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The PC-24 has, under the fuselage, a kind of huge support to hold the wings.

Pilatus PC-24

If we compare with the Citation CJ4 or the Phenom 300, this difference is quite significant. Why??

Citation CJ4 Phenom 300

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    $\begingroup$ What makes you think that bulge is support structure to hold the wings? Generally in aircraft, the wings hold the fuselage rather than the other way round, but I don't think that's the purpose of this feature either. $\endgroup$ Sep 26, 2019 at 5:56
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    $\begingroup$ @pericynthion I have no idea about structures, so probably is the other way around as you say $\endgroup$
    – Ana
    Sep 26, 2019 at 6:01
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    $\begingroup$ It could be a wing main spar carry-through (effectively the wing holding the other wing, albeit for bending loads), or luggage space, or a place for the main gear wheels to go (PC-24 has larger wheels than most similar sized jets), or an aerodynamic fairing to reduce interference drag... I'm not posting an answer because I don't know which of these (if any) it is. But it's not necessarily structural. $\endgroup$ Sep 26, 2019 at 6:04
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    $\begingroup$ It's not a wing support, it's a plane support. ;) $\endgroup$
    – J...
    Sep 26, 2019 at 22:11
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    $\begingroup$ The paint scheme there really accentuates the heck out of it. It might work better to select a picture that has a paint scheme comparable to the other pictures. $\endgroup$ Sep 27, 2019 at 4:49

3 Answers 3

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To add to the other answer...

The Pilatus PC-24 is billed as a business jet that can operate out of rough airfields. Comparing the pictures of the PC-24 to the Phenom 300 or CJ4, you can see that there is quite a bit of extra ground clearance. You can also see that the rear landing gear on the PC-24 are much more robust (dual wheels, longer travel length) than the other two aircraft.

If you look at this picture:

enter image description here Source: Wikipedia

You can see that the wheels also retract into that area. They probably wanted to keep the wing low to take advantage of ground-effect to improve short-field performance. So having a higher-up fuselage allowed them to put the engines up higher and avoid FOD damage, while the low wing allowed them an impressive short field take-off performance. The PC-24 can operate out of a field just 2,800 feet long. The Phenom 300 takes 3,200 feet and the CJ4 takes about 3,500.

So the "bump" is there to be able to handle additional loads, elevate the fuselage, and store the gear. I can't find any diagrams of the fuel system, but it's possible that it also houses fuel tanks or baggage.

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    $\begingroup$ Also I have seen that the interior in the pc-24 is flat, while the cj4 and the phenom have a step. Coult it be related? To have more space in the interior or something like that $\endgroup$
    – Ana
    Sep 27, 2019 at 6:42
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The huge support, or bulge, is a fairing, designed for reduction of wing root drag. So many people talk about wing tip vortex drag, but much more is created at the wing/fuselage interface, especially in uncoordinated flight. This improvement, first seen in 1930s vintage gliders (where else!), improves the airflow around the aircraft, resulting in significant fuel saving.

Importantly, this is a subsonic effect. Many super sonic aircraft actually reduce fuselage diameter near the wing to reduce drag, complying with "Area Rule".

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    $\begingroup$ Another reason for this fairing is to provide space for the main wheels to retract. The cabin needs to be circular to hold pressure and the wing isn't thick enough. I don't know why it should be different between these particular aircraft though... $\endgroup$ Sep 26, 2019 at 11:24
  • $\begingroup$ They might have some fuel/cargo in there too. Overall, very practical design, keeping wings nice and thin. Possible hatchling of airliner. $\endgroup$ Sep 26, 2019 at 11:35
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    $\begingroup$ It's also very pronounced on the fastest bizjet. $\endgroup$
    – user14897
    Sep 26, 2019 at 15:21
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    $\begingroup$ Transsonic transport aircraft are usually area-ruled to some extent, but in their case the biggest increase in cross-section is the engines, and the wing root fairing shape tends to be shaped to match—including here, the wing root fairing ends just before the engines. $\endgroup$
    – Jan Hudec
    Sep 27, 2019 at 6:11
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In that area, the wing spar is interfacing with the fuselage, and the retractable landing gear needs space.

In larger airliners, that space is largely inside the cylindrical fuselage, in the level underneath the passenger floor, and the cabin is still high enough to walk in the aisle.

The cross section of the business jets is usually quite small, so having the spar crossing the cabin would make a bump in the cabin floor. In order to not reduce cabin height, that structural space gets pushed "slightly" outside of the cylindrical fuselage body.

That said, other large airliners also have a large bulge - check the A380 for example.


edit:

I did some more digging into that.. Let's focus on a comparison of the PC24 and the CJ4.

I didn't find any reliable drawings or models, so while the bulge certainly seems bigger in the pictures, it's hard to tell how much bigger it really is. Also - designs are always a compromise, and we do not know all the boundary conditions Pilatus or Cessna had, so we can only make educated guesses as to why the bulge is larger on one than on the other.

In order to explain different designs, best look at the differences in mission profile. A big difference between the CJ4 and the PC24 I know of is that the PC24 has "rough field" certification. And here we can start to dig..

PC24 vs CJ4

The CJ4 has a "single wheel" main landing gear (one left, one right), which retract into the wing, so it doesn't need any space in the bulgy fairing. The PC24 has twin wheels, and they do retract into that bulgy fairing, likely because they are too wide to fit into the wing. The wing also seems to have a more elaborate flap system, likely leaving even less space there for a retracting landing gear. I would attribute both to the rough field certification:

  • you want larger / softer .. wheels with more footprint on soft soil
  • while I unfortunately didn't find comparable data for landing speed / take off speed / stall speed, I assume that the more complicated flap system is there to allow for slower landing/takeoff speeds of the PC24, while maintaining cruise speed.

Concerning the aerodynamics: the bulge doesn't strike me as particularly aerodynamic, and if it was, it shouldn't be a differentiating factor between the PC24 and the CJ4. Cruise speeds between the two are similar (Wikipedia: 440kn for the PC24 vs 451kn for the CJ4), assuming that cruise aerodynamic efficiencies are also similar, both planes should converge there in the design. Assuming similar efficiencies there is valid because good aerodynamic knowledge for cruise operating points is around for longer than both planes exist.

So .. Where do they differ? The PC24 as the younger plane might have profited from newer fluid dynamics methods, CFD, turbulence & separation prediction. If anything - these improvements won't be so visible during cruise, but maybe improve handling at slow speeds, stall characteristics etc. It would strike me as odd though, if the bulge improves stall characteristics.

Bottom line for aerodynamics: I strongly doubt that the bulge is larger in the PC24 for aerodynamic reasons.

Finally, another example to support my point: the Hawker 800 has also twin wheels on the main landing gear, and it also has a relatively large bulge:

pic[]

Note: all pictures are from Wikipedia.

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  • $\begingroup$ This is the correct answer. The crossover of a low wing with a narrow radius fuselage leaves bulgy stick-out failing bits, unless the wings are relatively thin. $\endgroup$
    – Koyovis
    Dec 27, 2022 at 9:41
  • $\begingroup$ @Koyovis: this is the correct answer to why in general that area is big on business jets. But the question is why on the PC-24 that are is bigger than comparable airplanes. The correct answer is actually this one. $\endgroup$
    – sophit
    Dec 27, 2022 at 10:48
  • $\begingroup$ @sophit I think this is even more so correct answer why the wing root fairing is bigger in PC-24—as mentioned in the other comment, most other bizjets do have a step in the cabin over the main wing spar, PC-24 does not. $\endgroup$
    – Jan Hudec
    Dec 29, 2022 at 14:13
  • $\begingroup$ @JanHudec: hm, the Citation CJ4 (which is in the question) has no step over the wing spar and has anyway a smaller bulge than PC-24 $\endgroup$
    – sophit
    Dec 29, 2022 at 14:22
  • $\begingroup$ @sophit my guess is that in that case the difference is aerodynamic—CJ4 is slower, with straight wings, so it can do with sharper transition. $\endgroup$
    – Jan Hudec
    Dec 29, 2022 at 14:25

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