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zionstrat
Topic Author
Posts: 225
Joined: Sat Apr 28, 2001 3:26 am

Confusion Regarding T-Tail Deep Stall

Wed Jan 09, 2002 11:33 am



I know precious little about this topic, so this question is probably rather stupid, but I am confused about 1 aspect of the deep stalls that affect T tailed aircraft.

If I understand deep stalls correctly, they are caused by the 'shadow' of the wing disrupting the slipstream over the elevator reducing its effectiveness.

This seems to make sense, but I don't understand why this is crucial as it does not sound as if the rudder would be affected considering that most of it is not as high as the elevator, and therefore not in the disrupted air.

So in a standard spin recovery, it seems that the rudder would still be available to counteract the yaw -The AC would now be in a straight dive, the wings would develop lift, the disrupted airflow stop and it seems to follow that you would them regain elevator control to arrest the dive. With this thinking, it might take a little longer to get the nose back up than with a conventional tail, but I must be missing something.

Can anyone help me understand what I’m missing?
 
POSITIVE RATE
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RE: Confusion Regarding T-Tail Deep Stall

Wed Jan 09, 2002 1:18 pm

When a t-tailed a/c is in a Deep stall it doesn't necessarily mean it's in a spin. Yes the rudder may still be effective but what good is that if the elevator is not! The disrupted/stalled airflow from the wings will be hitting the elevator rendering it ineffective and once this has occurred there can be no recovery and the airplane will definitely go down. Remember that a swept wing aircraft will want to pitch UP when it stalls and the ensuing high rate of descent will further increase the angle of attack thus perpetuating the deep stalled condition.
 
411A
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RE: Confusion Regarding T-Tail Deep Stall

Wed Jan 09, 2002 1:24 pm

A good reason why stall/spin 'chutes are fitted to flight test aircraft.
 
ThirtyEcho
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RE: Confusion Regarding T-Tail Deep Stall

Wed Jan 09, 2002 1:58 pm

Even if the stall progressed to a spin, full opposite rudder would not effect a recovery from the deep stall condition even if it did stop the spin. Some of you aerobats may know better than I do, but the full opposite rudder, deep stall condition might only suffice to flip the airplane into an inverted spin or an inverted flat spin.
 
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RE: Confusion Regarding T-Tail Deep Stall

Wed Jan 09, 2002 8:21 pm

There was a Trident destroyed on a test flight in the 1960's and that airplane was being flown by a test pilot and even he couldn't recover the airplane. They tried using assymetric thrust, deploying flaps, spoilers, droops, landing gear, rudder and nothing worked. This is why all T-Tailed jets are fitted with stick pushers.
 
Guest

RE: Confusion Regarding T-Tail Deep Stall

Wed Jan 09, 2002 8:35 pm


Scusi,Positive rate ,but I think you meant the BAC 1-11.
 
VC-10
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RE: Confusion Regarding T-Tail Deep Stall

Thu Jan 10, 2002 6:16 am

G-ASHG, the prototype 1-11 that went down was testing the stall situation with the C of G at the extreme aft end of it's range with 8 Degs of flap, so that didn't help the stall recovery position. In addition at that time the elevators were operated by servo tabs so as the vertical acceleration increased the elevators trailed upwards in the slipstream. Susequent 1-11's had powered elevators in addition to the stick push.

A Trident, G-ARPY, was also lost in a stall during flight testing. In this case it happened during a test with the stall protection systems inhibited. At 11,600 ft it entered the stall and started to pitch down but as the airspeed dropped off rapidly the a/c was seen to pitch up to 30-40 degrees and enter the deep stall. The a/c started a turn to port, then the stbd wing suddenly dropped and after a short burst of engine power entered a flat spin to stbd, hitting the ground 1.5 mins after the stall started. 'PY' hit the ground in a flat attitude with low fwd airspeed.
 
Guest

RE: Confusion Regarding T-Tail Deep Stall

Thu Jan 10, 2002 6:46 am

Well,you learn something every day.Didn't know about that Trident crash.  Smile
 
LZ-TLT
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RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 11, 2002 7:44 am

A second Trident(G-ARPI) went down under circumstances not fully cleared even until today. Along with some misconfiguration(premature slats retraction) and inappropriate crew actions, the Trident entered a deep stall and was unable to recover.
 
VC-10
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RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 11, 2002 8:16 am

There was also a near 'PI' incident at Rome a year or so before G-ARPI went down. In this case the crew soon realised they has retracted the drooped L/E's too soon and deployed them again
 
LZ-TLT
Posts: 427
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RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 11, 2002 9:36 am

VC-10

I know about this incident. Apparently, there is also at least one confirmed Trident incident(Dublin), where premature slats retraction has nearly caused a deep stall. I wonder just, was it just inappropriate crew action or was it something generally wrong with the Trident systems?
 
TAA_Airbus
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RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 11, 2002 10:29 am

Wasnt theren an incident where an aircraft pitot system was malfunctioning causing an over reading in IAS. Consequently, the pilots kept pulling back till the aircraft was in an tail down deep stall and stayed like that all the way from 30,000 feet.

My facts may be way off but I think it may of been a 727.

Another factor with deep stall recovery. T-Jets with rear mounted engines dont have the thrust stream available for empanage control surfaces which makes it even worse during stall recovery which in turn makes them rather useless.
 
VC-10
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RE: Confusion Regarding T-Tail Deep Stall

Sat Jan 12, 2002 7:57 am

LZ-TLT,

There were in two fact instances of premature slat retraction prior to the demise of 'PI'.

In the early days of the Trident the stall warning system was notorious for giving false warnings to the extent crews cancelled the warning as a matter of course.

In one case, shortly after TO the stick shaker operated for no apparent reason as speed and attitude were correct. Then the stick push operated, forcing the nose down. The Capt cancelled this and brought the nose back up to level. The stick push then operated again at 1000 ft so the Capt instructed the Third Officer to pull the circuit breakers for the system. As the co-pilot then turned to operate the stick push overide he noticed the droop lever in the up position. He immediately extended the droop leading edges and the a/c recovered. No crew member could recall retracting the droops.

Initially it was attributed to an a/c fault but after extensive checks no defects could be found. As the story of this incident got around BEA a Capt came forward and admitted he had experienced a similar incident a few years before.

These incidents could be attributed to poor design in that the Flaps and Droop L/e's were operated by seperate levers sited alongside each other. While the flaps were extended there was an interlock to prevent the droops being retracted before the flaps but, once the flaps were up there was no protection to prevent the droops being retracted below the retract speed of 225Kts

After the 'PI' accident a speed switch was installed to prevent to L/e's being retracted too soon.
 
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RE: Confusion Regarding T-Tail Deep Stall

Sat Jan 12, 2002 12:44 pm

TAA_AIRBUS you are indeed correct about the a/c type. It was a Northwest Orient B727-200 and it happened in 1974. I read about it in Air Disaster vol.1. The crew forgot to turn the pitot heat switch to on and as they climbed the pitot tubes iced over and so the ASI acted like an altimeter and kept reading higher and higher so the pilots kept pulling back on the control columns until they stalled, went into a flat spin and impacted a mountain(no pax on board fortunately)
 
allisontp
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Joined: Thu Sep 27, 2001 3:48 pm

RE: Confusion Regarding T-Tail Deep Stall

Sat Jan 12, 2002 5:36 pm

Hello friends,
Are there any preventative methods such as stick pushers incorperated into light GA aircraft (ex. Piper TurboLance T-tail), or will the inherent design of a strait wing simply yield a pitch down moment when stalled, relieving the situation? Just thinking back to the rule of keeping the CG ahead of the aerodynamic center to maintain proper stability.

Thanks
AllisonTP
 
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RE: Confusion Regarding T-Tail Deep Stall

Wed Jan 16, 2002 2:16 pm

Allison as far as i know there are no light a/c fitted with stick pushers. The PC-12 has a stick shaker but that is different. I think you are correct in saying that a straight wing will yield a nose-down pitch movement so it won't happen. I would say the Learjet/Citation probably would be anyone know???
 
zionstrat
Topic Author
Posts: 225
Joined: Sat Apr 28, 2001 3:26 am

RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 18, 2002 1:32 am

Great thread! Thanks for the info.
 
SSTjumbo
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Joined: Sun Jun 17, 2001 3:29 am

RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 18, 2002 12:50 pm

I hate to pop the big question, but is this in the right forum?  Acting devilish
I don't know, so this is my signature.
 
SSTjumbo
Posts: 2579
Joined: Sun Jun 17, 2001 3:29 am

RE: Confusion Regarding T-Tail Deep Stall

Fri Jan 18, 2002 12:53 pm

Oops, sorry. I thought I was in the Non-Av forum!!! Embarrassment
I don't know, so this is my signature.

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