Tail cone sizing bug

[askprash]

The tail cone sizing also is defaulting to using the skin bending stress limit to size the tail cone that needs to resist shear due to the vertical tail generated torsion:

TASOPT.jl/src/structures/fuseW.jl

Lines 139 to 142 in 90a11ce

	#--- cone material properties
	# (assumed same as skin, but could be different)
	taucone = sigskin
	rhocone = rhoskin

and

TASOPT.jl/src/structures/fuseW.jl

Lines 259 to 269 in 90a11ce

	Qv = (nvtail*Lvmax*bv/3.0)*(1.0+2.0*lambdav)/(1.0+lambdav)
	Vcone = (Qv/taucone)*
	(pi + nfweb* 2.0*thetafb)/
	(pi + nfweb*(2.0*thetafb+sin2t))*
	(xconend-xshell2)/Rfuse *
	2.0/(1.0+lambdac)
	Wcone = rhocone*gee*Vcone*(1.0+fstring+fframe+ffadd)
	xWcone = Wcone * 0.5*(xshell2 + xconend)
	tcone = Qv / (2.0*taucone*Afuse)

The shear strength (depending on the material) can be a fair bit lower than the YTS which would mean that the cone skin thickness is not adequately sized per the current code.

A little concerned that I'm misinterpreting something about the conceptual abstraction of the smeared out semi-monocoque design so I've reached out to Mark to confirm. Will update the issue accordingly.

[askprash]

Mark says this is tricky and is just one sizing criteria he used - the real cone sizing might be more complex and he's not really sure what sizes the skin gauge. Updated the cone to use the materials type for consistency in #57 and will look to see if any comparably tractable approach is recommended in the literature to better size the tail cone.