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Buiding larger engines for larger aircraft as opposed to smaller but many engines has proved to result in more fuel efficiency. Having established one of the main drawbacks or impediments to building larger engines are ground clearance for engines in light of aircraft/airliners in their current under wing configuration. This configuration in light of current technological advancements seems to offer the greatest advantage in terms of control of the aircraft, stability and weight closest to the center of gravity when flying among other things.

If space to store the retracted gear were not a problem due to some new design, are taller gear for airliners necessarily undesirable for landing and take off? Do they present some technical problem if for example they were strengthened, the extra length would present some problematic leverage problems as they get closer to the pivot and "load"?

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Tall landing gear has been used in the past:

enter image description here

The Tu-114 needed really tall gear to clear the really big props. The drawbacks are extra weight and volume, as described in the other answers.

The Tu-114 was so tall, on its first flight to the US (the prototype was used to ferry Nikita Kruschev to the UN for some shoe banging) the available boarding stairs weren't tall enough. That arrival raised some eyebrows in the West because they'd done a nonstop flight from Moscow (the Tu-114 was the first aircraft that made that possible).

enter image description here

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    $\begingroup$ How did they eventually get him down? $\endgroup$ – securitydude5 Mar 25 at 19:37
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    $\begingroup$ You can see that in the second photo. They placed a smaller set of stairs on top of the large one. $\endgroup$ – Hobbes Mar 25 at 19:50
  • $\begingroup$ OK otherwise great answer and thanks, much appreciated $\endgroup$ – securitydude5 Mar 25 at 19:55
  • $\begingroup$ The Tu-114, itself, was a case of lowering what was a high wing on the original Tu-95. Also the Tu-114 sorely needs a 4-pack of Progress D-27 engines; that would make it the coolest thing to ever fly. $\endgroup$ – Harper Mar 25 at 21:27
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    $\begingroup$ @AzorAhai en.wikipedia.org/wiki/Shoe-banging_incident $\endgroup$ – Bob Mar 25 at 23:39
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It's just more weight and volume.

The problem really arises when you modify an existing airplane with a new engine that requires more clearance. Changing the engine is not that big a deal (relatively speaking) when it is slung below the wing as its physical interface is through the pylon. It is an appendage.

The landing gear are smack, dab in the middle of the fuselage, and changes in that will ripple through far more systems. It is literally right in the middle of things.

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    $\begingroup$ @CrossRoads It's the flat bottom on the nacelle that led to the pitch-up moment at high AoA. Having the weight of the engines farther forward would result in a pitch down moment. $\endgroup$ – reirab Mar 25 at 18:21
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    $\begingroup$ @CrossRoads The engines on the 737 MAX aren't that much farther forward than on the NGs. They are higher, but not much farther forward at all. This is an NG (-800) and this is a MAX 8. There's currently a lot of misinformation and speculation spreading around in regards to the MAX, though. That's why we ban accident speculation here and wait for the official reports. $\endgroup$ – reirab Mar 25 at 18:32
  • $\begingroup$ Changing the engine can be a very big deal if it affects the wing's flutter stability margin. Boeing had a terrible time trying the design the original B747 wing to be compatible with engines from the three manufacturers, with maximum commonality of parts. $\endgroup$ – alephzero Mar 25 at 20:24
  • $\begingroup$ @reirab if the flat bottom of the nacelles leads to pitch up at high AOA this can be solved by adding area to the H stab, just like they did with the canarded Ascender years ago. I cringed when the MD-11 downsized theirs. A big one adds safety, and offers an option other than trying to fly the plane with a rapidly pitching trim function. $\endgroup$ – Robert DiGiovanni Mar 25 at 23:56
  • $\begingroup$ @RobertDiGiovanni The system isn't designed to fly the plane with it. It's designed to activate as the AoA nears critical to prevent stall, just like the almost identical system on every Airbus since the A320. $\endgroup$ – reirab Mar 26 at 2:36
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Taller landing gear do present challenges due to increases in leverage and greater amount of space, but this has already been dealt with in large delta designs such as the XB-70 and is certainly not insurmountable.

However, in the vast database of aeronautical endeavor there are solutions worth reviewing (wing? ah, review wing). Yes, many high wing designs would comfortably fit the larger engines.

Secondly, the trend towards fewer wing mounted engines has deprived air craft designers of a key safety feature from the tri-jet, using thrust to push the nose down. Nose tractors (prop) have it as down thrust, rear pushers (prop or jet) have it as tail up thrust. Add power nose down.

So let's look at the A-10 Warthog. Why not put the two large fan jets back there with future airliners!

Notice many major airliner designs trace unbroken lineage all the way back to the Me-262. Every time I see one, I think this is the first jet airliner ever. A great plane indeed.

But, with the development of more efficient and larger fan jets, designs such as the Ba-146 and many high winged transport aircraft can be brought into consideration to keep the thrust line in a manageable, safe, and even helpful configuration for wing mounts, if we wish to keep them there.

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  • $\begingroup$ Engines on the tail have the disadvantage of longer fuel lines. Fuel needs to be transported from the tanks to the engines and when those are not mounted below the wings, fuel actually needs to be pumped to the back and up to the engines instead of more or less straight down from the tanks. Under wing engines have the shortest fuel lines from almost anywhere on the plane. $\endgroup$ – Adwaenyth Mar 25 at 15:50
  • $\begingroup$ Why is thrust pushing the nose down a safety feature? My intuition from model aircraft would be that you'd want the nose to drop if the engines cut out, not under thrust. $\endgroup$ – Skyler Mar 25 at 16:21
  • $\begingroup$ You have it both ways. If you cut power the aircraft slows, sinks, and (properly designed) H stab pushes nose down. Thrust pushing nose down would also help, as does forward set CG! But with tractor props, torque makes this a questionable option at low airspeed. Rear jet thrust, at an angle, not only pushes nose down to help gain air speed, it also reduces trim needs at cruise. $\endgroup$ – Robert DiGiovanni Mar 25 at 16:49
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    $\begingroup$ "Notice many major airliner designs trace unbroken lineage all the way back to the Me-262." That really seems like quite a stretch. The Me-262 is a fighter and has little resemblance to airliners of any sort, aside from wing-mounted turbine engines. $\endgroup$ – reirab Mar 25 at 18:20
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    $\begingroup$ @Adwaenyth that ignores the fact that many airlines already have fuel pipes to the rear of the aircraft, because many airlines take advantage of using fuel as trim ballast by pumping it to tanks in the tail. The real disadvantage to engines at the rear is weight - the entire tail structure had to be strengthened significantly, while the wings and centre wing box are already a strengthened structure for bearing the weight of the aircraft, so additional strengthening for bearing the engine weight and thrust adds little extra weight. $\endgroup$ – Moo Mar 25 at 22:04
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I have no experience in aviation but just from physics, longer landing gear "legs" (I don't know the terminology) would mean that as the aircraft lands and experiences a force perpendicular to the landing gear, the torque on the landing gear would be higher. This could present a challenge in that the structural stability of the landing gear "leg" may be compromised from increased torque, especially if we're talking about a heavier aircraft.

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    $\begingroup$ You should add figures to illustrate your answer. $\endgroup$ – Manu H Mar 25 at 19:55
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    $\begingroup$ Another way to explain this is: a longer lever sticking out from the plane means it has to be stronger to not snap off in a crabbed landing, or during braking, or any other sources of non-vertical force. So yes, the landing gear has to scale up in thickness as well as length, so the weight required might scale as length^3 or at least length^2, not just linearly with length. (The question does already guess that this might be a problem, but still +1.) $\endgroup$ – Peter Cordes Mar 26 at 4:22

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