Pro's and Con's for a T-Tail
 

Hi,
I am trying to find out what the advantages and disadvantages are of having a T-tail like on the Embraer 145?

So far i've found: -

Disadvantages to be: -

• The possibility of a deep stall
• The vertical stablizer must be made stronger and stiffer to support the forces generated by the tailplane

Advantages to be: -

• The elevator and stabilizer are out the way of FOD being kicked up the the tires, engines etc etc.
• Allows high performance aerodynamics and excellent glide ratio as the empennage is not effected by the wing slip steam.

My question is 1) are these correct and 2) are there any more??

A T-tail does not cause deep stall, although it may well aggravate it when it happens.

The deep stall is caused by tip stall on a swept wing, which causes the lift centre to move inboard and forward.

At least incomplete LM, in theory a deep stall in a T tail light airplane is possible.
Ultimately it would depend on CoG and the momentum of the airplane

http://expertaviator.com/wp-content/...ailHighAOA.jpg
Source: FAA Flying handbook
http://www.free-online-private-pilot...deep_stall.gif

http://upload.wikimedia.org/wikipedi..._1-36_NASA.jpg

Let me google that for you

Another advantage is that the arm to the CG is longer so the elevator is more effective which means the manufacturer can make it smaller.
Smaller stabilizer and elevator means less drag.

My understanding of the deep stall was as the a/c is sinking the relative airflow now comes more from below the wing meaning you can have angles of attack of around 20 degrees with 0 degrees pitch angle. This is then exacerbated by the nose up moment from the centre of pressure moving forward.

The T-tail can make the deep stall impossible to recover from due to tail blanking, where the wash from the wing also disturbs airflow over the tailplane. This would make pitch inputs difficult/impossible.

Anyone please feel free to correct me if my understanding of this is incorrect.

Correct packo, but the aeroplane does not need a T-tail to get into deep stall.

No need for push-back (if engines is mounted at the back of course).

Oh dear- time to leave this thread.

DUH...I didn't make those pictures specifically for this thread you know.
I even mentioned the reference source.

- not necessarily those 'as illustrated' but some T-Tail a/c - ask the next-of-kin of Mike Lithgow? If you want a deep tech study (somewhat dated) T-Tail study

An advantage of a T-tail is that the endplate effect of the horizontal allows the vertical tail to be substantially (25% or so) smaller.

Another one is that the rudder is not blanked by the horizontal at very high angle of attack, making it more effective to get out of a spin.

A T-tail does not necessarily lead to deep stall tendencies, it depends on a number of factors such as wing loading, etc.

Your second listed advantage (wing slip stream) is questionable, I wouldn't list it as such.

--M

A t tail allows for placing engines on the rear end of the plane...this allows for more efficent design of the wing

it allows engines to be closer together and allows for a smaller rudder/vertical fin to compensate for engine out situations, thereby reducing drag.

however putting all that stuff on the tail makes the plane tail heavy and it must be carefully balanced up in the nose.

my favorite plane is the DC9...flys great...engines at the tail means they are slightly more protected and no one has been sucked into the engines on a DC9.


oh, and putting the engines in the tail means the whole plane is quieter.

Engines are higher off the ground, making them harder to service/maintain/remove. Uncontained failures can still sever hydraulic lines/control cables (in the tail).

If there are four engines on the tail, the engine pairs are very close to each other, increasing the risk of one affecting another.

Everyone (almost) here seems to have an opinion about a "deep stall"...

Lots of theory....all B.S. in the real world of commercial flight...

Get some experience you "puppies" and then hopefully you won't ever have to answer the "what happens in a deep stall?"

'Nuff said...

Additional advantages:

Mounting the tailplane high puts it in an area of low downwash gradient making the tailplane a more effective stabiliser and a more aft cg is possible. This also leads to the tail generally having a more positive loading and thereby reducing trim drag

The T-tail raises the tailplane out of the fuselage drag-hole which can reduce your tailplane effective aspect ratio by 20% or more.

Disadvantages:

Very messy loading and structural design.
Tailplane more difficult to clear snow off and access for maintenance and checking.
Tilting of the principle axes of inertia giving rise to coupling issues in roll.

Absolutely correct. B2N2, I don't find the definition above incomplete.
I agree that that FAA figures give a different perspective, but in my opinion they are depicting a slight different scenario, where the wing is NOT actually stalled even if close to. By design, in the real stall (with weight and balance within certified limits of course) the nose should drop forward so giving better authority to the elevators while in the real deep stall that doesn't happen, because the CG keeps moving forward to a point that the recovery is virtually impossible. Anyway, imho, i think that the same term "deep stall" is used to define 2 different situations.


FB

When it comes to military transport aircraft a T tail means you dont have to spend time worrying about parachute static lines fouling the elevator (as you have to for the Hercules).
Also I had to smile when I saw the size of the tailplane design grow on the A400M after we had supplied Airbus with data on the cg transients of a heavy drop.

Most medium to high wing loading T-tail airplanes will do this only with the help of a stick pusher and/or ventral fins, an additional complexity to take into account when selecting a T-tail. This pitching moment sign reversal with fully deflected elevator normally takes place in the post stall regime though, that's why it is also called deep stall.

Exactly.

--M

One advantage in a glider (sailplane) of having a T tail is that when doing an out-field landing it helps keep the elevators and horizontal stabilisor out of a tall standing crop.:(

It also looks pretty!

MB

T tail advantage: If the aircraft is a flying boat, it keeps the tail out of the water.

Disadvantages: Much more difficult to inspect for frost/snow/ice, and clean if required.
More complex elevator and trim control systems, and,
Depending upon aircraft type and configuration, a high tail might be out of the prop wash, which reduces pitch control on the ground (less able for soft field takeoffs).

Generally yes, though more vulnerable to snow and ice from the top of the fuselage being ingested.

When you present your thesis, don't forget to remove those silly apostrophes.

Looking at the historical use of T-tails, it seems that the primary advantage was to make room for tail-mounted engines. Which doesn't say much about the tail itself, other than - it had to be someplace the engines weren't.

(And even then, there were the L-1011 and DC/MD-10/11)

A lesser advantage - to make c. 1970-designed bug-smashers look "F-104 supersonic cool" even sitting still on the ramp. Generally, the lower the power, the more prominent the T-tail. :} Kind of the aviation equivalent of 1950s car tailfins.

The F-104 itself is worth some research - it was given a T-tail to counteract the likelihood of inertia coupling (wild gyrations on all three axes with control input) due to the long thin fuselage and stubby wings. In the event, it may have created as many problems as the one(s) it solved.

T Tail on Ground Effect Craft / ekranoplan
 

Alexander Lippisch (designer of Me163 rocket fighter and many sailplanes) used T Tails so that pitch control could be applied to prevent nose up pitch as the mainplane left ground effect. Note in GE the trailing vortices are cancelled out by the ground and therefore cause a low mounted tail to be even less effective.

Rostislav Alexeev helped develop the ekranoplan from hydrofoil boats and used T and high set V tails on the huge Caspian Sea machines:

Ekranoplan - MoscowTopNews.com

The advantages of GE are more to do with Drag reduction than any gains in lift.

The recent Gulfstream flight test crash report noted a reduction in stalling AoA for the main wing due to a reduction in the maximum Lift Coefficent in GE.

www.ntsb.gov/doclib/reports/2012/AAR1202.pdf

A real inconvenience at remote northern locations such as Canada/Alaska where normal de-ice facilities don't exist. Expensive solutions are very high and expensive and incovenient tall, mobile stairs stuctures or powered, vertical lifts to enable access for de-icing.

And the only real large turboprop options are T-tail such as the ATR and DHC-8.

With over 1000 hrs on the Dominie (HS 125) I can state with certainty that it did NOT have a stick-push system.

We regularly stalled the aircraft clean but only went to the calculated stick shaker speed / actual stick shaker (whichever was the higher) in the Approach Configuration - Gear and Approach Flap.

There was some discussion from Multi Engined Standards Sqn that a fully developed approach stall would be quite nasty.

PEI,
The reason for the T-tail in the 146 has more to do with the fact that it was based on the HS681, which was a VSTOL military freighter originally designed to support the Harrier II (P1154) fleet. The T-tail came with the usual military aft cargo ramp. 146 also needed 4 engines as the 681 was powered by twin Pegasus and thus four jet effluxes to effectively cover the flaps.

''One advantage in a glider (sailplane) of having a T tail is that when doing an out-field landing it helps keep the elevators and horizontal stabilisor out of a tall standing crop.''

True, but the main reason why gliders have T tails is that there are only two right angle jucntions instead of four. It reduces interference drag.

The VC10 had a T tail so that the engines could be mounted at the back. The main reason was that it was designed for the British ''Empire Routes'' which had short and high runways. Having the engines at the back meant the whole length of the wing could be used for high lift devices. Consequently had much slower take off and landing speeds that it's competitor, the B707. IIRC, the approach speed was some 20 to 25 konts slower for similar weights.

However, the extra structure of the suppot for the engines being away from the lift generators (the wings) added some seven tonnes to the structure and the VC10 was quite a bit heavier and more expensive to build than the B707 - one of the reasons it wasn't a huge commercial success and why only about 60 were built.

Incidently, Boeing got round the problem of the 707's inferior performance by getting the US Government to fund longer runways at all of it's destinations!

One interesting and unplanned benefit of the T tail on the Handley Page Victor when it was in it's testing phase was that on landing, the low set anhedral wings would be subject to ground effect whereas the tail would not. The resulting pitch would flare the aircraft almost perfectly for a very smooth landing and it prompted Handley Page to bill the Victor as "The aircraft which lands iteslf".

Unfortunately, the tail came off one of the prototypes during testing during a high speed low level position error test at Cranfield and the resultant re-design shortened the fin and the "autoland" effect was lost. It certainly didn't do that when I flew it, as anyone who suffered any of my Victor landings would attest!

De-icing T tails is easy if you have the right equipment!

http://i.dailymail.co.uk/i/pix/2013/...96_634x357.jpg

It looks like two doors are open at the top of the fin. I wonder why.

It's a C17. It has a ladder running up the inside the fin and two access doors to climb out of.

PEI,

The long fuselage came into service with the 600 series 125, the 700 series being essentially a cleaned up 600 but with the Garrett fan engines. These 'stretched' variants had the fuselage length increased forward of the wing root leading edge (ie the wing - tailplane distance was the same as the earlier variants).

AIRC neither were fitted with a stick push system.

The 800 series and later (800XP, 1000), of course, have a very different (totally redesigned) wing to the earlier aircraft, extended in span by some 4' 4.5" (as well as further fuselage extensions). I would guess that this new wing fundamentally altered the stalling characteristics, hence the stick push system?

Never posted on this forum before bur, here goes. I have always understood, like Dan Winterland, that a major disadvantage of tail mounted engines was the extra weight to compensate for so much concentrated on the (almost) centre line. An additional disadvantage is that, to keep the cg where it should be, the wings had to be a long way back thus reducing the tail moment. This requires a larger stabiliser to maintain its required power. Thus more drag. You have to admit the VC10 tailplane is a bit big. The BAC111 accident taught us that T tails are prone to locked-in stalls. There is always that danger despite some getting away with it. Hence the requirement to have a recovery parachute during trials of such configurations.

Pontifex,
Not sure if the VC10 was the one with too large a tailplane - IIRC from my apprenticeship it shared the same surface as the much smaller 1-11.

Not sure about that - the VC10's tailplane was huge. As big as a Hunter's wing, and almost as big as a Tornado.

[IMG]http://i210.photobucket.com/albums/b...ps3f18544c.jpg[/IMG]

ICT SLB

I take it that yopu mean that the VC10's tailplane shared the same dimensions as the 1-11's wing! You're probably right.

Pontifex et al,
No, I remember seeing the structural drawing for the 1-11's tailplane and being surprised to see that it was originally called out for the VC-10. Must have been for the inner member only as the 1-11's tail span is given here as 29 ft 6 inches, while the VC-10's is given as 43 ft 10 inches.

high tail is best....
 

Here's a summary of why the high tail on the C-141 (It saved 7800 lb TO weight)

Google this AIAA paper:

Effect of horizontal stabilizer vertical location on the design of large transport aircraft Byrnes, Hensleigh, Tolve



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