Looking at a variety of truss types (anticipating a steel tube fuselage structure), it appears that you can “tune” the geometric configuration for the types of loads you expect. For example – a Warren truss uses diagonal members both in tension and compression equally (no vertical members). A Pratt truss on the other hand uses vertical members in compression only, while the diagonals are in tension (only). In my mind, the Warren truss is applicable to competition aerobatic aircraft where positive and negative loads are expected in equal portions. The Pratt is most applicable where loads are primarily from one direction – such as a railroad bridge.
Based upon the wide variety of rag and tube aircraft design, it is obvious that both types can be constructed to work in either application, but if one is looking for the most efficient use of materials (best strength/weight), then there is something to be gained by selection of the geometric configuration. If my thinking is still on track, then a normal category aircraft is somewhere between the loads expected between the two – that is, it will see a given load in the positive, and something less in negative. In my design, I expect the ratio to be on par with a “normal” GA aircraft. Given all that, would a modified Pratt truss – one with slightly canted compression members and therefore slightly shortened tension members provide the structural bias and therefore efficiency I’m looking for? In other words, can I lean on geometry, or is it better to vary the number of cells/modify the size of the individual force members?
Clearly I have some math to do, but is there a general guideline here?
Based upon the wide variety of rag and tube aircraft design, it is obvious that both types can be constructed to work in either application, but if one is looking for the most efficient use of materials (best strength/weight), then there is something to be gained by selection of the geometric configuration. If my thinking is still on track, then a normal category aircraft is somewhere between the loads expected between the two – that is, it will see a given load in the positive, and something less in negative. In my design, I expect the ratio to be on par with a “normal” GA aircraft. Given all that, would a modified Pratt truss – one with slightly canted compression members and therefore slightly shortened tension members provide the structural bias and therefore efficiency I’m looking for? In other words, can I lean on geometry, or is it better to vary the number of cells/modify the size of the individual force members?
Clearly I have some math to do, but is there a general guideline here?
