P R O P E L L E R  D E S I G N  P R O C E S S

A short course on propeller design steps for most installations
Most of our applications are developed through selecting an "off-the-shelf" basic hub assembly (hub and the pitch change mechanism) and either selecting or tailoring an "off-the-shelf" blade design or custom tailoring to a new blade design. This effort is achieved subsequent to the customer providing us with a completed design questionnaire that provides appropriate aerodynamic and dimensional requirements of the propeller design. Our engineers then use this data in the conduct of various design trade-offs to achieve the best product that meets or exceeds the customer’s stated requirements. The analytical and manufacturing tools available today enable a custom tailoring to be performed in a very reasonable effort. Given the wide assortment of blade designs available today however, custom tailoring isn’t always needed. If the customer requests composite blades (for weight savings, or enhanced performance tailoring that composite structures may enable), the consequence of this decision means that the lead time to first prototype is substantially longer than for a conventional aluminum blade given the added requirement of a blade mold that is manufactured to the new geometry.

After aerodynamically determining the best blade design for a given application, the propeller is then analyzed for suitable installation vibratory stress levels. Turboprop installations lend themselves to a reasonably accurate assessment of the operating stress levels long before the installation is flown. Reciprocating engines, though, pose a more difficult challenge and the stress level predictions are often much less accurate, thereby leaving more to chance (in terms of failing certification tests or testing) when it comes time to test the product. With our extensive test data of different engines and propellers, it is often possible for us to minimize this unknown by matching blade resonant characteristics with a previously tested and successful, blade design.

After the blade design effort, it is time to test the product. A prototype propeller is manufactured and provided to the interested airframer or modifier and they flight test the propeller for a limited amount of time to verify suitable aircraft performance, handling, and noise characteristics.

Upon customer acceptance of the propeller performance, we then proceed with a stress survey on the propeller as installed on the aircraft and engine. With a properly instrumented propeller, we record the stress levels on appropriate points of the propeller while the aircraft is operated throughout its intended flight envelope. If the aircraft is aerobatic, the appropriate aerobatic maneuvers are performed. If there has already been a test of the same blade design on a sufficiently similar aircraft and engine combination, then this test might be waived and replaced by a similarity analysis to that other installation.

Provided the stress survey is successful, the certification data representing this design can be provided to the FAA for final approval of the propeller. To get to this stage though, various other certification requirements would have had to be met either by similarity to previous tests or by the conduct of the new tests. The timing of these other tests depend quite significantly on the degree of confidence in passing the higher risk certification tests as well as the schedule demands of the customer. Also woven into the process described above is design effort, as required, related to the control system, synchrophaser, spinner and de-ice system

Given the propeller approval, the airframer or modifier can take this data and add it to his aircraft data to support approval of the aircraft.

New Configuration Propeller Hub Assembly
Our design tools enable us to configure a new propeller construction fairly readily. Common themes involve variations in:

• The number of blades for a given hub construction.
• A different method of achieving reverse pitch operation.
• Start locks for minimum torque or zero thrust on engine startup.

Although we already offer a wide assortment of number of blades (2 through 6 bladed propellers), reverse pitch control techniques, and start lock devices, all combinations on all propellers are not available. Therefore, some additional design work on the hub assembly may be needed depending on the installation requirements.