T-tail aircraft

[Darlick]

I've tried searching without much luck for some info for a possible future interview. I'm trying to find out what the advantages and disadvantages are of t-tail aircraft and also aircraft with rear fuselage mounted engines.

I appreciate its a fairly basic question but my old ATPL notes don't seem to cover it much in any detail. Any help would be greatly appreciated.

Thanks.

[Rainboe]

There are lots of resources for this. Have you searched here properly, have you tried googling it? It is covered in Handling the Big Jets. I am suspicious you are going the lazy way and just asking everybody else to do your work for you!

[Confabulous]

Should be reasonably easy, even though I'm not an ATPL...


Advantages for engines at the rear:

1. Less worrying about CG issues, since the heavy stuff is already at the rear

2. Less wires, control runs etc

3. Engines are easier to replace (everything is grouped together)

Disadvatages:

1. Compressor blade loss could take out both engines and damage the tail if severe enough

2. Thrust changes on approach work the wrong way, i.e, more thrust = pitch down, less thrust = pitch up

3. More dangers for overrotation? (Just a thought)

4. Needs a T-tail design to avoid melting bits off the aircraft

Advantages of a T-tail:

1. Tail is in clean air

2. Can be a single unit, therefore can be stronger

3. Can be set further back from the CG, improving pitch authority

Disadvantages:

1. Possible pitchup following a stall, could be unrecoverable

2. Previous trait requires more effort designing the flight control system, wing mods etc

[Biggles' Apprentice]

Confab has covered most things but I'd also add:-

Advantages:-
Completely clean wings
Cabin noise is lessened- happy SLF!
Also...very minor but greater margin of error for crosswind landings as no low slung pods.

Disadvantages:-

Deep stall (arse down):- In T-tail designs, the turbulent wake of a stalled main wing "blanks" the horizontal stabilizers, rendering the elevators ineffective and preventing the aircraft from recovering from the stall. Stall issues and overotation are also compounded by the fact that in a pitch up attitude it is possible for the wing to also 'blank' clean airflow through into the engines, thus preventing using thrust for recovery and lessening power. I think a 1-11 was lost from this issue.

Also fatigue can be compounded by the 'lever' weight of the epenage and engines over a period of cycles. DC-9/MD-80 family have experienced this I believe.

You pays ya money and you takes your choice!

[Rainboe]

You lot have got to be stopped before you do any more damage!
<Advantages for engines at the rear:

1. Less worrying about CG issues, since the heavy stuff is already at the rear
Nonsense- heavy weight at the back makes load and balance more critical
2. Less wires, control runs etc
Nonsense

3. Engines are easier to replace (everything is grouped together)?
No. Watching an engine change on VC10/707- I think easier on a 707. The VC10 engine is higher and harder to get at.

Disadvatages:

1. Compressor blade loss could take out both engines and damage the tail if severe enough

2. Thrust changes on approach work the wrong way, i.e, more thrust = pitch down, less thrust = pitch up
Nonsense. The VC10 had no thrust/pitch change, the 737 has plenty

3. More dangers for overrotation? (Just a thought)
Why?

4. Needs a T-tail design to avoid melting bits off the aircraft
What?

Advantages of a T-tail:

1. Tail is in clean air

2. Can be a single unit, therefore can be stronger
There isn't much difference from a 707/737 tail which has a carry through structure.

3. Can be set further back from the CG, improving pitch authority
Engines at the back puts the CG further aft, so you need all the aft positioning you can design in
Disadvantages:

1. Possible pitchup following a stall, could be unrecoverable
That's why they have 'anti-pitch-up protectors' (stick push)

2. Previous trait requires more effort designing the flight control system, wing mods etc
***************
Confab has covered most things but I'd also add:-

Advantages:-
Completely clean wingsBut to pay for that, you have lost all this weight from the wings and put it on the fuselage, so lose all that wing bending relief. You need structurally stronger and heavier wings.
Cabin noise is lessened- happy SLF!
Also...very minor but greater margin of error for crosswind landings as no low slung pods. So the DC9 without those engines under the wings has a nice cheaper shorter undercarriage making the wings closer oto the ground so you are just as likely to scrape a tip or flap track! No better than a 737!

[TyroPicard]

Nice one, Rainboe - over 100 years of aviation and it's still bl**dy complicated!

TP

[ROSCO328]

NICE 1 RAINBOE

Amazing the crap some people talk!! Guys if your gonna help someone out atleast look up the correct info.

[World of Tweed]

Forgive me if my eyes deceive me but no has mentioned the BIG advantage of less yaw during engine out ops?

With pods close together (laterally) down the rear any yaw induced due to e.g. a V1 cut, will be markedly reduced compared to a underlsung-out-on-the-wing jobby.

Mucho Advantago!! Less yaw = less rudder required/introduced to airflow = less drag on the single engine.

Also (I minor Geekee point):

The T-tail by its nature will 'end-cap' or 'fence' off the end of the fin. By doing so it limits any tip vortices/marginal spanwise flow and improves the effectivness of a fin of a given length thus allowing a slightly shorter fin. There is also potential for an improvement to induced drag = fuel saving. So much so that Boeing toyed with the idea for the 757 to have a T-tail config with its current underslungs....but it must have looked stupid and I guess that old phrase 'looks right - flies right' got the better of them.

This is however only derived benefit and perhaps not a true 'driver' for the confirguration in the first place. In fact it is only by moving the engines from the wing that 'forces' the desginers to move the stabiliser in the first place.

From what I've read on the subject the T-tail rear engined designed aircraft all had the following Primary Design Driver in common:

They were all intended for regional/rough strip/commuter or high frequency ops with the minimum of infrastucture at airports driving the following:

>They needed to be self supporting(DC9/BAC1-11/F28),i.e. airstairs, low hand loaded baggage holds - short turnarounds which meant they had to be LOW to the ground = too little clearance for a traditional underslung engine design - not without forcing the engines to become part-time sandblasters. And if you can't put the engines under the wing and you have to put them on the rear fuselage...then where do you put the stabiliser?....on top of the fin......

>Also whilst operating from little used/'Empire' strips/commuter towns it made sense to mount the engines so that they would be effectively shielded (in part) by the main plane from FOD/gravel. This also conveniently then made them prone to ingesting less than silky smooth air from the wing which was an undesired consequence but they try to iron it out with more carefull geometry on subsequent designs. (altough the MD-80s used to munch down fuel Ice off the wings with alarming frequency in a certain air-touring airline I believe).

Even the Biz jets of today seem to be driven by this need to be low to the ground which then drives the rest of the configuration. Perhaps its not so much the case of advantages or disadvantages because in many ways t-tail rear engine is a pain in the arse......its more a point of what they need the aircraft to do and be good at.

Thats my take on it....but as for the VC-10...I think it was the pursuit of beauty that scultped it that way! ;)

Final Geek-fact - the original EMB145RJ design was an t-tail but with the engines mounted ABOVE the wing in pods and that did look stupid.

[seacue]

One should note that one of the very early rear-engined jets didn't have a T-tail.
http://www.airliners.net/open.file/962509/M/

[enicalyth]

The Tee does have an "end cap" effect and I believe that (discounting any top fairing) cutting down the number of roots fillets to two from four in the cruciform conventional tail is beneficial in drag terms or is that saying the same thing twice but in a different way? Having rear engines is a bit of a pity having to add structural strength exactly where the designer was hoping to finish off with a light touch. The rear-engined conventional tail jobs might have had more to do with stick shaker controversy and, poisonous thought, if someone comes across with a thoroughly bad attitude why not let the interview board in on the secret too. After all, once you can fake sincerity the world is your bottom feeding bivalve. Only half-kidding.

Tea break's over, back on yer heads

the "E"

[MileHigh76]

Just incase i missed on this, when reading thru'. A T-Tail design and rear mounted engine do require beefing up of the Tail structure, which ofcourse increases weight and design complications. Reference to the MD8X (alaska air if not mistaken) crashed off the west coast of the US after a THS screw jack problem, and a Movie which was circulated on the internet of an MD/DC xx during flight testing which experienced a total loss of the tail section on touch down-with a heavy landing.( i would suppose this happened before the beefing up took place.

[enicalyth]

G'day Mile High!!

Beefing up?

If something has to be beefed up my understanding of the term is that for some reason it was not structurally sound enough at the first attempt. Not the same as a different design process.

With rear-mounted engines the wings tend to experience more bending moment. Lift force bends them upwards and in a conventional wing-podded design the engines bend them downwards.

Without podded engines the wing is cleaner but will experience more bending moment "M" unless someone does something about it. For simplicity sake imagine a wing beam to be a solid rectangle. Its moment of inertia "I" or ability to resist bending is proportional to breadth and the cube of depth. Stress "f" is inversely proportional to the distance "y" from the plane of no bending called the neutral axis. Which is smack in the middle for a uniform solid beam. And the radius of bending curvature "R" is inversely proportional to a mechanical property called Young's modulus or "E". For simple elastic design M/I = f/y = E/R giving the designer several options to counter, for example, increased bending moment without increased stress and/or physical bending without too much sacrifice of competitive edge in weight and aerodynamics. Not so easy at the tail however where you now apparently want to hang the engines.

Make all the due allowances for the shape of real wing spars and the stress engineer can work to some compromise with the aerodynamicist and obtain a slightly thicker wing perhaps which truth to tell was beginning to happen as successors to the peaky airfoils were developed. But at the tail end of the aircraft is a vanishing point and consequently both stressmen and aerodynamicists want to be slimming the shape down not bulking it up. Sadly the solution usually means stronger and heavier materials going into the tail than would otherwise have been put there... if that is where you want the engines to be. Building a physically stronger tail might allow you to do other things with it but certainly now that it is heavier every little trick to reduce drag is crucial not trivial.

Neither situation of wing and tail is really a beefing up more a penalty of going down the chosen design path. Richard "DC-9" Shevell was happy to admit Doneless wanted to look different from Boering and he acknowledged that there was also a thing called "fashion"!

My father went through both brain drain in forward and reverse, leaving de Havilland for Douglas where he freely admitted jumping from frying pan to fire, and then back, this time to Airbus to tear out what hair was left!

Significantly he was not a Tee-tail aft mounted engine proponent so make of that what you will. Airbi still have the 5.4 metre diameter fuselage and an invisible "sea anchor" holding 'em back but of course the spin on that is "optimisation". Not my words but my old man's - "Optimisation my a**se". Tacitly he envied Boering their better aerodynamic research and was scathing of what became known as the :mad: :{ [expletives deleted].

I happily plied along in Boering products and Lord do I miss feet up "smokoe" with the old chap. He had a great life, him and all those others, and in the days when your job title was written in your passport, dad's said "Rocket Scientist"!

Have fun! If this ramble has helped pass the time and learn a little good-oh. If not, well I'm old meself now.

The "E"

[Thunderball 2]

Ramble with you anytime, "E".:ok:

[fortuna76]

Did somebody mention less noise in the cabin....:confused: Did you ever sit in the back of a DC-9? I had the experience a few years ago and I am still recovering from hearing loss. My worst ever experience on an airliner, what a bloody noise :{

[Tony M]

In terms of CG, rear mounted engines means the main wing is mounted further back in the fuselage, and in most cases, a small rear hold with a small moment arm. Forward of the wings and CG, are bigger holds with a much larger moment arm.

In practical terms; an MD87 needs about 200kg of balast in the nose when empty, but when full, the front hold cannot be fully utilised in terms of weight without cg going off the front of the envelope.

MD81/82/83 were better for some reason even though the long fuselage created bigger moment arms.

Most airliners were designed with a relatively light front SLF cabin for first/business class ops, but high density Lo/Co ops mean that many of 100-150 seaters used need lots of weight in the rear holds.

[Rainboe]

Fortuna, the very rear rows of a rear engined jet are noisy, but a bit further up the cabin it can be incredibly quiet. I flew on an AA MD-something (stretched modern DC9) and couldn't believe how quiet it was- far quieter than the rear cabin of a 737 with the jet blast. The biggie was sitting in the rear cabin crew seats on a 737-200. The 2 stewardesses were communicating on take-off with hand signals (women never stop gossiping), and I was looking at them in alarm at the sheer volume of racket- and that after piloting them for 6 years- I had no idea they were so noisy in the rear.

[Tony M]

Fortuna, the very rear rows of a rear engined jet are noisy, but a bit further up the cabin it can be incredibly quiet. I flew on an AA MD-something (stretched modern DC9) and couldn't believe how quiet it was- far quieter than the rear cabin of a 737 with the jet blast. The biggie was sitting in the rear cabin crew seats on a 737-200. The 2 stewardesses were communicating on take-off with hand signals (women never stop gossiping), and I was looking at them in alarm at the sheer volume of racket- and that after piloting them for 6 years- I had no idea they were so noisy in the rear.

I remember the old KLM DC9s that were VERY loud in the back, but the back of most MD80 series are much better, I think because of better engines and mounting technique.

Rainboe, did you notice how much the chuff moves about compared to the sheepskin seats, especially in yaw?

[Rainboe]

Could never keep the 200 pointing the right way- it used to wonder off something horrible on final approach. You'd always see them waggling the wings on approach- it wasn't me. I believe it was something to do with the BA ones and higher aileron response for the autoland system.

[barit1]

In Connie/DC-6/B377 days, first class was in the rear of the cabin - quieter than sitting in the plane of the props.

There's a reason 1st was moved to the front in jets.

[seacue]

barit1

I don't think that was universal. I recall sitting at the very rear of a Capitol Airlines Connie on a stormy night flight in 1958/9 from MDW to DCA. All the cupboard doors were rattling and I could swear I could see the fuselage twisting. I wasn't a very good flier/SLF back then. My typical route was Peoria to MDW on Ozark Super DC-3 and then UA/AA/Capitol to DCA.

seacue