For various reasons – none of them related to the aerodynamic advantages, oddly enough – I’m working on a nearly-unswept tailless sportplane design, broadly similar to Charles Fauvel’s AV36/AV45 motorgliders, but with shorter span.
My question revolves around flying qualities as perceived by the pilot; specifically in the pitch axis.
It’s possible to give the design as much or as little static stability as I want, but I’m pretty much stuck with very low damping in pitch. The pitch moment of inertia is also very low, fortunately.
My concern is that a ‘normal’ 7-12% static stability margin is probably going to develop pitch moments that could ‘overwhelm’ the low damping, possibly to the point where I have positive static stability, but negative dynamic stability. Obviously, that wouldn’t be desirable for a fairly low-time pilot such as myself. Even if things weren't that bad, a really strong static stability combined with weak damping could be disturbing to a pilot, especially in rough air.
My thinking is to reduce the static margin to be more ‘in harmony’ with the amount of damping available, such that the balance between the two results in a more ‘normal’ feel for the pilot. A little research showed the Hortens felt their aircraft felt ‘normal’ at 3.5-4.5% static margin, but then they seem to have had somewhat loose standards on flying qualities.
My ‘actual’ question falls into two parts:
1) Am I on the right track here? Should the static margin be reduced compared to a ‘normal’ airplane, such that static stability and damping are more in ‘normal’ proportion?
2) Short of a complete dynamic stability analysis – which I don’t have the math skills to do at this point – can anyone suggest a way to quantify (even a 'ballpark' fashion will do) this at the design stage, so that I’m not stabbing completely in the dark?
Thanks!
My question revolves around flying qualities as perceived by the pilot; specifically in the pitch axis.
It’s possible to give the design as much or as little static stability as I want, but I’m pretty much stuck with very low damping in pitch. The pitch moment of inertia is also very low, fortunately.
My concern is that a ‘normal’ 7-12% static stability margin is probably going to develop pitch moments that could ‘overwhelm’ the low damping, possibly to the point where I have positive static stability, but negative dynamic stability. Obviously, that wouldn’t be desirable for a fairly low-time pilot such as myself. Even if things weren't that bad, a really strong static stability combined with weak damping could be disturbing to a pilot, especially in rough air.
My thinking is to reduce the static margin to be more ‘in harmony’ with the amount of damping available, such that the balance between the two results in a more ‘normal’ feel for the pilot. A little research showed the Hortens felt their aircraft felt ‘normal’ at 3.5-4.5% static margin, but then they seem to have had somewhat loose standards on flying qualities.
My ‘actual’ question falls into two parts:
1) Am I on the right track here? Should the static margin be reduced compared to a ‘normal’ airplane, such that static stability and damping are more in ‘normal’ proportion?
2) Short of a complete dynamic stability analysis – which I don’t have the math skills to do at this point – can anyone suggest a way to quantify (even a 'ballpark' fashion will do) this at the design stage, so that I’m not stabbing completely in the dark?
Thanks!