The short answer is because efficiency doesn’t propel the airplane, thrust does. The most efficient propeller blade count for a particular aircraft is a function of the aircraft mission and a number of other factors. These include the amount of engine power, operating RPM for the propeller, diameter limitations, aircraft performance requirements (high speed cruise, takeoff, loiter, etc), noise requirements, and others. Depending on the combination of these parameters a 2-blade propeller may be most efficient, but as power increases additional blades are generally required to efficiently utilize the increased power.
Hartzell propellers are constructed from various materials, but the primary structural components in the hub system are high strength aluminum alloy or steel forgings. Blades are made from high strength aluminum alloy forgings or advanced composite materials (Kevlar® or carbon fiber).
Hartzell structural composite blades are composed of a metal blade shank retention section onto which is molded a low-density foam core that supports built-up layers of composite laminate.Beyond the obvious advantage of a substantial weight reduction over aluminum blades, there are additional advantages to the use of structural composite blades. These advantages are a longer service life and the ability to maintain a more optimum airfoil shape over the service life of the blade. This is possible because most damage to composite blades can be repaired and returned to service without adversely affecting the airfoil shape, while damage to an aluminum blade requires removal of material and it ultimately falls under dimensional limits for strength and must be retired from service.
No, Hartzell Propeller only makes constant-speed propellers.
The determining factors in choosing the correct propeller for an experimental home built aircraft are safety, performance, weight, cost, noise, diameter limits and durability. For most popular kit aircraft, Hartzell has developed propellers specifically for those aircraft and they are available directly from the kit manufacturer.
No; the costs involved with development and certification of new products is prohibitive to most individuals. New propellers and governors are usually developed with volume production as a goal.
Generally, it requires a large increase in propeller efficiency to produce a small increase in cruise speed on an 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 your specific installation.
Yes, in most installations increasing the number of blades helps to reduce noise. Cockpit noise comes from a variety of sources; engine, exhaust, air flow around the fuselage, and the propeller. Vibrations are also perceived as noise in the cockpit. In a single-engine airplane, the propeller blade wake will beat on the windshield producing noise. A 2-blade propeller produces two pressure pulses per revolution, where a 3-blade propeller will produce three smaller pulses per revolution (for the same amount of total thrust) which is inherently smoother and therefore quieter. The 3-blade propeller will generally have a smaller diameter than the 2-blade propeller that it replaces, which also reduces the tip speed and noise. In a twin-engine aircraft, the reduced diameter of the 3- 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.
Sweeping the blade tip back reduces the noise generated and may also improve the efficiency. Designers search for 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 for a typical airplane.