Video #8: Aircraft Drag

An explanation of Induced Drag and Parasitic (form and skin) Drag. It is really, really exciting.

I certainly hope you find this enthrallingly entertaining.

Script (with weird spellings and lots of comma’s to make the helpful American gentleman narrator speak properly!)

Having completed the previous video, I realised that I was tossing around terms such as ‘induced drag’ and ‘aeroplane’ without having really discussed what these are. I certainly hope that this oversight on my part, did not reduce you to a confused, weeping mass.

Drag in an aircraft is made of two components. Parasite Drag and Induced drag.

Parasite drag is not caused by a giant tic on your aircraft, although that would be pretty exciting, even if it did throw your weight and balance plans out of the window. Parasite drag is actually mainly made of skin friction drag, and form drag.

Let’s look at skin friction drag first. This is caused by the skin of the aircraft accelerating air particles in contact with it. These then in turn accelerate air particles in contact with them to a lesser extent, which accelerate air particles in contact with them to an even lesser extent, ad finitum. Accelerating trillions of tiny particles to the same speed as the aircraft is going to mean that some resistance is going to be felt, and resistance created in this way is known as skin drag.

Next, Form drag. When an air flows around an aircraft, it is going to change in speed and direction, and this also requires energy to be expended. You can think of this as how aerodynamic something is. Having a nice streamlined object, like the magnificantly designed british comet, will minimise this form of drag. It is generally considered that a disregard of form drag was the main cause of failure with this ilyushin aircraft.

Parasite drag is affected by the speed of the aircraft through the air, the configuration of the aircraft, like whether flaps or the wonderfully aerodynamic landing gear are deployed, and whether the airframe has any contamination on it, such as mud, ice, or snow.

Induced drag is caused by wingtip vortices. These are caused by the difference in air pressure below and above the wing, as you will remember from the creation of lift video number 2. At the tips of the wing, the higher pressured air below the wing can go around the tip, to satisfy it’s insatiable urge to get with the lower pressure air on top of it. This creates a circular motion of air at the tips of the wings, clockwise on the left, anti-clockwise on the right.

The vortices that this action creates affects the air around it. At the root of the wing, where vortex action is not felt too much, airflow is like this. However, at the wing tips, where there is a lot of the whole vortex action thing going on, airflow is altered to become like this. As you can see, the air is forced upwards in front of the wing, and then forced downwards behind it. This alteration of the air then means that the lift generated by the wing is slightly deflected backwards. This lift force going backwards now adds to the drag force, and is the induced drag of the aircraft.

The induced drag is affected by the lift being generated by the wing. The heavier the aircraft, or the tighter the turn, the more lift the wing is having to provide, the greater the pressure differential is on either side of the wing, and the greater the strength of the vortices. As I said in the previous video, more lift means more induced drag.

It is also affected by the speed of the aircraft. Higher speed reduces the downwash effect, and therefore the rearward deflection of the lift force.

Wings that are long and thin are also good for reducing induced drag, as less wing is covered by the vortex affected air. However, they are not so good are reducing parasite drag, so a happy medium has to be found.

I hope this has given you a better glimpse into this draggy world.

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