Letters to the Editor
Junk Box Air Compressor Critique
The November issue's article concerning "Junk Box Air Compressors" piqued my interest as it involved both my avocation and my profession. While I'm a devoted RC modeler on the weekend, during the week I am a senior development engineer with the largest compressor manufacturer in the world, Ingersoll-Rand Co. I have spent the last 18 years developing compressors and related equipment (mostly 50–500 horsepower diesel portables).
While I applaud the author's spirit of ingenuity, I feel compelled to discourage fellow readers from building such a device. As presented, the details of the design ignore some very important safety requirements of all compressed-air systems. These are required by law on commercially produced systems, and prudent design demands their inclusion even on a home-built unit.
First and foremost, a safety relief valve is an absolute must on any compressor or compressed-air system. This cannot be an either/or choice as presented. A pressure switch used to cycle the compressor between set pressures can fail, causing the pressure to rise until something explodes.
The author discussed the fact that an explosion could occur if the compressor is allowed to run unchecked, but did not insist on the use of a relief valve even if a pressure switch is present. Even if the pressure switch is in good working order, an explosion could occur during the first use while trying to adjust the switch settings.
Also, the relief valve must be sized properly. ASME code states that the valve must be able to flow the full output of the compressor at 110% of the valve set pressure. Temperature compensation is required in sizing the valve as well. I have experienced relief valve failure in cases where the valve was oversized by 50% or more due to chattering of the valve seat because of insufficient flow. As you can see, just choosing a correct valve involves some expertise that most people attempting the project would have no reason to possess.
The second major hazard involves the use of a Freon tank as a receiver vessel. Freon and propane tanks are not designed, intended, or code-rated to be used in an air system. The major problem is water that collects in these tanks, causing corrosion. Used tanks have no provision for draining water. A receiver should be drained daily to prevent rust. People have been killed or severely injured over the years; air tanks have exploded because fractures propagated through severe rust pitting the tank wall. You must know the pressure rating of the intended tank and ensure the safety valve is set below that pressure. Most Freon and propane tanks are marked with their pressure rating. Systems commercially available in the U.S. have tanks constructed under the auspices of ASME and are code-stamped by licensees of the organization. If you must build your own compressor you should obtain an ASME‑coded tank and a petcock drain. Probably the easiest way to get a safe portable receiver is to obtain an ASME‑coded tank with a drain.
More Input on Junk Box Air Compressors
I am forced to write in regard to your article "Junk Box Air Compressor" by Joel Hamm published in the November 1992 issue of Model Aviation, page 188.
This article certainly slipped past your safety committee! I was horrified to see such information and examples displayed and promoted in a safety-conscious publication such as MA. Please, please, please—print a retraction and/or safety warning to your readers.
My highest concern is the use of a discarded Freon can. It is absolutely illegal (federal law) to reuse these cans for any purpose involving internal pressure of liquid refrigerant. The can design does incorporate a safety blow-out patch in case the can becomes involved in extreme pressures, such as in a fire.
The problem is this device is actually a can, not a heavy, certified (read: expensive) pressure vessel. The sidewalls are very thin stamped steel. An air compressor installed in the unit will cause pressure rise due to compression and the moisture collected can corrode and scale the inside.
This moisture collects in the can. Certified compressors have a valve to drain this moisture. The Freon can does not. This collected moisture will corrode the thin walls and seams of the Freon can until they fail. A worst-case result is a shrapnel-type missile and is extremely hazardous.
Also, the pressure switch, if used, must have independent overpressure protection. A pressure switch is not a safety device. The relief valve must be sized per ASME requirements and tested to the specified pressure. It must have a tamper-proof adjustment and be protected so it cannot be blocked.
Furthermore, all connections, hoses, and piping should be rated for the maximum possible pressure and temperature. Use threaded fittings with pipe dope designed for gas service, and avoid soldered joints.
The second concern is the more obvious open-belt drive. Inexpensive covers are available or could be made and should have been a part of the article.
Daniel D. Kautz Plano, Texas
Is It Possible?
I am 15 years old, and I have been flying model airplanes for about three years now.
After reading "Letters to the Editor" numerous times in different issues of MA, I decided to send my thoughts in to let other modelers see the possible design of my one idea about model aviation.
My idea is to take the tail section of a V-tail and attach it to the wingtips of a normal plane. There would be no need for the tail section, for all of the control surfaces would be in the V on the wing. Up would be normal on a V-tail, except both of the V's would be in the up position; down would be the same.
Along with not needing a tail section, there would be no need for ailerons. To roll, simply make one V go up and the other go down. For a banking climb, raise one V more than the other.
These are the thoughts of a 15-year-old who is very anxious to see whether or not this type of design will work.
Derek Walton Port Neches, Texas
P.S. I imagined this plane to have a tricycle-style landing gear and for it to be a pusher.
Transcribed from original scans by AI. Minor OCR errors may remain.
