Plane cannot be stalled

[Beefitarian]

Heck you are not even stalled while flying a 0 airspeed if you are at 0g...

Agreed, I would call that levitating.

[Beefitarian]

.

[Frank Uckër]

you have no understanding of how a stall works

You have no idea what a full-stall landing is in a tailwheel aircraft.

Why would it be different for landing?

Do you land a tailwheel aircraft with the mains 3 feet in the air and the tailwheel on the ground? Of course you don't. In level flight after flaring to land in ground effect, that is the kind of stunt you would have to perform, to fully stall the aircraft before landing after a flare.

Or you could smash it on the ground with a high descent rate and no flare - that's the other way of getting a full-stall landing.

I don't see many people regularly doing either of the above.

Oh God, for an AOA indicator.

Just because lift decreases, doesn't mean you're stalled. It just means that you're slowing down, because lift is proportional to velocity SQUARED.

[Beefitarian]

Still, I think it's dangerous that people believe that a wing is stalled when the aircraft is parked in the hangar, or rolling down the runway, or that as soon as there is a nose up attitude and a descent associated to it it means you are stalled. Believing any of the three things I just named means one does not understand at all what a stall is.

I don't think those things if anyone is reffering to me. Though I understand how that rumour is taking hold after I was involved in trying to discuss the stall warning system.

I believe (and am still possibly completely incorrect) the moment of transition from creating enough lift to the point of not creating enough lift would be a stall. I am willing to accept that it is not, however it does tend to occur when you do actually exceed the CL with too great an angle of attack which I also believe to be a stall.

I fully believe that in order to land and have the wing stay in that condition it must make some transitions
from creating enough lift to climb, to only creating enough lift to fully support the weight of the airplane,
then in order to decend slightly less lift than required to fully support the weight of the airplane,
next to only creating enough lift to share support of the weight with the landing gear
and finally to not creating enough lift to support the airplane at all.
Depending on several things the time period required for that to happen can vary quite a bit.

I do understand that any stall will be broken as soon as the weight is fully supported by a surface.

I also understand the wing is not going to change it's position during post touch down taxiing, altering the angle of attack. However as was pointed out Ground Effect might.
The stall warning will not operate because the negative pressure component of lift might not move forward or it does so too rapidly to notice or a third possiblility being that as it did the airspeed was too low for sufficient low pressure to actuate it.

What happened here?

If I am flying my Cessna 206 at maximum forward C of G (typically two adult males in the front seats, reserve fuel, and a survival kit in the back to save the nose wheel) then as I flare I am at idle with full aft elevator and descending. I usually hit the ground before the otherwise perfectly functioning stall horn goes off

Anyone?

[photofly]

I believe (and am still possibly completely incorrect) the moment of transition from creating enough lift to the point of not creating enough lift would be a stall.

There are three ways you can transition from having enough lift to fly level, to having not enough lift, which means you descend. One of them counts as entering a stall, two don't. The three ways I can think of doing that are

1. To maintain a constant angle of attack and to slow down. Less speed -> less lift -> descent. You can do a nice soft landing like this, and without stalling. It won't be the shortest landing roll you could do, though.

2. To maintain a constant speed, and reduce the angle of attack by lowering the nose. It works great up in the air, for starting a descent, not so great for landing because it would be flying the plane onto the ground.

3. In the special case where you're level and the wing is already at the critical angle of attack, you can also descend by increasing the angle of attack still further (raising the nose, and entering the stalled regime.) If you do that just a few inches above the ground you will do a nice soft full-stall landing, and it will also be at a slower speed than case 1. If you do it a few feet above the ground it's going to be a hard landing. But the roll is still going to be short.

Cases 1 and 3 meet in the middle.

Frank's point (which is plausible) is that in case 3 it's possible that the aircraft geometry will result in a tail strike before you get there. However, flaps, by rotating the wing relative to the aircraft fuselage, prevent that in a 182.

I fully believe that in order to land and have the wing stay in that condition it must make some transitions
from creating enough lift to climb, to only creating enough lift to fully support the weight of the airplane,
then in order to decend slightly less lift than required to fully support the weight of the airplane, ...

It's not really a huge transition - a difference of a fraction of a knot, or a fraction of a degree AoA, will make the difference between level, and beginning a slight descent to touchdown.

I do understand that any stall will be broken as soon as the wieght is fully supported by a surface.

Um, no it won't. Stall depends only on AoA, not on the wing loading. If you land in a full stall the stall "breaks" as soon as the nosewheel comes down enough to lower the AoA below the critical angle of attack.

[Beefitarian]

How is the nose wheel not touching if the weight is fully supported by the surface? There seems to be a CofG issue now.

[photofly]

What happened here? (206... idle, descending, full back elevator, no stall horn)

The plane isn't stalled. The wing hadn't reached the critical angle of attack. The descent would have slowed if there had been enough elevator authority to raise the nose.

If the plane had been stalled, the stall warning would have been sounding, and in that case, raising the nose - had there been elevator authority to do so - would have increased the rate of descent.

[Beefitarian]

The stall warning did sound. Supporting my theory that the wing can stall after touchdown.

[photofly]

How is the nose wheel not touching if the weight is fully supported by the surface? There seems to be a CofG issue now.

Right. It's correct to say that the stall will have been broken by the time the weight is full supported by the surface (which taken to extreme, will only occur when the airflow over the wing has stopped entirely). It's also correct to say that the stall will have been broken by September of the year 4563. But in both cases there's no connection between the two. It's not the supporting of the weight that causes, or indicates, the fact that the wing is no longer stalled. That is determined only by the angle that the wing makes to the airflow.

[photofly]

The stall warning did sound. Supporting my theory that the wing can stall after touchdown.

You can stall after touchdown if you raise the nose sufficiently in the rollout. It's not a necessary part of the landing though or even a great idea: if you do it while moving too fast you'll take off again.

But that's a feature of pilot technique, not a necessary part of the aerodynamics of a landing aircraft.

[photofly]

To be clearer:

You can stall after touchdown if you raise the nose sufficiently in the rollout. It's not a necessary part of the landing though or even a great idea: if you do it while moving too fast you'll take off again.

If you raise the nose in the rollout, you can raise the angle of attack beyond the critical angle, and stall, while still rolling along the runway.

If you are travelling fast enough, and you raise the nose by a moderate amount, you may generate enough lift to become airborne. Or, if you raise the nose by more, you may become airborne and stall at the same time, in which case you have a significant problem to deal with.

[Beefitarian]

Ok, you mentioned the stall is not allways corrisponding with a decrease in lift. Though at low power I think it will cause you to slow down and that will cause a decrease in lift.

Is part of the reason for the sudden decent when you stall because the vector angle of that lift will no longer be vertical even though the vector representing weight remains vertical, so as you pass the CL the plane changes angle rapidly and you feel the drop?

I thought the break would be caused by a sudden decrease in lift.

[iflyforpie]

Why would it be different for landing?

Do you land a tailwheel aircraft with the mains 3 feet in the air and the tailwheel on the ground? Of course you don't. In level flight after flaring to land in ground effect, that is the kind of stunt you would have to perform, to fully stall the aircraft before landing after a flare.

I've already proven that there are tailwheel aircraft that exceed the stalling AOA in the three point configuration.

I've already pointed out the error in your measurments and using your own raw data demonstrated that tailwheel aircraft do indeed reach or exceed the stalling AOA in the three point attitude.

There are other errors you've made too, like the Cessna 172 you measured the angle on, but totally neglected the wing incidence angle (hint, it is not zero degrees when on the gear ).

Just because lift decreases, doesn't mean you're stalled. It just means that you're slowing down, because lift is proportional to velocity SQUARED.

But lift doesn't decrease, it remains the same if you are holding altitude. It remains the same in spite of decreasing velocity by increasing the angle of attack (what most people would call a flare or hold off) and you can continue to do so until the plane stalls or runs out of elevator authority.


Everybody seems to understand this but you. But given your poor attitude and inability to provide any convincing argument otherwise, I think I am finished here.

[Beefitarian]

Don't go, Trey went for pizza.

[photofly]

Ok, you mentioned the stall is not allways corrisponding with a decrease in lift. Though at low power I think it will cause you to slow down and that will cause a decrease in lift.

It might be an idea to refresh your basic physics, about forces, velocities, and accelerations. Stall -> extra drag -> slowing down -> less lift -> descending -> speeding up -> increased lift ... what really happens is you reach an equilibrium. You're descending at a constant rate, not accelerating downwards, therefore lift = weight again. Same lift as when flying level, same (engine power) only now you're going down like an express elevator.

Is part of the reason for the sudden decent when you stall because the vector angle of that lift will no longer be vertical even though the vector representing weight remains vertical, so as you pass the CL the plane changes angle rapidly and you feel the drop?

There isn't a sudden descent. There may (or may not) be a sudden nose drop, and - if you transition into the stalled regime rapidly - a sudden acceleration downwards. Additionally the descent may become rapid and can be prolonged; but the descent of itself is not sudden.

(I use the two different words acceleration, and descent, carefully.)

I thought the break would be caused by a sudden decrease in lift.

The "break" is the nose dropping because the centre of pressure moves. You can't rely on feeling a break, or anything sudden, to tell you you're in a stall. That poor AF crew rode a stall for 38,000 feet, all the way down to the sea, without noticing.

I massively recommend you read John Denker's online book at http://www.av8n.com/how. It's written by a CFI who's also a professor of physics, and you will get a rock solid understanding of stalls from him. Section 5.3, in particular.

[Beefitarian]

I thought the break would be caused by a sudden decrease in lift.

The "break" is the nose dropping because the centre of pressure moves. You can't rely on feeling a break, or anything sudden, to tell you you're in a stall. That poor AF crew rode a stall for 38,000 feet, all the way down to the sea, without noticing.

I massively recommend you read John Denker's online book at http://www.av8n.com/how. It's written by a CFI who's also a professor of physics, and you will get a rock solid understanding of stalls from him. Section 5.3, in particular.

Cool, and as it's not uncommon to not have a noticable break in a C-172, kind of annoying during flight training if you ask me. That was the part of the stall that the Original Poster was missing.

[trampbike]

Heck you are not even stalled while flying a 0 airspeed if you are at 0g...

Agreed, I would call that levitating.

I call it flying a Hammerhead maneuver and going over the top, or to be flying a steep inverse parabolla, experiencing 0g while transitioning from upward path to downward. In both cases, the airspeed is close or equal to zero and yet, you are not stalled. Did you know that stalled wings produce a whole lot more lift than wings in 0g flight? In fact, when flying at 0g, the wings produce no lift at all. Yet, this is not a stall. You have to produce some kind of lift to stall and to remain stalled.

[trampbike]

Cool, and as it's not uncommon to not have a noticable break in a C-172, kind of annoying during flight training if you ask me. That was the part of the stall that the Original Poster was missing.

In such case, if you want a clearcut stall, you can try to add a burst of power once the stick is fully aft and the airplane already very slow. Elevator authority increases greatly with the propwash.

[Beefitarian]

Did you know that stalled wings produce a whole lot more lift than wings in 0g flight? In fact, when flying at 0g, the wings produce no lift at all. Yet, this is not a stall. You have to produce some kind of lift to stall and to remain stalled.

That makes sense. There's been a suggestion that stalled wings generate as much lift as in regular flight. It's just the drag is huge and the lift is not in a direction opposing the center of gravity directly.

[photofly]

flying a steep inverse parabolla, experiencing 0g while transitioning from upward path to downward. In both cases, the airspeed is close or equal to zero and yet, you are not stalled.

Typo I think...

Ballistic trajectory, zero angle of attack, non-zero airspeed, all the way through.

[photofly]

There's been a suggestion that stalled wings generate as much lift as in regular flight.

True

It's just the drag is huge

Also true! Although perhaps better to say (my bad) that the drag rises very quickly as a function of angle of attack. There's no sudden jump in the drag from not-stalled to stalled.

and the lift is not in a direction opposing the center of gravity directly.

Not sure quite what you mean by that. The lift directly opposes and matches the weight, just as in regular non-stalled flight. There's no difference.

There's no difference between the lift in a stall, and the lift in regular flight!

[Beefitarian]

Aight, I'll remove, "Lot's of incorrect stuff."

[trampbike]

flying a steep inverse parabolla, experiencing 0g while transitioning from upward path to downward. In both cases, the airspeed is close or equal to zero and yet, you are not stalled.

Typo I think...

Ballistic trajectory, zero angle of attack, non-zero airspeed, all the way through.

You are right, a ballistic trajectory it is not exactly a parabolla (unless object moves in empty space)

You can however bring your airspeed indicator to show 0 (or very close to it) by zooming steeply and then pushing on the stick, full power might be necessary to have enough elevator authority to bring the nose down at such slow speed. The path of such a maneuver is not a parabolla, thanks for your correction.

[trampbike]

There's been a suggestion that stalled wings generate as much lift as in regular flight. It's just the drag is huge and the lift is not in a direction opposing the center of gravity directly.

Coefficient of lift decreases once you've exceeded critical AoA

[photofly]

If the lift is equal how can you decend? I was thinking because ... (lots of incorrect stuff removed)

Very important point, without which most of the discussion of stalls will make no sense at all.

An unbalanced force doesn't cause a steady descent. An unbalanced force causes an acceleration - i.e. an increasing rate of descent. That's why I suggested a review of the physics around Newton's laws (Force = mass times acceleration and all that).

If the rate of descent is constant, the vertical forces are balanced. Flying level is one case where the vertical forces are balanced, but so is a steady 500fpm climb, and so is a steady 2000fpm descent.