Wind shear, relative wind, airspeed and stalls

[1RTK1]

LOL, yes there were runway lights. The airport is out in the country with nothing around it so it is a big black hole to boot.

 

[MrManH]

You've lost me. Why would you need to pitch down? While the magnitude of the relative wind changes, the direction, which is all that matters for stalling, wouldn't.

Relative wind is always parallel and opposite to the direction of flight, at least this how I've always understood it. So if your relative wind suddenly becomes (theoretically) 90 degrees from the chord line and the nose doesn't drop to get chord line closer to the relative wind, you'd be in a stall.

It's time for you and your instructor to go flying with the airspeed indicator covered. Heck, hang a towel over the whole panel.

Bob Gardner

That was part of my instrument training. The point of my question is to understand what could happen in theory.

Here's a realistic example, the winds in Chicago were shearing and gusting around 50kts yesterday. If you were flying a LSA on final approach at 40kts and suddenly a 20kt headwind sheared into a 20kt tailwind, for a moment the airspeed indicator would indicate 0. Regardless of what the airspeed indicator indicates, the wings at that moment wouldn't be producing any lift. The aircraft would drop almost vertically (with a bit of forward motion thanks to inertia), so the relative wind would be a pretty steep angle towards the ground. If the pilot isn't touching the controls, what will happen to the nose of the airplane while this happens? If as some suggested the airplane maintains its trimmed attitude as the relative wind suddenly moves away from the chord line, then the critical AOA will be exceeded and the aircraft stalled. This is how I understand it and I'd like to know if this is correct.

Using the same example but this time the wind shears from a 20kt tailwind to a 20kt headwind and we are cruising at 40kts on final approach. The airspeed would momentarily become 80kts, twice its original value. If AOA was constant and the airspeed was suddenly multiplied by 2, the load factor would be quadrupled (4G). However a change in airspeed will also induce a change in lift, that change in lift will change the relative wind and therefore the AOA. If the airplane accelerates, more lift is produced which brings the relative wind closer to the chord line which reduces the AOA. Does that reduction in AOA offset the increase in airspeed and therefore makes wind shear (horizontal gusts) less of a concern when it comes strictly to load factor?

 

[flyingron]

Relative wind is always parallel and opposite to the direction of flight, at least this how I've always understood it. So if your relative wind suddenly becomes (theoretically) 90 degrees from the chord line and the nose doesn't drop to get chord line closer to the relative wind, you'd be in a stall.

You're still not explaining things. Relative wind is not always parallel and opposite to the direction of flight. If it were AOA would be constant.

I'm having a hard time imagining what case you have where the relative wind is suddenly going to have a 90 degree shift in relative wind. Remember relative wind includes not only the actual wind but the airspeed of the aircraft.

A shift from a 20 knot headwind to a 20 knot tail wind won't change the relative angle of attack much, just the magnitude of the relative wind. What will happen is that you'll have less lift and if you then pull back to try to regain it you will have increased the AOA.

In practice, gusty conditions don't cause stalls. A bigger issue when you have strong winds close to the ground is that they aren't just shifting from tailwind to headwind or whatever (which is relatively benign) but also got a lot of vertical compent. Again, the problem isn't STALLING (unless the pilot is stupid), but the fact that the airmass may cause such a sufficent descent that you can't outclimb it.

 

[luvflyin]

Again, the problem isn't STALLING (unless the pilot is stupid), but the fact that the airmass may cause such a sufficent descent that you can't outclimb it.

Good point. Loss of enough lift to counteract gravity and downward moving air if down drafts are present is what causes the landing short accidents. In the OP's scenario airspeed goes to zero because the head wind instantly changes to a tail wind and the assumption is that because there is no lift being produced that the wing must have stalled. An airplane sitting on the ramp on a calm day has zero airspeed, the wing is producing no lift. That is not a stall. An airplane in the air that "instantly" goes into a zero airspeed state is no different except that there is no pavement preventing gravity from making the airplane start going down.

 

[MAKG1]

You're still not explaining things. Relative wind is not always parallel and opposite to the direction of flight. If it were AOA would be constant.

I'm having a hard time imagining what case you have where the relative wind is suddenly going to have a 90 degree shift in relative wind. Remember relative wind includes not only the actual wind but the airspeed of the aircraft.

A shift from a 20 knot headwind to a 20 knot tail wind won't change the relative angle of attack much, just the magnitude of the relative wind. What will happen is that you'll have less lift and if you then pull back to try to regain it you will have increased the AOA.

In practice, gusty conditions don't cause stalls. A bigger issue when you have strong winds close to the ground is that they aren't just shifting from tailwind to headwind or whatever (which is relatively benign) but also got a lot of vertical compent. Again, the problem isn't STALLING (unless the pilot is stupid), but the fact that the airmass may cause such a sufficent descent that you can't outclimb it.

Having experienced a 15 knot headwind quitting during a short field landing, you can't explain the stall warning going off this way. That was the FIRST indication.

I think quite a few of us have experienced the stall warning chirping during a gusty takeoff.

Heck, the Cardinal set the stall warning off early in the landing rollout last week (yes, with the nose up, but a stalling pitch angle would have dragged the tail), and the conventional wisdom says that's impossible.

Wind gusts do change the AoA, or you can't explain any of this.

 

[Lachlan]

Of course wind sheer can change the AoA, otherwise there would be no reason to have different maneuvering speeds in turbulent conditions. Every student pilot learns this. Seems like most forget it, though.

 

[bobmrg]

My emphasis. Without help from the pilot the airplane will try to maintain the same angle of attack because that's the way the horizontal tail has been trimmed. The same thing would happen if you dove the plane without trimming and then sudenly released the controls.

dtuuri

It will attempt to maintain trim speed, IOW. Too few pilots/instructors know about or have been introduced to the concept of trim speed.

Bob Gardner

 

[MrManH]

It will attempt to maintain trim speed, IOW. Too few pilots/instructors know about or have been introduced to the concept of trim speed.

Bob Gardner

Wouldn't this depend on the stability of the aircraft?

I've asked my questions to instructors and they always try to oversimplify the concept and I'm not happy with the answer. I come from a scientific background and I know I dig in too much beyond what a pilot really needs to know to actually fly the airplane safely. What's your opinion on the 2 wind shear examples I gave regarding stall and load factor?

I have my PPL and IR so things like these haven't kept me from progressing and passing oral exams, but I'd still like to shed a light on some concepts.

We all agree that if a stall was to occur it would most likely be the result of the pilot trying to compensate for the sink rate by pulling back and exceeding the critical AOA. But what I'm trying to find out is what happens if we take the pilot out of the equation and let the trimmed airplane do its thing.

 

[dtuuri]

If you were flying a LSA on final approach at 40kts and suddenly a 20kt headwind sheared into a 20kt tailwind, for a moment the airspeed indicator would indicate 0. Regardless of what the airspeed indicator indicates, the wings at that moment wouldn't be producing any lift. The aircraft would drop almost vertically (with a bit of forward motion thanks to inertia), so the relative wind would be a pretty steep angle towards the ground. If the pilot isn't touching the controls, what will happen to the nose of the airplane while this happens?

It will nose down into the relative wind. This is what happens during a stall as the plane drops due to a loss of lift.

If as some suggested the airplane maintains its trimmed attitude as the relative wind suddenly moves away from the chord line, then the critical AOA will be exceeded and the aircraft stalled. This is how I understand it and I'd like to know if this is correct.

Not correct. The trim doesn't maintain attitude it seeks an angle of attack.

Using the same example but this time the wind shears from a 20kt tailwind to a 20kt headwind and we are cruising at 40kts on final approach. The airspeed would momentarily become 80kts, twice its original value. If AOA was constant and the airspeed was suddenly multiplied by 2, the load factor would be quadrupled (4G).

The dynamic pressure affects the tail and wing the same, so the angle of attack should stay the same as you say.

However a change in airspeed will also induce a change in lift, that change in lift will change the relative wind and therefore the AOA.

Right, because the aircraft pitches up.

If the airplane accelerates, more lift is produced which brings the relative wind closer to the chord line which reduces the AOA.

Nope. The one thing you can bet your life on is the horizontal tail surfaces control the angle of attack. As your speed increase is only for an instant, the plane will hunt for its trimmed AoA. If you had enough thrust, you could just add power and do a loop without changing AoA.

Does that reduction in AOA offset the increase in airspeed and therefore makes wind shear (horizontal gusts) less of a concern when it comes strictly to load factor?

There is no reduction unless you change your trim.

dtuuri

 

[dtuuri]

Of course wind sheer can change the AoA, otherwise there would be no reason to have different maneuvering speeds in turbulent conditions. Every student pilot learns this. Seems like most forget it, though.

A gust from below, certainly, but the inherent stability of the plane will tend to realign it with the relative wind, wherever it's coming from. A horizontal wind "shear" shouldn't change the AoA, at least not nearly as much.

dtuuri

 

[MrManH]

dtuuri thank you very much for breaking down and providing feedback on my statements, it's very clear now.

Understanding that the airplane will hunt for its trimmed AOA when the airspeed changes, what's your opinion on this:
"Using the same example but this time the wind shears from a 20kt tailwind to a 20kt headwind and we are cruising at 40kts on final approach. The airspeed would momentarily become 80kts, twice its original value. If AOA was constant and the airspeed was suddenly multiplied by 2, the load factor would be quadrupled (4G)."

Since the AOA will remain constant and the airspeed is now doubled, would the load factor become 4G in this situation?

 

[Vance Breese]

I've asked my questions to instructors and they always try to oversimplify the concept and I'm not happy with the answer. I come from a scientific background and I know I dig in too much beyond what a pilot really needs to know to actually fly the airplane safely. What's your opinion on the 2 wind shear examples I gave regarding stall and load factor?

In my opinion the reason the instructors you ask over simplify it is because you have overcomplicated it.
In my opinion exceeding the critical angle of attack is what stalls an airfoil.
It is not speed or weight.
It really is that simple.

 

[dtuuri]

dtuuri thank you very much for breaking down and providing feedback on my statements, it's very clear now.

Understanding that the airplane will hunt for its trimmed AOA when the airspeed changes, what's your opinion on this:
"Using the same example but this time the wind shears from a 20kt tailwind to a 20kt headwind and we are cruising at 40kts on final approach. The airspeed would momentarily become 80kts, twice its original value. If AOA was constant and the airspeed was suddenly multiplied by 2, the load factor would be quadrupled (4G)."

Since the AOA will remain constant and the airspeed is now doubled, would the load factor become 4G in this situation?

First a disclaimer: I am not an engineer nor fluent in higher math. I could (and I have) spent hours proving the Pythagorean theorem on a square cocktail napkin, trying to reach a clear, self-evident understanding of why it works and have failed. I'm more of a Stick and Rudder kind of guy. So, when it comes to how things vary, directly or indirectly with the square or square root of some other thing, I'm probably not the one to ask. But if I'm asked, I will give it a shot just the same.

Load factor is the ratio between lift and weight. Since lift varies directly with speed squared, and since speed doubles in your scenario, squaring a lift value that doubles results in a four fold increase. So my opinion is... you're right.

dtuuri

 

[MrManH]

In my opinion the reason the instructors you ask over simplify it is because you have overcomplicated it.
In my opinion exceeding the critical angle of attack is what stalls an airfoil.
It is not speed or weight.
It really is that simple.

I am not disagreeing that exceeding the critical AOA is what stalls an airfoil, no debate about that. The part that I was missing was that the plane would seek to maintain its trimmed AOA when disturbed. Which leads to my question: would this statement only be true for an airplane that has a positive static stability on the longitudinal axis (longitudinal static stability)? Or regardless of the airplane's stability, it will always attempt to maintain its trimmed AOA?

 

[MrManH]

First a disclaimer: I am not an engineer nor fluent in higher math. I could (and I have) spent hours proving the Pythagorean theorem on a square cocktail napkin, trying to reach a clear, self-evident understanding of why it works and have failed. I'm more of a Stick and Rudder kind of guy. So, when it comes to how things vary, directly or indirectly with the square or square root of some other thing, I'm probably not the one to ask. But if I'm asked, I will give it a shot just the same.

Load factor is the ratio between lift and weight. Since lift varies directly with speed squared, and since speed doubles in your scenario, squaring a lift value that doubles results in a four fold increase. So my opinion is... you're right.

dtuuri

Well your explanation made perfect sense to me So based on my example strong horizontal gusts could therefore become a destructive force for airplanes flying slow. As far as I know, wind shear awareness has always focused more on the dangers of the sink rate and potentially stalling the airplane while trying to outclimb the sink, but I've never heard it related to load factor. What's everyone's take on this?

 

[coloradobluesky]

Wind shear on landing can plop you down on the runway unexpectedly. The countermeasure is carry extra speed and bleed it off by leveling out just above the runway so if you do plop on, it won't be from very high up. Comes with the usual precautions, you have to have sufficient runway etc.

 

[Vance Breese]

I am not disagreeing that exceeding the critical AOA is what stalls an airfoil, no debate about that. The part that I was missing was that the plane would seek to maintain its trimmed AOA when disturbed. Which leads to my question: would this statement only be true for an airplane that has a positive static stability on the longitudinal axis (longitudinal static stability)? Or regardless of the airplane's stability, it will always attempt to maintain its trimmed AOA?

Part of certifying an aircraft is its airspeed stability so in my opinion it would be reasonable to expect a certified aircraft to attempt to return to its trimmed airspeed during a wind shear event.
I have flown experimental aircraft that are not well suited to returning to trimmed air speed after a wind shear event or in severe turbulence.
I admire your curiosity and relentless pursuit of knowledge.
I feel the fundamentals are simple and the devil is in the details.
I have found many fixed wing pilots don’t want to fully let go of the idea of “stall speed” and replace it with exceeding the critical angle of attack.

 

[dtuuri]

Well your explanation made perfect sense to me So based on my example strong horizontal gusts could therefore become a destructive force for airplanes flying slow. As far as I know, wind shear awareness has always focused more on the dangers of the sink rate and potentially stalling the airplane while trying to outclimb the sink, but I've never heard it related to load factor. What's everyone's take on this?

I had a nest of yellow jackets in my siding near my front door. I devised a scheme to rid them by setting up my shop vac with soapy water in the bottom and put the end of the tube near the hole. As the bees came back to the nest they'd get in the downdraft, try to fly up and suddenly vanish in the tube. Later, I read where some folks consider that to be torturing them, since their little wings fail from the stress. Now I feel bad about it. I still get to sleep though.

I don't think horizontal wind shear is likely to be a problem because the increased load factor is transient. The plane incurs an increase in drag at the same time and slows. The extra lift causes the plane to elevate, changing the relative wind to a lesser AoA and decreasing the added lift. The tail, meanwhile, feels an increase in down force and inclines the plane uphill, slowing it down even more. All these things work in favor of mitigating the effect of the gust.

dtuuri

 

[MrManH]

I had a nest of yellow jackets in my siding near my front door. I devised a scheme to rid them by setting up my shop vac with soapy water in the bottom and put the end of the tube near the hole. As the bees came back to the nest they'd get in the downdraft, try to fly up and suddenly vanish in the tube. Later, I read where some folks consider that to be torturing them, since their little wings fail from the stress. Now I feel bad about it. I still get to sleep though.

I don't think horizontal wind shear is likely to be a problem because the increased load factor is transient. The plane incurs an increase in drag at the same time and slows. The extra lift causes the plane to elevate, changing the relative wind to a lesser AoA and decreasing the added lift. The tail, meanwhile, feels an increase in down force and inclines the plane uphill, slowing it down even more. All these things work in favor of mitigating the effect of the gust.

dtuuri

No mercy for yellow jackets! As a kid I was a magnet for them and got stung around 20 times (not at once thankfully).

Your 2nd paragraph was my original understanding until you explained that the airplane is trimmed for a certain AOA (and corresponding trim speed assuming the power isn't changed):
"But it would also make sense to me that without any input from the pilot the AOA in a headwind would be decreased due to the change in relative wind. Since lift is the product of both airspeed and AOA, if airspeed goes up but AOA goes down, do they basically cancel each other out and the load factor remains around 1G?"

Airspeed increases, lift increases so the relative wind comes up (relative to chord line). But since the airplane is trimmed as you explained for a certain AOA, wouldn't the airplane pitch up to maintain that AOA?

 

[MrManH]

Part of certifying an aircraft is its airspeed stability so in my opinion it would be reasonable to expect a certified aircraft to attempt to return to its trimmed airspeed during a wind shear event.
I have flown experimental aircraft that are not well suited to returning to trimmed air speed after a wind shear event or in severe turbulence.
I admire your curiosity and relentless pursuit of knowledge.
I feel the fundamentals are simple and the devil is in the details.
I have found many fixed wing pilots don’t want to fully let go of the idea of “stall speed” and replace it with exceeding the critical angle of attack

I found this statement on Wikipedia that supports your opinion:
"Accordingly, the operating handbook for every airplane specifies the range over which the c.g. is permitted to move. Inside this range, the airplane is considered to be inherently stable, which is to say that it will self-correct longitudinal (pitch) disturbances without pilot input."

 

[dtuuri]

Your 2nd paragraph was my original understanding until you explained that the airplane is trimmed for a certain AOA (and corresponding trim speed assuming the power isn't changed):
"But it would also make sense to me that without any input from the pilot the AOA in a headwind would be decreased due to the change in relative wind. Since lift is the product of both airspeed and AOA, if airspeed goes up but AOA goes down, do they basically cancel each other out and the load factor remains around 1G?"

There's a timeline here. I took the longer view in my first answer. When the air current smashes into the plane, the AoA of the wing is reduced, the airspeed increased and the down force on the tail and negative AoA increased too. All of that would cause a pitch up into a climb as the plane hunts for its trimmed AoA (speed). If you meant the initial blast, I didn't catch it.

Airspeed increases, lift increases so the relative wind comes up (relative to chord line). But since the airplane is trimmed as you explained for a certain AOA, wouldn't the airplane pitch up to maintain that AOA?

That's what I would expect.

dtuuri

 

[MrManH]

There's a timeline here. I took the longer view in my first answer. When the air current smashes into the plane, the AoA of the wing is reduced, the airspeed increased and the down force on the tail and negative AoA increased too. All of that would cause a pitch up into a climb as the plane hunts for its trimmed AoA (speed). If you meant the initial blast, I didn't catch it.



That's what I would expect.

dtuuri

I guess what I'm trying to find out is the timeline. Are you saying this:
-initial blast: AOA decreased simultaneously as the gust is encountered
-overtime: plane increases AOA to return to its trimmed AOA

I feel like the timeline determines whether or not the load factor may be increased, even if it's only momentarily in gusty conditions.

Also would you consider the initial blast to be a question of static stability and overtime a question of dynamic stability?

 

[mscard88]

No mercy for yellow jackets! As a kid I was a magnet for them and got stung around 20 times (not at once thankfully).

That's right! No mercy for yellow jackets. My grandfather and I were in the woods when I was a tot and we got tore up by those little bastards!

 

[dtuuri]

Are you saying this:
-initial blast: AOA decreased simultaneously as the gust is encountered
-overtime: plane increases AOA to return to its trimmed AOA

Yes BUT the increase in airspeed would cause a net increase in lift, I would think, because it would more than offset the initial reduction in AoA. The tail would react to this and immediately weathervane the wing back to and probably beyond the original AoA. That's where you'd get the brunt of the load factor increase.

Also would you consider the initial blast to be a question of static stability and overtime a question of dynamic stability?

The response to a disturbance that results in a reinforcing feedback, I would say, is negative static stability. If the tail reacted by decreasing the AoA even more it would be a vicious cycle and to my mind negative static stability. No planes are designed like that. So, the reaction to the gust is positive static stability in the short run. Whether the hunting that ensues next becomes more or less dramatic determines the character of the inherent dynamic stability.

dtuuri

 

[MrManH]

The Vertical Damping section of this page seems to shed a light on the topic:
https://www.av8n.com/how/htm/vdamp.html