What Happened to the Aft Engine Mounted "T" Tail Design?

[L-38]

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?

[The Range]

I was thinking the same thing. But business jets are still being built with the engines in the rear.

[AtoBsafely]

Off the top of my head:

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.

[protectthehornet]

Boeing happened. boeing bought Mcdonnel Douglas and started to phase out the DC9/MD80 etc.

I had the rare opportunity to watch a brand new MD90 take off from KDCA...the sound level was so low, the pitch so high...it was magnificent.

I've piloted both the DC9 and the 737...no comparison...the DC9 is much nicer to fly, much quieter in the cockpit. Nicer on the controls.

while there is an advantage in the way the wing is loaded with engines on the wing, it is easily overcome by the beautiful, clean wing design, simpler leading and trailing edge devices as there is no engine to interrupt the flap.

I understand DELTA airlines is looking to buy every MD90 they can find as they have exceptional performance in some areas needed. Indeed, DELTA still flys more than 30 DC9's as they can find nothing that can truly replace them.


The DC9 series doesn't change pitch with power changes. IF you add power on a 737, the nose goes up, cut power and it goes down.


ITS all about money...and NO one on the ground has been sucked into a DC9's engines...but a number have been sucked into the 737's.

[barit1]

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.

[AdamFrisch]

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.

[barit1]

slam_click: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.

[barit1]

AdamFrisch:anything more than 2 was allowed to fly trans-atlantic distances over water/without alternatives.
And then there's this -http://farm3.static.flickr.com/2265/2247189364_7a42383bcb.jpg

...and of course the VC-10

[Dont Hang Up]

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?

[wiggy]

top mounted wing pods! Maybe the future?

As in back to the future?

VFW-Fokker 614 - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/VFW-Fokker_614)

[FlightPathOBN]

or the IL-62...

https://encrypted-tbn2.google.com/images?q=tbn:ANd9GcRzy3BkZonNpiLqZLeUCnD6KtS2IDa3nD_gEt6o0Zs gxLkZkD6OLA


love the interior!

http://cdn-www.airliners.net/aviation-photos/middle/9/0/6/1508609.jpg

[henra]

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.

[pattern_is_full]

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.

[TurningFinals]

(out of interest)

Did rear mounted engined aircraft ever have a problem with tipping whilst on the ground? Too much weight in the back during loading etc?

[Easy Street]

http://www.vc10.net/History/Images/VC10-Incident_small.jpg

Incidents and Accidents (http://www.vc10.net/History/incidents_and_accidents.html#XR806%20Brize%20Norton%2018%20D ecember%201997)

[TurningFinals]

That'll be a yes then.. Maybe a reason why the design isn't as popular anymore?

[FlightPathOBN]

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.

[Loose rivets]

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.)

Qantas 'Connie' Super Constellation Takeoff with flames (1080p HD) - YouTube

Much checking of load sheets by a very worried looking skipper.

[DaveReidUK]

Did rear mounted engined aircraft ever have a problem with tipping whilst on the ground? Too much weight in the back during loading etc?

I seem to recall the IL-62 having huge C of G problems.

It had the rear 20' or so of the cabin blanked off so that there was no payload located there, but even so it still needed its pogo-stick:

http://www.abpic.co.uk/images/images/1305176M.jpg

[Groundloop]

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.

[Owain Glyndwr]

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.

The VC-10 had a CLmax of 2.3, but the A320, from the same Weybridge design team achieved CLmax 3.2, albeit with a lower sweep wing, so it isn't obvious that wing mounted engines have worse high lift capability.

Going back to the original question, a couple of other considerations would be:

Susceptibility of T tails to being blanketed by wing wake at high AoA (deep stall problems), and
Great difficulty today to get clearance for the disc burst case where T tails with engines closely mounted opposite each can rapidly turn into a glider with no hydraulics or electrics.

[Volume]

Another important aspect is the position of the cabin and baggage compartments CofG in relation to the wing. Having the engines far aft you need to position your payload more to the front making the aircraft tail heavy at low loading and nose heavy at full load. With wing mounted engines this is much more ballanced.

The Trident had the forward bulk cargo compartment (requiring asymmetrical nose gear) to give more flexibility in placing the payload.

I seem to recall the IL-62 having huge C of G problemsThey call it differently ;)
Illushin claims that a major disadvantage of the VC-10 compared to their IL-62 was the position of the landing gear too far aft of CG producing high nose gear loads and requiring a larger tail for rotation. The IL-62 uses the pogo stick when empty and is balanced perfectly when loaded. But maybe that is just a good excuse :hmm:

[andrasz]

The IL-62 uses the pogo stick when empty and is balanced perfectly when loaded.

The Il-62 also had a water ballast tank up front that could take as much as six tonnes, to make it flyable with low payloads.

Bit off topic:

Recalling some hazy memories from my early career, Aeroflot used to oprate some weird fortnightly Il-62 services to west african destinations like Brazzaville, Bissau Guinea, etc. via BUD, being the last friendly port of call to uplift fuel. They arrived in the small hours with 8-12 grim looking guys in black leather coats as passengers, and usually there were a few green painted long wooden boxes in the hold that we were not allwed to go near to. They came in with a light fuel load from Moscow with the ballast tank always full, and left a mighty puddle on the apron before departing with full tanks...