Firewall Forward: Choosing a Prop for Your Project

In the first three articles of this series, we’ve looked at choosing an engine for your homebuilt project, with a brief departure dedicated to the Lycoming Engine School. Now we’ll look at propellers. Engines get a lot of attention from amateur airplane builders, but propellers need just as much attention as engines and at about the same time in the building process. They go together like a horse and carriage, as in the old song. There may be as many propeller choices as there are with engines, but they boil down to two basic options: constant-speed or fixed-pitch. Constant-speed propellers give better performance, but at the added cost of price, complexity and weight. Fixed-pitch propellers are generally less expensive and lighter, but they can only be optimized for a portion of the airplane’s performance envelope. There are many factors to consider as you weigh the pros and cons of each.

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Not every propeller that will bolt up to your engine is a good choice for your airplane. The science of matching propellers to engines is complex and ultimately based on a lot of field testing. Each propeller manufacturer will have a list of propeller and engine combinations that it has tested and found to be good matches. This information should be consulted and taken seriously. Damage to your airplane, including catastrophic failure, could be in store if you use the wrong propeller. Some will work on certain engines only if specific rpm restrictions are heeded. If that is the case for you, the appropriate warnings need to be on placards and on the tachometer. Other propellers will work fine on some engine models of a certain size but not on others. Be sure to ask the manufacturer if the propeller you want to use will work on your engine.

Fixed Versus Constant-Speed

If you are on a tight budget, a fixed-pitch prop should get the bulk of your consideration. A wide variety of brands and styles is available to give you pretty good performance for a reasonable price. These range from wood propellers by small manufacturers to metal and composite models from industry leaders such as Sensenich. As a general rule, a fixed-pitch propeller will be lighter and less expensive than a constant-speed propeller for the same engine. On the downside, either takeoff or cruise performance must be compromised to some extent when the pitch of the propeller is fixed, because the optimum pitch will vary depending on engine load and airspeed. The single pitch chosen by the manufacturer is always a compromise.

If you are looking at a smaller engine such as the Lycoming O-235, a fixed-pitch prop is the default option because of the engine’s solid crankshaft. (So-called hollow-crank engines have an oil passage through the center of the crank behind the prop flange used to feed hydraulically controlled props.) There are some ground-adjustable props that allow you to change the pitch to fit the particular mission, but they cannot be adjusted in flight. Also, there are some options for variable-pitch props using electric pitch-change motors; these tend to be expensive and somewhat rare.

As a practical matter, most people using the O-320 series of engines do not go with constant-speed propellers, either. Many versions of the O-320 can be used with hydraulic constant-speed props, but if greater performance is needed, it would make more sense to upgrade to a 360-series engine. An O-360 with a fixed-pitch prop will be less expensive and no heavier than an O-320 with a constant-speed propeller, so why not go that way and forgo the complexity of a constant-speed prop?

Benefits of Constant-Speed

For many builders, the superior performance of the constant-speed prop outweighs the extra weight, complexity and cost that come with it. The ability to have the optimum pitch for every situation allows the engine to produce maximum available power on demand. It also keeps engine speed constant even when the airplane experiences pitch changes that would increase or decrease engine speed if a fixed-pitch prop were used. This means maximum takeoff and climb performance and optimized cruise performance can be achieved with the same prop. Constant-speed propellers have come to dominate the airplane scene with engines of 180 horsepower or more for good reason. People like the performance and stability they deliver, even at the higher price.

Table 1 gives an example of the cost/weight/price trade-offs involved. This list is not meant to be comprehensive, but merely a sample of some possible choices. For the most current information, you should consult with vendors selling these products. The example in Table 1 is for a Lycoming O-360 engine like the -A1A that can be fitted with either type of propeller.

Propeller Materials

Props are made of three basic types of material: wood, aluminum or composite. The type you choose has implications for cost, performance and maintenance. Wood propellers typically cost the least, but are also the least durable and can require the most maintenance. Metal propellers are more durable, but they are also the heaviest. Composite propellers, both fixed and constant-speed, represent the newest technology. They offer good performance and light weight, but typically cost more than more traditional alternatives.

Wood propellers are the lightest and least expensive. A wood prop for a four-cylinder Lycoming will weigh about 15 to 20 pounds, complete with a short extension and bolts. That is less than half the weight of a typical fixed-pitch aluminum prop. Good wood props start as low as $ on up to $. Wood props are not sensitive to the kinds of vibration problems that can bedevil aluminum props, but they are sensitive to moisture and do not hold up well in heavy rain or float operations.

Aluminum props are the most common and have proven to be durable while still reasonably priced. There is a wide variety in both fixed and constant-speed versions, and just about any combination of engine and airframe has an aluminum propeller that will work for it.

Materials such as fiberglass, Kevlar and carbon fiber make the new composite propellers lighter than aluminum, just as strong and less prone to vibration problems. A composite MT or Whirl Wind propeller will weigh 10 to 15 pounds less than a comparable Hartzell metal prop. In response, Hartzell is now producing composite propellers for four-cylinder engines. A composite constant-speed prop will cost $ to $ more than a similar aluminum prop.

Pitch and Diameter

When considering propellers, there are two considerations that dominate decision-making: pitch and diameter. Pitch is the bite the prop takes of the air as it turns (or how far the prop tries to advance through the air for each turn). It is measured in inches. Greater pitch means greater aircraft speed, and as you might imagine, the greater the prop’s pitch, the more power it takes to turn it. Because power or thrust is opposed by drag, either more power or less drag is needed to take advantage of greater pitch, and thus go faster. And because thrust and drag are relatively fixed in any airplane, you must choose a pitch that gives the best compromise of takeoff performance (where less pitch is preferable) and cruise (where more pitch is preferable). All of this mainly applies to fixed-pitch propellers. Constant-speed propellers change pitch to provide near optimum performance in any situation. However, there are limits to the speed range best served by any model of constant-speed prop, so pitch is still something of a consideration.

Large-diameter propellers produce more thrust, at least to a point. But diameter also ends up being a compromise, because it is limited by things like ground clearance and tip speed. In a trigear airplane, clearance between the tip of the prop and the ground is a major concern. A prop with only 7 inches of clearance (minimum clearance as per FAR 23.925) must be landed and taxied carefully to avoid striking the prop on the ground. Nine inches of ground clearance is really better for trigear aircraft. Because of this problem, many trikes can only handle a prop of 70 to 74 inches in diameter. An 80-inch prop that might work fine on a bush plane with conventional gear and big tires may come much too close to the ground for safe operation when installed on a trigear aircraft with smaller tires.

Another limiting factor of diameter is tip speed. As a prop is turned at maximum rpm (usually to ), the tips can approach supersonic speeds if the diameter is too large. That means a loss of efficiency and an increase in noise, both undesirable. Finally, certain engine features may limit propeller diameter, mostly those related to the torsional vibrations set up in the crankshaft as a result of size and shape of the prop.

Choosing the Right Pitch and Diameter

Unless you are among the first to build a particular type of airplane, your best sources of information for pitch and diameter will be the airplane designer and those who have built the same type of airplane. The propeller manufacturers will also have recommendations for the more popular kits and plans. Sensenich, for example, has some specific recommendations for many Experimental models with the more popular engines.

If you are building something that is a bit out of the mainstream, or if you just want to do the math for your own edification, there is a formula that can get you close.

P = V x /R

where
P = pitch in inches
V = cruise speed in mph
R = engine speed in rpm

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The more your flying requires optimum takeoff and climb performance, the greater your prop diameter should be. Greater diameter produces more thrust, subject to limits, but the more you need to get off the ground (or water) quickly, the more you will be willing to push those limits. Bush planes and floatplanes both need big props to do their jobs well, which is why these planes typically have propellers of 80 inches diameter or more. However, it is still possible to have too much of a good thing. The tip speed of an effective prop needs to be kept to a maximum of 0.9 Mach (9/10 the speed of sound), and 0.85 Mach is better. To determine whether a prop is too long, you can easily compute the tip speed at maximum rpm. Here is the formula:

Vt = D x R / 229.9

where
D = diameter in inches
R = engine rpm
Vt = propeller tip speed in feet per second

For an 80-inch prop turning rpm, the tip speed is:

Vt = 80 x / 229.2 = 942 ft/sec.

Using ft/sec as the speed of sound at sea level, we get 942/ = 0.86 Mach, which is within the 0.9 Mach limit. If the prop diameter is increased to 83 inches, the tip speed goes up to 0.89 Mach, which is about as far as you can push things before you start to get into trouble.

Teamwork

The engine and propeller are a team when it comes to powering your airplane. As such they must work well together to provide good performance. There are good engines and there are good propellers, but not every good engine and good propeller mate well. Both components must be matched to each other and to the mission of the airplane. Spend as much time and effort looking for the best propeller as you do looking for the best engine. You will be glad you did. Take advantage of the technical support people available at the various propeller manufacturers. They definitely want to sell you a propeller, but they also want to sell you the right one for your project. Use the resources available to you. That’s why they’re there.

Up Next

With an engine and propeller selected, it’s probably time to think about the tools you’ll need to install the firewall-forward components in your aircraft. Builders who have experience working on cars or motorcycles will have most of the basic shop tools that are required, but a number of specialty tools are unlikely to be found in most builders’ toolboxes. We’ll take a look at these in the next installment.

Previously Owned Props

Used propellers can represent a good value for the amateur builder. But once again, knowing the difference between used and used up is vital. If a propeller has been overhauled by an FAA-certified repair station, it will have a yellow tag with it that identifies the prop and when it was overhauled. That should be reasonable assurance that the prop is serviceable. A propeller without a yellow tag needs to be checked out by someone knowledgeable, which means a prop shop, not just any mechanic. Propellers get cut down, reshaped and re-pitched over time, but without the proper information and careful measurements, it can be hard to tell if a prop is still within the limits of serviceability set forth by the manufacturer. Corrosion is also a major concern. A trained eye is best for spotting problems with that bargain prop you are considering.

Props have logbooks, too, or at least they should. You will want to know the propeller’s service history and, if applicable, that ADs have been complied with. Without logs you may have to pay for an overhaul to be sure you are getting a good prop. As with engines, any significant lack of information should be reflected in the price you pay.

Here are some things to consider when shopping for a used prop:

• Props that have bent blades need to be viewed with some skepticism. It is possible to straighten some bent blades if they are not bent too far and if the bend is near the tip, but you will need a prop shop to tell you if any particular one can be saved.

• Props that have noticeable corrosion, especially around the hub, can be unserviceable if the corrosion is too bad.

• A prop with no yellow tag and no logbooks should be sent to a prop shop for evaluation before purchase. Further investigation is required before you know what you are getting.

• A prop with a gouge in the leading edge that is more than an 1/8 inch deep may be ready for the scrap heap. Gouges of more than 1/16 inch deep on the face of a blade are also bad news. Each propeller maker has guidelines for what can be repaired and what cannot. If there is any doubt, consult them.

• If there are any signs of welding on any aluminum prop blade, move on. Welding is not permitted.

• Wood propellers with signs of cracking or splitting should be avoided. They will not stand up to the rigors of flight for long. Also be careful if there are any signs that the metal leading edge protectors of a wood prop are not secure and in good condition.

Damaged composite props need to be checked out by someone with expertise in that type of prop. This is an extremely specialized field, and only someone with expertise will be able to tell whether the damage can or cannot be easily repaired.

—D.P.

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