Keith Shaw on Props
A Tech Talk to the Midwest R/C Society, November 4, 1998 by Keith Shaw
(Courtesy of the EFO Newsletter)
Most people don't think too much about their props, although the prop is very important to the aerodynamics of the plane. There can be performance improvements of between and 30-40% selecting the correct prop for a job. A prop is a rotating wing with span, diameter and aspect ratio. The materials that make up a prop can vary. The characteristics of the material are important in selecting the correct prop. The way to measure the pitch varies, but in modeling it is measured to the bottom of the prop.
Early props were made of gum wood while many of today’s are made from rock hard maple. Wood happens to be good at keeping itself together when tremendous forces are trying to pull the blade off from the center. An important safety aspect is to remember NEVER to stand in line with a spinning prop.
Safety and Care of Props
Look for good wood in a prop. The grain should be close and going with the blade. There should be similar patterns on both blades. A lot of the available props have the grain cut at an angle. The same band of props can be made out of good and bad grained wood. The wood should be knot free. Wooden props are by far the better props. Plastic props are getting better but can't be modified like wood.
Plastic Prop Care and Feeding
The hub area on the plastic props is getting better. The Graupner props and APC props have a good size at the hub now. Be sure to remove the flashing on the trailing edges of a plastic prop. All props should be balanced.
Don't cut the prop down to fit a spinner. Make the spinner fit the prop. The prop should not touch the spinner holes. Use a reamer not a drill to enlarge a prop hole. A drill can cause an off center or angled hole.
Every prop needs to be balanced. Vibration kills airplanes. Vibration kills wire connections in the radio and its components, including the crystal, servos and NiCads. A lot of radio problems are caused by vibration. The structure of the plane can break. One reason that electrics live longer is because of less vibration. Propellers, even a little out of balance, can have a big effect.
Balancing is not just having the prop stay in a horizontal position but the prop should hold its position at any attitude. You can balance a prop by using clear dope, if the blade is only a little out of balance. If it's to far out of balance to use the dope method, you need to remove some material. To remove prop material use a single-edged razor blade held at 90 degrees to the front of the blade. Never remove material from the back of the prop. If the varnish is removed, remember to reseal the prop.
Another method is to use a clear tape. The ONLY tape that works is 3M decorator tape in clear. It is to be put on the back of prop. The tape has stayed on at up to 20,000 RPM.
When cleaning the plane you should clean the prop too! Clean off all of that green crud and inspect it. Is it nicked? Is it secure?
Why One Prop Works "Better" Than Another
Why one prop works better than another was always a question for Keith, and electrics have allowed him to compare props with better results than were possible with internal combustion engines. Keith noted that the best props off the shelf are Rev-up which have a nice airfoil all the way through the blade and the APC props, which seem to work well at high RPM. Keith considers the Zingers to be a good prop kit. He has found some of the best wood in Zinger props, as well as some of the worst! Besides the varying wood quality, the main flaw with a stock Zinger is the tip. At lower speeds, anything less than racing, the tip is best if it has an elliptical shape. The shape of the tip should be like that of a Spitfire wing.
There are two-stage props that have a larger pitch towards the tip. Everything happens in about the outer 1/3 of the prop. The lower pitch on the inside is to allow it to work easier, while the outside portion of the tip is working harder, at the same RPM.
Props do make noise, but it takes a really high rpm to generate it.
The diameter to pitch ratio is important. 12x6, 11x8, and 10x10 props absorb about the same amount of horsepower. Trainers and biplanes should use a diameter to pitch ratio 2:1 (i.e.. 12x6). Very draggy planes like a tripe might be better off using 3:1 (i.e.. 14x6). Sport planes can use a 1.5:1 (i.e.. 9x6). Fast planes racing planes might use a 1:1 ratio (i.e.. 10x10).
A close approximation to the prop speed is pitch in inches times RPM in thousands The prop pitch speed is NOT how fast the plane is flying. It is the theoretical speed that the prop would move, if it weren’t attached to a plane! Prop speed should match the type of plane.
P-factor is caused when the plane is flying in a different direction than the prop is spinning. It is particularly noticeable in a climb.