Wind and stalls (split)

[aaron886]

Yeah, and obviously right there you will stall, too.

 

Might have to elaborate, that doesn't make much sense. (To me at least.)

[BlueRidgeDx]

I'm having trouble deciphering what some of you are trying to say, lol.

 

It is true that the presence or absence of wind does not affect the AoA at which the aircraft stalls, but it is also true that wind can directly cause a stall. There are times when the airmass you are flying through is not as homogenous as some are implying.

 

In the "real" simulator, I would typically give the clients a microburst encounter either on final, or during takeoff. In the takeoff scenario, at some point during the takeoff roll before V1, the airspeed would stagnate as the tailwind component increased. If they didn't reject the takeoff, the usual result was an attempt to rotate early - below Vr - and they would get the stick shaker, usually followed by the stick pusher and the subsequent Red Screen Of Death.

 

The landing scenario was different because initially, you actually get increasing performance as you fly into the microburst since you're getting an increasing headwind. The headwind causes you to float high on the glideslope and the typical response is to pull the throttles back to flight idle. Even at idle, you might gain a full 20+ KIAS. Then the fun begins...you fly next into the strong downdraft core and emerge into an area of rapidly increasing tailwind. Even at TOGA thrust (which still takes 3-4 seconds on modern engines; 8-12 on older ones) you will lose 25+ knots in only a few seconds. There are many examples of microbursts exceeding the aircraft's performance capabilities. Delta 191 is a textbook example...you can even find the CVR audio online.

 

I'm purposely leaving out the details about keeping the engines spooled up on approach, and about the approach idle function of most FADEC engines, in order to illustrate a point. The point being that there are localized phenomenon such, as microbursts, that disprove the argument that the airmass surrounding an airplane is always some ideal homogenous mass.

 

All that being said, I don't think this has any bearing on the SFM/AFM comparison.

Edited June 13, 2012 by BlueRidgeDx

[smnwrx]

Just gonna jump in and stir the pot. An a airplane is chained down loosely on the ramp. Holy crap! 50kt steady head wind. The wing is now producing lift. Wind dies down and vuala! The wing is stalled. Not an engineer so I'm into sure if this relates at all.

I agree with what is posted above. A stong tail wind just pushes a plane faster over the ground thus keeping the IAS the same but a windshear near stall speeds can cause the wing to lose lift.

Edited June 13, 2012 by smnwrx

[159th_Viper]

 

All that being said, I don't think this has any bearing on the SFM/AFM comparison.

 

In the OP's defence, that was exactly the point he was trying to make, ie that certain conditions affecting flight might be present in the AFM that are not present in the SFM (such as but not limited to the factors you mentioned above), but availed himself of an example that did not really find applicability in the circumstances and was as a consequence picked apart.

 

Ta for the explanation :)

[Shaman]

and side-loading moments from cross-winds.

 

This is very correct answer for this thread and pretty much answers relation of wind causing airplane to stall. Crosswind is actually critical on apprach, life and death for General Aviation especially. Most stalls that ended up in tragedy happened during last moments in final approach or turn for finals caused by pilot used to fly certain speeds (and weights) and not increasing his airspeed enough to deal with lift problems caused by cross-wind scenario. Cross-wind can cause you too stall by reducing lift (even worse since it is asymmetric) on your little wings by disrupting and blowing off linear airflow too on normal approach speeds (typical for GA). You must add enough indicated airspeed speed during all phases of landing if you have cross-wind even on large planes.

And remember, landing with significant cross wind use smaller degree of flaps!

 

Edit: We had quite a lot GA disasters recently in Poland, not increasing IAS in crosswind scenario during landing phases is quite common pilots' error.

Edited June 13, 2012 by Shaman

[EtherealN]

that disprove the argument that the airmass surrounding an airplane is always some ideal homogenous mass.

 

I'm not sure this argument has been made though.

 

Windshear, microbursts, or even an unusually strong thermal bubble release can do funny things to an aircraft. First time I was in a glider (as passenger on a demo flight years ago) we actually couldn't get down because our assigned downflight sector had an unusually strong thermal in it (go figure...), and doing a descent in there would require either dangerous nose-down attitude or dangerous bank angle (where the differential air vertical velocity of air would threaten to flip the plane) if it wasn't for double Shemmps.

 

The cause of argument is, imo, clearly one of language; the use of the word "wind" does indicate that we are not talking about microbursts or other very localized phenomena; according to my encyclopedia and dictionary: "Wind: current of air moving in relation to the earth's surface."

 

That's basically my objection; and I've seen this misunderstanding very often where people somehow cannot "detatch" from the ground and think that a 50km/h wind might cause them to stall - during level flight at altitude. Essentially the common mistake of thinking about speeds in relation to the ground, not the air, as is also common during initial navigation training. (And I had a bit of trouble there too - when you intend to fly from a Point A to a Point B that are geographic points on the ground, but fly through a mass of air that is moving relative to these points, it can take a couple minutes to get your head around it the first few times.)

 

Microbursts, windshear and so on are special cases that you definitely need to be really careful with and that can ruin your day pretty quick. But these phenomena aren't causing problems because of air's relationship to the ground, they're causing problems because of their relationship to your aircraft or (potentially disastrously) having a different relationship to different parts of your aircraft.

 

Same thing with the landing scenario: it is easy to become "ground-fixed" such that you start flying only for the runway and forget the wind, and then cause yourself to enter stall conditions - but the key here isn't a specific aerodynamic effect of "wind", it is the pilot forgetting to fly the aircraft. (And of course, landing can be a pretty busy time of your life, especially if there are tricky conditions meaning that your approach is unusual or your margins for landing reduced etcetera, making this mistake easier to make.) One of my first solo flights saw me get blown way off course on final turn, whereafter I got sort of saturated fixing it and ceased to mind my airspeed: fortunately for me the resultant airspeed deviation was positive, not negative, so that was fine. (During no-instrument practice it turned out I always make errors in judgement there when I get really busy, but the errors are in the "right" direction so to speak - fortunately for me. :P )

[WildFire]

In the OP's defence, that was exactly the point he was trying to make, ie that certain conditions affecting flight might be present in the AFM that are not present in the SFM (such as but not limited to the factors you mentioned above), but availed himself of an example that did not really find applicability in the circumstances and was as a consequence picked apart.

 

 

Actually if they had to model it it would have been applicable. Just because they didnt have to model it because the effect was already dealt within their model doesnt mean it wasnt an applicable quantity. The original post had a few different variables that could have been included in the equations. Obviously there is not only one way to program stuff. There are many possibilities. I dont even know how and what equations they used to model theirs so I couldnt begin to say. I would venture to say that most if not all of us dont know the exact equations and everything they used to model the FM, part (see the opposite of all) of the reason why they cant just throw it out to others. Therefore because probably no one but the engineers know how they wrote the complete model means it is arbitrary to argue about what has been included or not.

 

Furthermore, there has been a lot of debate about speeds, IAS, TAS, GS... Granted moving in an air mass has no effect unless its a crosswind, then it would seem that while your plane may be moving in direction x but the wind is pushing you in the y direction. Or even Z or X. The moderators immediately assumed we were talking about IAS and came to the conclusion that they were right cause I was obviously wrong saying that wind has no relation on IAS which is correct, however my general statement was wind affects speed, mind you I did not specify which and therefore calling me blatantly wrong in this scenario is a blatant show of tour de force. In that very statement that moderator said that it may affect approach therefore agreeing with my post all along in that one specific circumstance of flight. I find that kind of funny he maintains his correctness any further than that point. You cant agree with someones point and then continue to call them wrong, this isnt politics, come on... There is also much argument on the wind aspect. Depending on where you look at the definition it can include gale forces, or microbursts or many others, or it may not. Picking one definition and believing that is the end all to prove your argument is the same as manipulating facts to support your argument, both are considered bad tactics in the professional debate world.

 

Noodle, I wasnt talking about angle of the aircraft, rather angle of the wind/airflow in relation to the engines and wings among other things. I probably shouldnt have used angle of attack in that case but I did not know a better way to say it. Thank you and thanks for your input shamandgg.

 

I also noticed in the original thread one of the mods went through and picked apart my original post in that thread when it clearly says at the bottom it was a basic explanation. Its not like my post was supposed to actually be physics.

 

See I am a science major, plenty of upper division physics and maths so I have no problem getting into those types of conversations, but in a public medium where people ask a simple question, a simple answer is warranted. It shouldnt be picked apart and judged as if whatever you said wasnt written from the bible (if that sort of thing is important to you). I firmly stand by my belief on these forums that the moderators go way way too far to prove a point to the point where it become a game of "Im right your wrong" and it degrades to a point where I'm no longer willing to debate with child like mentality. Its hard to try to help and answer peoples questions when people are blatantly calling you wrong when your example wasnt meant to be right, only simple. A stong man exhibits both assertion and restraint with equal measure in any argument. Im noticing a bit of a lack or restraint, at least in my opinion.

[WildFire]

but the key here isn't a specific aerodynamic effect of "wind", it is the pilot forgetting to fly the aircraft.

 

Another assumption to support your own theory. While it is the pilots responsibility to take into account the wind that doesnt mean wind is not a factor. Thats redundant. Wind is the factor the pilot must take into account. Argue just to argue if you must.

[EtherealN]

What I am saying there is that aerodynamically, and physically, there is zero difference between that case and just going nose up with throttle idle: the subject matter was Flight Model, not pilot behaviour. Thus things that are caused by the pilot making mistakes are not part of the subject matter.

 

This is also why we "assumed" IAS: we were discussing simulator flight model and stall. Not a pilot's capacity to remember his training.

 

Wind is a factor I have to take into account, yes: but not for a specific aerodynamic effect of wind. I have to take it into account so that I do not place the aircraft in a non-flying condition, which can happen if saturated during a complex landing scenario. What happens then could for example be that I fail to account for the wind when planning my base and final turns, and end upp stalling the aircraft (or for that matter the inboard wing and flip) due to focusing on the ground instead of the air. Withín the context of pilot training this is extremely important: failing it is not something that will flunk you from class, it's something that will send you to the cemetary. But it's not relevant to a flight model.

[WildFire]

Its semantics really, your trying to be really specific in order to prove your point. And its fail logic.

 

As a logical counter to your argument I will pose this:

Suppose wind is not an aerodynamic effect that we have to take into account (which is your point). That would mean wind has no effect at all, while we are flying, because aerodynamic effects are the only effects we are concerned with really. There are other effects like the basic forces of flight that are also to take into account but we already know those. So that is the only possible conclusion. So we have wind but because it doesnt affect the aircraft in any aerodynamic way then its really not a problem. Therefore regardless of the way we maneuver our aircraft we would not enter a stall condition. edit: at least not due to wind.

 

This is your stance correct? Does it not sound logically wrong to you? The point is wind is an aerodynamic effect that the pilot must take into account that if not countered for can help to produce a stall condition. Shamandgg said it, I've read about it, you said it happened to you, its true.

 

Also you cant assume I was simply speaking of IAS, because when programming anything at all it would be assumed in any specific instance you might be using the values for TAS, GS, or IAS. Its not a "well we only use IAS" thing. Flight principles and programming flight principles are two completely different things. Whether your taxiing on the tarmac is going to use GS for the FM and takeoff is going to use a combination of GS and IAS, its just not something you can nail down especially if you havent programmed it yourself.

 

The point is everyone got super ridiculous specific when my original statement about the FM was way way general. This debate never should have started in the first place, you guys are way too critical of the simple stuff people say.

 

And then it got twisted into assumptions about speeds, flight conditions, when the flight was taking place (level flight/approach/landing) all things that were never specified in the original post and were therefore exempt from most of the arguments, because the arguments were very specific. And the explanation was the exact opposite. Extremely critical, again I say it was all for show; tour de force.

Edited June 13, 2012 by WildFire

[y2kiah]

Guys there seems to be a lot of misunderstanding about the concept of "relative wind" vs. the definition of "wind" as the movement of an air mass across the ground.

 

The debate is pointless, there is no gray area in this topic, the science is in, we got it ;-). If you have never studied aerodynamics but you *think* you understand it using logic or common sense, you might want to do some homework.

 

The short answer without going into gory detail is that relative wind is an aerodynamic concern, NOT "wind" meaning the movement of an air mass across the ground. However, wind is a huge factor for pilots flying airplanes in the real world, specifically when it comes to tracking a course across the ground, takeoff, and landing.

[effte]

Therefore regardless of the way we maneuver our aircraft we would not enter a stall condition. edit: at least not due to wind.

 

This is your stance correct? Does it not sound logically wrong to you?

 

Not in the least, as that is the Way Things Are.

 

You appear to argue that a crosswind is a special case? Rest assured it is not - completely insignificant until you try to adjust your trajectory to match that of the unmoving ground, at which point wind does become a factor to contend with. Before that, a steady wind is merely a navigational concern.

 

Whether your taxiing on the tarmac is going to use GS for the FM and takeoff is going to use a combination of GS and IAS, its just not something you can nail down especially if you havent programmed it yourself.

 

We know that GS is in there somewhere, as we see it directly represented in the movement of the aircraft. Other than that, the implementation is all up to disguession, even though observation can give us a pretty good idea of what must be in there.

 

Cheers,

Fred

[WildFire]

Im gonna say pay special attention here to the words "until" and "at which point", because the original argument was about wind affecting stall (in any way, in any condition: not specified). People keep coming on and saying yes it does in some special way and then continuing with the overall point of saying it does not. Im over it. The flight model isnt really up for a discussion unless your the engineer who programmed it, which is why I've stated many times that the debate on what exactly is in an FM is pointless. You can say "they modeled handling with weapons mounted and handling without weapons which acts very different". But do you know what equations were used to program such, and what variables were used? Do you know the rates of change? Nope, and I would figure neither do the mods or testers unless they were involved with creating the actual equations.

 

This argument is dead, leave it.