Which is better? engines under wings or to the rear of an airplane?

I just want to know if there is a reason for the difference. I honestly dont understand why engines are under wings to begin with if the wings depend wind flow for lift. seem in the way to me. but i know nothing about the science behind it so im asking you professionals. none the less. flight itself is just amazing. those big craft flying like that.. just amazing.

8 Answers

  • 1 decade ago
    Favorite Answer

    There are several considerations:

    1. Really heavy engines affect the balance, by affecting the location of the center of mass of the aircraft. That center of mass needs to be located near the center of lift, otherwise when you apply lift at the forward end with the wings and weight at the aft end with engines, the plane will tend to pitch. With aft mounted engines, the wings are usually mounted further aft on the fuselage (e.g. DC9, DC10, L1011) to move the center of lift back near the center of mass. But with really heavy engines, you are better off just mounting them under the wings, which eliminates most of the balance issue immediately.

    2. Engines that are mounted low beneath the wings are more prone to foreign object damage than those mounted high on the tail. On the other hand, engines mounted on the tail are more likely to ingest water kicked up by the landing gear.

    3. Engines that are tail-mounted can have certain flight maneuvers that create severe inlet distortion. For example, a side slip at a high angle of attack may result in the airflow to the engine inlet on the "downwind " side being functionally blocked by the fuselage or the wing ahead of it. This can cause compressor stall and temporary loss of thrust at a critical flight condition. Since wing mounted engines generally have inlets ahead of the wing, this is not a serious issue for them.

    4. Not sure I understand the earlier statement about applied moments. The truth is that engines mounted beneath a wing will create a huge bending moment on the wing structure as thrust is applied. That's because the engine center line is well below the mounts, and even further below the wing structure that holds it. Tail mounted engines are mounted so close to the fuselage center line that bending moments, while still present, are greatly reduced.

    5. (Added) Finally, wing mounted engines result in a much quieter cabin than tail mounted engines, which tend to place that screaming fan right next to the aft seated passengers.

    So it's a tradeoff, like most things in aircraft design. Structurally, tail mounted engines are probably easier to deal with. From an aerodynamics, flight control / stability, and noise standpoint, I would guess that wing mounted would be more favorable. And once the engines become large enough and heavy enough, the balance issue simply will not allow a tail-mounted version.

    ADDED (2): Your concern about flow under the wing is valid, but the only disturbance that is really created (once the aircraft is moving) is the relatively narrow strut that attaches the engine assembly to the wing structure. That's not a huge effect.

  • 1 decade ago

    They're not in the way, they are hung under the wings on a pylon that lowers them enough to avoid disturbing the airflow around the wing.

    The advantage to mounting the engines on the wings is mostly structural. Since the wings already have to support the weight of the whole plane and survive the stresses of flight, it's not hard to build them to support the engines as well. If you mount the engines in the tail, the entire rear fuselage has to be reinforced to deal with the extra stress.

  • DT3238
    Lv 4
    1 decade ago

    Generally, for large airplanes, wing mounts work best.

    When a tail mount is used it is often to increase cabin volume and to allow shorter landing gear (big issue for business jets) which, by the way are SLOWER, not faster, than commercial jets.

    Why do podded wing-mounted engines win the trade?

    1. Flutter. Having a lump of mass in front of the wing improves the dynamic natural stability of the wing, because the center of lift, if behind the center of mass, naturally damps vibration. THis was discovered back in the '40s on the B-47.

    2. Wing bending and torsion relief. The weight of the engine is best carried on the wing (where all lift is) rather than through the body. Also, since the wing exerts nose up twist moment, this balances the nose-down twist the wing lift creates, lowering wing torsion inboard of the wing.

    3. Aerodynamics- the wing strut acts as a barrier to spanwise flow. At high angles of attack (stall) this reduces the tendency of tip stalls to snap inboard all the way to side of body, causing a spin.

    4. Systems weight- fuel is in the wing.... lighter to burn it where it is stored- less systems runs for fuel lines and fuel cooling.

    5. Center of gravity control (loadability)- having the engines on the tail moves the c.g. aft, reducing airplane pitch stability. This forces the wing farther aft, increasing body bending moments, and narrows the range between forward most and aftmost c.g., which reduces the possible seat/cargo arrangements. More than once on a rear-mounted engine regional jet, I have seen passengers have to be moved around to control the c.g. before takeoff.

    6. 727s had to have a tail prop under them- when empty, if it snowed they could tip back on the tail!

  • 1 decade ago

    Well think of how much larger commercial planes are than private planes. Private planes (learjets, gulfstream, etc.) have a JET ENGINE on them and are flying 20 people max. That's why they go higher and faster. They're not carrying 80+ people with all that weight. In addition, commercial airlines generally flight at typical altitudes depending on which direction they are flying (N/E/S/W).

    In terms of efficiency, I think its better to have less in the way of your lifiting forces. So to have a wing with no obstruction to the lift being created, i.e. no big engine attached to it, you're going to maximize the lift being created, regardless of the drag. Avanti aircraft are known for their efficient planes because they have several lifting forces all over the plane.

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  • 1 decade ago

    Having the engines under the wing reduces the bending moment on the wing which gives a weight advantage. It also puts the thrust of the engine closest to where the most drag is produced. (The creation of lift induces drag.)

  • 1 decade ago

    Good question but I wouldn't think there are a lot of reasons, Usually commercial airliners (Boeing, Airbus) have them under the wings and private jets (Learjets, Gulfstream) have them on the tail!! And private jets fly higher and faster the commercial so there is probably a difference!!! Although the MD-11 had 2 under wings and one on tail!!!!

  • Anonymous
    1 decade ago

    it doesnt matter where the engines are as long as the plane is made of the black box material!!!!

  • 1 decade ago

    under da wing

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