that affect aircraft. They include:
  1. Parasitic drag, which consists of form drag, skin friction and interference drag.
  2. Lift-induced drag
  3. Wave drag
Out of these, wave drag is only experienced by aircraft that are flying at transonic or supersonic speeds.

Parasitic Drag
Parasitic drag is the drag a body feels as a result of moving through some fluid. This drag is caused by the force of fluid particles impacting the surface of a body. It thus increases with increases in speed. Parasitic drag consists of three parts: form drag, skin friction drag and interference drag.

Form drag is the drag a body experiences because of its shape. Shapes with a large cross-section in the way of the flow will experience a higher drag. One can get a feel for this form of drag intuitively. A more streamlined body will pass through a fluid with less drag than a blocky, heavy one. This is why, to minimize form drag, bodies are shaped in streamlined ways.

Skin friction drag is the drag that arises from the friction of fluid particles against the surface of an object. It depends on the surface area of the object in question. As a body moves through the air, it causes a thin film of air to stick to it, much like how water sticks to your body after a shower. This layer of stuck air causes the layer above it to stick as well, and so on, until the body is dragging a layer of air around with it. The force required to overcome this is known as the skin friction drag. One way to reduce skin drag is to make the body longer and thinner. The length of an aircraft divided by its width is known as the fineness ratio, and increasing this decreases skin drag. For subsonic aircraft, it is usually around 6:1.

Image result for wing body fairing
Interference drag is the drag that is caused when two or more streams of air interfere with each other. It is caused when airflow from different parts of an airplane interact with each other. When two streams interact, there is an increase in turbulence and thus drag. For example, air that is going around the wing may interfere with air that is going around the fuselage, increasing drag. Interference drag is detrimental to all aircraft, and designers keep this in mind when building them. For example, one way to combat this form of drag is to introduce a wing-body fairing, which smooths the point of contact of the streams of air from the wing and fuselage. (Above right): The wing-body fairing of an airplane.

Lift Induced Drag
The process by which wings generate lift also generates drag. This drag is known as the lift-induced drag of a wing.

Lift is accompanied by different air pressure on the top and bottom of a wing. This difference is pressure allows air at the bottom of the wing near the tips to flow to the top of the wing. This happens on both wings of an aircraft, and creates two counter-rotating vortices on either wing-tip, which create eddies of air behind the wing itself. These vortices can sometimes be seen if there is a lot of water in the air; a thin stream of water will appear to be coming out from the wing tip. (Above right): The direction of the wing-tip vortices are indicated by the two grey arrows at the ends of the wings.

These vortices reduce the amount of lift which can be generated by the wing at that angle of attack. They also introduce a force in the downstream direction which is known as the induced drag.

Induced drag would not be produced by a wing of infinite length as there would be no wing tip for air to flow around. However, as wings cannot be infinitely long in real life, lift-induced drag exists and must be dealt with. It is added to parasitic drag as another form of drag on an airplane. 

(Left): Water which has condensed due to the low pressure-low temperature vortices created by air moving from under the wing to over the wing.

Wave Drag
Image result for sound barrier
Wave drag is drag that is experienced by aircraft that are moving at transonic and supersonic speeds. It is most commonly seen as a sudden increase in drag as an aircraft approaches the sound barrier. It is caused by shock waves that form around a body that approached supersonic speeds. The increase in drag experienced by aircraft attempting to break the sound barrier are so dramatic that, before the advent of supersonic flight, many believed that it would be impossible to even fly faster than the speed of sound. However, in modern times, many methods have been developed to negate wave drag as much as possible, although they will not be discussed here. (Above right): An incredible picture of an aircraft breaking the sound barrier.

The Power Curve

In aerodynamics, the power curve is a plot that shows the total parasitic drag and lift-induced drag vs. the airspeed. (Left): The power curve for an airplane.

As you can see from the graph, there is a particular velocity at which the drag is the least. This speed for powered aircraft is the speed at which the plane will achieve the greatest range, and is also the speed where it is flying at optimal efficiency. 

This particular curve does not include wave drag as it refers to a plane which flies only at subsonic speeds.

One point to take away form this curve is that, until the optimal velocity, it actually takes more thrust to maintain a lower speed, and it is not until you pass the optimal speed that you actually need more thrust to go faster.

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