You and Your Canopy
by Jan Meyer
Jumpers come in all sizes and shapes. Munchkin jumpers weigh less than 100 pounds and are barely 5 feet tall. Bowling ball types, of stocky build, and come in short, average and extra tall models. Most jumpers fall into the average category. This is also known as the Goldilocks category: not too short, not too tall, not too fat, not too skinny, but just right.
Way back when, all jumpers used the same type and size parachute. It was usually a military surplus T-10, 28' flat or Navy conical. Jumpers could get extra performance from these parachutes with modifications; Tojo cut, double L, inverted TU, etc. Some jumpers found landing to be difficult, hard and physically taxing.
Today, equipment is made in as many types, shapes and sizes as there are jumpers. Each jumper can find several parachutes that work well for his size. (Note that your total weight should include your harness and container, reserve parachute, camera equipment, etc.)
Performance and Wing Loading
A parachute can be matched to a jumper by considering the ratio of a jumper's weight to the planform area of a parachute. This ratio is known as the wing loading. Wing loading is the average force per area that is exerted on the parachute surface.
The total upward force must be equal to your weight. Wing loading is calculated be dividing your weight by the canopy area. It should be between 0.6 and 1.0 for most people. The wing loading will be over 1.0 for high performance canopies.
Wing loading influences how a canopy flies. The most noticeable performance characteristics are
Full glide speed increase as wing loading increase. As you jump smaller parachutes, your forward and descent speeds will increase. You may experience ground rush on your first few jumps on an itty-bitty parachute.
Toggle pressures increase with wing loading. More weight under a parachute means that each suspension line carries more tension. This tension is the toggle pressure you feel when you steer and flare your parachute.
Front and rear riser pressures are not always equal. Some parachutes carry more of your weight through the front risers than the rear risers. This makes rear riser turns, stalls, flares and toggle pressures very light and easy. However, the front riser turns become increasingly more difficult to perform. Trim tabs may be significantly harder to use.
Canopy controls become more sensitive as wing loading increases. A 6-inch toggle travel will cause a faster response and turn rate on a smaller parachute that the same control on a larger parachute. This is equivalent to saying: a smaller control movement, say 6-inches of toggle travel, on a smaller parachute will produce the same response as a larger control movement, say 12 inches of toggle travel, on a larger parachute.
Hands off steering is using your legs to turn your parachute. Leg steering is easier on smaller parachutes than larger parachutes.
Some malfunctions start as slow speed on large parachutes. Broken lines and blown panels, depending upon location, may remain as slow speed, correctable malfunctions, provided the parachute is large enough. With smaller parachutes, these malfunctions are high speed or start as low speed and degrade rapidly into high speed malfunctions. These malfunctions are easily identified by a collapsed and rapidly spinning parachute. This is a great advantage of higher wing loadings. You definitely know when you have a malfunction. There is no doubt in your mind. With a larger parachute, you might think the parachute is controllable and decide to ride it down. The risk you take is that the parachute may degrade when you are very close to the ground. Be cautious in your decision, constantly monitor the malfunction and be prepared for a canopy transfer until landing.
Jumper - Canopy Match
Before you can find the right wing loading for you, you need to decide what you want from your parachute. There is no such thing as MAXIMUM performance. You can only OPTIMIZE performance to your special needs. If you maximize one item, say full glide speeds, then you'll give up some other performance factor, say lightest toggle pressure. It makes no sense to compare a single parachute's performance when two different people jump it. You need to jump each parachute in high and no winds, and in turbulent air to see how each parachute responds.
You must decide if you want and are skilled in controlling a canopy the way you want to and under conditions you determine. Old injuries to your back, knees, ankles may indicate a need for slower full glide airspeeds and gentle, reliable flares. Shoulder, wrist or elbow injuries may require a parachute with low toggle pressures or minimum toggle travel during a flare. Artificial limbs may limit flare strengths, riser turns or landing ability. Photographers require soft, reliable openings and predictable flares. Your present skill and experience level may suggest using docile parachutes, before progressing to more sensitive parachutes. Based on what you need in performance, you can find the right parachute for you by choosing the right size parachute.
You can use the trends given above to guide you. You should always jump a parachute several times under different weather conditions to really test it out.
Originally published in Sport Parachutist's Safety Journal V1, #4 Nov./Dec. 1989.
Dedicated to enhancing sport parachuting safety by disseminating information about equipment, environments and human factors.