Why not mount airliner jet engines above the wings?

Question

It seems that most airliners put the engines below the wings, i.e. the Boeing 7*7 series and the Airbus A3** series. This requires long (heavy) landing gear, and the engines are close to the ground which may cause damage to the engines (ingest debris) or the ground.

Why not mount the engines above the wings? In addition to the shorter (lighter, stronger) landing gear, the plane would be easier to service and embark / disembark. What are the major disadvantages to having the engines above the wings for civilian airliners?

The question "How does the mounting location of a jet engine affect aircraft performance?" addresses the performance issues affecting engine placement but answers do not cover over-wing placement extensively and focus only on performance and not other operational issues or design advantages.

Answer 1

Maintenance

Having the engines below the wing makes them fairly easy to access, even if some kind of lift is needed. Putting them above the wing would make them much harder to get to. Dropped a tool? Better start on that wing repair too. This includes both general access to the engine and the procedures for removing and installing the engine.

Structure

An engine below the wing is fairly stable, since it is hanging from above. It naturally wants to keep hanging downwards. Mounting the engine above the wing would be less stable. It naturally wants to tip over.

Noise

If an engine is under the wing, it helps to block some of the noise from at least the mid cabin. An engine above the wing would expose much more of the cabin to noise. It could reduce the ground noise while in flight, though.

Aerodynamics

The top of the wing is the most sensitive part aerodynamically. This is why items like fuel tank access panels are placed on the bottom. Having an engine and pylon on top would affect the flow over the top of the wing.

Safety

Engines falling off the wings tends to be a bad thing. But if they were above the wing, they would tend to fall down onto the wing, which would still not be good. It would be harder to have then break away in a crash. Also, in the event of an uncontained failure, the wing would not provide any protection to the fuselage.

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Of course there are benefits, such as the reduced noise, less ground clearance required, no interference between the jet blast and the flaps, and less chance of FOD being ingested. As new engines get larger fan diameters, the constraints become more difficult. Some concepts of the blended wing body design put the engines on top of the body. But for the current design of aircraft, the benefits of having the engines below the wing are greater than those of the alternatives.

Answer 2

There was one design with overwing engines: The VFW-614. The number was chosen because it was the 4th project to be started in 1961 by the Vereinigte Flugzeugwerke (VFW).

The engine placement had several advantages:

• less risk of ingesting foreign objects
• less noise during take-off and landing, at least for the people on the ground
• smaller trim change with power changes

But the disadvantages (cabin noise and engine access) were the reason why this engine placement never took off. Also, low engines can get their fuel without pumps, just by gravity. However, pilots who flew both the 614 and the Boeing 737 told me that the 614 was more pleasant to fly since it needed no trim changes when the power setting was changed.

Placing the engine forward of the wing helps to dampen flutter, but then the main advantage of overwing placement, reduced noise footprint, is lost. Todays engines are mostly placed ahead of the wing for that reason, and below it so they are out of view for the passengers but easy to reach for maintenance crews. Placing the engine directly under the wing would help with feeding it pre-compressed air, but again the forward location makes little difference in terms of precompression but helps with flutter damping.

Answer 3

If we ignore the other engine placement options considered in the linked question, we can compare only over vs underwing placement. I'm no expert but here are some factors that occur to me:

• Underwing simplifies engine access, the engine is close to the ground. Since most engines are mounted this way it may be that engine-designers optimize placement of components with this in mind (though engines are often produced for a specific plane)

• Underwing mounting means that gravity helps hold the engine in position laterally. For overwing placement, the opposite is true and you might need a slightly stronger pylon and wing-spars?

• Overwing placement causes the torque on the wing from the weight of the engine to act in the same direction as the torque from thrust, underwing location reduces total torque - though this may not be significant.

• Engines are at maximum thrust at take-off. An overwing engine at maximum thrust would probably be more likely to exert a torque on the airframe that opposes lifting the nose for take off. I'm not sure if this is good or bad but it seems wrongish.

• Overwing placement certainly decreases the risk of FOD but may impede passengers view and increase noise.

• Aircraft landing gear may have to be a certain height anyway to keep the tail clear of the runway during take-off rotation and to absorb landing forces over a longer distance. This may not be significant as there are large aircraft with very short landing gear (e.g. some military and civilian transport aircraft). Perhaps they pay a penalty for this.

• There are probably more airliners than large airports, so it may be thought better to remove FO from runways than make general airliners more FOD-proof.

^{source: Bartosch}

Answer 4

I painted the new pictures myself but took the text from an external source:

You can see with the engine mounted over the wing, you have the centerline of thrust located above the longitudinal axis of the aircraft. This creates a nose-down pitching moment about the lateral axis, which is at the center of gravity (CG). While the weight of the engine does help counteract it, it must also be countered by an increased amount of downward lift created by the horizontal stab (a greater tail-down moment).

Answer 5

I will answer only for the landing gear length. When you consider large enough aircraft, you need a long landing gear to reduce the risk of tail strike when landing and taking off. Thus, you have enough place underwing.

For example compare the length of the nose landing gear of an aircraft whose engines are mounted underwing (e.g. this A320) with the one of an aircraft whose engines are not mounted underwing (e.g. this Caravelle). I choose photos with a man near to the landing gear to ease comparison.

Answer 6

Placing turbines in underwing pylons began with early US jet bombers, the B47 and B52, and the C135 transport - later to morph into the 707. In his book 747, Joe Sutter (the lead engineer on the 747) gave the reasons for underwing pylons rather than the in-wing engine placement that was used on other early large jets like the Comet, Victor, and TU-16, as:

• Maintenance

• Less damage in the case of catastrophic engine failure (early
  turbines weren't as reliable as they are today) or fire.

As to why over wing pylon placement isn't used... both reasons still apply.

Most maintenance on turbines is done on the aircraft - engine removal is not common, so the lower placement facilitates quicker average maintenance. Engine removal/replacement is facilitated with a forklift.

Catastrophic failure or fire... quite rare, but when it happens, the engine will normally drop off, away from the wing.

Also, under METO, the lower engine placement, under the CG, tends to pull the nose of the plane up rather than down, which is what one wants on takeoff or most situations where a pilot applies full power. Over wing placement would tend to pull the nose down on takeoff. A minor factor, but worth mentioning... no point in taxing the elevators more than necessary on takeoff, when control authority isn't the greatest due to minimal airspeed.

One rare exception to underwing pylon placement protecting the wing was AA Flight 191, the Chicago crash. Due to poor maintenance techniques, the rear engine mount on #1 broke, the engine pivoted upward, broke off, and went over and into the left wing, tearing out the hydraulics for the front spoilers. The pilots thought they were dealing with a simple engine failure, and followed the book for climb out with a single engine failure, maintaining an airspeed consistent with takeoff flaps set on both wings. Due to loss of lift from the damaged wing, the left wing stalled, and the aircraft rolled left into the ground. Not enough altitude to recover.

Having said that, this sort of catastrophic failure is quite rare, the most recent example being Qantas 32, an A380 that suffered a major inflight engine explosion. The engine did not drop off of the aircraft, and it was able to land safely, albeit with some degradation to control authority of the ailerons. Whether or not over wing placement would have resulted in more damage to the critical upper wing surfaces is a matter of conjecture.

One recent executive jet noted for over wing engine placement is the Hondajet. They include a short passage on the advantages:

A breakthrough in aeronautics, the Over-The-Wing Engine Mount was engineered and proven by Honda after more than 20 years of extensive research and development. This innovative technology not only breaks the conventional mold set by the aerospace industry, but also provides category-leading advancements such as a more spacious cabin, noise reduction, and increased fuel efficiency.

Answer 7

There is one very good reason to put engines just above the wing, as seen here on the NASA QSRA:

and the YC-14:

This is known as "upper surface blowing", or some variation on that theme. The idea is that the airflow from the engine follows the upper surface of the wing, so when the flaps are lowered the airflow follows their smooth bend until they are blowing downwards, which greatly increases lift. This is an alternative to lower surface blowing seen on the C-13:

and it's much larger sister, the C-17:

The USB solution has a number of major advantages over the LSB. The main one is that the airflow doesn't blow directly onto the flaps, which makes the structural loads much simpler. The airflow is also greatly smoothed out by its flow over the top surface of the wing, which also lowers loads. Finally, since the flaps are already accelerating airflow over their top surface - pretty much the whole idea - the overall design is actually far simpler. The end result is the same, both improve lift by bending the airflow downward via the flaps, but USB was mechanically much simpler and this was a major selling point for the C-14.

So... why didn't they use it? Well, ironically, one of the reasons is that it was too good. In the case of an engine-out, especially on the two-engine variety, one whole wing would lose lift and it was practically impossible to keep control. A four-engine variety addressed this, but the original program wasn't considering those and Boeing was pushing the two-engine layout as a major advantage (in maint terms). In contrast, the LSB models had less lift augmentation and had to rely on "more flaps!" to get the same performance (which, IIRC, they never did), but as a result they had better response in an engine-out condition.

It is not clear to me that the C-17 ended up with LSB for any reason other than that it came from McD, and that had anyone else won the contest they might have used USB.

Answer 8

Another thing that has been partially mentioned. The pylons holding the engines are designed to break before any extreme vibration or unbalanced load (due to a failed fan) causes severe structural damage to the wing. Underwing placement means the engine won't fall down onto the wing and should have plenty of clearance for the horizontal tail. Over wing placement would or could result in impact with the wing and horizontal tail or even with the fuselage if the bank angle was severe enough.

In addition having the thrust line above the wing would create a nose down torque along the wings axis which would be compounded by the pitching moment developed by the airfoil. This additional moment would require stiffer and therefore heavier wings.

Answer 9

Below the wing helps lift. Above the wing hinders lift. Below allows the air to go over the wing not spoiling the aerodynamics for lift. Above disturbs the aerodynamics for lift, depending how far back the engine is on the wing. Sure lighter landing gear is good, aircrafts don't use the landing gear when flying, and besides the engines weigh far more then the landing gear. It's better to have the engine under the wing. New models can raise the wings and can decrease the pylon length. However the lift is not yet enough. They need lift, every heavy cargo aircraft has the engines under the wing. They cannot afford to lose any possible lift.