Aoa VS airspeed

[McLovin242]

Hello all. I have a burning question in my mind and would love to pull some knowledge from the ever so experienced folks on here. So i've always been taught that an increase in lift is only achieved by increasing the AOA and that stalls only happen when the critical AOA has been reached and not due to speed. What i'm confused on however is that when you travel slower you need to increase AOA in order to stay level which means there must be some relationship between speed and lift which is not making sense to me. Because if there were no relationship, then every plane should be able to fly level at 10kts indicated without stalling and if it were to increase the aoa at all then that would be an increase in lift and it should then climb

Thanks in advance

[GAPILOT]

You just answered the question yourself. If the speed is low then the AOA required to maintain level flight will increase and vice versa.

Mathematically speaking lets say the lift required to maintain level flight is = Z. So that comprises of AOA and speed. Say AOA = X and Speed = Y so we get X + Y = Z. If that makes sense. We play around with speed and AOA to get the lift we require to maintain level flight.

And to be more precise Lift is directly proportional to the Square of Speed (i.e Speed x Speed)

[Beefitarian]

Now we can start this thread! I had a pretty long similar discussion when I first started here years ago.

I was confusing and intermingling stalling a wing with a wing not going fast enough to produce enough lift.

A bigger plane has more wing surface which equals more lift. It also equals more drag and more weight. That requires more thrust and finally more speed. It might fly and stall at very nearly the same angles of attack.

There is physical limits on the size of wing that will be useful on every plane, otherwise there might be bush planes with really giant wings. Gliders have long wings to make them glide better but with more power something like a Piper Arrow has shorter wings, that's ok as long as you don't lose the engine. There is a reason the Arrow is built that way but it's beyond my small amount of knowledge. I think it is for a better roll rate. Point is they don't glide far and stall a bit faster.

If the wing is not going fast enough there is little or no lift, so even though it won't stall at 10 knots, it can't stall, it also can't fly, because the air flowing around a wing at 10 knots won't create much lift. Maybe if we filled the plane with enough helium to counter the weight it might fly at 10 knots. The problem with that being volume, otherwise they would make blimps smaller.

If they could build an ultralight that could stall at 10 knots they would. It would make landing pretty easy.

As you mention the stall happened when the AOA got too big. At that point the air is not flowing over the wing in a way that can create enough lift and the plane accelerates slightly as it starts to fall.

That is part of why when I stall a 172, the solution is not to merely throttle up but to drop the nose and decrease the angle of attack.

So it is not angle of attack versus speed. They need to work together. Once stalled the only way to fly again is to reduce the AOA while maintaining the speed.

When you see those videos of planes (mostly models or migs) standing on their tails and flying. The wing is stalled but there is enough power to lift the thing using thrust. That's not normal and is only posible for much less than 1% of anything that can fly. Including models.

Power plus pitch or something equals performance? Help us out here AvCanada posters.

[digits_]

Hello all. I have a burning question in my mind and would love to pull some knowledge from the ever so experienced folks on here. So i've always been taught that an increase in lift is only achieved by increasing the AOA and that stalls only happen when the critical AOA has been reached and not due to speed. What i'm confused on however is that when you travel slower you need to increase AOA in order to stay level which means there must be some relationship between speed and lift which is not making sense to me. Because if there were no relationship, then every plane should be able to fly level at 10kts indicated without stalling and if it were to increase the aoa at all then that would be an increase in lift and it should then climb

Thanks in advance

Let's start by looking at the actual lift formula:

L = Cl * 1/2 * r * v^2 * S

S is the surface area of the wing, r (rho) is the air density, both of which can be ignored for your question. Assume both of them are a random constant value.

The important factors are Cl (lift coefficient) and v^2, which is speed squared, basically speed times speed.

The lift generated by a wing thus depends on the lift coefficient and the speed squared.

The lift coefficient depends on the angle of attack of the wing generating the lift.



As you can see, a greater angle of attack (AoA) results in a higher lift coefficient, up to a certain point, usually around 16 degrees.
What does this mean? As your angle of attack increases, the lift generates more lift. Typically, in cruise, if you raise the nose (increase AoA), you will get more lift, and initially your lift will increase and you will start to climb.

Now, if you increase the AoA soo much that you go past the 16 degrees (in the graph above), your lift coefficient will actually decrease again. You are now stalling the airplane.

The stall of an airplane ONLY depends on the AoA of the airplane, not on the speed.

However, usually when you are pulling up or down on the airplane, you are not only controlling the AoA, you will affect your airspeed as well. To make things "easier" for pilots, manufacturers will link this AoA value of the stall to an airspeed, assuming you are in unaccelerated level flight. Only in that case can you say with some kind of certainty that the plane will stall at your published stall speed. But the actual stalling only happens because you achieve a certain AoA.

When you start turning and banking the plane, you will notice that the stall speed is no longer correct. The plane will stall at a different speed. You can calculate this based on the load factor (pulling g's like in fighter jet movies), but think about it this way. The higher the AoA, the harder the wing is working for you. Past a certain AoA the wing gives up and you stall. If you are turning, you are using part of the force of the wing to make the turn, and part of it is used to keep the airplane in the air. Because not all the force of the wing is used to keep the plane in the air, the lift force keeping you in the air is less strong, and so you'll need a higher AoA to keep the plane in the air at the same speed.

For the second part in your question: can you fly a plane at 10 kts without stalling? Yes you can!
Imagine an airplane in vertical flight, which meanse you are flying straight up. A bit like fighter jet or even a simple aerobatic propeller airplane. When the plane is flying straight up, it is the propeller that pulls the plane forward. The AoA of the wings are going to be around zero degrees, because the wings are not generating lift. The plane is, however, not stalled. You will eventually run out of energy (unless you have a really strong engine) and the airspeed will start to drop down to near zero. At that point something will happen, you'll usually end up with the nose down somehow, but that discussion isn't relevant to your stall question.

Another question you should ask yourself is: can you stall your plane at 100 kts? If your plane is strong enough, yes you can! If you werre to pull really quickly and hard on the yoke (note: don't do this, talk to your flight instructor), the AoA of the wings and plane would change so quickly -because the plane sort of rotates in its spot and doesn't have enought time to change its flight path-, that you will immediately increase it beyond the stall angle and you will stall. A bit like a car's tires losing grip on a wet highway when you quickly turn the steering wheel.

[DanWEC]

The faster a regular speedboat goes the more it lifts out of the water, without changing pitch for the most part, right? Air is fairly similar to water.

Stick your hand out the window in the car going 100 and the slightest angle makes it go up and down, whereas at 10 it doesn't do anything. It's the mass of the air molecules per second that create more force. More molecules per second pushing on a surface, more pressure.

A good read for you might be "Stick and Rudder"

[photofly]

So i've always been taught that an increase in lift is only achieved by increasing the AOA and that stalls only happen when the critical AOA has been reached and not due to speed.

An increase in lift is achieved by a higher angle of attack (up to a limit, called the critical angle of attack, where the wing stalls) or by flying faster.

There is a strong connection between stalling and flying too slowly, as you clearly understand. The bit that you're missing is that the amount of lift you need depends on aircraft weight and the manoeuvre being conducted at the time: you need more lift in a level (wings-banked) turn, or pulling out of a dive, than you do in straight and level flight, and less lift "pushing over the top" from a climb than you do in straight and level flight.

So the link between AoA and stall is primary. But the link between airspeed and stall is secondary, because it depends on what you're trying to do at the time.

Pilots who focus on and remember only the speed at which their airplane will stall at a given weight in straight-and-level flight are apt to be surprised when the aircraft stalls at a higher speed than that when in a steep turn, or pulling "g" exiting a dive.

(I'm not sure speedboats disambiguate the question...)

[McLovin242]

You just answered the question yourself. If the speed is low then the AOA required to maintain level flight will increase and vice versa.

Mathematically speaking lets say the lift required to maintain level flight is = Z. So that comprises of AOA and speed. Say AOA = X and Speed = Y so we get X + Y = Z. If that makes sense. We play around with speed and AOA to get the lift we require to maintain level flight.

And to be more precise Lift is directly proportional to the Square of Speed (i.e Speed x Speed)

Now we can start this thread! I had a pretty long similar discussion when I first started here years ago.

I was confusing and intermingling stalling a wing with a wing not going fast enough to produce enough lift.

A bigger plane has more wing surface which equals more lift. It also equals more drag and more weight. That requires more thrust and finally more speed. It might fly and stall at very nearly the same angles of attack.

There is physical limits on the size of wing that will be useful on every plane, otherwise there might be bush planes with really giant wings. Gliders have long wings to make them glide better but with more power something like a Piper Arrow has shorter wings, that's ok as long as you don't lose the engine. There is a reason the Arrow is built that way but it's beyond my small amount of knowledge. I think it is for a better roll rate. Point is they don't glide far and stall a bit faster.

If the wing is not going fast enough there is little or no lift, so even though it won't stall at 10 knots, it can't stall, it also can't fly, because the air flowing around a wing at 10 knots won't create much lift. Maybe if we filled the plane with enough helium to counter the weight it might fly at 10 knots. The problem with that being volume, otherwise they would make blimps smaller.

If they could build an ultralight that could stall at 10 knots they would. It would make landing pretty easy.

As you mention the stall happened when the AOA got too big. At that point the air is not flowing over the wing in a way that can create enough lift and the plane accelerates slightly as it starts to fall.

That is part of why when I stall a 172, the solution is not to merely throttle up but to drop the nose and decrease the angle of attack.

So it is not angle of attack versus speed. They need to work together. Once stalled the only way to fly again is to reduce the AOA while maintaining the speed.

When you see those videos of planes (mostly models or migs) standing on their tails and flying. The wing is stalled but there is enough power to lift the thing using thrust. That's not normal and is only posible for much less than 1% of anything that can fly. Including models.

Power plus pitch or something equals performance? Help us out here AvCanada posters.

Thank you very much for the response. It just wasn't clear to me that speed is vital for lift but now this explanation raises another question.. You say that the critical AOA will result in a stall and you must reduce it (which I don't doubt at all). But then again, if lift = speed + AOA like you guys said, can I not also increase speed without touching the AOA thus increasing my lift? i'm not talking about hovering with thurst like those models but maybe maintaining 20 degree AOA (past stall angle) and then pushing the throttle to gain say 10-15 kts. can the extra speed get me enough lift to hold it?

Also another question. If greater speed increases lift does that mean you have to pitch down slightly as you speed up in order to maintain altitude? I'd love to see some kind of chart on this


edit: @digits_ just saw your edit. That's marvelous

[photofly]

ut then again, if lift = speed + AOA like you guys said, can I not also increase speed without touching the AOA thus increasing my lift? i'm not talking about hovering with thurst like those models but maybe maintaining 20 degree AOA (past stall angle) and then pushing the throttle to gain say 10-15 kts. can the extra speed get me enough lift to hold it?

This is a really great question.

It is possible to maintain flight past the stalling AoA. There are two difficulties that render it impossible for almost all aircraft though:

Firstly, the drag is extremely high and there is unlikely to be enough thrust to maintain sufficient airspeed.

Secondly, there are stability issues. Because of the positive slope of the lift curve at regular (non stalled) angles of attack, the aircraft is sufficiently stable in pitch to be flown by a human. The negative slope of the lift curve at stalled angles of attack creates an unstable pitch scenario and the aircraft would be unflyable.

If you have a computerized fly-by-wire control system and enough power, yes, you absolutely can fly level flight in the stalled regime. But those prerequisites are only met by a few military types of airplanes. Muddying the waters somewhat is the fact that at very high angles of attack those aircraft meet a significant proportion of the upwards force they need with engine thrust alone.

Also another question. If greater speed increases lift does that mean you have to pitch down slightly as you speed up in order to maintain altitude? I'd love to see some kind of chart on this

Yes. This is taught in your first or second flying lesson. It is usually expressed the other way around: flying slowly requires a higher nose attitude. Because of the non-linear relationship between airspeed and lift, the change in nose attitude for a given change in airspeed is much greater when flying slowly, close to the stall, then when flying at cruise airspeeds.

[digits_]

Thank you very much for the response. It just wasn't clear to me that speed is vital for lift but now this explanation raises another question.. You say that the critical AOA will result in a stall and you must reduce it (which I don't doubt at all). But then again, if lift = speed + AOA like you guys said, can I not also increase speed without touching the AOA thus increasing my lift?

Yes. You do that by increasing the throttle. In essence you increase your speed with a constant AoA, and you will start climbing. Note that you will have to keep the plane at a constant AoA, as the power changes might make it move a bit.

Note that the lift = speed + AoA is quite an oversimplification. Look at the graph in my reply above for a more correct relationship.

i'm not talking about hovering with thurst like those models but maybe maintaining 20 degree AOA (past stall angle) and then pushing the throttle to gain say 10-15 kts. can the extra speed get me enough lift to hold it?

I suggest you read what happens in slow flight.
Basically, by adding the throttle in the stall, the energy of the engine will first be spent on reducing the AoA before you pick up more speed. Note that the correct initial stall recovery method is to lower the nose, not adding power.

Also another question. If greater speed increases lift does that mean you have to pitch down slightly as you speed up in order to maintain altitude? I'd love to see some kind of chart on this

Correct!

[photofly]

When you start turning and banking the plane, you will notice that the stall speed is no longer correct. The plane will stall at different AoA.

@digits: You might want to fix this typo

[McLovin242]

This has all been wonderful information as i'm trying to get back to the basics and figuring out what's really going on. I truly appreciate it

I suggest you read what happens in slow flight.
Basically, by adding the throttle in the stall, the energy of the engine will first be spent on reducing the AoA before you pick up more speed. Note that the correct initial stall recovery method is to lower the nose, not adding power.

now this had me ponder. usually an increase in power will cause an abrupt nose up attitude. Would the AoA reduce because you are now traveling more vertically thus the wind will be coming from a slightly different angle?

[photofly]

Actually the opposite. An abrupt nose-up movement in level flight (before the flightpath of the aircraft has changed) will cause the relative airflow to hit the underside of the wings more, resulting in an increased AoA. In the short term (before the aircraft has a chance to slow) this generates extra lift, and the aircraft initially accelerates upwards. This is the mechanism used to cause an airplane to commence a climb.

Once the aircraft begins to move upwards as well as forwards, this reduces the angle of attack, reducing the lift and limiting the upwards acceleration. The new upward velocity is maintained, and the aircraft is in a climb. This all happens in a fraction of a second.

It is exactly this kind of self-stabilizing mechanism that doesn't occur with flight at angles of attack greater than the critical angle of attack, and would make the airplane unflyable.

[digits_]

When you start turning and banking the plane, you will notice that the stall speed is no longer correct. The plane will stall at different AoA.

@digits: You might want to fix this typo

woooooooooooops!

but hey, someone read my post!

[PilotDAR]

thus the wind will be coming from a slightly different angle?

For a more clear understanding of this topic, keep "wind" out of the discussion. There is wind, which is an atmospheric phenomenon, and not relevant in this discussion, and there is relative airflow, as Photofly states, which is pivotal in this discussion. If an instructor somewhere is allowing you to confuse "wind" and "relative airflow" with each other in a discussion about aerodynamics, give them the side eye, and ask for clarification, with a meteorology book as a starting point! I would agree that wind is equal to relative airflow locally on the surface of the earth (or a parked plane), but not around a plane in flight!

Remind yourself that a large portion of the lift provided by a wing is because of low pressure over the top of the wing. That low pressure is a product of uniform airflow. As the AoA increases, then yes, lift will increase, as long as the uniform airflow is maintained. Or, as speed is increased, you'll have more of/faster uniform airflow = more lift. Two distinct factors, which relate to each other, though not always directly. If that airflow becomes non uniform, as during a stall, it cannot cat to produce lift any more. Also remember that modern wings have a washout, and will not stall over the entire span at the same time, it's always progressive roots to tips. This washout angle is a variation of angle of incidence, which translates to AoA in flight. One wing has more than one AoA locally along it's span. But yes, the airplane is considered to have one AoA, for convenience.

Remember that indicated airspeed begins to have large errors due to alignment (of the pitot tube) error, so "speed" as read from the ASI loses direct relevance as AoA increases. You must revert to calibrated airspeed - for which you'll need a position error correction table from the flight manual (mis named, as it's both position (the static port) and alignment (the pitot tube) error factored into one chart). Or, flight test instrumentation.

And, as mentioned, this is for unaccelerated flight - pulling G's changes everything exactly as adding weight to the aircraft would. However, in the opposite, unloading G will indeed allow the wing to not stall down to a speed so slow that the ailerons will cease to be effective. 0G = zero expectation of lift, so a stall is really hard to achieve. The airflow at very low G over a streamlined aileron may not be separated, but will certainly separate as soon as you deflect the ailerons - they will stall, and not control roll well. Zero G is not sustainable for any length of time in useful flight, but the principle must be considered. Unaccelerated one G flight is not at the extreme of operations, but rather part way along a scale. 'Just not a part of the scale we spend much useful flying at!

There are other exotic ways of producing uniform airflow over a wing at slower speeds. They are not employed on GA aircraft, but if you read up on "blown wing" aircraft, it'll give you an idea.

Reluctantly, I must concede that yes, adding power at the point of the stall may increase airspeed, and seem to reduce or recover the stall. However, you will serve yourself better by thinking of a stall as an aerodynamic only event, and the recovery should be aerodynamic too. If power is required to prevent an excessive loss of altitude, that's somewhat separate. powering out of a stall is poor technique. I'm not saying it can't work, but it should not be the basis of understanding for GA pilots.