Source: Shreyas Narsipur, Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, NC

A propeller is a twisted airfoil, where the angle of the chord changes with respect to the location, along the radial station, as shown in Figure 1. Propellers are widely used in aircraft and watercraft propulsion systems thereby necessitating detailed characterizations of propellers to design high performance vehicles.

Figure 1. Chord, thickness, and pitch at a radial station.

One of the defining characteristics of a propeller is the pitch/twist. The pitch of the propeller, generally given in units of length, is the theoretical distance the propeller will travel through the air in one single revolution. However, due to the drag force on the aircraft and the propeller, the propeller never travels its theoretical distance. The actual distance travelled is referred to as the effective pitch of the propeller, and the difference between the theoretical or geometric pitch and the effective pitch is referred to as propeller slip, as illustrated in Figure 2.

Figure 2. Representation of pitch and slip.

In this demonstration, seven propellers are characterized using a propeller test rig in a subsonic wind tunnel. This is followed by a detailed parametric study to analyze the effects of variations in pitch, diameter, and number of blades on propeller performance.

1. Measuring propeller characteristics in a subsonic wind tunnel

1. Set up the propeller test rig in the subsonic wind tunnel using a 4-axis sting mount, as shown in Figure 3. A wind tunnel with a 2.6 ft x 3.7 ft test section and maximum dynamic pressure setting of 25 psf was used in this demonstration.

Figure 3. Propeller rig. .css-f1q1l5{display:-webkit-box;display:-webkit-flex;display:-ms-flexbox;display:flex;-webkit-align-items:flex-end;-webkit-box-align:flex-end;-ms-flex-align:flex-end;align-items:flex-end;background-image:linear-gradient(180deg, rgba(255, 255, 255, 0) 0%, rgba(255, 255, 255, 0.8) 40%, rgba(255, 255, 255, 1) 100%);width:100%;height:100%;position:absolute;bottom:0px;left:0px;font-size:var(--chakra-fontSizes-lg);color:#676B82;}