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Guest Erick_Cantu

Tail mounted vs. Wing mounted engines.

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Hi,Today I got an interesting comment from my neview. He asked me why some aircraft have the engines tail mounted, and some wing mounted.To be honest, I responded that I really don

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Just a few of my own thoughts, I'm not sure how they apply to the real world.Tail mounted engines would be better protected from Runway debris and would be better in an engine out situation, being closer to the center of the aircraft.Wing mounted engines would be much easier to work on or flat out replace if damaged. They are also right there where the fuel is wich would also be a strong point for maintinance.

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Tail mounted engines allow for some noise reduction in the cabin/cockpit. In the 727 cockpit it's quite difficult to hear the actual noise of the engines-same with a DC-9. You still hear a lot of wind, but very little engine. Also, another benifit of the tail engines is the ability to push back on reverse thrust, rather than using a tug, allowing the aircraft to serve smaller aircraft. This procedures isn't used on the 737 or any other wing engine aircraft I know of. Regards,MarcMarc Gibson

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Tail mounted aircraft have stricter requirements on aft CG loading because the thrust moment is some way behind (and above) the CofG. However, the wings on tail mounted aircraft are usually more efficient as they don't have large, drag-inducing protuberances hanging off them spoiling the lift.Wing-engined aircraft usually have shorter control runs and fuel lines and the engines can be more powerful for the size of aircraft as the thrust moment is reduced by being closer to the Centre of Gravity.The main operating advantage for wing-mounted engines is ease of access and ease of maintenance.The FOD example is interesting as until now I have always thought that tail-engines conferred better protection from foreign object damage but the new generation of regional jets, typically operating into and out of smaller airports the runways might not be up to international standards, are now switching to wing-mounted. Is it just a style thing as the smaller airrcaft try to emulate the big stuff? Or is there a valid reason, other than above?Allcott

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I think with the larger, newer jetliners, it is a matter of the size of the engines. The diamater of the engines of a 777 for instance are enormous. How would you put those up on the tail? :)Tony

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Maybe wing mounted engines have less chance of destroying rudder & elevator hydraulics if they loose a compressor or turbine blade. Remember the Souix City Iowa DC-10 incident? The Number 2 engine suffered a engine disk breakup and all controls in the tail failed due to hydraulic line ruptures by engine debri. A significant portion of wing mounted engines are forward of the wing leading edge and slat actuators are often located in the wing root or under the cabin floor in the center of the fuslage.

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Tail mounted engines give a plane the ability to deep stall. This is when at high angles of attack the engines flame out as they are in the turbulent airflow from the stalling wing. This stall is unrecoverable. I have to admit to being no expert but without looking it up afresh I'm sure this what I read once.John

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HiTail mounted engines have the disadvantage off bigger risk to get foreign objects into the turbines, ie. the SAS DC9 (MD8x?)that suffered both engines out immidiately after take off in stockholm, because wing-ice was sucked into both engines. Also ground objects from wheels could end up into the engines I suppose.

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Hello,the first jet engine wing mounted airliner was the B707 developed from experience on the B-47 Stratojet and the B-52 Stratofortress.General Lamay wanted bombers which did not have engines in wings as when they caught fire the wings burnt off and they fell out of the air as was common with the B-17 Flying Fortress.Boeing's solution was to put engines on struts offset and in front of the wing. Also broken blades would fly out of the engine and missing the wing. The B707 used the bomber technology and confirmed the engines act as mass dampers for the aeroelastic wings which was a problem with the B-47. The engine lateral spacing is critical.Source reference:http://www.amazon.com/exec/obidos/ASIN/068...product-detailsThe first jet engine tail mounted airliner was the Sud Aviation Caravelle (A

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Cheers to you Staffan Ahlberg, one of the greats!Peter Sydney Australia

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Apart from what has been said, I guess it's a bigger engineering challenge to build an aircraft with tail mounted engines. I think one particular british airliner featured 4 tail mounted engines but although being economic it wasn't successful for safety reasons or something like that.I like the wing mounted ones better, btw., those huge 777 engines - woah. Although - the blended 3rd engine in the Lockheed TriStar looks cool.-Daniel

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Hello Daniel,I was reading Torrenbeek last night for the wing vs. tail mounting explaination:http://www.amazon.com/exec/obidos/ASIN/902...product-detailsWing mounted engines on engine failure give a high yaw and roll monent and so need a large and heavy tail fin.Tail mounted engines need heavy mounting pylons and reinforced rear fuselage. Also the H tail will get in the way and is usually mounted as a T-tail but these can suffer locked-in high AoA stalls. The DC-10 was renouned for this and this was solved by renaming it the MD-10.The 4-engine you were refering to could have been the VC-10 which is still flying as a combined tanker and transport and was heavily used in Iraq to support the USAF and USN tanker shortage, hence the plan for B767 tanker conversions.The USSR produced many tail mounted designs and only moved over to wing mounted in the Ilyusen when they got this technology from Boeing in a trade for their Titanium welding technology. So are tail mounted aircraft easier to manufacture? Most biz jets are this way.Ian

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Hi Staffan,By searching the avsim-forums for the VC10 (as there has been a new payware-VC10 released recently), I found your topic "Tail mounted vs. Wing mounted engines" (http://forums.avsim.net/dcboard.php?az=show_mesg&forum=121&topic_id=161195&mesg_id=161195&listing_type=search).Though most of the pros and cons have already been mentioned I thought I tell you some points mentioned in the excellent book "Handling The Big Jets" by D.P. Davies (then Chief Test Pilot of the United Kingdom Airworthiness Authority).I skip the first paragraph (a) which deals with engines buried in the wing root (like on the COMET)."(:( Mounted in underslung wing pods:Advantages:1 Depending on geometry and design cruise Mach number, installation (interference) drag is minimised.2 Intake efficiency is rarely compromised by interference flows.3 The engines provide bending relief thusreducing wing structure weight.4 The wing profile is not compromised. (compared to a)5 At high incidence (and, by careful design, at other times) the pylons tend to act in a way similar to fences by controlling spanwise flow.6 There is less acoustic damage to the airframe.7 Thrust reverser design is comparatively uninhibited.8 Engine accessibility is good.9 There will be less overall damage in the event of a gear-up landing. Disadvantages:1 Unless the engines are mounted well inboard (which itself would spoil the wing bending relief provided by more generous spacing) the asymmetric yawing moment following failure is high: this demands good rudder control and inflicts, comparatively, higher critical speeds.2 With pods below the wing, roll freedom on the ground is limited.3 A low thrust line can have an adverse effect on longitudinal control (the nose down pitch on reducing thrust).4 On a swept wing four-engined type, the reversed thrust flow from the inners upsets the intake flow on the outers; this necessitates early cancellation of reverse on the inners.5 The low mounted engines encourage ingestion from the runway surface.© Mounted in, or in pods on, the rear fuselage:Advantages:1 The passenger cabin can benefit markedly from the reduction in engine noise.2 The asymmetric yawing moment following failure is small because the thrust lines are close to the centre-line.3 The wing design, freed from the restriction of engine mountings, can be optimised in terms of lift and drag throughout the flight envelope.4 An odd number of engines can be accomodated.Disadvantages:1 Passenger accomodation is reduced for a given fuselage length.2 Engine malfunctions are more likely due to the disturbed flow from the wing at high incidences.3 Care has to be exercised in the routeing and the protection of the fuel lines from the wing to the engines.4 The engines are well behind the C.G.; this gives the unloaded aircraft a very aft C.G. and in order to get the left near the C.G. the wing hasto be set well aft: this in turn (for a given tail arm requirement) requires a larger tail.5 The basic structure weight is higher because the wing itself has to be stronger (without the bending relief provided by wing mounted engines) and the rear fuselage has to be strengthened, both to support the engines and to take out through the fin structure the tail loads from the high set tail.6 the tailplane usually has to be set high up to be clear of the engines involving, in some cases, severe problems in terms of stall qualities.7 Where a third engine is centre-line mounted in the rear fuselage there can be difficulties in

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I didn't see this explanation so I thought I would add it to the mix. One advantage of tail mounted engines has to do with engine out conditions. When planes had 4 wing mounted engines, losing an engine did not affect the directional control as severely because the remaining engine on that wing could compensate for the loss of thrust. When the A/C has two wing mounted engines and one is lost the aircraft is going to be pushed severely in the direction opposite the remaining engine. In order to compensate for this aircraft designers had to design a rudder section large enough to keep the plane flying straight with one engine. Just compare the rudder section on a 737 versus a MD80. With tail mounted engines the loss of thrust has a far less significance as it impacts yaw on the plane. At one time, planes were designed in such a way that catastrophic failure of a single control surface would not render the plane unflyable. This design approach has more or less gone out the window with aircraft having two wing mounted engines. Their rudders are huge in order to compensate for an engine out condition, but, their huge size can cause severe problems. This is a partial factor in the two 737s that crashed several years ago because of apparent uncontrolled rudder movement.

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Just after posting my reply I read the post by sevenseas and he does touch on this subject.

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