Sizing for a drag link connecting a wing to a fuselage

[simkatu]

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?

 

[SWComposites]

I think there is a whole bunch of load cases defined in FAR 23.321 thru 23.562 which you have to consider. Suggest contacting an loads authorized DER or the applicable FAA ACO office. You should also look at FAA AC23-9 and AC23-19.

 

[rb1957]

you could use 3.8g LMT and remember that this lift can be generated at low AoA (high speed) and at high AoA (low speed). remember that the lift of the wing is 3.8*W+Ht, Ht = horizontal tail lift, assuming a conventional layout.

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 ?

 

[simkatu]

@rb1957 - It is definitely a work problem. I'm designing structure for a small jet airplane and was told by someone in the stress group that I needed to use [9g * mass of entire airplane] as a critical loading case for the drag link. I cannot see how anything in FAR23.561 (crash loads) pertains to loading on the drag link (fuselage to wing) other than perhaps the .5 coefficient of friction that would occur with a gear up landing on the wing and even that would only be necessary if we determine that seperation of the wing will rupture the fuel tank.

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.

 

[rb1957]

you're right (IMHO) that applying 9g (from 23.561) is excessive. it is meant of items of mass inside the cabin that can impact the passengers.

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 ??

 

[SAITAETGrad]

Please note that the following is third hand information:

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.

 

[rb1957]

being familiar with the DHC6 i can say with authority that dH did incorporate a mod to restrain the wings folding fwd in a crash ... it happened unfortunately. notwithstanding that, the good folks at Viking may have changed the install in some way.