Is it the end for tail mounted engines?

[Spirit MD-80]

Is it the end for airliners will tail mounted engines? Every aircraft companies planes look the same(Engine location and usaully tail). MDC was the only manufactuer, to have a medium range tail mounted airplane. Let me say, that those types of airplanes look very streamlines, and the way to the future, at many angles.

 

[KSUpilot]

I have to agree with you that they do appear more streamlined. And they are very sucessful aircraft. The DC-9 and MD-80 ruled the skies, and now the smaller Embraer 145 rules more regional jet fleets.
Boeing needs to do something different with their next aircraft. Airlines like the rear-mounted configuations like the 717 (look at Air Tran) the EMB-145 (look at just about any regional) the MD-80 (look at American). They also tend to be quieter for the passengers. The EMB-145 is quiet, just a bit tight with the seating.

Here is what I propose for the 737 replacement:

There is nothing wrong with the current 737 shape and size, its small and charter buslike, but that is what I like about it. But what if we take the standard 737, redo the entire fuselage with composites and place rear mounted, fuel efficient GEnx like engines on it? You could have a shorter version that is based off of the 737-300 or stretch versions based off of the 737-900.
The version with the smaller engines uese your standard small 717 sized engine, or something similar to the MD-80. You could also throw in a version that uses an updated, efficient version of the current 737 engine. Where the rear-mounted engine truly comes into play is when you add a larger engine to the 737. The current engine is stuck at its current size because of ground clearance. If the engine is mounted rearward, it has all the ground clearance in the world. With this we can have a longer, more powerful 737.

 

[ta152]

I had always read that they put engines in the rear to have a "clean" wing area but since Airbus and Boeing don't do it that way I guess the Fokker 100 is the last airliner to have rear mounted engines, although alot of business jets use the rear engines.

 

[KSUpilot]

I did some research on the advantages of Tail Mounted engines as opposed to wing mounted engines:

"One of the advantages of tail-mounted engines is that, the controllability is better if an engine fails because they are close to each other as compared to engines below the wings (they are further apart)."

"aircraft with tail mounted engines produce less overall drag and provides the aircraft with a clean wing another would be that wing mounted engines have the advantage of providing a counterforce to Lift (due to their weight) on the wing hence producing less wing bending (but this cannot be figured in when constructing an aircraft).
One other characteristic is that with tail mounted engines stall recovery is usually very straight forward because when you advance the thrust the engines produces a nose down moment (due to thrustline being above CG) which aides in recovery where on wing mounted engines you will experience a nose up moment which you need to be ready for when recovery commences. This little fact also plays a role in landing the aircraft. "

"Aft Fuselage Engine Placement
When aircraft become smaller, it is difficult to place engines under a wing and still maintain adequate wing nacelle and nacelle-ground clearances. This is one reason for the aft-engine arrangements. Other advantages are:

Greater CLmax due to elimination of wing-pylon and exhaust-flap interference, i.e., no flap cut-outs.

Less drag, particularly in the critical take-off climb phase, due to eliminating wing-pylon interference.

Less asymmetric yaw after engine failure with engines close to the fuselage.

Lower fuselage height permitting shorter landing gear and airstair lengths.

Last but not least - it may be the fashion.

Disadvantages are:

The center of gravity of the empty airplane is moved aft - well behind the center of gravity of the payload. Thus a greater center of gravity range is required. This leads to more difficult balance problems and generally a larger tail.

The wing weight advantage of wing mounted engines is lost.

The wheels kick up water on wet runways and special deflectors on the gear may be needed to avoid water ingestion into the engines.

At very high angles of attack, the nacelle wake blankets the T-tail, necessary with aft-fuselage mounted engines, and may cause a locked-in deep stall. This requires a large tail span that puts part of the horizontal tail well outboard of the nacelles.

Vibration and noise isolation for fuselage mounted engines is a difficult problem.

Aft fuselage mounted engines reduce the rolling moment of inertia. This can be a disadvantage if there is significant rolling moment created by asymmetric stalling. The result can be an excessive roll rate at the stall.

Last but not least - it may not be the fashion."

And here is an interesting one that breaks it down real nice:

"In article <airliners.2001.58@ditka.Chicago.COM>, Wolfgang Keller
<w_keller@gmx.de> writes:

What are the actual design tradeoffs today and how did the situation
evolve over the past decades to favor wing-mounted engines that much? Whatwould be the impact of, for example, drastically increased fuel prices and/or significantly more strict noise regulations (=> engines with bypass ratio >>10)?

Weight: Putting engines in the back requires extra load carrying structure, both for the thrust and the weight of the engine itself. On the other hand, little extra structure is required for the wing mounted engine since there's structure already there; the wing is mostly structure. Advantage: Wing Mount.

Noise: Clearly, advantage rear, except perhaps for those sitting in the back. Advantage: Rear Mount.

Maintenance: Most maintenance on wing-mounted engines can be performed at ground level without lifts or scaffolding. Significant appeal to maintenance staffs. Advantage: Wing.

Engine-Out Performance: Single-engine control is much better with rear-mounted engines over wing, simply because thrust is far less asymmetrical because the engines are so much closer towards the center. Advantage: Rear

Aerodynamics: Rear-mount allows for a cleaner wing design, and less conflicts with airflow and mechanics for slats and flaps. Advantage: Rear.

However, most rear engine configurations necessitate a T-Tail configuration for the horizontal stabilizer, which requires extra weight for structure at the top of the vertical stabilizer, as opposed to conventional horizontal stabilizers attached to existing structure at the end of the fuselage.

CG Considerations: There are differences in handling and stall characteristics that tend to favor wing with the engine weight near the CG vs the increased moment as a result of having a longer nose and all that weight in the rear. However, advanced control systems and pilot training tend to negate these differences.

Ground Handling: Many rear-mounted designs allow the use of reverse thrust for backing up the aircraft with limited danger of FOD ingestation, thus eliminating the need for a push-back at the gate. This is appealing to airlines as it means that less ground support required. Advantage: Rear.

That's all I can think of at the moment.

Personally, as a passenger, I prefer the MD-80 to the 737, mostly because of less noise, the 2x3 seating, and the 737 is "bumpier". But the reality is that what really determines what plane you will be flying is the economics."




And I found this in a post on Airliners.net regarding the old 7J7 concept:

"However, don't be surprised that Boeing does revisit many of the studies done on the 7J7 and comes up with a plane that will replace the Next-Generation 737.

It will likely sport the following:

1. A heavily composite structure like the 7E7.

2. A return to the T-tail design (with today's FBW controls, the issue of deep stall is much less so than it used to be), which allows Boeing to use larger front-fan versions of the CFM56 or V2500 engines that turn at a slower rate to really reduce the noise from the engine, similar to what Airbus did with the A380 engines that have relatively slow-turning front fans on the Trent 870 and GP 7270 engines.

3. A wider fuselage, probably about the same width as the A320 Family but with a more oval cross section for better headroom.

4. A new, highly-efficient wing with raked wingtips.

5. Seating capacity between 125 and 180 passengers in three fuselage lengths."

 

[ta152]

Nice research KSU. I remember in the mid 70's Fokker tried to sell a small airliner with engines mounted on top of the wing. Not many were sold but it was cute looking ! Maybe that will make a fashion statement in the future. 😛

 

[KSUpilot]

I believe Honda also looked into the engines mounted on the wing configuration as well.

Here is what I believe is holding the tail mounted configuration back in terms of a 737 replacement. Boeing just came out ahead of Airbus for the first time in years, with help from the 787. Originally the 787 was going to be the much more unconventional Sonic Cruiser. Totally new shape, something we have never seen before. Bin Laden and hs sick little helpers kill the airline industry and ultimatly the Sonic Cruiser on 9/11. In the aftermath of 9/11 the airlines all want cheaper and more efficient aircraft, therefore the Sonic Cruiser becomes the 7E7.
Remember when the first 7E7 images came out? It was still a much different shape. The nose was different and the tail looked completely new. Then the design changed again when the 787 name came on. This time the 787 looked just like every other aircraft Boeing and Airbus has come out with in the last 10 years.
But, this conventional shape allowed Boeing to surpass Airbus for the first time. Therefore inorder to keep that lead they will want to stick to conventional shapes when it comes to the 737 replacement. They are too timid to attempt something like a T-tailed rear engine aircraft. They are afraid that the airlines will reject it. But this is wrong, as airlines still fly T-Tailed aircraft today.
Had the 717 had a cockpit common with the other Beoing aircraft more airlines would have shown interest in it. Still that did not stop Air Tran from buying them. American Airlines loves their T-tailed, rearmounted MD-80s so much, that isntead of scrapping them, they are looking for more efficient engines.

There are more advantages to the T-Tailed, Rear engine designs than just a streamlined look.

 

[KSUpilot]

Here is something interesting I found on Airliners.net:

And an article I found through a Google Search:

'Muppet planes' on drawing board
Enviro-friendly aircraft get look from Boeing
By Dominic Gates
Seattle Times
Tucson, Arizona | Published: 06.06.2006
SEATTLE — When Boeing names an airplane design after a Muppet, it must be pretty different.
Two small teams at the company are re-imagining the airplane in futuristic configurations that sprout wings, tails and engines in unexpected shapes and places.
The research, illustrated in internal documents, aims in two directions: low-cost airplanes, and environment-friendly planes that will be quieter, use much less fuel and leave fewer pollutants in the upper atmosphere.
In the latter category is the "Kermit Kruiser," a low-noise concept airplane with main wings radically swept forward rather than back, and sporting miniature wings on the front.
Then there's the "Fozzie." It has a "Pi-tail" — two vertical tails joined by a piece across the top, and sips fuel because it flies slower using open-rotor jet engines that resemble the old-style propellers.
The concepts are "intended to help us focus technology on a future out beyond the horizon," said Dan Mooney, Boeing vice president of product development, who directs both research teams.
The documents show Boeing has looked at other concepts as well: a supersonic business jet; a megasize freighter; airplanes that use biofuels or hydrogen; and even a "reduced crew" airliner — one with no windows in the cockpit, judging by a sketch in the Boeing documents.
But of all the potential concepts, Boeing has prioritized the "low-cost" and the "green" planes for further research this year. Both teams have begun work with engine companies on the various propulsion alternatives.
The Boeing documents include assessments of very similar research projects that its rival Airbus has sketchily mentioned at scientific conferences.
In an interview, Mooney declined to discuss proprietary details of the designs but offered insight into what his research teams are up to.
He said the latest airplanes being sold today, such as the 787, are designed to meet airlines' projected requirements for about the next two decades. Designers strike a balance among cost, fuel efficiency, capacity, range and other factors based on those projections.
His concept teams, however, think "out beyond 15 or 20 years," where fuel costs, noise or other factors may become more important and reshape what airlines want.
"We need to be developing technology today to allow us to be ready for those uncertainties in the future."
The low-cost team, documents show, is studying the benefits of options such as long, thin wings and new engine types. That team has not yet envisioned new structural designs, however.
In contrast, the Green Team, with a broad mandate to address diverse issues of fuel burn, noise and emissions, has considered some widely differing airplane structures — each with its own whimsical code name. (The Muppet theme may be a reference to the song Kermit sang on "Sesame Street": "It's not easy bein' green.")
● "Kermit Kruiser": Low noise. The engines sit atop a twin-fin tail, so that the noise is reflected upward. The wings are placed so far back they join the fuselage right at the horizontal stabilizer. And most radically, the wings sweep forward, not back, lowering aerodynamic drag and increasing maneuverability at the price of some stability. Keeping this tail-heavy aircraft stable in flight requires a canard — those mini-wings up front. The plane would be a wide-body seating nine abreast.
● "Fozzie": Ultra-low fuel burn. The airplane is designed to cruise at a much reduced speed — 500 mph rather than the typical 600-plus mph of current jets. That would add an hour to the typical transcontinental flight.
Attached to a tail with twin vertical fins and a crossbar (called a Pi-tail because it resembles the Greek letter pi) are engines with an "open rotor" or "unducted fan" design.
The plane has a fanjet gas-turbine engine of the sort used on airliners today, but without the usual duct encasing the fan, Mooney confirmed. At slower speeds, this offers great fuel efficiency.
One internal drawing shows the rotors on the back of the engine; another shows them on the front, the more usual position.
● "Beaker": Low emissions. This airplane has the low fuel burn and same low cruise speed of Fozzie. It has low-emission engines and long, very narrow wings perpendicular to the fuselage. The wingspan is such that the wings must fold to fit an airport gate.
● "Honeydew": Low fuel burn. Another wide-body, this aircraft seems to be a meld of the traditional "tube-and-wing"-shaped airliner and the often-touted "Flying Wing" design that produced the B-2 bomber.
The resulting delta-shaped wing blends in a graceful curve into the fuselage. Yet there is still a distinct fuselage at the front.
The Flying Wing design is more aerodynamically efficient. One disadvantage is that most passengers are far from a window. Honeydew appears to be an intriguing compromise.
Since April, Boeing's Phantom Works research unit, in collaboration with NASA and the U.S. Air Force, has been doing wind-tunnel tests on a small-scale, 21-foot-wingspan prototype of a Flying Wing or Blended Wing Body aircraft concept. Flight testing of the prototype is planned for later this year.
The Air Force is interested in the design's potential as a long-range, high-capacity military aircraft.
So how realistic are these cool-looking airplanes?
"When you look at where energy costs are going in the next decade, it could be time for a change in the rules," said Jerry Ennis, a retired vice president at Boeing's Phantom Works who worked on prototypes.
Like the Detroit carmakers who wheel out fanciful concept cars that never reach the showroom, Boeing may never build an airplane that looks like any of these images.
Still, Mooney said, "Most likely there'll be parts of the technology or parts of the configuration that will find their way onto products of the future."

 

[ta152]

I think the Honeydew has possibilities, however getting conservitive business men to buy something different is a major task.

Beechcraft tried to sell the Starship about 20 years ago and hit a brick wall.

 

[KSUpilot]

I believe the Honeydew has possibilites when it comes to the 747 replacement. Boeing has been playing around with the BWB which looks more like the B-2 bomber. This is more of a mix of the BWB with a conventional tube and wing aircraft.

In the other thread about these aircraft I started there are some close up images of the Honeydew and the Beaker.

I think a more conventional version of the Fozzie design is possible...UDF engine with a normal T-Tail, basically a 7J7 rebirth. The Beaker has some possibilites as well.

But look at all those designs, they all have one thing in common, Tail Mounted engines. The Boeing engineers working on this project agree that a clean wing is the most efficient.