Which Flight Model Does the F15 Use?

[Cmptohocah]

2 hours ago, Exorcet said:

A stalled wing still produces lift, just less than the peak lift, and more drag. Anyway the peak recorded AoA isn't about stall. The plane can exceed critical AoA if it's stable enough, like the Su-27 performing the Cobra.

A wing in a full stall condition is producing exactly 0 lift force. When an airplane is in a cobra, it is still in the air yes, but it's no longer flying. The reason it seems that it is, is 'cause the maneuver is only temporary in nature, but if you were to leave the airplane in it it would drop out of the sky like a rock.
That's the reason why "peak" AoA doesn't matter, as there is only critical (read maximum) AoA above which the wing produces the same lift as an air brake for example. Once the critical angle of attack is reached, the airflow separates and wing can't produce any lift.

[Exorcet]

23 minutes ago, Cmptohocah said:

A wing in a full stall condition is producing exactly 0 lift force. When an airplane is in a cobra, it is still in the air yes, but it's no longer flying. The reason it seems that it is, is 'cause the maneuver is only temporary in nature, but if you were to leave the airplane in it it would drop out of the sky like a rock.
That's the reason why "peak" AoA doesn't matter, as there is only critical (read maximum) AoA above which the wing produces the same lift as an air brake for example. Once the critical angle of attack is reached, the airflow separates and wing can't produce any lift.

It's very difficult to get 0 lift out of a wing. Remember that both the top and bottom sides contribute to lift. While the flow over the top separates, the flow under the wing is still likely at high pressure. It's still generating lift, just not as efficiently. If what you said was true, NASCAR spoilers wouldn't work.

 

Here is a reasonable looking representation of a cobra in CFD, you can see that there is still a top to bottom pressure distribution on the aircraft. It's still producing lift.

 

 

Lift doesn't vanish at critical AoE, and flying at less than stall speed is possible, you will just have to fly at less than 1 g.

[Cmptohocah]

1 hour ago, Exorcet said:

It's very difficult to get 0 lift out of a wing. Remember that both the top and bottom sides contribute to lift. While the flow over the top separates, the flow under the wing is still likely at high pressure. It's still generating lift, just not as efficiently. If what you said was true, NASCAR spoilers wouldn't work.

 

Here is a reasonable looking representation of a cobra in CFD, you can see that there is still a top to bottom pressure distribution on the aircraft. It's still producing lift.

...

Lift doesn't vanish at critical AoE, and flying at less than stall speed is possible, you will just have to fly at less than 1 g.

Thanks for the images, they look awesome.

Well technically you are right, it's never 0, but effectively most of the lift is gone so we might as well approximate it to be 0, or "vanish" as you put it.

As you can see in the side views, the airflow is almost completely detached, so there's not gonna be any effective lift on those wings.

The (majority of) lift comes from difference in pressure between the top and the bottom surface of the lift (sub-sonic regime), and if there is no flow over the top of the wing there's no difference in pressure, so in the end it's the same thing. No airflow, no low pressure, no pressure difference... you get where I'm going with this.

Flying at lower speed than the stall speed is not flying, it's dropping. Technically it's not about the speed, it's about critical AoA, but more often than not this limit is expressed as speed - I guess it's easier for pilots to use. When you are at the stall speed, you are effectively reaching your AoA limit for a certain weight, 'cause at that point you've reached your max lift and the lift force is equal to your weight. If you slow down at this point, the only way to maintain level flight is to increase AoA and this will cause a stall.

Just a small explanation on what I mean by "not flying". If you throw a rock up in the air, for example, it will seem like it's flying, it's gaining height and has certain speed, but that does not mean it's flying, in a sense that it will come down eventually. A stalled aircraft is the same as that rock - it will eventually hit the ground, as long as the stall condition exists.

P.S. Technically I think you could (could be wrong also) produce lift by just lower part of the wing, but the amount of thrust required to do that is probably not realistically possible, so I did not even consider it.

Edited October 22, 2021 by Cmptohocah
Added missing information.

[Exorcet]

2 hours ago, Cmptohocah said:

Thanks for the images, they look awesome.

Well technically you are right, it's never 0, but effectively most of the lift is gone so we might as well approximate it to be 0, or "vanish" as you put it.

As you can see in the side views, the airflow is almost completely detached, so there's not gonna be any effective lift on those wings.

The (majority of) lift comes from difference in pressure between the top and the bottom surface of the lift (sub-sonic regime), and if there is no flow over the top of the wing there's no difference in pressure, so in the end it's the same thing. No airflow, no low pressure, no pressure difference... you get where I'm going with this.

Flying at lower speed than the stall speed is not flying, it's dropping. Technically it's not about the speed, it's about critical AoA, but more often than not this limit is expressed as speed - I guess it's easier for pilots to use. When you are at the stall speed, you are effectively reaching your AoA limit for a certain weight, 'cause at that point you've reached your max lift and the lift force is equal to your weight. If you slow down at this point, the only way to maintain level flight is to increase AoA and this will cause a stall.

Just a small explanation on what I mean by "not flying". If you throw a rock up in the air, for example, it will seem like it's flying, it's gaining height and has certain speed, but that does not mean it's flying, in a sense that it will come down eventually. A stalled aircraft is the same as that rock - it will eventually hit the ground, as long as the stall condition exists.

P.S. Technically I think you could (could be wrong also) produce lift by just lower part of the wing, but the amount of thrust required to do that is probably not realistically possible, so I did not even consider it.

 

It depends on the wing how close to zero the lift gets. Some can produce a decent amount of lift a few degrees after critical AoE.

Lift doesn't just come from the pressure difference on the top and bottom of the wing. There is also a component from momentum transfer. However we can just focus on the pressure difference. You're only describing a component of that pressure known as dynamic pressure. Pressure also has a static pressure component. These sum to give you total pressure. From the point of view of the wing the two components of pressure come about in different ways, but at the end of the day deltaPxwing area (simplified) is the lift force. Separated flow, like what you see on a wing beyond critical AoE, cannot produce low dynamic pressure but will have low static pressure. This allows the top to bottom pressure differential to exist even with separated flow. Again, this is how spoilers work. They spoil (separate) flow but can still generate a net lift force by using stagnation pressure (maximized static pressure) and losses in total pressure.

 

This image is obviously velocity, but the flow over the car is attached and out of the boundary layer, so you can estimate the pressure from it. In front/above the spoiler is slower speed (meaning higher pressure). Behind/below the spoiler is separated flow, which implies low total pressure and low static pressure. There is a net pressure difference producing negative lift.

 

I suppose you can define flying as maintaining level flight, but I'd argue that sub stall speed flying fits pretty well with the general idea of flight, which I'd define as something closer to "being able to control motion through aerodynamic means". As you pointed out "stall speed" doesn't really exist. That is just the speed at which critical AoA is reached. The AoA is what's really important. Nothing stops you from lowering AoA while under your stall speed. Not only is this possible, it's advisable for aircraft with tempermental stall behavior (wing drop). A lot of the time, a plane is more controllable under critical AoA than above, even if flow at speeds where it can't maintain level flight (1 g). This is unlike the rock you used as an example because the rock doesn't produce any notable lift. It's can't control itself aerodynamically.

[captain_dalan]

I think i've seen Cl charts that state the wings stall at 40 degrees or so. How much of it is available? I guess it would depend how high and how fast you are. And transient alpha could always go higher as usual. BTW, do wings break now on the F-15 or are these just naughty rumors circling around? 

[Cmptohocah]

Hi @Exorcet,
I kind of messed up on my initial explanation, so I will try to explain a bit better now.

I made a critical error when I said that the lift is equal to zero and you have a good point there. Now, having said that, let's consider a straight and level airplane flying:

vertically it will have 2 forces actin' on it: lift and weight and they are in balance. Let's also say that total lift produced is a combination of A, B, C, D... lift components, where a significant portion of this lift is produced by the wing hitting the airflow at some angle of attack. When in this condition, we say that aircraft is under 1G of load. Let's say that now we start slowing down and want to maintain flight. The only way to compensate for the loss of lift due to loss of speed is to increase AoA of the wings (I am assuming only wings produce lift in this example). As the airspeed goes down, less lift is produced and the AoA needs to be increased and this can be done until maximum (critical) AoA. At this point the wing's lift coefficient is the greatest and for that airspeed it can produce max. amount of lift. After the critical alpha is exceeded, the lift that the wing produces is no longer enough to keep the airplane in the air, ie. G load becomes less than 1 and the airplane is going only one way: down to the ground. In U.S. airforce and/or navy they call this "ballistic".

What is the difference between being "ballistic" and flying? Well if nothing changes, while in flight, the aircraft can stay up in the air indefinitely, while when being stalled it's going, you guessed it, down and will eventually collide with planet Earth. I used a rock as an example, but it could be anything really. Ballistic objects, unless they are leaving Earth's orbit, are going down regardless if they have thrust and some lift (like a stalled airplane) or dropping like a rock.

So in summary, yes there might be other components of lift still in effect on a fully stalled lift surfaces (like a wing), but they are not enough to keep the aircraft in the air (when in straight and level flight), nor are they enough to keep the airplane turning and in the air when flying in some other plane other than horizontal one.

This is the reason why I said anything over max AoA doesn't really matter, 'cause:
A. you can't stay up in the air
B. you can't turn and stay up in the air

It's an equivalent of someone physically chopping portions of your lift surfaces for the amount of lift lost due to airflow separation.

Edited October 23, 2021 by Cmptohocah

[henshao]

for those who didn't watch the video i made

tops out about ~35 sustained AoA in DCS (max thrust)

[draconus]

16 hours ago, captain_dalan said:

BTW, do wings break now on the F-15 or are these just naughty rumors circling around?

Yes, they do. Who says it's a rumor?

[captain_dalan]

On 10/23/2021 at 9:26 PM, draconus said:

Yes, they do. Who says it's a rumor?

Oh you know, with all the social networks around, the DCS community can often turn into a sewing circle

[Cmptohocah]

On 10/23/2021 at 3:54 PM, henshao said:

for those who didn't watch the video i made

tops out about ~35 sustained AoA in DCS (max thrust)

Could you test the max AoA without thrust? It seems to me that vertical component of thrust might be keeping the airplane "hanging" in the air.

[Rex]

On 10/20/2021 at 10:10 PM, gortex said:

I already saw one Hornet pilot on Growling Sidewinder showing surprise at the change, which is odd because I'm not sure why anyone was going for the paddle switch on a server that is 90% BVR.

Get painted, dive for cover behind the mountains, pull out of your dive then realize "#@% I'm going too fast!"

Suddenly, you are Tina Turner and the mountain is Ike. Worse still, you just ripped a loud, flapping fart while recording Proud Mary, and Ike is giving you the stare.

Current Options:

1) Pull the pinkie switch

2) Eject and possibly die

3) Die

Most opt for #1, and I'm embarrassed to say that it happens somewhat often.  The lack of G can make you forget just how fast you're going sometimes, and that pinkie switch is the difference between making it to the "Rollin', Rollin' on the river" chorus, or Ike asking to have a word with you in private.

Advice: Pull the pinkie switch. You don't want to talk to Ike in private. Trust me on this one.

 

P.S. With all that being said, anyone flying the Hornet in Growling Sidewinder may as just fly into the mountain since they're going to get an IR suppository that they don't see coming anyway.

Edited October 31, 2021 by Rex

[virgo47]

I found this "CPU" mentioned, yet totally unexplained, in the F-15C manual - and in this thread. Can someone please explain, what is the meaning of CPU acronym and also why it's used instead of AoA in the manual?

[Frostie]

2 hours ago, virgo47 said:

I found this "CPU" mentioned, yet totally unexplained, in the F-15C manual - and in this thread. Can someone please explain, what is the meaning of CPU acronym and also why it's used instead of AoA in the manual?

Cockpit units.

Some aircraft display AoA in degrees some display cockpit units.

[draconus]

8 hours ago, virgo47 said:

why it's used instead of AoA in the manual?

Keep in mind they're not the same and may be even not linear in conversion. They are also different for other aircraft. It's done for easier pilot understanding, getting rid of negative numbers, correction for flight dynamics and other reasons...

 

[henshao]

2 hours ago, gortex said:

This is what it's like to fly against the Flaming Cliffs F-15.  Any hydro-mechanical DCS jet would simply depart and crash.

 

obvious latency issue on the part of the hydro-mechanical jet in question

[draconus]

1 hour ago, gortex said:

you can do that with a good connection.

Track or it didn't happen. I know MP failures are a great motivator but they usually result in poor bug reports. And what type of controls has to do this? Or the F-14?

[henshao]

well I'm sorry you got killed by someone with a poor connection but tacview only exacerbates inaccuracies caused by high latency

[dundun92]

21 hours ago, gortex said:

This one looked less stable to me in-game, but tacview shows it a bit smoother.  After taking a hit pulls 12.5g, then transitions to 47.4deg AoA.  No snatching, no wing wobble for this F-15.

 

 

 

yeah that flight path looks pretty far from smooth and lag free

[BrotherBear_]

On 10/27/2021 at 3:51 AM, Cmptohocah said:

Could you test the max AoA without thrust? It seems to me that vertical component of thrust might be keeping the airplane "hanging" in the air.

I know it's several years late, but for anyone reading on... you can absolutely fly horizontal in a stall and you don't lose lift after you exceed critical AoA. obviously there is no aircraft designed such that you can produce the amount of thrust needed horizontally without a significant portion being vertical. Here is a Cl_alpha curve (Cl is just lift relative to the kinetic force of the wind or dynamic pressure) that kinda shows what happens after stall.
null nullnull

After the peak and break, you see that lift does some wacky things. it drops a pretty significant amount and then kinda sticks around 75%, but it does not fall to zero. However drag obviously skyrockets, especially pressure drag. Another important thing to keep in mind is that lift is always perpendicular to V_inf (aka relative wind) and drag is always parallel. That is part of the definition of lift and drag.

Also, for something to fly ballistic means for there to be no significant lift to alter the course of the object. that means only drag and gravity. ballistic and uncontrolled are two different things as most planes would have to be controlled into flying ballistically but something like a ballistic missile is inherently designed to fly ballistically by putting the aerodynamic center significantly farther aft of the c.g..