Aerodynamic forces acting on a wing.

 Aerodynamic forces acting on a wing.

While a lot has been discovered about why a paraglider flies, a lot remains unknown too (To me, but i know people who know everything). 

Like every aircraft, a paraglider has its own uniqueness. It's made of cloth and lines with no hard structure. The pilot hangs far below the wing, like a swing. It can fly backwards commonly known as tailslide. It can do spiral dives facing the ground and can tumble, it can Helico(spin on its vertical axis) and can change the center of rotation from outside in a spiral dive to inside in a SAT.

Even the best engineers struggle to explain everything but fortunately there is a lot known too.

Firstly, we know the broad shape of an aerofoil. When that shape moves through air, some air flows over it and some under it. We named the point at which airflow splits as Stagnation point.

DIAGRAM 1 

Then we drew a line from the stagnation point to the trailing edge and called it Chord.

We now put some weight hanging below and viola! It floats around, moving forward through the air, gradually descending, bending the trailing edge on one side, it can turn to that side and we have a magical machine that can fly a person from the top of a hill to the bottom and the person can turn it in the direction he wants 

DIAGRAM 2

We see that it flies and now, man being man, wants to know why it flies and hence the unending search begins. To me, it's like the search for god, We can see it everywhere and yet we will never completely understand it. 

Now we have a wing, a stagnation point, a chord and a glide path through air. Now we apply some logic and say if the wing is flying in a particular direction then the airflow is in the opposite direction. So now we have a direction of Airflow. We had already named a line as Chord and now we had these two meet at an angle and since we are a violent species we decided to name the angle at which they meet as Angle of Attack. I have always wondered who or what we are attacking! We usually attack each other and maybe it has some deep meaning I am yet to discover. Many people who know everything about paragliding do attack me if I make the slightest error in defining AOA. Maybe that’s it.

Now that we have started naming things that we can’t see, lets name a few more

Obviously if the machine is not falling out of the sky, it must be creating Lift. A force which holds the weights up. It also seems to be flying slow so there must be something holding it back and that's a Drag but something that's thrusting it forward and that’s a Thrust. Let's just stop, we might just end up naming too many things and forget what is what.

DIAGRAM 3

 To now place everything we named into perspective, we need a new terrifying name, Vectors. At this point, we have some clarity, after you try to understand Vectors, you will have none. But I chose to explain and I will go forward with my swords and shields….Ah Attack.

Weight!! I hate it when I have to ask this question to my students. My scales are almost always wrong. But if I ask in which direction is your weight acting, most people know its downwards. So Vectors have a magnitude and a direction. So my weight is 62 kg and yes its downwards. If it was upwards, we would be explaining how humans can walk on earth. So if me and everything that I'm flying with including the weight of the wing are flying, the All Up Weight is held up by the wing and that force has to be opposite and equal to the weight. As we named everything that we can’t see, we name this Resultant Force but we now somehow decide to say that its not a real force, its the Vector addition of Lift and Drag. As if Vector was not bad enough, we are adding Vectors now. I should just stop now.

Now that we have the aerodynamic forces at work, we could play around and see what happens. What happens when we change our famous AOA, we know the magnitude and direction of all the forces, weight downward, resultant force upwards, lift 90 degrees to the airflow, drag opposite to the direction of movement and thrust in the direction of movement. Weight and resultant same in magnitude, we can find lift, drag, thrust at different AOAs.

DIAGRAM 4

If you are with me till now, Congratulations but now I'm gonna really go at it. Well, a hard winged aircraft with a tail wing can change the AOA but a paraglider has no tail wing, its a pendulum, you can change the AOA momentarily but the pendulum effect reverses it and you cannot have a paraglider fly at a very different AOA. When you apply brakes/controls simultaneously the glider does pitch up but only momentarily. In fact, if you apply too much brakes for too long the glider falls out of the sky. So, are the changes in AOA creating the changes in aerodynamic forces?

DIAGRAM 5

What we do know is that the amount and the rate at which we apply the brakes/controls affects how a paraglider flies, maybe it's something else. Drag? Airflow? Maybe but as I said, we will create more theories. 

The unified theory of flight - Next Topic. By Khushroo Pithawalla.