Stalls in turbulence

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photofly
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Re: Stalls in turbulence

Post by photofly »

Your mathematics is correct. But the air conditions you're supposing don't seem very realistic, and you're ignoring the fact that as the pilot you have complete control of the aerodynamic load via the elevators. Stalls don't happen instantly; if you're concerned that you're loading up the wings, push on the yoke.

If your more general point is that strong gusty winds make landings more hazardous for GA aircraft with low approach and stall speeds, then you're quite correct. Strong gusty winds do make the landing phase more hazardous for GA aircraft. One of your roles as pilot is to assess whether the wind conditions at your intended point of landing are within your limitations and those of the aircraft. if they're not - don't go there.
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Last edited by photofly on Fri Jan 25, 2013 7:51 am, edited 1 time in total.
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Re: Stalls in turbulence

Post by Colonel Sanders »

If we are flying at 70 kts and encounter a gust which induces 1G acceleration, our load factor is 2G, so the airplane stalls at 70 kts
I suppose for an extraordinarily brief moment, you might "stall upwards" :wink:

Many, many times I have heard/seen the stall warning indicator flicker
ever-so-briefly on approach, at pretty good speeds. Is a non-event
because if it's incredibly brief duration.

What lasts is loss-of-headwind shear. That's what's going to bite you
in the bottom. The heavier the airplane, the worse the problem.

It's really easy to understand. You're trucking down final, then all
of a sudden you lose 10 mph (eg) and the aircraft sinks.

That's what you've got to worry about. Permanent loss of lift - not
brief, transient conditions.

If you don't want to stall, don't make the wing work. Unload it.
At the same time, use the rudder pedals to deal with any yaw,
and it's a snoozefest. I frequently recover from inverted spins
below 1000 AGL and it's really no big deal if you do what I tell
you to do.
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Re: Stalls in turbulence

Post by Rich_Pa »

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!
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Re: Stalls in turbulence

Post by Colonel Sanders »

gust ... you may even get a spin
Nope. Far more likely to cause an upset on final
is wake turbulence.
spin, is it possible to not have enough rudder to recover?
Some aircraft are not approved for spins. Others
that are approved for spins, may not be at higher
weights and aft C of G's.

I have done thousands of spins and what I have
learned is that if you arrest the yaw initially with
rudder - you do not allow it to develop into a fully
blown spin with auto-rotation - all is good. Roughly
speaking, the first turn of a spin is incipient, and
that's when you want to stop it. After the first turn,
the spin winds up and may be harder to stop.

What you need to know is that aircraft stall
on the infamous turn to final because pilots are
unconciously hoofing the inside rudder, because
they are trying to bring the nose around, while
keeping the bank low, which is what their instructor
erroneously taught them.

Aircraft aren't the problem when it comes to accidental
spins in the circuit. Pilots are the problem.
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Post by Beefitarian »

What plane are you flying Rich?

I'm not sure how to word this but.. You might want to look for an instructor that can explain things in a way that you think about them. Most instructors are trying to teach the same things but sometimes they explain those things very differently.

Once you find an instructor that seems to make more sense of things than usual, you should book a ground briefing to really discuss these things you're wondering about, I know it seems expensive but you might make some huge break throughs.

While you're starting to think about how spins could happen during stalls. Book a lesson for about an hour and a half in a plane certified for spins. Ask to fly around the practice area at a safe altitude to do upper air work. While in slow flight do lots of turns, you will get a chance to really use the rudder. Intentionally cause some stalls, dip the wing with aileron and do incipient spins. Really pay attention to how things happen and what the plane is doing, try to lose the least amount of altitude possible during recovery.

If you go in a mighty C-172 you can even climb a bit in slow flight while the stall horn is blaring away. It will be the slowest climb ever and useless in a way but you will realize even a partially stalled wing is still generating lift.
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Re: Stalls in turbulence

Post by Rich_Pa »

Currently a Cessna 172. Maybe you misunderstood my concerns. What I meant was that if one of your wings gets stalled at a speed far above 1G stall speed due to a gust, the lift difference between the two wings might be larger than the difference induced with full ruder as we use to enter a spin. So, you might not be able to overcome it.
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Re: Stalls in turbulence

Post by photofly »

even a partially stalled wing is still generating lift.
Even a fully stalled wing will develop lift, and large amounts of it. Enough to support the entire weight of the aircraft in fact. Why? Because if it didn't, the aircraft would be accelerating downwards. Once the plane reaches a steady rate of descent, lift once again equals weight(*), stalled or not. It's another myth that lift suddenly disappears at the stall. Various things disappear - pitch stability and roll stability among them, but not lift.

(*) more correct to say vertical components of lift and drag equal weight, since the aircraft is likely to be descending fast and the relative airflow at an angle to the horizontal - but the point is the same.
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Post by Beefitarian »

Rich_Pa wrote:Currently a Cessna 172. Maybe you misunderstood my concerns.
Fair enough and maybe you can't find an instructor that can or will do what I'm suggesting.

If you do enough slow flight and power on stalls. You can observe differences of the plane's behavior between - Using the rudder versus using the aileron at the wrong time. Eventually you will learn at least a couple of things no one can fully explain.

It's part of why many pilots talk about planes with conventional gear so much. Some things are sort of simple to do but nearly impossible to put into words. Doing them wrong could go badly.
Rich_Pa wrote:What I meant was that if one of your wings gets stalled at a speed far above 1G stall speed due to a gust, the lift difference between the two wings might be larger than the difference induced with full ruder as we use to enter a spin. So, you might not be able to overcome it.
I don't know if that's possible. As Colonel Sanders hints at. You seem to be ignoring inertia. A real nasty spin doesn't usually catch pilots by surprise without some help.

Plane is loaded wrong and going too slow while the pilot is texting perhaps stepping on the wrong rudder pedal...
Then POW!they hit a 2G updraft.
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Re: Stalls in turbulence

Post by Colonel Sanders »

if one of your wings gets stalled at a speed far above 1G stall speed
What you are describing is a snap roll.

For example, in an aircraft with Vs 70 mph, at 140 mph
if you apply full back elevator and full rudder, you will
perform a +4G snap roll, something like this in a Pitts:




Most people don't snap roll 172's - even unintentionally.
A 172 is the most docile aircraft in the world. It is
enormously tolerant of pilot goof-ups.
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Last edited by Colonel Sanders on Fri Jan 25, 2013 11:29 am, edited 1 time in total.
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Re: Stalls in turbulence

Post by dr.aero »

Rich_Pa wrote:Currently a Cessna 172. Maybe you misunderstood my concerns. What I meant was that if one of your wings gets stalled at a speed far above 1G stall speed due to a gust, the lift difference between the two wings might be larger than the difference induced with full ruder as we use to enter a spin. So, you might not be able to overcome it.
I've got a feeling you've been taught a few incorrect things or are misunderstanding some things.

What is the main reaction when the lift is higher on one wing than the other? The correct answer is roll. In this scenario you'd also have a bit of aileron drag causing adverse yaw but it'd be similar to just using the ailerons without coordinating with the rudder. If that's all that's happened I can assure you that a touch of rudder will bring the airplane back into coordination.

What were you taught to do when you have a wing drop at the point of stall?
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Re: Stalls in turbulence

Post by photofly »

dr.aero wrote:What is the main reaction when the lift is higher on one wing than the other? The correct answer is roll.
The correct answer is a roll-wise acceleration, a.k.a an increasing (or decreasing) roll rate. A steady roll requires equal lift on both wings.
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Re: Stalls in turbulence

Post by Rich_Pa »

Ok, so I understand I was very wrong in my spin assumption, actually you get a snap-roll, not a spin. Still not satisfied, I guess a snap-roll close to ground is a bad thing, but I figure out it is less likely to have one in a 172. Anyway, I guess you lose height getting stalled both wings (or having a snap-roll) due to gust.
dr.aero wrote: What were you taught to do when you have a wing drop at the point of stall?
I wasn't taught to pick-up the wing with rudder, if that's the answer you want to hear. I'm not sure if this is good or bad, but I was thaught just to avoid an incipient spin with rudder, eventually to arrest any further roll/yaw, but not to pick-up the wing to level. Having the opportunity, I hope you also help me to clarify this issue.
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Re: Stalls in turbulence

Post by dr.aero »

photofly wrote:
dr.aero wrote:What is the main reaction when the lift is higher on one wing than the other? The correct answer is roll.
The correct answer is a roll-wise acceleration, a.k.a an increasing (or decreasing) roll rate. A steady roll requires equal lift on both wings.
If you want a constant roll rate you will need to hold constant aileron input.
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Re: Stalls in turbulence

Post by dr.aero »

Rich_Pa wrote:Ok, so I understand I was very wrong in my spin assumption, actually you get a snap-roll, not a spin. Still not satisfied, I guess a snap-roll close to ground is a bad thing, but I figure out it is less likely to have one in a 172. Anyway, I guess you lose height getting stalled both wings (or having a snap-roll) due to gust.
dr.aero wrote: What were you taught to do when you have a wing drop at the point of stall?
I wasn't taught to pick-up the wing with rudder, if that's the answer you want to hear. I'm not sure if this is good or bad, but I was thaught just to avoid an incipient spin with rudder, eventually to arrest any further roll/yaw, but not to pick-up the wing to level. Having the opportunity, I hope you also help me to clarify this issue.
Well not really a snap roll either. If anything you'd get a bit of a wing drop. Remember this is a gust - it only happens for a very short time period. You need to hold control inputs at full deflection for at least a few seconds to get a C172 to spin... or do a snap roll.

That's good you weren't taught to pick up a stalled wing with rudder! Some instructors teach that religiously and it's a very dangerous procedure. You might get away with it in a Cessna, but not all airplanes are Cessnas!

I wrote a 4-page article on why you shouldn't use uncoordinated rudder in a stall recovery. This is a section from that article:
At the point of stall the air flowing over the wing is extremely turbulent and, therefore, not producing effective lift. The best way to produce lift is to have smooth airflow that flows perpendicular to the leading edge of the wing. Any spanwise flow is not conducive to lift. The large amount of spanwise flow on swept wing aircraft is part of the reason why they can go so fast, but also a reason why they have poor stall characteristics! If rudder, opposite to the wing drop, is used during the stall recovery it puts the aircraft into a slip, which progresses towards a skid as the wings are levelled. The slip, being used to decrease the lift and increase the drag on the wing for the purpose of losing altitude, goes against the goal of recovering with minimum altitude loss.

Imagine the aircraft has a wing drop to the left at the point of stall and the recovery procedure is initiated when the angle of bank is at 45 degrees. At this point, the elevator is effectively 50% elevator and 50% rudder. When pitching down you accomplish two things: reducing the angle of attack and reducing the yawing moment. Approximately ¼ - ½ second after initiating a pitch down movement, full aileron control and coordinated rudder can be used to bring the wings to level. Even at the point of stall, many modern general aviation aircraft have sufficient roll control due to the washout of the wing and other design features. If aileron control is applied at the stall in these aircraft, they will most likely roll in the direction applied, the opposite of what is taught will happen. It is true, mostly on high performance aircraft, that it’s possible to induce a spin by using aileron at the point of stall.

The other part to that example is what applying rudder does to the stall recovery. At 45 degrees angle of bank the rudder, like the elevator is equally split. It will provide 50% rudder and 50% elevator control. If you applied opposite rudder (right rudder in this case) the aircraft would reduce its yawing tendency and put it into a slip, but it would also increase the pitch of the aircraft, something that is opposite to what we want!

In summary, applying opposite rudder is not conducive to carrying out an effective and safe recovery from a stall. Here are a couple quotes from FAA documents on the stall recovery procedure: “Straight and level flight should be established with full coordinated use of the controls.” [3], “…straight-and-level flight should be regained with coordinated use of all controls.” [4] These two paragraphs from the FAA Airplane Flying Handbook, under the heading “Use of Aileron/Rudder in Stall Recovery”, summarize it well: “When the airplane is in a stalled condition, the wingtips continue to provide some degree of lift, and the ailerons still have some control effect. During recovery from a stall, the return of lift begins at the tips and progresses toward the roots. Thus, the ailerons can be used to level the wings.”, “Even though excessive aileron pressure may have been applied, a spin will not occur if directional (yaw) control is maintained by timely application of coordinated rudder pressure. Therefore, it is important that the rudder be used properly during both the entry and the recovery from a stall. The primary use of the rudder in stall recoveries is to counteract any tendency of the aircraft to yaw or slip. The correct recovery technique would be to decrease the pitch attitude by applying forward-elevator pressure to break the stall, advancing the throttle to increase airspeed, and simultaneously maintaining directional control with coordinated use of the aileron and rudder.” [4]
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Re: Stalls in turbulence

Post by Rich_Pa »

dr.aero wrote:Well not really a snap roll either. If anything you'd get a bit of a wing drop. Remember this is a gust - it only happens for a very short time period. You need to hold control inputs at full deflection for at least a few seconds to get a C172 to spin... or do a snap roll.
Let's say encounter a gust that lasts longer. Would it be a spin or a snap roll?


I found this experiment http://www.pilotsofamerica.com/forum...3&postcount=93. Is that difference in a descending turn so high that the inside wing might have a greater/significant stall speed? About how knots above the usual stall speed?

This difference is what kills the base-to-final skidders or glide-stretchers. If they allow their airspeed to reach stall speed, right?
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Re: Stalls in turbulence

Post by photofly »

dr.aero wrote:If you want a constant roll rate you will need to hold constant aileron input.
That's correct. To maintain equal lift on both wings, and therefore to maintain constant roll rate, requires aileron input. See if you can figure out why!

Hint: consider the direction the air hits each wing, taking into account that in a roll the up-going wing has a different flightpath to the down-going wing.

This is indeed not space. But the laws of physics don't vary from space to atmosphere. Zero angular acceleration implies zero turning moment.
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Re: Stalls in turbulence

Post by dr.aero »

photofly wrote:
dr.aero wrote:If you want a constant roll rate you will need to hold constant aileron input.
That's correct. To maintain equal lift on both wings, and therefore to maintain constant roll rate, requires aileron input. See if you can figure out why!

Hint: consider the direction the air hits each wing, taking into account that in a roll the up-going wing has a different flightpath to the down-going wing.

This is indeed not space. But the laws of physics don't vary from space to atmosphere. Zero angular acceleration implies zero turning moment.
Geez, I need to get more sleep haha. I wasn't thinking. You're correct.
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Re: Stalls in turbulence

Post by photofly »

@dr.aero:
I notice you like to "collect" aerodynamic myths (I do too) in order to squash them. If we agree that in a steady roll there is equal lift on both wings then there's equal drag too. So "aileron drag" as the cause of the need for inside rudder during the roll into a turn is another myth to be stamped out.

That poses three questions:

1. Why is inside rudder needed during the roll into a turn?
2. When does aileron drag occur?
3. Why is inside rudder still needed during the steady turn, when the roll rate is zero?
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Re: Stalls in turbulence

Post by dr.aero »

photofly wrote:@dr.aero:
I notice you like to "collect" aerodynamic myths (I do too) in order to squash them. If we agree that in a steady roll there is equal lift on both wings then there's equal drag too. So "aileron drag" as the cause of the need for inside rudder during a steady turn is another myth to be stamped out.

That poses three questions:

1. Why is inside rudder needed during a steady turn?
2. When does aileron drag occur?
3. Are there any other forces associated with turning flight that require rudder input?
I'll have to think about this more later.

3 quick answers:

1) I've thought about this before and the only reason I came up with was that the nose needs to be guided - elevator and rudder guide the nose so that the relative airflow remains head-on.
2) Aileron drag occurs anytime there is an aileron deflected. It's net effect on an airplane is more complicated.
3) Rudder's job in normal flight is to coordinate and prevent adverse yaw from affecting the airplane. I can't think of anything else right now that would require rudder.
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Re: Stalls in turbulence

Post by photofly »

Here are my quick answers. Feel free to think, argue, and disagree.


1. Basically I agree with you. The actual rotation of the aircraft body in space is a combination of yaw and pitch. The "tilted lift" from the banked wing will move the aircraft in a circular path. But it won't rotate the body of the aircraft to face in the direction of flight. The vertical stabilizer does that (because of very strong slip-yaw coupling) even without a rudder deflection, but without the rudder input it will ensure only that there's no sideways airflow over the fin. Because the aircraft is long and straight when it flies in a curved path the airflow can't be parallel with the flightpath everywhere along the whole fuselage. Minimum drag occurs when there's some airflow across the nose of the aircraft from the inside of the turn and across the tail from the outside of the turn. You can arrange that by deflecting the rudder into the turn. The steeper the turn, the more elevator you need because the rotation becomes more pitchwise.

2. Aileron drag occurs when the ailerons are deflected but before the roll has reached a steady rate. While there's roll-wise acceleration there's differential lift, and therefore differential drag. So - right at the beginning of the turn procedure.

3. while the aircraft is rolling, the lift vectors of the two wings are no longer parallel. The down-going wing has its lift vector tilted forward, and the upward-going wing has its lift vector tilted backwards (because lift is defined as the aerodynamic force perpendicular to the relative airflow, and the two wings have different relative airflows). This is "twisted lift" and exerts a yawing force opposite to the direction of turn. While rolling the aircraft needs rudder into the turn to counter this effect. Wikipedia has a good diagram.

There is also a certain amount of twisted drag, for the same reason. This twisted drag acts against the roll rate, which will require a little extra aileron into the roll.
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Re: Stalls in turbulence

Post by Colonel Sanders »

Translation for any student pilots reading this:

Step on the ball.
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Re: Stalls in turbulence

Post by white_knuckle_flyer »

So in a stall with wing drop, I was taught to 1) break the stall with forward pressure ( or just relaxing pressure )
2) input opposite rudder
3) throttle

I noticed in the article quoted above, the only thing missing is aileron input. I have flown nothing but 172s so far, but who knows eventually.
Have I been misguided ?

Also, is the article suggesting that in an unbanked stall, aileron input will drop the wing in the direction of the aileron input ? If so, is it different for climbing turn stalls ? Reason I'm asking is that more often than not, I was successful in getting a left wing drop while doing a climbing right turn stall.
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Re: Stalls in turbulence

Post by dr.aero »

Photofly...

1) "The vertical stabilizer does that (because of very strong slip-yaw coupling) even without a rudder deflection," - which is called weathercocking. Take a look at the rocket in this example: http://exploration.grc.nasa.gov/educati ... tcock.html

The change in velocity is completely done by weather cocking. There is no rudder deflection required. The same thing happens though - there is uncoordinated flight for a few seconds until the rocket aligns with the new effective flow direction (relative wind). In an airplane, without a rudder, the exact same thing would happen except that this uncoordinated flight condition will continue to exist until the airplane's wings are levelled and it assumes a new velocity (direction) where acceleration is reduced to zero. Like you said, there is airflow across the nose from the inside of the turn which is created since we'll assume this theoretical airplane is perfectly balanced and will fly so that the centre point of it's fuselage is tangental to the flight path in a turn. In a right turn (with no rudder input), due to the relative wind pushing the tail inward and the nose outward from the centre of the turn, you will continue to fly in a slip while in the turn. To fix that, applying inside rudder will oppose both forces created on the tail and nose.

Of note is that if you plot this out for an average airplane, flying at average speeds, you'll find that the angular difference in tangents at centre-point and at the nose and tail will be very small. As you know, a little bit of rudder is all that's required to keep it coordinated in a turn. You should also note that when the airplane is flown slower, at the same angle of bank, more rudder is required to maintain coordinated flight than when flying faster. This is due to smaller radius of turn at a lower speed and the resultant greater difference in tangental angles at the centre-point and at the nose and tail.

2) Aileron drag occurs at all times the ailerons are deflected from their neutral position (either positive or negative drag). However, this doesn't mean that there will be a net change in drag on a wing.

3) I want to pop open my flight mechanics book before I reply to this point.

White_knuckle...

The procedure I teach students is as follows:

1. Reduce pitch
2. Power full
3. Coordinated roll to wings level
4. Pitch for VY

The coordinated roll to wings level is where the ailerons come in. If I'm being very technical and lets say critiquing a precision flying maneuver, I'd tell you that if you got a left wing drop you might need a tap of left rudder as you're rolling out with right aileron, before you put in right rudder - this would ensure you recover to coordinated flight the quickest.

The pitch for Vy also has a lot of things to talk about. It's a generic rule and each airplane will behave differently, requiring slightly different technique. For example, a C152 doing a stall recovery with 1) zero power, 2) a 100 hp engine, 3) a 200 hp engine, will all require slightly different techniques. Some model airplanes you might have seen seem to defy physics (I don't like it when people say that!) with their stall recoveries consisting of jamming full throttle and entering a vertical climb. If only real airplanes had that power-to-weight ratio!
Also, is the article suggesting that in an unbanked stall, aileron input will drop the wing in the direction of the aileron input ?
In a C172? Yes. Try it next time you're practicing stalls. I'd bet a fair amount of money that if you entered a coordinated, power off stall, in a C172 and applied full aileron in one direction, the airplane will roll in that direction. The only reason I say that is because of washout. Put your eyes right at the bottom of the wing, at the wingtip, looking towards the fuselage. It's very hard to miss the change in wing shape about halfway down the wingspan. Instructors (myself included) will tell you to not use aileron in a stall recovery and I'll make sure you understand the real reason why you shouldn't do that. I found it pretty ineffective when my PPL instructor tried to show me that it was dangerous to use aileron in the stall recovery and the airplane happily rolled in the direction the instructor put the ailerons - not what he was trying to illustrate!
Reason I'm asking is that more often than not, I was successful in getting a left wing drop while doing a climbing right turn stall.
That's exactly what should happen because the outside wing in a climbing turn is at a slightly higher AoA than the inside wing. This diagram illustrates that: http://i.imgur.com/TIZZK.jpg

If that picture isn't making sense I can add some explanation to it for you.
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Re: Stalls in turbulence

Post by dr.aero »

Colonel Sanders wrote:Translation for any student pilots reading this:

Step on the ball.
That's a pretty good summary.

But I don't see why everything with regard to flying has to be kept suitable for the lowest common denominator.
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Re: Stalls in turbulence

Post by white_knuckle_flyer »

dr.aero wrote:White_knuckle...

Instructors (myself included) will tell you to not use aileron in a stall recovery and I'll make sure you understand the real reason why you shouldn't do that. I found it pretty ineffective when my PPL instructor tried to show me that it was dangerous to use aileron in the stall recovery and the airplane happily rolled in the direction the instructor put the ailerons - not what he was trying to illustrate!
Can you fill me in on what the real reason is ? I'm also curious as to why my instructor, and the examiner after him, were content to let me perform the ol' pitch, rudder, power trio. My instructor was no spring chicken, was an AME as well and seemed to hae a pretty good handle on all facets of flying. Is it just easier to throw out the 1,2,3 procedure since it works in a 172 99.2% of time ?

I'm good with the climbing turn and AOA on the outer wings. I remember a thread on that not too long ago and I poured over it for some time.

Now I feel like I have to take my new instructor up to the practice area and have him teach me this method. problem is, he IS a spring chicken !
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