We’ve been lifted and dragged into the Dark Side (like what we did there?) in our Aerodynamics for Dummies series. Now it’s time to join those forces together. Welcome Glide Ratio. How does this affect our canopies? And why can’t our geniuses give us a fixed number? Once again, we asked our Head Designer, Julien, to explain.
As covered in our previous article, the significance of Drag in our canopies has been well-established. First inventors were all about increasing Drag to slow our descent to the ground. Whilst Lift is really just here for the fun times. Nowadays, as our wings evolve, people seek more from them. Not surprisingly, the goal to achieve faster landings has turned into a worldwide contest. Even if it means potentially meeting the ground face-first.
Of course, this desire for improved performance has meant parachute manufacturers have had to look for new ways to meet this demand. Enter Glide Ratio (GR).
This metric encompasses two aspects:
- the balance of forces between Lift and Drag (GR = L/D, for example, GR = 5 indicates lift being 5 times greater than the Drag), and
- how far the canopy can fly horizontally compared to its vertical descent (GR = Distance travelled forward/Distance travelled downward, e.g., GR is 5 if you cover 500m forward from a 100m altitude)
These two definitions and concepts are very different at first sight. However, they end up being the exact same number when the canopy is flying in a straight and balanced flight. This is thanks to the magic of physics and geometry.
These ratios are describing the aspect ratio of the rectangles in the figure below.
Now for the science part
Physics comes first. If the canopy is well designed and is stable, it’s going to find the position where the aerodynamic force is going to balance the weight. Weight being vertical (downward), the aerodynamic force is also going to be vertical (upward).
And then comes the geometry. By definition, Drag aligns with the trajectory, and the aerodynamic force aligns with the vertical distance. Consequently, the angles match, leading to both rectangles (see diagrams) having the same aspect ratio. And this aspect ratio is none other than Glide Ratio. Ohhhhhhhhh (mind blown).
To improve Glide Ratio, designers have two choices: increase Lift or decrease Drag. However, there’s a problem. Lift and Speed work together to counterbalance the pilot’s weight (Speed² = Weight/LiftCoefficient). If Lift is too efficient, Speed doesn’t have much work to do, so we end up with a slow paraglider. Good for paragliders, not so much for skydive canopy pilots. That’s why canopy designers tend to maintain a reasonable Lift and focus on optimising Drag.
So the challenge is to move a large volume of air downward to generate lift without creating vortices in the wake and wasting excess energy. That would actually be possible in a non-viscous fluid. Unfortunately for us, it only exists in aerodynamicist fairy tales. In fact, viscosity is the way movement is transmitted from a particle to another (just like conduction for heat). So since the canopy is moving a few quadrillion particles, it is likely to create a big mess in its wake. Fun times.
So Glide Ratio is pretty important to our canopy’s performance. But what about that Golden Number? And how does this apply to JYRO canopies? We’ve got more to tell about Glide Ratio in our next episode of Aerodynamics for Dummies…