Spartan07

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

mcartier713

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

Cubdriver

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!

Spartan07

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.

mcartier713

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.

Spartan07

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.

tangoindia

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

fatmike69

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.

flightest

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

Airbum

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.