Stalls in turbulence

[Steve Jeff]

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?

[WLIU]

You have an imperfect visualization of how the atmosphere works. Imagine an invisible river where one layer is moving in one direction and the neighboring layer is moving in another. You climb to the top of the first layer and discover that you can not continue your climb into the second layer for a time. You ride the boundary until you accelerate enough to be able to continue your climb in the second layer. You do not fail out of the sky unless you do something really dumb that causes the airplane to stop flying in the first layer of air. If the two layers of air are are moving in really different directions or a greatly different speeds, you are likely to find turbulence at their boundary, but surprisingly, some days you do not.

A light wing loading means that you go up and down more as you move from one air mass to another. But your airplane is moving relatively slowly and it does not weigh a lot. You do NOT necessarily put higher loads on your aircraft. The G that results from a 50fps gust is different depending on aircraft speed and wing loading.

Momentary G is a different hazard than the sustained G of a steep turn. If an updraft momentarily bumps you to 2G, the inertia from the mass of your aircraft keeps the change in aircraft attitude relatively small. And then you are flying again, moving the controls to correct the undesired yaw or pitch. But that is why when you have a lot of turbulence on final you fly a slightly (5%) higher airspeed.

Go fly,

Wes
N78PS

[Steve Jeff]

If the two layers of air are are moving in really different directions or a greatly different speeds, you are likely to find turbulence at their boundary, but surprisingly, some days you do not.
And there, considering my example, 180 change in direction, that turbulence will cause your speed to drop, right? And thus having a stall, right?


A light wing loading means that you go up and down more as you move from one air mass to another. But your airplane is moving relatively slowly and it does not weigh a lot. You do NOT necessarily put higher loads on your aircraft. The G that results from a 50fps gust is different depending on aircraft speed and wing loading.

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?


Momentary G is a different hazard than the sustained G of a steep turn. If an updraft momentarily bumps you to 2G, the inertia from the mass of your aircraft keeps the change in aircraft attitude relatively small. And then you are flying again, moving the controls to correct the undesired yaw or pitch. But that is why when you have a lot of turbulence on final you fly a slightly (5%) higher airspeed.


So, it might stall accelerated when it bumps at 2G or not?

[WLIU]

As you climb to the higher air mass, you will stop climbing but should not lose altitude. Your airspeed in relation to the lower air mass will not drop. Stall is in relation to the air mass around you.

Va decreases as weight decreases yes, but your attempted example is flawed for that very reason. If the airplanes have the same wings but are flying with different weights, they must have different Va's.

As for momentary gusts, you are trying to suggest that when a wing has a gust exceed its critical angle of attack for a second, total control of the aircraft is lost. That does not happen due to inertia. As you slow towards the stall speed, turbulence causes more AoA excursions and you will provide more and larger control inputs to correct in between the "gusts". As you slow, there will be a speed where the time that the wing is having AoA problems exceeds the time that the wing is flying. Somewhere around that time you will effectively lose control of the aircraft. That is why you fly the landing pattern at a higher speed on turbulent days.

Each aircraft has what is called a V-N diagram that charts the airspeed vs G caused by pilot maneuvering or "gusts". The V-N diagram has an outline drawn on it that shows whether a particular G at a specific air speed results in the wing stalling (exceeding its critical AoA) or the airplane actually breaking. But to effectively use the V-N diagram you need to have a G meter installed in addition to your airspeed indicator. My observation is that the majority of pilots grossly over-estimate the G's that they think they have experienced. Anyway, look at your V-N diagram, look at the airspeed that you fly final on a bumpy day, and see how large or small your speed and G margin is. That will answer the question that you seem to be asking.

Hope this makes sense.

Go fly,

Wes
N78PS

[martymayes]

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...

No because a light plane will react to the 15 kt change in airspeed almost immediately. It's unlikely you would notice. Your speed over the ground would change by 30 kts, however.


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

Yes, an updraft would + load the airplane, for a second. Then the plane would respond to the gust by ballooning above the desired glidepath, not much else.

[Steve Jeff]

As you climb to the higher air mass, you will stop climbing but should not lose altitude. Your airspeed in relation to the lower air mass will not drop. Stall is in relation to the air mass around you.
Not necessary climbing, let's assume I'm flying straight and level at 150 ft AGL when the 15kts headwind suddenly changes its direction 180 degrees. My airspeed will drop 15kts or 30kts?

Va decreases as weight decreases yes, but your attempted example is flawed for that very reason. If the airplanes have the same wings but are flying with different weights, they must have different Va's.


As for momentary gusts, you are trying to suggest that when a wing has a gust exceed its critical angle of attack for a second, total control of the aircraft is lost. That does not happen due to inertia. As you slow towards the stall speed, turbulence causes more AoA excursions and you will provide more and larger control inputs to correct in between the "gusts". As you slow, there will be a speed where the time that the wing is having AoA problems exceeds the time that the wing is flying. Somewhere around that time you will effectively lose control of the aircraft. That is why you fly the landing pattern at a higher speed on turbulent days.
Few knots added gives just a small G margin in addition. What I thought was that when the critical angle of attack of one wing or both will be exceeded, the airplane will stall and possible to have also a wingdrop and considering we are talking about flying at a higher airspeed than 1G stall, the stall induced by a un upward gust will have a much greater effect, that's whi I thought it might induce a spin. The differential lift between the two wings will be really high assuming that only one wing stalled.


Each aircraft has what is called a V-N diagram that charts the airspeed vs G caused by pilot maneuvering or "gusts". The V-N diagram has an outline drawn on it that shows whether a particular G at a specific air speed results in the wing stalling (exceeding its critical AoA) or the airplane actually breaking. But to effectively use the V-N diagram you need to have a G meter installed in addition to your airspeed indicator. My observation is that the majority of pilots grossly over-estimate the G's that they think they have experienced. Anyway, look at your V-N diagram, look at the airspeed that you fly final on a bumpy day, and see how large or small your speed and G margin is. That will answer the question that you seem to be asking.

I guess it's not very large, that margin.





Yes, an updraft would + load the airplane, for a second.
Why that one second is not enough to induce a stall (with wing drop if it's stalled just one wing)?

[martymayes]

Not necessary climbing, let's assume I'm flying straight and level at 150 ft AGL when the 15kts headwind suddenly changes its direction 180 degrees. My airspeed will drop 15kts or 30kts?

If it were an instantaneous event, the change in airspeed would be 15 kts. Example: Flying straight and level at 115 KIAS. Doesn't matter if it's a 15 kt headwind or a 50 kt headwind, the plane's speed relative to the air is 115 kts. Wind shears to 15 kts of tailwind, the change in airspeed is 15 kts. 115 - 15 = new speed relative to the air of 100 kts. Now, thrust is > drag so the plane accelerates back to 115 KIAS. If the pilot did not change the angle of attack to compensate for the decrease in lift when the speed changes, plane would lose altitude but not 150 ft. If the pilot did increase angle of attack, his stall margin would decrease but not enough for the plane to stall.



Why that one second is not enough to induce a stall (with wing drop if it's stalled just one wing)?

When the plane is unloaded (i.e. the gust is past), it resumes flying like nothing happened. A gust is a momentary event. It would happen so fast the pilot would go "duh" and the event would be over.

I think your imagination is getting the best of you. Perhaps a flight lesson or two will clarify some of these misconceptions.

Under the right conditions, you can impress your friends by flying the plane backwards! All you need to do is fly at 40 kts into a 50 kt headwind.

[Steve Jeff]

If it were an instantaneous event, the change in airspeed would be 15 kts. Example: Flying straight and level at 115 KIAS. Doesn't matter if it's a 15 kt headwind or a 50 kt headwind, the plane's speed relative to the air is 115 kts. Wind shears to 15 kts of tailwind, the change in airspeed is 15 kts. 115 - 15 = new speed relative to the air of 100 kts. Now, thrust is > drag so the plane accelerates back to 115 KIAS. If the pilot did not change the angle of attack to compensate for the decrease in lift when the speed changes, plane would lose altitude but not 150 ft. If the pilot did increase angle of attack, his stall margin would decrease but not enough for the plane to stall.
If not 150ft, how much?

If the pilot did increase the angle of attack, his stall margin might decrease to critical if it was already high before he pull the yoke.




When the plane is unloaded (i.e. the gust is past), it resumes flying like nothing happened. A gust is a momentary event. It would happen so fast the pilot would go "duh" and the event would be over.

I think your imagination is getting the best of you. Perhaps a flight lesson or two will clarify some of these misconceptions.

Not necessary to take flight lessons. I am a private pilot. I usually fly in good weather. However, I know that these scenarios don't happen. I just know to understand why. There were moments when I felt the G for a few seconds, I think a few seconds would be enough to stall the airplane. If I suddenly pull the yoke, I can stall accelerated the plane in less than 1 seconds. Why the gust doesn't?

[martymayes]

If not 150ft, how much?

Not much. In the real world, a light airplane will start responding and never see the 15 kt speed change. In a heavier airplane, windshear is more of a problem because of higher inertia. In airline operations, during a critical phase of flight, like takeoff and landing, a -20 kt windshear can trigger a windshear alert which requires the pilot to perform an escape maneuver.


If the pilot did increase the angle of attack, his stall margin might decrease to critical if it was already high before he pull the yoke.
Perhaps if he was practicing minimum controllable airspeed when the shear occurred....even then recovery would be a non-event.


There were moments when I felt the G for a few seconds, I think a few seconds would be enough to stall the airplane. If I suddenly pull the yoke, I can stall accelerated the plane in less than 1 seconds. Why the gust doesn't?

A 2g load increases the stall speed by ~40%. It's harder to apply a sustained 2g load than you realize, a momentary event doesn't count. So you can fly a 172 at 75 kts, which is pretty slow, apply a 2g load and it won't stall because it's still within the flying envelope.

[Steve Jeff]

Not much. In the real world, a light airplane will start responding and never see the 15 kt speed change. In a heavier airplane, windshear is more of a problem because of higher inertia. In airline operations, during a critical phase of flight, like takeoff and landing, a -20 kt windshear can trigger a windshear alert which requires the pilot to perform an escape maneuver.


Perhaps if he was practicing minimum controllable airspeed when the shear occurred....even then recovery would be a non-event.



A 2g load increases the stall speed by ~40%. It's harder to apply a sustained 2g load than you realize, a momentary event doesn't count. So you can fly a 172 at 75 kts, which is pretty slow, apply a 2g load and it won't stall because it's still within the flying envelope.

On final you are 65 kts, I would say you'll stall if 2G load is applied. Sustained? I don't know here, but sometimes it feels like the updraft load is sustained 2-3 seconds when you encounter it suddenly.

[WLIU]

You should go take an hour of aerobatic dual to start to get an understanding of how "G", airspeed, and aircraft inertia interact.

But I will note that on final approach, you are unlikely to encounter 2G turbulence. That is really severe. Having a lot of experience with both sustained and momentary G's from flying aerobatics, I will offer the opinion that moderate turbulence is maybe +1.5G to -0.5G. And it is the minus G that gets folks most concerned. Getting jerked against the seat belts makes most straight and level pilots really uncomfortable.

Light airplanes float up and down more on final where airplanes with heavier wing loading, which means a higher approach speed, "punch" through a "bump" with less change to their flight path. But the airplanes with the higher wing loading typically stall at lower angles of attack! So what is going on? Due to the higher speed and higher weight, the time exposure to the "bump" is less and the greater inertia means that the airplane reacts less. In turbulence, mass and inertia are your friend.

Best of luck,

Wes
N78PS

[martymayes]

On final you are 65 kts, I would say you'll stall if 2G load is applied. Sustained? I don't know here, but sometimes it feels like the updraft load is sustained 2-3 seconds when you encounter it suddenly.

An inflight load applied for 2-3 sec is a transient condition.

Steve, when the plane encounters an updraft on final, the pilot pushes the stick forward to keep from ballooning above glidepath which unloads the airplane. A stall would be more likely if you encountered a downdraft on final, cause the pilot would pull back on the stick, which increases load + angle of attack. When wind shear, updrafts/downdrafts/gusts and turbulence is expected on final, you fly a higher approach speed which gives extra padding for those things.

[Steve Jeff]

When wind shear, updrafts/downdrafts/gusts and turbulence is expected on final, you fly a higher approach speed which gives extra padding for those things.

Usually 5 or max 10 kts above normal app speed. I would say that is small difference for updrafts, downdrafts.

[WLIU]

Increasing your approach speed from 65 kts to 70 kits is 7%. That is large in airplane stall margin terms. Going from 65 kts to 75 kts the increase is 15%.

If you fly with a G meter and an AoA display on a bumpy day you will find that you are overestimating the effects of the turbulence that you have encountered. Perhaps there is a homebuilder near you that has the gear and would go flying on an otherwise bad day to let you observe the data.

You will overcome your fears by simply flying on less than idea days. Start easy and work up.

Fly safe,

Wes
N78PS

[martymayes]

Usually 5 or max 10 kts above normal app speed. I would say that is small difference for updrafts, downdrafts.
Considering that updrafts/downdrafts have little impact, small difference = more than adequate. Can't keep adding speed because it impacts landing distance. How many airplanes have stalled and crashed on final in the last 10 yrs due to turbulence? I suspect the number is very, very small.

[Steve Jeff]

Increasing your approach speed from 65 kts to 70 kits is 7%. That is large in airplane stall margin terms.

Yes, there is a substantial changde in 1G stall, but considering gust also increases G, the difference will be smaller in stall margin terms. Perhaps it will give you just 0.1G in plus.


How many airplanes have stalled and crashed on final in the last 10 yrs due to turbulence? I suspect the number is very, very small.

Don't know, there are lots of crashes due to stall/spin in traffic pattern. But I hope all of them are due to maneuvering, inadequate speed maintained by pilot etc.

[WLIU]

The statement "Perhaps it will give you just 0.1G in plus." is based on your fear, not on looking at the V-N diagram for your airplane.

I get the distinct impression that the original question is posed in a manner that is looking for support for the notion that flying in weather that is less than perfect is far too dangerous for a mere mortal in a light aircraft. The folks contributing to this thread are using logic and fact based information, but the info provided isn't being accepted as the answer to the question.

I will suggest that none of the folks who have provided info in this thread started out as masters of the subject, but rather started flying on nice days, studied the performance of their aircraft, and flew on progressively more challenging days. Not instant gratification. Can't get it all from a book or an internet forum. You have to fly.

Perhaps flying only on nice days is appropriate for now? After all, that is a perfectly fine approach to enjoying aviation.

Fly safe,

Wes
N78PS

[Steve Jeff]

The statement "Perhaps it will give you just 0.1G in plus." is based on your fear, not on looking at the V-N diagram for your airplane.
Right, it's about 0.25G.



I get the distinct impression that the original question is posed in a manner that is looking for support for the notion that flying in weather that is less than perfect is far too dangerous for a mere mortal in a light aircraft. The folks contributing to this thread are using logic and fact based information, but the info provided isn't being accepted as the answer to the question.
No way, I just want to understand exactly what is the margin and how likely is to have problems.

[Frank Giger]

The only turbulence I fear in the pattern or on final is wake turbulence - and it's fairly simple to avoid.

However, get bit once and you'll add quite a bit of distance more than what the book suggests from then on, particularly if one is piloting a Champ! Or so a friend tells me. ;)

OTOH, I've had wind change directions on me while on final more than a couple of times - heck, I've had wind from two directions on the runway (kinda funny when the two windsocks on the ends of the runway are pointing in different directions) - and it's not that big a problem if one is sticking to the basics and not trying to do anything fancy. A little more power (just a little) helps out a lot. "Squirrely" air and gusts are actually more challenging and have resulted in more go-arounds than changing wind conditions for me.

But my advice is to find a day that is outside of your personal minimums and put a trusted CFI in the rear seat for some pattern work. I've done it and it's worth gold in terms of understanding where my skill level was and how much I underestimated myself - as well as pointing out how much more I have to learn. And also when it's time for me to throw in the towel and put it in the hangar.

The bonus is that with a few other simple tasks thrown in one can get WINGS credit and have it count towards the biannual (er, flight review). I've already given this guy a wheelbarrowful of cash for the basic license; might as well throw in a few more bits of paper and have him really do some work for me after all.

Then again, I have no shame and zero pride when it comes to becoming a safer pilot. There's a Champ driver at our field that is simply amazing at slipping to a super short field landing and I basically begged him to take me around the patch and show it to me first hand. Maybe when I have 30 years behind the stick of one I can pull it off like he can; until then I'll just be jealous.

[Steve Jeff]

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!

[David Pavlich]

I'm a non-pilot, but basic aerodynamics tell you that a spin is a stall that has one wing stalled more than the other which creates the environment for autogyration. Being a non-pilot, I've been following this thread with more than a little passing interest. It seems to me that if these veteran pilots have given you proper answers, it rests with you whether or not you choose to fly in anything but calm weather. It seems to me, however, that you will spend the majority of your time on the ground or flying within 20 miles of your home airport since weather changes from minute to minute. Good luck!

David

[Steve Jeff]

I'm a non-pilot, but basic aerodynamics tell you that a spin is a stall that has one wing stalled more than the other which creates the environment for autogyration.
My examples are related exactly to this aspect.


It seems to me that if these veteran pilots have given you proper answers, it rests with you whether or not you choose to fly in anything but calm weather. It seems to me, however, that you will spend the majority of your time on the ground or flying within 20 miles of your home airport since weather changes from minute to minute. Good luck!

David
I really appreciate all the help here, but all I want is to understand why are things so, that's why I asked for help. I have no intention to argue or to contradict. Also, I flew in bad weather, it is true that not very often, but that's because I generally prefer good weather, not necessarily those reasons. What I want to know is more theoretical understanding of issues relating to flight in bad weather. Thanks!

[WLIU]

I will offer once last piece of analysis.

Looking at the Cessna 150 manual, you typically fly down final at 65mph at full flaps. Some folks teach to fly 70mph. The 1G stall speed in that condition at gross weight is 48mph and the 2G stall speed is 67mpg. So you are not close to having a problem with turbulence until at least 1.5G of acceleration. Not an issue unless you choose to fly when there are airmets out for wind shear and severe turbulence.

An hour of aerobatic instruction will answer the question about how stalls and spins work.

Fly safe,

Wes
N78PS

[pacerpilot]

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?

Steve, it seems to me you're overthinking this and have a bit of misunderstanding about wind/turbulence. By reading your post I think what you're concerned with is the "feeling" of "updrafts/downdrafts" and a theoretical 180 degree change in wind direction. Many people visualize these conditions in their minds as "columns" and "sheets" of air moving independently. It really doesn't happen/look that way at all. Normal turbulence as we know is just regular old bumpy/wavy air caused by the usual conditions. It's uncomfortable sure, but it won't make your airplane fall out of the sky. Wes and Marty did excellent jobs of explaining those issues and should have dispelled any worry about that. What I have seen personally is pilots trying to constantly compensate for turbulence and they work themselves, and the airplane, like they're pulling giant aluminum taffy! I guess you could overcompensate and force yourself into an accelerated stall. It makes sense since that's basically how we practice them so if you "felt" a descent and honked back on the stick at a low airspeed you could make a dent in the ground. Doing so in a turn obviously makes it worse. The "up/downdraft" condition is most commonly horizontal gusts which result in momentary slight increases or decreases in airspeed with associated vertical movement of the airplane. This is what I understand as "shear". We're not flying through vertical columns like that of a thunderstorm (at least I hope not!) but small changes in speed which cause the up and down movement. It ain't much but it feels that way sometimes. We also encounter lumpy air close to the ground due to surface friction, but it usually doesn't do as much as gusts.
I was trained not to "chase" the airplane through these conditions. I've been flying one of the types with lighter wing loading and I have to be careful not to work too hard in turbulence. She bounces and goes vertical but I remember the basics-attitude, airspeed, etc. As Marty and Wes illustrated the wind shear thing really is a "heavy" airplane problem. Anyhow, just a non-technical response to your question. Hope it helped a bit.

[pacerpilot]

Steve, I just got an email from the FAA informing me of a Webinar on "How to Avoid Losing Control". Sounds like a good one, you should check it out. There free and usually very well presented.

[steveinindy]

Usually 5 or max 10 kts above normal app speed. I would say that is small difference for updrafts, downdrafts.

Plus figure in that the approach speed (at least as specified in most design manuals; whether people actually pay attention to it is arguable) is Vso x 1.3 or something along those lines. You shouldn't be bumping along just above stall unless you're crossing the fence.


Don't know, there are lots of crashes due to stall/spin in traffic pattern. But I hope all of them are due to maneuvering, inadequate speed maintained by pilot etc.

That probably has more to do with the pilot coming in slow and then trying to whip around base to final, overshooting, winding up cross-controlled and exiting both controlled flight and their range of experience almost simultaneously.

Most of the "he stalled due to encountering an up- or downdraft" stories that get circulated become rather clear cut attempts at protecting the reputation of a friend and colleague than something based on reality. If you're flying cautiously and conscientiously, you probably have effectively zero chance of a stall event due to turbulence. The only scenarios (outside of wake turbulence as Frank pointed out) I can think of that would yield a crash would be someone being already too slow or if you encountered something more along the lines of a microburst level downdraft. The latter is only a problem if you're stupid enough to try to land or take off in anything (airliner all the way down to an ultralight) with convective weather that close in. You're not, as WLIU brought up, going to see 2 g or higher in the pattern in VMC unless you're flying well beyond the techniques taught to private pilots (that is, trying to do a mini-airshow tight and steep turn on base to final, etc)

I am not a master of the subject and have far fewer flying hours than most who have contributed thus far but this is my conclusion based on what I know about aircraft and weather. If I am wrong, someone please teach me where I fall short.

[Flyfalcons]

Stalls in turbulence just isn't a concern in the real world.

[Steve Jeff]

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?

[pacerpilot]

The wing isn't stalled and, you're in ground effect. A crab is a wings level condition so AOA isn't going to change. If you pull back on the yoke/stick you'll get an AOA change. The ailerons don't affect AOI or AOA, the elevator does. Remember, we "stick" the tail to the runway in taildraggers to prevent bouncing and floating once the tailwheel touches or is about to touch. Once in the landing configuration, even slightly above stall full back stick cannot cause the plane to climb once the tailwheel is on the ground. In a nose dragger, you can balloon because the tail can be lowered further by this action. But if you're slow enough you should stall/land perfectly. But still, ailerons won't change AOA/AOI.

[Steve Jeff]

The ailerons don't affect AOI or AOA,

Why on some airplanes opposite aileron induces a spin or at least further wingdrop if you try to pickup the wing at stall using ailerons?

[pacerpilot]

Actually all airplanes will behave that way. Think back to stall training, most instructors jump on us for trying to aileron the wings level, we use rudder. By trying to pick up a wing in a stalled condition will worsen the condition. Also, the spin/rotation is effected by the rudder. You're yawing round and round the vertical. That's why we don't-or shouldn't be-picking up speed once stabilized in the spin. A spiral is twirling around the roll axis, usually not stalled. I re-read your original post and wasn't sure if anyone had answered your first question too: "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...". The answer is no. You'll still be flying 65. Your ground speed will go from 50kts to 80kts though. Remember, your airplane is moving through the airmass-with the airmass. If you're indicating 65mph, the wind could be blowing 100 mph, and you'd be flying backwards over the ground at 35 mph. The airplane is still flying 65mph through the airmass. If the windspeed changes, you're still going 65. But as Wes and Marty described, it may be possible-although remote-to see a momentary airspeed change. Which is actually a true wind shear condition, i.e. a sudden wind speed change or gust (not a vertical column of air blasting up or down). There was an article in our magazine a while back addressing the "downwind turn". Refering to a potential airspeed reduction from crosswind to downwind. It just don't happen. To sum up, use your training, don't chase the airplane through every little bump or "feeling" of vertical accelerations and keep the ball centered-you'll be fine.

[Steve Jeff]

Don't understand. You sad initially that ailerons don't change AOA and after you seem to agree with me.

And reagrding my first question, it was about a sudden gust, I know that steady wind means nothing to airspeed.

[pacerpilot]

Well, just trying to help but it seems I'm not. I'd suggest a flight instructor at your local field.

[Matt Gonitzke]

The ailerons do affect the local angle of attack for the region of the wing where the ailerons are located. However, most airplanes are designed such that the wing root stalls before the tip so that there is still aileron control after the wing (root) has stalled. This is done by either twisting the airfoil leading edge downward over the length of the wing, or with different airfoils along the span so that the angle of attack at the tips is lower.

Example: my glider stalls at about 40 mph, and on landing rollout, I still have aileron control down to about 20-25 mph or so. Look at a single engine Cessna wings sometime...the wing twist is very visible and obvious.

[Steve Jeff]

However, most airplanes are designed such that the wing root stalls before the tip so that there is still aileron control after the wing (root) has stalled. This is done by either twisting the airfoil leading edge downward over the length of the wing, or with different airfoils along the span so that the angle of attack at the tips is lower.

Example: my glider stalls at about 40 mph, and on landing rollout, I still have aileron control down to about 20-25 mph or so. Look at a single engine Cessna wings sometime...the wing twist is very visible and obvious.
Do you think airplanes not designed so, will have a wingdrop tendency when having a full stall landing during decrab in crosswind?

[Bill Greenwood]

Steve, Wind does not shift instantly, it is not like driving a boat over a waterfall, except maybe if you flew into a tornado.

Also there is some kind of stall warning in most planes, the best kind may be airframe buffet such as from the tailplane when turbulent air from the wing hits it; there are also warning horns bells, stick shakers in jets, etc. so that a pilot should know when a stall is near.
If you release stick back pressure at the first sign of a stall, good airplanes recover quickly and don't continue into a stall or spin.

I think you are overanalyzing and worrying about a stall too much; go out and fly and practice stall approach, stalls and recoveries and I think you'll find that while you need to be aware of it, you don't need to be on the edge of your seat about a stall sneaking up on you, IN NORMAL FLIGHT. Now, if you are doing acro or have just lost the engine after takeoff that is another realm.

[Cary]

Steve, you're getting a lot of good advice here, but you're not responding that you understand it. One of the best pieces of advice was to take some aerobatic lessons. It sounds to me as if your initial private pilot training really left out a whole lot about aerodynamics, because many of your questions should have been answered long before you went for your checkride. So now you owe it to yourself and to your passengers to make up for those deficiencies, but to do that, you have to pay attention to what you're being told. While you don't think you're arguing with the advice you're getting, in fact you are. So before you respond again, go back and re-read everything that has been said. Honestly, it's good, and it's accurate. I did not see one inaccurate item.

I speak from the perspective of 40 years of flying mostly singles, a few years of instructing back in the late 70s and 80s, and gobs of hours in turbulence. Don't try to reinvent the wheel--but learn from those who have been doing it, and the only way to do that is to pay attention--and stop arguing.

Cary

[Steve Jeff]

I have no intention to argue. I just want to understand how things really work. I pay attention and I really appreciate any advice, but I want rational help for my doubts. Just saying don't worry and listen to what we say it's not enough. I don't understand what's wrong to clarify these doubts. Of course I haven't understand some things as long as they were pointed to what I wanted to find out. When I came back with my thoughts, you stop answer and think I'm arguing with you, which is wrong. Don't try to reinvent anything.

[martymayes]

Why on some airplanes opposite aileron induces a spin or at least further wingdrop if you try to pickup the wing at stall using ailerons?

Because an aileron does two things; it makes lift and it makes drag. A spin requires yaw to be imparted at the stall. Using the rudder to do that usually works pretty well, however, using ailerons can certainly exacerbate the situation, because they are working on a long lever arm. A couple of similar looking trainers, one by Piper and one by Beechcraft made for very interesting lessons when misusing ailerons during stalls.

BTW, I don't think you're being argumentative, I don't think you need aerobatic lessons (although you might enjoy it), you just have some misconceptions that should have been cleared up when you were training. No big deal. Live and learn.

[Steve Jeff]

Thank you very much, I really appreciate.

If you have time and patience to help me with some questions that, as you also said, were not clarified during my training, actually because I haven't know about them then.

Regarding my first question, headwind shifts to tailwind, you said 15 kts and sincerely I don't get it why it's so. I figure out that if the headwind will decrease suddenly, there should be a change in airspeed, e.g. 15kts headind changes to 5 kts headwind, if it's suddenly, the airspeed decreases 10 kts. So, why when it suddenly shifts from 15kts to 15 kts tailwind there is no a 30 kts drop?

Also, what is not clear to me is the difference between a wingdrop and an incipient spin. I figure out that you may have a wingdrop even if you are coordinated, because the wingdrop may be induced by engine power effects and other factors, not necessary yaw, so, with ball centered you may have a wingdrop, but not a spin, right?

If that wingdrop is abrupt, what be will be the outcome? The wing will tuck under and the result will be a spiral with nose pointing down?

During a coordinated turn descent, I know that the AOA of the inside wing is higher than of the outside, in this case, you may have a spin even if you had ball centered?

And during decrabbing, when you introduce pro-spin controls, i.e. rudder to decrab and opposite aileron just to prevent banking in the direction of the rudder, why there is no wingdrop? Considering a stall landing, the airplane is very close to stall when we introduce these controls.

[pacerpilot]

Thank you very much, I really appreciate.

If you have time and patience to help me with some questions that, as you also said, were not clarified during my training, actually because I haven't know about them then.

Regarding "my first question, headwind shifts to tailwind, you said 15 kts and sincerely I don't get it why it's so". I figure out that if the headwind will decrease suddenly, there should be a change in airspeed, e.g. 15kts headind changes to 5 kts headwind, if it's suddenly, the airspeed decreases 10 kts. "So, why when it suddenly shifts from 15kts to 15 kts tailwind there is no a 30 kts drop?"

Also, what is not clear to me is the difference between a wingdrop and an incipient spin. I figure out that you may have a wingdrop even if you are coordinated, because the wingdrop may be induced by engine power effects and other factors, not necessary yaw, so, with ball centered you may have a wingdrop, but not a spin, right?

If that wingdrop is abrupt, what be will be the outcome? The wing will tuck under and the result will be a spiral with nose pointing down?

During a coordinated turn descent, I know that the AOA of the inside wing is higher than of the outside, in this case, you may have a spin even if you had ball centered?

And during decrabbing, when you introduce pro-spin controls, i.e. rudder to decrab and opposite aileron just to prevent banking in the direction of the rudder, why there is no wingdrop? Considering a stall landing, the airplane is very close to stall when we introduce these controls.

Steve,

I know you're not going to want to here this but you need to get with a flight instructor. Not the same one who did your initial training either. Whoever that was left you with a serious and dangerous lack of knowledge and understanding of aerodynamics and flight conditions. Your misunderstanding of simple airspeed verses groundspeed and wind is disturbing. How do you plan cross countries? Using your current understanding, you could run short of fuel far from a destination-or needlessly stop to refuel. The whole stall thing has been rehashed ad nauseum here in the last few days. I am sure you're not "refusing" to accept information, your just not "seeing" what everybody is telling you. A short ground session will clear all your questions up.

[martymayes]

One at a time:


Regarding my first question, headwind shifts to tailwind, you said 15 kts and sincerely I don't get it why it's so. I figure out that if the headwind will decrease suddenly, there should be a change in airspeed, e.g. 15kts headind changes to 5 kts headwind, if it's suddenly, the airspeed decreases 10 kts. So, why when it suddenly shifts from 15kts to 15 kts tailwind there is no a 30 kts drop?

When a plane leaves the ground, it flies in a mass of air. All the plane senses is it's motion relative to that mass of air. The whole air mass may also be moving, however, all that does is affect the airplane's speed and track over the ground. Example: You are flying South in an airplane at 120 kts, relative to the air. The airspeed indicator reports 120 kts. Now the whole air mass begins moving North at 15 kts. The airspeed indicator still reports 120 kts, however, the airplane's speed over the ground is now 105 kts, 120 - 15 = 105. That is the ONLY way you know there is 15 kts of headwind, the groundspeed changes. Speed relative to the air mass, or airspeed, does not change. Now the air mass begins moving South at 15 kts. The airspeed still reports 120 kts, however, the airplane's speed over the ground is 135 kts, 120 + 15 = 135. Had that change occurred instantaneously, it would represent a 15 kt change relative to the mass of air. So for the sake of discussion, that's what you "could" see on the airspeed indicator if the plane did not react, a 15 kts loss, not a 30 kt loss.

In the real world, even if the change in speed relative to the airmass is instantaneous, i.e. windshear, the airplane reacts. It can't react immediately because it has inertia (although for a light plane, not much inertia). So you may see the airspeed fluctuate slightly, like 3-4 kts. You won't see more because the airplane has already begun to adjust to the new airmass. A 50 ton airplane has more issues in an abrupt wind shear because it can't react as fast and it may be several seconds before it regains lost AIRspeed, that is it's speed relative to the air mass.

[Steve Jeff]

Steve,
How do you plan cross countries? Using your current understanding, you could run short of fuel far from a destination-or needlessly stop to refuel.

Maybe I should tell you that I know the fact that headwind decreases groundspeed and tailwind increases i.e. more time to reach destination with a headwind. In my doubts above I was talking about sudden wind changes which CAUSE airspeed to change, so what is your point?


One at a time:

Had that change occurred instantaneously, it would represent a 15 kt change relative to the mass of air.

Let's take an other example which bothers me to not understand it. We have 20 kts headwind, airspeed 150. Headwinds change to 10 kts. Had this change occurred instantaneously, would it be a momentary change in airspeed? Of course groundspeed will increase 10kts. But what about airspeed?

[CarlOrton]

Let's take an other example which bothers me to not understand it. We have 20 kts headwind, airspeed 150. Headwinds change to 10 kts. Had this change occurred instantaneously, would it be a momentary change in airspeed? Of course groundspeed will increase 10kts. But what about airspeed?

Think of it this way: You're in your airplane, flying along. Imagine if you were in a giant bottle. The bottle is hurtling thru the atmosphere at 100 kts. Now the bottle starts flying thru space at 120 kts. You're still flying at the same airspeed as when you when you started, since the air around you hasn't changed. The change in headwind is not affecting your airspeed. The entire mass of air is travelling at that speed. Yes, it affects groundspeed, but YOU are still flying within that mass, then entire mass of which has changed, *not* just the air hitting your wings/prop/fuse/etc.

I haven't chimed in here before, but I'll agree with the others. The $50 or so you'll spend for an hour of groundschool with a seasoned instructor will pay huge benefits. I've been flying since 1978, and I *still* learn new ways to comprehend stuff from *every* instructor I use. Ask around to see who is good. Age is irrelevant. Some instructors are just better communicators than others. I've even learned stuff from this thread.

[martymayes]

Let's take an other example which bothers me to not understand it. We have 20 kts headwind, airspeed 150. Headwinds change to 10 kts. Had this change occurred instantaneously, would it be a momentary change in airspeed? No. The airspeed indicator does not display aircraft speed + headwind.

[Steve Jeff]

So, in headwind to tailwind shift, reversing its direction makes the airspeed momentary changed and not the mass speed change itself?

[martymayes]

So, in headwind to tailwind shift, reversing its direction makes the airspeed momentary changed and not the mass speed change itself?

In your example, the airplane's AIRspeed didn't change, only the ground speed changed.

http://www.physicsclassroom.com/mmedia/vectors/plane.cfm

[Steve Jeff]

In your example, the airplane's AIRspeed didn't change, only the ground speed changed.


One at a time:
Had that change occurred instantaneously, it would represent a 15 kt change relative to the mass of air. So for the sake of discussion, that's what you "could" see on the airspeed indicator if the plane did not react, a 15 kts loss, not a 30 kt loss.


:eek::confused:

[martymayes]

:eek::confused:

"Windshear" is a change of wind direction and/or velocity that abruptly occurs within the airmass. The only way for the airspeed indicator to report a change in airspeed would be to fly through a windshear, imagine the transition occuring over a distance the thickness of a sheet of paper.

[Steve Jeff]

Yes and all my examples were related to a windshear event, although I said abrupt/sudden change instead of using the term "wind shear". So, all what I ask is windshear, not gradually change. Well, why is there a change in airspeed when it shifts in direction and there is no airspeed change when it shifts in speed? This is what you seem to say and it makes no sense for me. Maybe I'm too dumb, please explain me why is so to clarify it.

[martymayes]

Yes and all my examples were related to a windshear event, although I said abrupt/sudden change instead of using the term "wind shear". So, all what I ask is windshear, not gradually change. Well, why is there a change in airspeed when it shifts in direction and there is no airspeed change when it shifts in speed? This is what you seem to say and it makes no sense for me. Maybe I'm too dumb, please explain me why is so to clarify it.

15 kts of headwind shearing to 15 kts of tailwind does not arthmetically add up to 30 kts because the 15 kt headwind vector is not part of the original airspeed measurement.

[Flyfalcons]

15 kts of headwind shearing to 15 kts of tailwind does not arthmetically add up to 30 kts because the 15 kt headwind vector is not part of the original airspeed measurement.

No there's still a 30 knot difference there.

[martymayes]

but YOU are still flying within that mass, then entire mass of which has changed, *not* just the air hitting your wings/prop/fuse/etc.



Carl, one of my friends used to fly surveillance in a C-182. On a particularly windy day, he landed and said:
"Everytime I turned downwind the cowl flaps blew open"

.....it almost sounded believable.

[Steve Jeff]

15 kts of headwind shearing to 15 kts of tailwind does not arthmetically add up to 30 kts because the 15 kt headwind vector is not part of the original airspeed measurement.

If so, how is the difference between 0 kts and 15 kts tailwind, part of the original airspeed measurement?

[Cary]

FWIW, I'm beginning to think we're all being played, which may explain the unwillingness to accept the good advice that has been given by so many, as well as the lack of basic aerodynamic knowledge of the OP. There is no Steve Jeff in the FAA's pilot data base. There is only one person with the last name of Jeff, and he's a commercial pilot of very long standing, some 55 years. Reversing the name, there is no Jeff Steve in the FAA's data base. There are 3 persons with the last name of Steve in the data base who have addresses showing, only one is a fairly recently certificated pilot, and his name would not easily include Jeff in it. Of the remaining 7, one has no certificate, one is a mechanic, one was certificated some 30 years ago, one was certificated 59 years ago, one is a commercial helicopter pilot of more than 20 years, one is a commercial pilot certificated 24 years ago, one is a private pilot certificated almost 2 years ago.

Now granted, many people create handles for posting on forums, so this tiny bit of research doesn't prove anything, but it may explain things a little.

Cary

[Steve Jeff]

Ha ha, really funny offtopic Cary, for you FAA is the whole world? What about JAA? You heard of it? Anyway, no further offtopic, if you can help on the topic issue, please do it, thanks.

[PA11]

Thinking about all these vectors, these math problems, numbers...sheesh. Y'all are taking the fun out of flying. :P

[miemsed]

FWIW, I'm beginning to think we're all being played, which may explain the unwillingness to accept the good advice that has been given by so many, as well as the lack of basic aerodynamic knowledge of the OP. There is no Steve Jeff in the FAA's pilot data base. There is only one person with the last name of Jeff, and he's a commercial pilot of very long standing, some 55 years. Reversing the name, there is no Jeff Steve in the FAA's data base. There are 3 persons with the last name of Steve in the data base who have addresses showing, only one is a fairly recently certificated pilot, and his name would not easily include Jeff in it. Of the remaining 7, one has no certificate, one is a mechanic, one was certificated some 30 years ago, one was certificated 59 years ago, one is a commercial helicopter pilot of more than 20 years, one is a commercial pilot certificated 24 years ago, one is a private pilot certificated almost 2 years ago.

Now granted, many people create handles for posting on forums, so this tiny bit of research doesn't prove anything, but it may explain things a little.

Cary

i agree it is prob best to end this.

[Sam Buchanan]

The same individual posted the same question on VAF. Here is his public profile:

Biography:LSA, PPL, working on CPLLocation (City, State):BulgariaInterests:Aviation, ITOccupationProgrammer

33 posts in the VAF thread, probably enough to give ol' Steve plenty to think about. :P

[PA11]

... And on the Super Cub forums as well.

[pacerpilot]

I just checked, he's been on VAF doing the same thing as well. I'd say we keep an eye on this one.