Deep Stall? Any ideas?
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Deep Stall? Any ideas?
I was reading a fabulous old book called 'The History of Commercial Aviation' and there was a chapter devoted to the emergence of the T-tail jets in the late sixties, namely the 727, Trident and the One-Eleven. . .. .It mentioned that several of these aircraft were lost during test flights due to 'deep stall'.. .. .It showed a picture of a One-Eleven that had force landed on Salisbury Plain after one such test flight.. .. .I understand that 'deep stall' is something to do with the stalling of the wing and the high mounted elevators at the same time.. .. .How would you recover from such a situation and what exactly causes it?. .. .Ta. .. .RU
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Something to do with the high angle of attack of the wing creating such a volume of turbulent airflow behind it that the stabilizer and elevator are not effective enough aerodynamically to push the nose down. The CAA required a stick push system on Tee tail aircraft but the FAA didn't. So our three holers at Dans had a pneumatic stick pusher but over here in the USA they dont have them. I believe Mike Lithgow was the test pilot in the Salisbury 1-11, and as I recall in a most exemplary demonstration of the test pilots art, was trying different configurations and reporting on their effect all the way down. Per Ardua..........
In a deep stall, the turbulent airflow off the wing completely blankets the horizontal stabiliser, making it totally ineffective. That's one reason why development commercial aircraft carry a tail-mounted drag chute - deploying it in a deep stall pitches the aircraft nose down and puts the tailplane in clear air, I think.. .. .Yes, that was Mike Lithgow in that prototype BAC-111, along with, I believe, BAC's chief aerodynamicist and a significant proportion of the aerodynamics department as that flight was specifically investigating the phenomenon. It was not a forced landing, they never made it out of the stall and all on board were lost. I was in the second year of my aero engineering degree course at the time and I seem to remember quite a lot of discussion of it in the following weeks.. .. .Any high tailed aircraft could be susceptible to it but, nowadays, the implications are understood.. .The Gloster Javelin was a martyr to it (big delta wing and a T-tail right behind it), quite a few were lost to deep stalls. I think Hunter pilots were told not to bounce Javelins because several were lost trying to manoeuvre away from their attackers.
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The thing that distinguishes deep stall from ordinary stall is that in deep stall the aircraft continues to pitch nose up, taking it deeper into the stall. . .. .Under normal circumstances, if both wings stall simultaneously, a straight winged aircraft will pitch nose down, taking itself out of the stall. This is because as the wing stalls, the C of P moves aft, creating a nose down pitching moment.. .. .With a swept back wing the situation is somewhat different. Spanwise flow out towards the tips produces a thick sluggish boundary layer over the wingtips. This separates more easily than the higher energy boundary layer close to the roots, so the wingtips stall before the roots. But the wingtips are further aft than the roots, so the tip stall causes the C of P to move forward. This causes the aircraft to pitch nose up taking it deeper into the stall. The loss of lift then causes the aircraft to descend, making the angle of attack still greater. The overall effect is that aircraft goes ever deeper into the stall.. .. .The position of the tailplane becomes a very important factor at this stage. A high tailplane (T-tail) is likely to be enveloped in the turbulent flow coming from the stalled wings. This will drastically reduce its effectiveness in regaining pitch control. This is why the T-tailed aircraft you referred to have particular problems with deep stall. It is also why such aircraft must have stall prevention devices such as stick shakers and pushers. With a low tailplane the turbulent flow is likely to pass well above it leaving full pitch authority available.. .. .The fact that many T-tailed aircraft also have rear fuselage mounted engines just makes matters worse. The turbulent airflow from the stalled wings is likely to cause engine surge/stall, turning the aircraft into a (very bad) stalled glider!. .. .The important point to note as far as the JAR ATPL exams go is that it is the tip stall of the swept back wings, rather than the T-tail, that started the train of events.
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With the 1-11 incident, my understanding is that the crew were investigating the stall with aft centre of gravity. The weights in the cabin were hydraulically powered back and forth and at the fatal stall, the engines also stalled depriving the crew of the means to move the weight forward.. .. .Whether moving the C of G forward would have had any bearing on the outcome is open to speculation.. .. .It is possible that the 727's and DC9's did not suffer from the deep stall in the same way as Tridents and 1-11's because of the high mounted tail and rear set wing. Indeed the VC10 actually developed a pitch down at the stall. I'm sure though that somebody can go into more detail.
A Trident, piloted by Peter Barlow, also failed to recover from a deep stall, whilst on a test flight in 1966. Again, sadly all on board were lost, including a number of company employees.
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Robert, the VC10 had such a powerful stick push system, the idea was to totally prevent a stall completely. I do not believe it ever stalled. I was involved with testing the stick push system on a test flight- it was a frightening experience-the AofA hovered around 15, then jumped to 17. I think if it went any higher you were dead. The stick push system operated with much fanfare- one of my awful moments of aviation- I was petrified and not happy at experiencing this! The B727 and DC9 were just as much at risk, but benefited from the experience of the 1-11 and Trident. In fact, I believe at least one 727-100 was lost pulling out of steep descents when it got into a high drag situation. It got a reputation as being 'difficult' under such circumstances. I'm not sure how much at fault was the slow response from idle of the early jet engines on it.. . . . <small>[ 27 March 2002, 21:58: Message edited by: Notso Fantastic ]</small>
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The MD80 could suffer from deep stall and the development aircraft at Long Beach still had a tail chute attachment to assist recovery from that situation in test flights. The MD90 actually has little elevators on the engine pylons for this purpose!
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twistedenginestarter: there is a WHOLE lot more to the unfortunate 'PI disaster than that, not the least of which is the fact that the irascible Capt. Key may have been in the process of having a heart attack!
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The F101 also had this unlovable feature. It was called pitch-up. A stick shaker delivered the first warning of onset followed by a 40 lbf pusher. These could be disabled by a switch on the control column. The pitch up was divided into 3 stages. At stage 1 moving the control column forward got you out. Stage 2 required the deployement of the landing drag 'chute. Stage 3 required the use of the ejection system. In stage 3 the engines probably didn't work any more OAO compressor stall so there really wasn't much point in keeping the aircraft anyway as it wouldn't glide.
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Also a problem in fighter AC with T-tail.. .Stalling the F-104 Starfighter caused a sudden uncontrollable pitchup momentum. Therefore even preproduction models type YF-104A(17 build) received a strong kicker called APC(auto pitch control) which fired at critical limit and pushed the nose down. Additionally in landing flap configuration (slow flight)a boundary layer control was developed, which guided highpressured air from the engine compressor evenly over the wings to stabilize the airflow. . .2 big problems were never solved:. .1) At low level flights the APC could malfunction and fire unintentionally. It happened and was always a killing item.. .2)Boundary layer control duct broke on one side, causing sudden unsymmetric lift conditions, hence high roll rates. No chance to recover. . .Regards. .. .Hello flash2001, you beat me by 1 minute. <img border="0" title="" alt="[Smile]" src="smile.gif" />. . . . <small>[ 28 March 2002, 00:15: Message edited by: Captain104 ]</small>
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Well,in addition to the fighters, and the MD-80/90 series, the other American manufactured aircraft would include the Lockheed Jet Star, the Canadair Challenger, and the Boeing 727. Additionally at high altitude the DC-9-10 series could also exhibit deep stall, but not at lower altitudes. Apparently, the prototype Challenger (No. 1) was lost due to deep stall. Not a uniquely British phenomena.
One of the reasons that the Americans didn't have that many deep-stall accidents was that after the 1-11 accident BAC decided to share it's knowledge with the American manufacturers. Several people from BAC went to visit Boeing and McDonnell Douglas and others, and explained all the data and experiences to them. This saved the Americans from having to try it for themselves..... .. .The VC10 is one of the few t-tailed aircraft to have a natural nose-down pitching tendency at the stall. In just one extreme configuration did the certification pilots have doubts on whether the pitching moment would be strong enough, and to cater for this extreme case the CAA wanted the stick pusher fitted. Brian Trubshaw explains the phenomenon in his book 'Test Pilot'.. .. .On Trident G-ARPI: apart from captain Key's heart attack the main reason that the aircraft got itself into the deep-stall situation was retraction of the LE slats at too low a speed (since known as a configuration stall). The full report can be found on: <a href="http://www.aaib.dtlr.gov.uk/formal/garpi/garpi.htm." target="_blank">http://www.aaib.dtlr.gov.uk/formal/garpi/garpi.htm.</a> I think the report on the 1-11 crash is also somewhere on that site.. .. .<a href="http://Fly.to/VC10" target="_blank">http://Fly.to/VC10</a>
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Had the opportunity in the early eighties of performing full stalls in a TriStar (with a former Lockheed production test pilot in the RH seat). These were done about ten miles west of Jeddah from nine thousand feet, both clean and approach configurations. Must say that the handling was impecable, not a hint of wing drop...unless a bootful of rudder was applied of course. As a line/training pilot, never had the opportunity of doing this, was an interesting experience. Don't think I would want to do these maneuvers every day however....don't much care for the very high angles reached. <img border="0" title="" alt="[Eek!]" src="eek.gif" /> . .Gives a new meaning of "blue side up"....way up!. . . . <small>[ 28 March 2002, 14:36: Message edited by: 411A ]</small>
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reverserunlocked. .. .Your original comment about seeing a picture of a 1-11 following a forced landing on Salisbury plain is quite correct. This happened during the development programme for clearing the changes to the type to fix Mike Lithgow’s problems. The development aircraft streamed its tail chute during one recovery and then neglected to jettison it afterwards. Full power would not maintain height so a forced landing was carried out with nobody seriously hurt.
