Pugilistic Animus... That was shocking!
Originally Posted by
tdracer
There are several big ones. First off, tail mounted engines are relatively high off the ground, making servicing and maintenance more difficult and time consuming. Not a big difference, but over a ~25 -30 year service life of an aircraft, it mounts up. Second, during the early design phases of a new aircraft, the engine weight is little more than an educated guess. Worse, the engines seldom get lighter - they are usually heavier, often a lot heavier. Now, if the engine is mounted on the wing - it's located near the aircraft CG, so a heavier engine is generally not a big concern to the overall design. Tail mounted engines - by definition - are way behind the aircraft CG. So when the engine gets heavier, the wing is suddenly in the wrong place. The only solution (short of starting over with a new wing position) is to add ballast to the nose - effectively doubling the weight penalty. According to people I worked with that worked on the MD-90, this hurt the MD-90 massively. After the basic design was frozen, the FAA required much better blade out and uncontained failure protection - resulting in a massive increase in the engine weight. Between that and the associated required nose ballast, the resultant aircraft was so heavy that it lost most of it's fuel burn advantage from the new engines (relative to the MD-80).
Structurally, putting the engines on the wing has significant advantages relative to the tail. I've been told there are some aerodynamic advantages as well although I admit it's not obvious to me what those might be.
Very interesting as always,
tdracer. Your point on the aft CG is only too pertinent to the VC10, as it was to the One-Eleven. Empty ferries were always a problem. They usually happen after an unplanned diversion to an aerodrome where the airline has limited facilities, and typically involve a short flight to the original destination. Loading fuel into the VC10's centre tank as forward ballast could be a hostage to fortune, and ballast is not necessarily available. The usual expedient was to gather as much of the remaining catering and its canisters into the forward galley. A CG near aft limits played a part in the accident that caused the demise of the VC10 ex-prototype, G-ARTA, landing at Gatwick on a positioning flight from Heathrow..
In addition to the weight of the engines, sticking them on the rear fuselage pushes the tail-plane/horizontal-stabiliser (referred to on the VC10, oddly, as the TPI) upwards. On the Caravelle, it went about half way up the fin, but on most aeroplanes to the top. So, on the VC10 the "bullet" that housed the pivot point and the top of the screw jack was around 38 feet above the ground.
Am no expert on structures and aerodynamics, but there are further disadvantages for tail-mounted engines in both those areas. For the
non-cognoscenti:
1) Aircraft have a maximum zero-fuel weight (MZFW) that is limited mainly by the strength of the wing root, and affects the maximum payload, So - in effect - every ton of equipment or structure added to the fuselage or empennage reduces the maximum payload weight. (That also applies to fuel in any belly-mounted tank, as in the VC10's centre tank.)
2) It follows from (1) that the engines being supported in flight via the wing root is less efficient than mounting them on the wing, because - everything else being equal - the wing root has to be thicker and creates more drag.
3) Less obvious to the lay person is the aspect of wing-bending relief, a subject on which, as a non-engineer, I'm going to have to tread carefully and stand to be corrected. Pilots of big jets know (and flight engineers take for granted) that even aircraft with wing-mounted engines like to keep the outboard wing tanks full of fuel as long as possible, particularly at high ZFWs. From a structural viewpoint, it's best to place weight as close as possible to where the necessary aerodynamic lift is being generated. Emptying the outboard tanks prematurely would result in the outer wing bending upwards. Although engines are of a fixed weight, distributing them at intervals across the wing enables a lighter wing structure, as well as a thinner wing root.
There's little doubt that, versus the B707, DC-8, CV-880 and CV990, the VC10 suffered cruise penalties from having rear-mounted engines. Crews will remember the so-called aileron-upset system. At high all-up weights, it was deployed after flap retraction. The ailerons were angled up slightly to move the wings' centres of lift slightly inboard - presumably to reduce the stress on the wing root and/or the bending up of the wing tips. Perhaps someone can remind me, however, why that was no longer necessary above 24,000 ft?
To stray off topic for a moment: based on the argument I've offered in (3), above, I'm also curious about the conflicting factors of two wing-mounted engines versus four on long haul.