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Old 08-21-2009, 06:06 PM   #1  
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Default Aerodyamics Question: When lift starts/stops:

Ok, I've been putting off asking this question here for awhile, but I need to get some input on this. It is a private pilot type of question coming straight from a private pilot, so bear with me here.

So, everyone learns in private pilot ground school that you should land and takeoff within 'the bowl' for wake turbulence reasons, as the wings of an aircraft (as told by my instructors and text books) are not creating lift until the point at which the aircraft is either rotating on lift-off or when the nose gear touches down (more or less speaking).

My issue is with the validity of that 'fact' regarding when lift actually begins to generate and cease. Everyone seems to tell the story as if to suggest that lift basically 'shuts off or on' at these two critical moments in flight, and that there is nothing in between.

I contest that wings are always generating lift so long as any amount of air is moving over the wing, forward to aft, regardless of whether the wheels are on the ground. Any why not? Why should a wing care if another separate part of the airplane is touching the ground in order for its aerodynamic functions to work? Physics, right?

You have two identical airplanes moving in parallel: Airplane 1 is flying at 120kts just off the deck, gear up, at the same altitude it would be at if it were supported by its gear. Airplane 2 is rolling on the ground on its wheels at 120kts. By what I've been "taught", Airplane 1 is the only airplane generating lift, due to the 'facts' listed further up at the top.

So where is the truth??

Ends filibuster.
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Old 08-21-2009, 06:21 PM   #2  
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Default Miyagi says: Lift-On, Lift-Off

This answer is coming from a student, so I'm hardly authoritative, but here's my best understanding.

For physics purposes, lift is variable, true.

But for many flight-purposes, lift is either greater than aircraft weight ("On"), or less than aircraft weight ("Off").

It doesn't matter if your wings are generating lift (in the physics sense), if the lift is less than the aircraft weight. You're still falling. So for flight-purposes, you have no usable lift.

If the lift generated exceeds aircraft weight, you're flying.
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Old 08-21-2009, 07:16 PM   #3  
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When Lift Fails

By Dave Esser

When it comes to aerodynamics, student pilots are taught about stalls early on. They are told that an airfoil stalls at one critical angle of attack, when air no longer flows over the upper surface in an organized pattern but instead separates or burbles away. Students are also told that an airfoil can be stalled at any attitude and at any air speed. At this point, the discussion of stalls is generally over, except for the admonition to "keep your air speed up and don't let this happen to you." But there's actually a lot more to learn. This paper will examine the aerodynamics behind a stalled airfoil, with the hope that the reader will come away with a better understanding of the physics of airfoil stall and with increased knowledge of how to avoid a stall.

Before we begin, it should be noted that the word "stall" as it applies to airplanes is a bit of a misnomer, since it makes one think of an automobile engine stalling -- a completely different problem. The term was probably coined because an airplane climbing at a steep incline is reminiscent of a car stalling as it attempts to climb a hill that is too steep. The word has remained in the aviation lexicon even though it causes confusion in aircraft accident reports.

In the flight training of stalls, students may think they are going aloft to practice losing control. In fact, they are learning how to maintain control. The goal in stall practice is not to get good at stalling an aircraft -- many dead pilots achieved that -- but to be able to recognize a stalled condition and then to recover with minimal loss of altitude. Another objective of stall practice is to make students aware of slow flight. Failure to attain or maintain a safe flying speed is the cause of a significant number of mostly fatal accidents.

To understand the stall, or lack of sufficient lift, one must understand how an airfoil creates lift in the first place. The camber, or curvature, of the airfoil increases the airflow over the upper wing surface, and the lowered pressure creates the upward force called lift. The pilot controls the amount of lift by adjusting the wing's angle of attack. The angle is measured between the chord line and the relative wind. As the angle of attack is increased, the coefficient of lift increases to the maximum coefficient of lift (Clmax). The Clmax is attained at the critical or stalling angle of attack. This stalling angle of attack is constant. That is, a given airfoil will always stall at the same angle of attack. (For a further discussion of this topic, see "Lift" in the January/February 1999 issue of Woman Pilot.)

The pressure on the upper surface of the wing is lowest at a point known as the center of pressure (CP). The exact location of this point depends on the airfoil, but is usually located between 30% to 50% of chord (the line connecting the leading and trailing edges). In other words, the CP is typically found roughly one-third to one-half of the way between the leading and trailing edges. A high-pressure stagnation point develops on the leading edge of the wing while a lower-pressure stagnation point develops at the trailing edge. A pressure gradient is determined by dividing the pressure difference between two points by the distance between them. The higher the difference in pressure and the closer the points, the greater the pressure gradient. A favorable pressure gradient is from high pressure to lower pressure, the natural direction of airflow. Due to a wing's pressure distribution, the pressure gradient is favorable from the leading edge to the CP and unfavorable from the CP to the trailing edge.

As the angle of attack increases, the pressure at the CP decreases. The lower the pressure at the CP, the greater the adverse pressure gradient to the trailing edge. At a particular angle of attack the adverse gradient becomes so great that the airflow begins to move from the trailing edge forward to the CP. As this forward-moving air meets the rearward-flowing relative wind, the air stream has nowhere to go but up and away from the airfoil. This burbling, felt by the pilot as a buffet, is the first indication of an imminent stall. As the stalled area of the wing increases forward from the trailing edge to the leading edge, the center of lift moves forward. When the entire wing is fully stalled, the loss of this forward upward force results in a nose-down tendency.

All airfoil-stall progression is from the trailing edge forward to the leading edge.The reason for this progression is the pressure gradient mentioned previously. The planform, or top-down view of the wing, plays an important role in the stall-progression pattern.

The most common design for light trainers is the rectangular planform. The use of wash-in and wash-out creates a stall pattern that starts at the trailing-edge root and develops forward and outward as the angle of attack is increased beyond the critical angle of attack. Wash-in and wash-out refer to the decreasing angle of incidence (angle between the chord line and the aircraft's longitudinal axis) as one moves from the wing root to the wingtip. The advantage of this pattern is that the ailerons are effective (not stalled) up to the most fully developed stall condition. The rectangular planform also has a lift load distribution advantage, with most of the lift created inboard, reducing the bending moment on the wing spar. The inboard stall initiation warns the pilot as the burbling air flows rearward onto the horizontal stabilizer and elevator. This creates a buffeting that can be felt on the control yoke, providing a natural stall warning. The use of stall strips on the leading-edge section of the wing can also induce stall initiation at the inboard sections of the wing.

The tapered wing is another kind of planform. In this wing the chord (the distance between the leading and trailing edge) decreases as one moves from the wing root to the wingtip. Tapering allows the inboard lift distribution, described earlier, that is desirable for spar-bending moment reasons. The taper causes the stall to initiate along the entire trailing edge, moving forward as the angle of attack is increased. The problem with this progression is that the ailerons begin to stall at the same time as the root. As soon as any buffet is felt, lateral control is reduced by the stalled ailerons. The shorter the chord (as at the trailing edge), the sooner the stall begins, because as the chord is decreased the distance also decreases between the CP and the higher pressure at the trailing edge. The shortened distance increases the adverse pressure gradient. Thus, the decreased chord at the wingtip would tend to stall first if it were not for the wash-out. Overall, the wash-out of the wingtips cancels the decreasing chord and the entire wing trailing edge stalls at once. This stall-progression pattern is similar to that of an elliptical airfoil.

The most unfavorable stall characteristics are found with the swept wing, which is used in transonic aircraft to reduce wave and parasite drag at cruise speeds. The sweep causes high induced drag and stall conditions at low speed. The sweep creates an induced upwash angle of the relative wind at the leading edge that increases from wing root to tip. This means that the wingtip is flying at a higher angle of attack than the root. Thus, it will reach its critical angle of attack first. The wingtip is the worst possible location for an initial stall, since loss of aileron control is the result. The burbling airflow off the wingtips does not encounter the tail assembly and thus does not give the pilot a natural stall buffet warning.

Another disadvantage of the swept wing is that as the stall progresses the area producing lift shrinks forward and the center of lift load has a pronounced forward shift. As this upward force shifts forward, the aircraft develops a nose-up tendency. The upward pitching moment can make stall recovery difficult. The moral of this story, of course, is that swept-wing aircraft should not be allowed to stall. To counteract the poor natural stall warning on a swept wing, an angle-of-attack sensor activates a "stick shaker" to give the pilot the simulated sensation of a stall buffet. Other airliners have a computerized fly-by-wire system that prohibits pilots from exceeding the critical angle of attack. Control inputs from the pilot that would result in a stall are simply ignored. It reminds one of the HAL 9000 computer in the movie 2001: A Space Odyssey:
"Pitch up higher, HAL!"
"I can't do that, Dave."
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Old 08-21-2009, 07:16 PM   #4  
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High-performance aircraft also use laminar-flow airfoils. In this design, drag is reduced by maximizing smooth air streams called laminar flow. This smooth airflow tends to separate more abruptly from the wing at high angles of attack, resulting in a full stall that occurs immediately after exceeding the critical angle of attack. Like the swept wing, this design improves high-speed efficiency at the expense of low-speed performance.

Pilots know that an airfoil can be stalled at any attitude and at any air speed. The part of this statement referring to attitude is easy to understand. Because the relative wind is opposite to flight path, the relative wind comes from beneath the airfoil when an aircraft is in descending flight. Even with a pitch attitude level with the horizon, the stalling angle of attack can be exceeded with a steep angle of descent. The air speed part of the statement is a bit more complicated. If the stalling angle of attack is always the same, why isn't the stalling air speed always the same? It's because four factors affect the stalling air speed: gross weight, load factor, altitude, and location of the center of gravity.

First let's examine the lift equation L = Cl S Greek letter sigma Vktas2/295, where L = lift in pounds, Cl = coefficient of lift, S = wing area in square feet, Greek letter sigma = air density ratio to that of standard sea level, and Vktas = true air speed in knots.

If the aircraft is in straight and level flight, lift is equal to weight and the equation becomes L = W = Cl S Greek letter sigma Vktas2/295.

Solving for velocity, the equation is arranged as Vktas2295 W / Greek letter sigma S Cl and Vktas = the square root of 295W / Greek letter sigma S Cl.

As the air speed is decreased, the Cl must be increased by increasing the angle of attack to keep the lift equal to weight. As the critical angle of attack is reached, the Cl has reached the maximum Clmax and the air speed is at the minimum speed, or stall speed. This speed is abbreviated as Vs.

When calculating an aircraft's stall speed the equation is Vs (knots true air speed) = the square root of 295W / Greek letter sigma S Clmax. From this equation, each factor affecting the stall speed can be evaluated.

As the weight of an aircraft increases, so do the stall speed and the required angle of attack. If a heavy aircraft and a light one are both flying at the same speed, the heavy aircraft is at a higher angle of attack. If both aircraft decrease their air speed at the same rate, the heavy aircraft will reach the critical or stalling angle of attack first, at a higher air speed than the lighter aircraft. As the center of gravity is shifted forward, the greater nose-down moment must be offset by a greater tail-down force. This tail-down force increases the effective gross weight of the aircraft, increasing the stall speed as described previously.

The extension of flaps has a pronounced effect on stall speed. Some flaps, such as Fowler flaps, will increase the wing area (S) and thus decrease the stall speed. The extension of flaps also increases the maximum coefficient of lift by increasing the wing camber (curvature), and, with some flaps, the boundary layer energy. The lowering of the trailing edge with the extension of the flaps will increase the angle of attack for a given pitch attitude by increasing the angle of the chord line to the relative wind. (For a further explanation of flaps see "High Lift Devices" in the May/June 1998 issue of Woman Pilot.)

As altitude increases, the density ratio (Greek letter sigma) decreases. The higher the altitude, the higher the true air speed of the stall. The indicated stalling air speed remains the same (as does the calibrated and equivalent air speed) with increasing altitude, but the actual speed through the air mass (TAS) increases.

An aircraft's bank angle in a turn has an important effect on stall speed. As an aircraft banks, the lift vector is displaced away from the vertical. The horizontal component of this deflected lift vector acts to horizontally accelerate (turn) the aircraft. As the lift vector is deflected, less of it remains in the vertical direction. To maintain level flight, the vertical component of lift must remain equal to the aircraft's weight. To accomplish this, the entire lift vector must be increased by increasing the angle of attack. Pilots learn that banking an aircraft requires an increased back pressure on the control yoke. As the bank angle is increased in level flight, the angle of attack is increased, and will eventually reach a stall, no matter how high the air speed. Hence, the aircraft can be stalled at any air speed.

Load factor is the ratio of the lift the aircraft is producing to its weight. In level coordinated flight, the load factor is equal to the inverse of the cosine of the bank angle (1/cos Greek letter theta). In wings-level flight, the load factor is 1 (cos 0 = 1). As the bank angle increases, the load factor increases. At a bank angle of 60 (cos 60 = 0.5) the load factor is 2. The stall speed increases with the square root of the load factor as it does with weight. At low speeds, an increasing load factor will result in a stall. At high speeds, structural damage may occur before the stall. For example, normal-category aircraft are designed to withstand a load factor of 3.8 g's. If attempting to turn with a 75 bank, the stall speed is approximately doubled. If flying at an air speed that is less than twice the normal stall speed (Vs x 2), the aircraft will stall. If flying above this speed, structural damage may occur because a load factor in excess of 3.8 would be experienced. This is why the speed of an aircraft must be reduced to a value known as maneuvering speed (Va) before attempting maneuvers that might possibly exceed the maximum load factor.

A spin is an aggravated stall in which one wing is more fully stalled than the other and thus experiences less lift and more drag. In the resulting autorotation, the aircraft yaws and rolls while descending. A spin requires a significant amount of altitude to recover, and some aircraft can become unrecoverable in a fully developed spin. Pilots should practice spins only in an aerobatic aircraft with a certified flight instructor.

To summarize, an airfoil stalls when it exceeds its critical angle of attack. To recover from a stall, the angle of attack must be decreased, usually by lowering the pitch attitude. As the pitch attitude is being adjusted, full power is added to minimize altitude loss. The proper stall recovery technique can make the difference between life and death. That's why competent pilots routinely practice stall recoveries in various configurations. It also helps if pilots understand the aerodynamics of the stall. In this case, an inch of prevention is definitely worth hundreds of feet of cure.
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Old 08-21-2009, 07:18 PM   #5  
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Quote:
Originally Posted by abelenky View Post
This answer is coming from a student, so I'm hardly authoritative, but here's my best understanding.

For physics purposes, lift is variable, true.

But for many flight-purposes, lift is either greater than aircraft weight ("On"), or less than aircraft weight ("Off").

It doesn't matter if your wings are generating lift (in the physics sense), if the lift is less than the aircraft weight. You're still falling. So for flight-purposes, you have no usable lift.

If the lift generated exceeds aircraft weight, you're flying.
I agree with all you've said there. I suppose my beef with it lies in the absoluteness that instructors tend to put on the matter, which I suppose is just fine for students who haven't or don't need to study any basic physics for the purposes. I just needed a little peace of mind!
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Old 08-21-2009, 07:21 PM   #6  
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Default Here's another one

Critical angle of attack and stall speed are related to each other, but they are not dependent on each other. There will be no one-to-one correspondence.

The POH gives stall speeds as a function of weight and flap/gear setting with the airplane in unaccelerated flight and wings level. So lets ignore the variation with flaps and gear. Just consider the flaps up problem on a fixed gear airplane.

You'll notice that the stall speed varies with the weight. The more the airplane weighs (or the higher the g factor) the more lift the airplane must generate to maintain altitude. If you're already flying at the critical angle of attack the only way to get more lift is to go faster. Hence the higher stall speed for the heavier airplane, even though the critical angle stays the same.

So if you're flying around 10 knots above stall speed at 1g you have a little margin relative to the critical angle. You have extra speed and don't want to climb, so you've set the wing at an angle of attack slightly lower than the critical angle. Start turning and you'll add some g's; effectively making the airplane heavier. It won't take much of an increase in g's to eat up your small margin on angle of attack and stall 10 knots above the stall speed for 1g.

If instead you increase speed to 20 knots above stall speed you set the wing at an even lower angle with an even larger margin relative to the critical angle. Now start a turn and you have more margin to eat up before you get to the critical angle and stall 20 knots above stall speed.

And so on. The faster you go the less angle of attack you need and the greater your margin relative to the critical angle. The faster you go the more g's you can manage before you eat up all your margin.

Now, for Va.

The airplane is designed so that if you are at Va you will be able to pull enough g's to just get up to the maximum maneuvering design load (g factor) of the aircraft. Then you exceed the critical angle of attack and stall. If you go slower than Va you can't get to the design maneuvering g factor before you stall. If you go faster than Va you can pull more g's than the aircraft was designed to carry and overload the structure. So if you're going faster than Va you're supposed to avoid full/abrupt control displacement in order to avoid overloading the structure.

Back to speed.

You stall only if you exceed the critical angle of attack. Stall speed is just the speed you happen to be at when you stall. The speed is related to the critical angle by a complex relationship between many variables - weight, bank/yaw angle, c.g., air density, and probably a couple more things I can't think of just now. But if you stay at or below Va you won't overload the structure.

So critical angle of attack and stall speed and Va are all related, but different concepts.

If you can manage to go straight up at Mach 1 and force the angle of attack far enough you can exceed the critical angle and stall. If you can manage to go straight down at a very slow speed and force the angle of attack high enough you can exceed the critical angle and stall. Force the angle to far and you stall - upside down, right side up, on your side; fast, slow, doesn't matter.

Concept 1: Stall happens when you exceed the critical angle.

Concept 2: Stall speed is the speed you're at when you exceed the critical angle. It depends on a lot of things. But the speed doesn't make the airplane exceed the critical angle.

Concept 3: If you're flying at or below Va (in the specified configuration for that maneuvering speed - flaps up at a specific weight for instance) you'll stall before can damage the airplane.

I like to try to explain these kinds of things, but I'm not sure I do it very well. Hope I helped at least a little.

Study your ground school course hard and don't fall behind in your studies. Fly a bit less often if necessary to keep up with the book. You don't want to be out there not knowing what you're supposed to be learning. Learn first; fly with knowledge. It beats a lot of the other possibilities.
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Old 08-21-2009, 07:25 PM   #7  
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Wow, HSLD to the rescue!
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Old 08-21-2009, 07:27 PM   #8  
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Wow, HSLD to the rescue!
Cuttin' and pastin' like a champ
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Old 08-21-2009, 08:34 PM   #9  
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Money creates lift
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Old 08-21-2009, 08:44 PM   #10  
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To understand the stall, or lack of sufficient lift, one must understand how an airfoil creates lift in the first place. The camber, or curvature, of the airfoil increases the airflow over the upper wing surface, and the lowered pressure creates the upward force called lift. The pilot controls the amount of lift by adjusting the wing's angle of attack.
So, I'm a little confused.... does increased airflow (increased kinetic energy) create low pressure.... or does low pressure create increased airflow?

Also does Bernoulli's principle work in the boundary layer?
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