AIM-120 max lift coefficient issue

[stefasaki]

Hi everyone!

Even though this has been an issue for quite some time (since the last flight model update of the AMRAAM) I have now decided to ask about some clarifications over the behaviour of the AIM-120.

My concerns are mainly about the maximum lift coefficient of the missile. At the moment the AIM-120 is capable of pulling 35.1 G's at 1558 km/h TAS (as the picture below shows), which is a value that makes me raise my eyebrows a little bit. In fact, this value would imply (considering an empty weight of the missile of 75 kg) that the value of S*Cl is 0.25. This also means that the lowest speed at which the missile can sustain flight would be lower than 69 m/s (249 km/h or 155 mph). And I say lower than because the maximum lift coefficient at the fast edge of the transonic regime is quite substantially lower than the incompressible value.

Now,  I appreciate the hard work that you put in to come close to a high fidelity missile, but there has to be some kind of error in your CFD. The error may be subtle but is certainly a possibility, especially since CFD is not an exact tool and requires careful analysis to be trusted (I know this for a fact, having worked on CFD's myself). 

Instead of being just pedantic, I'd like to ask you to give me, or the community in general, the opportunity to look into the details of your calculations, as there may be someone capable of quickly identifying the culprit (I'd be looking at the mesh, most problems are there) and hopefully leading to a more trustworthy model. 

Pulling 35 g's while still in the transonic regime is a bit silly as a number, I am sure that if there are any aeronautical engineers in your team they would agree with this straight away. After all, if this were possible, the next supersonic transport would look exactly like an AIM-120.....

Edited November 30, 2021 by stefasaki
refined calculations

[Esac_mirmidon]

As far as i know the info bar is not a reliable source fot that kind of information. But i can be wrong about that

[Aquorys]

On 11/29/2021 at 11:50 PM, Esac_mirmidon said:

As far as i know the info bar is not a reliable source fot that kind of information

I'd also recommend to rather look at TacView. I have rarely seen missiles do any really hard maneuvering without failing, so I've always had the impression that the opposite is true: they can't maneuver well enough in DCS, and they're too slow as well.

[nighthawk2174]

On 11/29/2021 at 4:40 PM, stefasaki said:

Hi everyone!

Even though this has been an issue for quite some time (since the last flight model update of the AMRAAM) I have now decided to ask about some clarifications over the behaviour of the AIM-120.

My concerns are mainly about the maximum lift coefficient of the missile. At the moment the AIM-120 is capable of pulling 35.1 G's at 1558 km/h TAS (as the picture below shows), which is a value that makes me raise my eyebrows a little bit. In fact, this value would imply (considering an empty weight of the missile of 75 kg) that the value of S*Cl is 0.25. This also means that the lowest speed at which the missile can sustain flight would be lower than 69 m/s (249 km/h or 155 mph). And I say lower than because the maximum lift coefficient at the fast edge of the transonic regime is quite substantially lower than the incompressible value.

Now,  I appreciate the hard work that you put in to come close to a high fidelity missile, but there has to be some kind of error in your CFD. The error may be subtle but is certainly a possibility, especially since CFD is not an exact tool and requires careful analysis to be trusted (I know this for a fact, having worked on CFD's myself). 

Instead of being just pedantic, I'd like to ask you to give me, or the community in general, the opportunity to look into the details of your calculations, as there may be someone capable of quickly identifying the culprit (I'd be looking at the mesh, most problems are there) and hopefully leading to a more trustworthy model. 

Pulling 35 g's while still in the transonic regime is a bit silly as a number, I am sure that if there are any aeronautical engineers in your team they would agree with this straight away. After all, if this were possible, the next supersonic transport would look exactly like an AIM-120.....

 

Reaching up to 35G at that speed is probably not unreasonable we have gloading charts available for an AIM-9 with an AIM-120 style nose cone and much smaller fins and it was fully capable of reaching its structural G-limit of 40G at speeds as low as 1100kph. 

 

[stefasaki]

@nighthawk2174

Interesting, would you care to show such g loading charts? My guess is that for those charts they were accounting for motor thrust which would significantly increase the turning capability of such missiles, while my reasoning accounts just for the aerodynamic platform. A missile has a thrust to weight ratio that is so high that its thrust makes up for most of the centripetal force needed in a turn at low speed (at 30 deg of AOA half of the thrust is actually making you turn). That means that you just need 1100 kph in order for the fins to provide trim, but the actual turn is done by the rocket motor. 40 G’s at 1100 kph sound otherwise not believable, at least not without a thorough explanation.

Edited December 13, 2021 by stefasaki
added mention

[Hummingbird]

On 12/2/2021 at 7:27 PM, Aquorys said:

I'd also recommend to rather look at TacView. I have rarely seen missiles do any really hard maneuvering without failing, so I've always had the impression that the opposite is true: they can't maneuver well enough in DCS, and they're too slow as well.

I'd strongly advise against that, as TacView is known for sometimes showing wildly incorrect values. Meanwhile the infobar is a live recording/reading of exactly what is happening in the sim, and will therefore be as accurate as it gets; providing ofcourse you're in SP and not spectating someones else over MP were netcode becomes a factor.

 

Edited December 13, 2021 by Hummingbird

[nighthawk2174]

5 hours ago, stefasaki said:

@nighthawk2174

Interesting, would you care to show such g loading charts? My guess is that for those charts they were accounting for motor thrust which would significantly increase the turning capability of such missiles, while my reasoning accounts just for the aerodynamic platform. A missile has a thrust to weight ratio that is so high that its thrust makes up for most of the centripetal force needed in a turn at low speed (at 30 deg of AOA half of the thrust is actually making you turn). That means that you just need 1100 kph in order for the fins to provide trim, but the actual turn is done by the rocket motor. 40 G’s at 1100 kph sound otherwise not believable, at least not without a thorough explanation.

 

This is without the motor btw.

[Cmptohocah]

@stefasaki's screenshot shows an AMRAAM flying at ~1600km/h at 1000m alt, which roughly correlates to 1.3M.
On your AIM-9 chart @nighthawk2174 the missile can pull around 24-28G at that Mach no.
Yet his screenshot shows a 120 pulling 35G.
Or am I missing something here?

[nighthawk2174]

16 hours ago, Cmptohocah said:

@stefasaki's screenshot shows an AMRAAM flying at ~1600km/h at 1000m alt, which roughly correlates to 1.3M.
On your AIM-9 chart @nighthawk2174 the missile can pull around 24-28G at that Mach no.
Yet his screenshot shows a 120 pulling 35G.
Or am I missing something here?

The "variant" line is going to be more along the lines of an amraam due to being more similar in shape and at M1.3 its about 30-33G at M1.3.  Also the chart is for 10k feet or just over 3km.

Edited December 16, 2021 by nighthawk2174

[stefasaki]

On 12/14/2021 at 12:54 AM, nighthawk2174 said:

This is without the motor btw.

Ok, first thing first: a missile doesn't reach 40 g's at 1100 kph as you stated before, that wasn't logical at all. The true value at 1100 kph and 10 kft according to your graph is between 10 and 12 G's which is definitely a more reasonable number.

 

Then there's the original case: mach 1.3 at 3000 ft. Let's do a simple correction about the air density and your graph should still be useful. Rho at 3000 ft is 1.11 kg/m3 while at 10000 it becomes 0.9; therefore we can assume that your graph times  ~1.23 should yield a somewhat correct value.  The "variant" stands at roughly 25 (the line is closer to 30 but the graph is logarithmic) which is therefore corrected to 31. Now, since this value is around 15% less than what happens in the game, we can say that the model is in fact overestimating a bit.

 

Then there's the fact that your graph is not about an AMRAAM but refers to an unspecified AIM-9. To believe that this graph is somehow valid for both, we'd need at least their wing loadings, which for most Sidewinder variants it should be lower than that of an AMRAAM. This in turn would mean that the expected max G at a specified Mach of an AMRAAM should also be lower than that of such AIM-9, i.e. less than 31 g's at mach 1.3 and much less than what happens in the game. Let me know if you have an actual wing loading number,  we can only keep speculating otherwise.

 

If that graph happened to be for an AIM-9M (surely not an -X) we can expect a wing loading which is about 30% lower than that of an AMRAAM, a value which would linearly correct the max G to 22 G's for my original test case. This is the value I'd actually expect from an aim-120c at 3000 ft and mach 1.3.

Edited December 16, 2021 by stefasaki
grammar

[nighthawk2174]

5 hours ago, stefasaki said:

Ok, first thing first: a missile doesn't reach 40 g's at 1100 kph as you stated before, that wasn't logical at all. The true value at 1100 kph and 10 kft according to your graph is between 10 and 12 G's which is definitely a more reasonable number.

Original posts meant to say 1600kph like in your original image that was a mistake on my end.  And that's the number I was working with when writing up my posts.  I accidently just wrote 1100kph sorry about that.

5 hours ago, stefasaki said:

Then there's the original case: mach 1.3 at 3000 ft. Let's do a simple correction about the air density and your graph should still be useful. Rho at 3000 ft is 1.11 kg/m3 while at 10000 it becomes 0.9; therefore we can assume that your graph times  ~1.23 should yield a somewhat correct value.  The "variant" stands at roughly 25 (the line is closer to 30 but the graph is logarithmic) which is therefore corrected to 31. Now, since this value is around 15% less than what happens in the game, we can say that the model is in fact overestimating a bit.

From the standard atmosphere table air density for standard day at 3000ft is ~1.1226 kg/m^3.  Also fair some value between 25 and 30G is probably accurate for M1.3 for the chart.  However when brought down to 3000ft from 10000ft 30-35g is probably accurate and is why I said 30-33G in my last post.  I probably should have clarified that.

5 hours ago, stefasaki said:

Then there's the fact that your graph is not about an AMRAAM but refers to an unspecified AIM-9. To believe that this graph is somehow valid for both, we'd need at least their wing loadings, which for most Sidewinder variants it should be lower than that of an AMRAAM. This in turn would mean that the expected max G at a specified Mach of an AMRAAM should also be lower than that of such AIM-9, i.e. less than 31 g's at mach 1.3 and much less than what happens in the game. Let me know if you have an actual wing loading number,  we can only keep speculating otherwise.

I'm not saying its completely valid but rather that it is a rough indication that the amraam's performance in game is not impossible.  That's always what I meant by the use of the charts essentially just to say hey we have a rough analog here and it can reach nearly to the loading values seen in game at roughly the same speeds, when the fact the chart is for 10,000ft is considered.  There are additional factors, which I'll mention below, that can make up the difference if any.

5 hours ago, stefasaki said:

If that graph happened to be for an AIM-9M (surely not an -X) we can expect a wing loading which is about 30% lower than that of an AMRAAM, a value which would linearly correct the max G to 22 G's for my original test case. This is the value I'd actually expect from an aim-120c at 3000 ft and mach 1.3.

Something else to consider is iirc the 120's fins can deflect more than the sidewinders but don't quote me on that.  I want to say in game its 25deg, I could ofc be very wrong.  But if I am correct this would allow the missile to pull higher aoa and as such a get a higher Cl.  Tail control instead of nose control also would help getting to higher aoa's as well iirc.  Something else for the chart as well is that as far as I can tell this is for 0deg of roll.  And that for this paper 0deg roll means that if you were to look from the rear of the missile the fins would look like a + where if they were X in orientation you would get additional lift from the missile increasing the Gload further.

Edited December 17, 2021 by nighthawk2174

[stefasaki]

There’s another thing though which could limit the AMRAAM performance compared to the AIM-9 aside from the higher wing loading (I’ll try to do rough calculations to get some numbers about this as soon as I can): that is trim control. On the AIM-9 both surfaces are producing positive lift when in a turn, due to the CG being in the middle, while on an AMRAAM the rear wings most likely just provide trim control (likely a downforce as the CG is forward of the main wing but could also be a small positive lift if the nose cone develops a strong enough force). This further decreases the effective lifting surface of the missile. It’s true that the max AOA of the AMRAAM is likely much higher than that of an AIM-9 but in my opinion it should not be enough to compensate for the difference in effective wing loading between the two missiles (at very high AOA the lift coefficient curve is almost flat for low aspect ratio lifting bodies). I stand by my opinion that the max lift coefficient seems overestimated in the game by at least 15% (up to 50% for worst case scenario). Of course we should have a real manual with experimental data to confirm this, as at the moment CFD’s for highly separated flows are definitely not reliable, even with industrial level resources (read this as “do not try to do this CFD at home”)

 

@nighthawk2174 Just checked the geometrical wing loading (considering the area of the whole planform) : the aim-9 "variant" has a 26% lower wing loading compared to the aim-120C. If you account for the fact that the rear surfaces likely just provide trim on the AMRAAM, the difference in effective wing loading reaches at least 40%. This also means that for a specified lift coefficient and velocity the resulting g loading of the AIM-120C should be at least 40% less than that of the aim-9 "variant". Even agreeing that the AIM-120 is certainly capable of a higher max lift coefficient, this difference cannot be in the order of 40%. 

Edited December 23, 2021 by stefasaki
Added paragraph

[nighthawk2174]

Its not all about wing loading, the variant AIM-9 from the paper can produce substantially more G and for a longer period of time despite having smaller fins.  Especially as altitude goes up.

[stefasaki]

13 hours ago, nighthawk2174 said:

Its not all about wing loading, the variant AIM-9 from the paper can produce substantially more G and for a longer period of time despite having smaller fins.  Especially as altitude goes up.

We were comparing missiles with similar aerodynamic configurations, for which wing loading becomes the main difference. Comparing the standard case and the "variant" is pointless since their nose cone is very much different and as such this assumption cannot be made.

 

also the "variant" is lighter and the geometrical wing loading (considering the body) is not that different

Edited January 3, 2022 by stefasaki

[Cmptohocah]

Please correct me if I am wrong, but what does AIM-9 have to do with the AIM-120 performance.
Am I missing something here?

[GGTharos]

It was theoretical (wind tunnel study) a conversion of an AIM-9 to a radar guided missile, so closer in shape to an AIM-120.