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Why has placement of jet engines changed?

Why has placement of jet engines changed?

Old Nov 18, 19, 10:37 am
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Why has placement of jet engines changed?

I was watching the new series of the Crown and it struck me that commercial jetliners used to have jet engines positioned either on the sides of the fuselage or in the tail fin. These days jet engines are pretty much exclusively mounted under the wings. Can anyone give me a technical explanation of why this would be the case? Google isn't forthcoming.
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Old Nov 18, 19, 12:05 pm
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Power and aerodynamics
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Old Nov 18, 19, 12:20 pm
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The main reason for this engine placement relates to aerodynamics (airflow). Newer engines are more efficient, but they're larger in diameter. In short, they won't fit well on the rear of the fuselage. They are more efficient when located out on the wing where the airflow doesn't interact with the flow on the fuselage and tail.

(This is a simple explanation that doesn't consider things like center of thrust, engines pushing on the body vs. the wings pulling, ground clearance, maintenance, high- vs low-wing, cost, etc)
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Old Nov 18, 19, 12:20 pm
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Plenty of regional jet families such as CRJ and ERJ have rear-placed engines.
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Old Nov 18, 19, 2:08 pm
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Originally Posted by Allan38103 View Post
The main reason for this engine placement relates to aerodynamics (airflow). Newer engines are more efficient, but they're larger in diameter. In short, they won't fit well on the rear of the fuselage. They are more efficient when located out on the wing where the airflow doesn't interact with the flow on the fuselage and tail.

(This is a simple explanation that doesn't consider things like center of thrust, engines pushing on the body vs. the wings pulling, ground clearance, maintenance, high- vs low-wing, cost, etc)
Tail-mounted engines make fuselage stretches more difficult too, apparently.

Some designs also obstructed air flow to engine and made the a/c more prone to stalling and harder to recover, in certain situations.
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Old Nov 18, 19, 3:50 pm
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The wing lifts everything. Placing weight on the wings is better, as placing it elsewhere means a stronger (heavier) fuselage is needed to support it. A flying wing is a great shape, for cargo, not so for passengers. No window seats.

Something else to consider. The fuel tanks are in the wings. With rear mounted engines, there is a fuel pipe the size of your arm running under your feet back to the engines. Want that to break in a belly landing? (Assuming you don't always fly J and sometimes sit in the high number rows.)
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Old Nov 18, 19, 5:43 pm
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Originally Posted by Flaflyer View Post
Something else to consider. The fuel tanks are in the wings. With rear mounted engines, there is a fuel pipe the size of your arm running under your feet back to the engines. Want that to break in a belly landing? (Assuming you don't always fly J and sometimes sit in the high number rows.)
To be fair, some planes (at least the 747-400, but no other 747 models) stored fuel in the horizontal stabiliser. Added 1/2-1 hr range and to be fair, was probably the first tank to be emptied.
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Old Nov 18, 19, 11:50 pm
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Less commonly seen:

Engine is at the wing, not in a pod. Example, Boeing 737-100, Boeing 737-200

Engine is above the wing. Example, VFW-Fokker 614

Engine is buried in the wing. Example, deHavilland Comet

Pair of engines on each side of the rear fuselage. Example, Vickers VC-10

Pair of engines side by side. Example, BAe Aerospatiale Concorde

Engine within the vertical stabilizer. Example, McDonnell Douglas MD-11

Engine in the rear fuselage with intake above the fuselage. Example, Boeing 727-200, Lockheed L-1011 Tristar

What is common is engine is under and forward of the wing or under the wing.
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Old Nov 18, 19, 11:56 pm
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Originally Posted by rpjs View Post
Plenty of regional jet families such as CRJ and ERJ have rear-placed engines.
Some considerations include but are not limited to that rear placed engines allow shorter landing gear. Tall landing gears are heavy and otherwise not too useful if the plane is not too big. Engines in the rear also need more structural reinforcements in the rear instead of the wing only.

What about high wings, like in a BAe 146? Wings protrude in to the cabin, reducing space. Some say it's worse in a water landing but those are rare. Others say engine noise is greater in the cabin. What did you say?
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Old Nov 19, 19, 1:49 am
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Engines buried in the wing root (ala Comet) are harder to access for maintenance, and MUCH harder to remove and replace. The Nimrod ( Maritime patrol/AEW variant of Comet IV) flew on successfully for decades its true... but maintenance is less of a concern time/manpower wise for Air forces.
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Old Nov 19, 19, 5:18 am
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I read somewhere that part of the root cause of the 737MAX issues is that the engines for this variant of the 737 are placed much further forward than previously. This is because they are too big to fit under the wing, without stretching the landing gear.
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Old Nov 19, 19, 10:29 am
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Originally Posted by lhrsfo View Post
I read somewhere that part of the root cause of the 737MAX issues is that the engines for this variant of the 737 are placed much further forward than previously. This is because they are too big to fit under the wing, without stretching the landing gear.
Yes, that's what I'd seen too. Earlier 737 models had smaller engines but modern high-bypass turbofans on the 737 Max require larger air inlet diameter. So, they put the engines in front of the wing in order to be able to elevate them a bit, which induces instability that Boeing countered with the MCAS system that occasionally went haywire and crashed the plane.
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Old Nov 19, 19, 6:49 pm
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Originally Posted by Toshbaf View Post
Engine is above the wing. Example, VFW-Fokker 614
The HondaJet is a modern example of this.

In short, it depends on engineering tradeoffs. It happens that the configuration that's the most profitable for commercial airline operators tends to be the under-wing configuration more often than not.
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