Design Objective: Re-design the lift strut interface of the Arctic Tern to withstand increased static loading conditions.
January to June 2017
The Arctic Tern's lift strut design – the known point of failure during extreme wing loading – must be improved to withstand the loading conditions of the FAA certifications process. The main research goals were:
1. Characterize the failure of the existing lift strut design.
2. Design and develop an improved design.
3. Design and develop a testing procedure to validate the new design.
Analysis of Existing Wing Structure.
To gain a better understanding of the current design and its shortcomings, I sought to analyze the existing wing structure under the anticipated flight loads and characterize the previous failure of the front lift strut. In a former static test procedure, the maximum potential of the current wing was determined by continuing to load the wing structure, lift struts, and fuselage until a structural failure occurred.
Static test #22 failed front lift strut
Static Test Report #22 setup. The uncovered aircraft wing and fuselage are oriented upside-down and Four hydraulic cylinders apply the distributed static load to each rib.
Although the test report documented the failure as a net tensile failure at the highest bolt hole of the front lift strut, they did not know the exact load through the lift strut when the component failed.
Aircraft Load Model: Convert aircraft gross weight to load through strut to determine design load
The first step in analyzing the current design was determining the maximum load that the front strut would experience during flight and designing for it. To find this design load, I built upon the previous aircraft load model STOL Aviation had developed and analyzed the plane at all corner of the flight envelope required by the FAA.
Free body diagram of aircraft wing structure
The aerodynamic loads applied to the wing structure are supported by the wing lift strut and the wing root connection to the fuselage. This force balance determined the resultant load, R through the front lift strut.
Sample output from MATLAB model
The Normal and Chordwise forces applied along the aircraft wing are calculated for a positive vertical acceleration load factor of 3.8g and for Arctic Tern’s maneuvering speed, cruising speed, and dive speed.
The inputs to the model are the aircraft gross weight, and the velocity and load factors specified by the FAA. Using these variables, the model solves for the distributed lift and drag forces along the wing. Next, those distributed loads are resolved into internal loads on the wing’s ribs and spars. Finally, the model calculates the force through the lift strut. The maximum load through the front lift strut - experienced at the Arctic Tern's Dive speed and a positive load factor of 3.8 - was used as a benchmark for analyzing the existing design and future designs.
Existing Design: Stress Analysis
Having determined the theoretical limit loads for the front lift strut, the existing structure was analyzed under the new load conditions to determine where the existing design needed to be improved. Hand calculations were performed to analyze the net-section tensile stress , the bearing stress, and the bending stress of the strut and fitting. I also performed Finite Element Analysis of the strut assembly with the SolidWorks simulation package.
The findings from both classical hand calculations and FEA indicated the current lift strut design did not meet the strength requirements of the increased static loading conditions.
After considering numerous design alternatives, a final design was established that was a simple modification to the original design. The fitting features a press fit steel bushing at the fuselage mount to improve the bearing strength of the lug hole. The lug width was also increased to strengthen this connection further. The fitting was elongated and an additional bolt hole was added to take advantage of the greater cross sectional area further down the lift strut extrusion. The fitting thickness was reduced to remove material where it was not necessary for additional strength.
On the strut extrusion, the counterbore on the bolt holes was eliminated to increase the bearing strength and the net-section tensile strength of the extrusion. As a result, curved washers were added to accommodate the removal of the counterbore.
To validate the new design’s strength and confirm the predictions of the old design’s failure mode, I developed a component level testing procedure that simulated the full scale static loading of the lift strut. I designed and fabricated custom test fixtures to perform ultimate tensile loading on an Instron Machine.
Testing: Results and Discussion
Three ultimate tensile tests were performed on three different configurations of the front lift strut interface. The new design demonstrated a 30% ultimate tensile strength improvement - enough to withstand the design load with a factor of safety of 1.5.