Ok, just lost my complete post to a computer crash, so a slightly shorter one.
I have a shoulder-wing design with classical sandwich composite wing construction, no landing gear in it and the requirement for a straight trailing edge (flaperons), so polyhedral is not an option. Large sump tank (2 hours of fuel) in the fuselage, gravity feed to the sump. Filling of the wing tanks solely via the fuselage sump (simplicity again), so that the highest fuel level of the wings can never be higher than the fuel cap.
First, I had two fuel tanks per wing, one behind and one in front of the spar, both out to roughly half span. This didn't work (flutter) and thus I need two tanks in the wing, in front of the spar, one inner, one outer. A single big tank is not an option since the pressures during a spin would be so huge that the skin would rip apart and fuel shift during asymetric flight would be unacceptable too.
Gravity feed for me is essential (simplicity, reliability, build time). Now I got thinking about using some dihedral. While it might cause excessive roll stability, it makes feeding from the outer tanks feasible. Since the spar is at the "lowest" part of the airfoil in cruise, feeding the fuel behind the spar doesn't help. I was thinking about laminating three 1/2" tubes in the fuel tanks. One in the inner tank, against the lower skin and front of the spar, to feed the fuel from the tiptank to the fuselage, the other two against the upper skin and spar to vent towards the tip.
A straight wing would immediately leak fuel when you don't park it on perfectly even ground. A further requirement is that the empty wing can be lifted at the tip to fold the wings easily (no fuel only). A winglet with a vent on the top would be perfect. Winglets though are very complex to design from an aero point of view. Canted tips aren't. I'm thinking about something like this:
This way you can continue a normal airfoil, still achieving some vertical heigth for the vent. By keeping it short (7" or so) lifting the wing at the end of the straight section still isn't a problem.
With a wingspan of just under 30 ft, more than 3.5 degrees of dihedral (per wing) would put the tip tank above the filler cap. 3 degrees of dihedral would give the outer tanks a head to the fuselage of 4".
Is that enough? And if it is, would that cause excessive stability (I didn't do the math yet). What is an acceptable number of "unlevelness" of an airstrip where the vents starts to leak fuel?
I have a shoulder-wing design with classical sandwich composite wing construction, no landing gear in it and the requirement for a straight trailing edge (flaperons), so polyhedral is not an option. Large sump tank (2 hours of fuel) in the fuselage, gravity feed to the sump. Filling of the wing tanks solely via the fuselage sump (simplicity again), so that the highest fuel level of the wings can never be higher than the fuel cap.
First, I had two fuel tanks per wing, one behind and one in front of the spar, both out to roughly half span. This didn't work (flutter) and thus I need two tanks in the wing, in front of the spar, one inner, one outer. A single big tank is not an option since the pressures during a spin would be so huge that the skin would rip apart and fuel shift during asymetric flight would be unacceptable too.
Gravity feed for me is essential (simplicity, reliability, build time). Now I got thinking about using some dihedral. While it might cause excessive roll stability, it makes feeding from the outer tanks feasible. Since the spar is at the "lowest" part of the airfoil in cruise, feeding the fuel behind the spar doesn't help. I was thinking about laminating three 1/2" tubes in the fuel tanks. One in the inner tank, against the lower skin and front of the spar, to feed the fuel from the tiptank to the fuselage, the other two against the upper skin and spar to vent towards the tip.
A straight wing would immediately leak fuel when you don't park it on perfectly even ground. A further requirement is that the empty wing can be lifted at the tip to fold the wings easily (no fuel only). A winglet with a vent on the top would be perfect. Winglets though are very complex to design from an aero point of view. Canted tips aren't. I'm thinking about something like this:
This way you can continue a normal airfoil, still achieving some vertical heigth for the vent. By keeping it short (7" or so) lifting the wing at the end of the straight section still isn't a problem.
With a wingspan of just under 30 ft, more than 3.5 degrees of dihedral (per wing) would put the tip tank above the filler cap. 3 degrees of dihedral would give the outer tanks a head to the fuselage of 4".
Is that enough? And if it is, would that cause excessive stability (I didn't do the math yet). What is an acceptable number of "unlevelness" of an airstrip where the vents starts to leak fuel?
