What Happened to the Aft Engine Mounted "T" Tail Design?
Today, jet transport manufacturers seem to be exclusively manufacturing medium range airplanes that feature a wing pylon mounted engine design. Clean "T" tail designs that have worked in the past (designs such as the MD-80, B-717, B-727, TU 54, Trident, VC10) are not being improved, and such a design style seems to be now becoming abandoned to history.
Could the sleek DC-9/MD-80 have been better engineered to outlive that of the ubiquitous B-737 and A-320?
Fuselage engine mounted "T" tail airplanes are quieter, not as susceptible to pod strikes or FOD, and have better single engine asymmetrical performance. . . a clean wing looks better too!
Even Embraer has switched to a wing mount design. . . . so why are such "T" tail designs a sudden historic thing of the past?
Engines under the wing help with the spanwise loading, allowing a lighter wing structure. The engines are also very accessible.
Aft mounted engines mean the wings need to be further back on the fuselage, reducing the moment arm for the empennage (larger tail surfaces needed). Also, more noise in the cabin. And fuel lines from the wings to the engines.
For larger aircraft, the aft engine design becomes less attractive.
BTW, you'll never get a pod strike on a twin with the gear close to the engine.
Last edited by AtoBsafely; 21st Jul 2012 at 16:13.
Reason: Afterthought
Heavier aft fuselage structure, i.e. longer load path from the mass of the engines to the source of lift.
Additionally - disturbed airflow over the top of the wing root gets swallowed by the engine inlet, leading to inlet distortion and operability issues. Aircraft from the Caravelle to the A-10 (& probably more) experienced flight test problems because of this.
The main reason back in the day was that ETOPS didn't exist, so anything more than 2 was allowed to fly trans-atlantic distances over water/without alternatives.
Low bypass vs High bypass engines would also be a large reason.
I wouldn't draw much of a distinction there; The Canadair RJ's engines (CF34) are bypass 4 or more. The TF34 (basically the same engine) in the A-10 likewise.
The picture above is an excellent illustration of the main reason.
That must be a very strong structure to carry those engines and transfer their thrust to the airframe. And so must add considerable weight.
Why not hang the engines from a horizontal structure that you already have i.e. the wings? And then indeed (as has already been mentioned) the wings need less strength because the lift that overcomes the weight of the engines does not need to be transferred to the airframe. The engines are also very close to their fuel supply.
One begins to wonder how rear mounting designs survived as long as they did.
Of course for business jets there isn't sufficient clearance under the wing so they continue with the rear mount approach. Except I seem to recall seeing a picture of a light jet (Honda?) with top mounted wing pods! Maybe the future?
One begins to wonder how rear mounting designs survived as long as they did.
Of course for business jets there isn't sufficient clearance under the wing so they continue with the rear mount approach.
That plus reduced noise levels in the rear part of the cabin. Otherwise I see not a single real design advantage of the rear mounted engine. Besides the additional required mounting structure and the weight of the engines fully adding to the moments on the wing box, tail mounted engines usually require T- tails or at least raised HS, further increasing the weight penalty. The other issues like longer fuel lines, unfavourably long forward fuselage or disturbed air entering the engines have already been mentioned. Therefore I fully agree to your first statement in the quote.
Tail-mounted engines are more protected from runway FOD on unimproved surfaces, thus their use on Russian airliners and the A-10 (and may have figured in Canadair's thinking, for use in the "Siberia of the West").
And of course the "three-holers" had to have at least one (L-1011, DC-10, MD-11) or more (727, Trident) engines in the tail anyway, as a stepping-stone before engines were powerful enough for two to be sufficient on heavier long-range planes.
With wing-mounted engines, the wing root carries less weight load in flight - but more thrust loads.
There was a design "cachet" to T-tail designs in general in the 60's + 70s, beyond simple functionality. They had that F-104 "Starfighter coolness", even if they were pedestrian planes otherwise (Tomahawk, Skipper, Seminole, Duchess).
But absent any of those "special needs", two or four under the wing is likely the pattern for the future except for small planes that don't have room there. Or have one engine.
Last edited by pattern_is_full; 23rd Jul 2012 at 21:35.
yes, tipping over, and as with the L1011, the upper engines were more difficult, ie, checked less for ice buildup, and accordingly the associates issues.
Last edited by FlightPathOBN; 23rd Jul 2012 at 23:55.
Location: A Whilom nimble brain. With 31 million posts.
Age: 73
Posts: 3,381
That's reminded me. Early to mid 60s, one of the most beautiful aircraft of all time sat on her tail at Liverpool as I was looking straight at it. Such was the speed of rotation, that an engineer on a nacelle was thrown upwards. ( He landed on the servicing gantry and seemed okay.)
Otherwise I see not a single real design advantage of the rear mounted engine.
The VC-10 had rear mounted engines so that the entire wing could be fitted with high lift devices (ie full span slats and flaps/ailerons) to provide an amazing short field performance for hot and high operations off short runways on BOAC's African network. Wing mounted engines required gaps in the high lift devices.
Last edited by Groundloop; 24th Jul 2012 at 07:48.
