Their wings can be folded or collapsed back against the Stormbird's chassis in some manner when the spacecraft was not in-atmosphere. A Stormbird was bulkier and larger than a Thunderhawk and also less agile in flight. However, the Stormbird could carry up to the equivalent of a modern Space Marine company, between 60 and 100 Space Marines. The Stormbird had missiles slung under both wings and a powerful cannon mounted on its nose. Stormbirds were piloted by two Space Marines sitting back to back along with two Servitors hardwired into the vessel. It is not known what a Stormbird actually looked like, yet early drawings of the Thunderhawk show organic, large, and Vertical Take-off and Landing (VTOL) capable Imperial landing craft dropping Space Marines on a target and it is assumed that the Stormbird looked much like these early pattern Thunderhawks, only larger.

Anyway, the novels generally describe the Stormbird as being much more graceful and elegant compared to the Thunderhawk, as well as substantially larger; it had forward-sweeping wings and a conical nose(unlike the square nose of the Thunderhawk or Marauder). It also said that the Stormbird had both frontal and rear assualt ramps, with rear-facing cage seats and an elevated crew compartment (which contained two pilots and two servitors, if memory serves me right).

They are larger and more 'sleek' than Thunderhawks. Still, they are described as 'fat bodied' and 'heavy-duty'. There are several different variants in service (the Luna Wolves, for example, use the 'Warhawk VI' pattern). They have some kind of 'underwing cannons', as well as cannons mounted underneath the cockpit, and 'rotary cannons in forward pintle mounts'. In addition, they are armed with racks of missiles under the wings. They are often described as looking somewhat similar to birds of prey, so judging by this I would have thought their wings are much larger in proportion rather than the relatively small wings on a Thunderhawk. The wings are described as folding inwards while landing as well.

Stormbirds are heavy, armoured delivery vehicles. They are described as 'monstrous, fat-bodied flyers' (False Gods P. 91-92). Despite their great size and heavy armour, they are honed and sleek. Their wings when extended are 'like hawks' and they are flared. They wings are folded upon landing.

The engines are positioned under the wings, as are missle racks. Wide rotary cannons are seated in forward pintel mounts

The disembarking/assault ramp is at the rear of the Stormbird and it has a series of skids upon which it lands.

In the frontal cone of the Stormbird there are forward slit-ports.

Details taken from Horus Rising P. 139, 149 and False Gods P. 91-92, 103, 183

Stormbirds:

Description:
Its actual pattern type is a Warhawk VI, and they were originally manufactured for the campaigns of the Unification Wars. It is possible that it was initially designed as a sub-orbital combat transport, as they were built in or around Indonesia (Yndonesic Bloc in the book) for use against forces based in and around the Pacific (the Panpacific tribes) (Horus Rising P139).

The stormbird is a heavy, void capable, armoured transport and delivery vehicle that is also sleek and honed for atmospheric work (Horus Rising P139). In comparison Thunderhawks are smaller and described as 'not as sleek' (False Gods P205),

Fuselage:
It also described as having its 'wings extended, like hawks waiting to be thrown to the lure.' (Horus Rising P139), though this may also refer to the folding wing design as the wingspan is too large for the entrance of the embarkation decks (False Gods P183). In Flight Of The Eisenstein A Stormbird is described in the following fashion whilst landing "The ship settled and folded its raptor wings to its fuselage." (P162). The folding mechanism appears to be powered by the craft itself, and not a manually handled operation, though whether this is a form of swing wing design, naval folding wing design or some design unique to that age, is as much your guess as mine.
Described as 'dangerously graceful birds of prey' (False Gods P29), they get the adjective 'Sleek' when described when compared to the 'Heavy' Thunderhawk (Flight Of The Eisenstein P62).

It's nose, based on the description of the cockpit, seems to a cone with 'forward slit-ports for the pilots to look out of, either servitor-pilots (Horus Rising P149) or human ones (Fulgrim P88), though the cockpit layout may not be standard. The shape of the forward section of the Stormbird may contribute to the 'bird of prey' analogy.

Carrying Capacity:
The troop capacity is at least 50, with 12 being able to carry a Company of 600 Space Marines (Horus Rising P139 & 141). This, to a certain extent, is a also borne out by the fact that Horus uses six Luna Wolf Stormbirds from one embarkation deck (False Gods P88), which are joined by 12 more Luna Wolf Stormbirds, and two Word Bearer Stormbirds, (False Gods P98), twenty in total, to carry 4 detachments of nearly 200 Luna Wolves, each led by one of the Mournival (False Gods P109) for the attack on Davin's moon, as well as himself and a possible honour guard. Though there are references to 5 Companies of Luna Wolves (False Gods P89) and a detachment of Word Bearers in the force (False Gods P135).

Weapons Payloads:
They have the capacity to carry racks of missiles beneath each wing (False Gods P91), and bear cannons slung beneath the cockpit (False Gods P88), though it is uncertain if these are the same as the rotary cannons in forward pintle mounts (False Gods P91). (Pintle mounted refers to a weapon being mounted on a vertical rod allowing to be rotated.)
A Stormbird can be modified to carry extra missiles and stowage compartments at the, as Ferrus Manus's Stormbird (Fulgrim P147).

Internal Arrangement:
For the Luna Wolves' Stormbirds there is a central 'spinal' aisle, and the troops sit in rear facing 'cage seats', whilst the cockpit holds four crew, 'two flight officers sat back to back, facing wall panel consoles' though it is possible that they are separated by an aisle to allow access forward to the two pilot servitors hardwired to the helm consoles (Horus Rising P149).
But the Stormbirds of the Dark Angels seem to have a different layout for the passenger compartment with at least some Astartes sitting against the fuselage (Descent Of Angels P384), and sat across from brother Astartes (Descent Of Angels P385).

Other Details:
A Stormbird appears to have at least a frontal assault ramp (Fulgrim P147) and a rear debarking ramp (False Gods P103), as well as other means possible of egress as Dark Angel Stormbirds are described as having 'assault doors' (Descent Of Angels P387).

The landing gear appears to be landing skids (False Gods P103, Fulgrim P147), probably similar to those of the Thunderhawk.

The Firebird:
It is possible that Fulgrim's Firebird is a variant of the Warhawk pattern type, and may even be a Stormbird variant. It is described has having a greater wingspan than the Stormbird with the wings curved in a graceful backwards sweep and a hooked prow (Fulgrim P86 & P144). It even has landing claws (Fulgrim P202).

Model details:
In models terms a Stormbird's fuselage is probably of a similar size to that of Forge Worlds Tau Orca dropship. And in the case of the Stormbird described in Descent Of Angels, a similar internal configuration.

'The Stormbird touched down...as the disembarking ramp dropped from the rear of the Stormbird.' False Gods P. 103

There are currently unanswered questions about the Stormbirds design. For example:

Was arrangement of cockpit and passenger compartment like the Thunderhawks? The cockpit on an elevated deck with a single deck for the passenger compartment and access to a frontal assault ramp under the cockpit as well as a rear ramp, like a modern Hercules military transport.

Or was the main fuselage section on two decks? With the cockpit and passenger compartment sharing an upper deck allowing for vehicle or cargo transport on the lower deck.

Or was it a single deck aircraft with the assault ramp part of the lower section of the cockpit area's nose-cone?

With the basic description of 'pintle mounted' for the structure holding the rotary cannons, could this possibly allude to a mounting like the sponsons of the M40 Predator? Or would they be some form of turret, possibly like the ones found on a M40 Razor Back, or underneath the wing of Thunderhawk?

Waaay back in November I think, I had a little chat with Dan Abnett as to how he pictured and thought of the Stormbird. He told me that the thing looks like the Naboo Space ship from Star Wars Episode 1:The Phantom Menace (you know, the one where the Queen, Ob-wan and the other guy fled to corusant), but made more 'Imperial' and not as shiny .

I saw this take on a stormbird on deviant art years back (at least it feels that way) and have had it saved in my bookmarks ever since

Maybe a little too BFG esque but I think it looks cool

Very cool but that nose would pretty much make it drop like a stone, as it would push the ship down.

Aerodynamics, however, apparently don't apply in the 41st (or 31st) Millenium.

Very cool but that nose would pretty much make it drop like a stone, as it would push the ship down.

Aerodynamics, however, apparently don't apply in the 41st (or 31st) Millenium.

I'm going to use this DRM Dropship I converted a bit as my Stormbird ...

Good idea! Those engine pods MaxMini jump packs? (put a LR next to that guy too )