by afernan » Fri Mar 28, 2014 1:08 pm
Main guidelines to design your copter free of frequency problems. Summary
1.- To be safe, use soft arms and legs combined with high RPM (small propellers). This is the MK style: small tube section (10x10) and long alu tubes. Then you´ll have the excitation frequency f(motor) high enough from arm ones. Typically we can get f(arm) = 20-30 Hz, f(motors) > 100Hz.
2.- As you increase the propeller size, rpm decrease, so freq.excitation. For instance a typical HEXA (mass = 2.5Kg, propeller = 12x4) f(motor) = 60-70Hz. With arms around 320mm, 16mm we have f(arm) = 35 - 40 Hz that is OK. Is suitable in this case to have longer arms to reduce more the arm frequency. Don´t use large diameter arms (18+mm), or shorter arms since this will increase f(arm). Torsion is not an issue (very high at f>400Hz).
3.- For propellers > 15" a lot of care must be taken, since the f(motor) can be very low (<50Hz). So we need to decide to put f(arm) freq behind or below f(motor). I prefer below (soft and long arms), it´s safe.
4.- Last but not least, the legs: usually we want to be behind f(motor). So use short, light and stiff legs. A typical "T" leg, 200mm long has f(leg) > 100Hz, that´s good.
Hope this will clarify general design ideas.
Angel