need track replay AIM-120C Seems to defy laws of physics - low/no drag and impossible turn rates

[Cmptohocah]

Looks like there is a slight issue with the AMRAAM's drag: it is capable of sustaining high angle of attack (~25 deg) without really loosing any airspeed and all this with it's rocket motor off.

Another issue it that is capable of producing very high turn rates at almost 1G (+/- 0.2G) at extremely low air-speeds (below 600km/h). Also it seems to be able to turn by pulling less then 1G which is physically impossible as the G force is the one responsible for creating turns.

Here are some screenshots from TacView:

1. Title: High turn rates at less than 1G!!!

2. Title: AMRAAM losses almost no speed and even accelerates at high AOA In the following chart one can see that during very high AOA (>25deg) the missile not only does not loose speed, at one point it actually starts accelerating:

This missile flight was recorded in multi-player. I don't have a track per se as it is quite large, but I can upload it upon request.
 

[KlarSnow]

It is stalling at this point, so its AOA is going to be pegged, as to the turn rate, is the missile rotating, or is its flight path actually moving, these are two different things, in a stalled state of flight the missile isn't actually changing its flight path its just rotating as it falls. This is also why it accelerates, it's falling. Essentially you are saying that an aircraft spinning with a 50 degree per second or faster spin rate is unrealistic because of how much G they are pulling. They are still pulling 1 G, but tacview doesn't show lateral G (yaw) at all, and that can be very disconnected from flight performance.

Like just to be clear, based on the airspeeds you have presented this is a missile that has missed its target and is falling at terminal velocity (360Km/hr), completely stalled (max AOA 25 degrees pegged) and apparently autorotating at some rate. This doesn't appear to be remotely affecting guided performance or its ability to hit the target. like look at the turn radius vs the turn rate. This looks much more like an autorotating stalled ballistically falling missile than anything else. IE it's not a factor to anything. 

Do you have the entire tacview of this missiles flight, because based on the parameters shown this is my conclusion, not anything untoward in the missile dynamics or flight model. If you had the entire tacview of the missile and its flyout instead of just the snippets that would help illuminate what is actually going on.

[Cmptohocah]

45 minutes ago, KlarSnow said:

It is stalling at this point, so its AOA is going to be pegged, as to the turn rate, is the missile rotating, or is its flight path actually moving, these are two different things, in a stalled state of flight the missile isn't actually changing its flight path its just rotating as it falls. This is also why it accelerates, it's falling. Essentially you are saying that an aircraft spinning with a 50 degree per second or faster spin rate is unrealistic because of how much G they are pulling. They are still pulling 1 G, but tacview doesn't show lateral G (yaw) at all, and that can be very disconnected from flight performance.

Like just to be clear, based on the airspeeds you have presented this is a missile that has missed its target and is falling at terminal velocity (360Km/hr), completely stalled (max AOA 25 degrees pegged) and apparently autorotating at some rate. This doesn't appear to be remotely affecting guided performance or its ability to hit the target. like look at the turn radius vs the turn rate. This looks much more like an autorotating stalled ballistically falling missile than anything else. IE it's not a factor to anything. 

Do you have the entire tacview of this missiles flight, because based on the parameters shown this is my conclusion, not anything untoward in the missile dynamics or flight model. If you had the entire tacview of the missile and its flyout instead of just the snippets that would help illuminate what is actually going on.

Hi @KlarSnow,

thanks for your reply. It's a bit difficult to format quote of your post, so I will leave the replies here:

1. I am talking about the missiles ability to turn - please note I am not talking about its rotation around the axis of symmetry, i.e. its roll. What I am referring to is its ability to change heading, that is to turn. Turn rate is a function of G and velocity. What the TacView screenshots are showing is that the AMRAAM is creating turning performance beyond its physical potential to do so. Unless TacView is wrong, it's not possible for a flying object to create any sort of turn rate while its G load is at or below 0. To put it simple: no G, no turn and there fore no heading change (turn rate).

2. You mentioned the missile being ballistic. Just to clarify, objects falling ballistically are not capable of making any sort of maneuvering, i.e. heading change. Ballistic objects get their initial conditions at the time of the launch and then it's up to physics to "guide" them rest of they way. Think of throwing a rock - that's a good example. Problem with this particular track is that the missile is not ballistic as it's performing a 110degree turn while stalled at a very high AOA and pulling impossible Gs. To be honest, it's not clear from my post that the missile was turning, but it will  clearly be visible once I upload the track.

3. You are right, the missile is not posing any immediate threat, but the point is that the underlying physics is something to be looked into. Also this AMRAAM in particular had the added benefit of harassing my RWR since it could actually "track" the entire time.

4. I do have the entire TacView  file and I will upload it here after trimming some time latter today.

Edited August 17, 2022 by Cmptohocah

[KlarSnow]

I would really compare the turn rate to the assessed turn radius, because those are tied to the speed. 13 degrees per second turn rate vs a 2km turn radius at 360 km/hr don’t match up for an actually turning object. The turn radius if it was actually turning 13 degrees per second at that speed should be something in the vicinity of 4-500 meters, not 2000. A 2km turn radius at that speed matches a 2.8 degree turn rate. I feel like you are looking at a transient rotation of the missile here. The same btw applies to all three of the snippets you presented. The Turn radius matches a 2-3 degree turn rate for that airspeed. The turn rate seems to match nothing.

the turn radius is a much better representation of The actual flight path of the object than the turn rate.

As to the airspeed increase. A pitch angle would help here. The missile could be pointed straight down at max AOA in which case gravity will make it accelerate.

[AeriaGloria]

A spinning and falling rock might have a turn rate of 100 degrees per second, but is still experiencing 0 G. 
 

I wouldn’t confuse turn rate either with actual change in flight vector. When tacview tells you turn rate it isn’t telling you “this is how fast it’s changing course” it’s only telling you how fast the rotation is. You have to compare AOA with turn rate to really get how that turn rate compares to the flight vector, or just watch it in tacview. 
 

Also, you point on your screenshot of it holding 25 degrees AOA and not de accelerating. But the missile is basically at 400 knots, as your tacview graph shows that is basically the stall speed of the missile. So when it falls from stalling, it’s defiantly going to accelerate a bit or de accelerate less. A plane that stalls and holds max AOA might still accelerate as gravity brings it down and causes spin/autorotation. At such stall speeds, the only way to lose even less speed would be to go straight up into 0 G so you can’t stall, and then that could only be maintained until the missile would be turned downwards and can’t maintain 0 G anymore. 

[BIGNEWY]

If you think there is a bug here we would need to see track replay examples. 

thanks

[Cmptohocah]

3 hours ago, BIGNEWY said:

If you think there is a bug here we would need to see track replay examples. 

thanks

Hi @BIGNEWY,

This happened on a MP server. Shall I still go ahead and upload the track?

[Cmptohocah]

Hi @KlarSnow,
the TacView track is attached below (it has been trimmed to save space). You are looking for AIM-120C launched by "VOODOO 1-1".

aim120-issues.zip.acmi

[Cmptohocah]

4 hours ago, AeriaGloria said:

A spinning and falling rock might have a turn rate of 100 degrees per second, but is still experiencing 0 G. 
 

I wouldn’t confuse turn rate either with actual change in flight vector. When tacview tells you turn rate it isn’t telling you “this is how fast it’s changing course” it’s only telling you how fast the rotation is. You have to compare AOA with turn rate to really get how that turn rate compares to the flight vector, or just watch it in tacview. 
 

Also, you point on your screenshot of it holding 25 degrees AOA and not de accelerating. But the missile is basically at 400 knots, as your tacview graph shows that is basically the stall speed of the missile. So when it falls from stalling, it’s defiantly going to accelerate a bit or de accelerate less. A plane that stalls and holds max AOA might still accelerate as gravity brings it down and causes spin/autorotation. At such stall speeds, the only way to lose even less speed would be to go straight up into 0 G so you can’t stall, and then that could only be maintained until the missile would be turned downwards and can’t maintain 0 G anymore. 

I think you might be confusing some terms here. A falling rock can't have any turn rate as it can't pull any Gs.

In order for a flying object to turn it has to provide some sort of force that will allow it do so. This force is called G and it is responsible for making the object turn, that is change its direction. The more G, the higher the turn rate, or if you will more degrees per second. That's why if an airplane or a missile is flying under 1G it can only go one way - straight. At 1G it's heading change per unit of time (second) is zero.

Stalled airplane or missile reached its maximum AOA and is producing maximum drag force. It cannot accelerate if the AOA stays the same as the drag force is constant. From the graph above you can see that the angle-of-attack stayed pretty much constant. I didn't understand the rest of the "zero G" portion of what you wrote at the end.

9 hours ago, KlarSnow said:

I would really compare the turn rate to the assessed turn radius, because those are tied to the speed. 13 degrees per second turn rate vs a 2km turn radius at 360 km/hr don’t match up for an actually turning object. The turn radius if it was actually turning 13 degrees per second at that speed should be something in the vicinity of 4-500 meters, not 2000. A 2km turn radius at that speed matches a 2.8 degree turn rate. I feel like you are looking at a transient rotation of the missile here. The same btw applies to all three of the snippets you presented. The Turn radius matches a 2-3 degree turn rate for that airspeed. The turn rate seems to match nothing.

the turn radius is a much better representation of The actual flight path of the object than the turn rate.

As to the airspeed increase. A pitch angle would help here. The missile could be pointed straight down at max AOA in which case gravity will make it accelerate.

Well if the TacViews telemetry is correct, anything more than 2-3 degree TR, as you have suggested, is wrong and opens up possibility for investigation into why is it happening.

 

[BIGNEWY]

40 minutes ago, Cmptohocah said:

Hi @BIGNEWY,

This happened on a MP server. Shall I still go ahead and upload the track?

if you can reproduce in a short example yes, long tracks will be useless for debugging. 
 

Our team will also watch for it, but as of yet no issues seen.
 

[Dragon1-1]

20 minutes ago, Cmptohocah said:

I think you might be confusing some terms here. A falling rock can't have any turn rate as it can't pull any Gs.

Depends on how you (or in this case, TacView) defines turn rate. If you define turn rate as a rate of flight path heading change rate, a falling rock can't have any. If you define turn rate as nose heading change rate, then it can (if you define a "nose" on it). I strongly suspect TacView uses the second definition. Normally, flight path rate and nose rate are closely coupled, but if AoA is increasing during the turn, then it means the nose is rating faster than the flight path (this is the cause of the phenomenon known as "accelerated stall" or "high-speed stall"). Moreover, in a tumble, nose position uncouples from the flight path and can take wild values totally unrelated to flight path rate. 

Also, it's quite possible to turn with less than 1G, it's called an unloaded turn. In fact, if you turn 90 degrees and unload the rudder to 0G, you can load the stick to any value you wish and your heading will change. Just remember that you're in freefall on the horizontal axis and thus accelerating towards the ground.

[Cmptohocah]

1 hour ago, Dragon1-1 said:

Depends on how you (or in this case, TacView) defines turn rate. If you define turn rate as a rate of flight path heading change rate, a falling rock can't have any. If you define turn rate as nose heading change rate, then it can (if you define a "nose" on it). I strongly suspect TacView uses the second definition. Normally, flight path rate and nose rate are closely coupled, but if AoA is increasing during the turn, then it means the nose is rating faster than the flight path (this is the cause of the phenomenon known as "accelerated stall" or "high-speed stall"). Moreover, in a tumble, nose position uncouples from the flight path and can take wild values totally unrelated to flight path rate. 

Also, it's quite possible to turn with less than 1G, it's called an unloaded turn. In fact, if you turn 90 degrees and unload the rudder to 0G, you can load the stick to any value you wish and your heading will change. Just remember that you're in freefall on the horizontal axis and thus accelerating towards the ground.

For me when I talk about turn rate, I mean the flight path heading change rate. Nose position is completely irrelevant in this case, as it means nothing in a sense of turn performance. I am not sure what TacView considers as turn rate - I certainly hope not the nose position.

Could you please elaborate a bit more about the "less than 1G" turn? Where is the turning force coming from? How are you loading the stick and not producing any G forces? Thanks in advance.

[KlarSnow]

2 hours ago, Cmptohocah said:

Hi @KlarSnow,
the TacView track is attached below (it has been trimmed to save space). You are looking for AIM-120C launched by "VOODOO 1-1".

aim120-issues.zip.acmi 425.85 kB · 3 downloads

Ok on reviewing that I really don't see anything untoward, the missile basically swaps its nose from max AOA to the right to max AOA to the left, the actual flight path does not change. It is still stalled and the turn rate you are seeing is just the nose moving. As to the acceleration, its falling downhill. Stabilized in a stall at max AOA, it will pick up speed as it falls so again I dont really see any issue here.

Like you still can point the nose of an aircraft post stall with control surface movements, that's all that's happening here, it has a very high nose movement rate for a second as it swaps from one side to the other, but it is stalled so the actual flight path barely moves.

[Dragon1-1]

4 minutes ago, Cmptohocah said:

Could you please elaborate a bit more about the "less than 1G" turn? Where is the turning force coming from? How are you loading the stick and not producing any G forces? Thanks in advance.

You can think of G force as the sum of forces other than the force of gravity. When you're under 1G, you're in equilibrium with force of gravity, so you're not accelerating. Either the wings or the ground is pushing up up with the force equal to the one that pulls you down. When you're under 0G, you're in freefall, that is, accelerating towards the Earth's center of gravity at the rate of 1g (small g is standard acceleration due to gravity). Freefall doesn't have to be straight down, if you enter it with any sort of horizontal velocity, you will keep it, indeed, true freefall means you have to compensate for the drag with, say, engine power. This is how orbiting works, you just need to get high up enough so you can go fast enough without air getting in the way (ISS is affected by drag and needs to occasionally fire its engines to keep orbit).

A less than 1G turn is any heading change made in freefall. If your wings are perpendicular to the airflow, or if you use your rudder to turn, you will generate a force that will cause a heading change. As it doesn't have to oppose gravity, it can be however low you want, you will turn. With rudder, it won't show on the G meter because of how these instruments work (they typically only show vertical G), but with wings, it would. 

9 minutes ago, Cmptohocah said:

Nose position is completely irrelevant in this case, as it means nothing in a sense of turn performance.

Nose position is very important, for two reasons. 1. It's fixed to the body of the airplane. 2. That's where the gun is. A sustained turn is one in which AoA doesn't change, but some fights are won by instantaneous turn rate, in such turns the nose rates faster than the flight path vector, AoA constantly increases and the aircraft bleeds speed. Judging from this:

1 minute ago, KlarSnow said:

Ok on reviewing that I really don't see anything untoward, the missile basically swaps its nose from max AOA to the right to max AOA to the left, the actual flight path does not change. It is still stalled and the turn rate you are seeing is just the nose moving. As to the acceleration, its falling downhill. Stabilized in a stall at max AOA, it will pick up speed as it falls so again I dont really see any issue here.

It looks like TacView indeed shows nose rate not flight path vector rate. 

[Cmptohocah]

3 minutes ago, KlarSnow said:

Ok on reviewing that I really don't see anything untoward, the missile basically swaps its nose from max AOA to the right to max AOA to the left, the actual flight path does not change. It is still stalled and the turn rate you are seeing is just the nose moving. As to the acceleration, its falling downhill. Stabilized in a stall at max AOA, it will pick up speed as it falls so again I dont really see any issue here.

Like you still can point the nose of an aircraft post stall with control surface movements, that's all that's happening here, it has a very high nose movement rate for a second as it swaps from one side to the other, but it is stalled so the actual flight path barely moves.

It can't be stalled as stalled aircraft can't change its flight path - the missile did a 110turn before it hit the ground.

It also can't pick up any speed as a constant AOA produces constant drag so where is the acceleration force coming from? No force, no acceleration.

[KlarSnow]

A stalled aircraft can definitely change its flight path, you can demonstrate this with any aircraft in DCS, pull the nose up to max AOA and then as it buffets and stalls kick the rudder in one direction or the other, or unload the stick letting the nose fall back through vertical, in this instance it appears like the missile went from fully loaded in one direction to fully loaded in the other, I see zero issues with this.

There is an AOA dip right as the missile is changing direction, with the nose below the horizon, so this also contributes to the small acceleration you see there.

The force is coming from gravity, again a rock thrown through the air is stalled, does it accelerate?

In addition as to it making a 110 degree turn, the missile is dead (battery death) approximately 5 seconds after it swaps its nose from one side to the other, it then falls with its controls in their final position, continuing to turn (at a 2-3 degree per second rate) as it falls to the ground. Again this is all expected based on what is happening here. 

Look at the turn radius, not rate. Looking at your tacview, it appears the rate you are observing is solely related to how tacview interprets turn rate, as nose movement of the aircraft, not how quickly it is moving along its flight path. The turn radius and airspeed shown in the tacview match the max performance you calculated, which also matches the actual flight path of the missile. if it was actually turning at 13-18 degrees per second turn rate, there would be a 500M turn radius and a much tighter circle instead of the giant barely moving curve presented.

I see absolutely nothing wrong here.

Edited August 17, 2022 by KlarSnow

[Cmptohocah]

Ok there seems to be some confusion here. Let me put in more simple terms what I mean:

Turning motion is by nature accelerated motion. In order for anything in this universe to turn, i.e. not fly move in a straight line, there must be acceleration, that is "a force", present. On the ground for vehicles, this force comes from friction between wheels and the ground and in the air it, usually, it comes from lift (neglecting pro

pulsion here). To make a circle (turn) in any plane (vertical, horizontal or oblique) there has to be a force present. Without force there is no circular movement and consequently no turning. That's why @KlarSnow mentioned turn-radius, it's a radius of a circle

42 minutes ago, KlarSnow said:

The force is coming from gravity, again a rock thrown through the air is stalled, does it accelerate?

It accelerates until its drag force equals the gravity acceleration force at which point it stops accelerating and drops at a constant speed, also known as "terminal velocity". At that point both the drag force and the weight are in equilibrium (tough word to spell :D). See when you let go of a rock from a balloon, for example, it's initial speed is zero and so is its drag force, but as the rock accelerates (and picks up speed), the drag force increases until the two match.

53 minutes ago, Dragon1-1 said:

...Nose position is very important, for two reasons...

Sorry I meant it's irrelevant to the topic I am trying to discuss - that is: it does not matter where the nose is pointing or how fast IT changes direction. What matters is how entire aircraft/missile changes direction, i.e. turns, i.e. "draws" a circle.

Edited August 17, 2022 by Cmptohocah

[KlarSnow]

The acceleration occurs when the missiles flight path goes from above the horizon to below the horizon,and when it briefly unloads AOA during the nose swap,  what exactly is wrong there. And as to the turn the only thing I have concluded is tacview is assessing turn rate based on change in nose position.

Edited August 17, 2022 by KlarSnow

[Cmptohocah]

40 minutes ago, KlarSnow said:

The acceleration occurs when the missiles flight path goes from above the horizon to below the horizon,and when it briefly unloads AOA during the nose swap,  what exactly is wrong there. And as to the turn the only thing I have concluded is tacview is assessing turn rate based on change in nose position.

 

The AOA does not change. Have a look at the graph please.

[Exorcet]

1 hour ago, Cmptohocah said:

It also can't pick up any speed as a constant AOA produces constant drag so where is the acceleration force coming from? No force, no acceleration.

Gravity is a force.

If you want an example of a heading change at 0 g, fly any plane in DCS and push the nose down and hold 0 g. You'll change direction.

[KlarSnow]

45 minutes ago, Cmptohocah said:

The AOA does not change. Have a look at the graph please.

The point you indicate here

is when the flight path and nose of the missile goes from  above the horizon to below the horizon. Why would it not accelerate as gravity is no longer slowing it down but increasing its speed. 

the AOA dip immediately after (red line) when it changes direction is also commensurate with a 10 knot speed increase.

like to be clear you are talking about a small 10-20 knot increase in airspeed as the missile goes from pitched up to pitched down. That is very much not unreasonable, nor does it defy physics.

Edited August 18, 2022 by KlarSnow

[Dragon1-1]

7 hours ago, Cmptohocah said:

Sorry I meant it's irrelevant to the topic I am trying to discuss - that is: it does not matter where the nose is pointing or how fast IT changes direction. What matters is how entire aircraft/missile changes direction, i.e. turns, i.e. "draws" a circle.

It's not entirely irrelevant because the reasons I gave are why TacView shows you the nose rate and not flight path rate. As pointed out by multiple people, that value is fully expected to act like this in the given situation. The "value that matters", is not the one that you see in TacView, because TacView devs considered nose rate to be a more useful measurement, for  reasons unrelated to our discussion.

[okopanja]

In general I think funky things occur with flight model of missiles in general.

Just for the clarity observe this view from above, taken here shortly before that AoA/G dip there. Intersting is what happens after the dip.

 

 

Edited August 18, 2022 by okopanja
updated graph

[KlarSnow]

Which is entirely related to the nose position of the missile swapping from one side to the other, please, this is all just the “turn rate” in tacview being tied to nose position, not actual turn rate. Any time there is a transient and a mismatch as the nose is moving there will be a high transient “turn rate”. This is not the same as the actual flight path turn rate. Which appears to match the steady state turn rate and turn radius the entire time.

again nothing appears remotely wrong here. Y’all seem to be latched on to the high “turn rate” which is more accurately described as nose position. Actual turn rate of the flight path once it stabilizes the AOA is exactly in line with what is possible. The only way what is shown happening is not possible is if the control surfaces have no authority to affect the nose of the aircraft/missile.

you can replicate this exact behavior with any aircraft.

[KlarSnow]

It is very easy to recreate this behavior, just hold the stick back and as you get slow step on the rudder in either direction as demonstrated here.

The "turn rate" is just heading change, IE the nose swinging around, it is not the flight path turn rate until the nose has stabilized and it is fully loaded up again.

19.4 Dps turn rate, 1.1G, at 231.0 Km/h CAS

17.5 Dps turn rate, 1.4G, at 256.1 Km/h CAS

17.7 Dps turn rate, 1.4G, at 263.2 Km/h Cas

also note the stick was held back the entire time, the AOA pegged, and my aircraft increased airspeed as I descended.

Tacview-20220818-173925-DCS.zip.acmi

Edited August 18, 2022 by KlarSnow