Stalls in turbulence

[Rich_Pa]

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?

[white_knuckle_flyer]

I'm going to be watching this thread very carefully to see how Question 1 is answered. I've struggled with it in the past, but am still confused about how quickly and at what point the plane completely yields to the airmass it has just entered. So I'm still trying to figure out why your airspeed doesn't drop off the second you become airborne ( or does it ? ) since as soon as you leave the ground you are now part of the airmass and hence, the headwind you were once experiencing on the ground is nonexistent ( unless gusting ). So the whole headwind-becoming-a-tailwind scenario doesn't seem possible to my tiny brain, unless there is a period of a few seconds where the plane is still accelerating through the airmass and has not yet fully yielded to its movement.

So like I said, I'll be watching this one.

[photofly]

1. Yes. If you encounter 30 knots of sudden windshear as you describe, then your airspeed will drop suddenly by 30 knots. If your airspeed was only 65 knots to begin with, it will suddenly halve and you will likely be in a stall. The nose of your aircraft will drop (and if it doesn't you will push it down) to regain airspeed at the expense of altitude, just as in practice stall exercises. If the wind shear was quite screwy I guess you could end up in a spin; it doesn't seem likely though.

Windshear is dangerous, and causes airliners to crash. A lot of money is devoted to sophisticated radar to detect windshear close to airports to alert pilots. However: 30 knots of WS is a lot; the pilots of the Q400s where I fly start bitchin' when there's about 10 knots WS.

Incidentally: WS is not generally that the wind at one place has instantly started to blow in a different direction; it's usually layered: you encounter it by leaving a layer of air moving in one direction and climb (or descend) into a layer of air moving in a different direction.

This is also a good reason not to climb out of ground effect with too small an airspeed margin. Just in case.

[photofly]

at what point the plane completely yields to the airmass it has just entered. So I'm still trying to figure out why your airspeed doesn't drop off the second you become airborne

The aircraft is in the airmass all the time, not just at takeoff. It's "rowing" itself through the air all the way down the runway, with the propellor, so it doesn't make any sudden difference to the aircraft if the wheels are on the ground or not. So why would you slow down?

[white_knuckle_flyer]

at what point the plane completely yields to the airmass it has just entered. So I'm still trying to figure out why your airspeed doesn't drop off the second you become airborne

The aircraft is in the airmass all the time, not just at takeoff. It's "rowing" itself through the air all the way down the runway, with the propellor, so it doesn't make any sudden difference to the aircraft if the wheels are on the ground or not. So why would you slow down?

You know...because on the ground, your relative airspeed is your ground speed plus the headwind, but once off the ground, you are now part of the headwind. Similar I guess to the idea that the aircraft may weathervane on the ground in a crosswind but once airborne, there is no weathervaning.

Am I nuts ?

[photofly]

Am I nuts ?

Hard to tell. You'd need to see a physician to be sure. You may be confused, perhaps, but that's not a medical issue (despite what some people on this forum will try to tell you ...)

You know...because on the ground, your relative airspeed is your ground speed plus the headwind, but once off the ground, you are now part of the headwind.

Once you leave the ground, your airspeed remains your ground speed plus the headwind. No change occurs.

Similar I guess to the idea that the aircraft may weathervane on the ground in a crosswind but once airborne, there is no weathervaning.

When it comes to crosswinds, the difference is because the tires grip against sideways movement. That is their purpose in life. On the ground - sideways grip - potential to weathervane. Off the ground - no sideways grip - no weathervaning.

But (with the brakes off) the wheels have essentially no resistance to moving forwards and backwards. Again, that's what they do - allow free movement backwards and forwards, but not sideways. So as far as headwinds go, there's no difference whether the tires are on the runway or off.

To reinforce the point: imagine trying to take off with the brakes on. With the (locked) wheels on the runway you're stuck to the ground, instead of "in" the airmass. I suppose - if it helps - you can think of the aircraft starting to be affected by the headwind at the point the brakes are released. After all, without the thrust from the prop, at that point the aircraft would start to be blown downwind. The propellor has to do some work even to keep the aircraft stationary!

[white_knuckle_flyer]

In reading it over, I've definitely done a terrible job trying to explain what I wanted to say in the last post. What I mean is, if the plane is just sitting on the runway with a 10 kt headwind, then I figure that the AI would register 10 kts if it was capable of doing so. Then, say, you start down the runway and at 60 kts indicated, you leave the ground. So I'm figuring, in a perfect world where the plane is is flying perfectly into the headwind and not climbing at an angle to it, my AI should only read 50 kts since the plane is now part of the airmass that is itself, doing the 10 kts.

As far as weathervaning goes, as you pointed out, there is no weathervaning once you leave the ground, but is this because tire friction is gone or is it because you are now moving inside of the airmass and therefore, no crosswinds ( ignoring gusts, ws for the moment ) are attempting to push your tail sideways ? Perhaps it is both, but I feel that the biggest reason is the latter. Once you're in the soup, your speed through the airmass is all there is. So it just makes sense to me that once off the ground, the speed of the air over your wings should theoretically decrease, although in practice, things like ground effect, the fact you are at full power, and that it takes a while before you are assimilated into the airmass, are all working to increase your airspeed.

[photofly]

In reading it over, I've definitely done a terrible job trying to explain what I wanted to say in the last post. What I mean is, if the plane is just sitting on the runway with a 10 kt headwind, then I figure that the AI would register 10 kts if it was capable of doing so. Then, say, you start down the runway and at 60 kts indicated, you leave the ground. So I'm figuring, in a perfect world where the plane is is flying perfectly into the headwind and not climbing at an angle to it, my AI should only read 50 kts since the plane is now part of the airmass that is itself, doing the 10 kts.

In a perfect world, the airspeed indicator will continue to read 60 kts, since the air continues to flow past the pitot head at 60 kts. That's true in the moments before you leave the ground, and continues to be true after you leave the ground.

The 60 kts of indicated airspeed is made up of 50 kts ground speed plus a windspeed over the ground of 10 kts. It would be quite remarkable if the airspeed indicator was able somehow to adjust itself downwards to show you your groundspeed - and of course it can't do that.

As far as weathervaning goes, as you pointed out, there is no weathervaning once you leave the ground, but is this because tire friction is gone or is it because you are now moving inside of the airmass and therefore, no crosswinds ( ignoring gusts, ws for the moment ) are attempting to push your tail sideways ?

Let's be clear about what we mean by weathervaning. The crosswind pushes sideways on the aircraft. If the aircraft is on the ground, the tires resist the sideways force. But the tires and the air push sideways at different places. The air pushes on the aircraft more towards the tail, whereas the tires only push at the location of the undercarriage. The two forces are equal & opposite - but act in different places - resulting in a torque, which twists the aircraft into the wind. The weathervaning is a product of the twisting action of the two opposing forces.

As soon as the aircraft leaves the ground then the crosswind still pushes on the aircraft but there are no tires to provide a counter-force. The aircraft accelerates sideways - downwind - until it's moving sideways at the same speed as the crosswind, at which point the sideways force disappears. (Of course the pilot has the option of turning the nose towards the wind and flying into the crosswind to track the runway centre line if he or she wishes.)

As a matter of fact, even when the aircraft has left the ground it still has an initial tendency to turn nose into the crosswind because the tall rudder fin on a long arm means the sideways force isn't acting through the centre of mass (this is an example of slip-yaw coupling). Once the nose points into the relative airflow, the yawing force goes away.

[Beefitarian]

What I mean is, if the plane is just sitting on the runway with a 10 kt headwind, then I figure that the AI would register 10 kts if it was capable of doing so. Then, say, you start down the runway and at 60 kts indicated, you leave the ground. So I'm figuring, in a perfect world where the plane is is flying perfectly into the headwind and not climbing at an angle to it, my AI should only read 50 kts since the plane is now part of the airmass that is itself, doing the 10 kts.

The airspeed indicator measures the difference between the static or still air pressure and the pitot or moving towards you air pressure. It's not electric or anything so it will even do that tied down if there is a wind blowing toward the plane from the front.

The faster you go the harder you push against the air increasing the pressure in the pitot.

In flight you're also riding along if the air is moving.
If the air is moving toward you it makes you go over less ground. Slower ground speed.
If you are moving in the same direction as the air mass (wind) you move over more ground. Higher ground speed.

That's why you want a headwind instead of a tail wind when landing. When the wheels touch the ground they will be doing the ground speed along it.

The speedometer in a car measures how many turns your tire makes over time. So that would be ground speed. Doesn't matter what the wind does.

If your plane had a speedometer in your example it would read 50 knots as you went 60 knots airspeed towards or into the 10 knot wind.

Once you lifted off the wheel would slow to a stop and the speedometer also would slow down until it read zero. Even though the AI continued to read the same speed if you continued at that speed.

[Rich_Pa]

So what about my questions guys?

[AEROBAT]

You won't enter a spin just because you stalled, you have to introduce yaw. If you are climbing at best rate of climb I doubt you would even stall.

[Beefitarian]

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...

What is causing the spin? I am not positive this is the same but I experienced a drop of a few hundred feet once flying through a valley. It was very abrupt and seemed like it took less than a second. I was flying often at the time but also didn't really have time to do anything to prevent the plane from dropping a wing. I was flying more or less straight and level when it happened.

Maybe it was fourtunate that I was not turning but typically you may turn slightly on take off to get out of the flight path. Usually you won't turn until you're over 1000' agl.

By the way a 35 knot change in wind direction is quite rare, especially in a concentrated spot. I would suggest it's unlikely to happen unless you're in mountains or something.

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

On final you're decending, as you encounter an updraft you will start to ascend. You will most likely react by reducing power and pushing the yoke forward. At this stage of flight the power reduction will have a slightly better affect on preventing the climb. Lowering the nose will help but also will increase your airspeed. That will help prevent a stall but will also be counter productive to a safer landing.

As we were discussing your airspeed is forward motion in the air. It also affects your ground speed. The lower your ground speed on landing the easier it will be to keep the plane straight and centered on the runway. Also you want to have an airspeed that is high enough for optimum control of the aircraft while low enough that as you touch down or even better just before, the plane does not want to lift off or fly again. That way all your aircraft's mass will transfer to the support of the landing gear and tires onto the runway surface.

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?

The wings will be loaded less by less mass.

Some people will be by soon to correct my answers. Hopefully you can sort through it and learn some things.

[Beefitarian]

So what about my questions guys?

I must ask why you ignored this excellent answer to one of your questions?

1. Yes. If you encounter 30 knots of sudden windshear as you describe, then your airspeed will drop suddenly by 30 knots. If your airspeed was only 65 knots to begin with, it will suddenly halve and you will likely be in a stall. The nose of your aircraft will drop (and if it doesn't you will push it down) to regain airspeed at the expense of altitude, just as in practice stall exercises. If the wind shear was quite screwy I guess you could end up in a spin; it doesn't seem likely though.

Windshear is dangerous, and causes airliners to crash. A lot of money is devoted to sophisticated radar to detect windshear close to airports to alert pilots. However: 30 knots of WS is a lot; the pilots of the Q400s where I fly start bitchin' when there's about 10 knots WS.

Incidentally: WS is not generally that the wind at one place has instantly started to blow in a different direction; it's usually layered: you encounter it by leaving a layer of air moving in one direction and climb (or descend) into a layer of air moving in a different direction.

This is also a good reason not to climb out of ground effect with too small an airspeed margin. Just in case.

[Rich_Pa]

What is causing the spin? I am not positive this is the same but I experienced a drop of a few hundred feet once flying through a valley. It was very abrupt and seemed like it took less than a second. I was flying often at the time but also didn't really have time to do anything to prevent the plane from dropping a wing. I was flying more or less straight and level when it happened.

Maybe it would be more appropriate to say wingdrop instead of spin.

On final you're decending, as you encounter an updraft you will start to ascend.

When you ecounter an updraft, it puts aerodynamic weight on your airplane. That makes it to stall at a higher airspeed. Some like an accelerated stall. That is what i thought about.

I must ask why you ignored this excellent answer to one of your questions?

I really appreciate it.

[photofly]

I don't think an updraft causes an accelerated stall; an accelerated stall occurs when the aircraft is pulling more than 1g (as far as I understand it.) So if you're straight and level and fly into an updraft the aircraft isn't pulling more than 1g, the load on the wings remains equal to the normal aircraft weight, and the stall speed is the same regular stall speed.

I also dislike the term accelerated stall - it sounds like a stall that happens more quickly than a regular stall. Which it isn't. A stall is a stall is a stall, and occurs when the angle of attack reaches the critical angle; be that regular, accelerated, decelerated or intra-turbo-funkboosted with the convective sigmet sprinklies on top (I like the chocolate ones best).

[photofly]

I want to amend my previous answer about what the effect of a 30kt windshear is when you have an airspeed of 65kts.

The velocity of the air over the wings drops suddenly. The angle of attack doesn't change. So you can't be said to have "stalled". But you no longer have enough lift to support the aircraft. If you raise the nose of the aircraft and increase the angle of attack in a futile attempt to restore the lift to that needed for level flight then you will enter the stalled flight regime.

If you don't change the angle of attack and either pitch the nose forward, or allow the aircraft's natural pitch stability to do it for you then you will dive and accelerate downwards to convert altitude to airspeed in order to get enough lift for level flight.

The outcome is the same; I just wanted to be more precise about the "stall" element.

[Beefitarian]

I must ask why you ignored this excellent answer to one of your questions?

I really appreciate it.

Ok cool, you didn't really say much about it and I don't use this guy to indicate I'm happy and or thankfull.

Moving on...
Wing drop that is not caused by a piloting error I made is usually not a big deal. It's when I cause the wing drop that it's a problem. Once I had been away from flight training for a while when taking my PPL. The instructor and I went to the practice area to do upper airwork. Turns, slow flight, everything is going ok. He says, "Ok, let's see a stall." I start to slow the plane down but did not close the throttle. He says, "Oh, going to try it with some power on?" So I decide I might as well since I'm part way into the manouver by now and reply, "Sure, I can handle it."

The stall would have probably been more abrupt but not a big deal until. Mostly because I had not been flying for a while I was probably in car driving mode treating the yoke like a steering wheel. The wing dropped so I incorrectly tried to bring it up with the aileron and we entered one of the few spins in a C-172 that ever got my attention. We had plenty of hieght and the instructor knew me pretty well so he laughed and left me to figure out how to recover. Now I'm keeping the ailerons nuetral, power to idle, opposite rudder, ease out of the dive. Increase the power and climb back up to our practice height.

When you ecounter an updraft, it puts aerodynamic weight on your airplane.

While an updraft might increase your wing loading. I don't know about using the term "puts aerodynamic weight on your airplane".

If we discuss this a bit you might be exactly right with saying that.

When it comes to updrafts, they are typically pretty mild and fairly predictable. While windshear is much more abrupt. The second part is the plane starts to move up instead of down. While that is quite annoying at times and you will need to do something to counter it in order to get the plane to go toward the runway threashold again. There are much fewer things to cause problems to your flight above you. I have never encountered the change in airspeed that would cause a stall in an updraft.

While I understand it is helpfull to ask questions about this sort of thing. Discussion is good but... If you're not getting in some sort of real airplane to experience how the plane acts in stalls, slow flight, cliimbing, turning, etc. with an instructor. You can develop some funny ideas. I know I have sometimes.

[Rich_Pa]

You won't enter a spin just because you stalled, you have to introduce yaw. If you are climbing at best rate of climb I doubt you would even stall.

During turbulence, the airplane might induce suddenly roll, yaw, you know, it's impossible to fly it coordinated 100% in these situations.

So if you're straight and level and fly into an updraft the aircraft isn't pulling more than 1g, the load on the wings remains equal to the normal aircraft weight, and the stall speed is the same regular stall speed.

Strange, I think a sensitive G meter will demonstrate the opposite. Have you ever felt G when you suddenly ecountered an updraft?

I also dislike the term accelerated stall - it sounds like a stall that happens more quickly than a regular stall. Which it isn't. A stall is a stall is a stall, and occurs when the angle of attack reaches the critical angle; be that regular, accelerated, decelerated or intra-turbo-funkboosted with the convective sigmet sprinklies on top (I like the chocolate ones best).

Usually the stall behaviour tends to be more aggresive during an accelerated stall, at least in simple practical observations.

While I understand it is helpfull to ask questions about this sort of thing. Discussion is good but... If you're not getting in some sort of real airplane to experience how the plane acts in stalls, slow flight, cliimbing, turning, etc. with an instructor. You can develop some funny ideas. I know I have sometimes.

Actually I do it, unfortunately I don't have a gust inducer to experience those scenarios.

[Beefitarian]

Ah keep at it. You'll get updrafts often. I have felt brief light "G" when entering an updraft. More often you notice you're gaining altitude then just as you've adjusted to that you fly past the updraft and now you're descending and need to bring up the power.

[photofly]

Strange, I think a sensitive G meter will demonstrate the opposite. Have you ever felt G when you suddenly ecountered an updraft?

I guess you're right. If you're flying at Va in a Utility category aircraft, through a gust that causes a stall, then you're going to be pulling 4.4g before that stall happens. (Ouch.) So I suppose that counts as "accelerated". Is it a useful distinction?

[Rich_Pa]

Strange, I think a sensitive G meter will demonstrate the opposite. Have you ever felt G when you suddenly ecountered an updraft?

I guess you're right. If you're flying at Va in a Utility category aircraft, through a gust that causes a stall, then you're going to be pulling 4.4g before that stall happens. (Ouch.) So I suppose that counts as "accelerated". Is it a useful distinction?

Yes, it is because a gust which induces 2G it's going to stall you on final at 65 kts. In my scenario it's the difference between having a stall or not.

[Colonel Sanders]

a gust which induces 2G it's going to stall you on final at 65 kts

Not something you have to worry about. I suppose
you could also worry about getting hit by a meteor
on final, too.

As a pilot, what you need to know, is how to precisely
control your airspeed on final. Not many low-time pilots do
a very good job of that.

Once you have mastered airspeed control, the next thing
you need to learn is how to deal with sudden decreases
in airspeed on final.

These often occur at inconveniently low altitudes, and at the
risk of sounding like the infamous pdw, are not due to transient
conditions, but rather due to your descending through layers of
air which are travelling at different speeds over the ground.

That's what you need to know, to operate an aircraft. If operating
an aircraft doesn't matter to you - frequently the case here, it
would appear - pardon my interruption.

[Rich_Pa]

a gust which induces 2G it's going to stall you on final at 65 kts

Not something you have to worry about. I suppose
you could also worry about getting hit by a meteor
on final, too.

So, to understand from that it's almost impossible to encounter a gust which induces 1G acceleration on airplane?

[photofly]

Yes, it is because a gust which induces 2G it's going to stall you on final at 65 kts. In my scenario it's the difference between having a stall or not.

I don't think it's such a big deal. If you feel the aircraft start to rise in such an alarming manner, simply pitch forward to relieve the aerodynamic load on the wings, and accept the increased airspeed excursion. If you can't get the airspeed under control in time, go around.

An updraft improves your energy situation. Windshear is much more scary - it makes your energy situation much worse.

[Rich_Pa]

Yes, it is because a gust which induces 2G it's going to stall you on final at 65 kts. In my scenario it's the difference between having a stall or not.

I don't think it's such a big deal. If you feel the aircraft start to rise in such an alarming manner, simply pitch forward to relieve the aerodynamic load on the wings, and accept the increased airspeed excursion. If you can't get the airspeed under control in time, go around.

My idea (possible wrong) is: when you encounter the gust, it pust "aerodynamic weight" on your airplane, smth like you pull the yoke and load factor increases. As we know, if the load factor is high enough for the airspeed flying, the plane stalls. That is, as an example, we fly straight and level at 150 ft, the stall speed is 50 kts. 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. Put that also with some yaw induced by turbulence and it becomes an incipient spin.. anyway, even being just a stall, a stall at 100 ft might force you to "kiss" the ground faster than you think. Is my theory wrong here?