Why did larger airliners move away from tail mounted engines?

Discussion in 'Technical Corner' started by sevensky, Jan 12, 2020.

  1. sevensky

    sevensky Filing Flight Plan

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    Shower thought here. With all the NEO, and Max reengineering to add larger turbofans why did tail-mounted engines disappear?
     
  2. Stewartb

    Stewartb Final Approach

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    Two engines are more efficient than three?
     
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  3. AKBill

    AKBill En-Route

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    I think the OP was thinking in the line of the DC-9. Maybe ease of installation under the wing and the size of engines.
     
  4. Spring Ford

    Spring Ford Pre-takeoff checklist

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    I think this is pretty clear. Tail mounted engines are great because the cabin is a lot quieter however there is a substantial structural weight penalty which in these days of fuel, fuel, fuel is unacceptable to airlines.

    The reason for the penalty is that the lift to hold the engines in the air has to be transmitted from the wings all the way to the tail - this takes structure. If you mount the engines directly on the wings, where the lift is, the result is less structure required.

    I am not clear why small jets still use tail mounted engines. I think there is one that has over-wing engines, or is that still a proposal? Found it, very much alive and flying.

    [​IMG]
     
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  5. Spring Ford

    Spring Ford Pre-takeoff checklist

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    Further analysis here -

    http://www.vc10.net/Technical/wing_shapes.html
    "Because of the 'hot and high' airfields that BOAC wanted the VC10 to operate to (Sir George Edwards mentioned Nairobi in particular) the aircraft needed very good take-off and landing performance. This drove the decision to put the engines at the back as that freed up the wing to deliver as much lift as could be generated from it. In doing this the design team also took away what's known as 'wing bending relief' from the engines, basically the weight of an engine hanging from the wing opposes a bit of the lift force that's trying to bend the wing upwards, therefore requiring less structure weight in the wing itself. That is one of the reasons that especially the Standard VC10 is more expensive to operate than a 707, its structure is slightly heavier for the same number of passengers (25.7% of MTOW as opposed to 24.6% for the 707)."
     
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  6. Grum.Man

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    I have no evidence only hearsay but the thinking is for passenger jets it was safer to have the engines on the wings instead of running fuel lines all the way down the fuselage to the rear engines. Most APU's are rear mounted so not sure how valid that is. I imagine smaller jets still have rear engines to keep the airplane lower to the ground for ease of entry, fueling, and maintenance.
     
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  7. Dave Theisen

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    Don’t stand up too quickly after thinking about airplanes in the shower. :)
     
  8. flyingron

    flyingron Touchdown! Greaser!

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    The 737 initially was going to have two tail-mounted engines (think about a 2-engined 727). The engineers involved determined that you can get by with less structure (read cheaper) if you mount them on the wings.
     
  9. Bell206

    Bell206 Pattern Altitude

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    While engine thrust at the airframe centerline is the most preferred, as I recall the reason behind the change to wing-mounted was mainly due to CG vs weight/arm, control authority, plus loading schedules, i.e., too much weight aft as engines got bigger. With the engine weight near the MAC then you can basically increase aircraft size as you extend cabin areas forward and aft of the MAC. It's a bit more complex but that is how I understood it to be.
     
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  10. Greg Bockelman

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    Can you imagine the high bypass engines with the huge fans mounted on the tail?

    I think, logistically, it is not possible to mount the current crop of efficient engines on the tail.
     
  11. Doc Holliday

    Doc Holliday Line Up and Wait

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    [​IMG]
     
  12. FormerHangie

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    I wouldn't say an MD-90 has current generation engines. Here's an A321 neo:

    [​IMG]

    While I don't doubt you could mount them on the back, they'd be out there.

    Having said that, if you do mount them on the fuselage, then the structure that joins the wings to the fuselage has to support the engines as well as the fuselage, and you're stuck with a T tail. As @Spring Ford and @flyingron noted, wing mounted engines allow for a lighter structure.
     
  13. Sac Arrow

    Sac Arrow Touchdown! Greaser!

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    Also I think weight and balance (as in, the balance part of it) would be more critical in a plane with tail mounted engines.
     
  14. Brad Smith

    Brad Smith Line Up and Wait

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    DC-9 was modified to carry from 63 to 172 passengers in different configurations. The Boeing 737 from 85 to 200 passengers in different configurations. Seems that whether the engines are mounted under the wing or by the tail makes no difference when it comes to stretching the fuselage, though some bad decisions were made when the final stretch on the 737 Max was accomplished ( another entire topic for discussion.)
    The HondaJet uses an unusual engine mounting position and I would wager a guess that this will not become an accepted practice from Boeing, Airbus or any other manufacturer, though it has its advantages. The aesthetics are surely a factor and no large manufacturer is willing to make the financial investment to find out.
     
  15. midwestpa24

    midwestpa24 Pattern Altitude

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    I think on larger aircraft a lot of it has to do with maintenance. Much easier to work on and inspect at ground height. For corporate aircraft, they may be operating at smaller fields with less emphasis on FOD prevention, so mounting them higher is a safety consideration.
     
  16. Tantalum

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    This is what I've read and been told also:
    The Piper Aerostar was originally supposed to have wing mounted engines.. I was told that engines go on the tail typically for ground clearance issues so passengers can easily get on and off the jets without needing loads of airport equipment, etc. Private jets care less about ops costs.. aesthetically they also start to look a little "funny" ..
    upload_2020-1-13_13-4-14.png

    For what it's worth tail mounted engines look sick.. the VC-10, B727, TU-154, Falcon 7 have to be some of the most beautiful planes out there
    upload_2020-1-13_13-6-15.png
    upload_2020-1-13_13-7-26.png
    upload_2020-1-13_13-8-17.png
    upload_2020-1-13_13-9-19.png

    AND... there's even a tail mounted amphib piston GA plane out there too.. with some of these flying in the us:
    upload_2020-1-13_13-12-8.png
     
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  17. Spring Ford

    Spring Ford Pre-takeoff checklist

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  18. Tantalum

    Tantalum En-Route

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    I was mildly obsessed with it for a while.. very slow though. I wonder how it would perform with TSIO-550-K powerplants instead

    Russia tends to built some unique planes.
     
  19. -KLB-

    -KLB- Pre-takeoff checklist

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    Is some of it the move from needing 3 or more engines for ETOPS type operations to those routes now being serviceable with twins?
     
  20. flyingron

    flyingron Touchdown! Greaser!

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    Eh? There's no such thing as ETOPS operations with other than two engines.
     
  21. X3 Skier

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    The L-1011 and DC-10 had tail mounted high bypass turbofans to meet the overwater one engine out criteria. When the regulators allowed ETOPS, the need disappeared and the underwing twins took over. Two vs three mega million engines is a real cost saver.

    If you want to see some structural penalty associated with tail mounts, look up Banjo Fitting on the DC-10.

    Smaller Biz jets tend to default to tail mounts mostly so they can have a minimum ground clearance to navigate with airstairs. It also minimizes gear length and gear wells.

    The Honda Jet solution keeps lower ground clearance and no structural penaly. It was a marketing gamble if the “funny” look would sell. AFAIK, it is a good seller.

    I would guess big Gulfstreams maintain tail mounts because that’s the way they started:cool:.
    Cheers
     
  22. Skip Miller

    Skip Miller Final Approach

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    Ya think?

    [​IMG]

    -Skip
     
  23. Larry in TN

    Larry in TN Cleared for Takeoff

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    Yes, there is.
     
  24. Kenny Phillips

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    It's a bit quieter without the engines bolted to the fuselage, and there's not 'carry-through' structure needed. It's beautiful! But I'm biased, I worked on the HF120 engine program.
     
  25. Kenny Phillips

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    Technically, there is, as ETOPS doesn't mean what it did before; now it's simply "Extended Operations". Of course, it's easier to get certified if you have more than two engines, but it still needs to be done. But how many new airliners will have more than two engines? The era of the super-jumbo is past. Two GE90 or GE9X turbofans will fly a very large plane.
     
  26. skier

    skier Line Up and Wait

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    Are there any turbulence effects with boundary layers from the fuselage affecting tail mounted engines? Visually, it seems like the wing-mounted engines may be in clearer air, but I’m no aerodynamicist.
     
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  27. Pilawt

    Pilawt Final Approach PoA Supporter

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  28. Pilawt

    Pilawt Final Approach PoA Supporter

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    In the 1970s there was a German-built 44-seat airliner, the VFW-Fokker 614, with a similar over-wing engine arrangement.

    [​IMG]

    [​IMG]
     
  29. Spring Ford

    Spring Ford Pre-takeoff checklist

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  30. Spring Ford

    Spring Ford Pre-takeoff checklist

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    Almost looks like it started out as an Ekranoplan (ground effect craft) and then they thought "to heck with this, we need WINGS"?

    upload_2020-1-14_1-19-47.png
     
  31. jonvcaples

    jonvcaples Pre-takeoff checklist

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    Interesting to see how trends form and evolve especially in aviation. For a long time flying boats were the way to go before there were many airports. The Brits insisted pilot's need to feel the air in their face. Engines under wing versus on the tail...2 v 3 v 4 engines. Interesting stuff driven by many revolutions and evolutions across numerous nanotechnologies. Intertwined with changes to certification processes.
     
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  32. Let'sgoflying!

    Let'sgoflying! Touchdown! Greaser! PoA Supporter

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    After UA232, Al said 'no more of that tail-mounted chit for me'.
     
  33. Tantalum

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    That's really cool!
     
  34. flyingron

    flyingron Touchdown! Greaser!

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    It ain't the tail mounted engines that are the problem. No L-1011 or 727 crashed like that. It was MDs hideous hydraulic system design (coupled with their general bad corporate philosophy towards safety and QA) that caused the problem. Oddly, the Aviation Safety gurus were railing on that long before it happened. Of coruse, the previously worse DC-10 crash was when the wing engine fell off of the one in Chicago. While losing the engine didn't help that one, what really killed them was erroneous single engine procedures.
     
  35. PaulS

    PaulS Final Approach

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    The biggest reason for going from 3 or 4 engines to 2 is cost. It is much cheaper to acquire and maintain 2 engines versus 3 or 4. There is also weight savings and complexity savings. The engines now are much more efficient and powerful. They are supposed to be more reliable too, and maybe are, but ETOPS (engines turn or passengers swim) rules require quick action on problems that crop up and most time problem engines are removed from service and fixed before they turn into a shutdown.
     
  36. flyingron

    flyingron Touchdown! Greaser!

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    It's not just acquisition and maintenance. Two bigger engines are usually more efficient than three smaller one. Passenger load divided by Fuel Cost is a big factor in airline profitability.
     
  37. Tarheelpilot

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    Hmm didn’t md merge with Boeing
     
  38. Larry in TN

    Larry in TN Cleared for Takeoff

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    Engineering is about compromises.

    Tail-mounted engines allows the airplane to sit lower. This works well for smaller airplanes and for giving easier access for stairs and baggage loading. You also have the thrust vector closely aligned with the longitudinal (i.e. through the CG) which virtually eliminates pitching moments from thrust changes. The primary disadvantages have to do with weight and structure.

    Under-wing engines are supported directly by the wing. That means that the load for supporting the weight of the engines does not have to be transferred to the fuselage through the wing attach structure. You can also have a simpler tail structure as a T-tail is not required. Engine maintenance is also facilitated by the engines being closer to ground level.
     
  39. Kenny Phillips

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    Kind of like Supercar meets Twin Commander and a canoe.
     
  40. Tantalum

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    Oh God.. you just described Boeing
    :rofl:

    MD sucks

    Amazing how an entity that built something as venerable as the DC3, and other prop giants like the DC6 became such a pathetic player after the DC-9