Rear Engines...

[PhatAJ2008]

Why is it no aircraft manufacturer is producing larger planes like the MD-80 with rear engines anymore?

 

[skid]

I just saw today that China is going to produce a 150 seater...(I think). The picture showed it looking like a 717/MD-90. Also the russians are producing something similar to it. I can't off the top of my head remember what it is called though. I think Tupolev is making it.

If I had to guess the reasons Airbus and boeing no longer do it is probably pax comfort and such. I know the 717 is a real nice ride but all the MD aircraft were super loud! Also besides the 727 boeing was never a big builder in this kind of aircraft.

 

[MTV]

If the Russians are building it you can be sure it will have at least 5 crewmembers on the flightdeck and will have twice the amount of engines and tires as its western counterpart, and it will be called the 717ski.

 

[geshields]

Best guess.....I would say it is mostly a thrust and noise issue. You need a turbofan to meet the upcoming noise requirements and you can't get enough thrust out of a small turbofan. It needs to be fairly large in diameter.

Imagine the 737 or Airbus engines mounted on the tail..not too pretty of a picture...

Although I could be TOTALLY wrong...

 

[Marko Ramius]

er.

Imagine the 737 or Airbus engines mounted on the tail..not too pretty of a picture...

Although I could be TOTALLY wrong...

You ever get a look at a MD-90? Those things carry IAE V2500's. I think they are the D5 model. This is essentially the same engine as the V2500's that are on the the A320 series which are the A1 and A5 variants. I think MD-90's have something they call a thrust performance reserve, so I would think that they are closer to the A1 variants that are used on some older A320's that have the 'thrust bump' deal. I've even heard of people proposing 2 engined 727's using V2500's, so you can mount a pretty big turbofan on the tail of aircraft if a manufacturer desires.

 

[~~~^~~~]

The reason why no "new" aircraft are configured like an MD88 is because it is less efficient - particularly with new technology which alows pylon design to be optimized.

The reason Douglas and early jet makers used aft engines was to keep the wing clean. The British VC10 had the first supercritical wing - which just could not be done as well with engines disturbing the flow. Airflow around pylons was not completely understood, especially transonic flow. There are still advantages to a clean wing and new designs like the GV, Global Express and Citation X make the most of their efficient wings.

However, as the airplane scales up you have several problems:

First, aircraft with tail mounted engines typically get nose heavy with every pound of weight. The LEMAC is back there with the weight of the engines. Empty the opposite is true and I've heard some business jets can't go out with two pilots and full fuel due to aft CG limitations. Since airplanes are more efficient with an aft CG, having an airplane get more efficient at higher weights is a good thing.

Second, by suspending the weight of the engine mid span some of the stress can be removed from the wing spar & spar box. One "advantage" the DC-10/MD-11 had on the L1011 was that its' engines were mounted further out mid span and the forces were better distributed. The L1011 had all kinds of work arounds and still had spar problems. Many operators actually had to replace the rear spars in mid life.

Third, the lighter load on the fuselage allows for thinner - less heavy skins.

Fourth, the entire fuselage, keel, and systems weight of an all up aircraft can be reduced with everything located in the middle. Fewer fuel lines, bleed air lines, wires and what not make for a lighter airplane.

With new technology allowing engineers to whip up computational fluid dynamics models in almost real time on a PC wing mounted engines are starting to make sense in jet about every application. Look at the new Honda Jet, or an E170 - really elegant engineering solutions to get the most our of the least thrust and fuel burn.

 

[spiffomatic]

great response! I had vaguely remembered it to have something to do with being lighter and more efficient - and also remember it having to do with better ability to stay w/in c.g. with varying loads... something about aft mounted engines giving you a tighter ranges on how you can place cargo/pax.

Thanks for posting good info.

 

[taxicabdriver]

Good response...I all the sudden feel inadequate of my lack of knowledge.....

 

[michael707767]

Second, by suspending the weight of the engine mid span some of the stress can be removed from the wing spar & spar box. One "advantage" the DC-10/MD-11 had on the L1011 was that its' engines were mounted further out mid span and the forces were better distributed. The L1011 had all kinds of work arounds and still had spar problems. Many operators actually had to replace the rear spars in mid life.

While not wanting to start a DC-10 vs L1011 argument, and being unfamiliar with any maintenance issues regarding either aircraft, you actually have it completely opposite.

The L1011 had its engines further out on the wing than the DC-10/MD-11.

 

[~~~^~~~]

You know, now that I think of it, you probably are right. Having the engines positioned further out required the tail with the S-Duct for engine out stability. Lockheed then cut the weight of the spar as far as they could & that is what started the problems when they did IGW versions. It has been about 20 years since I read that book.

 

[michael707767]

You know, now that I think of it, you probably are right. Having the engines positioned further out required the tail with the S-Duct for engine out stability.

you are exactly right. Having the engines further out on the wing drove the need for a bigger rudder. The DC-10, having the engines closer in, was able to get away with a smaller rudder, and thus a straight through engine nacelle.

 

[ACL65PILOT]

He is so smart. I think that Delta should hire FIN

 

[~~~^~~~]

Me too.

 

[HoursHore]

The fact that you need a 30 foot tall stand to service a DC-10 or MD-11 #2 engine might have turned the tide from those designs.

This is purely a WAG, but Pod mounted engines seem entail less plumbing, while Aft Mounted engines need All sorts of stuff to get Bleed Air, Hydraulics, and fuel Back and forth.

 

[CopilotDoug]

Having the engines further out on the wing drove...

I do recall reading that an added benefit (Not sure if it was intentional) was a quieter cabin

There are some benefits to tail mounted engines however: One is the reduced susceptibility (sp?) to FOD. And there's an aerodyamic benefit to the T-Tail config. The fin doesn't have to be as tall since the horizontal stab at the top provides some sort of "End-Cap" benefit.

However, having the T-Tail config requires the tail section to be beefed up, adding more weight (Not sure if it's really significant though)

Wish they made more aircraft like the DC-9 and DC-10s, at least it brings some variety to view.

 

[skid]

It has been about 20 years since I read that book.

Just wondering what book you are talking about. Sounds like it might be interesting.

 

[Tref]

There are some benefits to tail mounted engines however: One is the reduced susceptibility (sp?) to FOD.

Yes and no. Less likely to pick up FOD, but more likely to ingest spray from the nose gear on contaminated runways, hence the need for chined tires or spray deflectors.

 

[airplane wizard]

tail mounted engines look cooler

 

[Oh-ryan]

Though some of my ex-girlfriends would disagree, my opinion is that it is always better in the rear.

 

[Tref]

Though some of my ex-girlfriends would disagree, my opinion is that it is always better in the rear.

Ummm, you might want to re-phrase that. :laugh:

 

[retired guy]

The rear pylon mounted engines on the DC9-MD designs dictate a T tail design. The post stall tendencies are nasty. The foward fuselage blanks out the tail, and the DC 9 has been known to go into some nasty( on its back) maneuvers. The MD series has a stick (make that a yoke) pusher. If allowed to get too close to a stall, the slats deploy and the yoke is buried in the instrument panel. Another among many good reasons to hang the engines on the wings.

 

[enigma]

The rear pylon mounted engines on the DC9-MD designs dictate a T tail design. The post stall tendencies are nasty. The foward fuselage blanks out the tail, and the DC 9 has been known to go into some nasty( on its back) maneuvers. The MD series has a stick (make that a yoke) pusher. If allowed to get too close to a stall, the slats deploy and the yoke is buried in the instrument panel. Another among many good reasons to hang the engines on the wings.

I've done my best to dump Douglas knowledge, but I seem to remember that the autoslat function of the MD80 series only works from midseal flaps (11 or so). If you're clean when you get slow, the wing stays clean. If you're at more than flaps 14 IIRC, you already have full slats. The autoslat function was added after developmental flight testing showed a nasty flaps 11 stall. Also, not all MD80's possess a stick pusher. I spent hours trying to troubleshoot the system during the preflight of a flight test series only to find that the airframe never had one. I believe that it's original owner was an Irish company.

 

[Big Duke Six]

Actually, it's a simple function of demographics. Airplanes with engines in the tail are gay, and are disappearing because of the resultant low birth rate.

 

[PhatAJ2008]

So there will never be another a/c like this once the MD is gone?

 

[Quimby]

never say never....but, if it was a great idea, they'd still be doing it.

 

[flyboyike]

If the Russians are building it you can be sure it will have at least 5 crewmembers on the flightdeck and will have twice the amount of engines and tires as its western counterpart, and it will be called the 717ski.

Actually, it's called Tu-334, has a 2-crew cockpit with full glass.

 

[FLYLOW22]

Plus, nobody like a pilot who can't find the engine sync.


Woooo, wooooo, woooooahhhh, woooooooooahahahahhaa, woo.

 

[Scrapdog]

Though some of my ex-girlfriends would disagree, my opinion is that it is always better in the rear.

Spoken like a true Navy clown! Thanks for proving what 6 months on a boat with a thousand other dudes will do to you...

 

[skyward80]

I think Dutch Roll characteristics (especially in the unlikely event that all yaw dampers are lost) could be another issue. I could be wrong, but long, skinny tubes with engines in the back can draw perfect figure 8s through the sky with the nose without yaw dampers. I've had this demonstrated in a much smaller BeechJet 400. Turn the yaw dampers off, kick a rudder (gently of course) and the aircraft will oscillate until keel effect dampens the oscillations.

 

[Alcatraz]

And don't forget about the dual engine failure (or at least dual FOD) possibility on takeoff if you have frost/clear ice on the top of a cold soaked wing... that can break off as the wing flexes on takeoff and be ingested on a tail-engine airplane. Hence the "wing blanket" on the MD80.

Plus, ya gotta use wing heat during any icing conditions (anti-icer), whereas at least some wing-engine airplanes the wing heat is a de-icer (like the 737).