Safety Comes First!
NOTE: The title of this month's column ends with a question mark. Whether you agree or disagree may depend on whether you are currently flying or building the biggies. Last fall I picked up two hobby magazines and found both had editorials about the safety of big models. The two editors took totally different viewpoints: one accused the AMA of being too timid in establishing absolute size limits; the other felt established limits were too stringent. I won't add fuel to that fire by stating my opinion on maximum model size, but I do believe it's worth discussing safety topics largely confined to the new breed of big models.
Collision risk and loss of control
Many people worry that a collision between a quarter-scale model and a person would be significantly worse than with a conventional-sized model. Debating the relative severity seems pointless: even a very small model can cause fatal injury if it strikes a vulnerable body part. We know of a case in Europe where a .19-powered trainer caused the death of a 6-year-old boy after loss of control.
Losing control is probably the principal cause of injury from collisions. Loss of control can result from:
- Pilot error
- Radio malfunction
- Mechanical failure
I doubt that pilot error or radio failure is any more likely with larger models, but there is evidence that mechanical failure can be more consequential on big models.
Mechanical failures and control-surface flutter
Control-surface flutter can happen on small models, but large models are less forgiving of sloppy workmanship. Examples observed include:
- Separation of ailerons on a large-scale model where the owner had installed a more powerful drive; adding mass balances solved the problem.
- Breakage of a threaded brass rod in an aileron linkage due to flutter; the linkage hardware was undersized for the barn-door ailerons, resulting in extremely sloppy controls.
Fortunately, neither of those incidents resulted in a crash — both models were landed using the rudder for lateral control.
If there is a safety concern specifically associated with big models, it is most apparent in the realm of mechanical failure. Modelers who like to build the biggest model in the field should pay extra attention to structural strength, particularly control linkages.
Hardware, cables and pushrods
The market is responding with accessories aimed at the big-model builder. The small extra cost for heavier-duty hardware is negligible compared with the total investment in the model, and is well worth it for added security.
One notable option is operating control surfaces via cables instead of the usual pushrods. Unless pushrods can be kept very short and rigid, builders of big models should consider:
- Using cables from both sides of the servo output arm to horns on either side of the control surface (common in full-scale aircraft).
- This approach can improve mechanical strength and, for scale competitors, may net extra static points as well as a sounder control system.
Servos and control forces
Heavy-duty servos are another product category for big models. A practical question is: when must one switch from, say, a KPS-15 to a high-torque servo? We don't have a precise answer. It would be useful if someone versed in aerodynamics could publish graphs or tables showing control forces needed to deflect various-sized surfaces at typical airspeeds.
From observation, KPS-15s (or equivalent) can handle the control forces in chainsaw-powered models of around 25 pounds with no apparent difficulty.
Editor's Note
We have faced the "one big servo versus two normal ones" dilemma. Problems other than safety often make two servos preferable, but two can be inconvenient because of the multitude of wires, fittings and long leads to servos near the ailerons. FM systems are often considered an ideal solution. Our servos are located in the wing roots with mechanical linkages out to the ailerons — a reminder of several problem areas recently mentioned in print, all of them safety related.
Gasoline fuels and transportation
The frequent use of spark-ignition (chainsaw) engines in big models has raised concerns about gasoline-based fuels and their transport. Key points:
- Cheap "gas cans" sold at supermarkets or drug stores are sometimes poorly sealed and can leak or seep.
- Gasoline vapors are more hazardous than glow fuel vapors. It has been said that "a gallon of gasoline stored in the trunk of an automobile has the explosive power of 14 sticks of dynamite."
- A car trunk parked in the sun can heat up, increasing pressure in a gasoline can and increasing the chance of vapor escape.
Better-quality "safety cans" available at auto-parts or industrial suppliers are often U.L.-listed and include caps that relieve pressure above a set threshold. If you must carry gasoline with you in a vehicle, take sensible precautions: use a U.L.-listed safety can, avoid leaving fuel in a hot trunk, and transport fuel in a well-ventilated area whenever possible.
Construction and materials
Good construction practices reduce the chance that larger models will be less safe than small ones. Practical tips:
- Use sturdy medium balsa for major fuselage members (longerons) that take motor loads. Leading and trailing edges should be medium weight, not too weak.
- Wing spars should be medium-hard, stringy balsa that doesn't snap cleanly. If the design lacks a spar cap or rib tops, consider adding 1/16" square strips to prevent wing warp when covering is tightened.
- For hinges, consider thin beer-can aluminum at least 1/4" wide rather than weak wire or narrow strips that are easily knocked out of adjustment.
- Use light balsa for non-structural fuselage struts, but keep strength at the wing attachment and front end.
- For tail surfaces, one medium main spar for the horizontal stabilizer and one for the vertical fin, with the rest of the structure in the lightest balsa available, is a good rule. Making these spars at least 1/16" thick and sanding them to an airfoil section helps if it doesn't add undue weight.
- Some designers increase tail area about 10–15% over scale as a general practice.
- John Preston
Transcribed from original scans by AI. Minor OCR errors may remain.
