Air Foil

Aerodynamics

CF-Aspects of Aerodynamics in Larger Formations

It’s all about wing loading

A ram air parachute like the PD Lightning or the Triathlon for instance is an air foil. As we will see, the aerodynamics of a ram air parachute are quite complex.

The glide angle of such an air foil is invariant as long as the drag does not change. That means, if you look at a canopy of a certain size, the glide angle will not change as a function of the wing loading.

The forward speed on the other hand grows with the wing load! This applies to all types and sizes of ram air parachutes and air foils.

If we look at different sizes of the same type of canopy however, we will see that the performance is gaining with the size of the canopy and will decline with the drag.

  1. The parasite drag. That is the drag mainly caused by the canopy front, the lines and the body of the jumper.
  2. The induced drag. That is, the loss of lift caused by the air bypassing the sides of the canopy (vortex) following the pressure difference at the air foil (higher pressure at the lower surface and lower pressure at the top surface).
Air Foil
Air Foil

The relative parasite drag is changing with the size of the canopy. While the surface area of the canopy (causing the lift) is measured length by width, the added sides of the canopy (causing the parasite drag) are only twice the length, the lift is growing faster than the parasite drag. That means that the overall performance of a canopy is getting better the bigger the canopy is or the other way round: smaller canopies perform weaker than big ones. On the other hand, smaller canopies are usually jumped by smaller persons and big canopies by taller persons. This means that part of the advantage of bigger canopies versus smaller ones is being eliminated by the parasite drag of the jumpers body. However, a slim and tall person will experience more parasite drag than a rather ball-shaped person with the same weight under the same size canopy.

Knowing all this, we want the biggest canopies on top of a big diamond and heavy wing loadings along the centre line and small canopies with little wing loading on the free wings. In a free wing position it is easier to dock with a rather small wing loading because that canopy will not fly faster than the formation so that it is less likely to come around before it will be locked.

Ther PETi

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