On the previous page, dannysoar mentions that the hourglass style of prop blank which Aerodyne sells, and is often shown on model plans, is quite difficult to describe without pictures. Fortunately, Jim Anderson has provided the following illustrations, as well as a spreadsheet for juggling numbers for designing your own propellers using this type of blank. In order to make the spreadsheet as accessible as possible, It is saved with formulae visible. You wil likely want to hide the formulae, and show the calculated values before use.
This image shows half of the hourglass prop block calculated by Jim's spreadsheet.
This spreadsheet is intended to calculate the dimensions of an hourglass propeller block based on the initial information you provide. In the example given, I have used the dimensions for a "New Gollywock" and have chosen to start with a block of 1.75" x 13.5. The length is the maximum diameter allowed for this model under SAM rules and 1.75" will give the blade width I want.
The shape of the block is based on those shown in Don Ross' book "Rubber Powered Model Airplanes" and by Bill McCombs in "Making Scale Model Airplanes Fly".
This spreadsheet concept could be extended to provide the block dimensions for just about any pitch distribution, however for SAM and FAC type models, I believe this application should serve the purpose.
Comments, improvements, variations and corrections will be welcomed.
Click to download Excel spreadsheet.
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Copyright 2004, Thayer Syme. All rights reserved