Propellor Performance

Actuator Disk Theory

The performance of the piston propellor combination can be estimated using a simple fluid mechanics model based on the momentum theory. In this model, flow details including the design of the blades are ignored. The analysis uses the assumption of ideal flow. It supposes a pressure jump at the propellor/disk location. It is also used for estimating preliminary performance of helicopter rotors.

Continuity:

Momentum

Blade Element Theory

The blade element theory computes the performance of the propellor in detail by actually calculating the aerodynamic information based on the blade profile. In the following discussion it is apparent that the blade must be twisted to account for the influence of the rotational velocity at each blade section on the angle of attack. The built in twist is also referred to the blade pitch (angle) distribution. Remember that the aerodynamic calculations are based on the relative velocity at the blade section.. The main assumptions of the blade element theory are

1. At each section of the propellor is an airfoil (NACA 4415 a good profile airfoil)

2. The airfoil chord line in the cross section changes from almost parallel to the free stream velocity at the root to almost perpendicular at the tip. The angle the chord line makes with the plane of rotation is the pitch angle

The aerodynamic calculations are based on the lift and drag calculation - these are airfoils

Most blades will have

(a) Highly twisted sections along the blade

(b) Variable chord length along the blade

(c) Changing airfoil sections along the blade

In practice, the propeller cannot convert all of the energy delivered by the engine. This effect is captured using an efficiency called the propeller efficiency

The non dimensional parameter that characterizes propellor performance is the advance ratio defined