6.1 Introduction

For certain missions and speed ranges the propeller-driven aircraft is the most efficient. Some examples are:

1. speed range of 0-450 kn

2. certain STOL missions

3. long endurance maritime reconnaissance missions

Recent NASA research has shown it possible to extend the cruise speed of propeller-driven aircraft to 0.8 Mach while maintaining a propeller efficiency in excess of 80%. Therefore for certain applications the propeller will be around indefinitely, and may be reintroduced in such areas as air transport.

6.2 Propeller Theory

There are three theories now used in the design of propellers. They are:

1. momentum theory

2. blade element theory

3. vortex theory

Until recently only the first two theories were in use. This is primarily due to the mathematical complexity of the vortex theory.

Most of today's propellers were designed using blade element theory. The problem for the designer is to find an existing propeller design that can be adapted to their requirements.

Since few propellers have been designed using vortex theory we will limit our discussion to momentum theory and blade element theory. Those interested in vortex theory should read Barnes W. McCormick's Aerodynamics for V/STOL Flight, Chapter 4 (Ref. 1).

6.2.1 Momentum Theory

Derived from Newton's second law:

(6.1)

Assumptions:

1. Propeller does not add rotation to the air.

2. No profile losses.

3. Air is inviscid and incompressible.

4. Propeller has an infinite number of blades.

Since most of these assumptions are unrealistic, this theory is only useful in predicting ideal or maximum propeller efficiencies.

The mass of air passing through...