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


Cmptohocah
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37 minutes ago, Exorcet said:

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

Not much I can reply to this, except you might want to re-check on how lift is generated by a lift surface.

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17 minutes ago, Cmptohocah said:

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.

But where does the inability to accelerate come from? More drag doesn't imply that acceleration is impossible, you need to consider the net force, which can change while maintaining constant magnitude by changing direction.

8 minutes ago, Cmptohocah said:

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.

AoA is the angle between chordline and flight path. The pitch attitude has nothing to do with AoA. You can see this in DCS. Fly at 0 AoA and you will likely pitch down.

8 minutes ago, Cmptohocah said:

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.

Imagine this situation:

Speed = 0.00000000001 knots, AMRAAM weight = ~200 lbs (no fuel). Drag force is approximately 0. Gravity force is 200 lbs. Acceleration results in a increase in absolute speed. AoA doesn't prevent acceleration.

Maybe you're too hung up on velocity magnitude. If you use vertical velocity instead, then there is no change in acceleration direction. The missile is always acceleration downward (around the moment of flight we're talking about).

 

  

Just now, Cmptohocah said:

Not much I can reply to this, except you might want to re-check on how lift is generated by a lift surface.

You're not taking the whole picture into account. In level flight, lift must equal weight. So in level flight, the lift force does not vary with speed. From the perspective of the pilot, the plane is trimmed to maintain 1 g as speed changes.

If we move outside of level flight, then L = .5*rho*CL*A*V^2 becomes valid. But in level flight, L and CL change and the difference cancel out, no matter what happens to V.


Edited by Exorcet

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Just now, oldcrusty said:

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)

The missile just locks its controls and continues turning as it was until it stalls out or hits the ground. IE the fins just stay in whatever the last position they were commanded to. You can see this in the tacview posted at the start of this thread, the battery is dead at 100 seconds of flight, and it just continues its turn falling to the ground.

There is nothing remotely questionable about any of the physics at display in this tacview or any of what has been presented except the Sawwing back and forth that happens prior to the "physics defying" acceleration and turn. That to me appears to be a net lag artifact since this is not the shooters tacview. Other than that I dont have an explanation for why the missile did that. Its acceleration and deceleration and AOA performance all match what any object moving through the air should do.

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4 minutes ago, oldcrusty said:

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)

If I am not mistaken batteries ran out near that last AoA dip. 100 seconds. It seams to have reacquired briefly. I can not check now.

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4 minutes ago, KlarSnow said:

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.

How can a pitch change occur with AoA staying the same? Maybe TacView mesures AoA of the control surfaces?

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Just now, Cmptohocah said:

How can a pitch change occur with AoA staying the same? Maybe TacView mesures AoA of the control surfaces?

Fly a plane in DCS at 0 AoA.

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Just now, okopanja said:

If I am not mistaken batteries ran out near that last AoA dip. 100 seconds. It seams to have reacquired briefly. I can not check now.

It did not reacquire, it just continued in the turn it was in prior to the battery death.

Just now, Cmptohocah said:

How can a pitch change occur with AoA staying the same? Maybe TacView mesures AoA of the control surfaces?

Fly an aircraft at a constant AOA with the flight vector pointed up. Your aircraft will not remain pointed up if you maintain the AOA and turn your engines off. That is what is happening here.

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1 minute ago, okopanja said:

So the flight control surfaces stay in the last commanded position?

Yes, dead missiles are stuck in turns very often. You can see this on missiles with fin animations likes AMRAAM.

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Just now, okopanja said:

So the flight control surfaces stay in the last commanded position?

Yes, they have done this for well over a year at this point in the AIM-120 IIRC when they fully modeled the flight controls of it. As it and the sparrow are the only missiles on the new API that models all of that, they are the only ones that exhibit that behavior. Everything else just goes completely neutral controls after battery death.

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8 hours ago, KlarSnow said:

Yes, they have done this for well over a year at this point in the AIM-120 IIRC when they fully modeled the flight controls of it. As it and the sparrow are the only missiles on the new API that models all of that, they are the only ones that exhibit that behavior. Everything else just goes completely neutral controls after battery death.

What is preventing the control surfaces from going back to neutral if there is no energy source to counter the relative wind force imposed on to them? Mechanism friction?

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2 minutes ago, Cmptohocah said:

What is preventing the control surfaces from going back to neutral if there is no energy source to counter the relative wind force imposed on to them? Mechanism friction?

That very much depends on how the control actuators work. This is how ED has decided they work. There is nothing unrealistic about it. Non reversible gearing is a plausible thing in a fin actuator like this. I don't know if the real AMRAAM behaves like this after battery death, but I also don't know if the real thing doesn't behave like this. Both are plenty plausible.

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On 8/19/2022 at 5:56 AM, 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?

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.

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:

 

 

Just wanted to say, in such a scenario I think Induced drag is actually the same, becuase the lift is the same. AOA does not cause drag force to “face” further back. So with same induced drag, it comes down to if the 100 kmh decrease in speed causes enough of a parasitic drag loss to outweigh the increase of parasitic drag from higher AOA (bigger frontal area). So I’m not quite sure you would need more throttle in such a situation, I think that depends on the L:D curve/bucket speed. 
 

I get what you mean about how if you sum the force of lift and drag you get a line that can point more up or back, this depends on the L:D ratio. And where optimum L:D is in AOA I think is a much more complicated subject. But increasing AOA I don’t believe causes a worse lift to drag ratio as a rule, becuase if you keep decreasing AOA eventually you will get into all drag and 0 lift (sum of drag and lift pointing directly behind). Most planes have a speed and AOA where L:D is the best, and it’s never at one extreme. 
 

As for pitch not changing with constant AOA. That’s pretty curious. Imagine you are turning in a plane at a constant AOA. You can add a vertical portion to the turn and change pitch angle just fine. You can take plane like F-16/18/JF-17, go near max AOA and change pitch with throttle, just like when a F-18 lands, all you have to do is be trimmed right. 

As for the discussion for oscillations of the AMRAAM, I haven’t viewed the whole flight but couldn’t it just be oscillations from stalling, then pitching down until it increases speed and decrease AOA enough for the locked control surfaces to cause a new stall? And just hasn’t “settled” or gone into equilibrium yet? I’ve seen this behavior called porpoising before 

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On 8/17/2022 at 5:57 PM, 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.

That's interesting that the missile retains that nose pointing even now that is is 'dead' because of the ability to maintain control in the post stall

I guess that comes from HOBS tapes making the missile otherwise 'smart' enough to maneuver here without putting itself into an uncontrolled spin or tumbling aimlessly?

Something I've always be curious about is this behaviour coming from missiles that are not the R-77 AIM-120 AIM-9X and R-73

Like missiles who's guidance law is so primitive they're susceptible to orthogonal rolls like SA-2 SA-3s
Should there missiles also exhibit these characteristics of nose pointing while 'dead' and stalling?

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Its not really stalled tbh its just at max AOA and slow, and as I stated even in a stall you can still move control surfaces and affect the nose of an aircraft, just look at the hornet, or the Mig-29 or the flanker, or really anything. Push the rudder from one side to the other ad the nose will fall off in that direction. Thats all thats happening here.

IE just because the wings are stalled doesn't mean the tail is stalled.

I think you are seeing this behavior in the AMRAAM because it is one of the only missiles on the new API that has a full 6DOF flight model going on. I don't think the older API missiles are quite there yet.


Edited by KlarSnow
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54 minutes ago, KlarSnow said:

Its not really stalled tbh its just at max AOA and slow, and as I stated even in a stall you can still move control surfaces and affect the nose of an aircraft, just look at the hornet, or the Mig-29 or the flanker, or really anything. Push the rudder from one side to the other ad the nose will fall off in that direction. Thats all thats happening here.

IE just because the wings are stalled doesn't mean the tail is stalled.

I think you are seeing this behavior in the AMRAAM because it is one of the only missiles on the new API that has a full 6DOF flight model going on. I don't think the older API missiles are quite there yet.

 

I think you misunderstand my question.

I am curious if missiles with older, more simplistic guidance laws should also exhibit such nose pointing IRL, even at incredibly low speeds.
Could an example of this maybe be an AIM-9B?

Or control lay outs that don't lend themselves to be good at low speed control and flying as it may not be relevant to the misison, like a Nike Hercules
 


Edited by TaxDollarsAtWork
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It honestly isn't really relevant, Like this is all stuff that happens well after the missile is kinematically done and has no chance of hitting anything that isn't flying at the missile and trying to run into it. IE from a gameplay perspective the simulation could just let it go dumb once it hits these kinds of slow speeds, so the "realism" of it is just extra. IE I don't know if an AIM-9B can do that kind of thing, and it really isn't all that much again, think of a plane, when you are at max AOA at 100 knots, can you unload and point your nose down instead of up? That's all the AIM-120 is doing here, Its just loaded up in a turn, so it unloads and then loads up in the other direction. Any thing that has an AOA limit should be able to point its nose within that limit regardless of speed (assuming it has enough airflow over its control surfaces to be controllable).

The only way to improve on this is that IRL it probably would not be that stable at that AOA after the battery/autopilot system died, and would probably tumble out of stable flight eventually as transients made it pitch over a stable AOA, but again, I really see no point in modelling that in DCS since its quite a bit of extra work for a portion of the missiles flight that is incredibly irrelevant to gameplay. 

The much more important thing is getting the missile API good enough, and then getting all the older API missiles moved over onto it so they all have this kind of advanced 6DOF flight modeling going on.


Edited by KlarSnow
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