Can someone help me understand metal fatigue and it's relevance in light aircraft design.
From what I can find it seems like most homebuilt aircraft designs ignore fatigue? Online literature from manufacturers typically gives allowable (REF, Sonex: +6/-3), but don't say anything about expected fatigue life of different airframes. Can I really expect to be able to pull +6/-3 for 2000 hours, or can I only do +6/-3 for 150 hours? When designing a wing spar, do people usually use fatigue curves to determine max allowable stresses, or do people usually focus on yield?
2024-T3
Ultimate: 57-70ksi
Yield: 42-52ksi
Fatigue: 18ksi @ 1*10^8 cycles, stress ratio of -1
It seems like if I were to design the spar for the fatigue strength @ 1*10^8 cycles, I shouldn't have to worry about ultimate strength of the wing. But the wing may get too heavy.
If I want to look closer at fatigue, are there methods for determining the number of cycles at different load conditions for the life of the aircraft? How many cycles due to taxiing over a bumpy taxiway, and what loads should be expected? How many cycles due to turbulence, and at what loads?
Maybe I just pick a data point, design for that, but make one member slightly weaker than the others in an easy to inspect location. That way during the condition inspections, I can check the weak point. If the weak point is ok, then I don't have to worry about the other members. If it breaks, then I should build a new wing.
Just a bit confused here
As a side note, when renting old spam cans from your local flight school, do you ever worry about fatigue of the aircraft?
From what I can find it seems like most homebuilt aircraft designs ignore fatigue? Online literature from manufacturers typically gives allowable (REF, Sonex: +6/-3), but don't say anything about expected fatigue life of different airframes. Can I really expect to be able to pull +6/-3 for 2000 hours, or can I only do +6/-3 for 150 hours? When designing a wing spar, do people usually use fatigue curves to determine max allowable stresses, or do people usually focus on yield?
2024-T3
Ultimate: 57-70ksi
Yield: 42-52ksi
Fatigue: 18ksi @ 1*10^8 cycles, stress ratio of -1
It seems like if I were to design the spar for the fatigue strength @ 1*10^8 cycles, I shouldn't have to worry about ultimate strength of the wing. But the wing may get too heavy.
If I want to look closer at fatigue, are there methods for determining the number of cycles at different load conditions for the life of the aircraft? How many cycles due to taxiing over a bumpy taxiway, and what loads should be expected? How many cycles due to turbulence, and at what loads?
Maybe I just pick a data point, design for that, but make one member slightly weaker than the others in an easy to inspect location. That way during the condition inspections, I can check the weak point. If the weak point is ok, then I don't have to worry about the other members. If it breaks, then I should build a new wing.
Just a bit confused here
As a side note, when renting old spam cans from your local flight school, do you ever worry about fatigue of the aircraft?
