Airfoil bending stress
Two main loads on the airfoils are the centrifugal force and the aerodynamics
drag and lift. The rough comparison of these forces at below shows that
the aerodynamic forces are small compare to the centrifugal force for
this model’s material, solidity, and operating speed, so it was
neglected in the following calculation. To find the centrifugal force,
the rotor was estimated (prior to testing) to have a maximum rotor tsr
of 3 and subjected to maximum wind speed of 5m/s.
The strength property of the jelutong wood was not available at that
time so a simple test was conducted to test its strength. The airfoils
are NACA0015 with chord of 7cm and length of 50cm. The rough sanded airfoils
weigh 0.149, 0.141, and 0.126kg (almost 20% difference) respectively.
These airfoils will be supported at the centre, so the centrifugal force
will cause highest bending stress at the centre where the airfoils joint
to the support arms. To test the airfoil strength, both sides of the airfoil
were loaded with dumbbell weights before it was lifted off by holding
(balancing using hand) the centre of the airfoil as shown in the figure
below.
Testing the strength of jelutong wood airfoil
At maximum tsr of 3 and wind speed of 5m/s, the total centrifugal force
on the airfoils will be 0.149kg times 90, which is 14kg. To produce ‘equivalent’
maximum bending stress at the centre of airfoil, the point load needed
at the centre of both sides of airfoil will be 14kg / 2 = 7kg. The airfoil
was tested with maximum dumbbell mass of 6.5kg at both sides with distance
of 12.5cm from the centre The deflection of airfoil under this load was
clearly visible but the wood airfoil did not break. Stress concentration
is definitely lower when balancing using hand instead of the actual support,
which is four pieces of flat washers.
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