Whatever happened to the rear engine design?

Question

For awhile, you saw lots of DC9's., 727's. BAC 111,'s, L1011-s. and Dc10's, The 1011's and DC10';s had one engine in the rear, the others had all in the rear.
Now, you see rear engine designs in regional jets and B717's.
Everything else, Boeings and Airbusses, have their engines foreward, under the wings.
Why did the rear engine mount fall into disuse??

Answer 1

Ease of maintenance, for one. Kinda hard to service an engine that's 20' off the ground. Second, have you seen the size of some of the engines in service today? The newest models, like the GE-NX or Rolls-Royce Trent 900 have inlets over 10 feet in diameter. The engines these days NEED to be that big in order to produce the thrust needed.

The bigger the engine, the higher the bypass ratio can be, and so the more efficient the engine in terms of fuel consumption.

Another thing is the "quality" of the air getting sucked into the engine in the rear-mount configuration. Engines in rear mounts have to deal with what we call "dirty" air---air that has been disturbed by the wings and fuselage. Turbulent air adds drag and also lowers certain pressure and temperature qualities that engines need for high power and good economy. Putting them under the wing solves all these problems.

Small aircraft like regional gets or business aircraft use lower-power engines anyway, and are considerably smaller than their commercian counterparts, so the problems above are minimized.

Answer 2

None of these people have any idea what they are talking about, they didnt just strap those engines on back there for no reason, it was because the aircraft needed more thrust but didnt need four engines. .
The L1011 was a result of this thinking and its efficiency is higher than any 4 engine aircraft made before it.
In the 80's engine technology took a leap forward and you see a huge change in designs because of it.
The 757 and 767 could reach the same amount of performance as the earlier 3 engine designs but with even better efficiency. The L-1011 had three 40,000 lb thrust engines, and the 767 has 2 60,000lb thrust engines, so they have the same 120,000lbs of thrust. The 747 had four 45,000lb thrust engines, but it was also a larger heavier design that carried more people.
The 747 has 360 lbs of thrust per passenger, and the 767 and L-1011 have 500lbs of thrust per person, and the thrust ratios are similar, the 767 has a better ratio than either the L-1011 or 747.
The 757 and 767 were the most efficient aircraft in the world until the 777 came out. The 777 provided the same capability as early model 747's but is twin engined.
Before the 757 and the 767, twin engine aircraft were limited in size and performance was terrible, the 737 carried half the people half the distance for the same amount of fuel.

Answer 3

The main advantages of rear engine designs is the totally clean wing that results. The aircraft can also sit lower to the ground. The engine thrust is also nearer the center line of the aircraft so and engine out situation does not cause the same yaw motions as does under wing engine placement. They have not been used much lately on large airliners but are almost the norm for anything smaller like regional jets and lite-jets.
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The disadvantages of three engine rear designs is the problem of the placement of the third engine. The S-duct on the 727 and L1011 are responsible for a lot of weight and complexity to an aircraft design. The DC-10 straight through design causes the engine to be extremely difficult to service as the engine is so far from the ground. Boeing considered both when they were looking into a three engine 747 idea. They tended to favor the L1011 type S-duct arrangement.
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Three engine designs like the DC10 and L1011 came about because the airlines wanted a plane somewhat smaller than the 747 (300 passengers) but capable of continental and intercontinental ranges. The added safety of a three engine design especially for over water routes was pushed for by the airlines. Thirty years of large turbofan development has seen thrust increases four fold. The 777-300 has two 115,000 lb thrust engines and carries the same amount of passengers as did the DC-10 and L1011. Jet engine manufacturers have shown that these large turbofans are so reliable that two engine airliners regularly cross the oceans. During development the aircraft have to show they can travel on one engine for a certain distance depending on the routes. Usually 45 minutes or an hour.
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Answer 4

To answer your question is quite simple and Boeing figured it out back in the 1950’s. If you have time try and find a book called “Wide Body” by Clive Irving detailing the B-747 and why the engine pods are under the wing.

By placing engines in pods under the wing the nacelles can be designed to create their own lift on the pylon and fly by themselves independent of the aircraft. This save a lot of weight/fuel needless to say it is also a great safety factor having the engines away from the fuselage. There are many advantages having wing pods like fuel plumbing, controls, weight (C.G.) and list of other to long to list.

The only draw back is on take off it requires speed to get the pods to fly after that is a money saver and engne failure with lots of drag out on the wing station. Having swept wings reduces wing drag and pods are ideal for this. The smaller jets you see do not have the swept wings as much as Boeing aircraft so placing the engines on the rear is a trade off (they accept the drag/safety factor) for a stiff wing.

The hardest part of an aircraft to design is the wing. If the wing is not right the whole aircraft will be a dog or unstable and Boeing has lead the industry with wing design and manufacture. You may notice the boeing wing is twisted out toward the end as well as being swept back (prevents dutch roll).

Answer 5

There is no reason really other than economics.

Boeing bought McDonnel Douglas and the MD-80 and -90 were competing with the 737s.

Airbus just decided to go with wing mounted engines because of convention (though the BAC 111 successor was going to have rear mounted engines, Airbus was formed before it was built).

But most of the RJs and biz jets have rear engines.

As far as being harder to maintain? Rubbish! So you have to go and get a scissor lift. You have to anyways to work on the larger wing mounted engines.

More lift with wing mounted engines? You can't put flaps in the blast area of wing mounted engines (thats why they put an aileron or a cutout there for wing mounted engines). But with rear mounted engines you can have full span flaps.

More hydraulic lines and fuel lines for rear engines? Not really. Either the lines go through the fuselage or through the wings. Even though wing mounted engines are mounted by the fuel tanks they still have to have cross feed lines. But the fuel cross feed line on the 727 is only about a few feet long.

As far as weight goes the wing mounted engines will be lighter because the wing fittings don't have to support the weight of the fuselage and the engines but it is negligible.

There can be a problem with ice damaging rear mounted engines but proper deicing procedures on the ground and the use of anti ice and fuel heating prevent that from happening.

The rear engined aircraft (except with center mounted engines) are much easier to stretch and upgrade. They stretched the puny DC-9-10 progressively into the huge MD-90 and all they did was add some structural beefs to accommodate the longer fuselage and bigger engines.

When they stretched the 737-200 in to the -300 and -400 and put bigger engines on it, they had to redesign the landing gear, redesign the CFM-56 engine with a smaller fan and with the accessories on the side instead of the top. Then they had to mount the engine in front of the wing instead of underneath it with five degree up angle to get as much clearance as they could (sure it produces some thrust down, but any time you turn airflow, you increase drag and fuel burn). The engine installation also interferes with the airflow around the wing so they had to do a bunch of aerodynamic band-aids to fix it.

Answer 6

"As far as being harder to maintain? Rubbish! So you have to go and get a scissor lift. You have to anyways to work on the larger wing mounted engines."
calnick probably never removed a tail mounted engine.
Wiing mounted engines can be removed and replaced in 1/10 the time as a tail mounted, because they are removed pylon and all as an assembly. The 747 has a couple of extra mounts between engines to mount an engine for transport on either side.

Answer 7

nicely, i am going to respond to question 2 for you. It helps to comprise how a jet engine is began, so it is the way it truly works. First, air is blown down each of the fan blades to start up spinning them. once the fan blades are spinning quick sufficient to compress the air by an hourglass-like structure, then the air will change into really warm. a mist of gas is sprayed into the nice and comfortable air and it explodes. this explosion shoots out the decrease back end of the engine and through a series of fan blades that are appropriate quickly with the fan blades at the front of an engine. so so long because the explosion is blowing by the decrease back blades, then the front blades save turning aswell and compress the air for the gas to burn on. it truly is a self-protecting cycle.

Answer 8

design efficiency. maintenance, hydraulic lines, fuel tank location, weight and balance, all factor in how an aircraft is designed. when you take a heavy part of the plane and put it out back on the tail, you have to put some heavy weight in the front of the plane to prevent instability during flight. with the engines on the wing, oyu dont have the balance problem.

Answer 9

Much easier to do maintenance on wing mounted engines than tail mounted ones.

Less plumbing, piping, weight, for wing mounted engines.

Answer 10

Engines under the wing also add more lift than rear mounted engines.