Build Your Own Propellers! Science Olympiad FLIGHT

[@user-qs5fu1of2q]

Hi, when using these balsa propellers I’m having an issue where the entire flight itself is shaking while torqued with rubber, when we let it go it shake a lot in the front and doesn’t fly. May you advise us on how to fix this? The propellers are made well and pitched correctly but it seems that it won’t fly cuz of this shaking.

[@aeromartin1533]

There are two kinds of balance, static and dynamic. When a propeller can stop rotating at any position, static balance is achieved. Sounds like the dynamic balance on your propeller is off. Since pitch is true/same on the two blades, my guess is that flexibility of the two blades are not the same. Balsa wood density and grain distribution vary a lot. Extra care is needed when picking the wood for the two blades. We always cut the two blades out of the same sheet of wood, sometimes along the grain, sometimes side by side, depends on the grain distribution. Hold the wood sheet against light to see the density and grain, to plan your cut. See the first few slides in the video to see how I paired up the wood. With enough extra care, both static and dynamic balance could sometimes be achieved at the same time. 😎 Good luck! 👍

[@1fly2fly24]

Where can I find the 3D print files for the items that you are using?

[@aeromartin1533]

Here is the direct link to my latest OpenSCAD program for creating balsa-blade propeller molds. www.thingiverse.com/thing:6720495 (August 2024) To run the program, you also need to install OpenSCAD, openscad.org/, on your machine. My program has many adjustable parameters, e.g. propeller diameter, P/D, blade width, washout, stem location (flaring), blade profile, etc. See screenshot on the Thingiverse page for more. Depends on which indoor free flight class you are competing in, you may fine tune these parameters before printing out your mold. (In OpenSCAD, open my prop mold program, adjust parameters you like, hit F5 to preview, F6 to render, and F7 to export the .STL file.) Feel free to contact me at aero.martin at yahoo dot com if you have any questions on how these parameters should be adjusted. Good luck! :-)

[@charlesmaulion3014]

Hello Can you explain me the parameter pitch over diameter ? and the angle writ on the mold ? for exemple if i need to propeller 200 of diameter x 150mm of pitch, what i need to enter on the software thanks

[@aeromartin1533]

Happy to answer any questions. The P/D ratio, calculated by dividing the pitch by the diameter, is a fundamental parameter in propeller design. In your example, 200mm diameter divided by 150mm pitch is about 1.3. P/D=1.3. My program takes this value and then calculates a "representative" pitch angle to put on the mold. Propeller pitch conventionally is measured at 75% radius, i.e. at 150*0.5*0.75=56.25mm from the hub. The "representative " blade angle is calculated at this 75% radius location, i.e. pitch_angle=atan(pitch_over_dia/0.75/PI) ~= 29.5 degrees in your example. After balsa blades are formed/dried with the propeller mold, you may use my 3d printed pitch gauge or other pitch gauges to help set this 29.5 degree angle at 75% blade radius. This practice of measuring pitch at 75% radius assumes that all blade's cross-sections reach the same distance after one revolution. I.e. every blade cross section has the helix angle derived from the same pitch. Although many indoor flight propeller building blocks are designed with the same assumption, a helix propeller may not perform the best... In real-world, the incoming airflow speed varies across the propeller blade span. It is not uniform. The optimal blade angle at one cross-section may or may not be the predicted helix angle. Tweaking blade angle at each cross-section can help further improve a propeller's performance. Check out the washout angle setting, and the starting point for that, in my mold program. It might help your propellers, as it did for mine. Another thought. For full-scale airplanes, same propeller (if adjustable) is normally set to lower pitch for takeoff, and, higher pitch for cruise. Lower pitch at takeoff provides optimal thrust and acceleration. A higher pitch setting reduces drag and maximizes efficiency at cruising speed. In other words, a good P/D depends on the forward speed as well as blade's rotational speed and several other things. Other indoor flight classes' best P/D values can be a good reference, but might still be off a bit for the airplane you are flying. Similarly, a good P/D value for an airplane propeller may or may not work for a hovering helicopter’s rotor, esp. Science Olympiad helicopters, which stay at the ceiling most of the time. Please adjust P/D value for your propellers or rotors accordingly. Good luck! Cheers! -AeroMartin

[@charlesmaulion3014]

@@aeromartin1533 thanks i try to print mold for 200x150mm propeller

[@charlesmaulion3014]

@@aeromartin1533 hello how to save the face stl files on your software? When I try I've a error message because it's not 3d files. Can you also able to modify the code to be able to désign smaller propeller? Charles

[@aeromartin1533]

@charlesmaulion3014, In OpenSCAD, once you have dialed in all parameters, hit F5 to preview the blade mold one more time. Check mold for any void area. Adjust mold height if need to. Then hit F6 to render the 3d mold objects before hitting F7 (export as STL file.) If you are trying to save the "flat blade only" profile, check "File->Export->Export As DXF." (Use DXF file to laser cut precise balsa blades.) In OpenSCAD, flat blade is a 2D object. It can not be saved as .STL file. See Thingiverse for a screenshot on this. For smaller (