Safety Comes First
Dave Gee | [email protected]
A story about battery terminals: touching and short
Good old Murphy! If it weren't for his law, I might be writing pet obituaries somewhere instead of enjoying a position here at the pinnacle of aeromodeling journalism. "If anything can go wrong, it will go wrong."
Our miniature aircraft can be complicated things, and there are plenty of ways that they can go wrong. My e-mail brings new stories all the time.
Mark Miller wrote to me about his exciting experience with a simple, inert box of batteries. He stores his Li-Poly cells in a plastic box rather than in the aircraft. Check. Each pack is in its own compartment, to prevent shorts. Check.
Mark says he closed the lid of his storage box and then, as he wrote:
"... there was a loud pfffft! and the box filled with smoke. I was baffled at first—the pack hadn't been damaged or even handled roughly. It hadn't been over-discharged, and it was sitting in its own compartment of the plastic box.
"After the smoke cleared, the pack appeared scorched but not punctured. I use Deans Micro connectors with only one exposed pin, so I wasn't concerned about shorts.
"However, the tops of many balance connectors leave part of the balance pin exposed, although the connector itself is female. When I closed the lid of the box, it evidently pushed the bare power-connector pin into the exposed pin on the balance connector and pfffft!"
According to Mark's report, there was no damage beyond that one battery pack, but a serious potential problem has been exposed. He is now considering putting heat-shrink tubing over the balance connector so that those pins aren't exposed, or maybe rethinking his whole connector system. I wouldn't have predicted trouble from such a well-designed battery setup.
A local survey showed that some pilots put dummy plugs in their storage cases and plug the packs into a dead pin. Other fliers bag each pack individually, which prevents shorts but impedes cooling and ventilation. Even rubber-banding the leads into a neat coil would not be a 100% guarantee against this problem.
One of this month's photos shows my granddaughter, Ashley MacArthur, breaking into print modeling by displaying a variety of electrical connector styles from my workbench. All of the brands and types are well designed for a particular use, but will not work for every purpose. A close examination of the connector ends showed that some of them could be touched by metal when disconnected. I'm going to take a fresh look at how I manage and store my cells, and try to out-think Murphy.
Motor mounts and airframe strength
Things can "go wrong" with our airframes also. Phil Alvirez and I corresponded about the types of motor mounts used on electric airplanes these days. He is cautious, with more than a little engineering expertise, and he recommends Allen bolts and blind nuts behind the firewall. A layer of nylon cloth coated with cement completes the installation so that, as he wrote, "the motor ain't goin' anywhere." Phil and I both worry about models with motors mounted on a plywood box glued to the firewall. It works well most of the time, but it's nice to have reinforcement in the form of corner braces or angles and some bolts. Epoxy adheres only to the top layers of plywood. You might have seen such a joint "pop loose" under stress. This is why mechanical backups such as bolts or screws are important.
The ARF revolution is bringing in more new pilots who fly but do not build their models. Some are unfamiliar with airframes and other structures that are designed to deal with heavy stress.
When their airplanes are damaged, they might not know how to properly repair and reinforce the airframes to spread and absorb the forces of flight. Issues such as this are no longer covered in high school shop classes!
An ARF pilot might have quite a bit of flight time, but assembling a kit or scratch-building means another learning curve. An expert computer programmer or tax specialist will find that doing structural work on a model airplane is a new thing.
In the olden days, building model airplanes was considered good training for careers in drafting, aviation maintenance, or any mechanical field. (When was the last time you saw a drafting table in use?)
Now that drafting—and nearly everything else—is done on a computer, people aren't trained in basic, hands-on mechanical work. Those of us who do get this exposure eventually learn to spot design problems even without an engineering degree.
You might have seen structural failures on model aircraft, sometimes because of faulty design, but more often from poorly done repairs or improperly stressed modifications. By "stressed" I mean designed to withstand flight stresses by absorbing them and passing them throughout the rest of the airframe.
Even without formal education in airframe design, many builders have learned what works and what doesn't from the good and bad models they have seen. Experienced model designers and builders can look at a picture of a pioneer-era aircraft and describe exactly why it won't fly and where it will break when the engine runs. Those airplanes were designed by hit and miss, when nobody had sufficient experience with flying machines.
I had occasion to compare model-construction styles with full-scale aircraft structure. I was highly interested in the construction of a certain Bellanca Citabria, because I got to go up in it for some full-scale aerobatics.
The original plan was to use a Pitts two-seater biplane, but there was too much of me and not enough cockpit, so we went with the roomier airplane. The Citabria has a welded-steel-tube fuselage structure with numerous diagonals and crossbraces to spread the G-forces of the pilot and her large, nervous passenger.
Models are built the same way, because we sometimes do things with them that no onboard pilot would try. The engine mounts were integral with the airframe so that forces of torque and thrust could dissipate throughout the entire airplane.
My pilot was Judy Phelps of CP Aviation in Santa Paula, California. She is an experienced aerobatics instructor and competitor. Judy took very good care of me, because the cockpit is so hard to clean. I was even given the stick for a while, and when she complimented my nicely coordinated turns, I had to confess that I was trying to fly straight.
Aerobatics are way different from inside the aircraft! I would not have been able to even identify some of the maneuvers we performed if Judy had not narrated. The airplane seemed to take forever to do moves that my models do in an instant, but my models don't get airsick.
Judy asked me how I was doing after each stunt, and there came a point when I told her we were finished for the day—or else. Back on the ground, she looked as fresh as a daisy while I was draped over a chair, recovering. Just another day at the office for her!
A full-scale aerobatics machine is carefully and meticulously maintained, since any failure could be fatal. Our models are different in many ways. We can afford to be slightly more casual with them, since they are unmanned and they are usually simpler and easier to inspect and service.
Pound for pound, they are also much stronger than big airplanes. If you don't believe me, imagine picking up a full-scale airplane by its rudder or by a propeller blade. Oops! It would be different with a small to moderate-size model, right? This overengineering can lead to overconfidence too.
Rubber-scale builders pay close attention to structural design, since their fuselages must bear the torque of a tightly wound motor. Some of us are a bit tightly wound ourselves. I proved this by carefully examining the Citabria before my flight. The design was close to what a midsized rubber model would use, swapping balsa for steel tube.
Electric models don't need the same strength in the same spots, but their motor mounts have to withstand tremendous loads in all directions. Imagine the stress on an outrunner motor mount when the pilot performs a tight outside loop at full throttle!
This all comes back to the issue of what is strong enough and good enough for motor mounts.
Next month I will have the results of my question about propeller-saver systems. Meanwhile, I would appreciate your input on motor mounts you have used. Which ones worked, which types failed, and what is your best technique for spacing the motor up to the nose from the firewall?
There is just enough space here to slip in a little e-mail quote I got from an official AMA message:
"The safety code is not an official part of the safety code so the rules change system doesn't apply ... thanks for your inquiry."
This glitch was sent by Gregory Hahn, who is an extremely competent fellow and handles technical issues at AMA Headquarters. And, yes, he told me it was okay to print it.
If a man as sharp as Greg can slip up now and then, we can all feel better about an occasional lapse in attention. It's no big deal at the keyboard, but at the field it could be a lot more serious, so we should try to use an appropriate level of care with whatever we're doing.
Sources:
- Dave Gee
Box 7081 Van Nuys CA 91409
- CP Aviation
(805) 525-2138 www.cpaviation.com
Transcribed from original scans by AI. Minor OCR errors may remain.
