orion
R.I.P.
Don't you just hate it when you've been doing something for years and then discover a glitch in your process?
There being no other sources of software for aircraft design (as opposed to analysis), years ago I undertook writing my own. Over the years it has culminated into a rather large program but one that fits in quite well with the design process that I employ for configurational development. One of the aspects it covers is the derivation of the allowable CG range based on the Neutral Points and elevator effectiveness (forward limit). For years I knew that the program was conservative but verification runs of production aircraft were reasonably satisfactory so I never dug into it any further.
Recent work however (including answers I gave herein regarding stability and tail volume coefficients) was starting to bring certain questions to my mind, specifically as to why the program was so consistently conservative and finally, as of about 2am last night, I got it.
One of the terms of the static stability equation deals with tail volume, tail efficiency and downwash. It turns out it's the middle term that was the culprit. When I wrote the original version of the program, I calculated the elevator factors as those for a flight surface with a large chord flap. This still has relevance however the efficiency term is slightly different here since it deals with flow losses. The ratio it represents is the dynamic pressure the tail sees, versus free stream dynamic pressure. The losses in the dynamic pressure are a function of losses incurred due to a windmilling prop, interaction with the wing and the boundary layer buildup on the fuselage.
Calculating the efficiency factor as presented in McCormick (for flaps) is actually inaccurate since it results in much lower percentages than what we might see in calculating energy loss.
Well, the good news in all this is that what I had was conservative so no harm done. But still, I would have like to find things like this sooner than later.
There being no other sources of software for aircraft design (as opposed to analysis), years ago I undertook writing my own. Over the years it has culminated into a rather large program but one that fits in quite well with the design process that I employ for configurational development. One of the aspects it covers is the derivation of the allowable CG range based on the Neutral Points and elevator effectiveness (forward limit). For years I knew that the program was conservative but verification runs of production aircraft were reasonably satisfactory so I never dug into it any further.
Recent work however (including answers I gave herein regarding stability and tail volume coefficients) was starting to bring certain questions to my mind, specifically as to why the program was so consistently conservative and finally, as of about 2am last night, I got it.
One of the terms of the static stability equation deals with tail volume, tail efficiency and downwash. It turns out it's the middle term that was the culprit. When I wrote the original version of the program, I calculated the elevator factors as those for a flight surface with a large chord flap. This still has relevance however the efficiency term is slightly different here since it deals with flow losses. The ratio it represents is the dynamic pressure the tail sees, versus free stream dynamic pressure. The losses in the dynamic pressure are a function of losses incurred due to a windmilling prop, interaction with the wing and the boundary layer buildup on the fuselage.
Calculating the efficiency factor as presented in McCormick (for flaps) is actually inaccurate since it results in much lower percentages than what we might see in calculating energy loss.
Well, the good news in all this is that what I had was conservative so no harm done. But still, I would have like to find things like this sooner than later.
