T-Tail advantage & disadvantage ?!

[Raynald]

Hi,


Could you explain me the good & bad sides of a T-Tail airplane like the BAe 146 ? and also your opinion about that config.

It would really clarify some discussion ........


ThanX

R@y

[Intruder]

Pro: Horizontal stab is in "clean" airflow in virtually all flight regimes (deep stall excepted, but uncommon in air carriers).

Con: Vertical stab structure must be stronger to carry the loads.

[G-ALAN]

One advantage is it's not likley to be hit and damaged by anything thrown up by the tyres, especially on a rough strip or gravel strip. I fly a T tail and one thing I find very annoying is the fact I can't see the top of the elevators on the walk around. Also T tails don't perform quite as well in stalls, infact if the stall is deep enough there is a risk you won't have enough elevator authority to recover.

[safetypee]

A ‘T Tail’ increases the elevator effectiveness by giving a larger effective span. High elevator effectiveness is required for high lift wings, which are often mounted above the fuselage, thus the tail has to be out of the down wash. The end plate effect also gives more rudder effectiveness for a shorter tail arm. i.e. YC-14/15, C17, Dash 6/8, and 146/RJ.

The 146 still has the highest Clmax for any civil airliner. Deep stalls? For the 146 in theory yes, but the test team tried very hard to find it yet never experienced any during development. The stick push gives protection, but during certification it was also a quick method of determining the expected stall speeds so that performance testing could progress quickly.

[747FOCAL]

G-ALAN,

Not sure I agree with you on the t-tail and stalls. Maybe in something like an MD-80 where the designers put the wings closer to the tail than they should have, but not on something like a 727. I have also heard wind up stalls can be a beeeaaaaattcchhhh in a t-tail.

:)

[G-ALAN]

Ok I stand to be corrected :O I've always been taught low wings and a T-tail are not a good combination in a stall as turbulent air from the wings will flow over the elevators thus giving them little authority.

[DanAir1-11]

On the subject of T-tail stalls. A stall situation in a T tail is neither more nor less critical than that of any other configuration, providing it is recognised at the onset and recovery is effected promptly and correctly. The real problem occurs when the stall is allowed to develop into a deep stall, from which many aircraft have been lost.
Two notable examples being a BA Trident over Staines UK in the 60's where the PF retracted the leading edge 'droop' devices too soon leading to aerodynamic deep stall at a fairly low altitude, and a Northwest 727 on a ferry flight, where the young crew were mislead into the stall after pitot tubes became iced and subsequently blocked, the crew became aware of high airspeed in the climb, which they attributed to having a light aircraft. They pulled her back and let her climb, even mistaking the onset of the stall buffet as Mach buffet. As in the first incidence, the entered the deep stall and were unable to effect recovery.
The vast majority of my experience has been with T tails, 1-11's which were a pleasure to fly, as was the 727. THere have been many things put in print about tyhe pros and cons of the aerodynamics of the design, If I don't forget, i will come back to this thread and add some links later on.

Regards

[Loose rivets]

The deep stall of Mike Lithgow's 1-11 comes to mind.

DanAir 1-11, were u there when CC and CP turned up? I recall that all the training was done on the real a/c...the stall through to the push. Not sure that i would want to do that now.

[DanAir1-11]

Loose rivets,

That doesn't half bring back some memories!.
Was PNF on the famous phantom engine fire flight out of Manchester on AXCP (BAC 1-11 401AK - build# 87 for those confused by this) , she certainly earned her "smoke-bomb" handle! I - Re the stall training, never had the pleasure at that time, and i'm fairly sure that was a good thing!.
As I recall Bryn Wayt did something similar?

[Volume]

Greatest advantage of the T-Tail is, that is far obove those ´well trained´ service vehicle drivers, and therefor not hit by catering trucks, stairs, fuel trucks etc. which damage conventional tails nearly every day on some airport on this planet. :}

[DanAir1-11]

volume,

ain't that the truth!. Was out at a dirt strip last weekend and it was fairly warm, somewhere in the region of 48 degC open shade
Ramped next to a Cessna 'something' and a not so bright chap decides that he will park his ute (pick-up) under the wing in the shade, whilst working on her. Only one problem, he apparently forgot about his drum of kero on the tray complete with pump sticking out of the top end a proceeded to remove the stbd flap assy and puncture just forward of the mounts! Think he did most of the damage reversing attempting to extricate himself.

[BigGreenPleasureMachine]

Dan Air makes an important point, when the tail is unaffected by the down wash from the wing, natural stall warning doesn't exist. Hence the stick shakers/pushers on aircraft of these configurations, which works well as the tail won't be in the turbulent air at the stall warning speed.

A more problematic characteristic is the tendency for t-tails with aft mounted engines to pitch up on stalling, making things worse if the tail has already become ineffective. i imagine the abscence of this quality on the 146/RJs is to do with their engine installation position.

Regards, BGPM.

[Tinstaafl]

I've often considered what one could do to try to exit a deep stall. I think there's two or Given enough height I reckon I'd try:

1. Stick out all the low speed devices

2. Have everyone down the back see how close to the cockpit they can get + gear down to try to lower the drag line.

3. combined cycling power & elevator to try to destabilise the thing. May not be all that effective if there's not much power induced pitching.

4. Try to roll using aileron & spoilers far enough to allow gravity & airflow to cause a yaw towards the ground ie into the airflow.

5. Also try to roll, but using asymmetric power & rudder. Risk of spinning but at least there's the chance of an unstable bit while transitioning.


Discussion? Alternate suggestions?

[DanAir1-11]

Best remedy for a deep stall?

Don't let it get there in the first place.

[Field In Sight]

You could also fit retro rockets near the cockpit facing upwards.

a la The Space shuttle.

FIS.

[ft]

Suggested method to get some jet fighters out of a deep stall is rocking it out ( \m/ ;)) of it. The elevator authority remaining won’t allow you to get the aircraft out, but you can get it to sway around the stable null point. Keep feeding the oscillations with elevator input, and eventually the nose will be low enough on the downswing to let you get out of it. There’s a manual override on the elevator control in the F-16 to enable you to do this. I’m hoping for someone to fill in on the exact name of the switch and just how the elevator works in that mode.

Stick pushers might provide a stall warning, but if that was their only purpose they’d still be stick shakers. Keeping you out of a deep stall is the thing which makes them a required item.

IMO, T-tails were largely a fashion fad, along with rear-mounted engines. Those two sorta go hand in hand anyway.

An advantage yet to be mentioned is that the elevator won’t blank the rudder in a non-inverted spin where having a bit of rudder authority just might come in handy.

Con is structural, having all that weight up there on that long moment arm. Look at gliders groundlooping, it is not uncommon for the fuselage to snap due to the torque put on it by the T-tail. OTOH, T-tails are generally preferred due to the fact that landing out in crop will have a good chance of damaging the elevator of a non-T stab severely.

G-ALAN, of course you were right.

Cheers,
Fred

[Bre901]

Slightly O/T but still about flying machines

For gliders it is more than a fashion : since the Phönix (1959), 90% of plastic gliders have been designed with T-tails, notable exceptions being the Std-Libelle and pure aerobatic gliders (for structural reasons as mentionned by ft).

The determining pros are indeed the interaction with crops and the rudder authority in spins. Gliders are probably the most stall & spin prone airplanes (aerobatics excepted) as people tend to fly as slow as possible (min sink or even less) when cirling in thermals.

Never heard of deep stalls for gliders. (or seen anything in flight manuals). I guess the layout is not favourable (forward position of wings)