Safety Comes First
Gary A. Shaw
Box 4520, Milton FL 32572; E-mail: [email protected]
PROP SAFETY REVISITED
In several previous columns, I've touched on issues surrounding potential concerns about using one type of propeller compared to another (wood, fiber-filled, nylon, carbon-fiber, etc.).
Most discussion focused on propeller shearing ability, performance, and personal preference as it relates to material characteristics, and why metal propellers were banned.
My intent in the discussion was not to imply that I am an expert in materials or the physics that make or break propellers, but to highlight that no matter what type of propeller you use, it is critical to remember that they're inherently dangerous and deserve our respect.
A reader asked why metal propellers were banned from AMA. I commented that I would ask for history on this topic. To date, no answer has been forthcoming.
In several columns thereafter, I continued the discussion of propeller safety by presenting a number of reader opinions from the mailbag. I've received stiff criticism from the readership for providing material to you without technical review. Perhaps it's time to set the record straight on this issue.
AMA does not allow the use of metal propellers—and has not for years—and I hope the organization will not change its position. My limited research shows that metal is more prone to fatigue than other materials in use (especially in small, high-performance engines). There are plenty of safer choices available.
That's not to say we don't incur injuries and material failures from what we have in use today, but we would have much greater issues with metal if it was brought back to the sport.
Metal requires more examination when in use than most modelers provide, and Murphy's Law would always be there to enhance the effects of poor maintenance.
How can I validate that? Without input from a metals expert, what I write is personal opinion and common sense.
I received a letter from Malcolm Child that provides insight into why metal is not good for us. Although the letter is critical of a modeler's comments provided in the April 2001 column, the discussion is as important as the topic.
"I am an RC (Radio Control) flier and mechanical engineer who designs high-speed rotation machinery (gas turbines). I have been following the discussion in Model Aviation about the relative safety of metal propellers and I have noted that the correspondents have been missing the key issue about metal prop safety.
"I realize that MA is not a peer-reviewed technical journal, and consequently MA unintentionally often contains statements that are misleading or often false. The letter you published in your April 2001 column I found particularly disturbing.
"As you pointed out at the end of the column, the main point is that composite (wood, plastic, and carbon-fiber) propellers are dangerous. I wholeheartedly agree. However, the letter you used implies that metal propellers are no more dangerous than composite props. This is not true.
"Metal propellers are much more dangerous than composite propellers for two reasons:
- Fatigue
"The first and main reason is fatigue. Composite materials are not subject to fatigue, but metallic materials are. It is on this subject that the letter you used is completely off base. The description of bent propeller blades on belly-in aircraft illustrates a lack of understanding of the difference between fatigue and yielding in metallic materials.
"The bent propeller blades were yielded beyond their elastic limit by the impact with the ground—not fatigue. The consequence of fatigue were 'discovered' by the spectacular failures of the de Havilland Comets in the late 1950s. Very simply, yielding occurs when metal is loaded beyond its elastic limit and a permanent deformation, or bend, is imparted on the material.
"Fatigue is a cyclical process where a metal part that is repeatedly loaded and unloaded will eventually fail. How many times, or cycles, the part can be loaded and unloaded is determined by the magnitude of the stress cycle and the properties of the material in question.
"The difference between yielding and fatigue can be demonstrated by a simple experiment. Straighten out a paper clip. The part of the clip that used to be bent and is now roughly straight has been yielded—or more correctly—plastically deformed.
"Now bend the paper clip back and forth over a 90° arc. Keep the bending in about the same location each time. Each time you bend the clip, the metal is yielding. Each back-and-forth motion is a fatigue cycle. These are two different phenomena.
"Keep bending the clip back and forth until it breaks. You have now exhausted the fatigue life of the material and it has failed. It is important that I point out that yielding of the clip material has greatly accelerated its failure by fatigue. It probably took less than 30 cycles to break the clip. However, it is not necessary to load a metal into yield in order for it to fatigue.
"Materials like aluminum will fatigue at very low load amplitudes, which, if the bent propellers that your writer mentions were subjected to, they would be fatigued, and more to the point, they would probably have broken off or had serious cracks after only one cycle.
"Metallic propellers experience fatigue cycles from several sources. One source is going from idle to full throttle and back. This will principally affect the hub and the root of the prop, as this is where the greatest centrifugal stresses are located.
"Another source occurs when the aircraft is flying at an angle of attack relative to the plane of the prop. This will cause one side of the prop to be loaded more heavily than the other (asymmetrical loading).
"The difference in loading from one side of the prop to the other is relatively small, but the number of cycles experienced can be enormous. A prop spinning at 10,000 rpm could accumulate 100,000 such cycles in 10 minutes. That's 1,000,000 cycles in 100 minutes of flight.
"Granted, the number of cycles from asymmetrical loading will depend on the flying style and the aircraft in question (i.e., lots of lazy eights or extreme aerobatics), but my point is that the high revolutions of our small model engines will accumulate a lot of cycles in a short amount of time.
"Fatigue is an invisible and continuous process. There is no simple way of determining how much fatigue life is left in a part. Continued fatigue cycles will eat up the life of a metallic prop until it fails catastrophically, either by throwing a blade or by a complete hub burst.
"The number of cycles it takes for a part to fully fail can be predicted analytically with fairly good accuracy.
"However, the analysis has to take into account differences in individual flying styles, airframe configurations, and other parameters, and the user must keep very accurate records of the number and duration of all flights.
"Even then, the part must be discarded before the fatigue life is exhausted, in order to assure that the likelihood of a failure is minimized.
- Energy Released on Failure
"The second reason metallic props are more dangerous than composites is that there is more energy released when they do fail. The density of aluminum (the lightest metal used in props) is more than twice that of composites (2,700 kg/m^3 for aluminum vs. 1,100 kg/m^3 for nylon). The density of a wood prop made from ash is only 600 kg/m^3.
"An aluminum blade thrown from a prop will have 2.4 times the kinetic energy of a nylon prop, and 4.5 times the kinetic energy of a wood prop. A steel prop (best fatigue properties) has a density of 7,850 kg/m^3. That's more than 15 times the kinetic energy of an ash prop.
"What does this mean?
"The writer you quoted describes a hub burst he experienced with a plastic prop. His comment about the momentum of the pieces and the danger of the event are quite correct. All our model airplane props have considerable momentum when being spun at thousands of rpm. Any prop failure is very dangerous.
"However, if it had been a metal prop, it would have been much more dangerous.
"Regarding the incident you quoted, '...I found a neat slit in the lower edge of my pants leg where the piece of blade passed by. If the piece of blade had actually hit his leg (thank God it didn't), it probably would have stuck in like a knife. If it had been a steel blade, it would have gone completely through his leg, or broken it if it hit the bone.'
"We all accept some degree of danger in everything we do. Every time we walk down the street or drive our cars we are putting ourselves at risk. However, our duty, like common sense, is to minimize the risk.
"Spinning model propellers are very dangerous objects, no matter what they are made of. Metallic props introduce an increased and unnecessary risk over composite props. Metallic materials are subject to fatigue.
"Composite materials are not immune to fatigue; it is often invisible and cannot be detected by simple methods. It will lead to failure at any time with no warning.
"The kinetic energy and consequent damage potential of a failed metal prop is two to 15 times that of a composite prop. Damage that can cause a composite prop to fail can usually be seen by careful visual inspection.
"Inspect your props often. If you do see a nick or ding in any prop, or even local discoloration of the material, throw it away immediately. Even better, break it first so no one will be tempted to use it again.
"There are probably cases where the increased risk of metal propellers are acceptable due to the small performance increase that can be achieved with metal props. However, these cases would be confined to the upper end of our sport, e.g., Unlimited racing, extreme aerobatics, and speed records.
"Use of metal props will require rigorous design and maintenance practices (as currently used in military aircraft and general aviation) that are beyond the reach of all but the deepest pockets. Metal props have no place in the field box of a typical model aircraft enthusiast and are quite rightly forbidden by the AMA Safety Code.
"Nuff said."
That's all for this month. Thanks for the input, and be careful! MA
Transcribed from original scans by AI. Minor OCR errors may remain.
