I'm designing a regional turboprop for a university assignment. The design requirements are very similar to the Dash 8 Q400 in terms of passenger capacity (90 pax), speed, range, runway length requirements. The difference is the payload mass required is 35% greater (8,500kg to 11,500kg).
So I've sized my wings and my engines and control surfaces and flaps and they're all bigger. My issue now is the fuselage. Since the passenger capacity is the same I have less motivation to increase the dimensions.
Increasing the diameter for the sake of more internal volume seems inefficient; I don't need more aisle and armrest space. On the other hand, increasing the length (without increasing diameter) would mean a heavier fuselage structure.
Even once the fuselage is lengthened, where do I put the cargo? For the Q400, cargo holds are right at the aft and just before the cockpit. But if this is my solution I'll be shifting the centre of gravity and creating even greater bending moments.
My ideas so far are to create a near c.g. cargo hold that would likely split seating or to utilise space available under the floor of the cabin. I haven't seen any designs that support these ideas though (for narrow bodies).
I tried to look for twin-engine turboprops with similar payloads, the only one I found was the C-27J Spartan (military transport), but it's practically designed for cargo.
Could someone give me some wisdom on how I should proceed? I've done quite a bit of reading and research but can't resolve this aspect of the design.
Here's the 74 seat cabin layout for the Q400.