Low wing passenger jets - why?
 

The following thread Antonov has got me thinking, if high wings have so many advantages, ie lower floor height, protection from FOD, U/C weight etc, why are most jets low wing? Is it just down to windows for the PAX? or does it go deeper than that?

All design is an exercise in compromise. Some relevant considerations in a global sense relate to -

(a) undercarriage weight and cost

(b) for larger aircraft, cabin clear height, as the carry through structure is more easily buried in the lower fuselage which has the depth there anyway for tanks and holds.

(c) operational environment which plays a big role in considering engine location.

Noise level in the cabin is also a consideration. For the low winged a/c, presumably the engies are underslung, the wings can help to deflect the noise of the engine to other directions away of the fuselage.

But couldn't you then just stick the engines on top of the wing? Indeed, IIRC, there was a small commuter jet with overslung (?!) engines back in the seventies.

Might be a bit of a problem with the high turning moment, though...


[This message has been edited by Wrong Stuff (edited 23 May 2001).]

Easier to do line maintenance on low wing aircraft.

Wrong Stuff

Yes, there was the german built low wing VFW 614. Only 14 were built, and the failure to capture the 50 seater marked now owned by the CRJ and ERJ was apparently the high maintenance cost not only due to the unusual location of the powerplants but the entire design was not very operational friendly.

Embraer also designed what later became the ERJ145 with engines atop the wings but it seems that configuration isn't quite nice aerodynamically as the underslung version. Maybe something to do with clean flow over the top being more important than round the bottom.

Imagine how "unreachable" engines atop a high-wing plane would be for maintenance (also for FOD as well :-)

There were some experiments by Boeing and Antonov with engines on top and engine flow attached to the wing and over the flaps for better low-speed lift but that seems to be a specialty application and the exhaust gases cause corrosion on the structures exposed to them.

It seems a few years from now every new airplane will look like a 737/320. Even the new "big" regional airlines (Dornier/Embraer) have this layout and only those too small to make a higher gear viable keep the engines in the back.....

The wing-fuselage intersection is a significant cause of drag, and this is supposedly much worse on a high wing than a low wing. It is also very inconvenient for undercarriage mounting arrangements. Look at the RJ100 for example. The whole arrangement of sponsons to house the main gear and heavy frames to transmit loads from the undercarriage to the wing root are very awkward, and add a lot of weight to the airframe. Low wing configuration is structurally simple and also a good comprpomise aerodynamically.

Thanks for the replies, much clearer now

Here are some things I could think of, some mentioned by others, other not.

- Wing spar location. Designing a cabin that will seat pax comfortably is hard as it is w/o having a wing spar through the top of the cabin. Much better to bury the spar in the cargo bay/fuel tanks... or better yet, have a wing box under the floor. Placing the wing spar in a hump above the fuselage as in some designs of course has a significant aerodynamical penalty.

- Wing load carrying structure. For a high wing, the wing part of the fuselage becomes a load carrying member of the structure and has to be made heavier. You have the fuselage and cargo hanging under the wing rather than standing on it.

- Gear design. With a high wing, you're stuck between a rock and a hard place. Body gear, which will be relatively narrow with the associated drawbacks. Furthermore, you will once again end up with the fuselage a load carrying member of the structure as it will have to transmit the gear loads to the wings, heavy with fuel and engines.
Your other option is to have a tall wingmounted gear (F50). Heavy, takes up lots of space, draggy when extended etc etc.

- Crash safety. If you have a high wing, you better make sure that it will not come smashing down through the cabin in a crash. Also, in case of ditching, it's much preferable to float on a low wing. Fuselages make very poor submarines! :)

- Emergency exits. The choice between high/low wing has all kinds of impacts on how you are allowed to design your emergency exits. I can't remember the pros and cons off the top of my head right now - it's been a while since I looked at that - but they were numerous.

- A high wing will enable you to put prop engines below the wing, giving you a larger effective wing area. Things hanging below the wing will not impact lift anywhere near as much as things sticking out of the top, as a previous poster suggested. Engine pods for jets are also beneficial. Fuselage mounted engines again place loads away from the wings where the lift is created, making the fuselage carry loads. Comet style engines probably won't happen again. Fire endurance, inlet/exhaust duct length related problems, the simple geometrical impossibility of fitting a modern high BPR engine in a wing...

- Etc. Etc. And Etc.

Compromises, compromises!

Cheers,
/ft

And another thing . . .

Low mounted engines - throttle back - nose goes down - safe.

Hi mounted engines - throttle back - nose goes up - whoops.

Yeah, I know these days someone would overcome this with some clever auto-trim gizmo but I'd rather have a naturally stable aircraft any day.

Underslung engines on a high wing are very near the centre of gravity, and give a neutral pitch response. This is easier to fly than the large trim changes inherent with underslung engines on a low wing.

And don't forget the Anhedral; Dihedral arrangement and the aerodynamic characteristics for both.

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beauty is in the eye of the beerholder!

So what about the C17?? That's obviously a really bad design??

The topic is "Low wing passenger jets - why?"! The C17 isn't a passenger jet.