Propeller for the EMG-6-250
We have had to undertake the task of making our own propeller for the Polini 250 installation. because of the limited propeller clearance issue on the EMG-6, the noise requirements, the horse power that we have to absorb with the 250, the propeller loading requirements, the moment of inertia limitations, and the airspeed that the aircraft will fly at. we have searched the world over and been unable to locate what we feel will be an appropriate propeller for the aircraft.
We are using some propeller software to assist in the manufacturing process of the EMG-6-250 propeller. This propeller software is licensed by Jan Carlson. And although it’s not complete enough for us to make a perfect judgment about the propeller this necessary for the aircraft the math is accurate enough that we can get a really reasonable starting point to have a baseline that we can work with.
The software has the capability of outputting XYZ data plots for the propeller blade design.
Working with this data we can incorporate and imported into a solid works file that we can then interface with a propeller hub. Once we have the propeller design complete in solid works we can now use solid cam to generate the G code for manufacturing the propeller blade
We can then section out the propeller blade into the thickness of each board, create individual laminates that can be laminated together into a basic building block.
The group of laminated individual boards can then be positioned on a jigging fixture and we can begin the milling process by milling out the hub area which will be used to position the propeller blade for all of the many different cuts that are necessary.
Because of the limitations of size of the milling machine we will have to mill one blade at a time and limit the z-axis travel to about 2 inches making it necessary for about 4 operations per side and then an additional 8 operations for the flipside of the blade.
We started off the process by doing a rough cut but this required so much additional milling machining time that we soon switch to just doing a finished cut only.
After each pass the blade has to be removed and reinserted 180° out to complete the other blade. the ability to position the blade becomes critical.
It’s necessary to monitor the actions of the machining process hundred percent of the time. While one side of the blade is being machined we can hand sand the opposite blade.
Once the aft side of the propeller blade has been machined it’s time to flip the blade over and duplicate the process for the front side of the blade.
In addition to the hub supporting the propeller blade the tips of each blade have to be supported to prevent any warping during the cutting process especially since we have eliminated the rough cut. in this picture here were just finishing up the last blade.
Once the machining has been completed we can move the propeller to the workbench, mounted in place and begin the final hand sanding detail work necessary before we can finish the propeller blade.
Once the final sanding has been complete we will apply multiple coats of a clear polyurethane finish
Once the finish has dried we can install the propeller on to the hub of the 250.
The final installation of the propeller blade is using a 48 inch diameter blade but we have a significant chord with that we are anticipating will be just slightly more than what we need to absorb the load of the 36 hp Polini 250. We have looked at other propeller blades that may work as well including the Power Fin 3 blade 60 inch diameter which we have measured the moment of inertia right at around 3000 kg/cm² where as we have measured our wooden propeller closer which is closer to 1200 kg/cm² less than half the moment of inertia with the power fin 3 blade. And although we have not yet been able to determine what the maximum mass moment of inertia rating of the gearbox. The design is obviously small enough that moment of inertia is going to be a substantial variable and concern when related to the long-term viability of the gearbox. Because the gearbox also incorporates a centrifugal clutch in the oil bath this also will be helped by maintaining a very low moment of inertia propeller. A “B” gearbox on a Rotax engine is limited to a maximum of 3000 kg/cm². That gearbox is substantially more robust than the gearbox on the Polini.
