PPC Parachute Wing Basics
What every PPC pilot needs to know
By Betty Pfeiffer
The sport of powered parachuting has come a long way in a relatively short period of time. As with every air sport we have seen a marked evolution in the vehicles, the wings and the pilot education necessary to fly safely. This article will address parachute design, construction, maintenance and airworthiness. For the purposes of this article I will refer to the parachute part of the PPC as the wing.
The most important element of PPC safety is the pilot. As a pilot it is your responsibility to regularly inspect your PPC, maintain your engine and cart, preflight the total vehicle, evaluate the weather conditions, keep up with new developments and be aware of any problems.
Bill Gargano of Quantum Parachutes, Inc. has been designing wings for PPCs for the past 17 years. "One of the biggest problems I have had to address is the hinge action between the cart and the connector links where the parachute is attached. The pilot hits the power and the cart pitches up, if he abruptly powers back he gets pitch oscillation. The faster wings have less pitch oscillation but higher rate of descent, higher fuel consumption and higher foot bar pressure."
Design is a series of compromises. The PPC wing designer has been given a unique set of problems to solve. Since the wing is not a solid structure, the designer must allow for fabric stretch, loss of permeability over time, distortion caused by weather conditions or turbulence, line stretch over time. Thrust values with different engine and prop configurations and payload weight fluctuation due to a double seater units being flown solo contribute to the challenges faced by the designer.
The designer must ask: Do we want a good climb rate or a faster flying speed? Do we want a quick turning wing or more stability? Are we willing to pay more for materials that last longer or do we want to engineer in obsolescence? Can we cut corners in the manufacture process and produce our wing at a lower cost or will that jeopardize safety?
Eventually there will be a consensus among designers as to what features are most important in a good design. Eventually designers will find new solutions to old problems and need to compromise less. Eventually the evolution of PPC wings will plateau. Until that time it is important for you, the pilot, to understand your wing and the conditions under which it may need replacing.
Understanding wing construction
My first flight on a powered parachute happened to be with a wing I had manufactured. Suddenly the importance of using the proper size thread, the correct type needle, the appropriate stitch size and tension, inspecting all the reinforcement structure to make sure it was applied "dead-on line" was fully realized. Even though I had test flown hang gliding harnesses for the past 20 years, it was not the same. The world sure looks different from under a parachute that just a week before was a roll of material under the cutting table.
As a PPC pilot trying to evaluate your wing, it is important to understand some basic concepts in construction. They are:
The bottom line is this: If something on your wing does not make sense to you, it is your responsibility to ask questions.
Fabrics used on powered parachutes have included 1.9 ounce rip stop, 1.1 ounce rip stop "sport chute" material, light weight silicon treated rip stop nylon, ¾ ounce spinnaker cloth or equivalent urethane based nylon, 1.1 ounce silicone impregnated cloth and, most recently, ¾ ounce stabilized MO Cloth.
Each fabric has its compromises. There is a tradeoff among weight, tear strength and longevity. The chemical make up of coatings and how they are applied to the fabrics also affect the final product.
Fabric qualities most suitable for powered parachutes include the ability to maintain good tear strength even after many hours exposed to Ultra Violet rays (UV), low permeability to keep the cells pressurized, bright colors for ease of visibility, and light weight for better inflation, and fuel consumption
As a PPC pilot it is important that you pay attention to the fabric from which your wing is made. Do a thorough inspection of each cell inside and out looking for holes, broken or missing stitching, loose threads, tears, snags, faded material, discoloration due to unknown substances, pinched or severely wrinkled material, hard or stiff areas. Inspect all reinforcement webbing for nicks or broken tapes. (For a complete inspection guide refer to "Troubleshooting Your Powered Parachutes Parachute Before Trouble Hits " by Betty Pfeiffer and Bill Gargano in Ultra Flight Magazine, May 99 Volume 6, #5)
PPC lines have gone through an evolution of their own. Among the materials used for lines are: Nylon that is coreless or sheathed, treated and untreated, Dacron (Polyester) Kevlar , Spectra, Dyneema, and most recently QR Spectra round.
Again each line has unique qualities. For example: Kevlar is lightweight and low stretch, but it has poor UV qualities and can be damaged by repeated flexing. Dacron is readily available, bitter ends can be inserted into the line to form clean junctions but has up to 11 percent elongation which means it will go out of trim after time. The newest QR Spectra has low stretch (1.5% elongation), small diameter, is lightweight, and is available in multiple colors. The down side is that the bitter end cannot be inserted into itself and therefore requires each line to be folded and zigzag stitched.
Parachute lines will stretch from use. As they stretch parachute performance changes.
Parachute lines can be damaged when caught in the prop, the wheels or overstressed by carelessness. They can be damaged if they come in contact with hot engine parts. The best way to inspect your lines is to do a symmetry check. To do this, fold the parachute to match up lines from opposite sides. Using 10 to 15 pounds of tension on each line compare line lengths. If you see 1" or more difference in line length, consult the manufacturer. While you are checking the lines be sure to run your fingers up and down each line. Feel for any hard spots. This is a good indicator of heat damage and can easily be missed by a visual inspection.
Parachute Performance & Safety:
Your parachute should have been designed to best perform with your specific vehicle. PPC Wings should not be switched between vehicles with different riser lengths, hard point distances, or vehicle cg locations. Even though the parachute may fly on a different vehicle, you may, unknowingly, be pushing the performance envelope by changing the geometry designed into the wing.
Since I entered the Powered Parachute Wing replacement field, I have been very surprised to learn that even 4" difference in hard point locations can make a huge difference in parachute stability, stall characteristics, and inflation dynamics. The result of this awareness has required us to trim each parachute according to the specific vehicle it will be flying. We have found that vehicles produced the same year by the same manufacturer have had different distances between the hard points.
Grounding your Wing
How do you determine that your wing is no longer airworthy? Assuming there is no structural damage, the first indicator is your evaluation of its fight characteristics. Are you experiencing inflation problems, Do you have longer take-off distances? Is your climb rate deteriorating? Do you have a faster rate of descent? Does something "just not feel right?" If you answer yes to any of these questions it is time for an inspection.
The effects of aging on your wing can be very subtle. Your parachute shows obvious signs of aging when you see faded material. Even when the material appears bright you should conduct the following tests:
Permeability Check: Hold a portion of the upper surface material in such a way that it stretches out to form a flat drum like surface. Put your mouth up to the fabric and try sucking air through the fabric. A new wing should allow no air to penetrate the fabric. As the coating wears down and the fabric stretches small amounts of air may escape through the material. As the amount of air going through the fabric increases you will experience more problems with your wing.
Stretch test: Most fabric currently used on PPC wings is rip stop. You will see small squares in the weave of the fabric. If you hold the fabric in your fingers and pull about 2 pounds tension the same direction as the squares line up, you should feel very little stretch. When you repeat this test on the diagonal you will feel more stretch. The material should return to its original shape within a minute or two after this test. If it holds the stretch it is losing memory and thus showing signs of age.
Round-tipped Needle Puncture Test: This should only be performed with a special round tipped needle no larger than #18. If you are not sure if you have the appropriate needle do not attempt this test. Pierce the upper surface fabric with the round tip needle. Closely inspect the hole using a magnifying glass. If the threads have separated to allow the needle to pass, the fabric is good. If the threads have broken, you may have a problem and you need a professional inspection. After the test you can realign the threads around the puncture by stretching the material parallel to the ripstop squares and moving your hands slightly up and down. Turn the material 90 degrees and repeat. The area you test will be very slightly damaged, therefore only perform this test when you suspect fabric problems. Limit the number of punctures to 1 or two suspect spots.
Other reasons to ground your wing are obvious abuse or neglect. This includes: mice or other rodents nesting in the fabric, bugs crawling into your wing and dying, your parachute experiencing runway abrasion, damage from barbed wire fences or getting snagged on other structures, the parachute coming in contact with heat and showing signs of melted areas just to name a few. If damage is isolated to a small part of the wing, the manufacturer usually can repair it. If you have wide spread damage, you may want to consider a new wing.
Considering a replacement parachute wing for your vehicle
Why would you want to change your wing? Physical damage, effects of aging on performance or upgrading to a design better suited to your flying.
Remember that wing design is a series of compromises, which includes shaping of the wing, fabric selection for each component, line geometry and construction issues. The end result is usually a wing that has a safe flight range given a specific vehicle-pilot weight when flown at sea level on a standard day. It is a mistake too assume that a wing designed for a single person at sea level is appropriate for two people taking off at a field elevation of 5000 MSL.
When considering a new wing be sure to ask the right questions. What is the safe operating load of the wing? What should my climb rate be? What rate of descent can I expect engine out? How was it tested? Under what conditions can I fly this with a passenger? What materials were used in the wing? How many hours can I expect it to last given my flying style?
If this were a perfect world you would have several wings for your vehicle. You would select the wing to fly each day depending upon the weather conditions, your total suspended weight, and your flight goal.
Since we do not live in a perfect world it is important for you to constantly be aware of the condition of your wing and the effects of aging. You need to look for improvements in wing design and materials based on your flying needs. When selecting a replacement wing, you need to make informed decisions based on objective testing.
Powered parachute wings are evolving. New is not always better so evaluate the "latest and greatest" with a critical eye. Ask questions. The ultimate responsibility for safety lies with you the pilot. When choosing your wing look at it as though your life depends on it. Most of all .fly safely!
Betty Pfeiffer, president of High Energy Sports, Inc., has been building parachutes and harnesses for the hang gliding and paragliding industry since the early 80s. She has been working with powered parachute designer Bill Gargano of Quantum Parachutes, Inc. since the early 90s. Together Betty and Bill run safety clinics throughout the United States. Betty has just released her new Powered Parachute Replacement Wings designed by Bill Gargano. For more information contact your local dealer.