Clocking the speed of propeller tips
Dave Gee | [email protected]
Propeller tip speed
Have you ever wondered exactly how fast the tips of our propellers go? I lurked through an Internet discussion on this subject, and sure enough, somebody did the math and shared the answer. The formula is not difficult if you passed geometry and algebra. Hank Sperzel must have gotten an A in both classes, because he showed and explained the math.
The propeller diameter (and pi) gives you the distance that the tip travels, and then the engine rpm tells you the speed. A typical gas engine can easily drive the propeller tip faster than 500 mph, and a hot engine with a large propeller can exceed 600 mph.
There are several options for dealing with this fact. We can take up RC gliders, which are great fun and quiet. We could all meet at my indoor site, which is relaxing and even quieter. Or we could keep our hands out of the propeller arc!
Imagine the harm that a 600 mph piece of wood or plastic could do to a finger. Propeller strikes continue to be a common occurrence, despite the fact that all of us know to keep clear of such an obvious hazard. I would love to have an entire month go by without a report of some bloodshed from an otherwise experienced and alert modeler.
I do not claim to have perfect experience or alertness, but the finger that types these words of warning still bears a little scar from when I put my own hand into a running propeller. It was long ago, but I do not recommend it.
Attitude of modelers vs. the public
We modelers have unique viewpoints on some matters, don't we? We accept that our models may harm us a little or a lot if we are inattentive. Our miniature flying machines tend to have fairly short lives, despite the long labor we sometimes put into their construction. They might crash or just fly away, and we are (usually) okay with those possibilities. A typical modeler is almost as impressed with a nice piece of support equipment as with a beautiful aircraft. These attitudes set us apart from normal people, and we should be aware of the different ways the nonflying public sees our hobby and us.
Tale of Two Tethers
The AMA, always looking to promote the hobby, has developed an excellent program that uses simple model airplanes to teach middle school math and science. The AeroLab CD contains instructional videos, classroom handouts, and more, so that a nonmodeler teacher can help kids build and fly tethered, rubber-powered model airplanes. They use the flights to scientifically determine airspeed, distance flown, and how variables (such as ballast) affect results. I have helped present this program to teachers across the country. They are excited to see it, and they rave about the response from their students.
The AeroLab CD shows the tether rig as an ultra-simple nail in a wood block, taped to an upended trashcan. Being modelers, the demo team initially got spiffy custom-printed traffic cones with specially made tether assemblies that fit neatly into the tops of the cones. They looked great.
The teachers were impressed but kept asking where they could get the orange rubber cones and the machined tether pins. After a while it occurred to us that this was another instance of nonmodelers seeing things differently from how we do. The teachers couldn't imagine doing without the fancy equipment we had shown them. We model airplane people figured that since we would be using the demo setup so much, it was worth making a rig of which we could be proud. We expected the schoolroom tether to be more makeshift, since it was temporary.
After adjusting our mindset to match that of our clientele, we relegated the cones to traffic control around the circle and got an old box or trashcan to serve as the tether tower. Sure, it looks lousy, but the whole demonstration is now more effective than ever. The teachers see an easy-to-replicate setup instead of the fancy special stuff that modelers love.
I built my own tether tower for local use, and my utter lack of craftsmanship produced a primitive-looking rig. Perfect! Then my father, Ralph (happy 85th birthday, Dad), asked to borrow it, just to "throw some paint on it."
A consummate builder, he sanded and finished my shoddy product into a furniture-grade masterpiece, complete with telescoping tether pole and a folding carry handle at the balance point. I love to use this classy setup for fun-flying, but I hate to see Dad's face when I tape a block of wood to an upside-down paint bucket for teacher demonstrations.
Public perception of RC flight and club efforts
This difference in viewpoints between modelers and the public can also be seen when someone sees RC flight for the first time. Kids will commonly ask, "Can I try it?" with the expectation that they can instantly fly as well as the pilot. Although they probably can fly better than I can, it proves that the training and effort we put into our hobby does not always show to outsiders. They see things differently.
The Skyhawks RC club in West Virginia understands this "viewpoint" issue. The members carefully arranged their flying field with protective barriers in the right spots and then realized that their lush climate would allow a more scenic treatment. The club upgraded the field landscaping with bushes and shrubbery to augment and conceal the flight-station guard structures.
The grass in front of the flightline is left higher than normal to detain errant models. A maze pattern of hedges between the flight station and the pits provides protection and beauty. Visitors must be impressed with the extra-green airfield.
Club President Neil Gibbins says that the cost was low because the group used extra plantings donated from members' homes. The phrase "going green" is overused, but these modelers did it with style and practicality.
Children on the field — AMA Safety Code and exceptions
It's time again to scare the legal department with my politically incorrect opinions. I've been corresponding with several people lately about the kids-on-the-field issue. The AMA Safety Code is clear; it reads, "Children under six (6) years old are only allowed on a flightline or in a flight area as a pilot or while under flight instruction."
What about kids running loose in the pits? That can be a more dangerous spot than a pilot station. Is the pit area a flightline? Why the arbitrary age of 6? Eight-year-olds aren't known for their mature judgment. (By that standard, I should be required to remain in the car.)
This is a rule for which I tend to make exceptions based on circumstances. I remember being taken to the field as a small child and the dazzling, thrilling place it seemed to be. I was even allowed to operate the transmitter for a moment. The joyful impression has lasted all these years, and I would like all youngsters to experience that pleasure.
It isn't tough to figure out the reasoning behind the AMA rule. Kids have a tendency to escape our attention and get into trouble. There's plenty of trouble available at a busy RC field.
Valley Flyers outreach event
The Valley Flyers members took on quite a mission recently at the local field. They hosted more than 100 children aged 5–10 and made sure that the kids found out what our hobby is.
The highly organized event had a rubber-powered free-flight (FF) airplane for each child to build and fly while waiting his or her turn on a buddy box with a real gas RC airplane. The kids were carefully supervised and kept in safe areas until they could be escorted to the flightline. How those children will cherish the memory of their day.
Check out the blurry photo of the guys chatting in the pit area. I had to take it in a hurry with my cell phone camera to capture the moment. Look carefully, and you will see Pat Kinne with his grandson, Andrew, tucked close.
Andrew has his hands over his ears and a big grin on his face as he takes in the scene. Pat is chatting with the guys, but he keeps one or both hands on Andrew at all times. There was a very strong bond between the two. It was clear that Pat would have lifted the boy from harm or shielded him with his own body if need be. It doesn't get any better than this, and I would have a hard time strictly applying the AMA rule to such a circumstance.
This is not an official AMA position, but I think that sometimes there can be flexibility in the rules, depending on the situation. In this case, the risk was so tiny and the benefit was so large that an exception was in order.
Closing and contact
I've got my flak jacket on now, ready for dissenting e-mail. Please put "MA" in the subject line so I know that your message isn't from some lowlife spammer. Paper letters can be sent to the address below. Your comments, suggestions, and corrections are always welcome. MA
Dave Gee Box 7081 Van Nuys CA 91409
Sources
- AMA's AeroLab program: www.modelaircraft.org/education.aspx
- AMA Safety Code: www.modelaircraft.org/files/105.pdf
- Tri State Model Flyers Inc./Skyhawks Limited Partnership — Robert Garner: [email protected]
Transcribed from original scans by AI. Minor OCR errors may remain.
