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Aviation Forum / General / Soaring / February 2009Glider Tail Stall
Recent threads highlighted tail stalls in powered aircraft experiencing icing. The thing that concerned me was the recovery being exactly opposite to the wing stall that we all practice and demonstrate, and thus have ingrained as almost automatic. It also sounded like the thing that distinguished a tail stall from a wing stall was buffet in the controls rather than in the airframe. This distinction is pretty subtle to me, and in the heat of the moment I wonder if I would apply the proper recovery. Does anyone know, for a modern 40:1 glider, how violent a tail stall pitch up would be?Also, if a glider has a totatally benign wing stall, eg, non-violent wing stall break, would this imply that a tail stall would also be non-violent? I don't fly in icing situations, I don't have flaps, and control seal checking is part of my preflight, so this is probably all academic. Still, I'd like to know... -John snip > Does anyone know, for a modern 40:1 glider, how violent a tail stall > pitch up would be?Also, if a glider has a totatally benign wing stall, > eg, non-violent wing stall break, would this imply that a tail stall > would also be non-violent? snip > -John John, Since the tail is providing down force in normal flight, if it stalls due to icing the tail will lose downward "lift". the tail will then go up causing a nose pitch *down*. I doubt that benign normal wing stall behavior would have any effect on how violent a tailplane stall might be. bumper The aircraft pitches DOWN during a tail stall. The video link posted earlier provides a pretty good overview. I don't think that a benign wing stall indicates a benign tail stall. It's about the seperation characteristics of the tail airfoil ( changed by ice ) not the wing airfoil. Todd > Recent threads highlighted tail stalls in powered aircraft > experiencing icing. The thing that concerned me was the recovery being > exactly opposite to the wing stall that we all practice and > demonstrate, and thus have ingrained as almost automatic. Why would the recovery be different? The tail is an inverted wing producing a down force. You stall it by pulling back on the stick increasing its AOA until it stalls. Releasing the back pressure initiates a recovery - same as a wing stall. It also > sounded like the thing that distinguished a tail stall from a wing > stall was buffet in the controls rather than in the airframe. This > distinction is pretty subtle to me, and in the heat of the moment I > wonder if I would apply the proper recovery. It doesn't really matter. With many trainers, the buffet students are taught to recognize as wing stall is, in fact, tail stall with a little bit of turbulence from wing root flow separation thrown in. Allowing the tail to stall limits up elevator authority so the wing can never get into a full stall. Cessna 152's and SGS 2-33's are examples. There's a simple test for this. With the stick full back and the glider exhibiting pre-stall buffet, apply aileron and if the glider responds normally in roll, the wing wasn't stalled. If the wing was stalled, the glider would probably try to spin with the application of aileron. > Does anyone know, for a modern 40:1 glider, how violent a tail stall pitch up would be? If the tail stalls, and the CG is within limits, the glider will pitch nose down, not nose up, and this will help effect the recovery. If anything, modern gliders are even more benign than older designs. Also, if a glider has a totatally benign wing stall, > eg, non-violent wing stall break, would this imply that a tail stall would also be non-violent? Tail stall just runs out of up elevator authority. With one exception, tail stall is benign. This is the exception. When the nose up moment is being produced by something other than the elevator, the stick will be forward as the pilot tries to limit the pitch up. In this case, the tail is producing an up force and if the it stalls, the nose will rise further risking a wing stall. Two things can produce this situation. One is an aft CG and the other is a poorly located CG hook used on a winch launch. Slab type all moving 'stabilators' are more susceptible to this than fixed stab/ hinged elevator type tails. The fix is to be very aware of your CG and to winch these gliders carefully. Bumper, Toad - thanks. Sorry for my confusion about pitch direction - you're right, it would pitch down. The recovery with a tail stall, though, is to pull back on the stick, not push forward. That's why I'm worried about differentiating a tail stall from a wing stall. Bill, please see comments embedded in your reply. > Why would the recovery be different? The tail is an inverted wing > producing a down force. You stall it by pulling back on the stick > increasing its AOA until it stalls. Releasing the back pressure > initiates a recovery - same as a wing stall. According to the videos, if the tail stalls you need to recover by pulling back on the stick, not pushing forward on the stick as we usually do. > It doesn't really matter. With many trainers, the buffet students are > taught to recognize as wing stall is, in fact, tail stall with a [quoted text clipped - 8 lines] > stalled, the glider would probably try to spin with the application of > aileron. I'll have to try this. I think I've never tried to move the ailerons once I feel the pre-stall buffet - I just center the stick. > If the tail stalls, and the CG is within limits, the glider will pitch > nose down, not nose up, and this will help effect the recovery. If > anything, modern gliders are even more benign than older designs. Why would it help recovery? The videos say the recovery for a tail stall is to pull the stick back. > Tail stall just runs out of up elevator authority. With one > exception, tail stall is benign. [quoted text clipped - 10 lines] > hinged elevator type tails. The fix is to be very aware of your CG > and to winch these gliders carefully. OK, thanks - my CG is at 60%, I have a fixed stab/hinged elevator, and I don't winch. -John >According to the videos, if the tail stalls you need to recover by >pulling back on the stick, not pushing forward on the stick as we >usually do. I hate to risk adding to the obvious confususion but.... The NASA video deals with a specific case where a contamined tail results in a uncommanded sudden downward motion of the elevator, which in turn results in a forward stick motion, and a nose down pitch. The situation would appear to be completely different from an aerodynamic stall of an uncontaminated tail surface. The use of the term "tail stall" for the icing induced pitch down seems misleading to me since the tailplane could not be at critical angle of attack if returning the elevator to its pre-displaced position restores the downward tail force. A significant difference between the two scenarios is the stick motion associated with the event. Iced up tail - nose pitches down as stick moves forward. Aerodynamic tail stall - nose pitches down as stick moves aft or stay where it was. Linking to the thread on stall awareness and recovery, the iced tail situation that results in down elevator and uncommanded forward stick motion may be hard to distinguish from a stick pusher event, and the required recovery for these is exactly opposite. Andy Andy, Let me reduce my original question to a specific situation. Suppose you're flying in 60 degree weather, so no surface is contaminated with ice. Suddenly, the nose pitches down and the stick stays where it was. My question is: how do you know if you've suffered a wing stall (where the recovery is to push the stick forward), or a tail stall (where the recovery is to pull the stick back). -John > I hate to risk adding to the obvious confususion but.... > [quoted text clipped - 21 lines] > > Andy > Let me reduce my original question to a specific situation. Suppose > you're flying in 60 degree weather, so no surface is contaminated with > ice. Suddenly, the nose pitches down and the stick stays where it > was. I don't know how to answer that without knowing more about the flight condition and the stall margin. If the sudden pitch down was associated with a loud noise I'd bail out! Andy > > Let me reduce my original question to a specific situation. Suppose > > you're flying in 60 degree weather, so no surface is contaminated with > > ice. Suddenly, the nose pitches down and the stick stays where it > > was. If you are flying within the CG limits and at a reasonable speed (i.e. within the envelope), then the main wing will stall before the tail. Any modern aircraft will be designed to have good stability when loaded within the CG limits and flown at reasonable speeds. A propensity for the tail to stall in these conditions would be similar to what would happen when an aircraft is loaded aft of the CG limits and stalls (look up "deep stall"; and once you understand it you will never load an aircraft aft of the recommended CG limit _ever_). I've seen this happen with experimental R/C aircraft that I was testing (as a hobbyist, not as a professional). Of course, flying outside of the CG limits and/or pulling high-G aerobatics (wherein you might induce sudden and extreme AOA changes in the flying surfaces) are another situation entirely... The beauty is if you're not fying an aircraft with contaminated flying surfaces, both a tail tall and a main wing stall should have roughly the same recovery procedure: Allow the aircraft to regain flying speed and reduce the angle-of-attack (once air is flowing normally over the aircraft, you're fine), and then return to a normal pitch-and- bank attitude. The only difference is in recognizing what you need to do in order to regain flying speed. If you're holding the stick back and the airspeed needle is down around zero, stop holding the stick back! If you're holding the stick forward and the ASI is resting on its stop, relax the forward pressure! The vast majority of the time, bringing the controls closer to neutral is the right thing to do - mostly because of the built-in stability of nearly all aircraft produced in the last 60 years. But again, this is true only when the aircraft is loaded within the CG limits. Kirk - I really lament the fact that so few pilots truly understand what the air is doing as it flows over their aircraft. Seeing the diagram in a pilot handbook and memorizing answers for the FAA test is just not good enough. SO MANY "pilot-error" accidents could be avoided, if only people had a better grasp of the cause-and-effect relationship between the cockpit controls, the flying surfaces (as the controls are moved), and the airflow over the aircraft. Being able to understand "if I do X, then Y will happen" (in terms of the airflow and the aerodynamics) goes a long way towards knowing how to get out of trouble, or avoid it altogether! Without this understanding, I think a lot of pilots don't really know what risks they're taking as they blithely fly along, and they don't understand how their safety- margin is changing as they take different actions in the cockpit. *sigh* OK, rant over... :-P --Noel > Andy, > [quoted text clipped - 8 lines] > > -John I don't know how you would encounter a tail stall in a glider without ice buildup or some severe damage to the tail. It's just not a realistic scenario. In other words, don't worry about it. Todd 3S >I don't know how you would encounter a tail stall in a glider without >ice buildup or some severe damage to the tail. It's just not a >realistic scenario. In other words, don't worry about it. > >Todd >3S I am pretty sure I have encountered a tail stall due to a gust experienced in straight and level flight in a K18 at 50kts. I was alarmed to find myself suddenly pointing at the ground! Normal recovery worked. Thankfully has never happened again. I know how gliders work...magic. Is it really necessary to know how stuff works to fly a glider? Design and build one yes, but to fly one? Jim On Feb 21, 2:15 am, Jim White <j...@boffins.co.uk> > I know how gliders work...magic. Is it really necessary to know how stuff > works to fly a glider? Design and build one yes, but to fly one? > > Jim Define "necessary". If you find minimal competence acceptable, chancing that you will never encounter a situation beyond your skill, you might get away with an ignorance of aerodynamics. But, if you want to achieve anything beyond that, If you want to significantly enhance your safety, yes, it is necessary. >On Feb 21, 2:15=A0am, Jim White >> I know how gliders work...magic. Is it really necessary to know how [quoted text clipped - 11 lines] >But, if you want to achieve anything beyond that, If you want to >significantly enhance your safety, yes, it is necessary. I know my glider's VNE, Va, and stall speed. I know its G limits, CofG limits, flap speed limits. I know that it is cleared for loops and spins but not inverted flight. I know that stall speed will increase by 1.5 at 60degrees of bank. I know that contamination (rain, ice, or bugs) on the leading edge spoils the magic and increases stall speed by an undefined amount. Oh, by the way, I have been taught about high speed stalls. What other aerodynamics stuff do I really need to know? If I stay within the flight envelope and concentrate on my flying, lookout, and xc tactics I reckon I should be safe without actually knowing how it actually works. It is a bit like met. I can't do the maths but I can read a tephi. Jim In message <717782dc-84ad-4166-a177-d5c1d73eba6d@j10g2000prn.googlegroups.com>, bildan <bildan@comcast.net> writes <snip> > If you want to >significantly enhance your safety, yes, it is necessary. It is? If something goes wrong I'll be too busy trying to aviate my way to safety to think about the aerodynamics. Hopefully they have informed the training I have received (which is what I successfully used to resolve in the only significant incident I've been in so far) , but the training is devised by much better and more experienced pilots than myself.  SignatureSurfer! Email to: ramwater at uk2 dot net I had a long post I started to write on this topic, but I can shorten it down to two things: 1) No one is saying you have to be able to do aerodynamic calculations or design an airplane, just that you understand aerodyanmics in general and the cause-and-effect nature of your control inputs and how the glider is going to react to different situations (angle of attack, horizontal and vertical gusts, etc). 2) I continue to be astounded by the pilots out there who are perfectly happy to GAMBLE THEIR LIVES by taking part in an activity where they don't know what it is they're doing; AND they will even state outright that they don't CARE that they don't know (like some of the people who said so in this thread)!! Do they really put so little value their well-being? Or do they just think that an accident will never possibly happen to them, no matter how poorly (_or_ how well) they fly? Bottom line: If you don't know WHAT you're doing to the aircraft and the air, then you have no way to judge risk or know how safe you're being with your flying. So you _are_ GAMBLING - its not a "measured" risk, and you aren't able to manage your level of risk if you don't know what it is! And the penalty for failure is injury or death... Seems like education and understanding are a small price to pay! When you are in a stall or a spin or having an emergency, the glider won't care if you were "taught a certain way" by more experienced pilots. The aircraft also doesn't care what the textbook says. And the atmosphere is _going_ to behave in ways that don't go along with the textbook examples. All the glider cares about is proper airflow and aerodynamic principles - and if you don't know those things then you aren't guaranteed to be able to recover from the problem. Good luck with that... --Noel > I had a long post I started to write on this topic, but I can shorten > it down to two things: [quoted text clipped - 33 lines] > > --Noel I'd say that in any of these situations, knowledge of aerodynamic principles will not be of much use. Because you will not have the time to work out a course of action from those principles DURING the flight. The time to work out the course of action is ON THE GROUND, BEFORE THE FLIGHT. Todd 3S >I'd say that in any of these situations, knowledge of aerodynamic >principles will not be of much use. Because you will not have the >time to work out a course of action from those principles DURING the >flight. The time to work out the course of action is ON THE GROUND, >BEFORE THE FLIGHT. .and presumably based on some knowledge of aerodynamic principles ? Ian On Feb 23, 9:45 am, Ian Cant <remove_to_reply.ianc...@inreach.com> wrote: > At 13:06 23 February 2009, toads...@mac.com wrote: > [quoted text clipped - 7 lines] > > Ian Of course. But the beautiful thing about learning on the ground, is that you can draw on the knowledge of others to work out the plan of action during flight. Todd 3S At my club we had some tail stall situations with a Cirrus Standard during winch launches. With some of the lighter pilots the glider would tend to overrotate on takeoff and when the stick was pushed forward to lower the nose it only got worse. Since it´s an all flying stab, pushing forward would increase the already large angle of attack and stall the tail. A fast thinking pilot found that a quick pull on the stick would unstall it. Sometimes it required ´´pumping´´ a couple of times to regain full pitch control. After a few unpleasant launches we did a weight and balance and found that the glider was tail heavy. After correcting this condition the overrotation tendency was greatly reduced but it´s still possible. I guess that´s one of the reasons they don´t make gliders with all flying stabs anymore. Regards, Juan Carlos Jim, Thanks for sharing your story. What I'm most gratified to learn is that normal recovery techniques worked. -John > I am pretty sure I have encountered a tail stall due to a gust experienced > in straight and level flight in a K18 at 50kts. I was alarmed to find > myself suddenly pointing at the ground! Normal recovery worked. > At 20:47 20 February 2009, toads...@mac.com wrote: > [quoted text clipped - 14 lines] > > Jim Jim, Was the gust vertical ? and in a downward direction ? That's what I would think was necessary to produce a tail stall. A downward gust would exceed the stall angle of attack on the tail and produce a nose down pitch. Of course a strong gust can produce pitch excursions without causing tail stall specifically. On knowing how gliders really work, I think that some knowledge of aerodynamics is mandatory to be a safe pilot, but some is optional. For example: Mandatory knowledge: HOW lift relates to angle of attack (especially stall). Optional knowledge: WHY airfoils work the way they do. Mandatory knowledge allows a pilot to correctly fly the aircraft. Optional might help support that, but isn't critical to the flight. Being an aerospace engineer, I hear pilots explain a lot of flight physics completely wrong, but they are competent and safe pilots. Conversely, I hear some pilots explain the physics correctly, but would not allow a loved one to fly with them. IMHO all the aerodynamics that I pilot needs to understand is in "Stick and Rudder" by Wolfgang Langwiesche. Todd 3S > I am pretty sure I have encountered a tail stall due to a gust experienced > in straight and level flight in a K18 at 50kts. I was alarmed to find [quoted text clipped - 4 lines] > works to fly a glider? Design and build one yes, but to fly one? > Jim before reading this thread I did not know anything about tail stall, I did not even know that such thing existed nor I new that horizontal tail push down not up...... but I think I did experience one tail stall on a K21: I was flying from the front seat with a young passenger holding the stick for the first time in his life to keep the glider flying straight at 50kts, while I was monitoring the front stick to dumpen sudden movements if any, the K21 from level flight pitched the nose down at least 30/40 degrees. It was fun, as usual, and recovery was normal. It is interesting to note the K21 from level flight, full stick back will not stall, but only sinks more with mush controls, to make the K21 stall and have the nose pitch down we normally climb at about 30 degrees. maybe it was a tail stall due to a gust............ too bad I did not have a vid cam at that time!!! because after the flight when I asked for explanations, I did not get any. ......... they were staring at me, looking funny. On Feb 23, 5:06 am, toads...@mac.com wrote: > I'd say that in any of these situations, knowledge of aerodynamic > principles will not be of much use. Because you will not have the [quoted text clipped - 3 lines] > > Todd A general course of action is good to work out ahead of time, yes. HOWEVER, if you look at some of the famous accidents and close-calls in aviation history, you'll find that the people who _intuitively_ understood what was going on and were able to correct for their unique situation are the ones that made it out OK. There are plenty of "pilot error" accidents where people took action based on reflex, and it made the situation worse (instead of better) and killed them. Emergencies aren't always straightforward and they don't work out like the textbook. If you find yourself in an unusual situation you have to be able to detect what's different or unique, and modify your pre- planned course of action to suit the situation. That's why I think learning stall and spin correction "by wrote", for example, is very bad. Instead of teaching people to "push the stick", its better to teach them how to un-stall the wing by reducing the angle of attack. Now, in _most_ cases, that means reducing back- pressure or a forward stick input - but not in _all_ cases. If you simply have a Pavlovian response to situations and don't understand the situation or what your reaction is doing, sooner or later you're going to get it wrong and make a situation worse instead of better (for instance: in a spiral dive if you react like you're in a spin). Now, some of you may be thinking "geez, this guy is talking about these long chains of reasoning that you must go through to diagnose the situation and then figure out the appropriate response - there's no WAY you can do that while flying!" To those people I say this: You don't think that catching a ball is too hard, do you? But your brain is doing some serious math to calculate the trajectory of the ball so that your hands are in the right position to catch it. And even people with modest IQs can catch a ball quite well (as a side note: I make no claims about my IQ _or_ my ability to catch!)... The difference is that you've been catching objects with your hands since you were very young, so its all very normal and intuitive. Flying is something that most of us don't do on a daily basis through our formative years. In flying, you just haven't spent enough time practicing and working on understanding the knowledge so that it becomes a fast or intuitive process. You probably felt similarly overwhelmed when you first learned to fly and were trying to handle the aircraft controls while visually scanning all around the aircraft at the same time - especially if you then had to add juggling radio calls on top of it! But with practice and understanding, it became easier and was no longer overwhelming. The same thing is true when it comes to thinking about the aerodynamics of your flight, as you fly. Take care, --Noel >HOWEVER, if you look at some of the famous accidents and close-calls >in aviation history Please tell us about these famous accidents and how a knowledge of aerodynamics would have averted them. >(for instance: in a spiral dive if you react like you're in a spin). Geez Noel. Are you saying I don't know the difference between a spiral dive and a spin? That was learned very early in my gliding career. Or maybe you are saying that because I don't understand how a wing works I won't consider whether I am in a spin or spiral dive and so apply the wrong control inputs? Does anyone else see the connection? I don't. There is always plenty to do in my cockpit other than thinking about aircraft design. My glider simply does what it was designed to do and in the way the manual tells me it does. Jim > Does anyone else see the connection? I don't. I don't claim any expertise in aerodymanics but I have never felt that it placed any limitation on my ability as a pilot. With a perfect knowledge of aerodynamics and a full understanding of the characteristics of the glider being flown, all pilots still lack complete knowledge of the airmass they are flying in. How can the aero solution be derived without a knowledge of the airmass behavior? I very much doubt there is a good correlation between pilot proficiency, particularly cross country flying skills, and the theoretical knowledge of aerodynamics. If I'm wrong we should see Noel in the National team quite soon. Andy >> Does anyone else see the connection? I don't. > [quoted text clipped - 9 lines] > theoretical knowledge of aerodynamics. If I'm wrong we should see > Noel in the National team quite soon. I have to agree with Andy. I've flown gliders for 33 years, 6000 hours (5500 cross-country hours), and instructed for about 12 years. I am geeky to the point of annoyance about aerodynamics (I'm sure pilots around the world hesitate over the "send" button because they are afraid I will whip out my "Fundamentals of Sailplane Design" and ruin a good argument). And yet, I can't really think of how my knowledge of aerodynamics has kept me safe all these years. Same for my students. Sure, we talked about aerodynamics, but it was to reinforce the lesson or to satisfy curiosity, not to equip them to deal with a flight problem (literally) "on-the-fly". And frankly, most of us have such a simplistic understanding of aerodynamics, it's probably best we don't try to use it to deal with a flight situation. Shoot, we still have discussions about which way the elevator is 'lifting', yet pilots aren't crashing because they don't know the right answer. I'd like to hear of examples where a knowledge of aerodynamics saved the day. I don't have any I can recall, but maybe someone's comments will jog my memory. SignatureEric Greenwell - Washington State, USA * Change "netto" to "net" to email me directly * Updated! "Transponders in Sailplanes" http://tinyurl.com/y739x4 * New Jan '08 - sections on Mode S, TPAS, ADS-B, Flarm, more * "A Guide to Self-launching Sailplane Operation" at www.motorglider.org Glad to see people Dog-piling on me and exagerating my comments... *smirk* Makes me wonder why are you folks feeling so threatened and defensive. I'm not advocating a college course in order to get your pilot's license, I'm not calling for any changes at all in my comments... Why are folks blowing this out of proportion? Please re-read my original post carefully. I never said you had to have a "perfect" understanding of aerodynamics. I also never said it would make you a fast race pilot. I'm saying that pilots should understand what is happening - aerodynamically speaking - when they deflect the control surfaces on their aircraft. The sum total of your knowledge should not be "I push the stick to the right to make the airplane roll right". It should be "I push the stick to the right which makes the left wing aileron go down, increasing angle of attack and therefore the lift on that part of the wing. Simultaneously, the right wing aileron is doing the opposite movement with the opposite effect. The net _effect_ is that the aircraft rolls toward the right - with a bit of adverse yaw because the increased lift on the left wing has a bit of rearward action/angle on it." And since people are taking me so literally, let me clarify that I'm not advocating you talk through this whole sequence with each control movement - but it _should_ be something you intuitively know is happening. In a cause-and-effect system like flying, you need to understand the "cause" bit - because the "effect" is only guaranteed under certain limited conditions. What I am claiming, is that knowing this stuff will make you a better pilot and perhaps a more consistent pilot. Most of all it will make you a safer pilot. As Bob Wander is fond of saying, "your aircraft is your life support system". I think its mind-boggling that people are willing to use a life-support system they don't understand. Regarding the sarcastic "National Team" comments: I'm working on that. :-) I've only been flying for 2 years (well, 2.5 if you count powered aircraft as "flying"). I will be competing in my first Regionals here in late April at Warner Springs. Last summer I bought a DG-300 and took my first 7 flights out of an unfamiliar airport (Ephrata). I made 3 flights in excess of 400km (at 82 - 97km/hr); and all 7 flights were in excess of 200km - even the first couple of familiarization flights and on days with OD and rain. I'll let this die now - I'm sure many of you will post follow-on comments that will tempt me to jump back in and clarify - but I'll try to resist. If you're dead set on arguing this, nothing I say will change your mind - so I'll try not to waste everyone's time. --Noel > Glad to see people Dog-piling on me and exagerating my comments... > *smirk* Makes me wonder why are you folks feeling so threatened and > defensive. I'm not advocating a college course in order to get your > pilot's license, I'm not calling for any changes at all in my > comments... Why are folks blowing this out of proportion? ... > --Noel Noel, Maybe are responding to the tone of this post by you: > 2) I continue to be astounded by the pilots out there who are > perfectly happy to GAMBLE THEIR LIVES by taking part in an activity > where they don't know what it is they're doing; AND they will even > state outright that they don't CARE that they don't know (like some of > the people who said so in this thread)!! After they had stated reasonable positions on how much aerodynamic knowledge is needed during flight. So don't be suprised that people kick back. Todd Smith 3S Many thanks to those that supported my argument. My reaction was indeed against the SHOUTED proposition that I was a dangerous numb head because I felt that a knowledge of aerodynamics was not fundamental to my flight safety. Noel, seems I have an edge on you. I have been flying 15 years and have flown in 7 UK Nationals competitions, sometimes doing quite well. I suspect that when you have built up some experience you may temper your assertions. Jim Argh, so much for my resisting a follow-up: 1) I need to apologize. My flight experience may be small, but my computer experience stretches back to age 6, long before there was an "internet". In the good old days of dial-up modems (300 baud, baby) it was common to use ALL CAPS as a replacement for bold text, and _underscores_ to indicate italics. The "CAPS is SHOUTING" standard evolved later (sometime in the late 80's if I recall correctly). I understand it, and hate shouting as much as everyone. But I still sometimes slip back into all caps when I'm trying to really stress certain words or make things stand out. My intent was not to yell _at_ people, but rather to raise my voice in disbelief that people don't see (or care about) the connection between their understanding of how air flows over their aircraft and how it affects their safety. The whole "lack of vocal inflection in typed communications" thing still rears up and bites us all from time to time. 2) One last time (and I promise, it really is the last time in this thread I'll post): I think people took my initial comments too far. My main point is that I see too many pilots talking (and worse, too many instructors teaching students) about moving the controls and how the aircraft reacts. And the connection to aerodynamics is lost. Sure, people read in a textbook about lift, drag, and gravity. And they see a pretty picture about angle of attack. But then once they take the FAA written test, the knowledge falls out of their brain and they never think about it again. The thing is (and this is my central point) - the aircraft doesn't _care_ what you do with the controls or what the textbook says. The aircraft is going to react to what the air around it is doing; and the controls are just a means to manipulate that airflow (and only to a limited extent). However good or however safe a pilot may be, I personally believe they could be even _better_ if they keep that rattling around in the back of their brain. Not thinking through every step or any calculations; but just the general ideas and concepts and an understanding of how it all works. And I believe a pilot who doesn't have this knowledge and doesn't think about this stuff is at a much greater risk of reacting improperly in an emergency situation (or unintentionally doing something that increases their risk-factor during a flight). Alright, I promise I'll really let the thread die now... :-P --Noel P.S. Jim - just a note about experience: I flew R/C gliders for a few years before going "full-scale". My favorite thing was to fly 24" wingspan flying wings on small hills (20 - 30 feet tall) using ridge/ slope lift. The total CG envelope was 1/8th of an inch in size, and total movement of the control surface was about 3 millimeters (full back-stick to full forward-stick). And we flew 1 - 2 feet off the deck a lot of the time... Its different from flying full-scale stuff, but it really trains the hands and makes the pilot focus! ;-) It is impossible to separate the aerodynamics that you learn from the books from the total learning experience putting things you learned in the book together with things you learned from your Instructor. The total package is what makes us better pilots. I know that if you get into a spin because of a bad turn from base to final, you are not going to have time to decide, 'did my tail stall' or what. I think this topic got started when the video came out about the videos about icing causing a tail to abruptly go down when the area of stagnant air moved back to the rear of the stabilizer. This caused a movement that is not expected and the response has to be a learned not an instinctive one from regular 'stall' warning signs. You have to have the whole package of book facts and mechanical skills learned in the cockpit to make a safe pilot. >>> Does anyone else see the connection? I don't. >> [quoted text clipped - 31 lines] > day. I don't have any I can recall, but maybe someone's comments will jog > my memory. > Bumper, Toad - thanks. Sorry for my confusion about pitch direction - > you're right, it would pitch down. The recovery with a tail stall, [quoted text clipped - 18 lines] > > can never get into a full stall. Cessna 152's and SGS 2-33's are > > examples. What? I would love to see you provide evidence to back this up. The buffet is primarily caused by the wing stalling (i.e. causing airflow separation) near the root of the wing, and the ugly airflow from this striking the aft fuselage and tail as it goes by. A tail is an upside-down wing in a conventional aircraft (canard aircraft are the exception) - therefore raising the angle of attack of the wing *decreases* the angle of attack of the tail. NOTE: There is a difference between a tail stall and an aircraft who's tail is sized and limited in elevator travel to prevent the pilot from really getting a good full stall with a break. My DG-300 is an example of this - I can get a stall break if I perform an accelerated stall; but if I start in slow flight and gradually increase the stick pressure, the aircraft doesn't exhibit a true "stall break" - it just buffets a lot and develops a good rate of descent. > > There's a simple test for this. With the stick full back and the > > glider exhibiting pre-stall buffet, apply aileron and if the glider > > responds normally in roll, the wing wasn't stalled. If the wing was > > stalled, the glider would probably try to spin with the application of > > aileron. Uhhh, no not necessarily. You are ignoring the effects of wing taper, twist, and span-wise distribution of lift. Wings can (and ARE) designed to stall near the wing root first, and then progress outward along the span. So you can get "stall buffet" and still have aileron control because the outboard parts ofthe wings are not stalled (they are flying at a lower effective angle of attack and therefore still have clean airflow over part or all of the wing chord). It has nothing to do with the tail stalling! Oh, and before you do that test, please remember that applying aileron input will make one of the ailerons move _down_ - thus increasing the angle of attack on that part of the wing. If you've got a partially- stalled wing and you increase the angle of attack on the outboard (unstalled) part of it, you may stall the whole wing. Meanwhile, you are _decreasing_ the angle of attack on the wing that has the aileron deployed upwards. Thus that wing will likely _not_ completely stall. Everyone knows what happens when one wing stalls and the other doesn't completely stall, right? Spin time! This is why we teach people not to use much (or any) aileron input when feeling the pre-stall buffet. Again, this has nothing to do with the tail stalling. --Noel To add to the confusion: Many modern gliders are elevator limited - full aft stick just won't result in enough AOA to stall the wing. Nothing to do with the tail being stalled. In fact, it's "lifting" just fine, just in the "down" direction (which could be up if you are at the top of a loop). If the tail did actually stall, you would get a brisk nose down pitch rate, I would think! It would be almost the same as the horizontal tail falling off. Think about it.... Also, the tail on modern gliders do not always push "down". Depends on speed, AOA, G-load, CG location, flap position, etc. You have to look at the entire lifting contributions of all the parts of the glider. Stability is due more to decalage. Look it up. It's amazing how little most pilots know about how their aircraft really work....including yours truly! Kirk 66 Just remembered that there is a good discussion about glider-induced towplane tailstalls - both horizontal and vertical - in the BGA towplane manual. The rudder one kinda sounds like fun, actually, as long as I had a parachute and lots of altitude! Sorry, no link, I'll try to find it and post later. kirk 66 > Bumper, Toad - thanks. Sorry for my confusion about pitch direction - > you're right, it would pitch down. The recovery with a tail stall, > though, is to pull back on the stick, not push forward. That's why I'm > worried about differentiating a tail stall from a wing stall. > > Bill, please see comments embedded in your reply. I think the others made the essential point that the video subject was a special situation where ice on the tail leading edge led to a separation bubble on the lower surface with flow re-attaching aft of the hinge line. The re-attached flow was sucking the elevator down (nose down) so the pilots were advised to pull back to counter this force to retain control of their aircraft. The violent pitch down was due to this effect. The video also talked about non-linear elevator effects which could lead to PIO's As for the tail/wing stall discussion, unless you've flown a glider with yarn tufts on the wing and tail, this is hard to visualize. Yes, of course, the wing is designed to stall at the root first. In fact, unless there is a very effective root fillet, there will be some root flow separation visible at any AOA. In every case I know of, the yarn tufts show full flow separation on both upper and lower surfaces of the tail when as nose 'breaks' suggesting a wing 'stall' to the pilot. At the same time the wing tufts show attached flow over the majority of the wing which is why the ailerons work as expected. You can say the tail just loses effectiveness or you can call it a tail stall - the distinction isn't really important. |
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