Angle of Attack - Relationship of Lift and Drag

4. Defining a Stall

Many people have a misunderstanding what a "stall" is and thought it was merely just the speed at which a plane "drops out of the sky." Even I did when I was a kid. However in my case I merely just did not understand the terminology or definition of "Stall" but I did at least understand the basic concepts before even coming anywhere close to why a wing is shaped funny. What I understood back then though, is that increasing the angle of the wing to the airflow, after a certain angle the wing wouldn't produce more lift and would start to decrease. Else people would be flying like helicopters at 90 degrees with infinite lift.

You could try thinking it of it this way, the preserving of kinetic energy and obtaining the maximum acceleration.

Refer to the secenario below.

Assume the ball does not bounce and the wall has zero friction as if you're playing Quake 3 Arena while pogo-jumping and hitting a vertical wall while moving as an example. The velocity gets "clipped' each time it touches a new surface and only the vector component along/parallel the surface is maintained.

In the first example the ball hits the wall... and stops. There's no conservation of energy here.

In the second example we add a 45-degree wall. The conservation of energy equates to cos(45°) * cos(45°) = 0.5 meaning after hitting both walls, the ball will be going perpendicular to its initial direction at half the speed.

As we add more smaller plates with smaller differential angles, the conservation of energy approaches the value of "1" where the corner becomes a perfectly smooth circular/oval arc.

One could ask, "What angle of a single deflection plate would provide the largest change in direction?" - That is, how much could one plate deflect airflow efficiently?

The thing about Aerodynamics is that we deal with fluids rather than solids. So it is a little bit different. Also, I've been ignoring the facts of surface friction which also influence airflow which actually slow down the air in the vicinity of the wing surface and the airflow has a higher tendency to buffet a bit even if smooth but we won't cover that in this article.

So here we introduce things like "eddy currents" (like those circular whirlpool like things forming behind a fast moving boat) which are the result of the fluid trying to fill the gap left behind by the moving object we're dealing with, which is a wing.

A wing deflects airflow downwards. Most of us who know how Newton's physics works know that a wing gains lift by pushing airflow downwards and the momentum of the air has an equal and opposite reaction of pushing the wing upwards. However atmospheric pressure pushes the air that has passed ontop of the wing back down onto the wing which actually ends up producing a little bit of effective thrust while partially cancelling out the deflected pressure under the wing. Aerodynamics also gets a little more complicated here as well because after you have pushed the airflow downwards, the force to continue pushing it downwards isn't that great so simply increasing the wing area longitudinally doesn't necessarily increase the lift that much for the drag you get, and longitudinally long wings also suffer increased induced drag as the atmospheric pressure can more readily catch up behind the wing, creating eddy currents, however this all changes when you get faster towards the speed of sound.

The thing about eddy currents is that once the wing exceeds an arbitrary angle of attack, that we call the "Critical Angle of Attack" - the eddy currents start to flow back onto the wing as shown:

Because the airflow starts to fall back onto the back of the wing (and pushing it down, penetrating, and splitting the laminar airflow) even at what can be a visually small angle of attack, and that the failure to maintain the reduced pressure and thus the strong pressure differential between the top and bottom of the wing,the top of the wing is often more impotant and so people in the aviation industry usually refer lift production as the lack of airflow pressure on the top of the wing rather than the bottom of the wing pushing airflow down. Atmospheric pressure naturally meets up at the back of the aerofoil more readily than the front with the typical teardrop shape, resulting in the centre of lift being relatively far forward along the wing, along with the inefficiency of momentum conversion (ball and plate example). However, the teardrop shape provides the best strength for the wing thickness and least drag at subsonic speeds, and is typically more rounded on the top than the bottom with low speed flight having concave bottoms to maximise lift at low speeds and high speed aircraft having symmetrical aerofoils to minimise parasite drag. The ideal aerofoil is one that morphs in relation to angle of attack but that is currently still not a feasible 

When the eddy current is considered to have met with the aerofoil enough to the point that increasing angle of attack no longer provides increased lift, the aerofoil is considered aerodynamically stalled.

To summarise stalls:

• The start of a stalling point is usually defined as the point where increasing the angle of attack results in no additional lift production. In other words, the wing to airflow angle exceeding the critical angle of attack.

  • There are two main limiting factors that contribute to this:

    • The inefficiency of conversion of momentum (See the ball and plate example)

    • Eddy currents forming on the wing (See above image)

Because fluids are more unstable and have a tendancy to "fill the gap" usually what causes the "stall" is point number "2." However just because eddy currents start forming onto the wing doesn't actually mean it produces "no lift" unlike what a lot of combat flight simulator gamers say. Which is WRONG. It still produces lift. Just not the desired amount, and pulling back on the stick results in lesser lift production. Unless you happen to be in a deep stall with an angle of attack around 90 degrees and your flight path going simply straight down (no forces pushing off course from going down to the earth, that is to say, the forces received do not influence the trajectory of the plane and only effect speed) and isn't in autorotation where the reaction component somehow happens to be 100% drag.

Even in Autorotation you still produce lift. It's the reason why it's so hard to get out of a spin - because your wings do not want to duck under the other one due to your own aircraft's stability.

More notes:

Airflow has a higher tendancy to form eddy currents as airspeed increase so that becomes a more limiting factor and angle of attack where the Critical Angle of Attack occurs is not actually consistant and can vary from as high as 22 degrees (or even higher) to very low, so low that the plane could be in its "coffin corner" altitude. "Coffin Corner" is called so because there is an airspeed which most jetliners are capable of meeting where their critical angle of attack is lowered by high airspeed (usually in form of a high mach number) and increasing airspeed further can cause structural failure (i.e. wings coming off, fuselage skin ripping off etc.) but on the flight envelope of aircraft given to pilots are usually provided with 15% buffer.

Usually the stalling effect on control surfaces (ailerons, elevator flippers, and rudder) is dubbed as "compressibility" issues but really is just the airflow gone wonky and not flowing constantly anymore, or the pilot is unable to provide the force to deflect the control surface. These are two completely different things indeed but that's how pilots dub it. Blame the ones who defined it.

Summery:

My understanding of a "stall" also goes a lot further than this explanation but for now just accept that it's basically the point where increasing angle of attack results in decrease of lift and decrease of angle of attack results in increase of lift. War Thunder players generally experience the negatives of this instability by getting into a more stable spin condition. But just remember, A STALL IS NOT A SPIN, but a SPIN IS CAUSED (NOT THE RESULT) BY THE UNSTABLE STALLED CONDITION!

You can actually be in a stalled condition and still fly while avoiding the initial spin stage if you balance your plane well enough using the rudder. Examples from War Thunder you can experience by pitching back only:

-R2Y2, F2H (Banshee), Gloster Meteor, Contraprop Spitfire FR 47 etc. - Anything that doesn't produce an overall yaw and rolling torque with engine power

-He 51, Ki-10, Bf109 E (with prop feathered) etc. with engine off. Note that many aircraft in WT have a default trim that counters the engine torque slightly.

5. Key Points in Angle of Attack (Lift and Drag)

Refer to image below:

"C" - Coefficient scale. Not to scale. By that I mean it does not reflect the exact true curve, but it is close.

"aAoA" - Absolute Angle of Attack - That is, referring to the Zero-Lift angle of the wing to Airflow rather than Leading Edge to Trailing Edge chord to Airflow which is a rather obscure definition.

"drag" - Total Drag co-efficient curve

"lift" - Lift co-efficient curve

[Numbers below the aAoA axis] - In Approximate Degrees of aAoA. Most aircraft have their Angle of Attack values equate to their aAoA - 4 degrees so the critical angle for that specific wingform would be 14 degrees if its aAoA critical angle was 18.

    ~7: Largest Lift to Drag Ratio - Also known as the Gliding Angle of Attack, and on some aircraft, Best Climb Rate Angle of Attack. It provides the largest gliding distance. Also, it is the Least thrust required however does not mean least fuel consumed which is why "Best Endurance" is a different value. It is also met at the Service Ceiling (maximum operate-able altitude at 1g lift load). This angle to airflow ratio produces the maximum lift coefficient over drag coefficient ratio.

    ~12: Largest Lift over Drag (Literally just the largest overall Lift take Drag value for a given Angle of Attack) - Provides the highest approximate raw energy-efficient turn rate. Don't confuse this however with "Best Endurance Angle of Attack" which is defined as "least power required for lift" or rather "most excess thrust for the drag present." It is actually somewhat related however one value does not equate the other because in an engine-less glide your thrust is practically nil with gravity pulling you forward on a slant angle. This Angle of Attack provides best gliding endurance as well (meaning you stay in the air the longest) but its endurance isn't that much greater than the best drag ratio so if you do get an engine failure just aim for the best glide ratio as you'll need the kinetic energy to manoeuvre. I am unsure about the exact average value of most wing forms so 12 degrees is just an estimation. I expect the actual value to be somewhere between 10 and 14 degrees. In War Thunder you should generally fly with this angle of attack if your aim is to drown out the enemy's energy state while being manoeuvrable yourself. Its advantages are more noticeable in jet games where it takes longer to accelerate to your standard combat speed.

Note that "Power" refers to Force (Thrust) times Distance (Work) over Time (Power).

    ~18: Critical Angle of Attack - The highest co-efficient of Lift that can be obtained. It is rather an unstable condition since if you tried to maintain speed without the required thrust your AoA would exceed and decrease into a stall. A "stall" is usually defined as AoA going past this point. Overly long (forward-back wise) wings will generally have a smaller critical Angle of Attack while smaller ones will have more tolerance before stalling since Airflow starts to become turbulent in form of eddy currents similar to those behind a boat moving faster than one meter per second.

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