Safety Comes First
Gary A. Shaw
5063 Benton Boulevard, Pace FL 32571
BTA Autopilot
Since I wrote about the BTA autopilot in the November and December 1994 columns, I've received several cards, letters, and calls from individuals who have found it difficult to get in touch with the company. The following address and telephone number should solve the problem:
BTA Automatic Piloting Systems Ltd. 2422 Richmond Way Costa Mesa CA 92626 (714) 979-8948
Most of the mail regarding the autopilot has been from folks needing flight assistance due to varying degrees of physical disability. It's nice to know there's something on the market that promises to open horizons to the otherwise disadvantaged. A unique, somewhat opposing viewpoint was in a letter sent by Tom Cooperider of Cameron, Missouri, who recommended a "go slow" approach in promoting its use. Tom feels that use of the product by modelers might have a strong tendency to make lazy pilots:
"Students that learn to fly with an autopilot may never learn to fly without one. The autopilot has many good applications, but let's not try to sell it as a cure-all."
Although certainly not supported as a cure-all, its ability to save aircraft affected by signal loss and/or provide flying assistance for those requiring it — in any capacity — continues to make it a worthwhile investment, in my opinion.
More on PET Power
In the September 1994 column I wrote about the perils of using Polyethylene Terephthalate (PET) containers as compressed-air vessels for driving air-powered retract systems. To help illustrate my concerns, I included photos sent by modeler Bob Rodgers of Orchard Park, New York, which depicted the scattered remains of his 1/4-scale B-25. A sad condition for an otherwise proud aircraft. What happened to it? In a nutshell, the overpressurized PET container (a two-liter plastic soft-drink bottle) used to power its retract system erupted violently, leaving behind a balsa junk heap.
Many of you who read the column sent cards and letters describing your own concerns and/or offering different viewpoints. Olin McDaniel of Florence, South Carolina (retired Du Pont worker) set the record straight by clarifying what PET really stands for:
"It's not quite Polyethylene Tetrahyde, actually; that name really fails to identify any chemical structure. PET really stands for Polyethylene Terephthalate, a polymeric chain of repeating links, each link containing an ethylene unit and a larger unit which is really a modified benzene ring. This modified benzene ring is a derivative of terephthalic acid."
With the correct definition out of the way, another piece of correspondence by Franklin Vassallo of Lancaster, New York, arrived via AMA recently with some very interesting PET container pressure-testing results. Using the information gathered during his trials, Franklin feels there are many applications for which PET containers could safely be used (safety being the first consideration).
Franklin Vassallo — Tests and Findings
- Franklin has used PET bottles as the air source for RC compressed-air models for several years. He flies models of about one pound total weight with up to 500 square inches of wing area and uses combined two-liter PET bottles to provide the required air volume at pressure.
- He found two-liter bottles manufactured in various configurations — some suitable as pressure vessels, some not. Observed variations include size, neck flange, upper shape (nearly hemispherical to nearly conical), and base type. Small size is preferred. Conical upper shapes performed best in his experience.
- Bottle bases are important: bottles with a square molded base are unacceptable. Only bottles with a glued-on cup base (cup removed) should be used. Better types have a thickened section or visible plastic sprue.
- In tests, few two-liter bottles could be pressurized beyond 130 psig before significant damage occurred. Residual deformation usually appears as whitish stress lines in the neck area. Ruptures tend to initiate in the neck region, although sidewall failures have been observed around 120 psig.
- Because bottles vary in tolerance, Franklin emphasizes that each bottle must be tested to at least the desired pressure level. Pressure should be maintained with an accurate pressure gauge, preferably the same gauge that will be used during actual use of the tank. The bottle should be inspected after use for signs of residual deformation, particularly near the neck.
- Bottles exhibiting bulging sidewalls should be discarded. Creep failure (slow dilation under extended pressurization) can cause long-term failures; Franklin induced several such failures in bottles that showed slight circumferential bulging.
- One-liter bottles come in shapes similar to two-liter types and follow the same rules. Because of their smaller diameter, one-liter bottles can withstand significantly higher pressures — usually over 150 psig. Where size permits, combined one-liter bottles can provide the same gas volume as a two-liter bottle with a larger safety margin.
- Three-liter bottles of the best variety rarely withstand beyond 100 psig without risk of failure and would need to be wrapped for higher pressures.
- Both aluminum and plastic caps have been found capable of containing pressures above 175 psig; Franklin prefers plastic caps, which show less radial expansion under pressure and provide a better seal.
Wrapping and Reinforcement
- Over-wrapping bottles with braided nylon fishing line or lacing cord can significantly increase burst tolerance. For two-liter bottles, approximately 70 axial strands of 15-lb test nylon wound on the tank, plus an additional 24 hoop strands in the midsection, can extend burst pressure to about 150 psig. Using the tank at 120 psig then provides roughly a 30-psi safety margin.
- In experiments with 70-lb test Kevlar fishing line using the same wrap configuration, burst pressure was extended beyond 150 psig. Franklin stresses that each wrapped tank must also be tested to the intended use level. With wrapped varieties, tests may be performed at pressures considerably above the expected operating pressure.
- All of the above testing was performed in an enclosure so that a bottle rupture posed no safety concern. Pressures were measured using a pair of accurate pressure gauges. The supply was from a regulated nitrogen bottle.
Safety Precautions and Anecdotes
I recommend that the usual precautions be taken: safety glasses and ear defenders.
During development of experimental tanks, Franklin once experienced a tank failure due to a misdesigned bottle interconnect. The tank burst in his hand. Other than the shock of the tank's disappearance, he sustained no injury.
"The plastic fragments have little mass, so that eye protection is probably sufficient to ward off serious injury from a bursting, exposed tank (I was wearing goggles at the time, but no ear protection). Installation of the tank within a structure might pose other hazards, due to flying debris. Hence greater safety margins in pressurization must be used for those applications. Here, wrapping on the tank is essential if use at pressures of 120 psig is desired.
"Over the past several years I have flown literally hundreds of compressed-air RC flights using prepared PET bottles at pressures up to and beyond 120 psig with no burst failures during filling or in flight. Their use as pressure vessels can be very safe if attention is given to tank preparation, filling, examination, and proper use of safety equipment."
Conclusion
I agree with Franklin that a bursting exposed tank is probably much less hazardous than one that bursts inside the structure of a model — hence the destroyed remains of the model depicted in my original column. You'll also get no argument regarding the need for standard safety equipment on the flightline.
What matters most is that unless properly designed for such uses, many modelers are unlikely to follow independent and appropriate safety precautions. Most prefer convenience — no wrapping hassles or stress marks to contend with — and unless they are experimental-minded, many assume safety is already designed into the product being used.
Although many will continue to use PET containers as pressurized vessels until something better appears, I believe those who err on the side of caution will avoid their use.
Transcribed from original scans by AI. Minor OCR errors may remain.
