Lnewqban

I would like to hear different opinions about the pros and cons of the two airliners' designs that are more common: engines under wing-tall landing gear and engines close to fuse and tail and short landing gear.

rmh

A Gulfstream V is all I can afford so I really can't comment on the 737/757 etc., types

jgg215

That argument seems to be over with the demise of MDC as both Boeing and Airbus use underwing designs. Also, the gear length is more related to the rotation angle required without a tail strike. Take a look at a 737, engines under wing and fairly short gear.
even the next level down appears to be mostly under-wing engines. Embraer has taken most of that market and the new Chinese design is also underwing. Part of it has to be aesthetics. The larger MD-80s and the MD-90 look more like lawn darts because the stretch needs to occur forward of the wing for balance.
John

rmh

The old DC9 -using the fuselage mounted engines shared a common problem with the 727 (this was a plane from the past )
Engine noise and resonances transferred into the cabin
I hated these things - at low power they could drive you to distraction with the "wow wow" out of sync vibes.
Passenger comfort IS important.
Shoving fuselage area way forward the CG is a really lousy idea- -it can be very destabilizing
just like some Canards -
If you have not experienced this destabilization you probably don't realize the issue.

captinjohn

destabilization....is the right term for my flying RC planes this summer. I messed up 2 good airplanes....shoot anyway.............

Jet_Plane

As noted above, in the real world the argument seems to have been convincingly won in favour of underwing mounting.

off the top of my head i'd say factors involved are:

Easy access for maintanance on low slungunderwing engines.
Less noise transmitted to cabin on underwing engines.
Tail mounted engines fed with turbulent air possibly reducing efficiency.
Tail mounting can cause blanking of stabiliser and deep stall (esp. with large high bypass engines?)
Tail mounting forces 'T' tail configuration which is more complex and heavier.
Tail mounting results in short tail moment and long nose which is destabilising.
Tail mounted engines may have to be 'handed' for left and right installation, not so with underwing (maybe?)

About the only plus for tail mountedI can think of is ground clearance.

Steve

Louis9624

Under wing engines increase efficiency.  The lower center of thrust wants to turn the plane up; this is counteracted by increasing lift of the horizontal stabilizer.  This in turn takes lift out of the main wing and transfers it onto the horizontal stabilizer, therefore sharing the weight of the plane. This is common in all large airliners.  Have you noticed the kind where an engine intake is above the fuselage at the tail and reroutes it way down to the fuselage level where the thrust is.  Lower center of thrust is not the only reason to mount engines under the wing, the airplanes' weight becomes lower to the the ground.  This helps a little to naturaly keep the plane level, so it does not need as much dehydral angle in the wing, thereby also increasing effieciency.

rmh

The best reason - the noise transfer -
Trying to sleep in the rear of a 737 was sometimes impossible
I used to live on those things.
By the way - look at engine mounting on new Honda twin- that's a puzzler.

jgg215

I agree with Dick about the unpleasantness of sitting in the back of the older DC-9 and derivatives. Newer ones with modern engines have sync on the engines and the beat frequencies go away. Only thing worse was flying connies and dc-6s across to Europe for 18 hours back in the 50s. That was real vibration and noise.
Modern airliners do not have lifting stabs but they do try to reduce drag by getting as as close to zero downforce as possible. Some airliner stabs even have an upside down semi-symmetrical airfoil. An example of tail engines without a T-tail would be the Caravelle, DC-10 and L-1011. The MD-11 addressed the issue of tail drag by having a cruise fuel trim tank to move the CG back during long distance cruise to reduce drag(very slightly). They would not have done this except they did not meet their fuel consumption guarantees.
Both the DC-10 and the L-1011 have tail mounted engines but no T-tail. I believe T-tails were done to improve tail blanking during stall recovery but were clearly not required for fuselage mounted engines. The Honda overwing solution reduces the gear length substantially, probably at a small increase in fuel consumption which is probably offset by a decrease in fuel consumption from the decreased gear weight. BTW, it does have a T-tail.

John

Jet_Plane

Louis,

Interesting ideas but I'm not so sure they are true in practice. If airliners wanted their tails to lift then they would be designed to do so, when in fact the opposite is true, they actually lift downward in normal flight. The pitching up under power is perhaps an argument against under-wing engines, this can cause the plane to enter astall if power is added quickly at low airspeed, this may have been an issue inAir France 447 crash.

As for dihedral, you will find the opposite of what you propose is true. Planes wing underwing engines usually have quite marked dihedral whearas jets with engines in the tail have little or none, have a look at some airliner photos on line and you will see this is the case. The reason is nothing to do with stability. On underwing mounted engines dihedral has to be added to keep the engines sufficiently clear of the ground. Dihedral isnt required for stability on most jets because the sweep of the wings gives them more than enough lateral stability. In fact adding dihedral to keep the engined clear of the deck is a slight problem as it makes the plane excessivly laterally stable which causes dutch roll. Swept wing airliners require yaw dampers to control dutch roll.

HighPlains

My Feedback: (1)

The under wing engines make better wheels up landings.

Lnewqban

Thank you all; very interesting posts and points of view.

It is my understanding that both configurations are still produced and that both are competitive; hence, pros and cons are more or less balanced.

As for forced landings, there is a lot of damage to be expected on the expensive engines if they are low under the wings.
Even more, for forced landing on water, like the Hudson river one, they are in the way of a desirable sliding over the water, and can be ripped off from the wing, as it actually happened for one.

There is also a difference in inertia respect to pitch (more for tail engines) and roll (more for under-wing engines), as well as crabbing during one engine failure.

Jet_Plane

Good catch Lnewqban, thrust asymmetry on single engine operation is an issue that would on the face of it favor tail mounting, however the difference in engine offset from center is perhaps not so muchdifferent for later high bypass engines?

The only two tail engine airliners i can think of in production today are the verylong in the tooth Embrayer ERJ's and the Bombardier CRJ designs.Underwing layout seems to have a monopoly in more recent designs.

Steve

pimmnz

And yet, from the Star Alliance, a few ideas on the next generation narrow bodies...
Evan, WB #12.

Jet_Plane

Which have about as much chance of making it into production as the 'doodles' i sketch when I'm bored at work....

Roberto B.

Let's talk also about underwing's wing being lighter because they are more easily torsionally neutral, because of the engine mounted way in front of the wing, acting like a counterbalance.
Plus, there is a better distribution of the weight along the wing span.

G4guy

My Feedback: (13)

That's a very good answer, I can remember reading that is why Boeing were able to make the swept,long thin wings on the b-47 and B-52 stand up to flexing by using the engines as counter weights. I think it came from WWII German research