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Stalls in turbulence

SteveJeff

Member
I have some doubts which I don't master regarding the possibility of having a stall in turbulence. Considering I do most of my flights in good weather, I don't have much practical experience reagrding these issues.

1. Let's say I took off, climbing at 65kts and there is a 15 kts headwind. If at 100ft, SUDDENLY the wind direction changes and becomes a 15 kts tailwind, my airspeed will suddenly drop to 65-30=35 kts, right? I guess it will end up in a spin and being too low to recover...

2. On final, I encounter an updraft, I noticed that updrafts are +Gs, so is this scenario at risk for an accelerated stall?

3. How aircraft manufacturers make that all certified aircrafts are able to sustand the same amount of gusts? I mean all are certified to sustand 50 ft/s up gust (I think I remebered it correctly), but considering the fact a light sport aircraft will be more loaded due to low mass (inertia) than a heavier aicraft for the same gust?
 
Not sure I understand the questions completely. but...


1. wind shear is definitely something to be concerned with. you won't spin unless you stall uncoordinated.

2. perhaps I don't understand, but an updraft would affect the entire aircraft, not just push up on the nose of the plane. Turbulence is another matter that would make controlling the airplane in various axis go from bothersome to unable depending on the severity. see #3

3. This is where Maneuvering Speed (Va) comes in. The Va is defined; based on weight, category, etc, to stall (unload) the wings before exceeding the maximum defined load factor.
 
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I guess it will end up in a spin and being too low to recover...
Well, I am not going to attempt to address the details of your questions specifically,I will leave that to those who are more qualified to do so than I am, I would like to comment on one part of your first question. The above statement did bother me. Why are you assuming that, even if your statement is true, your plane would immediately spin? Just because you stall an airplane does not mean you will spin it! A spin requires "uncoordinated" control inputs while in a stall. If you are coordinated, no spin, just a stall.
 
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I have some doubts which I don't master regarding the possibility of having a stall in turbulence. Considering I do most of my flights in good weather, I don't have much practical experience reagrding these issues.

1. Let's say I took off, climbing at 65kts and there is a 15 kts headwind. If at 100ft, SUDDENLY the wind direction changes and becomes a 15 kts tailwind, my airspeed will suddenly drop to 65-30=35 kts, right? I guess it will end up in a spin and being too low to recover...

2. On final, I encounter an updraft, I noticed that updrafts are +Gs, so is this scenario at risk for an accelerated stall?

3. How aircraft manufacturers make that all certified aircrafts are able to sustand the same amount of gusts? I mean all are certified to sustand 50 ft/s up gust (I think I remebered it correctly), but considering the fact a light sport aircraft will be more loaded due to low mass (inertia) than a heavier aicraft for the same gust?

1. If you are climbing at 65kts IAS, your airplane doesn't know anything about a headwind or tailwind. It only knows about the air molecules passing by it. If those molecules suddenly start passing by 30kts slower than they were one second prior, you will feel an acceleration since your frontal drag has decreased significantly while power output has remained constant. Will you stall? Hard to say.

2. An updraft doesn't change the AOA on the wing usually, so no, an accelerated stall from this event is unlikely. My experience is that updrafts are usually paired with downdrafts, however, and if you pull back hard enough in that downdraft to maintain altitude you might stall.

3. Good question. Searching through FAR Part 23, I didn't see the word "updraft" appear. Are you familiar with manuvering speed, otherwise abbreviated as Va? That, tied to the particular category (standard, utility, etc.) holds the answer I think.
 
Scenario #1. Yes, if the wind shear is instantaneous, the plane will stall. It will not spin unless you haven't been keeping the ball centered. But on stalling it will descend back into the headwind, and start flying again. In the real world the shear layer tends to be a few feet thick, at least, and the plane tends to simply stop climbing and mush along at that altitude, until it accelerates enough to get back to steady state.

Scenario 2 and 3 are really the same, just that "updrafts" tend to be mild and continuous, while "gusts" are of short duration. In the updraft the plane initially accelerates upwards, until steady state is attained. After that the accelerations are zero (back to 1g on the meter) even if you are going up. Gusts are short enough that steady state is never attained. Just a qualitative difference. When the gust hits the main effect is to change the wing's angle of attack, because the direction of the relative wind has changed. Suppose you were smooth and showing 1g on the g meter, when you hit an updraft which changes the angle of attack by a factor of two. Then the wing's lift will double from what it was, and the g meter will show 2g. Notice that the mass of the plane does not apear in the answer, so light or heavy is the same, if both had the angle of attack double. And yes, if the increase in angle of attack exceeds the critical stall angle, it will stall.

BTW, maneuvering speed Va is defined as where the accelerated stall speed is equal to the design load limit. So, if you are at Va (note it varies like the square root of the weight) on final and you hit a strong upward gust, the wing will not stall as long as the acceleration is below 3.8 g (or whatever the plane is designed for)? If the upward acceleration would have exceeded 3.8 g it won't happen, the wing will stall instead. Better stall than break the plane. Faster than Va and you might break something. Slower and you might have an accelerated stall at less than 3.8 g.
 
Steve,
Flying on the east side of the Rocky Mountains it's typical to experience some rather brisk and bizzare winds. Changes in airspeed of 50 knots are not common but not as uncommon as I'd like. It's extreamly rare to see a change like last long enough too cause a big problem. One minute you're flying at Va and the next minute the stall horn is blaring and the bottom falls out. With just enough time to push on the stick but not enough for the plane to react, the wind changes and you're flying again. It's a little more exciting on short final but I've never seen more than a 25knot drop in speed when that close to the ground. That's more than enough to get your attention but momentum helps carry you thru some of it. That's the strangest lesson to NEVER drag it in on final.
 
Had unusual winds here at 52F on 11 Jan. I took off with less than 6 knots wind from the South, but had 51 knots at pattern altitude ~1000 ft AGL! No turbulence, but made for interesting patterns (170 kts GS on downwind!) There was no abrupt shear but more of a smooth transition from 51 kts to 6 kts at touchdown. The -10 held the KIAS I wanted with no special effort.

In hindsight I wish I had done a run at 2000 ft and taken a picture to have over 220 kts GS during cruise to post it on the appropriate thread.:)
 
stall?

Stall is A.O.A dependent. A loss of airspeed to produce enough lift is just that, not a stall. That would be like saying that my aircraft's wing is stalled untill moments before take off.
 
Searching through FAR Part 23, I didn't see the word "updraft" appear.
Gust/flight envelope.

Notice that the mass of the plane does not apear in the answer, so light or heavy is the same, if both had the angle of attack double. And yes, if the increase in angle of attack exceeds the critical stall angle, it will stall.

For an airplane, Va descreases as weight decreases.

But let's say same gust acts on 2 airplanes flying at the same speed (also they same the same Va), but they have different weight. Plane A is 600 kg, plane B is 300 kg. On plane B the gust will have a double acceleration effect. So, what differs that these 2 aircrafts still can have the same gust envelope as required by certification?

BTW, maneuvering speed Va is defined as where the accelerated stall speed is equal to the design load limit. So, if you are at Va (note it varies like the square root of the weight) on final and you hit a strong upward gust, the wing will not stall as long as the acceleration is below 3.8 g (or whatever the plane is designed for)? If the upward acceleration would have exceeded 3.8 g it won't happen, the wing will stall instead. Better stall than break the plane. Faster than Va and you might break something. Slower and you might have an accelerated stall at less than 3.8 g.
I think you mistyped smth in the last part here, it doesn't make sense at all.
will not stall as long as the acceleration is below 3.8 g (or whatever the plane is designed for)? If the upward acceleration would have exceeded 3.8 g it won't happen, the wing will stall instead

Why are you assuming that, even if your statement is true, your plane would immediately spin? Just because you stall an airplane does not mean you will spin it! A spin requires "uncoordinated" control inputs while in a stall. If you are coordinated, no spin, just a stall.

There are planes which in some circumstances even being coordinated there is a significant wingdrop at stall. So, I guess just a stall at 100 ft, it is not really good. And if the stall was trigered by an updraft where the angle of attack of one wing changed, I think the differential lift will be pretty high and even being initially coordinated will lead to a spin.

Let's assume a strong updraft hits the left wing and its aoa becomes critical. The right wing it's still developing lift considering the speed of the airplane was well above stall. The differential lift between the two wings will be really high, right? It will lead to a spin maybe even very hard to recover, am I missing smth?
One would say that spin needs yaw. But I think that gigh differential lift between the two wings will be enough to induce a spin hard to recover. If not, why opposite aileron induces a spin when you are stalled and coordinated? Or it doesn't?

I may be wrong here, I hope someone can clarify things.

Stall is A.O.A dependent. A loss of airspeed to produce enough lift is just that, not a stall. That would be like saying that my aircraft's wing is stalled untill moments before take off.

Interesting assumption. I can't get it at all on my own. So you figure out it will not show a stall behaviour in that situation? Just starts a momentary descent until speed is regained? Isn't that bad at low altitude, too?
 
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But let's say same gust acts on 2 airplanes flying at the same speed (also they same the same Va), but they have different weight. Plane A is 600 kg, plane B is 300 kg. On plane B the gust will have a double acceleration effect. So, what differs that these 2 aircrafts still can have the same gust envelope as required by certification?

FORCE = MASS X ACCELERATION

Force breaks wings. Not acceleration. The force exerted on the two identical wings is the same. Mass is half so acceleration is double...but force breaks wings and that's the same.

There are second order effects of course but I don't think that's what we're talking about..
 
There are planes which in some circumstances even being coordinated there is a significant wingdrop at stall. So, I guess just a stall at 100 ft, it is not really good. And if the stall was trigered by an updraft where the angle of attack of one wing changed, I think the differential lift will be pretty high and even being initially coordinated will lead to a spin.

Let's assume a strong updraft hits the left wing and its aoa becomes critical. The right wing it's still developing lift considering the speed of the airplane was well above stall. The differential lift between the two wings will be really high, right? It will lead to a spin maybe even very hard to recover, am I missing smth?
One would say that spin needs yaw. But I think that gigh differential lift between the two wings will be enough to induce a spin hard to recover. If not, why opposite aileron induces a spin when you are stalled and coordinated? Or it doesn't?

I may be wrong here, I hope someone can clarify things.
Yes, there are planes that will drop a wing at stall. However, a dropped wing is still NOT a spin. Can it lead to a spin without pilot corrections? Sure! Is it a given that it will? No! Being close to the ground (100 ft) can sure cause you some pucker time. However, if you are that close to the ground and stall the plane it is highly probable that you will have troubles to deal with that will have nothing to do with a spin. So, I agree that the STALL at 100 ft AGL is going to be a bad situation. I am just saying that to talk as if the SPIN is inevitable if you stall in the situation(s) you described is NOT true. In every situation you have described you CAN stall the plane without sending it into a SPIN. The plane will not just instantly go into a spin and it will surely not do so with an engaged and informed pilot in control of the situation that reacts appropriately to the cues his plane is telling him.
 
Coordinated turn can spin...

A stall in a coordinated turn can degrade rapidly into a spin... most stall 'training' lacks focus on the dangers of a turning stall on base to final or an aggressive turn to crosswind with a high angle of attack (watched this become terminal first hand). With some altitude nurse your plane along at MCA in a coordinated turn and nudge it just a bit more and see how quickly the world will rotate past the canopy!
 
Stall Recovery

One of the main purposes of Phase 1 flight test - and recurrent training thereafter - is to become intimately familiar with the low speed handling qualities of your aircraft, including stall recovery from straight and turning flight.

Perhaps I am fortunate and all the builder-induced flight control systems errors cancelled each other out and I have very stable low speed / post stall flying qualities with a pronounced stall break and no wing drop. Let go of the stick and recovery is immediate, with little nose drop - assuming nose trim was in the ballpark.

If for whatever reason your plane has a post stall wing drop tendancy, figure out the optimum recovery technique (probably more rudder than aileron) to get the wings stabilized in the shortest time, roll wings level and pull the nose up slowly enough to avoid an accelerated stall. You will not spin an RV unless pro-spin controls are maintained after the stall. You cannot spin any aircraft at zero angle of attack (of course, altitude is a concern).

Better to avoid the ground by a few feet in controlled flight than squander altitude for time in an overreaction with the flight controls that causes a departure.

Works for me - your technique/priorities may vary.
 
1. The poster who said the plane won't stall if you don't increase the angle of attack is correct. Problem is many pilots encountering the shear described try to force the plane to climb by pulling back. If you don't do this, the plane will just settle on at the edge of the shear layer, and as it accelerates you'll start climbing again. Only a problem if there are obstacles ahead.
2. I think the OP is confused about upward gusts. The main effect is not the gust pushing up on the whole plane; the dominant effect is the change in direction of the relative wind, resulting in an increased angle of attack. It's the wing which then accelerates the plane upward.
3. The poster who emphasized force over acceleration is correct for the wing, but only the wing. E.g. If the wing can tolerate a force which causes a 4g acceleration at 3000 lbs, then the same force will accelerate the the plane at 8g when it is only 1500lbs. Problem is, everything else on the plane is affected by that acceleration. If your engine weighs 400 lbs, then at 4g the engine mount has to support 1600 lbs. At 8 g the mount is supporting 3200 lbs! Same for the seats, battery mount, etc. Do not exceed max g loads. Having the wing intact is not of much help if the engine has fallen off.
 
Newton applies to wings and fuselages

3. The poster who emphasized force over acceleration is correct for the wing, but only the wing. E.g. If the wing can tolerate a force which causes a 4g acceleration at 3000 lbs, then the same force will accelerate the the plane at 8g when it is only 1500lbs. Problem is, everything else on the plane is affected by that acceleration. If your engine weighs 400 lbs, then at 4g the engine mount has to support 1600 lbs. At 8 g the mount is supporting 3200 lbs! Same for the seats, battery mount, etc. Do not exceed max g loads. Having the wing intact is not of much help if the engine has fallen off.

I really don't mean to get into a Statics 201 debate here but what you did was reinforce the point. It's force that breaks things, not acceleration. 1600 lbs, 3200 lbs are measures of force. If you're designing an airframe, you have to do a lot of detailed analysis about how forces act on a structure (and all of its individual components, like engine mounts) and its very complex...but that's why finite element analysis software exists. I don't think any of this is permanent to what the original poster asked however...he just wanted to know why two airframes one with half the weight could have the same Va envelope when one would accelerate more than the other...so [end detour]
 
1. The poster who said the plane won't stall if you don't increase the angle of attack is correct. Problem is many pilots encountering the shear described try to force the plane to climb by pulling back. If you don't do this, the plane will just settle on at the edge of the shear layer, and as it accelerates you'll start climbing again. Only a problem if there are obstacles ahead.
And if you were leveled, you'll stay leveled or you start to descend?
2. I think the OP is confused about upward gusts. The main effect is not the gust pushing up on the whole plane; the dominant effect is the change in direction of the relative wind, resulting in an increased angle of attack. It's the wing which then accelerates the plane upward.
What if that resulted angle of attack is greater than critical?
3. The poster who emphasized force over acceleration is correct for the wing, but only the wing. E.g. If the wing can tolerate a force which causes a 4g acceleration at 3000 lbs, then the same force will accelerate the the plane at 8g when it is only 1500lbs. Problem is, everything else on the plane is affected by that acceleration. If your engine weighs 400 lbs, then at 4g the engine mount has to support 1600 lbs. At 8 g the mount is supporting 3200 lbs! Same for the seats, battery mount, etc. Do not exceed max g loads. Having the wing intact is not of much help if the engine has fallen off.
So how that different weights at same speed have the same G load margin?
 
What if that resulted angle of attack is greater than critical?
you stall


So how that different weights at same speed have the same G load margin?

They don’t.

There are two ways for a wing to produce more lift: either increase the Angel of Attack (AoA), or increase the airflow (speed up).

All else being equal, for 2 identical airplanes, except one is carrying more weight… If both airplanes are flown with the wings at the same angle of attack… the heavier one will need to fly faster. If both wings are at the same AoA then they will both have the same G load margin regardless of speed. Therefore: Va increases with a weight increase.
 
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All else being equal, for 2 identical airplanes, except one is carrying more weight? If both airplanes are flown with the wings at the same angle of attack? the heavier one will need to fly faster. If both wings are at the same AoA then they will both have the same G load margin regardless of speed. Therefore: Va increases with a weight increase.

I don't mean identical planes. I mean 2 planes, different weights, same speed, how do they have the same G margin? I figure out their wing loading will differ and as I know, a low wing loading will experience larger accelerations... or maybe here I'm wrong. :confused:
 
AOA 101 - Navy Carrier Ops

Carrier approaches are always made at an optimum AOA. Gage was calibrated in some kind of "units" - but it didn't matter. The needle was supposed to be kept at 9 (or 3 - how soon we forget) o'clock - or the indexers displaying a donut vice an up or down chevron.

Approach speed at optimum AOA varied with gross weight - for the A-4 two or three kts increase per 1000 lb if I recall correctly.
 
I don't mean identical planes. I mean 2 planes, different weights, same speed, how do they have the same G margin? I figure out their wing loading will differ and as I know, a low wing loading will experience larger accelerations... or maybe here I'm wrong. :confused:


Then I apologize for misunderstanding the question. Can you provide a specific example of what you mean?
 
We have 2 airplanes. The first one is a light sport, max weight 400 kg, its speed about 90 kts. The second one, max weight 600 kg, same speed.
Assuming inertie due to mass difference, the lighter one will get higher loads due to gusts. So, how they are certified in the same envelope to have the same G margin?
 
Let me take another stab at your gust question, with a specific made-up example. Identical planes, but A weighs 2000 lbs, B weighs 1000 lbs. Both flying at the same speed, level, unaccelerated.
Let's say the angle of attack of A is 4 deg. Then B's angle of attack must be 2 deg. A's wing is holding up 2000 lbs, B's wing is holding up 1000 lbs. The engine mount in both planes is holding up an engine which weighs 400 lbs.

Now, assume a vertically upward gust hits. The relative wind now points upward a bit, let's assume 2 deg (note the faster the plane goes, the smaller the change in relative wind angle for a given gust).

So A's angle of attack is now 6 deg (4+2); its wing is generating 3000 lbs of force (6/4 times 2000) and A's g meter reads 1.5 g (3000/2000). A's engine mount is supporting 600 lbs (1.5 times 400).

B's angle of attack is 4 deg (2+2); its wing is generating 2000 lbs of force (4/2 times 1000) and B's g meter reads 2 g (2000/1000). B's engine mount is now supporting 800 lbs (2 times 400).

Notice A's wing has to withstand a greater force than B's; but B's engine mount has to support more than A's. This is how it works, in general; engineers must make sure the wing is strong enough at gross weight, but the other components (like engine mounts) strong enough at minimum weight, at the specified gust.

This whole subject is very complicated. For certified aircraft the specified gust load is not applied instantly, but rather over a small but finite time. This is important, because many wings tend to reduce the effect of a gust by twisting (forward spar is stronger than aft spar; under load the trailing edge bends up more than the forward edge, reducing angle of attack).
 
In your example if you have different airplanes, they have different wings. Lighter one may have a smaller wing so the actual force exerted by a gust may be smaller than the heavier airplane with a larger wing. You need to look at each design on its own. Remember it is not the gust directly blowing the aircraft upwards. It is the gust changing the angle of attack on the wing.
 
Really good discussion. Practical comment: I have stalled inadvertently once in 1200 plus RV4 hours. I use 1.3 till close to the numbers...shear has been a factor dozens/hundreds of times, and almost always a factor...that said, we can always stop the process: power up, point away from the hard stuff and fly away. For my ops, it has been a non-issue. Not to say that someday I might wind up landing in a microburst or something that overpowers my plane's power/acceleration at a given load and density...so far, have had the judgment to avoid the extremes...but, I would rather be strapped in my 4 than anything else I can fly if I do....so's, take yer chances, but they are such safe planes compared to a gross weight 172 (pick your single engine non-turbo plane) at 6000 feet on a hot day....
 
So A's angle of attack is now 6 deg (4+2); its wing is generating 3000 lbs of force (6/4 times 2000) and A's g meter reads 1.5 g (3000/2000). A's engine mount is supporting 600 lbs (1.5 times 400).

B's angle of attack is 4 deg (2+2); its wing is generating 2000 lbs of force (4/2 times 1000) and B's g meter reads 2 g (2000/1000). B's engine mount is now supporting 800 lbs (2 times 400).

From these two examples I figure out A is able to handle more turbulence than B. So how do they have the same gust envelope? That is both of them are able to support a max 15 m/s gust (or whatever) at the same Va.
 
Sometimes I felt the G lasts few seconds when you suddenly ecounter the gust. So I figured out in 2-3 seconds you may even get a spin, if the turbulence also induces some yaw. What if just one wing is hitted by a strong gust and it exceeds its critical angle of attack? The differential lift will be large due to the other wing is still flying. Maybe these scenarios are far away from how things really happen. I don't know, that's why I have these doubts.

Btw, usually, in a spin, how large is the difference in lift between the two wings? How much differential lift is the rudder able to overcome to recover from a spin? Although slightly embarrassed, I'll confess what I figure out: if a strong gust makes you spin, is it possible to not have enough rudder to recover? I guess the differential lift in a normal spin is not very high, cause you can't induce differential lift larger than using full ruder, but I guess if only one wings is stalled due to gust and the other is flying, that is larger than whatever spin you induce with controls. Again, maybe I have a very wrong idea of how a spin works. I'm sure you can clarify my strange doubts. Thank you so much!
 
Sometimes I felt the G lasts few seconds when you suddenly ecounter the gust. So I figured out in 2-3 seconds you may even get a spin, if the turbulence also induces some yaw. What if just one wing is hitted by a strong gust and it exceeds its critical angle of attack? The differential lift will be large due to the other wing is still flying. Maybe these scenarios are far away from how things really happen. I don't know, that's why I have these doubts.

Btw, usually, in a spin, how large is the difference in lift between the two wings? How much differential lift is the rudder able to overcome to recover from a spin? Although slightly embarrassed, I'll confess what I figure out: if a strong gust makes you spin, is it possible to not have enough rudder to recover? I guess the differential lift in a normal spin is not very high, cause you can't induce differential lift larger than using full ruder, but I guess if only one wings is stalled due to gust and the other is flying, that is larger than whatever spin you induce with controls. Again, maybe I have a very wrong idea of how a spin works. I'm sure you can clarify my strange doubts. Thank you so much!

Good and interesting aerodynamic questions, but the answer to the real world problem lies in your training more than anything else. If folks were properly trained these days in the use of the rudder, accidental spins would never happen. Even before I had aerobatic training, my first instructor taught me how to do a "falling leaf" in a good old Cessna 150. If you get in the habit of raising a dropping wing using the rudder instead of the aileron when the aircraft is stalled or close to stalled, you will never spin. A gust of wind can't make you spin unless you refuse to react properly. It's very, very easy to prevent an aircraft from entering a spin. I can't think of a single engine aircraft made that the rudder would not be effective enough to prevent a spin. A twin with one engine out is another matter.

Find a good instructor familiar with falling leaf maneuvers. Practice regularly. They are fun and will increase your confidence, as well as making you a better, safer pilot.
 
Meanwhile I found that it is more likely to get a snap roll assuming my theoretical scenario with high differerential lift at speed well above 1G stall.

It's very, very easy to prevent an aircraft from entering a spin. I can't think of a single engine aircraft made that the rudder would not be effective enough to prevent a spin.
Right, you can prevent it to develop, but I think the issue is that if you get a wingdrop and the wing tucks under, the nose will drop, and in some airplanes it might happen really fast, right - it's not the case of a C172, and as soon as you got the nose down it would be impossibe to recover at low altitude i.e. on final, or after takeoff. So it's still an issue at low altitude.

Also during flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings levek, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing? I guess it is not yet fully stalled tohave the tendency to drop a wing, right?
 
I apologize if this is rude, but....

Are you flying with an instructor that can demonstrate basic stick and rudder flying techniques? Some of your questions appear on the surface to indicate a lack of practical training. You can certainly learn things here, but some knowledge should come from practical "in the cockpit" training and practice.

There are well known techniques taught in early flight training to address the air movement issues (shear, crosswind, gusts etc) you have raised.

And similarly, speaking as someone trained in aerospace engineering, there are fairly basic techniques for tailoring aircraft designs to meet particular V-N plots (loading diagrams or "envelopes"). These are instructed early, just like the stick and rudder skills are instructed early in pilot training.
 
Also during flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings level, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing? I guess it is not yet fully stalled to have the tendency to drop a wing, right?

One reason that some airplanes have gentle characteristics is that the airfoil does not stall abruptly. You're assuming linear behavior up to an instantaneous stall. A properly designed wing will have a much more gradual stall than that, even with control surface deflection.

For example, the non-RV I regularly fly will roll wings level if stalled in a fully-developed slip. It'll do that either direction, with flaps down or not, but I've only tried it power off. It shakes a lot and the stall warner comes on. You can't miss it.

Less than a full slip, it's a nicely consistent and controllable as you'd ever want. I approach and flare to a full stall landing in crosswinds quite routinely. That is, I don't fly a crabbed final approach. I slip into the wind and keep doing that right through the three-point landing and the roll-out.

Dave
 
Meanwhile I found that it is more likely to get a snap roll assuming my theoretical scenario with high differerential lift at speed well above 1G stall.


Right, you can prevent it to develop, but I think the issue is that if you get a wingdrop and the wing tucks under, the nose will drop, and in some airplanes it might happen really fast, right - it's not the case of a C172, and as soon as you got the nose down it would be impossibe to recover at low altitude i.e. on final, or after takeoff. So it's still an issue at low altitude.

Also during flare, after using the crab method to compensate for a crosswind, when you de-crab you are cross-controlling close to stall speed, why there's no wing drop or even an incipient spin behaviour? I figure out it may be a slip here, but you are wings levek, not banked as in a slip and being so close to stall, why opposite aileron doesn't increase the angle of attack of the opposite wing? I guess it is not yet fully stalled tohave the tendency to drop a wing, right?

To answer your last question first, the reason you don't get a wing drop is because the inputs in a crosswind landing, even if using full rudder and aileron, are not increasing the the angle of attack enough on the retreating wing to stall the wing. What you say about landing level is confusing to me, I land in a slip during the x-wind, slipping into the wind at the same rate as the wind is trying to blow me off the runway. I surely don't land level - my upwind wing is down, the upwind wheel hits first and I am not level until all wheels are on the deck, and I have stopped flying.

Go do some falling leafs, you will understand more what I am talking about. Try it in a Pitts - go up to 5000' AGL in the Pitts, - which drops a wing very quickly when stalled uncoordinated, btw. Pull the stick full back and hold it there. The aircraft will stall and the nose and one wing will probably drop. The nose will bob up and down, and you can use the rudder to bring up the low wing. After some practice you will be able do this for a very long time, dropping several thousand feet, holding the stick back all the time, bringing up one wing and then the other, dancing on the rudders. You will be using full, fast, hard rudder inputs. You can do it power on and power off, it's more challenging with power on. It's fun, and once you get good at it, you won't ever again worry about an accidental spin. You will have a feel for the plane, and won't need airspeed or AOA's either. You will feel uncoordinated flight, and when you are nearing the critical angle of attack. It's nice to know you can still fly safely if the gadgets in your plane stop working. My instructor made me learn to do landings with the airspeed indicator covered, using attitude and feel to control airspeed. That''s the way I was trained, anyway, and it still makes sense to me.

As far as the first question, if you are so low to recover from a stall where you get a wing to drop, then you are probably too low to recover from the stall anyway, much less a spin. You are not really in a spin by definition until after two full turns, and if you can bring a wing up using the rudder, you are preventing the spin. You are likely to add rudder input as soon as you feel the wing start to drop, once you have some training under your belt. If you still hit the ground, you stalled to low. If you are trained and practice to use your rudder instead of aileron when a wing drops, then your reaction will be instinctive, whether it's a Pitts, RV or 172. But too low is too low, and if you stall to close to the ground to regain enough lift to prevent the aircraft from contacting the ground at a dangerous rate, then you are too low, period, and it doesn't make any difference whether a wing drops or not.
 
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What you say about landing level is confusing to me, I land in a slip during the x-wind, slipping into the wind at the same rate as the wind is trying to blow me off the runway. I surely don't land level - my upwind wing is down, the upwind wheel hits first and I am not level until all wheels are on the deck, and I have stopped flying.
No, if you don't use crab to sideslip transition, the tecnique is just to kick the grab out, so you use opposite aileron to prevent opposite bank due to rudder application to kick the crab out. So you are not banked, you are wings level.

But too low is too low, and if you stall to close to the ground to regain enough lift to prevent the aircraft from contacting the ground at a dangerous rate, then you are too low, period, and it doesn't make any difference whether a wing drops or not.
Right, but the greatest difference is to have the nose dropped or not. If the nose also dropped due to incipient spin/hard-wingdrop, even if you prevent it immediatly to develop, the nose down attitude will be fatal close to ground. If the attitude during stall remains close to straight and level, you have a decent chance to recover, this is what I meant.
 
No, if you don't use crab to sideslip transition, the tecnique is just to kick the grab out, so you use opposite aileron to prevent opposite bank due to rudder application to kick the crab out. So you are not banked, you are wings level.

You guys are only going to bang your heads against the wall if you continue to keep up a discussion with SteveJeff, a self-admitted 120-hour 172 pilot who has, on another forum, proved his total reluctance and inability to learn or accept concepts that are contrary to what he's learned so far in his supposed 120 total hours as a pilot, in a 172. So if you want to be thoroughly worn out by confusing, unrelenting, and pointless theory babble, take a look here: Same poster as the OP here, different alias. It won't end...he already knows more than you.

http://www.pilotsofamerica.com/forum/showthread.php?t=56065
 
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ymmv

in general, fast jets, low profile high performance wing - land in a crab (T-38, F-16) the gear and such are able to withstand the (sometimes significant) crossloads as the airplane aligns itself with the direction of travel (and can create a downwind roll if not countered with aileron as it does so!) upon touchdown.

caveat - some of the bigger platforms can't land wing low with a low performance wing due to engine nacelles hanging mere feet above the runway. watch the 747 crosswind landing videos and you'll see a crab to touchdown and subsequent fuselage alignment with the appropriate 'wing low' crosswind controls being input as the aircraft does so, to prevent the downwind roll tendencies that develop when doing that maneuver. i have no personal experience with these airframes so am just going off inductive reasoning and discussion with those who do.

in general, slow airplanes, high profile low performance wing - land wing low (T-37, C172, C208, J3 Cub, C120, RV8, RV4, RV6), you already have crosswind controls applied at touchdown and usually need to moderate (increase) the amount of control input as the airspeed bleeds off through the rollout and the controls lose effectiveness, until you're back to taxi inputs for the various wind directions across the airframe as you make your way back to parking.

using the wing low you don't 'kick out the crab at the last moment' because the rudder is actually already being applied to keep the fuselage pointed where you're going (hopefully on centerline straight down the runway!) ie there is no crab. you're in a controlled cross control/slip (opposite rudder than aileron) all the way to the runway including through the roundout and flare. even using a crab, you normally don't 'kick it out' at the last minute, that takes away everything you were doing right prior to doing so and would more than likely induce an immediate downwind drift that would require the appropriate crosswind controls or you're probably looking at side loads on the gear at touchdown. not a good thing on fixed or retract platforms.

you do actually touch down, while still flying, one wheel (the upwind wheel, which is 'lower' due to bank into the relative crosswind) before the other and if not intending to full stop, can do a one wheeled touch and go, maintaining centerline the entire time using appropriate aileron inputs and fuselage alignment with the direction of travel using appropriate rudder inputs.

oh, and try teaching the 'falling leaf' to Iraqi Lieutenants in a C172, some who had no experience with driving a car or riding a bicycle. Fun times!! but they GOT IT!

no math analysis, the above is based on personal experience only, it's worked for me for over 6000 hrs and a 26+ yr aviation career.

like the title suggests...ymmv. moving on...
 
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You guys are only going to bang your heads against the wall if you continue to keep up a discussion with SteveJeff, a self-admitted 120-hour 172 pilot who has, on another forum, proved his total reluctance and inability to learn or accept concepts that are contrary to what he's learned so far in his supposed 120 total hours as a pilot, in a 172. So if you want to be thoroughly worn out by confusing, unrelenting, and pointless theory babble, take a look here: Same poster as the OP here, different alias. It won't end...he already knows more than you.

http://www.pilotsofamerica.com/forum/showthread.php?t=56065

Yep, definitely a troll. He has posted the same question on the EAA and Super Cub forums.

Don't feed the troll.
 
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