# Tag Info

35

This is called stall hysteresis. You have two different situations and the flow reacts differently in each of them. When increasing the AOA The flow is attached to the wing and the boundary layer is resisting the adverse pressure gradient as much as possible. At some point the flow detach from your profile and you have stalled let's say at 18°. At this point ...

16

I think this has been at least hinted at in other answers, but, just to put it succinctly, the diagrams aren't actually intended to show the airflow as always being parallel to the horizon. The airstream is just shown flowing along the horizontal axis of the graph because it's convenient to illustrate it that way. There is no implication intended in the ...

10

An INS gives your speed & orientation in space, but without reference to what the airmass that you're flying in is doing. The Pitot tube and AOA sensors give speed and pitch orientation in relation to the local airmass, only. An indicated airspeed of 60 knots, measured by the Pitot tube, may correspond to a stationary aircraft pointed into 60 knots of ...

6

First of all, the relative wind is relative to the airfoil, it has nothing to do with any other direction. We almost always show the relative wind as horizontal in aerodynamic diagrams, because it is the reference flow for the diagram. Which way up the plane is, or which direction it is moving, does not affect the aerodynamics. All that matters is the ...

5

There are several, equivalent explanations: The airplane has positive static longitudinal stability. As the angle of attack increases, wing lift grows more slowly relative to tail lift because the coefficient of lift on the wing is larger than that on the tail. The airplane exhibits a growing nose-down pitching moment as the angle of attack increases in ...

4

Some vertical stabilizers will have a slight angle of incidence to counter left turning tendencies. Older model Cessna 172s and 150/2s have a little metal trim tab that can only be adjusted by manually bending it from the outside. This will serve to adjust the vertical airfoil chord line angle in the aircraft slipstream or relative wind. The degree of ...

4

Yes, but the term "incidence" normally only applies to horizontal surfaces. With a fin it's usually called "fin offset" and is very common on single engine tractor aircraft. The offset helps align the fin into the spiraling airstream coming off the prop so that a counteracting rudder input isn't required (or at least a smaller input is required) to ...

4

No, they don't. The slanted intake is slanted because the top part produces a compression shock wave that hits the lower intake lip so all of the compressed air is ingested and nothing is wasted. In order to do this reliably up to Mach 2.5 (where the Mach cone angle is 73.8° from the vertical), the whole intake needs to be slanted by those 73.8° (actually a ...

4

Look at a typical diagram illustrating angle of attack. Where on the diagram do you see the horizon? How do you know the horizon is parallel to the bottom of the picture frame? It's easy to fall into the trap of thinking that the relative wind is parallel to the horizon, because so many aircraft spend so much time flying so that the relative wind is parallel ...

3

Thank you for your question. Your most recent edit points to where your misunderstanding stems. Relative Wind is a function of flight path relative to the airfoil chord line. It is how the airflow interacts with the wing surface. Your frame of reference is the airfoil, not the ground. Align the Relative Wind with the flight path of the airfoil/wing. For big ...

3

S.M. Nawab, your question demonstrates an intuitive understanding of “relative wind” which is something that many others don’t always get right away. So, your root question is: “why in normal planes do we have to think about straight coming relative wind and accordingly the angle of attack against it?” The short answer is that we DON’T have to think about ...

3

There is no difference between "normal" planes and "acrobatic" planes as far as stall behavior. If the angle of attack exceeds the critical angle, the wing will stall, period. This is true for any airplane. I think your confusion lies in this idea that the chord line of the wing is always parallel with the wind flow. That is wrong. As the ...

3

I found your exact question a little confusing, so let me answer with a quote from a current British PPL exam preparation book: The wing's angle of incidence - the angle between the wing chord-line and fuselage - is fixed by the construction of the wing and the angle at which it is bolted onto the fuselage Source: AFE Book 4 - Aircraft General Knowledge ...

3

The question mentions Speed and Orientation which are broad terms that could include: Group1: (Air Data sensor + computation) IAS - Indicated Air Speed CAS - Calibrated Air Speed EAS - EquivalentAir Speed. TAS - True Air Speed AoA - Angle of Attack (HDG(M) - Magnetic Heading)^^ Group2a : (Inertial/Rate sensor + computation) GS - Ground Speed HDG(T) - ...

2

In sustained horizontal inverted flight, the wing is generating a skyward lift force. In most aircraft the pilot will be exerting a significant "push" force on the stick or yoke, causing the stick or yoke to be well forward of the position it would be in level upright flight at the same airspeed (or at any airspeed for that matter.) Regardless of the ...

2

This is easiest to understand with a fully symmetrical wing, which indeed are popular with aerobatic aircraft. The direction of "lift" has nothing to do with sky or ground and everything to do with your angle of attack to the relative wind. When inverted, you simply (as the professor said) push forward on the stick, which makes the wind strike the "top" of ...

2

Yes, it is active. I have seen and done high AOA exercises in the simulator and the green dashes (PFD markings) remain on the pitch limits even after the high angle of attack protection activates. That tells me as a pilot, the aircraft still has pitch attitude protections active. Here is a snapshot I took from a windshear recovery in an A320 (level D ...

2

Of course not. Thin Airfoil Theory produces a beautiful result that relates the zero-AOA lift to the mean camber of the airfoil, as well as the lift slope vs AOA of any thin airfoil is $2\pi$. But that's assuming: Airfoil is asymptotically thin: such that the mean camber line represents the flow path Camber is close to the chord: such that the vortex sheet ...

2

Let us assume level flight. Then the forces acting on the aircraft are shown in the following sketch (not necessarily to scale): The forces are : the total aerodynamic force $F_A$, which is split into two components: lift $L$ (perpendicular to direction of motion) and drag $D$ (parallel to direction of motion) the weight $W$ the thrust $T$, here acting ...

2

How does AOA change? The angle of attack (AOA) is defined as as the angle between the chord line and the relative wind. When flaps and slats are deployed, the effective chord line changes. This is already nicely illustrated for trailing edge flaps in the article you linked: (boldmethod.com) As you can see, the new chord line has a higher angle compared to ...

2

The question should really read, "how the pitch must change along the length to keep lift distribution reasonably constant while respecting stall considerations" or something like that. Rotor blades twist nose down going outboard, like propellers but with less twist, to account for the higher velocity toward the tip, so that the slower moving root ...

2

Wolfgang Langewiesche is right to appropriate order of approximation. For every elevator position a statically stable airplane settles to a specific equilibrium angle of attack. That's how static stability works. Airplane is statically stable if and only if increasing angle of attack causes higher increase in coefficient of lift on the aft airfoil (tail for ...

2

The answer to the title question is no, sideslip angle and drift angle are not the same thing. (in this figure, or any other...) What the diagram shows is drift angle, and drift angle only. If you are moving through a uniform airmass that is itself moving relative to the ground, your path across the ground (ground track, or course) will be affected by the ...

1

The direct answer to the question as it is ("What´s the relationship between AOA and Airspeed?") is simple: none whatsoever -- that is, until you introduce context and some conditions. Your context is: we want an airplane to fly. And not just 'fly' but to keep level, at least. For that, you need a certain amount of lift. Lift is used to counteract ...

1

A practical answer to help you understand critical angle of attack and stall speeds would be the operation of fast jets. The quickest way to land a jet is to join for a ‘run-in and break’ this could be at any speed but typically 350kts. A high-g turn would be used from overhead the runway at circuit height to the downwind position. The aircraft would be ...

1

"Can we get into a stall without reaching the critical AOA?" -- no. "We know that any aircraft will stall at its stall speed (for a specific weight, CG position, etc.)"-- we need to add "G-loading" to this list of parameters. The "stall speed" we usually talk about is the 1-G stall speed. Change the weight or the G-...

1

It's quite a bit of a simplification, but I can't see anyone pointing this in the other answers: If your plane is not climbing or descending (level flight) then your motion will be parallel with the horizon and so will be the relative wind (albeit in oposite direction). If the wing (well, plane) was in a climb then that blue arrow should indeed be pointing a ...

1

Ground effect creates more circulation around an airfoil (either lifting or downforce-ing). Not going into details about how ground effect works, but the 'mirror-vortex' in potential flow explains this very easily. Increased circulation will manifest in the pressure field as a higher pressure differential between the top and bottom surfaces of the airfoil. ...

1

Interestingly, a high G maneuver may have lower AOA than a stall maneuver. The MiG 21 spike inlet is 1950s vintage, also seen on the SR-71, used to solve the problem of slowing supersonic inlet air to subsonic flow that was then fed into the turbojet. The MiG 21 was designed more for hit and run and is not as versatile as the F-15 Eagle, which suceeded it ...

1

The angle of attack airplanes fly at while cruising is called the optimum angle of attack (around 4 degrees) which meets the max.(L/D).

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