If your balloon has started to show increased fuel consumption, and you are not quite ready to buy a new envelope or system, you may want to consider some practical alternatives.
Increased fuel burn can come from loss of coating effectiveness (porosity) or from parachute valve rigging problems. A top which does not seal can cause fuel consumption to increase noticeably. Flying a balloon with a rigging problem also causes fabric damage around the edge of the top as the hot air goes past the fabric and heats it. If the fabric near the edge of the vent opening is crisp or discolored, the leak has been there for several flights. The fabric in the parachute valve must be pull tested to the manufacturer’s specifications, and the rigging must be adjusted.
A Velcro top envelope is also subject to leakage as the Velcro ages, and should be checked for fabric discoloration and Velcro grip strength.
If the top has been out of adjustment long enough to seriously discolor the fabric, or change its flexibility, you may find that some fabric replacement is required.
An inflation test with a qualified observer can diagnose a rigging problem in an envelope. Rigging problems can be a result of an improper repair, or, more frequently, a result of heat, aging, and shrinkage of fabric and centering or shroud lines.
Perform the following test:
Inflate the balloon under calm conditions.
When the balloon is stood up and ready to launch, set the top, and heat the envelope to buoyancy. Look up into the envelope from all directions around the basket.
You should see a clean, sharp edge all the way around the intersection of the parachute valve and the envelope opening. You should also see slight wrinkling in the top itself. If you see a fuzzy line around the edge, light shining through the opening, and no wrinkles, the centering lines are too tight and should be adjusted. This can indicate an immediate problem, or it can provide an opportunity to prevent leakage around the parachute valve from developing.
Fabric porosity is another cause for increased fuel consumption. Balloon fabric is made of nylon or polyester. This fabric has small openings at the intersections of the threads. The fabric is subjected to various treatments to close or fill these openings so the fabric will contain the hot air in the envelope.
Calendering is a treatment used for some fabrics in which the woven fabric is flattened between heavy rollers. This spreads the threads to fill the openings. Exposure of calendered fabric to high temperatures eventually causes the threads to relax and open the thread intersections.
Most production balloons in service today are coated on one side with a polyurethane coating. The polyurethane coatings can break down for several different reasons.
Mechanical abrasion while unpacking and packing the balloon can cause the coating to crack and lose some of its effectiveness. Extended operation at high temperatures can accelerate this process.
Packing a balloon away wet can allow mildew and hydrolysis to damage the coating.
Porosity in nylon envelopes in humid areas, such as the Midwest or Southeast can be caused by penicillin mold rot. Penicillin mold, when it does not have bread to feed on, is quite happy to feed on polyurethane. This is a progressive problem, which will cause the envelope to become more porous over time, whether flown or not.
Flying the balloon more frequently may retard the progression of the penicillin mold. The recommended prevention or arresting measures are storing the envelope in a very dry area, or in cold storage, such as a meat locker or a freezer.
Turning the envelope inside out and leaving it in the sun for a few hours, and rolling it periodically, is recommended for killing the mold. Loading up the balloon and operating it above the normal operating temperature is said to work—according to a balloon manufacturer who said “this is not an official recommendation, but it works.” It will also void the fabric warranty, so check with the manufacturer first if the envelope is still under warranty.
If the problem is bad enough, you may not be able to overtemp the envelope.
The Hyperlast/HTN-90 coatings are not susceptible to the penicillin mold, as they do not have a polyurethane coating. Some fabric coatings are now treated with a biocide to prevent mold growth, but its effectiveness is not yet proven. Polyester normally won’t hold enough moisture for the mold to survive unless it’s packed damp. It is possible to create the right conditions for it.
Some of the new coatings have provided better durability in accelerated testing, but only real world experience over time will prove their value.
At one time, The Balloon Works offered a recoating service. They no longer provide that service. They reported that the results were not consistent, and sometimes the coating remained sticky. The cost was between $1500 and $3000, and the life extension was unpredictable.
Progressive fabric replacement from the top down is a better investment than recoating. This is a solution that both the manufacturer and the repair station will stand behind. You will be able to use the latest version of the manufacturer’s fabric, and keep the balloon in service for a few more years.
Be aware that the problems will work their way down the envelope, and the panels below the new fabric will encounter increased stress. The hot air will try to go through the highest weak coating it can find. This heats the threads, and eventually weakens the fabric.
Within 2 to 3 years, you will probably have replaced fabric down to the equator, and will have paid a reasonable percentage of the cost of a new envelope.
Work with your repair station to determine the condition of your envelope, and the prospects for a successful progressive rebuild.
Alan Sanderson is a commercial balloon pilot residing in Campbell, California. Gen Sanderson and Lindsay Miller hold FAA Repairman Certificates and repair balloons at Lindan Hot Air Service Center, in Los Gatos, California.
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