Propellers By James Goss      In this article I would like to first describe the props we use in our hobby so anyone new to the hobby can have a better understanding of props. Next I would like to go a little deeper into this subject of props and explain prop efficiency and how a progressive prop such as the 26 x 8 - 12 operates. I know there has been a lot of confusion about progressive props and why they are used. I know this because I am the one that was confused until I did some research on the subject. The prop is just as important as the wing or anything else on the plane. Without a prop you could have the best engine money can buy and not be able to move your plane one inch. A propeller is really a spinning wing and if you look at a cross section of a prop you will see that it has an airfoil just like a wing has. It has an angle of attack and this angle changes along the length of the prop.      Our propellers rotate counterclockwise when looking at the plane from its front. As the prop's leading edge begins to rotate or move downward, air is forced down and behind it. This creates a void of air above the prop at its trailing edge and there is now a pressure differential created between the leading and trailing edges of the prop. That is to say that the area around the bottom of the prop is now under greater pressure that the area at the top of the prop. A positive pushing pressure at the bottom and a negative pulling pressure at the top is another way of stating it. As the prop rotates around, the reference to the top and bottom is constantly changing so we will just think of the top and bottom as being the top and bottom of the plane. Do you think of your plane as being pushed or pulled through the air by its prop while it is flying? Well it is both; the prop is pushing and pulling at the same time. The prop is a pump just like any other pump and is pumping air, so it has a high-pressure side and a low-pressure side. It pulls air in the front and expels it out the rear at a higher velocity and forms a jet stream. This jet stream leaving the prop is smaller in diameter than the size of the actual prop and is the key to understanding the operation of a prop.      By the jet stream being smaller in size than the prop, the velocity of the air will increase and provide what we call thrust. Thrust is measured in ounces or pounds of thrust. A good example of thrust can be found in your garden hose. When you adjust the nozzle on the end of your garden hose so the water will reach to its maximum distance, what is happening? Are you adding more pressure to the water? It was discovered many years back that the volume of liquid entering a pipe must always equal the volume leaving that pipe. If the pipe has a taper toward one end then the water will be restricted and resistance to the flow will develop. The same amount of water must still leave the pipe as is entering so in order for this to occur one thing must happen, the water must accelerate through the pipe. Its velocity will be increases in order to maintain an equal volume flowing through the pipe. On your garden hose the water leaving the nozzle has less pressure than the water entering the nozzle, but more velocity. The obstruction or taper in the nozzle becomes an orifice and has a high and low-pressure side. When you increase velocity, pressure will go down.  Our propellers work the same as water flowing through a pipe. By the diameter of the jet stream being smaller than the prop's diameter the air has to accelerate in order to maintain the same volume of air flow. This increased airflow is the thrust that our planes need to fly through the air. The diameter of the jet stream is smaller because the prop is turning in a circle and forces the air into a funnel some distance behind the prop. Think of it as having a giant piece of flat rubber that stretches from the prop to the tail of the plane and the rubber is the same diameter as the prop. As you turn the prop by hand the rubber will reduce in diameter at some distance behind the prop as the rubber twist. With the prop turning at high speeds the air will do the same thing and form a funnel shape.      The diameter of a prop tends to determine how much thrust is developed by a prop. It is not the pitch of the prop that determines thrust; pitch determines the speed of your plane. The larger diameter props will develop more thrust than one of less diameter. This was puzzling to me at first but I soon realized that it was just like the garden hose. A larger diameter prop tends to produce a jet stream that has a smaller diameter ratio to that of the prop. It's like turning the nozzle on your garden hose to get more distance. A larger diameter prop has the power to reduce the jet stream to a tighter funnel, thus more velocity for the air and more thrust for our planes.      As stated above the pitch determines the speed of your plane. Pitch is rated in inches and is stated as 10 x 6 on a prop in this example. 10 being the diameter of the prop and 6 is the pitch. If the prop was 100% efficient, each time it makes one complete revolution your plane would advance 6-inches through the air. It is like screwing a screw into wood, each time you rotate the screw it advances further into the wood. If the prop was turning in a solid material it could advance 6-inches, but in air it will not. Unlike a solid the air will compress and distort as the prop turns and this creates what is known as slip. Prop slip is the difference between the calculated speed of the air through a prop and the actual speed of the air. A 10 x 6 prop may only be rated as 85% efficient, so each time it rotates it will only pull the plane 5.1 - inches forward and not 6 - inches. What I would like the manufactures of props to tell us on each prop is how much slip they have or their efficiency rating. Knowing this we could better make decisions about which prop to buy. It could be stated as 85% efficiency at 8,000 rpm or something like that. By and large we need to know more about our props from the manufacture because like I said above, the propeller is one of the most important components on our plane if not the most important. Another rating I would like to see on each prop is its weight so we could compare props without having to weigh them. Is this asking for too much?      Now comes the good part, progressive pitch propellers. These are confusing so I would like to give you my thoughts on the subject. An example of a progressive pitch prop would be a 22 x 6 - 12. The pitch is stated as being 6 and 12, this means that the pitch of the prop progresses from 6 to 12 in a linear manner. At the center of the prop, known as the root or hub, the pitch will be 6 and progress to the tip where it has a 12 pitch. Remember that pitch is simply describing the twist angle of the blade. Here is an example of what I mean about this subject being confusing. This question was posted on RC Universe: PROGRESSIVE pitch Props like 18x6-14        Someone help me understand what a... say for instance a 18x6-14 pitch prop does for me as compared to a 18x6 or 18x7 pitch prop. I have a S.T.2300 in a 25% Cap 232. Just wondering what would be the best power prop in this airplane. Another One I thought it is the other way around???? I have a 14x6-10 and it sure looks like the root has a higher pitch than the tip. Here is my reply      You are right; it does look like the highest pitch is at the hub. This is the case with all props; you can see that a higher angle is definitely at the hub. This is because as you approach the hub from the tip of the prop, that portion of the prop is moving slower than the outer portion of the prop while rotating through the air. As you move toward the tip it actually looks like there is less pitch to the eye, but because the prop in that area is traveling faster it generates the same pitch equivalent, or air pumping ability, as at the hub. If the prop maintained a true pitch that you could see all the way down the length of the prop, the tip would process much more air than at the hub and this wouldn't work. So the pitch of a prop is an average determined by the increased speed of the prop as you travel toward the tip. I think a progressive pitch prop will be more efficient because of the following. Think of prop pitch as being the same as the pitch on the gears in your car. Higher pitch will equal better top-end performance and lower pitch will improve low-end performance. The prop will be more efficient at low speeds with the high pitch section of the prop and more efficient at high speeds with the low pitch section of the prop, or is that backwards? No props are 100% efficient because of a thing called slip. Slip is the difference between the calculated speed of the air through the prop and the actual speed of the air. A constant pitch prop may be 85% efficient in pumping air, that same style prop in progressive pitch may be rated at 88% efficient when flown at various rmp. It would depend on what type flying you do as to where you would benefit from a progressive pitch prop. James Goss      Let me say again that you can easily see the angle of the pitch is greater at the hub than at the tip on any prop. Again this is because this section of the prop travels through the air at a slower rate than does the tip. As an example lets say that we have a 12 x 6 prop turning at 10,000 rpm. The section of the prop 1-inch from the hub will be traveling at 59.5 miles per hour while the tip will be traveling 356 miles per hour.  From this you can see the section of the prop at the hub will need a larger angle to maintain the same airflow as the tip. A progressive prop will allow the tip end to produce a great deal more airflow than at the hub. High pitch props tends to have more slip and low pitch tends to have less, so you get an overall improved average efficiency rating for the prop that is going to turn at multiple speeds.     If a prop had the greatest angle of pitch at the tip instead of at the root, it would be referred to as a regressive pitch prop. I know they use this type prop on some boats for special applications. I wonder how it would work on an airplane? Well I know it would create a lot more noise with a higher angle on the tip because so much more air would be placed in motion at that high tip speed. This arrangement would also distort the air coming off the tips and that would change things. When one blade of a prop cuts into the air it leaves a trail of upset air for the next blade to cut into. The trailing blade is a little less efficient because of this distortion of the air. Of course the most efficient prop would only have one blade for that reason, but it would be a real pain to balance. That reminds me; always balance your prop because some of them will really be out of balance a great deal, especially the glass filled props.