By Peter Pfalzgraf
In the early days of CReW (as canopy formation skydiving used to be called) I wrote and published a little booklet for those who were interested in learning the new sport. Things have changed a lot since then. So, I found it necessary to publish something new to make general information on our sport available to everybody interested. This essay combines techniques and hints from Europe and America. The latest input came from the training camps for the new canopy formation world record attempts aiming for a 100-way formation.
Canopy and Harness (Figure 1)
The times are gone where they had all-around canopies that could be used fairly well for any kind of skydive. Such canopies as the Cruisair, ¬U¬nit, Pegasus, Cruislite and Fury were once very common.
Today, the best parachutes for canopy formation parachuting (briefly known as “CF”, in the old days) are 7-cell canopies. These parachutes are the safest concerning deployment and stability in flight, especially in turbulent conditions. It is no coincidence that reserve parachutes and canopies for BASE jumping are mostly 7-cell canopies. Triathlons and Spectres with Dacron lines can be used for casual CF jumps, although Lightnings are the CF canopies of choice. If you intend to do a CF jump today you are well advised to use a canopy that has been designed for this kind of activity.
As a compromise for your first attempts, you may use a 7-cell canopy with a thick profile (for instance a student or accuracy canopy). Never attempt CF jumps with elliptical canopies, canopies with wide wing spans (in comparison with the depth of the profile/length in direction of flight) and/or a low profile.
Today’s CF canopies are available in different sizes for different weight classes. It is important that people intending to join a CF jump use the same type of canopy with the same line length and trim. Furthermore, the wing loading (weight under canopy per square foot) should be reasonably identical to guarantee similar flight characteristics.
The container of your harness should be big enough for the packing volume of the canopy to make sure it can be closed properly (safety!). There should be no handles, container flaps, pop-top pilot chutes or anything else sticking out that could get caught in another jumper’s lines. Those things could result in unintentional reserve openings or problems on separation. The bridle should be short or even better, self retracting to prevent your pilot chute from being caught in the other jumper’s lines while you are in a formation. You can imagine that separating a plane formation with one jumper’s pilot chute entangled in the other jumper’s lines will most certainly result in serious problems.
- The altimeter should be worn in a way that makes sure it does not get snagged and can always be seen while you have your hands in the toggles.
- The helmet should provide not only head protection but also allow good hearing. It should not cause wind noise that might affect your hearing.
- Every CF jumper should carry a hook knife that can be used in case of an entanglement or wrap. Sometimes it only takes one line to be cut to get free and save a reserve ride.
- The hook knife of course should also be worn in a way that prevents it from being caught or ripped away.
- Your shoes must not have any hooks. They should fit loose enough that you can get rid of them if need be in order to get free.
- It is very useful to wear long socks to protect your legs against bruises and line burns.
Aerodynamics of the Airfoil
Lift is the force that keeps a canopy in the air. Opposite to a round canopy that only creates a big air resistance to slow down your descent using a large area of fabric, the square parachute, or airfoil, really produces lift like the wing of a glider for instance. The square footage of an airfoil is approximately one-sixth to one-quarter of a round canopy, and yet has a far lower rate of descent.
The lift of the airfoil consists of 1/3 high pressure under the profile (similar to the round canopy) and 2/3 low pressure on the top surface created by the undisturbed airflow. This effect is the important matter for us.
The physical reasons for this effect aren’t pertinent to this particular discussion. What is most important is to know that the main part of the lift depends on the undisturbed airflow on the top surface of the square canopy, forward speed and the shape of the profile. The lift increases with forward speed and grows with the thickness of the profile. The air resistance of the canopy slows the forward speed down.
Due to the different pressure at the upper and lower surface of the profile air will flow around the sides of the canopy from the bottom to the top following the pressure difference. This effect reduces the lift and is called induced resistance. As a consequence of this, the maximum lift is in the centre of the airfoil. The stabilizers on either side of the profile are designed to reduce the loss of lift by hindering the airflow from the bottom to the top surface. Another way to reduce the loss of lift is to make the airfoil very wide so that the percentage of wing area being affected becomes comparably small. A good sample for this solution is the paraglider. This shape of airfoil is, of course, not good for CF activities.
Because of the airflow around the sides and also around the tail, a parachute gliding through the air leaves a track of turbulence. The side turbulence creates a zone of turbulent air on either side beginning at the trailing corners of the airfoil. The turbulence of the tail creates a turbulent zone rising from the trailing edge of the canopy.
It is clear that a parachute or canopy formation flying into such a turbulent zone will be affected by losing lift. Even aircraft flying through the turbulence of a canopy formation will lose considerable altitude.
Figures 2 and 3 show the shape and position of the zones of turbulence.
In a formation these phenomenon cause effects that can be noticed clearly. Just the top canopy in a formation gets only undisturbed airflow and has the maximum possible lift. All canopies having a body in front or on a front corner will lose lift due to the turbulence caused by the body out in front.
Aerodynamics of the basic Formation Types
Knowing the things mentioned above, we can predict that any type of formation cannot perform as well as a single canopy. That is useful for building formations as it gives the single canopy the potential to approach and dock on a formation due to its extra lift and forward speed.
In principle all formations consist of a few basic types. Let’s look at the conditions in these.
The stack (Figure 4) is the most basic formation. To build a stack one jumper sits on top the other jumper’s center cell and hooks his feet behind the center A-lines of the lower canopy similar to sitting on a chair. There will be some tension on the lines because the lower canopy in this type of formation will have a little less lift. In a stack the body of the top jumper is in front of the center cell of the lower canopy. The turbulence of that body meets the airflow of the lower canopy exactly at the point where the main lift is created and leads to a considerable loss of lift. This means that a stack will sink far more than a single canopy.
The plane (Figure 5) is created out of the stack. The top jumper climbs down the center A-lines and hooks his feet behind the front risers of the lower jumper underneath the slider. The lower jumper supports him by putting on some brakes to increase the lift of his canopy. For this formation type one or two cross connectors are required, which either connect the front connector links (one line) or two lines that connect the front and rear connector link on either side. Otherwise the top jumper might slide back up due to his plus of lift and pull up the slider, which would result in a collapsed lower canopy.
In a plane formation, the turbulence/loss of lift effect is not as strong. As the top jumper’s body is beneath the two canopies, one might think there is no loss of lift at all, yet the line contact causes deformation in the lower canopy and additionally forces both canopies into a new aspect ratio. This leads to a loss of performance. A single canopy will perform better than a biplane.
If a plane becomes bigger there will of course, be canopies with bodies in front of them. Planes with more than four canopies will sink faster as the plane formation grows larger.
In a stairstep formation (Figure 6) the top jumper is positioned outside of the lower canopies end cell. He takes a foot grip on the outside A-line. Flying the stair step requires much more experience than flying a stack or plane formation because the link is not as stable. It is more of a pivot point than a stable connection. The lower jumper has to compensate for the influence of the upper jumper’s body to the flight of his canopy. To prevent the lower canopy from coming up and around you can put some tension on the outside front riser of the lower canopy or the lower jumper can stretch the outside leg while lifting the inside leg.
The stair-step is far less turbulent than the stack and plane. The body of the top jumper only causes turbulence on the outside corner of the lower canopy and that is a part of the airfoil that contributes only a minor amount of lift. That means that a stair step formation is only slightly less efficient than a single canopy.
Because only one side of the lower canopy is affected, the other side will perform better and cause the canopy to rise and drive forward. If the lower jumper does not compensate for this, his canopy might come up and around, leading to a wrap. To prevent that the lower jumper will start to compensate for the difference as soon as the top jumper has taken his grip. Possible ways to compensate include putting tension on the outside front riser and stretching the outside leg while lifting the inside leg. One might also apply some slight inside brake.
You can determine how much trim is needed by easing up on the risers or brakes after the canopy is set. Sometimes the formation will fly well with only slight trim or none at all. Nevertheless, the lower jumper should keep a watchful eye on his canopy to immediately compensate, if necessary.
That wraps up part one. Join Peter in a week or so for Part 2, when we'll dive deep into building and controlling these formations.