Why are some aircraft engines position differently?

Question

The common ones are under the wings, just curious. Why do some big airlines such as b717 position it at the rear?

Does it improves the airplane's performance? .. What's the main difference (in terms of aircraft performance, if any) for the 2 very different positions?

Answer 1

Positioning of engines is dictated by the designers while the aircraft is still on the drawing boards. There are many factors which lead to positioning of engines at various positions. The main deciding factors are the aerodynamics of the aircraft and the flight envelope. Rear mounted engines give a clean wing area. A combination of front/rear engines (DC10 etc) had the advantage that the thrust was close to the center line and in case of an engine failure, the yaw  would not be very severe. Then you had the VC-10 with all four engines in the tail. High mounted engines are designed for operating in areas where sand/FOD could be ingested. At the time of designing, the engineers obviously worked to create an eficient and safe aircraft.

All in all, each design has its own pros and cons and sadly, in this answer all these cannot be covered.

Answer 2

Generally airplanes with engines in the back are smaller, like the  

ERJ/CRJ in the case of the 717 its a derrivitive of the DC9/MD80 and im pretty sure engine placement has to do with aerodynamics, in order to put them under the wing the airplane needs to be tall and have a Wider body

Answer 3

Advantages to Rear mounted engines:

- Less noise in the cabin.

- Thrust directed closer to the center line of the aircraft, greater stability with thrust acting from one point.

- Engine loss will cause less yawing effect.

- Less chance of ingestion of debris with engines mounted higher.

- Wings are more aerodynamic and lighter.

- Secondary flight controls can cover the whole leading and trailing edges.(slats & flaps)

- Saver for ground personnel.

Advantages of wing mounted engines.

- Weight is closer to centre of gravity.

- Heavier and larger diameter engines can be installed underwings.

- Thrust acts closer to centre of gravity less effect of pitching moment.

- Engine fires are easier to isolate.

- Line maintenance is easier to preform.

- Less fuel lines passing to the aft of the aircraft.

- Greatest structural support is already present at the wings.No need for extra strength at the tail.

- No need for a 'T' tailed design therefore less structural reinforcement required. Can use a trimmable horzontial stabilizer.

Answer 4

aerodynamics and lift is usualy why  jet plane engines are placed where they are

Answer 5

The B717 was a left over from the old DC9 series after Boeing bought the MD company.  Rear mounted engines give the advantage of thrust nearer to the center line causing less yaw if one engine fails. One engine out causes a plane to turn to the right or left (yaw).

It also gives the advantage of a clean wing, devoid of engines that disturb the airflow.  Rear mounted engines are quieter for the passengers but the placement can complicate the rear end of the aircraft resulting in fewer seats.  Rear mounted engines usually result in the T tail design which can result in much different flight characteristics.  

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Under wing mounted engines usually have the advantage of being closer to the ground for maintenance and service of the engine.

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Answer 6

The designers and engineers of the aircrafts which have engines emplacements at the rear thought that it can improve lift for the aircraft for a shorter takeoff. The wings are in front of the engines and are swept, therefore it can increase lift. Another thing is that the engines are not podded on the wings so it can have more airflow than most of Airbus aircraft which has podded engines on the wings. Perhaps they pod them at the wings to that it won't be back-heavy and have a better weight-distribution.