Calculations
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## Calculations and Principal Assumptions## Propeller Operating ConditionsThe principal assumptions are that the propeller is operating at a constant speed and constant rotation rate under moderately light disc loading. This latter assumption means this code should ## The Effect of InflowThe light loading means that inflow velocities are small relative to the aircraft velocity. This assumption is used to estimate the inflow. A constant inflow velocity is assumed over most of the propeller disc, which is close to optimum. Inflow near the tip is corrected to account for the effect of a finite number of blades. This correction is also reflected in the pitch distribution and the element efficiency. A short discussion of inflow with example calculations taken from my web site is at Inflow . Other distributions of inflow will have lower efficiency and slightly different pitch distribution from the one selected here. A tip correction is applied to account for the effect of the inflow and lift reduction at the propeller tip caused by non-uniform (in time) inflow. This effect is greater when fewer blades are used and causes the 'effective' propeller diameter for one or two blades to be less than 90% of the 'infinite number of blades' configuration. ## The Effect of Mach NumberThe effect of Mach number is accounted for by a simple Prandtl-Glauret correction to the lift curve slope. Because the blade elements are designed to operate at the best subsonic angle of attack, the rule used reduces the angle of attack at non zero Mach numbers by a factor of SQRT(1-M^2). This factor may not create enough reduction in angle of attack at Mach numbers greater than 0.8, however, it is significantly better than using the best subsonic angle of attack at all Mach numbers. The lift/drag ratio can be maintained to moderately high Mach numbers by using thin sections. A discussion and graph of these effects of transonic Mach number as measured in a wind tunnel are on the Mach Number page ## The Effect of Reynolds NumberNo account is made for the effect of Reynolds number on blade section aerodynamics. One of the data elements in the output file is the chord required to produce a constant Reynolds number ( In general lower ## Blade ShapeThe constant A discussion of some aspects of efficiency taken from my web site is at Efficiency . |