If two-blade propellers are the most efficient, then why don’t all propellers have two blades?The short answer is because efficiency doesn’t propel the airplane, thrust does. Efficiency is the ratio of the power coming out of the propeller to the power going into it. A two-blade propeller is capable of achieving a higher efficiency than a three-blade propeller and so on, but at the same time it uses less power and produces less thrust.

If you were to operate a propeller at a lower power setting than that for which the efficiency is at its peak, you would have a lower thrust and also a lower efficiency. Likewise if you operate at a higher power setting, the thrust will be higher but the efficiency is lower there also. There is therefore an optimum power setting for each propeller where its efficiency will be highest. If conditions require more thrust than is available from this optimum power setting, then the power must be increased and prop efficiency begins to fall off from its peak value. There reaches a point where a propeller operating at a power higher than that which results in peak efficiency has the same efficiency as a prop with more blades operating at less-than-optimum power. Further increases in power favor the performance of the propeller with more blades. This is because the propeller with fewer blades is no longer operating at its peak efficiency.

Do you have a prop for an (airboat, hovercraft, or wind tunnel)?Hartzell propellers are designed and certified for aircraft use only. While there may be other places where they would work, we cannot make a recommendation without a detailed design analysis of the application and a series of tests.

Do you make fixed pitch props?No, Hartzell Propeller only makes constant speed props.

Could I get a 5 or 6 blade prop for my 200 hp single?Hartzell does manufacture propellers with 2,3,4,5 and 6 blade hubs for piston and turbine engines. However, all engine mounting flanges are not the same. Currently, the propeller hubs that will mount on aircraft piston engines are available with 2,3 and 4 blades. The 5 and 6 blade hubs are only manufactured for turbine engine installations. The optimum number of blades for an installation will depend on the diameter limit, power, speed, and altitude. More blades will always weigh more and cost more. All of these factors must be considered in the propeller selection.

How much faster will my airplane go with your prop?Generally, it requires a large increase in propeller efficiency to produce a small increase in cruise speed on general aviation aircraft. If the propeller was well matched to the engine and airplane performance on the original installation, it may not be possible to increase the cruise speed by a significant amount. However, this is not always the case, and in some installations

there have been measurable speed gains with Hartzell propellers. Check with the TC / STC holder to see if there has been any accurate performance tests conducted on the specific installation.

Will a three-blade prop make my airplane quieter inside?Yes, in most installations. Cockpit noise comes from a variety of sources; engine, exhaust, slipstream, and the propeller. Vibrations are also perceived as noise in the cockpit. A two-blade propeller produces an inherent once-per-revolution vibration that shakes the airframe, so a three-blade propeller will be inherently smoother and therefore quieter. In a single engine airplane, the propeller blade wake will beat on the windshield producing noise. Changing from two to three blades reduces the wake intensity and also increases the frequency of the beating, which is perceived as being quieter. In a twin, the reduced diameter of the three blade propeller will result in less tip generated noise and a greater clearance between the blade tip and the fuselage. Both of these characteristics will reduce cabin noise.

Why is the scimitar blade better than a conventional straight blade?Propeller designers try to achieve maximum performance while meeting limits on diameter, weight, strength, and noise. High efficiency comes from running the blade tip speed close to the speed of sound. This is also the source of most of the propeller noise in a typical piston powered airplane. Sweeping the blade tip back reduces the noise generated and may also improve the efficiency.

What is the difference between an oil-to-decrease-pitch and an oil-to-increase-pitch propeller? Are they interchangeable on a given installation?It is not advisable to interchange propeller types on a given installation. The following explanations show why each propeller type is selected for a particular aircraft.

For most general aviation propellers, an oil-to-increase pitch propeller is the simplest, lightest and most affordable form of controllable propeller. When the prop is rotating, centrifugal force acts not only to pull the blade firmly against the hub, but also to twist the blade toward low pitch. Therefore control force only needs to be supplied in one direction – to increase pitch. The absence or reduction of this control force naturally results in a reduced pitch. The governor continually monitors the engine speed. When it senses that the RPM is higher than the setting selected by the pilot, it supplies oil to a hydraulic cylinder on the front of the propeller. The high-pressure oil pushes on a piston, which causes the blades to increase in pitch until the RPM returns to its preset value. When the governor senses an underspeed, it allows oil to drain from the cylinder, decreasing blade angle, until the RPM increases the desired amount. A spring inside the propeller ensures that the blades return to low pitch when the RPM is too

low to generate sufficient centrifugal force. This type of propeller is used on most single engine airplanes.

Conventional twin-engine airplanes need propellers having the ability to feather (where the blades are aligned parallel with the direction of flight) when there is a loss of power or a loss of oil pressure. The oil-to-increase pitch propeller would present a safety hazard to a conventional twin airplane because it defaults to a low pitch position in the absence of power or oil pressure. If power is lost on only one engine, its propeller will windmill creating negative thrust, which when combined with the positive thrust from the remaining engine, makes the airplane difficult or even impossible to control. A feathered propeller has very little drag, so there is less asymmetric thrust with one propeller feathered than with one propeller windmilling and the aircraft is more controllable.

For a propeller to default to feather instead of low pitch requires that both the propeller and the governor be significantly different from that of the oil-to-increase pitch propeller. Each blade is generally fitted with a counterweight, which is sized and positioned such that centrifugal force on the counterweight assembly causes a twisting action toward high pitch. A large spring is also used to force the blades into the feathered position as RPM diminishes and the counterweight becomes ineffective. Control oil must now be supplied to decrease pitch, opposing the forces of the counterweights and the feathering spring. The governor for such a propeller operates in the opposite sense from that described previously, supplying oil only if it senses an underspeed.

Acrobatic airplanes also benefit from using oil-to-decrease pitch propellers. These propellers are nearly identical to feathering propellers except that their high pitch angles are limited to values similar to those used in oil-to-increase pitch props. Due to the attitudes in which acrobatic airplanes operate oil pressure fluctuations sometimes occur. These fluctuations not only affect engine lubrication, but also change the speed of the propeller. If acrobatic aircraft were to use oil-to-increase pitch propellers, this could result in a momentary overspeed at the same time the engine is starved for lubricating oil. Using an oil-to-decrease pitch propeller results in an underspeed in this situation, which helps to protect both the propeller and the engine.

Are custom or specialized propellers and governors available from Hartzell at my request?The costs involved with development and certification of new products is prohibitive to most individuals. Therefore, new propellers and governors are usually developed with volume production in mind. This allows the up-front investment to be recovered gradually over the sale of many units, therefore keeping them affordable.

Hartzell has a large selection of propellers, blades, spinners and

governors and is often able to suggest a suitable propeller system without embarking on a costly development project.

Can I use this (paint, filler, glue, sealer, grease, etc.) instead of your specified (paint, filler, glue, sealer, grease, etc.)?The materials that Hartzell specifies are selected based on extensive testing and evaluation to determine that they adequately perform their desired function while not having any adverse affects to other components or materials used on the propeller. The testing and evaluation has to determine that the material will function as desired throughout the entire operating environment and envelope of the propellers it may be used on. If the specified material is unavailable, in some instances, an alternate may be approved. Requests for approved alternate materials must be submitted, in writing to Hartzell Product Support and must include all supporting documentation. If the proposed material is a direct replacement, approval can normally be granted. If additional testing is required, approvals will normally be denied pending completion of the additional testing.

What can I do when my propeller is outside of the airworthiness limits and I'm away from my home base?Contact Hartzell Product Support for an evaluation of the part in question. In some instances we may be able to approve limited operation for a ferry flight (a one-time non-commercial flight for the purpose of transporting the aircraft to the maintenance facility - FAA approval may be required in some instances) to a location where maintenance can be performed. Please note, ferry permits in conjunction with an FAA Airworthiness Directive must be obtained from the FAA Aircraft Certification Office.

Where do I get Service Bulletin Information and does it cost anything?Most commonly requested service documents are available for free download in the Product Support section of this site. Requests for documents that are not available on this site should be made directly to the Hartzell Product Support department. Up to 3 Service Documents may be provided free of charge. Requests for more than 3 documents will have charges applied. Those persons who regularly work with Hartzell propellers should maintain a complete set of Hartzell Service Documents.

The complete set of service documents is available in paper format as a bound set identified as Manual 126. All Hartzell Service Documents are also included on the Hartzell Technical Documents Library on CD ROM. To purchase either Manual 126 or the Hartzell Technical Documents Library on CD ROM, contact our New Parts Sales Dept. at newpartssales@hartzellprop.com.

Continuing revision service for Manual 126 in paper format is available for $350 per year. Continuing revision service for the Hartzell Technical Documents Library on CD ROM is available for

$175 per year. Contact our Technical Publications Dept. at manuals@hartzellprop.com.

I’ve found a good deal on a propeller and it fits the engine in my homebuilt. I’ve cut the diameter of the blades to that recommended by the designer and had it balanced, so now there is no concern about vibration. OK?Not OK. Engines and propellers are designed to be vibrationally compatible with one another. Propeller manufacturers conduct tests called vibration surveys on many installations and maintain a list of those engines and propellers which are known to be safe when used together and their operating limitations. The vibration which the prop manufacturer is looking for during one of these surveys is not the same as that which you can eliminate by balancing. It is a high frequency vibration that exists naturally in piston engines and propellers eagerly respond to it. It seldom shakes the airplane like an out-of -balance prop does. When a propeller is vibrating in synch with the engine, the amplitude of the vibration can blossom, resulting in dangerously high stress levels with no indication to the pilot. Continued operation under these conditions can eventually result in fatigue failure of a blade or hub.

If you have questions concerning the compatibility of an installation, Hartzell will gladly provide you with the information needed to make a safe choice.