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Can airliners recover from a stall?

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Can airliners recover from a stall?

Old 18th May 2005, 15:45
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Ignition Override,

You have correctly addressed a significant problem encountered in tests to the fully developed stall, that is, that (1) any assymetric thrust or slip is to be avoided at all cost, and (2) assymetric engine spool-up during recovery is to be avoided. Your quote -
when pulling both throttles back to where the bleeds are barely closed, about 52-55& N1, it is easy for one throttle to be set just a bit below what you want, and one engine can accelerate faster
If I may refer back to my earlier post, I think that these two items were covered -
During deliberate stall tests on jets, itís a case of feet OFF the rudders, engine synch ON, Yaw Damper OFF Ė all to avoid any yaw moment.
All of that in the interests of avoiding assymetric thrust during the stall, PARTICULARLY the Engine Synch to ON. On aircraft without Engine synch, it was necessary to judiciously maintain the engine parameters equal, and
apply forward elevator, and only when the aircraft has responded with a pitch down, apply full Thrust.
During the recovery it is essential to FIRST unstall the aircraft before applying thrust. In so doing, the wings are at an AoA somewhat below the stall, and small thrust assymetry can be tolerated. Preventing engine stall is a secondary but important consideration.

Like 747FOCAL, I have seen the world upside down from B727, DC9 and other cockpits, I don't ever want to see it that way again. Even the best stall warning system does not 'know' the wing condition, and the worst I've seen was a test on a 'dog' aircraft with paint overspray on the upper wing surface, the full stall ocurred about 20 Kt above schedule, and before the stall warning activated. I think that this instance alone stands to justify such stall training as is possible for flight crews.

Regards,

Old Smokey
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Old 19th May 2005, 02:24
  #42 (permalink)  
 
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engine synchronizer question

Forgive me for being a decade or two behind the times, but the last synchronizer I can recall working on was a turboprop. What turbofan aircraft/engine configuration(s) offers or requires a synchronizer? What flight modes is it used in? Is it strictly used to avoid assymetric thrust, or to tune out noisy heterodyne beats?
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Old 19th May 2005, 06:09
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I'm sorry to step back down the thread here, but a question please to clear something up for my simple brain! I fly a variable geometry swept wing military jet (not a Flogger), and was a bit confused about Old Smokey's
Approach to stall ...where itís RAPIDLY correct wing drop with roll control
Now I can understand that thinking with my toy as we have no ailerons and the spoilers should do the trick on the other wing I guess; then again the downwash on the taileron will be a nightmare - I hate knock-on effects. Nevertheless, I always thought that as soon as unintentional wing drop occurred , the AoA on that side wing increases. Is it not true that dropping an aileron (increasing your effective camber (cue FMS)) will further increase your AoA of the downgoing wing. Which I always thought was detrimental for lift?

Just for information, when we get towards the stall, wings level unloading seems to do the trick. Actually did the 'trial' once when a little disorientated during a fight, and can confirm that technique does work to below HUD reading speed (50 kts). Avoided use of the rudder then too. Probably best to avoid that regime with passengers however!

Finally, we have a hopeless, no visual simulator where I am at the moment, but I still think that there is benefit from going through the procedures required approaching the stall. Notwithstanding the valid comments about realism and simulator performance envelope, if nothing else, one can practice the cadence of the recovery. I really can't see there being too many negatives in that - and at least if you have to do it for real, it won't be for the first (and consequently perhaps last) time.

Looking forward to the help!

Psy
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Old 19th May 2005, 06:44
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Use of aileron - rudder

Psy-Clops and Old Smokey,

You both address the sensitive matter of controling bank during stall.

It is so, that down moving aileron increases the angle of attack on that wing, thereby possibly exceeding stall aoa and aggravating the roll tendency. For this reason military training taught keeping the wings level by secondary effect of rudder.

Although not at all a fan of rudder in flight on large aircraft (done to death on other threads) I have used this technique on air tests of the MD-80 series. If used sensitively it worked well. MDC once had a DC-9 invert on them during such a test, because one wing leading edge had been cleaned and the other one was covered in a layer of crushed flies.

By the way, we went to full stall (but not to stick pusher, which was some percent beyond the stall) on these flights.

FC.
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Old 19th May 2005, 10:15
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A quick reply - In transit.

What the last 2 respondants say is perfectly correct for an aircraft which only has aileron for roll control. For aircraft using aileron and roll spoilers, the spoiler is far more effective in roll control. Near the stall the spoiler dumps the lift on the rising wing more effectively than does the aileron, and, as a bonus, produces a little yaw in the direction desired.

Regards,

Old Smokey
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