AoA Question

[JG7_X-Man]

It is my understanding for an aircraft to fly at a constant altitude it's AoA = 0 correct or am I mistaken?

[AndyJWest]

A symmetrical airfoil generates no lift at zero AoA. A cambered one will, but whether it generates enough (i.e. lift equal to mass)  will depend on the characteristics of the airfoil, the airspeed, the air density and the mass of the aircraft. Most aircraft are designed to operate at a small positive AoA at cruising speeds, as this is generally most efficient.

Edited January 18, 2020 by AndyJWest

[JG7_X-Man]

2 hours ago, AndyJWest said:

A symmetrical airfoil generates no lift at zero AoA. A cambered one will, but whether it generates enough (i.e. lift equal to mass)  will depend entirely on the characteristics of the airfoil, the speed, the air density and the mass of the aircraft. Most aircraft are designed to operate at a small positive AoA at cruising speeds, as this is generally most efficient.

 

Hi Andy! Thanks for your response.

I made some recording and opened them in TacView intending to do some test on max Gs when g-LOC occurs on different aircraft, but I was sidetracked by this finding

 

Test Scenario: Quick Mission

Test Map: Moscow, Winter 1941-42

Region: Single aircraft Rzhev (Grid#: 1005 ) 

Conditions: Default(12:00, Clear, Clouds 1000m, wind/turb = 0)

Start Altitude: 1500M

Test Method: Start Mission, throttle down to cruise and press record (no other interaction with the aircraft).

 

Screen shots taken after approx. 2 minutes of flight time.

 

        Spit Vb                  Spit IX       Tempest V          MIG-3                    Yak-1                  Bf 109F-4         Bf 109K-4                  Fw 190D-9              P-47D

                   

P-51D

My question here is, shouldn't the AoA of all these aircraft be constant? They are cruising at co-altitudes (wing level on).

 

On a different note, I think there is an issues with the Fw 190D-9 for sure! There is no reason G-Force shouldn't be 1.0G. After a few seconds of spawning is, every aircraft settled to 1.0. All except for the Dora.

Edited January 18, 2020 by JG7_X-Man

[AndyJWest]

None of your image links work.

[JtD]

Angle of attack is the angle between the reference line (datum line) and the flight path. It doesn't matter if you go level, or up, or down - the angle is always given against the flight path.

 

The reference on an airfoil typically is the chord, which goes from the frontmost point to the rearmost point of the cross section. However, wings can have different airfoils or airfoils at different angles (wing twist), so it's not as clear. Additionally, wings are often mounted at an angle against what the designers chose as the datum line of the aircraft, which is completely arbitrary. It can be, but doesn't have to be, the thrust line, i.e. the angle the engine/propeller is mounted at. It can be, but doesn't have to be, the sight line of the gun sight. It can be, but doesn't have to be, the line through the frontmost and rearmost point of the aircraft. Plus, in Il-2, the programmers chose the reference/datum line. It can be, but doesn't have to, identical to the historical thing.

 

You will also notice that angle of attack changes with speed, as the aircraft at high speed has the nose further down than at low speed, where it has to raise the nose - against the flight path.

 

Wiki has a few pictures that probably make it easier to understand: https://en.wikipedia.org/wiki/Angle_of_attack

 

Angle of attack is not the angle between flight path and horizontal, that would be the angle of climb.

Edited January 18, 2020 by JtD

[JG7_X-Man]

Andy please check again - I can see them

28 minutes ago, JtD said:

Angle of attack is the angle between the reference line (datum line) and the flight path. It doesn't matter if you go level, or up, or down - it's always against the flight path.

 

The reference on an airfoil typically is the chord, which goes from the frontmost point to the rearmost point of the cross section. However, wings can have different airfoils or airfoils at different angles (wing twist), so it's not as clear. Additionally, wings are often mounted at an angle against what the designers chose as the datum line of the aircraft, which is completely arbitrary. It can be, but doesn't have to be, the thrust line, i.e. the angle the engine/propeller is mounted at. It can be, but doesn't have to be, the sight line of the gun sight. It can be, but doesn't have to be, the line through the frontmost and rearmost point of the aircraft. Plus, in Il-2, the programmers chose the reference/datum line. It can be, but doesn't have to, identical to the historical thing.

 

You will also notice that angle of attack changes with speed, as the aircraft at high speed has the nose further down than at low speed, where it has to raise the nose.

 

Wiki has a few pictures that probably make it easier to understand: https://en.wikipedia.org/wiki/Angle_of_attack

 

Sorry - I forgot a key part of the test. When I spawn into a quick mission air start - wing level is already set, so there is no attitude change during the test.

I merely reduced the throttle settings so to not overheat the engines, waited a minute and started recording.

Edited January 18, 2020 by JG7_X-Man

[AndyJWest]

1 hour ago, JG7_X-Man said:

Andy please check again - I can see them

 

I can't. No idea what you have done wrong, but 'image.png.beba371627bd634d7d89f536e2163a40.png' for example doesn't even look look a valid filename. 

[JG7_X-Man]

18 minutes ago, AndyJWest said:

 

I can't. No idea what you have done wrong, but 'image.png.beba371627bd634d7d89f536e2163a40.png' for example doesn't even look look a valid filename. 

Sorry for wasting you time - they are fixed! 

[-=PHX=-SuperEtendard]

It will depend on speed, a plane flying at constant altitude means the wing produces exactly the lift to counter the plane's weight. If a plane goes slower the lift decreases, but the weight stays the same, so angle of attack will have to be increased in order to produce more lift.


For example this photo of two modern fighters, a Mirage 2000 and a Rafale flying in formation with an old late 40s prop transport plane the Flamant. The Flamant is flying so slow the delta wing fighters have to pull a good bit of angle of attack to stay at constant altitude and not stall. All these three planes are flying at constant altitude but all of them have different angle of attacks in order to mantain it at this speed, since all have different wings and weigth


 

Edited January 18, 2020 by -=PHX=-SuperEtendard

[AndyJWest]

JG7_X-Man, as I wrote earlier, the AoA necessary to maintain level flight depends on all sorts of factors. And as JtD says, there isn't even necessarily an agreed datum from which to measure the AoA against. A difference of two or three degrees for different aircraft isn't that surprising.

 

As for the G reading for the Fw 190D-9, it certainly should be 1.0 G in level flight. Is that constant, or just a temporary glitch?

[JG7_X-Man]

Ahh - Thanks -=PHX=-SuperEtendard!

So what I gather here is by throttle back, I caused each aircraft to change its AoA to best maintain that speed it currently at (like a snapshot in time).

If I for example had left the throttle wide open, each aircraft would eventually reach an AoA of 0.

 

For example - the picture I posted above of the Tempest V. Notice how it's a negative AoA. Which makes sense as 590 is tad high for it need to decreae it's AoA to matian that speed.

 

Now I am more confused because with a -AoA, would the aircraft lose altitude?

 

6 minutes ago, AndyJWest said:

JG7_X-Man, as I wrote earlier, the AoA necessary to maintain level flight depends on all sorts of factors. And as JtD says, there isn't even necessarily an agreed datum from which to measure the AoA against. A difference of two or three degrees for different aircraft isn't that surprising.

 

As for the G reading for the Fw 190D-9, it certainly should be 1.0 G in level flight. Is that constant, or just a temporary glitch?

 

rgr! I will retest the Fw 190D.

[AndyJWest]

1 hour ago, JG7_X-Man said:

If I for example had left the throttle wide open, each aircraft would eventually reach an AoA of 0.

 

No.  Nobody has said that. 

 

Quote

Now I am more confused because with a -AoA, would the aircraft lose altitude?

 

As I said in my first post,  a cambered airfoil generates lift even at zero AoA (measured relative to the chord line). The zero-lift angle will be a negative number, and any angle greater than that will generate some lift. With the exception of the P-39, I think all IL-2 GB aircraft have wings with cambered airfoils.

 

In any case, we don't know what these angles are being measured relative to, and the angle relative to the aircraft datum used by IL-2 GB (which is quite likely what TacView gives) may well be different from the angle relative to the aerodynamic chord (which, since the wing quite likely has a twist, isn't constant from root to tip anyway).

Edited January 19, 2020 by AndyJWest

[19//Moach]

I think I see where the thinking goes here (maybe) 

 

One should understand that every airfoil has a specific "Angle of Zero Lift".  This varies a lot depending on the airfoil's shape, particularly it's camber amount.

 

This angle does not change with respect to airspeed. 

Nor does it vary under any normal (non-transonic) flight conditions. Not unless the shape of the wing is itself changed somehow, such as by way of extending flaps or by courtesy of German bullets blasting random bits off of it.

 

 

What does change with speed, is how much force this airfoil generates at any given angle beyond this zero-lift point.   This force is ultimately determined by the well known equation of lift. (where AoA plays as one of the many factors of Cl)

 

In purely theoretical flight conditions, as an aircraft accelerates towards infinity (and theoretically does not explode somehow), it's lift would increase such that a smaller and smaller AoA can provide enough force to keep it level. (that is:  the exact same force to counter the weight of the airplane)

 

This reduction of AoA is asymptotic, that is: With speed increasing towards infinity, (only possible in theory, that magical place where your wings never fall off)  this Level-Flight Angle approaches ever closer to our Zero-Lift Angle, yet it would never actually reach it. Like an infinitely wide bowl filling up, the water will never reach the rim as it moves towards it ever more slowly. This is an asymptote.

 

In (still very theoretical) "practice",  that airplane would become more and more sensitive to AoA changes, as the gap between the angle which produces no lift at all and that which produces enough to balance out all weight gets smaller.  

 

Note, however:  The angle at which no lift is produced does not change.  (Not in our perfect theoretical airspace, where the speeds of sound and light are casually waved away and told to sit quietly where they don't bother anyone.) It is the resulting lift force that changes in regard to airspeed and whatnot other factors.   That is to say: Flying at the Angle-of-Zero-Lift would be analogous to multiplying lift by zero. You could fly as fast as you want and get no lift at all at that angle. Any other angle would get you some non-zero amount of lift, including even negative lift. (fancy flying upside down? )

 

Of course, a real plane would most likely come apart at every available seam plus several new ones way before that happens - That is what that red line in the IAS gauge is there for.

 

 

And naturally, different kinds of airfoil (such as those in different kinds of airplane) have a different angle where no lift results.  This is primarily why you see such difference in the AoA required for level flight at constant airspeed.

 

 

 

Does it make any sense, or have I made things worse again?

 

 

Edited February 18, 2020 by 19//Moach

[1PL-Husar-1Esk]

In general   wings must generate lift so positive a/a is required  , wings are built to be inclined to generate enough lift to balance weight and drag , easy to see on the ground.

There is the Coanda paradox effect where a cambered wing develop some lift at zero degrees a/a.

To reduce the lift to zero , wing would have to be set to negative a/a. Zero lift a/a.  Vertical dive is condition where wings do not generate lift also (zero lift a/a).