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Negative angle of attack

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Negative angle of attack

Old 10th Jul 2019, 00:51
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Negative angle of attack

Is it possible to achieve flying at negative angle of attack? Let's skip the part where it would be more like fall than a flight.

Sure in theory it is not a problem. I have airfoil, I put it into a wind tunnel and I can have any aoa I want.

But in reality? Let's asume I have normal real aircraft with some normal angle of incidence. My point is that even if I nose dive the flight path vector will be always "under" the airflow. I can imagine reaching negative alpha during the pushing. For a while but once I fly down it gets and stay possitive.

Even if I look at aoa indicators they have mostly only possitive values and books or so show you lift coeficient curve but I have not seen anything which shows how alpha changes in real flight.
I know it may look simple or ridiculous but I'm really not sure.
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Old 10th Jul 2019, 02:21
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I know it may look simple or ridiculous but I'm really not sure.

Not at all - the only silly questions are those we don't ask when we don't know the answer ...

Consider an aerobatic aircraft flying upside down in level flight - the wing sees a "negative" angle of attack. The forces are similar for both cases (positive and negative AoA) albeit that the fuselage attitude will differ, depending on the wing rigging incidence.

Often, the text books tend to show only the positive CL curves but you can find graphics for a wider AoA range. For instance,

https://www.sciencedirect.com/topics...ft-coefficient

(you need to scroll down a bit and there is a useful graphic on the RHS of the page).
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Old 10th Jul 2019, 03:23
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Couldn’t you just flip the graph upside down? (Joking!!)
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Old 10th Jul 2019, 08:17
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Originally Posted by Rarife View Post
Is it possible to achieve flying at negative angle of attack? Let's skip the part where it would be more like fall than a flight.

Sure in theory it is not a problem. I have airfoil, I put it into a wind tunnel and I can have any aoa I want.

But in reality? Let's asume I have normal real aircraft with some normal angle of incidence. My point is that even if I nose dive the flight path vector will be always "under" the airflow. I can imagine reaching negative alpha during the pushing. For a while but once I fly down it gets and stay possitive.

Even if I look at aoa indicators they have mostly only possitive values and books or so show you lift coeficient curve but I have not seen anything which shows how alpha changes in real flight.
I know it may look simple or ridiculous but I'm really not sure.
The two cases I can come up for sustained negative AOA are inverted flight and multiple outside loops (a loop made by pushing the control while in level flight and keeping the input until you are level again).
Also parabolic flight can have a temporary negative AOA, as long as you push enough.

Most wings are designed to give positive lift at 0 AOA, but exactly at what (negative) G-load your AOA becomes negative is above my pay grade.

​​​​​(I think the angle of incidence isn't relevant, the AOA is the angle between wing and air, the angle of the fuselage really doesn't matter for this...)
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Old 10th Jul 2019, 15:46
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From video and stills showing unusual combinations of AoA and attitude.
AOA versus pitch

Pitch +5 AoA -15 (estimating AoA from smoke trail)
Check pitch - I have never flown inverted and I am not sure if I am describing the pitch correctly. Nose seems above horizon and so I think pitch is +ve.
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Old 10th Jul 2019, 19:20
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Unless an aircraft is specifically designed for aerobatics, it is likely that the wings will fall off at sustained negative g.

Fun fact, a wing cannot be stalled in a zero g state.
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Old 10th Jul 2019, 20:26
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This is a really interesting one.

All wings will produce SOME lift at zero AOA. Look at a typical lift curve slope, you have peak CL at something like 1.0 or so at an AOA of 15 degrees (just before the stall), and a CL of (in this example) about 0.1 at 0 degrees AOA. I don't have enough posts to post a link, so google is your friend!

Typical cruise AOA would be about 3-4 degrees, for a CL of, say, 0.4.

If we go to our lift equation, we can see that lift = 0.5 x CL x rho x S x V^2. Double the velocity, and you can quarter the CL for the same lift.

So if you are, say, in steady level flight at 4 degrees AOA, CL of 0.4 at 100 KIAS, then doubling your airspeed to 200 KIAS requires one quarter the CL, i.e. 0.1.

You are in steady level flight at 0 AOA with a CL of 0.1. Any faster, and you will be in steady level flight, with a positive CL, and a slightly negative AOA.

Magic!
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Old 10th Jul 2019, 20:48
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Originally Posted by Deskscribbler View Post
All wings will produce SOME lift at zero AOA.
A symmetrical aerofoil doesn't.

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Old 10th Jul 2019, 21:02
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Originally Posted by DaveReidUK View Post
A symmetrical aerofoil doesn't.
OK... a symmetrical aerofoil doesn't. But any standard cambered aerofoil fitted to the vast majority of flying things will. Horizontal stabs are often inverted aerofoils to produce 'negative lift' at 0 AOA.

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Old 11th Jul 2019, 01:30
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Not negative AOA but many helicopters also have an inverted aerofoil tail stabiliser to improve level flight in the cruise.
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Old 11th Jul 2019, 02:32
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Originally Posted by Jim_A View Post
Unless an aircraft is specifically designed for aerobatics, it is likely that the wings will fall off at sustained negative g.

Fun fact, a wing cannot be stalled in a zero g state.
Maybe you should check the G limitations for a non aerobatic aircraft. Airliners are typically -1 G to +2.5 G when clean. While there are plenty of reasons why they will struggle with inverted flight, "wings falling off" is not one of them.
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Old 11th Jul 2019, 02:51
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Unless an aircraft is specifically designed for aerobatics, it is likely that the wings will fall off at sustained negative g.
Or (my bold)....

Sec. 23.337

Limit maneuvering load factors.

(a) The positive limit maneuvering load factor n may not be less than--
(1) 2.1+ for normal category airplanes, except that n need not be more than 3.8 nor may it be less than 2.5;
(2) 4.4 for utility category airplanes; or
(3) 6.0 for acrobatic category airplanes.
(b) The negative limit maneuvering load factor may not be less than--
(1) 0.4 times the positive load factor for the normal and utility categories; or

(2) 0.5 times the positive load factor for the acrobatic category.
(c) Maneuvering load factors lower than those specified in this section may be used if the airplane has design features that make it impossible to exceed these values in flight.
As long as an airfoil is producing lift, there will have to be either some AoA, or an effective unsymmetrical airfoil. A variation on this will be wings with very effective flaps. Though the whole wing will still have a positive AoA to create lift, with the extension of flaps, the non flapped section of the wing might have a slight negative AoA, and no longer be producing lift. This is why some STOL planes have ailerons which droop with extension of flaps.

Aside from an AoA indicator calibrated to an airplane with effective flaps as described above, I cannot otherwise think of a reason for having an AoA indicator which read into the negative. I don't imagine that aerobatic pilots are considering the AoA indication during inverted flight.
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