Newcomers

Newcomers

Bob Underwood Box 40, St. Peters MO 63376

WHAT GOES AROUND, comes around.

At least it better, or your powered trainer isn't going to fly. I'm talking about the propeller — that twisty object that spins away, causing your model to taste the air in flight.

The choice of propeller (prop) for a trainer is not as critical as it is for a sophisticated Pylon, Pattern, Scale, etc. model. Maximum thrust or speed is not a primary goal; you are looking for something that will haul the model skyward, allow it to cruise nicely, let the engine run smoothly, and stand up to the abuse of bounced landings.

In recent years, significant progress has been made in producing efficient, quieter props. The materials have gone from the widespread use of wood to composites of various types. Tip shapes, airfoils, and different ideas about "loading" the engine are all part of this revolution. There's also been a move from using the low end of the engine size range to using the upper end. The philosophy that you can reduce the throttle setting rather than trying to squeeze an extra 100 rpm has come into play.

Which prop to use? During my newcomer years, the prop recommendation chart that accompanied the engine was standard. You usually saw a 10 x 6 on a .40, an 11 x 7 on a .60, and smaller sizes on the .15 and .19, etc. The engines huffed and puffed their way through 10,000–12,000 rpm unfettered by mufflers, but fitted with a butterfly-style exhaust stack cover that helped provide back pressure so that you could get a reliable (hopefully) idle. Now the engines are propped for lower rpm, and the sound level is brought down with mufflers, tuned pipes, and efficient props.

When you are selecting a prop, consider the following factors:

1. What does the engine manufacturer recommend?

• Use that as a starting point. Since you probably won't need to strive for maximum performance, there shouldn't be too much of a problem.
• Read engine reviews in modeling magazines for further information. Reviews generally cover a wide range of sizes and pitches; look for those that fit the engine's "comfort zone."
• Engines are tuned or timed to work most efficiently in a particular rpm range. Two-stroke engines are more often happier singing along at a higher rpm than four-strokes. Four-strokes are restricted to lower speeds because of the valves, which tend to float at higher rpm.

1. Should I use wood or composites?

• Good question — difficult to answer. Wood doesn't tend to last as long. A ding on landing, a scrape here, a nick there, and a wood prop is history. However, wood is inexpensive compared to most composites.
• There is a possibility of inconsistency in the stiffness or hardness of a wooden propeller because of the nature of the wood itself. A hidden knot, a vein of softer wood, etc., contribute to create an unknown. Composites generally will be more consistent.
• Whether wood or composite, check the prop frequently for cracks or fatigue. Grasp the blades and twist or bend them to detect problems. Wooden props may develop cracks at the hub and along the length of a blade; composites tend to flex and create stress at the hub. If you have a spinner, these fatigue points may be hidden inside it.
• It's a good idea to check prop tightness at the start of each flying session. Wood props tend to compress over time and loosen. Props were designed to help models fly, not fly themselves!

1. Balance your props, and keep them balanced!

• Engines tend to vibrate enough without putting an out-of-balance "stick" on the front turning 10,000 rpm! While holding a model that is running way out of balance may be stimulating, imagine what it does to the engine and airframe.

Illumination

• Engine Loading: Every engine has a "happy zone" where the combination of prop, glow plug, fuel, revolutions, etc., produces a cool-running, efficient operation. The key is to find it. Start with the manufacturer's recommendations and experiment, but change only one item at a time.

• Back Pressure: Radio-control engines need a degree of back pressure on the exhaust outlet in order to idle well, and the muffler generally provides this naturally. Look at the muffler outlet size (small) and the exhaust area (large). Now you know how back pressure happens.

• Tuned Pipes: Kind of a muffler, but not really. The pipe is a chamber specifically designed and tuned to the engine that creates a resonation allowing the exhaust to move freely. Properly set up, tuned pipes increase rpm while at the same time "softening" the exhaust note.

• Valve Float: In a four-stroke engine, the valves are driven much the same way they are in an automobile. If you try to make the engine run too fast, the pushrods, springs, etc., just say "the heck with it," and never fully open and close. This is not a good thing.

• "Chicken Stick": A stick, usually covered with a plastic shield, that is used to turn the prop blade for starting. No, Gracie, it's a finger-saver, not a snack for between flights!

1. Diameter and pitch:

• The diameter is not hard to figure: it's the total length of the prop.
• Pitch is the twist or angle at which the blades are set. When you see a number such as 10 x 6, the first number refers to the diameter in inches, and the second is the pitch. Both blades will (should) carry the same pitch.
• Some props carry a "washed out" pitch, such as 12 x 6-10. This means the pitch of the blades varies from the hub to the tip. For the average newcomer, this type of prop is not a necessity, but it may be worth a try if you're having trouble getting the right rpm/"bite" combination to pull your model.

1. Other:

• A host of differences exist between brands of props, or even within the same brand. For instance, you'll find those indicating they are for four-stroke engines; some feature scimitar-shaped blades. Others feature wide blades, and still others are very narrow. Wide blades are designed for creating greater thrust at lower rpm. Narrow blades tend to be speed props where the thrust is created at much higher rpm.
• It is often suggested that for the APC line you increase either the diameter or the pitch by one number for optimum results. A 10 x 6 would suggest using an 11 x 6 or 10 x 7.
• Years ago, many of the "plastic" (nylon) props required specific steps before they were used to prevent fatigue, especially in the hub area. The newer composite materials generally do not require much preparation. Their construction is usually resin with fibers impregnated in the resin. Some use bits of fiber, while others use continuous strands running the length of the prop.
• Be certain to read and follow any manufacturer instructions. In the case of composites, the thin trailing edge may be very sharp. Dulling that edge is wise. You don't want a lawnmower-blade-type surface near your fingers, especially with four-stroke engines, since they have a propensity to backfire under certain conditions. If you are hand-cranking, this can be dangerous. Actually, you should always use a protective device to start an engine — from an electric starter to "chicken sticks" to a heavy glove.

Safety

A large proportion of accidents occur because of a turning propeller. While one would assume that it doesn't take a rocket scientist to figure out that getting some part of your anatomy caught in something turning 10,000 rpm will hurt you, day in and day out someone does just that. There are some safety steps you can take:

• Paint the prop tips a bright color so you can see the arc.
• Never put your body in the prop arc. It's common to see people lean over a model with their faces just inches away from the prop.
• Adjust the needle valve from behind the prop arc. Don't reach around! Engine manufacturers have recognized this, and most new engines have the needle valve angled away from the prop or allow you to place it in a remote position.
• Plan where you will put the starter box, cord, glow plug wires (if used), and transmitter to keep those items out of the prop. Make this placement a ritual.
• Watch loose clothing such as jackets. If you use a transmitter lanyard or strap, make certain it is tucked away. It will naturally swing into the prop if you're not careful.
• Have someone hold the model — firmly. Work out signals ahead of time so that your helper knows what to do.

Throw away a damaged prop. Break it so that others won't be tempted to retrieve it from the trash can. Don't try to glue it, splice it, or anything else. I've heard individuals state, after grinding a bit of the blade tips on takeoff, "Oh well, it's probably still balanced since I ground off both tips." Don't believe it! Besides, you may not see the split that runs lengthwise in one or both blades.

Give yourself plenty of runway-to-prop-tip clearance; a minimum of one inch is advised. Remember that even a model with tricycle landing gear will lose clearance on a hard landing!

Through the years I've watched modelers lavish attention on the cleanliness of the model, the engine, and their toolbox, and never give the prop a glance. That's probably one of the most important pieces on a model! Without a prop — a good one — you're going to need a really good throwing arm, a tall building, or to take up sailplanes. On second thought, the latter probably wouldn't work, because if you didn't take care of the engine, you probably wouldn't take care of the launch mechanism for the sailplane!

Last month I said I'd talk about props and field etiquette. I guess I lied! Too many prop revolutions, not enough thrust! Next month I'll discuss field behavior and assorted sundry. How's that for being redundantly repetitive?

Transcribed from original scans by AI. Minor OCR errors may remain.