Instructions for Using the Propeller Conversion Table

Download: www.reorganisation.ch/modellflug

All clubs are permitted to make the table available for download on their websites.

The propeller conversion table is used to find a different propeller for an existing engine with a matching propeller—one that changes the aircraft's flight performance or appearance while placing the same or a similar load on the engine as the propeller recommended by the engine manufacturer.

• A 2-blade propeller is to be replaced with a 3-blade propeller, and the load on the engine is to remain the same. In most cases, the aircraft’s performance decreases.
• A different two-blade propeller with a steeper pitch (resulting in a smaller diameter) is to be used so that the aircraft flies faster, while the load on the engine remains the same. This results in less thrust. The takeoff distance increases and the landing speed increases. Standstill thrust and idle RPM decrease.
• A different two-blade propeller with a lower pitch (resulting in a larger diameter) is to be used so that the aircraft has more thrust (e.g., for towing), and the load on the engine is to remain the same. The takeoff distance becomes shorter and the landing speed lower. The static thrust increases. The idle RPM may increase slightly.

Blue fields to be filled in (results are displayed in the green fields) Enter the data for the propeller recommended by the engine manufacturer in the 3 fields under “Existing Propeller (Engine Manufacturer’s Recommendation).” Enter the desired change in the 3 fields under “Desired Propeller.” There are 4 options:

• Switching to more (e.g., 5) or fewer blades: If the pitch remains the same, the result will be the new diameter; if the diameter remains the same, the result will be the new pitch.
• Changing the diameter: The result is the new pitch with the same number of blades as the existing propeller.
• Changing the pitch: The result will be the new diameter with the same number of blades as the existing propeller.

Enter all dimensions freely. You also have the option to enter all dimensions freely and view the effects on the motor load. This feature also allows you to experiment with the results under “New Dimensions” and, for example, with a catalog in hand, find the right new propeller.

The results must be rounded. The results under “New Dimensions” are rounded to two decimal places. Propellers with these dimensions are not available. Therefore, the dimensions must be rounded to standard sizes. Enter the values in the blue fields under “Enter the values rounded to standard sizes here.” The result shows the change in load on the motor. Only propellers from the same manufacturer and of the same type (e.g., APC E) can be compared with one another. The fields in the table that have already been filled in as examples are based on a 2-blade 18×12 propeller from any manufacturer. So, for example, if an APC E 18×12 propeller is to be replaced with, say, a Menz wooden propeller, the results should be treated with caution, since Menz uses completely different profiles than APC and the wooden propeller is heavier. However, the table does provide an indication of what is likely to be fairly accurate in such a case.

The example in the table with the 2-blade 18×112 propeller shows that a 2-blade 16.6×118 propeller would also fit the motor. That would, of course, be ideal for flying our racing glider faster with the current motor. Assuming we can’t find such a propeller, let’s make it ourselves. We’ll take a larger propeller with this pitch and reduce its diameter. We’ll shape the blade tips to be roughly the same as the original and balance it very carefully! The thing is, the propeller is actually much too heavy because it’s built more solidly. And the blades are also deeper. That’s why the diameter should be chosen a little smaller. By how much, I don’t know.