Slip

The ‘slip’ of the propeller can be defined as the ‘difference between the advance per rev and the geometric pitch’.

When the advance per rev equals the experimental pitch, the angle of attack is slightly negative and produces zero thrust. However, in normal operating conditions, the angle of attack is around positive three degrees and the advance per rev is considerably less than the geometric pitch. Diagram 4, Geometric Pitch, shows the advance per rev (B-C) plus the slip (C-D) is equal to the geometric pitch (B-D). For the propeller to provide maximum thrust and efficiency, and because air is not a solid medium, slip must be present. Maximum efficiency is only obtained when the slip, expressed as a percentage is around 30% of the length of the geometric pitch. In other words, slip is the difference between the actual distance the prop travels forward in one revolution (B-C or effective pitch) and the distance the prop would theoretically travel in on revolution if its advance per rev were equal to the geometric pitch (B-D). Slip is a percentage of distance.

Given the prop RPM, prop pitch in inches and true air speed in knots, the slip can be found using the following formula:

RPM x pitch x 60

6080 x 12

Given: RPM = 2400

Pitch = 69 inches TAS = 100 knots

2400 x 69 x 60

6080 x 12

= 136.18 inches = 26.57%

It must be emphasized that slip is related to the geometric pitch and the advance per rev. As stated above, zero thrust occurs at a negative angle of attack (experimental pitch) and thrust increases as the angle of attack increases through the geometric pitch up to some positive angle of attack. Diagram 4, Geometric Pitch, shows a small amount of slip (C-D) is present when the prop blades are operating at zero angle of attack (geometric pitch). To produce thrust, slip must be present with the maximum prop efficiency occurring at around 30% slip, or 30% of the geometric pitch. It must be remembered slip is related to geometric pitch, which is the distance the prop travels during each revolution.