Stalls
 

ok, so how retarded am i if i'm studying for my commercial written and i still dont understand stalls? every chapter seems to describe them differently. we have stall speeds, Vso Vs1, everything related to stall speeds.. but yet, stalls have nothing to do with speed, airplanes stall because of angle of attack, well, then what the heck are the stall speeds for? and how do they relate?

Put you hand out the window at highway speeds. As you angle your hand up you experience lift. Now, at a certain angle the up force stops and your hand is just forced back against the air flow. Your hand has stalled. The air must flow over and under your hand to make lift. The minute you angle your hand too much lift stops and your hand is pure drag.

Crude yes but the model should make it clear. If you wish, get a model airplane wing and try the same experiment. (Go slowly with someone else driving) The more you angle the wing the more lift you generate for a given speed. At a certain angle the lift will stop. You will feel a dramatic increase in drag and loss of lift.

Does this make sense?

The reason for this is because lift is actually the force generated by ACCELERATING the air downward. The air flow must be deflected down in the direction opposite the opposing force (gravity) Low pressure on top of the wing is a byproduct of this action. F=Ma is the ruling equation. Source? The Proficient Pilot By Barry Schifft. It is one of my favorite books.
http://www.overstock.com/Books-Movie...cid=80486&fp=F

VS0 and VS1 are airspeeds where the angle of attack is expected to reach the critical maximum for a particular set of inputs, and they are by no means absolute numbers. The angle of attack is the absolute criterion for stall, and the corresponding airspeed will vary according to a bunch of factors (if you can think of any pls. add), things like cg location, weight load, trim tab setting, gust and load factors, boundary layer fences, vortex vanes, leading edge extensions, and even dirt buildup. They are merely predictions based on typical inputs, they are not absolute airspeeds for stall. Many of the variables are covered in the FAA Airplane Flying Handbook and are included in Certified Flight Instructor programs.

Be that as it may, stall speeds are predictable enough to allow performance computations for landing distances, manuever limitations, and structural limitations to a fairly high degree of accuracy. This is why it is a good idea for a pilot flying a new airframe to take it out for some stalls first, to see what the actual indicated stall airspeeds will be dirty and clean under 1G conditions. A lot of times new pilots struggle with short field work mainly because they have no real idea where the stall is. Familarity with the stall behavior of an airplane helps one implement lower approach speeds while maintaining a 2-3 knot margin for error. Bear in mind stall speeds at altitude are somewhat higher than in ground effect, due to the additional lift available from the low-drag airflow as the airplane nears the ground.

Mike- I am a fan of Barry as well, I like his articles on flight training.

-Cub

Here, this might help you visualize the disruption of airflow over the top of the wing when it reaches the critical angle of attack.

http://www.youtube.com/watch?v=J-xxCkebdZs

The stall is actually your best friend - think Va - The stall becomes a pressure relief in a way - and keeps you from tearing your aircraft and you apart...

To understand a stall I had to embrace Va

Think center of pressure in relation to cg and as you pitch up with more angle of attack - the center of pressure moves toward the leading edge until the airflow becomes so turbid and basically the pressure is dumped...

Very crude explanation - but this is why I love the stall and really try to know my aircrafts weight so I can determine Va for that specific weight :)

I understand why the wings stall, I understand the concept in both speed and angle terms, I just don't understand the difference and why it's expressed in both terms.

I donno if that makes sense :-/

the best example that I can think of is when flying aerobatics, doing a hammerhead or something vertical. you are way below stall speed right as you rudder over, yet you dont stall?

I think the lift formula might come in use: L=CLQSA L(lift) CL(Coefficent of lift) Q(rho or dynamic pressure) SA (Surface Area). The only place that airspeed has a factor is Q and CL. As the speed (velocity) changes, the lift changes proportionaly. Remember from the CL curve that that as airspeed is decreased lift is decreased and an increase in angle of attack must be made in order to maintain altitude (Slow flight or MCA). Sometimes the pitch increases so much with out an increase in power that a stall occurs. hope I explained this ok.
Aerobatic aircraft use symetrical airfoils, that means that they have to have a positive angle of attack to have a higher velocity over the upper surface of the airfoil, because the thickness and camber is identical along the upper and lower surfaces of the airfoil in relation to the chord line.

anyone else have a good way to explain it?

Keep these definitions in mind:
Relative wind is the flow of air directly opposite the direction the aircraft is traveling (which is NOT necessarily the same way the nose is pointed!). If you want a good illustration of the difference between relative wind and pitch angle, look at the attitude indicator during slow flight. Although you're traveling at a fairly constant altitude, the nose is pitched up significantly.

Angle of attack is the angle between the aircraft's chordline and the relative wind.

Understanding these two concepts is imperative if you're going to understand stalls at any level.
An aircraft stalls at the critical angle of attack - that is, the AOA at which the wing no longer produces lift. This AOA is a constant number for all weights of the aircraft. The number referred to as a stall speed is a general speed at which you might expect the aircraft to stall. However, this speed changes as weight changes, as well as with different aircraft configurations (thus we have Vs1 and Vso).

Maybe you are a little fuzzy on what happens around an airfoil in a fluid flow. If you're asking why does it suddenly quit lifting, be aware that visualizing airflow is not especially easy. Smoke streams are used to help illustrate the flow around an airfoil, which is a 2-D "slice" of a wing. Generally, at less than critical AOA's the smoke will work its way around the airfoil on both sides, top and bottom, and since the distance around the top is longer, regardless of any camber present, it has to speed up to get around the airfoil and meet the other half of the airflow that went around the bottom of the airfoil. It is a law of aerodynamics called the Kutta Condition these two smoke trails must meet at the back of the airfoil. The speeding up of the smoke on the top causes a drop in pressure by Bernoullis Law. This is where the lift comes from. When the critical AOA is finally reached around 12-16 degrees (it varies for different cambers), the amount of speeding up the smoke on the top has to do to meet the flow from the bottom is so much that it can no longer do so without flying off the surface of the airfoil. It becomes turbulent flow, and this is the stalled condition. For the time being, ignore the lift from reactionary forces where the air bounces off the bottom of the wing, and lift from reactionary forces from air being thrown off the back of the airfoil in a downwash. The basic principle is that the air cannot stay attached to the airfoil when the distance gets to be too high. It's that simple. And the reason airfoils are studied is they represent the most basic, 2-dimensional flow situation. You can see the same behavior in 3 dimensions by attaching yarn tufts to a wing and observing them flip around, but it gets a lot more complicated because some of the wing may not stall, some of the wing will be affected by the open sides of the wing, some of the wing will have a different camber, and it is harder to observe effects related to the stall. Does this help or is another aspect unclear? Richard Shevell's Fundamentals of Flight Second Edition is a useful reference work if you need more than the FAA or Jeppeson have on the subject.

I tried not to mention Bernoulli, that's why I posted the youtube video... I smell a Bernoulli/Newton debate coming on now ;)

ok I guess here is a question... when you have a stall speed, Vso or Vs1, at what angle of attack are they referring to?... "an aircraft can stall at any airspeed and any attitude"

its that line that confuses me I guess, one side said the stall speed is X and the other one says it doesn't matter because airfoils can stall at any airspeed.

am i just going in circles? lol

Spartan- there's no conflict, they are valid aspects of lift. The watered down versions you see in some sources omit the Newtonian aspects- and a lot of other ones- because it's too complicated to explain in brief treatments. The full mathematical treatment of everything involved in incompresiible flow is found in the Navier Stokes Equations of Incompressible Flow. Full fledged mathematical analysis of flows, whether compressible and incompressible- is so complicated the greatest supercomputers fail to reach closed form solutions. As a result, simplifying assumptions are used as a matter of course in aerodynamic analyses.

McCartier- I promise you there is a fixed number that will be the stall AOA for any given wing presented. It's some number for every single one, say 13 degrees or 12.5 degrees. The reason the airspeed may vary is, a wing may not have used up all its available AOA and reached that limit, if various conditions happen to be in play. But when it does, it's like a mousetrap that suddenly goes snap and it is always that same AOA no matter how fast, how slow, how heavy the airplane, etc. You know that if you take all the people off your plane it will be flying at a lower angle of attack. If somehow you could drop people onto the airplane from outer space or something, you would gradually have to add angle of attack to keep the lift equal to the weight. At some point, snap you have reached the critical AOA and that's all there is, the wing stalls. Notice the airspeed never changed up to the point of stall. Now if you take the same airplane and the same load of people and speed the airspeed up, you can lower your AOA down from the near-critical one, because you have increased the dynamic pressure acting on the wing and it is able to meet the lift demands on it that way rather than by increasing the AOA up and up. So this shows that airspeed had nothing to do with a stall, AOA did.

Peace out, I have to get up early!

I think I recall hearing that something akin to 13 degrees of AoA is normally the critical angle at which the wing stalls for most general aviation, trainer type aircraft. Keep in mind, this is the angle of the wing in relation to the relative wind, not the attitude. If you yank the stick back hard and the aircraft pitches up faster than it's momentum can change direction then the wing will stall. That's kind of what they mean when they say at any speed, in any configuration, and at any attitude. It all depends on the aircraft's direction of actual movement as opposed to basically which way the nose is pointed.

P.S - I don't know if this helps, and I know it repeats some of what has already been said but maybe someone on here can say it just the right way for you.

is it like.... if you're at/close to stall speeds and you pull back on the stick there is insufficient speed to keep the AOA under the critical angle? but at higher speeds there is much less risk of hitting that AOA with a hard pull? o_O lol

spartan I wrote this before I read what you just said, and yeah thats what I'm talking about.

This is nigh impossible to type out but here goes... What I think you need to see is that speed is irrelevant. The only thing that stall speed does for you is say that below this particular speed the aircraft will not be flying fast enough to produce lift. This lack of lift will cause a drop in altitude (1 stall indicator, not a stall). If you exacerbate this loss of lift by trying to maintain altitude with back elevator then the relative wind will shift further down the airfoil increasing your angle of attack and inducing a full stall. However, if you pay attention to the stall indicators (High rate of descent, loss of control effectiveness, and of course the stall warning system in the aircraft if equipped), which by the way, is the whole purpose of stall training, and you correct the stall before it happens then it never occurs.

The stall doesn't occur because of low airspeed. But low airspeed will cause a stall.... make sense?

P.S - Someone feel free to step in if I said something incorrect or misleading.

Ever tried doing a stall with a headwind....and then with a tailwind?...what was difference?...a headwind increases the flow of air over your wings, therefore the plane is a lot harder to stall...as mentioned before everything is about the smooth flow of air over the wings. When that smooth flow (laminar) reaches a certain point the wings are just not producing ENOUGH lift to hold the weight of the plane so it drops in an attempt to regain the flow of air => lift

Well I'll give it a try: The reason an airfoil can stall at any speed has to do with weight. When you slow an airfoil, the only way to generate the same amount of lift is to increase the angle of attack. Eventually the critical angle of attack will be reached, and the airfoil no longer produces lift. This is the "stall speed". Now, a heavier aircraft obviously requires more lift to stay afloat. So think of it this way: take 2 identical aircraft, A and B. Aircraft A is loaded with 1000lbs more cargo than B. They are both cruising at 100 knots. Of these two IDENTICAL aircraft, which one is going to be cruising at a higher angle of attack? Aircraft A of course, because it is heavier, and requires more lift. So....what does this mean in relation to stall speeds? Aircraft A will stall at a higher speed than B, because the critical angle of attack will be reached before B's. Remember, these are IDENTICAL aircraft, and will stall at the SAME critical angle of attack. Aircraft A will just reach its critical angle at a higher speed due to the extra lift it requires due to weight. So, aircraft A might stall at 80 knots where B might stall at 75 knots, even though they are identical, because of weight.

Now let's take it a step further. Excess weight can also be imposed on the wings by load factor. Think about how it feels when you maintain a constant altitude in a 60 degree bank. You feel like you're being compressed into the seat, and you are experiencing a 2g load factor because of the excessive bank. This actually causes the wing to support 2 times it's actual weight, so the aircraft "thinks" that it is much heavier. Again, with an increase in weight, angle of attack must be increased to maintain altitude, and you increase the stall speed. In a 2g load factor, this can cause stall speed to increase dramatically, about 40% or so if my memory serves me right. This is because to maintain altitude in a 2g turn, the required angle of attack might be like 8 degrees to maintain altitude, where it might only be 1 or 2 degrees in unaccelerated, level flight at the same airspeed. So, the critical angle of attack is reached at a much higher speed than normal, thus causing the aircraft to stall much faster than it would normally. This is why an airfoil can stall at any speed. The stall speeds published in the POH are usually at max weight with no load factor. Overload the airplane and yank and bank it, and you'll stall much faster than normal.

Hope this helps.

mcartier713:

Here's the answer to your question: The published stall speeds you ask about are the speeds that correspond to the critical angle of attack under a +1G symmetric flight condition, (no roll rate), for the particular configuration.

FT

Its because most likely you don't have and AOA gauge in the aircraft. The wing on the plane you are flying stalls at a specific AOA. This stall AOA is the same regardless of aircraft weight or speed. The different V speeds represent the cheap simply way to show the pilot he is at the "stall" in 1 g flight.

haha thanks for all the help guys... I think I can finally picture it in my head the way I want to. fatmike, I think your explanation was the one that helped the most.

hopefully this thread can be of some use to other pilots now :)

FT and airbum, those helped too.. they have AOA gauges? lol