Why does this analysis show very high stress in a propeller? I have been trying to evaluate the stress on aluminium propeller:

Diameter: 78", 2600RPM

Have fed load numbers from here, applied distributed mass model onto the blades.

Static Thrust (2600 rpm):
MT 2-blade, 83" composite, 920 pounds
MT 3-blade, 80" composite, 900 pounds
McCauley 2-blade, 82" aluminium, 860 pounds
McCauley 3-blade, 78" aluminium, 825 pounds


The results are rather scary - 30 MPa out of 55 max, for 6061 class Aluminium (the visual deformation is increased 20x for better representation),
resulting in +-1cm vertical bending.

Is this normal? 

Is there any info on industry's practice for safety margin (numbers show its less that 50%)? Or any kind of test procedures applicable to manufacturers?



The shear stresses are negligible - 150 Pa out of 150 MPa


The two propeller case is really scary looking
Below is a distributed mass (375Kg) - 80% of Max. yield stress


And centrifugal force - over-stressed by 200%:


Finally, a combined non-Linear analysis (incl. torque) - over-stressed by 250%:
(notice that everything not in blue will bend and likely crack, with only the blade tips surviving the torture)
The plots of your propellers seem to use a regular airfoil all the way to the root. Real propellers become almost cylindrical there, however. Also, aluminium propellers are hollow, so the outer part is only a thin shell, saving mass and reducing centrifugal loads significantly.

After all, real aluminium propellers work, so there must be a way to keep the stress in operation below the yield stress. FAR 23.907(c) even demands that a non-wooden propeller must be life-safe:


  The applicant must perform an evaluation of the propeller to show that
  failure due to fatigue will be avoided throughout the operational life
  of the propeller using the fatigue and structural data obtained in
  accordance with part 35 of this chapter and the vibration data
  obtained from compliance with paragraph (a) of this section.


Below is a drawing of a wooden propeller, but the outer shape of a metal propeller is not much different.



Drawing of a wooden propeller for the Fokker D VII (picture source)