Aerodynamics for Dummies is back! Our second lesson from Professor Peelman is taking on the Dark Side of the Force (feel free to read that in a Darth Vader voice). Drag, friends, is truly on the dark side. But why? Julien explains more below. It seems it’s mostly to do with avoiding massive craters in the ground…
Drag, the Dark Side of the Force
To understand the world, people need to separate the good from the bad. Draw a line between the Light side and the Dark side. Aerodynamicists are people too… So they decided to split the Force generated by wings between Lift (perpendicular to the relative wind, makes our canopies fly) and Drag (parallel to the relative wind, slows them down). This is totally artificial, but satisfies the human mind more than any other way of thinking. However, even though Drag is usually seen as Lift’s evil sister, it can be very useful to us skydivers. Let’s see how!
In fact, Drag is the last step of an energetic transfer chain that started millions of years ago when oil was created by huge compression forces under the surface of the Earth. This oil is chemical energy – about 35.000 kiloJoules (kJ) per litre released when it explodes. It is turned into mechanical energy by the turbine of the Porter, then into speed (kinetic energy) by the propeller and finally altitude (potential energy) by the wings.
A lot energy is lost on the way through heat, noise and other useless things but at the exit, at 13000 ft (4000m) each jumper is loaded with about 3300kJ. 3200kJ of potential energy due to its distance from the ground and 100kJ of kinetic energy coming from the speed the plane is travelling at. If you look into the eyes of a tandem passenger when the door opens, you can tell that they instinctively know that this energy is enough to dig a big hole into the ground. And indeed, it is equivalent to about 8kg of explosives. So yep, a very big hole.
How do we avoid causing those great big craters?
Basically, we need to find an outlet for this energy. Whatever way we choose to fly, ultimately we share the same goal. Dissipate most of it before the ground – otherwise it will be released all at once in an ugly manner! Wingsuiters will dissolve it slowly and efficiently (20kJ per second), even when landing far away from the PLA. Where as freeflyers will burn it really quickly (70kJ per second) and transfer most of it to the air in their wake. However, at 3300 ft (1000m), right before opening, a jumper is still loaded with about 1000 kJ. Which means they need to find a better way to waste this energy.
The opening in itself burns roughly 150kJ, but 850kJ is still potentially enough to land at 500km/hr! That’s more than twice the average speed of freefall. Here is where the Drag comes in! Thanks to viscosity, the canopy will produce vortices in its wake that will help us transfer this energy to the air. The big vortices are then broken into smaller ones, which break into smaller ones themselves and so on. Finally, at the end of the process, it turns into molecular movement that is also called heat. This process is called the Kolmogorov cascade of Energy after the Russian mathematician who put it in equation in the 40’s but the English mathematician Lewis Fry Richardson described this theory in a more poetic form 20 years before when he wrote:
“Big whirls have little whirls that feed on their velocity, and little whirls have lesser whirls and so on to viscosity.”
And that’s the story of how drag finally saved your ass!
Simply by dissipating the energy of a few litres of petrol into heat, we master the most complex processes of energy transfer without even thinking about it. Of course, we all want to stay in the air longer and glide further. However, it turns out having some drag is useful when it comes to dissipating energy in a controlled way.
Tune in next time for our third lesson from the Prof – Glide Ratio. There’s a golden number…