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Whirl Propeller design & selection

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One of the results of the tremendous work accom- induced drag), horsepower, and propeller efficiency all plished by the Wright Experience as it researched and play an important role in determining an airplane’s exactly reproduced the 1903 Wright Flyer for EAA’s speed. The impact on airspeed by changing any of Countdown to Kitty Hawk was an increased apprecia- these is proportional to the cube root, which unfortution of the obstacles the brothers overcame in achiev- nately means it takes a big change in each factor to ing powered flight. One of the biggest obstacles the result in a significant change in airspeed. Wrights surmounted was developing a practical aircraft A propeller’s installed efficiency typically ranges propeller, which allowed them to succeed where between 65 and 85 percent, depending on the many capable individuals had failed. flight condition. For comparison, recent wind Propellers will be with us for some time tunnel testing at Old Dominion University during the second century of flight, so let’s Neal Willford, showed that the Wright Flyer propeller operlook at what affects their performance and ated at 75 percent efficiency at its flight EAA 169108 speed! Very impressive, those Wright brothdetermines their dimensions for a given application. A new spreadsheet is available to ers. download from the EAA website (www.eaa.org) Numerous factors affect a propeller’s efficienby scrolling down and clicking the EAA Sport Aviation cy, and propeller theory can get mathematically cover. This spreadsheet will help estimate the needed nasty in a hurry, so we’ll only discuss the simplest verdiameter and pitch and determine the ideal blade sion, called the momentum theory of propellers. shape for the highest efficiency. Suppose a propeller manufacturer could build the best theoretically possible propeller. Instead of a normal Propeller Efficiency propeller it would look like a thin disk of some diameHow propeller efficiency affects performance is best ter. illustrated by this formula: Vmph = (BHP x Propeller This “ideal” propeller would produce thrust by accelEfficiency x 146625/Drag Area) 0.33 erating the approaching airflow. Half of the acceleraIt shows that the total drag area (parasite and tion would occur before the air passed through the 56

JULY 2004

DEKEVIN THORNTON

LEEANN ABRAMS

LEEANN ABRAMS

JIM KOEPNICK

MIKE STEINEKE

Figure 1. Wood Propeller Diameter Chart

efficiency. As the thrust drops with increasing speed, so does the propeller’s airflow acceleration, and the propeller’s efficiency reaches its maximum at the airplane’s design speed. An airplane’s maximum rate of climb occurs at roughly 60 percent of the maximum speed, so its propeller efficiency in climb won’t be as high as the maximum speed efficiency at cruise. This is true whether it has a fixed-pitch or constant-speed propeller. Finally, a large diameter propeller leads to greater efficiency because more air flowing through the disk requires airflow acceleration for a given amount of thrust. However, there are practical limitations to increasing the diameter that we will now consider.

Diameter

Figure 2. Wood Propeller Performance for Various Pitch/Diameter Ratios

disk, and the other half after. How much thrust it produces depends on the weight of the air going through the disk times the acceleration of the air caused by the disk. Unlike regular propellers, the disk would not lose efficiency to drag on the blades or in the rotating slipstream—but losses would still occur. The momentum theory shows that high propeller efficiency occurs 58

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when: ■ the airflow acceleration is small; ■ the airplane is traveling fast; ■ the propeller diameter is large. The first two items are related. A propeller’s thrust is highest at low speeds and actually decreases with increasing airspeed. High thrust means high propeller airflow acceleration, and consequently, lower

We don’t fly in a theoretical sky, so instead of imaginary disks the job of accelerating air to create thrust belongs to the propeller blade—usually two or three blades on small airplanes. Among other things, a propeller’s diameter is limited by ground clearance. FAA certification requirements call for at least 7 inches of prop clearance when a taildragger is in a level position. An airplane with tricycle gear needs at least 9 inches of clearance. These are good minimum limits for homebuilt airplanes, too. Let’s assume that the gear legs are long enough so propeller clearance is not a limitation. There is an ideal diameter that depends on the airspeed, altitude, temperature, horsepower, propeller rpm, blade airfoil, and blade planform. Fixed-pitch props are always a compromise, so builders must decide which phase of flight has the best performance. The design condition is usually either maximum speed at sea level or maximum speed at 75 percent power, rated engine rpm, at 8,000 feet. Engine manufacturers provide graphs that show how horsepower output varies with engine speed. At low power settings the curve that

traces horsepower versus rpm is essentially a straight line. As the rpm increases though, the engine’s internal friction and losses in intake and exhaust efficiency increase, which results in less power being developed. This happens where the curve rounds off at the top as it reaches maximum rated power. The propeller speed is the same as the engine rpm unless it has a gear reduction drive. In this case the propeller rpm is the engine rpm divided by the gear drive reduction ratio. In the 1920s and ‘30s, systematic testing of a family of wooden and metal propellers gave airplane designers the information they needed to select the best diameter for the airplane’s design condition. Figure 1 (from Reference 1) shows the best wooden propeller diameters based on the design condition. Similar information can be found in References 2 and 3. The three curves in Figure 1 show that a prop’s ideal diameter also depends on its primary purpose. Climb propellers give better takeoff and rate of climb performance at the expense of top speed. Recalling the momentum theory, a larger diameter means a greater airflow through the propeller disk, and consequently less acceleration is required to get the needed thrust. The result is higher efficiency for those conditions. It’s no surprise that a climb propeller has a bigger diameter than one designed for cruise. A cruise prop emphasizes greater efficiency at top speed, or for a given cruise airspeed, a lower rpm. Its optimum diameter is smaller, partially because the tip speed limit depends both on the propeller rpm and the airplane’s speed. To stay below the tip speed limit, the prop diameter must decrease as airspeed increases. At cruise, because the airflow is approaching the prop at a higher speed, it doesn’t need a bigger diameter to accelerate the air as much to get the needed thrust. As a bonus, the smaller diameter has less EAA Sport Aviation

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blade drag, which helps increase the efficiency. Between the climb and cruise props is the “general service” propeller that is a compromise between both ends of the performance spectrum. This is probably the best choice for most applications. Propeller efficiency drops off rapidly as the tip speeds approach the speed of sound, so tip thickness and the airfoil used there play a large role in limiting diameter. For struc-

tural reasons wooden propellers need thicker blades, and this limits their tip speeds to about 850 feet/second. Aluminum propellers can use thinner blades, which increases their tip speed limit to about 950 feet/second. Tip speed is a major contributor to propeller noise, which is an environmental factor everywhere. Smaller diameter props or lower propeller rpm reduce tip speeds— and noise. Flyover noise is a grow-

ing issue in the United States, so the conditions and then added a third “airscrew,” which is a good analogy. prop diameter on future designs may blade of the same size, the prop A screw’s pitch tells how far it will in one revolution. be dictated more by noise than per- couldn’t effectively absorb the advance formance requirements. engine power at the design condi- Propellers are usually described by The propeller blade shape (partic- tion. Adding the third blade is like diameter and pitch, such as 72 by ularly on the outboard portion), widening the chord, because it 70, indicating a 72-inch diameter design lift coefficient, and airfoil reduces the ideal diameter. Propeller and a geometric pitch of 70 inches. used also affect the optimum diame- research showed that the diameter Pitch is usually measured at the 75 ter. How much power a propeller of a three-bladed propeller is typi- percent blade radius and indicates blade can effectively absorb depends cally 3 to 6 percent smaller than its how far the bottom of the blade at on blade shape; assuming the same two-bladed equivalent. A properly this station would advance in one airfoil, a wider blade chord will make designed three-blade prop also has revolution if screwed into a solid. A the ideal diameter smaller. We’ll dis- essentially the same propeller effi- propeller like this would require cuss blade shape and airfoil in more ciency as a two-blade prop for the increasingly steep angles on the inboard portions of the blade in detail later, but the main point is that same application. order for each section to advance the information in Figure 1 and the the same amount. This type of prop other references is valid for propellers Pitch with similar shape and the same air- A propeller is sometimes called an has uniform pitch, like the props on a rubber band-powered model. As foils. Otherwise, the recommended we will see shortly, this is not diameter may not be correct. desirable for most airplane Finally, the ideal diameter propellers. depends on the number Many homebuilts use metal props, and some Because air is of blades. If we sized a builders cut down larger fixed-pitch and constant-speed not a solid, how two-bladed prop props to achieve the necessary ground clearance. Builders should far a profor particular be aware of the problems choppin’ a prop creates, and EAA Technical Counselors should be on the lookout for modified props. Here are some of the harmful possibilities Jack Kane suggested to a builder/flier who wanted to trim 7 inches off the blades of a constant-speed prop and mount it on a solid-crank IO-320. ■ Unlike the higher-powered O-360, the IO-320 crankshaft does not have torsionalabsorbing counterweights. In certificated form, the 320 most likely had a yellow band on the tach that defined an rpm range to avoid because it’s where harmful interactions between the engine and prop blade frequencies resonated. ■ Before its blades were shortened 7 inches, the 84-inch Hartzell was probably designed for a Lycoming 540 or Continental 520/550 and found to have no potentially harmful interactions between the six-cylinder engine excitation (third order) and any blade natural frequency. ■ Shortening the blades changed one or more of the blade natural frequencies (most likely causing them to increase), but without tests the magnitude of the change is unknown. The change could easily cause a serious interaction between a blade frequency and the second-order excitation of a four-cylinder engine. ■ Metal props on direct-drive piston engines pose a daunting set of vibration problems. The solutions are nontrivial and include blade profiles, mass distributions, root thicknesses, and a host of other factors. Consider that a prop that’s safe on a certificated counterweighted IO-360 with 8.7 compression is unsafe after the STC’d installation of 10:1 compression pistons. ■ If you’re determined to fly this cut down prop, have Hartzell do a vibration survey on your engine/prop/airframe combination. It’s not cheap, but it sure beats the prospect of trying to land after the loss of a couple of inches off one blade tip (probably followed soon thereafter by the entire engine)! A resonant frequency in a metal prop blade leads to destructive vibration. Provoking the vibration is the power pulse from each cylinder, transmitted to the prop via the crankshaft, which cyclically loads and unloads the propeller. When operated at the power setting (rpm) that generates the resonant frequency, it will lead to a blade failure as the vibration quickly gains amplitude. Compounding the problem of resonant frequency is that the blade material (typically aluminum) has no endurance limit in a fatigue environment. Therefore, because an aluminum part has run for x-number of fatigue cycles, there is no guarantee that it will not fail during the next hundred similar cycles. —Cy Galley

Choppin’Props

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peller really advances per revolution is called effective pitch, and it varies with airspeed and rpm. Researchers found that dividing the effective pitch by the diameter was a good way to compare propeller performance. They called it the advance ratio and you can calculate it with this equation (the airspeed is in mph and the diameter is in inches): Advance Ratio, J = (1056 x Airspeed)/(rpm x Prop Diameter) Along with the diameter, pitch is the most important factor in determining a propeller’s performance. The same design condition that helps select diameter also determines the needed pitch. The Wrights were probably the first to observe that a propeller is essentially a rotating wing. A wing’s efficiency is highest when it’s flying at an angle of attack that gives the highest lift/drag (L/D) ratio. In a propeller’s case, lift represents thrust and drag and the amount of engine torque required to turn the propeller at the particular condition. High L/D is therefore an indicator of high propeller efficiency. Determining a wing’s angle of attack for maximum L/D is easy. It’s more difficult for a propeller. The airspeed, rpm, and airflow acceleration all play a role and must be considered at each position along the blade. To better understand these factors a lot of propeller research was conducted between the world wars. Figure 2 shows the results of some of this research, which in this case is for wooden propellers. Aluminum prop performance looks similar and can be found in the previously mentioned references. Figure 2 shows that each curve has a similar shape. The efficiency is lower on the early part of the curve because the blade sections are operating at a fairly high angle of attack, which means higher thrust but also higher drag. The angle of attack for the blade sections decreases as the airspeed increases, resulting in more favorable L/D ratios. Where each curve reaches its maximum indiEAA Sport Aviation

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cates that the blade sections are operating at the maximum L/D. The advance ratio where this occurs should represent the design condition. Further increases in airspeed result in the blade sections operating at angles of attack so low that they produce little or no thrust. The blades are still generating a lot of drag though, and require engine

power to overcome it. The result is a steep drop in efficiency. The black curve drawn through the peaks of the different curves represents the “general service” propeller, indicating the maximum possible efficiency for a given design condition. Similarly, the dashed black curve represents the maximum possible efficiency for a

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Figure 3. Pitch and Efficiency Chart for Wood Propellers at Design Condition

Figure 4. Typical Propeller Airfoil Sections

Figure 5. Approaching Airflow Affected by Cowling 62

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“cruise” prop. Both curves show that a propeller’s maximum efficiency increases with higher pitch/diameter ratios. High performance airplanes benefit from this, because they often need propellers with pitch/diameter ratios greater than 1. The shape of the efficiency curves shows why a fixed-pitch propeller can only be optimum for one condition. In some ways, a fixed-pitch prop is like a single-speed bicycle. The single gear is a compromise between ease of starting off and excessive pedaling at high speed. A multispeed bicycle is much easier to pedal in the lower gears, and allows the rider to develop more power at slow speeds and therefore accelerate more quickly. The same is true when comparing a fixed-pitch and a constant-speed propeller. The curves in Figure 2 are like different gears on a multispeed bike. A constant-speed prop doesn’t have distinct settings like gears, but the pilot can control the propeller rpm by changing the blade angle. At lower speeds pilots reduce the blade angle so they will operate at higher L/D ratios. The resulting lower blade drag allows the engine to develop more horsepower, and consequently more thrust for a given airspeed. If the propeller rpm is constant, then the dashed curve in Figure 2 represents the maximum efficiency of a constant-speed propeller over the speed range of an airplane. Figure 3 (also from Reference 1) shows the recommended pitch/diameter ratios and propeller efficiency for wooden propellers at the design condition. This information is only for propellers using the RAF 6 airfoil (developed in 1912!) and modified for propeller use by making the bottom flat so that it would be easier to carve. It is shown in Figure 4. It was the propeller airfoil well into the 1930s. It is still a good choice for fixed-pitch propellers with moderate to high pitch/diameter ratios. Propeller manufacturers used

other airfoils, and the Clark Y soon found favor. Figure 4 shows that like the RAF 6, it also has an essentially flat bottom. It has a better L/D ratio than the RAF 6, contributing to a slight increase in propeller efficiency. The higher efficiency is achieved at a lower lift coefficient of 0.4 (compared to 0.5 for the RAF 6), which requires the blades to be at a lower angle for the design condition. This results in the design pitch being 3 to 5 percent lower than a comparable propeller using an RAF 6 airfoil. The amount of thrust depends on the lift coefficient and the blade area and this means that if the diameter is not changed, a propeller optimized for a Clark Y airfoil will need a wider chord than one optimized for an RAF 6. Reference 3 suggests that the Clark Y is a better choice for lowpitch applications (like a climb prop), whereas the RAF 6 is better for higher pitched propellers. Over the years the Clark Y has been used extensively on constantspeed propellers. Today, flat bottom airfoils are not as critical to keep manufacturing costs down. Propeller manufacturers now use computer-controlled machines to make blades with airfoils optimized for the different blade stations. Using optimized airfoils provides some increase in efficiency, but compared to using the correct pitch and diameter, the airfoil efficiency has a secondary impact on airplane performance. Earlier we saw that the design airspeed plays an important role in determining the needed pitch. The airflow approaching the propeller would be equal to the design airspeed all along the propeller blade if it were mounted on a long shaft. However, Figure 5 shows that the airflow is not uniform as it approaches the propeller. It is slowed down by the presence of the cowl—just how much depends on how blunt the cowl is. Using a propeller with uniform pitch EAA Sport Aviation

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would result in the inboard portions of the blade flying in air that is slower than the design conditions. This results in that portion of the blade operating higher than the desired angle of attack. This leads to higher blade drag and possibly stalling of the inboard blade sections, with both contributing to lower propeller efficiency. The solution to this problem is to reduce the pitch distribution inboard (and consequently lower

blade angles). Complicating this solution is the shape of the fuselage behind the propeller. It’s not uniform, and the windshield, cooling inlet “bug eyes,” and lower cowl shape all result in different airspeed distributions along the blades as the propeller rotates. The best the propeller designer can do is to get the average velocity distribution at each blade station and repitch the propeller to accommodate it. This is the

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method used in this month’s spreadsheet and much of the efficiency loss due to the fuselage can be eliminated this way. The propeller efficiency shown in Figure 3 includes the losses due to the presence of the fuselage. Compared to Figure 2 (which was for propellers on a long shaft), the loss in propeller efficiency was not that much.

Blade Shape Aerodynamic research showed that a wing with an elliptical planform has the lowest drag due to lift. This is not the case for a propeller, however. The propeller’s rotation changes the ideal lift distribution along the blade from elliptical to a lopsided one that peaks out at about the 75-percent blade radius position. It wasn’t until the 1940s when National Advisory Committee for Aeronautics (NACA) researcher Theodore Theodorsen developed the necessary theory to determine the ideal propeller blade shape for a given design condition. This shape theoretically results in the highest possible propeller efficiency. The mathematics behind it are bad enough to send most of us running for cover, but fortunately NACA developed design curves that made using it pretty painless. Reference 5 describes the method and it is also used in this month’s spreadsheet. The ideal blade shape looks similar to that found on most propellers except that it tends to be narrower towards the tip and wider at the root. Using the ideal planform also can result in slightly higher blade strength when compared to using the blade shape often found on wooden propellers (shown in Reference 6).

Materials Propellers are typically made from wood, aluminum, or composites. Wood, such as walnut, oak, birch, and mahogany, has been used since aviation’s early days and still is a good choice for fixed-pitch 64

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propellers. It has high strength for those wanting more information the weight and it dampens out the on the subject. engine’s power pulses. Wood’s Composite propellers are main drawback is its poor resist- becoming more common on ance to erosion due to rain or run- homebuilt airplanes. Both fiberway debris. The leading edge glass and carbon fiber have been material put on by most wood successfully used, and can result propeller manufacturers in propellers lighter than those reduces this problem made from aluminum. though. Some propeller We Aluminum is manufacturers one of the best like MT use a don’t fly in a propeller wooden blade theoretical sky, so materials. It’s that is durable and sheathed in instead of imaginary disks fairly cheap. fiberglass. the job of accelerating air to Probably its As you b i g g e s t can see, create thrust belongs to the drawback is there are propeller blade—usually that an alumany conminum prop siderations for two or three blades on can act like a determining small airplanes. tuning fork. the best propeller Engine power pulsfor a particular es, rpm, and aerodyinstallation. A fixednamic forces can make it respitch propeller will always be onate at certain frequencies, some sort of compromise, so don’t which can cause the propeller expect it to do everything. Just pick a blades to fatigue. Airframe and propeller that is best suited for the propeller manufacturers often type of flying you most want to do, work together to run propeller or better yet, follow the designer’s stress surveys to find and elimi- recommendations. Many homenate dangerous frequency prob- builders have carved their own prolems. This process can be expen- pellers, and References 6 and 8 will sive and time consuming. be helpful for those interested in Reference 7 is good reading for doing so.

References Engineering Aerodynamics, 1st Edition, Walter Diehl, 1928, Ronald Press. “The Aerodynamic Characteristic of Full-Scale Propellers,” NACA TR-640, Hartman, 1938. Aircraft Propeller Design, Fred Weick, 1930, McGraw-Hill. Airplane Propeller Principles, Wilber Nelson, 1944, Wiley and Sons. “Application of Theodorsen’s Theory to Propeller Design,” NACA TR-924, John Crigler, 1945. “Design and Build Your Own Propeller,” Fred Weick, EAA Sport Aviation, December 1960 (reprint of NACA TN-212). “Propeller Fatigue,” L. Sunderland, EAA Sport Aviation, November 1972. Propeller Making for the Amateur, Eric Clutton, n.p. (All NACA reports can be downloaded from http://naca.larc.nasa.gov/)