29
$\begingroup$

Why have modern airliners converged on a configuration with two or occasionally four underwing engines, while modern business jets have converged on a configuration with two or occasionally three rear-mounted engines?

I am of course aware of the many past airliners with rear-mounted engines, with the DC-9 and MD-80 and later derivatives continuing into production until rather recently, but it still seems fair to say that the underwing configuration dominates modern airliner design.

Meanwhile business jets exclusively use the rear-engine design, unless one counts small airliners put into business service such as the Boeing Business Jet. Does the underwing engine design simply not scale down well to something the size of a smaller business jet?

Here's a related question with good answers but still doesn't really answer my question-- How does the mounting location of a jet engine affect aircraft performance?

One key factor now occurring to me is that business jets are boarded with self-contained airstair doors/ steps so a fuselage sitting high off the ground would be problematic. Airliners with long fuselages perhaps may need rather long landing gear to provide adequate ground clearance for rotation anyway-- although the DC-9 / MD-80 with its wing and landing gear far to the rear did sit rather low to the ground-- still in the airliner situation perhaps there simply is not as much benefit to having a low-sitting fuselage as there is in the business jet situation.

Of peripheral interest is this Wikipedia article on "airstairs", pointing out some cases where full-blown airliners are equipped with airstairs. One of the more interesting lines is:

The most unusual airstair design was found on the Lockheed L-1011, which was a full-height airstair which was stored in a cargo compartment and allowed access from the right aft passenger door to the ground. This design was ultimately so large and heavy, and it took up valuable cargo space, that it was rarely used.

$\endgroup$
  • 1
    $\begingroup$ And the longer fuselage length of an airliner requires longer landing gear for adequate ground clearance for rotation anyway? Maybe a key factor is that business jets need to be able to reach the ground with self-contained airstair doors/ steps? $\endgroup$ – quiet flyer Jul 3 at 12:40
  • 1
    $\begingroup$ Go ahead and post that as answer, It was your earlier comment (now deleted) that got me thinking along those lines-- $\endgroup$ – quiet flyer Jul 3 at 12:52
  • 1
    $\begingroup$ You assume that all business jets are small. Boeing makes the BBJ from the 737 airframe. Airbus also has a corporate jet division that uses their airliner airframes. $\endgroup$ – Juan Jimenez Jul 3 at 12:58
  • $\begingroup$ A related question with good answers about why regional jets are moving toward a configuration with underwing engines -- aviation.stackexchange.com/questions/25636/… $\endgroup$ – quiet flyer Jul 4 at 18:38
  • $\begingroup$ Related-- "Regional and private jets, which often have fuselage-mounted engines, also tend to serve such smaller airports. These planes were designed with the engines mounted higher specifically to reduce the risk of ingesting FOD, thus allowing them to safely power back when needed." -- from aviation.stackexchange.com/questions/67219/… $\endgroup$ – quiet flyer Aug 2 at 21:39
43
$\begingroup$

Underwing engines have more advantages for larger planes, but there must be enough room underneath the wing to mount them.

This was tested by Boeing in the 1960s, when they designed the B737 and had two competing teams pre-design both configurations. The underwing config won. When Airbus designed the A320 two decades later, the underwing config won again with engines that were a good amount wider.

enter image description here

Image source.

Advantages of underwing mount:

  • Engines mounted in pods underneath the wing reduce the upward bending moment of lift creation, and therefore allows for lighter construction.
  • If the pods stick out forward of the wing, the backwards aerodynamic twisting moment is also reduced: lighter construction.
  • The engines can be easily reached for maintenance.
  • All of the fuselage can be used for useful load - tail mounted engines have structural reinforcements in the fuselage between the engines.

Advantages of fuselage mount:

  • The yawing moment of a single engine fail is smaller, allowing for a smaller vertical tail.
  • Fuselage can be lower to the ground and allow for low door sill height.
  • Landing gear can therefore be shorter.
  • Clean wing configuration.
  • Lower trim change due to thrust.
  • Less noise in most of the cabin.

In smaller planes, the wing bending & twisting moment is a relatively smaller constructive issue than in larger planes. And indeed, being closer to the ground allows for easier boarding via air stairs.

$\endgroup$
  • 5
    $\begingroup$ For a great example of the "must be enough room" issue, look at the Boeing 737. Originally, it had underwing engines, but as the engines got bigger, they migrated forward in order to get enough ground clearance. $\endgroup$ – Mark Jul 3 at 21:06
  • 2
    $\begingroup$ There is a rarely used 3rd option: over-wing engines. Same light construction as underwing, but with a low door sill height. AFAIK only the HondaJet and the unpopular '60s airliner VFW-614 use this configuration. $\endgroup$ – Adam Jul 3 at 21:13
  • 1
    $\begingroup$ @Adam - coincidentally just this week we had this question showing the Beriev Be-200 with over-wing engines. $\endgroup$ – davidbak Jul 3 at 21:48
  • 3
    $\begingroup$ Also, rear fuselage-mounted engines tend to be a bit quieter for occupants, a non-trivial concern for private (jet) aviation customers $\endgroup$ – costrom Jul 3 at 21:49
  • 3
    $\begingroup$ Fuselage mount would also have the advantage of reduced ingestion of debris, wouldn't it. $\endgroup$ – Craig McQueen Jul 5 at 0:15
11
$\begingroup$

Airliners are designed for loading from elevated passageways at airport terminals. The terminal comes to the door you might say, so it doesn't matter so much how far off the ground it is. And absent a terminal gate, you can always use an external staircase you move to the plane for the passengers/cattle to climb.

Business aircraft are designed to be boarded from the surface, traveling from limo to cabin in a luxurious, exclusive manner, usually using an airstair door. You need a fuselage close to the surface for an airstair door to work. This pretty much precludes underslung engines.

And if you did insist on trying to squeeze in a set of underslung engines under the wings, they'd be so close to the surface they'd ingest every sand grain and pebble.

$\endgroup$
  • 10
    $\begingroup$ +1 but I'll note that the boarding from the ground isn't only (and perhaps not even primarily) about luxury and exclusivity. One of the major advantages of travel by private / business jet is the ability to use small airports that are more numerous, not crowded and are more conveniently located than major airports with airline service. These smaller fields don't have jetways and often don't even have mobile passenger boarding stairs available. Hence airstairs. $\endgroup$ – pericynthion Jul 3 at 15:35
  • 3
    $\begingroup$ @pericynthion: And even when landing at major airports, business jets (and other private aircraft) generally use facilities separate from passenger terminals. For instance, at my local commercial airport, the passenger terminals are on the northwest side of the field, private facilities on the east and south sides. $\endgroup$ – jamesqf Jul 3 at 16:48
  • $\begingroup$ Many of the Boeing 737s of Ryanair have built in stairs for the front door, so it's certainly possible to be self-sufficient in terms of getting on and off the plane even for planes with larger ground clearance. $\endgroup$ – frederik Jul 4 at 20:39
  • $\begingroup$ Isn't that a chicken-egg phenomenon? We have elevated passageways because airliners are as high as they are. And we build airliners as high as they are so they fit the existing passageways. However, if it needed to be, passageways could be built at any height. $\endgroup$ – Quora Feans Jul 6 at 17:41
  • $\begingroup$ The passageways can go up and down quite a lot to accommodate various sizes of airliners. Main thing is airliners are boarded from controlled terminal facilities, business aircraft are boarded from the surface anywhere and everywhere and need airstair doors. Airliners taller than CRJs (which are business jet derivatives), which include pretty much anything with underslung engines, are too tall for airstair doors to work. $\endgroup$ – John K Jul 6 at 19:24
8
$\begingroup$

Ground clearance

The single reason why an underwing design is unfeasible for smaller jets is just ground cleareance.

There are several reasons why smaller jets have lower ground clearance.

For once, higher ground clearance requires longer landing gears. These have to be stored in flight and take up valueable space and weigh more. On larger planes, the gear has to be sturdier anyways thus they need heavier and larger gears anyways since they have longer fuselages and more overall weight. In turn this reduces maximum range and payload of the aircraft and that is generally something you'd want to avoid.

Also a lot of smaller jets operate on smaller airports. Smaller airports have fewer gates with passenger bridges / stairs available (if they have any at all) and using a gate with one usally is attached to quite a hefty fee.

Considering that many business jets also have long durations of waiting on the ground, they are unlikely to ever be used at a gate with a passenger bridge anyways. Thus passengers have to be able to (dis-)embark easily without stairs / passenger bridges as they often will use GA parking spaces.

You can see that mostly on smaller jetliners too like the smaller Embraer or the McDonnell Douglas planes. The smaller designs tend to use rear mounted engines as they have a very low clearance below their wings.

$\endgroup$
  • 3
    $\begingroup$ The 737-200 had underwing engines. $\endgroup$ – Koyovis Jul 3 at 13:06
  • 2
    $\begingroup$ @Koyovis no one said otherwise. A 737-200 is rather tall with a total height of 37ft compared to an MD80 with 29ft although their passenger capacity is quite similar. $\endgroup$ – Adwaenyth Jul 3 at 13:20
  • $\begingroup$ "engine design simply [does] not scale down well" - on some airliners their enclosure is flat on the bottom.... Where you gonna put 'em, half way into the ground? +1 $\endgroup$ – Mazura Jul 5 at 18:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.