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-From Kim Kramar  General Information Sometimes we become so involved with the various types of model engines, performance ratings, and displacements that we overlook something important. Just as a fully race-prepared car engine is useless until its power is transmitted to the ground, a model engine is useless until its power is transmitted to the air. Most often, model aircraft use propellers for this job. Our engines also need to burn fuel to produce power, and they need to ignite that fuel. These functions, which are directly connected to the engine, are this month’s subject. A model’s engine is only as good as its propeller. The propeller’s size, shape, and composition determine how much of the engine’s power is transmitted to the air and the manner in which the aircraft can best use that power. The best combination of propeller characteristics for a particular model is a compromise. The pilot must choose a propeller that produces the best performance based on the aircraft’s mission (training, racing, aerobatics, combat, etc.), the engine’s power range, and the flying-field conditions. A racing airplane would do best if its propeller were designed solely to produce high airspeeds while rotating at the same rpm at which the engine produces maximum horsepower. This is the right choice even if durability, climb, and acceleration rates are sacrificed. Choosing the right propeller requires understanding and a few “prop tips,” one of which is that a propeller’s blade rigidity is important. A propeller is nothing more than a rotating wing. All propellers have airfoil shapes and direct their lift in a horizontal path, called thrust, instead of a vertical direction, as does the aircraft’s main wing. Thrust pulls the aircraft forward.
  The 2.5-inch spinner reduces propeller draw while streamlining the model's front end. Removing the spinner reduces the engine's top rpm by 450—a 4% power loss. Imagine how much of your aircraft’s wing lift would be lost if the outer third of the wing were to flex enough that its incidence—its angle of attack (AOA) to the oncoming airstream—significantly decreased during every turn or climb. In the same way, a propeller in which the tips flex does “flatten out,” reducing its incidence during acceleration and climb, thereby losing thrust when it is most needed. Unlike a wing, which develops lift along almost its entire span, a rotating propeller produces the majority of its thrust centered around the 75% point of each blade’s length. This makes the thrust lost caused by tip flexing even more critical. Stand slightly behind and to the side of the spinning propeller and watch the tips. If they follow a wavy path, that signals excessive pitch loss (lower propeller AOA), which results in power lost transferring the engine’s energy to the air. The first 20% of a propeller blade’s length—its span—produces much drag but little thrust. This section is the area where the propeller’s round center—the hub—tapers into the working “wing” of the blade, which does all the work. There is little “wing area” here. This area also moves the slowest through the air since it is closest to the center of the “disc” formed by the rotating propeller. However, this inner section does rotate and therefore produces air drag. This is why spinners make propellers more efficient. The next 50% of the blade’s span is the area where the LE-to-TE width—the chord—increases to maximum and the airfoil becomes fully developed. Some thrust is lost until the blade is fully formed, and more is lost because the center-section rotates more slowly than the remaining outer blade area. Since a wing’s total lift depends, in part, on its airspeed, the lift produced by different blade sections depends a great deal on their rotational speeds. How different are these rotational speeds? The blade section 1 inch out from the hub of an 11-inch-diameter model propeller rotating at 11,000 rpm has an “airspeed” of just 96 feet per second (fps), or 60 mph. The middle of the blade is rotating through the air at 260 fps, or 180 mph, and the 75% point is moving at 396 fps, or 264 mph. Even though the blade’s area near the tip (90%) is much less than that near the middle, it is moving nearly twice as fast, at 475 fps, or 317 mph, and is therefore producing more thrust than the center-section is. Please study that last rotational speed. The tip itself is moving at 530 fps, which is approximately the same speed as some .45-caliber bullets. If you want to know what happens if you are careless enough to put a hand into a spinning model propeller’s arc, envision pointing a Colt .45 at your hand and pulling the trigger! Not an attractive image. Please be careful. Tune your engine while standing behind the propeller, never stand directly to the side of a spinning propeller, and keep children away from your engine at all times. Since rigidity is important to propeller performance, a major factor to consider when choosing a propeller is its construction. Today they are usually made from one of four basic materials: fiberglass-filled nylon composite, fiberglass-reinforced nylon, wood, or carbon fiber (CF). The 2.5-inch spinner reduces propeller draw while streamlining the model's front end. Removing the spinner reduces the engine's top rpm by 450—a 4% power loss.
Propeller Pitch
The easiest way grasp the concept of propeller pitch is to draw a parallel to the gearing in your car.
Propeller Material Wood propellers are the lightest and present the smallest load to an engine assuming all else is equal (diameter, pitch and shape). They are capable of turning higher RPM than a heavier propeller. Wooden propellers are also the most easily broken. My opinion is that wood propellers are the most efficient and best performing in the air. Others disagree and they may be right because I'm not a good enough pilot to really have an opinion. In any case, if you nose over your planes often enough then wood propellers are probably a poor choice for you. Pure Nylon propellers were once manufactured, but for the most part they have been replaced by nylon composite construction. The fiberglass-filled nylon propellers are safer and stiffer than the old nylon-only variety, but nylon remain the most flexible kind. This flexibility is a major advantage for newer model pilots. The blades bend well on those poor landings that bend the nose wheel back nearly far enough to touch the fuselage bottom. Fiberglass-filled nylon propellers bend backward and usually do not break in those situations. Nylon props also last the longest when flying from paved runways. However this flexibility is not all good. Nylon propellers are so flexible that they twist in use which means they are constantly changing pitch. This flexing also creates a lot of vibration. The end result is akin to spinning the wheels of your car — a lot of energy is going to waste. Many pilots don't use them for these reasons. Fiberglass-Filled Nylon propellers are the heaviest propellers and also the most durable. This durability saves money and keeps newer pilots flying on those days when they would have exhausted their supply of more rigid propellers. Most RTF trainers are equipped with the fiberglass-filled nylon variety for exactly these reasons. These are a good choice for beginners because they hold up better than other types to propeller strikes. They are less efficient than wood or carbon fiber propellers, however. Most fiberglass-filled nylon propellers have large blade areas to improve their performance. They produce excellent thrust for a given rpm but tend to rotate more slowly than same-size propellers of different construction. These propellers suffer the most thrust loss as the airplane climbs steeply since the outer blade areas flex the most under stress. Carbon Fiber propellers are very rigid, but extremely expensive. I have seen carbon fiber propellers only for large engines. They may be available in smaller sizes in the future. All of the above propeller materials maintain their shape well under load. Wood and carbon fiber are best. Fiberglass-filled nylon propellers are the most flexible of the recommended propellers , but not enough to cause significant problems. Pure Nylon propellers are always a poor choice unless you crash every time you fly. If that's the case, then nylon propellers aren't the answer for you. Stamp collecting is.
Weight Heavier propellers have the advantage of flywheel action. Flywheel action will allow a lower, more reliable idle. Note that all properly designed engines idle reliably anyway, so this is really a moot point.
Another consideration is balancing the aircraft. Personally, I think the best propeller should be chosen for flight qualities, not for weight, but if the aircraft is close to being in balance then changing to a propeller of different weight may put the CG on the money.
Shape Low RPM engines, such as four-strokes, use wide blade propellers because four-strokes turn fewer RPM. The lower RPM means the air that the trailing blade is entering is less disturbed. Additionally, four-strokes have more torque than two-strokes. Because of this, a four-stroke can swing a wider propeller efficiently. High RPM engines, such as two-strokes are more efficient with narrow blade propellers due to the more disturbed air caused by the higher RPM.
I can't tell the difference between a scimitar blade and any other shape. I don't even know what the difference in performance is supposed to be so I can't help you here.
Determining the Range of Suitable Propellers for an Engine Engines like to do work. They are happiest running with a load. An engine can over-rev and be destroyed in short time if it has too little of a propeller load. If the engine has too much of a load it will be sluggish and tend to over-heat. Any engine can swing a wide range of propellers provided the propeller load is within the engine's comfort zone As a rule of thumb each inch subtracted from one property (pitch or diameter) allows one inch to be added to the other. This theory permits any of the following propellers to be used on the same generic .40 size engine:
This rule breaks down as the propeller size moves farther from the center of the range.
For example the engine above may not be able to swing a 12" propeller of any pitch without over-heating. Or it may have to drop additional pitch. The 12 x 4 propeller may not work, but a 12 x 3 might if such a thing is actually available.
Also note that the load presented by any given propeller varies by manufacturer and material the propeller is made from. One manufacturer's 6" pitch propeller may be the same as a 7" pitch from another manufacturer.
 Balancing a Propeller An out of balance propeller or spinner can cause a lot of problems. Here's a short, albeit incomplete list:
I do not bother balancing spinners. So far I have not had any real problems due to this, but as I mentioned earlier, I do not wring out my engines. If I did have a spinner that was noticeably out of balance I would probably make a couple attempts at balancing it and if that did not work I would just discard it unless it was particularly expensive. Typically a spinner is balanced by drilling small indentations into the backplate on the heavy side. It is a lot of trial and error and if your results are anything like my attempts, then you will have pits all over the backplate with no significant improvement in the balance. Obviously I am doing something wrong here. There are a variety of propeller balancers on the market. The worst of these are the type that wedge a propeller between two cones on a shaft that is held between your fingers. This is not real accurate, but it is better than nothing at all and it is inexpensive. Robart makes an excellent propeller balancer called the High-Point balancer. The High-Point has been very popular since it was first released because it is accurate and easy to use. It also uses two cones and a shaft, but the shaft rides on four large, free turning wheels. The idea is that the out of balance condition is magnified due to the smaller shaft having a leverage advantage — like a small gear turning a large gear. The High-Point is not limited to aircraft propellers. Anything that is has a hole in it can be balanced statically. This includes spinners, helicopter rotor heads, flywheels, boat propellers, etc. Before you attempt to balance a propeller, be sure to clean it. I wipe the propeller down with alcohol or wash it in warm, soapy water. Most propellers are close to being in balance when you buy them so they should only need a small amount of work to bring them into perfect balance. If the propeller is severely out of balance I return it because too much material would have to be removed which would significantly change the shape of the blade. If one blade is heavier than the other, then the usual method to bring the propeller into balance is to remove material from the heavy blade using sandpaper. Do not trim the tip of the heavy blade! The blade may balance statically, but it will not be balanced when the engine is running do to unequal mass distribution. Some people say to remove material from the face (front) of the prop and others say to remove it from the back. I have done both and have never noticed a difference. Usually I sand the face a little. If you use wooden propellers and you sand them to bring them in balance, you may also remove the fuel-proof coating. Obviously you don't want your propellers to become fuel soaked. Another way to balance a propeller is to spray a very light coat of clear fuel-proof paint on the light blade. This is an exceptionally good method because if you spray the paint evenly it distributes the added weight of the paint evenly over the blade. It also will not change the shape as much as sanding will. The only drawback to this method is that you can't check the balance until the paint has dried because you have to wait for the solvents in the paint to evaporate. |
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