Sizing for a drag link connecting a wing to a fuselage
Sizing for a drag link connecting a wing to a fuselage
(OP)
I'm trying to determine sizing for a drag link that is connecting an underbelly wing to a fuselage.
Assuming that this drag link is horizontal and only transmits loads in the forward and aft direction and assuming this aircraft is to be FAA certified to Part 23 standards for a normal, non-aerobatic, aircraft, what sort of loads will I have to design for? Do we just use 3.8g for part 23 aircraft as max maneuvering load or does that just apply to the vertical loading?
Or will I have to design using FAR23.561 crash loads using some sort of thing under (c)(ii), which indicate that I have to protect the occupant with gear up in a crash with coefficient of friction of .5 at the ground. (0.5 * weight of aircraft?), because I assume in such a landing case we'll need the wing to remain attached to the fuselage?
Assuming that this drag link is horizontal and only transmits loads in the forward and aft direction and assuming this aircraft is to be FAA certified to Part 23 standards for a normal, non-aerobatic, aircraft, what sort of loads will I have to design for? Do we just use 3.8g for part 23 aircraft as max maneuvering load or does that just apply to the vertical loading?
Or will I have to design using FAR23.561 crash loads using some sort of thing under (c)(ii), which indicate that I have to protect the occupant with gear up in a crash with coefficient of friction of .5 at the ground. (0.5 * weight of aircraft?), because I assume in such a landing case we'll need the wing to remain attached to the fuselage?





RE: Sizing for a drag link connecting a wing to a fuselage
RE: Sizing for a drag link connecting a wing to a fuselage
you could restrain the drag from a crash landing, or not, if losing the wing won't rupture the wing fuel tanks ...
what about landing gear ? attached to the wing ?? spin-up and spring back (from landing) usually create critical drag loads.
what about the flaps ?
i'd look into the ACs mentioned above and at airplane design books (Raymer, Torenbeek).
sure this is a work problem ?
RE: Sizing for a drag link connecting a wing to a fuselage
I'm thinking [9g * mass of entire airplane] is entirely too much to be designing towards, so I'd like some substantiation showing that this is an FAA requirement if it is.
I'm thinking that one of the in-flight maneuvering, flap down, gust loads conditions may be a critical case, and if not that, then one of the gear down landing cases may be providing the critical load.
I'm just trying to get some agreement that 9g crash loads are not an FAA requirement for the link.
The forward loading conditions in 23.561 are all conditions where the fuselage hits the terrain and either an occupant or an "item of mass inside the fuselage" continues on a forward inertial path. Nothing here would suggest we need to account for a wing only hitting the terrain and accounting for a 9g * mass of entire airplane moving forward in relationship to the wing.
RE: Sizing for a drag link connecting a wing to a fuselage
there are many design requirements for wing drag ... in-flight, landing, crash, flaps down.
if your stress group are telling you 9g then i think your best coarse of action is to show them how big this makes the drag link, then maybe they'll reconsider. you could ask them where the requirement comes from (they'll probably point to 561 and you can continue the "discussion").
what about the material in the ACs ? or design texts ??
RE: Sizing for a drag link connecting a wing to a fuselage
On the new Viking DHC-6-400 Twin Otters, they are incorporating a drag link modification that was not part of the original design.
This drag link reinforces the rear spar attachment for the crash load case.
Apparently, there was some past experience with the wings folding forward on a crash, driving the props into the cockpit.
Note that not every design consideration is triggered by a requirement. First, the aircraft must be safe. That is subjective, but in the Twin Otter example it was believed that the modification improved survivability enough to make the modification worthwhile.
Otherwise, I'm afraid I don't have an answer- not a wing designer. Keep digging.
RE: Sizing for a drag link connecting a wing to a fuselage