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cx007
19th Apr 2005, 12:47
I was reading the book " handling the big jets", came across Super Stall which appeared on 2nd generation airliners, e.g. 727 and some other MD.
Does super stall still happen on the later generation jets? Like 747? 777 and A340?
I believe it shouldnt since the engineers must have sorted it out. But i just want to know. And would someone who had such experience please explain how did you recover from the super stall? Many thanks.

spannersatcx
19th Apr 2005, 13:30
"Super Stall" is associated with high tail plane a/c. So it doesn't affect the likes of the 747 or 340.

Rainboe
19th Apr 2005, 16:43
Because of the high tailplane, the tail would be blanked by the flow from the wing in a stall, so there is no control input that is likely to get you out of such a stall. Therefore strong protection eventually was provided in the form of stick shakers, horns, and as a last resort a stick push system hopefully to prevent the stall developing completely.

Parapunter
20th Apr 2005, 08:02
Is that to say that in a stall, the turbulent airflow from the wings would prevent control authority over the elevators on a T-tail aircraft? (minds eye sees a nose high aiirplane) thus making the stall unrecoverable?:confused:

Regards, Paraglider flying know nothing:uhoh:

28L
20th Apr 2005, 08:11
That's right. And as Rainboe says, that's why the manufacturers put in all the prevention devices, ultimately the stick pusher which shoved the stick forward automatically.
Again, only an issue with high-tailed airplanes.

A-FLOOR
20th Apr 2005, 12:22
A superstall is only the end result, the conditions alluding to it are not so much the engines and tailplane ending up in the turbulent airflow, but in the case of airliners the violent pitch-up motion associated with stall onset on aircraft with high sweep.

If you want to know you should read up about the lessons learned the hard way on the first flight tests of the BAC 1-11.... for example ever since then most, if not all new T-tail aircraft have rockets installed in the tailcone to physically force the aircraft out of an eventual deep stall. (Only during flight testing of course :} )

cx007
20th Apr 2005, 18:44
Thanks for all informative replies, one more question now.
Why hight-tail? What's it advantages over later degisn with low-tail? Thank you.

A-FLOOR
20th Apr 2005, 21:06
One of the reasons for a T-tail or semi-T-tail (Caravelle, Falcon) on airliners and bizjets is the placement of the engines, which in turn is a result of the consideration that the plane has to operate from less-than ideal runways, keeping the engines clear from rocks, sand and/or wildlife they might ingest on takeoff.

Another one might be to keep the wings aerodynamically clean, making them more efficient and their construction lighter and easier to build.

These days, engines in pods under the wings are the preferred configuration as it makes them easier to maintain, and the behavior of such a configuration in the air is considered to be the safest and most predictable, also for other reasons besides the possibility of a deep/superstall :ok:

T-tails on high-wing jet aircraft (BAe-146, IL-76) are a different matter altogether though :)

Loose rivets
23rd Apr 2005, 04:43
Three major accidents come to mind. On two, the impact site was not much longer than the aircraft.

wilcoluca
26th Apr 2005, 09:43
I've always been taught that the effects of a super stall are caused by two factors essentially: T-tail and sweep back of the wings. Of the two, the main cause is the sweepback which produces, at high angle of attack, an even greater pitch up tendency, whose turbulent airflow strikes out the tail of the aeroplane making it uncontrollable.

Dick Whittingham
26th Apr 2005, 18:02
A basic swept wing setup, without any fixes, will pitch up with increasing alpha. No civil airliner with these characteristics would pass certification. Therefore, fixes are incorporated, mainly to induce root stall and delay tip stall, so the aircraft shows an increasing nose down tendency as alpha increases to the stall.

The problem arises at alpha above the stall, above critical alpha, when these fixes no longer work, and the setup reverts to a nose up tendency. Even then, all is not lost if you have sufficient tailplane authority to overcome this and lower the nose.

The attitude of the aircraft in a superstall, above critical alpha, is nose level or nose up, but it is sinking like a brick s...house, so alpha is 20 - 30 deg, and the "downwash" is going high over the tail. If you have a T- tail, this makes the tail ineffective, and delays or prevents recovery.

So the superstall itself is a factor of swept wing setups. Delayed or impossible recovery is a factor of T-tails.

Aircraft that are going to have difficulty recovering from superstall will have stall avoidance systems - not just stall warning. These will be stick pushers and possibly auto-throttle to full power. In fact, Airbus, in its various forms, has auto-protection of this sort.

Basic recovery is to select TOGA, flap to the optimum for nose down trim change, stick forward, wait, and pray.

Dick W

Old Smokey
26th Apr 2005, 22:22
Did somebody say "select TOGA"? A recipe for disaster.

If the aircraft has underwing (low slung) engines, this will cause a further pitch-up against minimally effective elevator pitch down, significantly worsening the situation (Taiwanese Airbuses come to mind). If the engines are rear mounted, which they usually are for T-tail designs, you're lucky to get a response at all, as the engines too, are in the disturbed air aft of the wing. Both T-tail, swept wing aircraft that I've done full back stick stall testing on had rear mounted engines, and the pop pop bang of the compressor stalls were very evident - I know of no recommendation to advance thrust on an engine suffering compressor stall (unless you want additional dire consequences on your hands).

In the flight testing I've done, both aircraft fortunately had excellent roll response down to and beyond the stall. One of the two had minimally effective "push" augmentation, which necessitated deliberate attempts to enter the "deep stall" during testing, e.g. APPROACHING full back stick, apply high thrust (not compressor stalling YET), rotate to much higher than the normal stall attitude, chop the thrust, and apply full back stick, (Now much banging and popping from the engines, DO NOT increase thrust) and wait to see the effectiveness of the inbuilt recovery system. It always worked, albeit slowly, but it was well accepted that elevator alone may not have been enough, and running stab trim forward was the only pitch option available. As a DIRE LAST RESORT, roll to the vertical and let the aircraft fall through with relaxed elevator.

In all cases, thrust was not increased until the angle of attack had been reduced to the normal stall angle. Engine synch on and feet on the floor to prevent any unwanted yaw.

It may sound odd, but at all times one had to have a mental picture of what the airflow was doing at every point on the aircraft, observation of a lot of wind tunnel testing was good value for this.

Thank God I don't do it any more, but if I had to go back again, I'll do the sissy coffin corner stuff and engine cuts at Vef and leave the deep stall stuff to hardier souls.

Regards,

Old Smokey

Laikim Liklik Susu
29th Apr 2005, 12:40
I believe one of the ferry pilots on a certain Australian airlines DC-9 entered a fully developed deep stall on a test flight in the USA "back then"

I also believe it was the first, and probably last, DC-9 to go supersonic during the recovery, and survive...

Old Smokey
29th Apr 2005, 13:51
Fairly close Laikim Liklik Susu, it was a training flight, initiated by the perpetrator inadvertantly applying the "deliberate entry" procedure I described, by sharply applying full BACK elevator as the stall warning sounded, taking the AoA beyond the stall. I'm not so sure that they went supersonic, but the said perpetrator's hair (and beard) turned white shortly thereafter. A severe roll and dive ensued.

The DC9 did obligingly drop a wing to the vertical in the fully developed stall (usually the right) providing a good lateral airflow to the tail assembly enabling recovery, and it was from just that that the 'last ditch' recovery was developed for other T-tail aircraft. Not all manufacturers may recommend it.

Not funny for the poor guy, but the compresser stalls on one such occasion were so loud that an engineer riding in the cabin thought that the main spar had "gone".

Regards,

Old Smokey

MrBernoulli
29th Apr 2005, 18:39
......... and thats why we are very careful with the VC10 - swept and T-tailed!

And would that be Dick Whittingham of 'Bristol Groundschool met lectures sometime way back' fame? I'm pleased to say that in 2 months time I will be putting that hard-won ATPL to use in a new job on a far more modern non-T-tailed electric Boeing. Hoorah!

chiglet
29th Apr 2005, 20:49
Heard from a "survivor" from a Javelin ejection.
A/c was in a "high nose up, low speed" attitde. Started to ocilate [sp] around both pitch and roll axies....Pilot tells Nav..eject x3. Nav bangs out.....canopy loss [+ lack of nav/seat] takes a/c out of regime...a/c lands ok.
May be an "urban myth" but told to me by two Navs. :ok:
watp,iktch

Laikim Liklik Susu
29th Apr 2005, 22:56
No, it is true. Since the curvature of the canopy actually provides a small amount of lift, the CoP was disturbed just enough, and the CoG moved sufficiently (thanks to GIB bang out) to effect a stall recovery and subsequent landing.

It was also the case on another aircraft, but I am trying to remember which! I know it was an RAF type - possibly the Hunter trainer (although not a deep stall, but an unrecoverable made recoverable by CoG and CoP movement after rapid weight loss :p )

ICT_SLB
1st May 2005, 04:51
To correct a few inaccuracies in previous posts.

1. Usually only the first prototype of a new design will be fitted with a spin chute (no rockets) which can be deployed by the crew in the event of a deep stall (as it is more commonly known with T-tailed aircraft) - and sometimes more important, jettisoned after use.

2. The Stall Protection System (SPS) usually has three phases (known as "firing angles" as they are triggered by Angle of Attack Vanes on either side of the aircraft) :
Lowest is Auto-Ignition that turns on the Igniters to ensure continued safe engine operation.
Then Shaker - to provide tactile & aural stall warning and Autopilot disconnect.
Finally the Stick Pusher will push the nose down to restore normal flight if the crew has not already done so.

The exact value of firing angle for a given model will depend on Flap & Slat setting, Mach Number & Lateral Acceleration (for sideslip).

Ignition Override
1st May 2005, 06:25
On the DC-9, one of the control checks done by the FO is to push the yoke all the way forward and look for a blue "Elev. Power" light. This indicates that the left hydraulic system (has accum back-up) is actuating both control tabs upward, which would help the elevators to go downward during a deep stall, lowering the nose . :ooh:

And the DC-9 uses "no stinkin" computers to fly-one needs to know how to be a pilot :eek: . "Mode managers" not wanted there, nor needed. And with no FE! :uhoh:

Dick Whittingham
4th May 2005, 10:48
MrBernoulli.

It is indeed I. Well done you, and all the best for the future.

Dick W