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

[Ironhand]

Everything else aside, that R-77's missile’s behavior was bizarre beginning roughly 1 minute into it's flight. It was tracking the aircraft, then suddenly began switching back and forth between something off to its left and the original target. Finally it shifted back to the original target and continued to track it for the next minute, even though the target was about 50° or more above the seeker head.

Edited August 18, 2022 by Ironhand

[okopanja]

1 hour ago, Ironhand said:

R-77's behavior

AIM-120C?

[Ironhand]

31 minutes ago, okopanja said:

AIM-120C?

 Yes. Got confused in my old age…

[Cmptohocah]

On 8/18/2022 at 2:23 AM, KlarSnow said:

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.

 

Ok the arrow is a bit skewed, that's my bad, but take that yellow line as a reference then. You see a dip in airspeed left of it? This means that there was some acceleration occurring while the AoA stayed pretty much the same.

P.S. Technically AoA was not constant, but what happened to the airspeed is not consistent with what the AoA was indicating in a sense of conservation of energy.

Edited August 19, 2022 by Cmptohocah

[Cmptohocah]

21 hours ago, Ironhand said:

Everything else aside, that R-77's missile’s behavior was bizarre beginning roughly 1 minute into it's flight. It was tracking the aircraft, then suddenly began switching back and forth between something off to its left and the original target. Finally it shifted back to the original target and continued to track it for the next minute, even though the target was about 50° or more above the seeker head.

 

Very good point, and if you actually look at the graph you can see that when this erratic angle-of-attack "dance" started, the loss of speed actually started to slow down, instead of speeding up. One would expect that with hard maneuvering the drag force increases, not the other way around.

I actually waited for a very long time, before I have decided to make this post. It seemed that AMRAAM's loss of energy was way below what it should be, but I never actually had any concrete evidence that this is happening - until this track that is.

Edited August 19, 2022 by Cmptohocah

[KlarSnow]

24 minutes ago, Cmptohocah said:

Ok the arrow is a bit skewed, that's my bad, but take that yellow line as a reference then. You see a dip in airspeed left of it? This means that there was some acceleration occurring while the AoA stayed pretty much the same.

P.S. Technically AoA was not constant, but what happened to the airspeed is not consistent with what the AoA was indicating in a sense of conservation of energy.

 

Again, that ten second period where it has swapped from being nose high, to buried nose low, you seem to be thinking this is an unexplained acceleration. If it was at the same AOA nose high, slowed down, and then maintained the same AOA nose low.... it should accelerate since it should have decelerated below the equilibrium while the nose was up. This tiny acceleration is entirely related to it re-establishing equilibrium as it falls, which appears (after the battery dies and it is completely ballistic) to be around 195 knots which it roughly holds until it impacts the ground. Again you would see this exact behavior in any aircraft that flew a similar profile.

This is an object that is going up, curving over the top and then coming back down. It MUST accelerate if nothing else changes. If it didn't in this scenario then it needs to increase its drag, which it cannot do since it is already maxed out at AOA. The small dip in AOA which matches the nose swap fromright to left also corresponds with it building up a few more knots, this is also expected. 

As to the Nose of the missile snapping back and forth, quite honestly that looks like a net issue more than anything else, so I wouldn't pin that on any flight model issues, remember you are looking at "your" tacview, which is not what is generating the aerodynamics of this weapon and will have latency and potential innacuracies compared to the shooters tacview. I'll bet if you looked at the shooters tacview, that strange sawing back and forth wouldn't exist.

Keep in mind the only "truth data" about what a weapon is actually doing is the shooters client, there can be significant network artifacts when looking at a track or tacview from a client other than the shooter.

Edited August 19, 2022 by KlarSnow

[Exorcet]

21 minutes ago, Cmptohocah said:

Ok the arrow is a bit skewed, that's my bad, but take that yellow line as a reference then. You see a dip in airspeed left of it? This means that there was some acceleration occurring while the AoA stayed pretty much the same.

P.S. Technically AoA was not constant, but what happened to the airspeed is not consistent with what the AoA was indicating in a sense of conservation of energy.

 

Why is this important? AoA =/= acceleration or speed.

  

15 minutes ago, Cmptohocah said:

Very good point, and if you actually look at the graph you can see that when this erratic angle-of-attack "dance" started, the loss of speed actually started to slow down, instead of speeding up. One would expect that with hard maneuvering the drag force increases, not the other way around.

I actually waited for a very long time, before I have decided to make this post. It seemed that AMRAAM's loss of energy was way below what it should be, but I never actually had any concrete evidence that this is happening - until this track that is.

 

There is a large ramp up of AoA before the erratic behavior. There is no increase in drag during the nose swinging, if you average out the erractic period, the AoA is roughly in line with what it was before the missile went crazy, so there isn't really a reason to expect it to slow down more.

Edited August 19, 2022 by Exorcet

[okopanja]

5 minutes ago, KlarSnow said:

'll bet if you looked at the shooters tacview, that strange sawing back and forth wouldn't exist.

Maybe slightly out of topic, but still relevant, but still relevant which one is actually relevant for simulation? I gather that there must be some sort of reconciliation between client worlds and the world on the server? (e.g. I noticed in the past rather discrepancies between tacviews generated from different universes, e.g. one 2 missiles being fired and having dilemma which one actually hit first. )

[KlarSnow]

3 minutes ago, okopanja said:

Maybe slightly out of topic, but still relevant, but still relevant which one is actually relevant for simulation? I gather that there must be some sort of reconciliation between client worlds and the world on the server? (e.g. I noticed in the past rather discrepancies between tacviews generated from different universes, e.g. one 2 missiles being fired and having dilemma which one actually hit first. )

Shooters client is law and always has been for any weapons employment, unless ED has changed that without telling anyone, its still like that as far as I'm aware.

 

The issue with a non shooter track or tacview is that any network inconsistencies get magnified. Tacview does not have a great sample rate since it is deriving data from positional data essentially, so you can see artifacts related to that. IE a tiny wiggle can get magnified to a big AOA change due to net lag and then tacview interpretation. This stuff is not super precise for looking at the actual mechanics that are going on. Your best bet is always and has always been the shooters track or tacview. Server is next best, other client is third.

Edited August 19, 2022 by KlarSnow

[Cmptohocah]

56 minutes ago, Exorcet said:

Why is this important? AoA =/= acceleration or speed.

  

There is a large ramp up of AoA before the erratic behavior. There is no increase in drag during the nose swinging, if you average out the erractic period, the AoA is roughly in line with what it was before the missile went crazy, so there isn't really a reason to expect it to slow down more.

 

Angle of attack is important because it dictates the ammount of drag force produced. Drag force oposes movement, think of it as a brake. Engine and gravity promote movement. 

For example, if you are flying straight and level, at 500km/h at 50% engine power if you slow down to 400km/h and still want to keep level, you need to increase your AoA to increase generated lift. You got more lift out of your wing so you can stay level, but since there is no free beer you also incresed drag and that 50% engine power ain't gonna cut it. So now you need more power to overcome that drag, otherwise you will continue deccelerating.

 

[KlarSnow]

13 minutes ago, Cmptohocah said:

Angle of attack is important because it dictates the ammount of drag force produced. Drag force oposes movement, think of it as a brake. Engine and gravity promote movement. 

For example, if you are flying straight and level, at 500km/h at 50% engine power if you slow down to 400km/h and still want to keep level, you need to increase your AoA to increase generated lift. You got more lift out of your wing so you can stay level, but since there is no free beer you also incresed drag and that 50% engine power ain't gonna cut it. So now you need more power to overcome that drag, otherwise you will continue deccelerating.

 

Yes, but that all only applies in level stable flight, the flight path is changing, so that will not remain constant. Again, the missile just before its airspeed gain was going up.... so gravity was decelerating it more than if it was in equilibrium, so it was NOT at terminal velocity, it than starts going down, so it will accelerate to regain that equilibrium, which in the instance you have shown, appears to be ~195 knots. 

Also there will always be transients, so no it will not sit at a static single lowest airspeed as it falls, it will vary around that equilibrium point.

 

It is not in equilibrium until the flight path angle is stable vertically, the airspeed has stabilized, and the AOA has stabilized, once all that happens then it will be static. The missile at all of the acceleration points you are pointing out, is doing none of those things, so its airspeed will change.

Again think back to throwing a rock straight up. When it reaches the apex of its arc, it is and always has been at the equilibrium of AOA, lift, drag etc... But gravity has slowed it down, none of its "AOA, or "Drag" has changed, but it MUST Accelerate because gravity is pulling it back down. This is precisely what is happening to the missile.

You seem to be assuming that for the given AOA that the terminal airspeed of an AMRAAM in DCS is the lowest airspeed it reaches during its flight, again, point it straight up, its lowest airspeed will reach zero, so should it not accelerate as it falls off the apex? Or as you seem to be saying it must sit at zero airspeed and hover in the air.

Edited August 19, 2022 by KlarSnow

[Exorcet]

13 minutes ago, Cmptohocah said:

Angle of attack is important because it dictates the ammount of drag force produced.

It is only one factor in the amount drag produced, and the drag is only one factor in acceleration

13 minutes ago, Cmptohocah said:

Drag force oposes movement, think of it as a brake. Engine and gravity promote movement. 

The engine and gravity's contributions depend on where they are pointing relative to the direction of motion, they can cause acceleration or deceleration. This is very relevant in this case since the missile isn't flying in a straight line, so gravity's contribution to acceleration is constantly changing. I think the graphs are actually making things harder to understand because they are masking the 3D nature of the problem.

13 minutes ago, Cmptohocah said:

For example, if you are flying straight and level, at 500km/h at 50% engine power if you slow down to 400km/h and still want to keep level, you need to increase your AoA to increase generated lift. You got more lift out of your wing so you can stay level, but since there is no free beer you also incresed drag and that 50% engine power ain't gonna cut it. So now you need more power to overcome that drag, otherwise you will continue deccelerating.

 

Your lift didn't change. The weight of the plane has stayed constant, so in order for the forces to balance out, lift must also stay constant. What you did by slowing down was shift your position on the L/D curve, and the effect of that depends on where L/D max is located. If L/D max is at 400, you actually lowered drag by slowing down, no matter what happened to AoA.

Edited August 19, 2022 by Exorcet

[KlarSnow]

The extreme example of this is what is the drag force of a stationary object. Zero. What is the AOA of a stationary object? Is the drag of a 5 Knot object the maximum it could possibly be? No its very low. Slower or higher AOA does not necessarily equal maximum drag. It depends very much on the situation, the aerodynamic properties of the object as Exorcet describes, and what is happening.

[Cmptohocah]

34 minutes ago, Exorcet said:

It is only one factor in the amount drag produced, and the drag is only one factor in acceleration

Yes that's technically correct, but I don't see your point. It is one of the contributors to drag, how does this change what I have stated?

34 minutes ago, Exorcet said:

The engine and gravity's contributions depend on where they are pointing relative to the direction of motion, they can cause acceleration or deceleration. This is very relevant in this case since the missile isn't flying in a straight line, so gravity's contribution to acceleration is constantly changing. I think the graphs are actually making things harder to understand because they are masking the 3D nature of the problem.

Again correct, but I was trying to explain how AoA influences the drag force. Of course if you fly 90deg straight up and have your thrust reversers on, then both gravity and thrust oppose movement.

34 minutes ago, Exorcet said:

Your lift didn't change. The weight of the plane has stayed constant, so in order for the forces to balance out, lift must also stay constant. What you did by slowing down was shift your position on the L/D curve, and the effect of that depends on where L/D max is located. If L/D max is at 400, you actually lowered drag by slowing down, no matter what happened to AoA.

 

This is incorrect, but when you slow down to 400km/h and keep your original angle of attack, the effective lift is less than what you had at 500km/h because the airflow over the wing is not sufficient anymore to produce the same amount of lift. The only way you can keep the same lift force at 400km/h is by increasing the AoA so that your lift surfaces produce the same amount of lift as if flying at 500km/h.

The effective lift force stayed the same, 'cause the weight did not change, but what happened with the increase AoA is that the drag component of lift (remember total lift has 2 components: one acting up to keep the airplane in the air, second one opposing the thrust) is now facing further back contributing to total drag. That's why you need to push the throttles forward in order to keep 400km/h.

You have lowered the parasitic parasitic drag, that is correct, but you have increased the induced drag.

Finally I think this quote sums it up really nice:

Quote

Typically at low AOA, the coefficient of drag is low and
small changes in AOA create only slight changes in the
coefficient of drag. At high AOA, small changes in the AOA
cause significant changes in drag.

 

Edited August 19, 2022 by Cmptohocah

[KlarSnow]

9 minutes ago, Cmptohocah said:

 

The effective lift force stayed the same, 'cause the weight did not change, but what happened with the increase AoA is that the drag component of lift (remember total lift has 2 components: one acting up to keep the airplane in the air, second one opposing the thrust) is now facing further back contributing to total drag. That's why you need to push the throttles forward in order to keep 400km/h.

Or your flight path falls, and the gravity vector increases your speed. You are ignoring the affect that the flight path angle has on all of this. You cannot be in equilibrium if the flight path is changing. The flight path of the amraam in your tacview is changing, so it will change airspeed. If its flight path has a more upward component it will decelerate because its drag component adds to gravity, If its flight path has a downward component then that gravity is removed from the drag vector and added to the velocity vector. If it goes from going up to going down, it must accelerate. This is what the AMRAAM in your tacview does.

The missile is NOT in static equilibrium as you are describing because its flight path angle (and pitch angle) continues to fall. Once it stabilizes, like it does around 195 knots after the battery dies, then all the forces are roughly equal and it will not accelerate anymore. You are removing the 3D component from all of this, just like you did at the start, you cannot do that, you must take into account the object going up or down and whether or not the flight path is changing or not. If it is changing, it is not in equilibrium.

Edited August 19, 2022 by KlarSnow

[Exorcet]

4 minutes ago, Cmptohocah said:

Yes that's technically correct, but I don't see your point. It is one of the contributors to drag, how does this change what I have stated?

I'm saying that it's a little more complicated than you're making it out to be. You can't look at a speed vs AoA graph and completely understand what is happening to the missile.

4 minutes ago, Cmptohocah said:

Again correct, but I was trying to explain how AoA influences the drag force. Of course if you fly 90deg straight up and have your thrust reversers on, then both gravity and thrust oppose movement.

It's not only in the 90 degree straight up case.

4 minutes ago, Cmptohocah said:

This is incorrect

It is not. Level flight must have lift = weight. You don't change lift by changing speed.

4 minutes ago, Cmptohocah said:

 

but when you slow down to 400km/h and keep your original angle of attack, the effective lift is less than what you had at 500km/h because the airflow over the wing is not sufficient anymore to produce the same amount of lift.

Only true is AoA does not change, but...

4 minutes ago, Cmptohocah said:

 

The only way you can keep the same lift force at 400km/h is by increasing the AoA so that your lift surfaces produce the same amount of lift as if flying at 500km/h.

So the lift does not change.

4 minutes ago, Cmptohocah said:

The effective lift force stayed the same, 'cause the weight did not change, but what happened with the increase AoA is that the drag component of lift (remember total lift has 2 components: one acting up to keep the airplane in the air, second one opposing the thrust) is now facing further back contributing to total drag. That's why you need to push the throttles forward in order to keep 400km/h.

Lift has no component in line with thrust. Lift produces a drag (induced drag) which is in line with thrust, but this is only one component of drag.

Where max drag is, is not obvious. But if you knew the L/D curve, then you could say something about total drag. And total drag can decrease with increased AoA.

4 minutes ago, Cmptohocah said:

You have lowered the parasitic parasitic drag, that is correct, but you have increased the induced drag.

Finally I think this quote sums it up really nice:

I agree with that, but again it's only a small part of the total picture.

 

Rather than the tacview graphs, I'd like to see a video of the missile in flight. Unless there are networking issues causing it to move unrealistically, I think a video would be much better in showing what is going on in an easy to understand way.

[KlarSnow]

11 minutes ago, Exorcet said:

Rather than the tacview graphs, I'd like to see a video of the missile in flight. Unless there are networking issues causing it to move unrealistically, I think a video would be much better in showing what is going on in an easy to understand way.

Here 

1

l

2

3

4

5

Tacview HUD view of the missile in question during the time period in question. It is falling, its pitch angle is decreasing, so it MUST accelerate.

Edited August 19, 2022 by KlarSnow

[KlarSnow]

Where it starts out 

Where it ends up

[Exorcet]

5 minutes ago, KlarSnow said:

Tacview HUD view of the missile in question during the time period in question. It is falling, its pitch angle is decreasing, so it MUST accelerate.

Thanks, this makes it very clear. The missile is initially going up, gravity is pulling down, missile slows. Then the missile goes down, gravity is still pulling down, missile speeds up. AoA doesn't matter, the missile is so slow that drag < gravity.

[KlarSnow]

Just now, Exorcet said:

Thanks, this makes it very clear. The missile is initially going up, gravity is pulling down, missile slows. Then the missile goes down, gravity is still pulling down, missile speeds up. AoA doesn't matter, the missile is so slow that drag < gravity.

This is why my first question at the start of this whole thread was we need to see more than just the graph and the telemetry, you cannot know anything without knowing what the flight path is doing.

[Cmptohocah]

31 minutes ago, KlarSnow said:

The extreme example of this is what is the drag force of a stationary object. Zero. What is the AOA of a stationary object? Is the drag of a 5 Knot object the maximum it could possibly be? No its very low. Slower or higher AOA does not necessarily equal maximum drag. It depends very much on the situation, the aerodynamic properties of the object as Exorcet describes, and what is happening.

Drag force on an object no moving in a fluid is zero - that is correct.

Quote

What is the AOA of a stationary object?

This question is not valid. There is no Angle of Attack for an object that is not moving through a fluid (air). The lift surfaces are not "attacking" anything since there is no flow, so I guess the correct answer would be "infinity"?

Maximum AoA (just before stalling) of a lift surface, in fact equals maximum attainable lift force for a given speed. So when ever you have max AoA you will have maximum possible induced drag happening for a given speed. The amount of that drag will vary with speed, at higher speeds more lift is produced and thus more drag is produced. What I am talking about here are not absolute values, like: is there more drag at 5km/h or at 500km/h? At both speeds and any other speed in between and around these, induced drag will always be at its maximum at max AoA.
To put it more simply: induced drag (and there for total drag) will be higher at max AoA than at AoA=0 at "X" speed. Now you can substitute the "X" with what ever speed you like, but the statements holds true.

[KlarSnow]

Until the highlighted line (flight path) stops changing, the missile is not in equilibrium, no matter what the AOA is. Thus it will change airspeed, up or down. depending on what that flight path angle is. Since it is changing and increasing its descent it must accelerate to reach equilibrium, equilibrium will be reached when that line stops increasing or decreasing and is steady.

[Cmptohocah]

15 minutes ago, KlarSnow said:

Here 

1

l

2

3

4

5

Tacview HUD view of the missile in question during the time period in question. It is falling, its pitch angle is decreasing, so it MUST accelerate.

 

Pitch of the missile does not matter here. First of all it's impossible for AoA to stay the same with a lower pitch, so there is something wrong in how TacView interprets pitch I guess.

Anyway at a constant AoA, from what it seems maximum attainable for AMRAAM in this case, there is no way its physically possible for it to accelerate. It just physics. Also notice in the last slide #5 it's pulling 0.1G than slides 2 and 3. And yet it's accelerating.

[oldcrusty]

6 minutes ago, KlarSnow said:

This is why my first question at the start of this whole thread was we need to see more than just the graph and the telemetry, you cannot know anything without knowing what the flight path is doing.

After reading the entire thread I'm obsessed now to see what happens after the missile motor shuts down, the batteries are dead and control surfaces are stuck...

For non-engineer types like me, the videos with whatever telemetry displayed in a replay with abilities to change viewpoints works best.

Runs to test dead missiles...   (yes I know, some basic flaws with the entire aerodynamic model might show up and I won't be able to tell anyway, lol)

[KlarSnow]

I'm sorry but you cannot ignore the direction and pitch angle of anything in flight. That is ludicrous. If things are changing you are not in equilibrium. Its pitch angle is changing, its heading is changing. The tiny .1G here doesn't matter, we are talking about miniscule increases or decreases in drag here that are more than made up by gravity as it falls. Again the object is not in equilibrium so it will not and cannot remain static, once it has stopped changing its flight path, then it can sit in perfect equilibrium as you describe.