The airfoils may have arbitrary shapes defined with either tabular data (splined cubic fits) or analytical functions typically used for NACA airfoils. The airfoil shapes may vary with span. Capability exists to smoothly "splice" together widely different airfoil shapes. The dominant basis for the primary airfoil shape used in most APC propellers is similar to the NACA 4412 and Clark-Y airfoils, except the leading edge is somewhat lower. Also, the aft region is somewhat thicker. This alters the zero-lift angle by approximately one degree and provides greater lift without having to twist the blade even more. All blades have some washout near the tip. For applications where Mach number effects become significant near the tip, either pitch washout or camber reduction tailoring minimizes Mach drag rise.

Cross-section geometry in and near the hub region is defined with specialized algorithms. The aerodynamic-dominant airfoil must smoothly transition into a structural-dominant shape in a manner that emphasizes strength consistent with milling machine tool constraints. Hub geometry for 3 and 4 bladed propellers is very complex because of the need to match mold parting lines at all points on the mold surface perimeter.

The propeller loads and performance software module has been correlated with measured (static) thrust for a broad range of propellers. During static operations, the blade is operating under dominantly stalled conditions. Use of empirical stall angle characteristics shows that the performance calculation software consistently under-predicts (static) thrust when the pitch to diameter ratios are relatively low, around 0.5. As pitch to diameter ratios increase to 1.0, very good agreement exists between predicted and measured thrust. A nearly linear relationship occurs when the ratio between predicted and measured thrust is correlated with pitch to diameter ratio. This correlation is used in the structural analysis software to increase stress loading for low pitch to diameter ratios. Application of these stall regime correlation factors to normal flight conditions is perhaps inappropriate. However, the practice is used to provide conservatism in stress calculations.

A propeller-unique stress analysis package is used to compute propeller peak stress and fatigue endurance. Blade cross-section geometric properties (needed for structural analysis) are computed numerically with a very fine chord wise mesh. Steady and alternating stresses are evaluated along the entire blade considering inertia and aerodynamic loads. Stresses are evaluated in terms of bending (thrust and drag), centrifugal (inertia), and torsional (acceleration) components. Fatigue endurance margins are estimated assuming Goodman, Gerber and Smith criteria.

Natural modes (frequencies) of the propeller are computed assuming that the hub is rigid. Both published and empirically determined ranges of composite modulus of elasticity (bending) are used to calculate dynamic amplification factors. These dynamic amplification factors are used to augment (intensify) cyclic stress components.

Torsional acceleration is typically highly sensitive to engine fuel mixture and is therefore rather uncertain. Design parameters have been empirically developed to seek reasonable upper bounds for maximum torsional acceleration loads. However, the uncertainty with this sometimes strong (especially for racing applications) contributor to cyclic stress requires that extensive operational testing be employed to verify stress margins for high performance applications.

Cyclic bending stress induced by precession during rapid loops is now included in the stress computations. However, this effect is very minor, even under extreme pitch rate conditions. Also, precession stress is normally out-of-phase with torsion vibration effects; therefore, it does not add to peak cyclic stress magnitude.

 

Modulus of elasticity (bending) is empirically determined using force-deflection tests with molded specimens that reflect the effects of "skinning " during the injection molding process. The bending modulus is sensitive to humidity.

The AeroSport data logger is a computer controlled data acquisition system that can record up to 30 minutes of data in solid state memory. The data are played back after flight recovery. While many options exist for the use of these data loggers, they are primarily used by Landing Products for speed and RPM measurements. The speed and RPM measurements from the data loggers rather consistently yield data very close to that determined from radar gun and audio recordings, providing considerable confidence in data accuracy.

Engine in-flight RPM is determined using a video camera to record engine audio, and, spectral analysis software to determine dominant harmonic content from the signal. Audio data are collected with the model flying both toward and away from the pilot. Differences in apparent frequency are used to determine and remove Doppler shift effects.

Conversion to 3 and 4 bladed propellers is performed by matching 2-blade torque for specified model speed and RPM conditions. This method allows efficient use of a rather broad data base that now exists for 2-blade propellers.