Hartzell Propeller Design, Safety, and Maintenance
By Mike Disbrow
Propellers seem to be one of the most visible and yet least respected components on the airplane. They lead a very difficult life, as they are blasted in operation by sand, rain, hail, etc., and are subjected to huge temperature extremes, all the while absorbing centrifugal loads of 10 to 20 tons per blade and punishing vibrational stresses induced by the engine and oncoming air stream. For all of this punishment, they are extremely reliable devices. Yet, like all mechanical components, they are subject to wear, fatigue, corrosion, and erosion, all of which can lead to failure if not kept in check. The failure of a propeller is usually a dramatic event. It does not simply result in a loss of thrust, as in an engine failure. If even a small portion of the propeller blade is lost, the resulting imbalance may tear the engine from the aircraft, making the aircraft uncontrollable.
A failure of the blade retention system is what motivated Hartzell to issue Service Bulletin 217, which led to FAA Airworthiness Directive (AD) 97-18-02. This bulletin and AD affects most of the propellers Hartzell produced in the 1950s and early 1960s and requires repetitive inspections of the blade retention system components. Before launching into a discussion of this issue, it would be helpful to provide a brief description of the construction of Hartzell propellers.
The constant speed propeller consists of three major subcomponents: blades, hub assembly, and spinner assembly.
The blades are what converts power into thrust. They are complex airfoils designed to optimize the performance of the aircraft over the broadest range of design criteria. Much of this design process is an exercise in compromise between the competing factors of takeoff/climb performance, cruise performance, noise, and ground clearance. Improvement in one of these areas may result in the degradation of another. At Hartzell, these designs are created using powerful computer programs to create the optimal combination of airfoil, twist distribution, and planform (shape).
The result is that most modern propellers are highly efficient (85 percent plus) at converting power into thrust. Blades also have to undergo a vibrational stress survey to ensure compatibility with the engine and airframe. Most propeller blades are made from a fatigue resistant aluminum alloy; however, many large propellers (greater than 1,500 shp) are being made from structural composite material (Kevlar or graphite). Composite blades are costly to manufacture, but extremely lightweight. This trade-off makes them advantageous only for large propellers and specialized applications.
The hub assembly is designed to contain the blades and the pitch change mechanism. Hartzell has two basic hub designs that are most commonly distinguished by their basic material: steel or aluminum. The steel hub design dates back to 1946. This design consists of a steel hub "spider" which pilots the blades. The blades are retained on the hub by the use of steel clamps around the shoulders at the shank or inboard end of the blade. This design utilizes both double- and single-shouldered blades.
The blade clamps are connected via a linkage to the pitch change mechanism, which is externally visible. Pitch is changed by high-pressure engine oil, which is controlled by the governor, moving a piston on the forward end of the hub assembly. This design was the mainstay for Hartzell into the early 1960s when the aluminum hub assembly was developed. Shown here is a cross section of a steel hub propeller assembly.
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