Letters to the Editor
Send Letters to: Model Aviation, 1810 Samuel Morse Drive, Reston, VA 22090
Let's Practice Safety . . .
As a power engineer with 25-plus years of heavy industrial experience, I read with alarm the article, "Junkbox Air Compressor," in the November 1992 issue of Model Aviation.
I believe that someone could be severely injured with possible resultant liability claims should a compressed air unit be built in this manner. Although an engine can be easily converted to function as a compressor, the materials recommended are not suitable for the temperatures and pressures involved (at 50 psi, the discharge temperature will be about 300°F; at 100 psi, about 500°F).
Specific comments are:
- Use of plastics that will soften if near the compressor discharge.
- Use of PVC valves that are not designed for the pressure.
- Use of soft solder for discharge joints (melts at about 450°F).
- Use of a water heater relief valve that is not designed for compressed air use. (Most are temperature reliefs — not pressure reliefs.)
- Use of a Freon tank that is not designed for re-use as an air receiver. (Most specifically state "Not for Re-Use" on the tank.)
- Use of a rubber stopper in engine conversion will eventually deteriorate because of high temperature.
- Lack of a receiver drain valve for condensation removal.
- Users should also be made aware of the possibility of internal explosion of a homemade compressor if hot spots inside ignite the lubricating oil/hot air mixture.
For safety's sake, I would highly recommend purchase of a small, properly designed compressed air system. Cost should be $150 or less — a small price to pay to prevent loss of an eye or a fire in one's home.
Robert J. Moffat Viscount, Canada
. . . And Here's Why and How
The article in Model Aviation, November '92, titled "Junkbox Air Compressor," by Joel Hamm, was creative and informative. But, as an AMA member concerned with safety, and as an air conditioning and refrigeration technician, I must protest the suggested use of an empty Freon tank as an air accumulator. These tanks are not protected internally against rust and corrosion. Refrigerant is a moisture-free product. A large percentage of air is water; therefore these tanks have a tendency to rust unnoticeably from the inside out, providing a potentially dangerous situation when used as an air tank. The actual warning label on the tank reads as follows:
"Do not refill or use for any purpose. Federal law forbids transportation, if refilled—penalty up to $25,000 fine and five years imprisonment. (49 USC 1809) Do not store in a wet or corrosive environment. After use, vent remaining pressure, leave valve open, and dispose of empty container properly. Never use as an air tank."
I would think a wiser move would be to go to your local auto parts store, hardware store, or even flea market, and purchase a DOT-approved portable air tank. A new one sells for $20 to $30, and a used one for about $10.
A small price to pay for safety.
Richard F. Canup Cocoa, Florida
Taking Exception
I am writing about deBolt's article, "A Primer on Aerodynamics," September issue. While I can't help but respect deBolt as a modeler, I also can't avoid saying that his explanation of how a wing produces lift is completely erroneous.
The sketch in the lower-left corner of page 28 shows how molecules of air flow over a wing, both for an airfoil that produces lift and a symmetrical one that does not. As the molecules pass over the wing, the sketch shows them coming closer together, both in the airstream direction and perpendicular to it, more so on the top surface than on the bottom.
This implies an increase in the air density (molecules per cubic inch, say). At the speeds at which models fly, that is just plain wrong. The molecules do indeed come closer together in the direction perpendicular to the airflow, but they go farther apart in the flow direction by an amount that almost exactly keeps the air density constant. The increased molecule spacing in the flow direction is consistent with an increased flow velocity which, according to Bernoulli's law, creates a decrease in pressure. That decrease is greater over the upper surface than the lower, and so the net result is lift.
Also, the sketches show the flow of molecules separating from the surfaces. While separation does indeed occur on large aircraft near the stall, and on small models to some degree under almost all flight conditions (but not in the manner shown in the sketch), that separation plays no part in the production of lift; in fact the lift would be greater if there were no separation.
As to his "gospel: air is considered to be stationary, and flight is created by a craft moving through it"—flight is equally well created when the air is moving and the aircraft is fixed, which is why wind tunnels work and why gliders can slope-soar.
Later, deBolt makes a distinction between a gas and a fluid. Equating a fluid to a liquid is a common error. Both gases and liquids are fluids; look it up in your Webster's. It is a common error; perhaps therefore excusable.
But he goes on to say that the "loosely knit" nature of air, as opposed to a liquid, is the characteristic that "makes flight possible." Wrong! At speeds well below the speed of sound, aerodynamic theory is based on the assumption that the air is incompressible. The theory and experiment agree quite well. The same theory works for water or any other fluid. Hydrofoil boats have wings that fly in water. Properly weighted model gliders fly very nicely under water.
Later he says, "Lift and drag increase as the square of increases in speed." That implies that if the speed increases by 10%, the lift increases by 10% of 10%, or 1%; when in fact it increases by 21%. More correctly, "lift and drag increase as the square of the speed" rather than "...as the square of the increases in speed."
The toughest part about explaining lift is explaining Bernoulli's law, which states that the pressure is reduced when the velocity is increased. To many, that seems contrary to nature.
Bob Meuser Oakland, California
Paper Planes and Young People
I applaud the efforts of the Hampton, Virginia, group described in your August 1992 issue ("World Record Paper Airplane"). They can be proud of their achievement. I hope this serves as an example for other groups around the country.
Paper airplanes seem to hold a natural attraction for youngsters. The AIAA and the St. Louis Science Center have cosponsored paper airplane contests in St. Louis in recent years that have attracted up to 100 entrants each year (mostly in the under-14 age groups), and that received good local media coverage. I think the modeling community is missing a sure bet by not getting involved in programs of this nature. Along with making a contribution to science education, there is a potential for recruiting many new young modelers.
One other Guinness paper airplane record may be of interest to your readers. The indoor duration record is 17.2 seconds and is held by my colleague and fellow McDonnell FJ club member, Ken Blackburn. This record was set indoors, and seems right down the alley of the indoor HLG fliers.
Tom McAtee Hazelwood, Missouri
Nats Critique
Having attended five Nats (Los Alamitos, Riverside, Lincoln, Reno, and Westover), I can say that this affair was by far the best organized, highest quality of them all. There were more than ample support, sanitation, communication, and individual event direction present to offer the contestants a good level playing field. Casual gripes aside, your staff of volunteers were there with endless energy and courteous help when needed. Kudos, then, from top to bottom for a splendid job. Any who were no-shows for whatever reason should regret it when the word spreads around. AMA bashers will have a shorter podium after this one!
A constructive criticism or two: We model people tend to be emotional most of the time, especially under contest stress, so consequently do not always do the best job of explaining ourselves to the public. I would suggest that photocopies of the contest rules as applied at the Nats be available in a central location for the benefit of both contestants and the public. Also, I think a more professional display—comprehensive photo boards with captions—would be worthwhile, though possibly difficult to produce. In fact, I would recommend an expansion of Nats onsite PR materials and displays; if the stuff is available, the public will appreciate it. Finally, videocassettes from this or prior Nats, if of good quality, should be available to the public for $15 or $20, I think.
John Oldenkamp San Diego, California
(Ed. note: Many of the photos you enjoyed in the November 1992 Nationals edition of Model Aviation were taken by this gifted photographer and hardworking AMA member. Thanks, John, for giving us a photo album of the year—the Nationals at Chicopee.)
Wonderful Memories—Wonderful Modelers
One of my goals upon retiring was to get back to old-time free flight model building. I started about where I left off so many years ago with a 1939 Zipper powered by a 1940 Ohlsson .23 ignition engine. The Hobby Horn kit was a dream to build, and at less than 30 oz., the completed airplane promised to be a real soarer. I was thrilled.
I attended the Bong Eagles–Chicago Aeronuts recent free flight contest as a spectator to once again familiarize myself with the activity. I took the Zipper along as an afterthought. After getting hyped up watching all the activity, I went off by myself to try a test flight. With an engine run of 15 sec. at very low power, the Zipper reached about 150 feet of altitude—which is where it stayed. It very slowly and lazily soared south, circling like a hawk. After chasing it on foot a mile or so, I came to the shore of a lake as the airplane continued across the water and out of sight over the woods and meadow beyond. It was a most successful flight, but it cost me the Zipper.
A few days later, I received a phone call from Mr. Jack Boone of Mequon, Wisconsin, a member of the Bong Eagles. Jack witnessed my flight, and that evening flew over the area in his own Cessna 172 until he spotted the Zipper near a cornfield. The next day he drove the 25 miles back to the scene and recovered the model before calling me. Is there any other activity in the world that could build such consideration for a perfect stranger?
But the story doesn't end there. As I left to chase the Zipper, my car was wide open and my flight box, a small paper carton containing a few essentials, was on the tailgate. When I cleaned out the box a few days later, I found a business card in the bottom from Mr. Leo J. Dringol of Northfield, Illinois. On the back of the card was a penciled note that read, "I hope you didn't lose that beautiful Zipper. If so, I fly out of Kenosha Airport, and will help you look for it from the air."
I have always felt that there are some attributes that make model builders unique people. Perhaps the natural skill, the patience, the dedication, the survivability, the adaptability, or the determination—all combine to create the modeling enthusiast.
As I reflect over the events of the past few days, as well as the many other wonderful contacts I've made through modeling, I sense the fraternal bond that exists among those who enjoy our hobby. It is this kind of bond that brings the Boones and the Dringols to extend themselves for others, even perfect strangers, as they did. But wouldn't all of us just as unselfishly share equipment, offer to assist, run to aid others in distress, and share the pain of loss at a crash? What a wonderful brotherhood we share through modeling!
Leroy J. Konrath Fox Point, Wisconsin
Transcribed from original scans by AI. Minor OCR errors may remain.
