Notes on Using Prop Block

NOTES ON USING PROP BLOCK SPREADSHEET

The Spreadsheet Example Block is a quick way of designing the appropriate prop pitch jig for any desired pitch distribution, diameter and pitch. All units are mm and degrees, mm being much easier to use to mark out appropriate dimensions.

Input Data

Fractional pitch ie 0.5 represents ½ maximum pitch at each selected station.

Station position as a fraction of the radius ie 0.7 is 70% of the radius from the root.

Prop Diameter

Maximum prop pitch.

Block width

Height of centreline above reference base

All data values are shown in yellow.

Pitch jig contour

The spreadsheet first calculates the actual pitch at each station and the angle of the jig surface relative to the plane of rotation of the prop ie zero angle equals zero pitch.

The overall article shows how to measure and make a jig to give the required angles. These are steep at the root and small at the tip. At each station, the centreline is at a constant height above the base of the block.

Now imagine the finished jig to be rotated about the centreline radius so that the angle at the tip is negative and that at the root angle is still positive. From a side view, the jig occupies much less space than before.

The spreadsheet then calculates the optimum rotation angle and the pitch angles resulting from the rotation. For each station (distance from the root), the spreadsheet calculates the heights of the leading edge and trailing edge above the base for the rotated block.

Output Values

Block rotation angle for minimum material use

Station values as distances from the root.

Height of leading edge above base for each station

Height of trailing edge above base for each station.

Jig manufacture

First, the block is cut to the specified width and deep enough to accommodate the maximum values of heights at the LE and TE. The maximum heights occur at the root for the LE and at the tip for the TE.

Then mark out the heights on the sides of the block or plot on to templates made from eg 1/64” ply. The templates are then attached to the sides of the block and mark the contours to which to carve the block.