Roll after launching AIM-9X

[lester]

Could someone explain me why Hornet starts rolling after launching AIM-9X? I also have AIM-7M and there's no anything close to that when I launch them (while 7M is much heavier I believe). Track is attached. I didn't observe such behavior earlier IIRC. Thanks

f18_7m_9x_trim.trk

Edited December 30, 2019 by lester

[falcon_120]

The hornet do some weird thing at times regarding weapon launches and roll tendency, I guess its WIP.

[lester]

Should it be moved to bugs then? It makes WVR fight involving 9x launch really annoying :(

[falcon_120]

Should it be moved to bugs then? It makes WVR fight involving 9x launch really annoying :(

I think so, that rolling is excesive from my non expert pov. OTH you should try a repair first because i cant recall experiencing such a extreme roll after aim 9 launches (sorry not at home for some days so i cant test it myself).

 

Enviado desde mi SM-G950F mediante Tapatalk

[CBStu]

I see some roll but nothing I feel is excessive. I usually just keep flying because I will be launching another 9 pretty soon.

[lester]

I see some roll but nothing I feel is excessive. I usually just keep flying because I will be launching another 9 pretty soon.

 

 

CBStu do you observe the same roll rate as in the track attached/video? Why didn't I see such roll (with even greater rate) after launching 7M?

[Fri13]

Hmmm..... I have nothing like that...

[lester]

Not sure it happens with every loadout/action sequence though. I did drop the fuel tank before launching 9X and used rather non-standard loadout of 7M+9X (plus used JHMCS - I don't think it's related, but who knows). Could someone try to reproduce it with the same actions/loadout please?

Edited December 23, 2019 by lester

[Sydy]

Could someone explain me why Hornet starts rolling after launching AIM-9X? I also have AIM-7M and there's no anything close to that when I launch them (while 7M is much heavier I believe). Track is attached. I didn't observe such behavior earlier IIRC. Thanks

 

 

 

Hi,

 

I watched the video and saw a normal behavior IMHO. You launched the right wingtip AIM-9 and the the acft rolled left. Normal. First because of the weight asymmetry and above all by the difference of induce drag due to asymmetric load. Many acft present this characteristic more than others. In some you even have to compensate with stick to the opposite side. Trim it you and life goes on.

 

All the best,

 

Sydy

[lester]

I watched the video and saw a normal behavior IMHO. You launched the right wingtip AIM-9 and the the acft rolled left. Normal. First because of the weight asymmetry and above all by the difference of induce drag due to asymmetric load. Many acft present this characteristic more than others. In some you even have to compensate with stick to the opposite side. Trim it you and life goes on.

 

Hi Sydy. Same question for you:

 

CBStu do you observe the same roll rate as in the track attached/video? Why didn't I see such roll (with even greater rate) after launching 7M?

[CBStu]

That is way more roll than I ever get. On the other hand I am usually cranking and banking all over the place so maybe I just don't notice it.

[lester]

OK, here is the video showing roll rate difference between 7M (~230 kg, 510 lbs) and 9X (~ 85 kg, 185 lbs) after launching:

 

 

and this is the diff between 120C (~150 kg, 350 lbs) and 9X (~ 85 kg, 185 lbs) (also note that I didn't drop the fuel tank in this case):

f18_120c_9x_trim.trk

Edited December 30, 2019 by lester

[lester]

On the other hand I am usually cranking and banking all over the place so maybe I just don't notice it.

 

 

I do too, that's why I initially couldn't understand what went wrong and why I needed to fight with the roll during intensive maneuvering. AIM-9x launch looks as if I would have launched AGM-62 Walleye instead.

Edited December 25, 2019 by lester

[turkeydriver]

I’d think(not researched) that rocket thrust blasted back toward a wing surface will case a large momentary increase in lift on that surface.

[GGTharos]

The 9 is sitting further away from the center of gravity than any other missile that the hornet carries so it has the longest moment arm on the aircraft. Perhaps the demonstrated amount of roll is excessive, but you should definitely be seeing it.

[lester]

I’d think(not researched) that rocket thrust blasted back toward a wing surface will case a large momentary increase in lift on that surface.

 

 

Hmm, maybe - but I didn't the see same behavior on previous OBs (and in Viper but its FM is even more WIP than Hornet's one).

[lester]

The 9 is sitting further away from the center of gravity than any other missile that the hornet carries so it has the longest moment arm on the aircraft. Perhaps the demonstrated amount of roll is excessive, but you should definitely be seeing it.

 

 

Hey GGTharos, thank you for reply! You are absolutely right and I should expect some roll. But the similar moment should be applied after 9M missile launch as it has similar weight, correct? Here is the video with the roll after 9M launch:

f18_120c_9m_trim.trk

Edited December 30, 2019 by lester

[GGTharos]

Interesting, seems like some sort of payload bug ... those 9Xs might be a bit heavy.

[Stubbies2003]

Lester you also caused testing discrepancies by not launching from the same wing station on the last two vids as the first one. So hopefully this terminology is somewhat similar to the F-16. Your first AIM-9 shot was from either station 2 or 8. Almost at the wing tip. Your second and third videos were station 3 or 7 shots which are closer to the centerline of the aircraft thus like what GGTharos pointed out. Even a 9X versus a 9X comparison would show different results based on where the lost drag/weight is at.

 

Now that being said I do agree that even with the location of the missile that seems to be an excessive amount of roll input. I've never seen that myself but I also never carry AIM-9s on that station. Always 120s.

[Sierra99]

OK, here is the video showing roll rate difference between 7M (~230 kg, 510 lbs) and 9X (~ 85 kg, 185 lbs) after launching:

 

 

 

and this is the diff between 120C (~150 kg, 350 lbs) and 9X (~ 85 kg, 185 lbs) (also note that I didn't drop the fuel tank in this case):

 

 

 

The 7M is heavier...but mounted far closer to the lateral center of gravity than a 9X on the wingtip is. I don't know where to look in NATOPS but I am sure there is a formula for computing lateral imbalance. I'm pretty sure it would show that releasing an Aim-9 from a wingtip results in the loss of more lateral moment than releasing an 7M from the cheek station does. The larger the change in lateral moment...the more pronounced roll reaction.

 

(Please note I am not saying or implying there isn't an issue...just offering a possible reason for the difference you see.)

[Sydy]

Guys,

 

Pylon launch does not have the same effect of the wing tip launch. The main deal is not weight, but induced drag.

 

Here goes a couple of nice readings:

 

https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/induced.html

 

https://www.key.aero/forum/modern-military-aviation/88842-wingtip-missile-rails

 

Take care,

 

Sydy

[The Falcon]

Guys,

 

Pylon launch does not have the same effect of the wing tip launch. The main deal is not weight, but induced drag.

 

Here goes a couple of nice readings:

 

https://www.grc.nasa.gov/www/k-12/VirtualAero/BottleRocket/airplane/induced.html

 

https://www.key.aero/forum/modern-military-aviation/88842-wingtip-missile-rails

 

Take care,

 

Sydy

So is this an increase in lift on the wing from where the missile was launched? I await the intervention of some dev

[lester]

Lester you also caused testing discrepancies by not launching from the same wing station on the last two vids as the first one. So hopefully this terminology is somewhat similar to the F-16. Your first AIM-9 shot was from either station 2 or 8. Almost at the wing tip. Your second and third videos were station 3 or 7 shots which are closer to the centerline of the aircraft thus like what GGTharos pointed out. Even a 9X versus a 9X comparison would show different results based on where the lost drag/weight is at.

 

Now that being said I do agree that even with the location of the missile that seems to be an excessive amount of roll input. I've never seen that myself but I also never carry AIM-9s on that station. Always 120s.

 

 

Hi Stubbies2003,

 

In the very first video of this thread I launched just one 9X from station 2 or 8 (as you correctly pointed out) to show enormous (imo) roll rate.

 

In the next two videos I tried to compare roll rate induced after launching 7M/120C and 9X. I do agree that you can't directly compare it here as different stations were used (3/7 for 7M/120C and 2/8 for 9X).

 

The very last video though shows similar launch parameters but with 9M instead of 9X: 3/7 for 120C and 2/8 for 9M and difference in roll rate is significant (imo).

[lester]

You launched the right wingtip AIM-9 and the the acft rolled left

 

Pylon launch does not have the same effect of the wing tip launch.

 

Hi Sydy,

 

In all videos published in this thread I didn't launch 9X/M from wing tips (stations 1/9). Maybe I missed something or maybe stations 2/8 are also considered wingtip stations?

 

Thanks for the links btw.

Edited December 26, 2019 by lester

[lester]

Added two videos with just 9X/9M launch from the same station:

f18_9x_launch.trk

f18_9m_launch.trk

Edited December 30, 2019 by lester