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Old 09-24-2007, 05:12 PM   #1  
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Default 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?
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Old 09-24-2007, 05:21 PM   #2  
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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

Last edited by mike734; 09-24-2007 at 05:27 PM.
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Old 09-24-2007, 05:32 PM   #3  
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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

Last edited by Cubdriver; 09-24-2007 at 05:51 PM.
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Old 09-24-2007, 05:33 PM   #4  
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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
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Old 09-24-2007, 06:02 PM   #5  
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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
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Old 09-24-2007, 06:10 PM   #6  
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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?
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Old 09-24-2007, 07:27 PM   #7  
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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.
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Old 09-25-2007, 03:59 PM   #8  
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anyone else have a good way to explain it?
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Old 09-25-2007, 04:31 PM   #9  
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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).
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Old 09-25-2007, 04:52 PM   #10  
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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.

Last edited by Cubdriver; 09-25-2007 at 05:08 PM.
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