Why a T- Tail?

[Teroc]

Ive been trying to find this answer for a while now.
When designing aircraft what are the main reasons for choosing a T- Tail over a conventional?
I realise with something like the 727 youve no real choice due to the proximity of the engines but are there any aerodynamic resons to choose one over the other.
Also what are the advantages/disadvantages?
Any help hugely appreciated.

Happy Xmas,

Teroc

[Genghis the Engineer]

Under normal circumstances a T-tail is completely free of any flow affects from the wing and fuselage, thus giving much more reliable and predicatable characteristics.

The problem with that is that at the stall, some T-tailed aircraft find the elevator getting blanked by wing downwash, with sometimes disastrous consequences.

G

[DABHAND]

Agree with the above. Also, with a T Tail the arm is greater, thus causing a greater pitching moment for a given deflection of the control surface, therefore the aerodynamic surface can be smaller, thus less drag.

[Jim lovell]

Does anyone know if having a T-Tail would be beneficial to STOL capability?? Most of the jet a/c with tails- 727,F-28,BAC-111,Trident,DC-9, BA-146 etc etc were all designed to be operated into and out of short strips in fairly hot and high conditions. I guess it could help due to the increased moment arm- anyone know??

[Oz_Pilot]

Another minus is the extra structure required to put it up there.

[Air Conditioned]

Not necessarily, Oz_P, the fuselage can be shorter if the tailplane is mounted on a swept fin which extends the moment arm. Have a look at the plan view of any of the aircraft mentiond. This also reduces wetted area and therefore drag. I would also expect that the designer gets more freedom to shape the back end the way he wants it.

Rear engine aircraft probably have high tails by necessity, and high wing A/C also e.g. 146, C-141 and Galaxy, the former structural and the latter aerodynamic.

It has also been said that the tailplane provides an endplate (winglet to regie spotters ) effect to the fin, improving efficiency of that surface and the rudder as well.

The Beech 200 may perhaps illustrate the advantages in that it seems to have a minute tailplane mounted well aft.

[FL390]

The trouble with a T-tail is that when the aircraft goes into a stall, the terbulent air created by the stall from the wing flows over the control surfaces on the tail, rendering them ineffective! Also, aircraft such as the 727 have their engines at the back; the air stalls them as well!:-(

So, if you create a stall with a T-tailed aircraft, get out of it EXTREMELY quickly.:-)

[This message has been edited by FL390 (edited 23 December 2000).]

[Genghis the Engineer]

Whilst I originally made that point myself, I'd suggest you make an unnecessary generalisation FL390. Some T-tail aircraft will do that, not all. For example there are well documented early problems with the original BAC 1-11, but I don't recall ever seeing problems at the stall with the series 500 aircraft we had on ETPS.

G

[Jim lovell]

I believe the Deep Stall happens in two stages. A T-tailed a/c like the 727 can be "conventionally" stalled just like any other aircraft- but if one does not recover from this "conventional" stall quickly the aircraft then enters the Deep Stall from which recovery is impossible. This is what destroyed the BEA Trident after take-off from Heathrow in the 70's- aircraft did not recover from stall and thus entered a deep stall.

[Air Conditioned]

It is a bit difficult to say what each aircraft will do. Several US machines had no stall protection on home ground but had pushers fitted to operate on the UK register. It was said that the BAC 1-11 had the deep stall designed out of it after the prototype was lost early in testing, but it had stick pusher. Fokker test pilots said the F-28 would not deep stall.

Re the Trident ex Heathrow; it actually recovered three times from the stall induced when the leading edge droop was retracted much below proper speed. The deep stall occurred when the stick pusher was incorrectly dumped.

[Flyerman 11]

The very first BAC 1-11 prototype was lost to a deep stall during flight test, the design was altered to include a stick pucher. Much attention is given to avoiding stalls during 1-11 training, this must work, since I never heard of another 1-11 being lost to a deep stall.
Aerodynamics change from one design to the next, each particular design is (supposed) to be equipped with the type of stabilizer that best suits it´s own particular flight envelope.

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...V1, Rotate!

[SevenFiftySeven]

I seem to remember seeing an early pre-production small model of a 767 with a t tail. Was a t tail seriously considered on the 767 and if so, why didn't they go with it?

Also Flyerman 11, I seem to remember reading about a bac 1-11 accident over staines where the captain had a heart attack during climb, the leading edge 'snoots' were retracted too early causing the aircraft to stall. The co pilot couldn't recover from it and it crashed killing all on board. I think this was late 70's or early 80's. Is this a different accident to the one you mentioned?

[Air Conditioned]

Flyerman, your second sentence could be ambiguos. Much attention is given to avoiding stalls in all aircraft, not the least jet transports of all ilk!!

The 1-11 could be stalled in training and it was required for C of A test flights. The Flight Manual contained the authorisation and procedures and the aircraft was taken to what amounted to the fully developed stall. This stall, used for certification speeds and etc, was defined by the Stall Identification System.

[This message has been edited by Air Conditioned (edited 27 December 2000).]

[Air Conditioned]

Sevenfiftyseven, our postings crossed, so I am back again.

What you may have seen was the original proposal for the B-757. This was a rewinged 727 with two engines, mounted underwing. Originally it retained the nose and tail of the 727, i.e. pointy three crew cockpit and the T-tail. With two-pilot cockpits becoming almost certain during development the 727 fuselage was redesigned to have a 767 - compatible cockpit and along the way it gained the low set tail. Presumably Boeing sought common ratings, initially 727-757 and then 757-767.

The accident you describe was not a 1-11. It was Trident G-ARPI which has been referred to above. Memory suggests it was 1975. The report is included on the UK AAIB website.

[This message has been edited by Air Conditioned (edited 27 December 2000).]

[Jim lovell]

When McDonnell Douglas was flight testing the MD-90 the test pilots flying the aircraft had to have parachutes on board in case the stall was taken too far and it entered a deep stall.

[Genghis the Engineer]

No test crew would ever contemplate conducting the stall tests on a new type without parachutes (worn not carried) or some equivalent means of escape.

G

[reverserdeployed]

Parachutes - really? If you got into a deep doodoo stall would you really have time to use them? Surely it would take a long time to get to the F/D door, get it open, get to an exit, jettison the door and jump out with sufficient height left for the parachute to open?

BTW, didn't one of the Concorde prototypes have an escape chute?

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"Something hit us - there's nobody left to fly the plane!!"

[Genghis the Engineer]

Means of abandonment, including drills on the ground, are always part of the work-up for a stalling trial. Yes, the prototype Concorde had an escape chute, I've seen photos of it being tested with dummies.

Stall tests are always (at least initially) done high enough to allow safe abandonment, and a mandatory abandonment height will be part of the sortie brief. (I recall doing Tucano spinning trials, the brief was that at x,000ft I would eject from the rear to try and recover it for the chap in the front to bring the a/c back - thankfully I never had to).

G

[Dockjock]

Some of the other advantages of a T-Tail are especially applicable to smaller jets. Basically it goes like this:

Small jet> short landing gear> must have aft-mounted engines> this necessitates t-tail

Engines on underwing pods are out of the question due to ground clearance. Also, since the fuselage is much shorter there is less yaw stability in an eng. failure so it is beneficial to have the engines mounted closer to centreline.

I've also read that aft-mounted engines (basically when discussing aft engines a t-tail is a given this is why I bring this up) allow greater fuselage interior space, and a quieter cabin but I'm not sure I can see that reasoning. I am not an engineer, but can anyone sort this out?

[Tallbloke]

Jim lovell,
I thought the primary factor in the Staines crash was the captain retracting the droops instead of raising the undercarriage whilst having just suffered a heart attack. The complete lack of CRM prevented the more junior members of the crew from doing anything about it until far too late. The stick shaker and pusher both activated but capt. disabled them. I think.
http://www.open.gov.uk/aaib/garpi/garpi.htm#Conclusions

[This message has been edited by Tallbloke (edited 30 December 2000).]

[Genghis the Engineer]

Dockjock,

A T-tail should free up a little space up the back since there's no need to put a tailplane mainspar through the cabin.

As to noise, I think the use of a T-tail in itself won't make much difference - maybe a little because the fin will act to insulate any tail noise from the fuselage - but frankly trivial. More significantly, T-tails often go with rear engines, which are significantly quieter in the cabin (compare, say, cabin noise in a VC-10 and 707).

G

[Roger Turbojet]

Because an aircraft has rear mounted engines, or even an tailengine (like the 727) is does not nesecerly need T-tail, just look at the L1011 Tristar. In each case, you get some bennefits, but must pay at another counter, so to say. Example, you get more rudder for less weight useing fuselage mounted stabilators, but you pay at the drag counter. Useing T-tail give better flow bennefits, but you pay at the stall "counter" Most T-tail aeroplanes have shorter fuselage from wings to stabilator, proberbly because the extra weight of the engines only allows a certan length. If designed with fuselage mounted stabilators, you run into flow problem, because the stabilators are to close to the wash from the wings. So putting them higher solves that problem.

One is not better than the other, it just depends a lot on how the aeroplane is designed.

[This message has been edited by Roger Turbojet (edited 30 December 2000).]

[twistedenginestarter]

Air conditioned.

The Trident sufferred a superstall loss during development. The BEA GARPI incident at Staines was as you say caused by the stall prevention system being over-ridden. It didn't recover three times from a stall. It three times went into stall prevention mode which would have saved the plane if only they'd have let the computers do the flying. Once the system was dumped it immediately went into a stall. Just the one was enough.

In this style of plane if you stall you are dead. You can't recover. Well maybe from 50000 feet if you are lucky.

Reverser deployed

A parachute on your body is not much use but I think they normally put one on the tail to pull it back up once it has gone into a stall - for development only of course.

[Mattvitale]

A question. What effect would a mid-mounted tail have, such as is found on the Sabreliners and Falcons? I would assume that they would still suffer from the same deep stall charicteristics, just perhaps a little harder to develop?

[Genghis the Engineer]

What's actually significant is the combination of the relative heights of the mainplane and tailplane, the distance between the two, and the downwash angle from the mainplane.

The design team SHOULD ensure that mainplane wash effects over the tailplane are ALWAYS minimal.

G