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.

enter image description here

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    $\begingroup$ Do you want to increase the payload volume, or the payload mass? You will get different answers for each. $\endgroup$ – jamesqf Dec 12 '20 at 3:15
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    $\begingroup$ If your number of passengers is given, so is their weight. ICAO has standard passenger masses which increase with trip range. What does the additional payload consist of? As @jamesqf says, it makes a difference if it's styrofoam blocks or bricks. $\endgroup$ – Peter Kämpf Dec 12 '20 at 4:29
  • $\begingroup$ @Jamesqf, Payload mass to 11,500kg (from Q400's 8,500kg). $\endgroup$ – Rory McDonald Dec 12 '20 at 16:07
  • $\begingroup$ @PeterKämpf Thanks I hadn't seen the standard ICAO masses before. The assignment brief specifies 90 passengers @ 106kg (!), but the aircraft must be able to carry a payload of 11,500 kg total. Strangely the design driver is for short-haul flights of 600km, I don't know why the passengers need so much luggage. $\endgroup$ – Rory McDonald Dec 12 '20 at 16:13
  • $\begingroup$ Thank you for the details. Should you assume ICAO's 161 kg/m³ payload density, or is another value given? Splitting the cabin in two is not recommended (emergency exits!), so make the fuselage wider and gain more space under the cabin floor. $\endgroup$ – Peter Kämpf Dec 12 '20 at 18:00

You really only have two options; stretch or swell.

The Q400 already has a "combi" configuration option with 50 seats forward and a cargo hold taking up the aft third of the cabin, so if you want to just add another couple of plugs, one (smaller plug) forward of the wings and one aft, to get 74 seats plus a large cargo area, you can do that and just adjust the structural requirements to deal with the even longer flying pencil that results. You can have a cargo hold aft and pax cabin forward with the appropriate weight and balance requirements and limitations for loading pax vs cargo same as any other split combi layout.

In the case of the Q400 itself, its configuration is already at the extreme limit of what you can do with fuselage stretches on that sort of 4-across tube diameter, having started with 36 seats in the original. The plane suffers from tail clearance problems because of this, due to limits on how much the gear legs could be extended to account for the long tail (landing with less than full flaps - there are a couple of intermediate landing flap settings - has a high risk of tail strikes and pitch attitude must be very carefully managed in the landing flare).

I think you'd get a better aircraft by going with "swell" and increasing the tube diameter enough to get a 5 across cabin (around 2 feet), with 2 on the left side and 3 on the right. A 74 pax 5-across cabin would only need 15 rows, leaving the rest free for cargo, and this extra width plus shortened passenger area might even allow you to shorten the fuselage a bit, leaving some stretch-ability potential in your initial design (you might even get bonus points for taking this into account).

enter image description here

  • $\begingroup$ Thank you, very informative. I guess that's part of the reason why people say the Q400 is difficult to land. I didn't realise that tail strikes put a limit on the coupling length (?) of the aircraft but it's definitely something I'll bring up in my report. I'm likely going to use the second configuration you recommended but move the seating backwards a little bit so luggage can be distributed both at the aft and the front. I posted a picture of the Q400 cabin layout in my post now, something like that just with 5 abreast seating. $\endgroup$ – Rory McDonald Dec 12 '20 at 16:21
  • $\begingroup$ That would work if you have the forward cargo hold on one side only.One issue with large aft cargo areas is you have to allow for a loaded cargo hold with an empty cabin on the ground, and you may be forced to move the gear location farther aft, requiring a larger horizontal tail, or worse, include a tail strut.So consider that in C of G calcs.Tail engine jets have this problem with no pax aboard cuz of the wt of the engines, and place the gear farther aft than necessary for the normal loaded C of G requirement, to avoid a tail strut, and requiring a larger elevator for rotation on TO. $\endgroup$ – John K Dec 12 '20 at 19:24

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