Loadout and roll rate question

[Furiz]

I was wondering if the roll rate is correct for the Viper, the loadout is like this

 

Aim120c - Aim9x - AGM65G - Fuel -Litening - no pylon - empty cheek station - Fuel - 2x GBU-38 - Aim9x - Aim120c

 

So what hapens is that my Viper is rolling to the side of the 2x GBU38 station, the bombs and its rack are heavier than the maverick side by about 10lbs. (as far as i could calculate that in the ME)

 

LAU117 + AGM65G + Litening = 1253lbs

BRU 57 + 2x GBU-38 = 1263lbs

 

The rest of the loadout is the same.

 

I know the position of the litening is close to the center of gravity, but for me that roll rate is too high,

 

I'm no expert but should it be that way?

[TobiasA]

The TGP is further inside, do the resulting roll force is less.

So to speak, the resulting lever arm is shorter. The further out a store is mounted, the more roll torque it will induce. The roll momentum of the TGP is very small, you can see that when loading a TGP on a symmetrical loadout. 

[Furiz]

Yep that is true, the TGP is further inside and it creates less of a lever force, but I still think it is a too much of a roll on the GBU-38 side. I would test but I have no idea how

[Machalot]

If you know the span-wise position of each store you can calculate the mass center location to determine which way it should roll.  Since it's mostly symmetrical, you can just focus on the asymmetries.  So 10 lbs times the centerline distance to the GBUs, and the TGP weight times its centerline distance. Whichever is greater, it will roll to that side. 

 

To estimate the roll angular acceleration from this asymmetry would require knowing the roll moment of inertia (Ixx, "eye ex ex") of the whole jet as configured, which I don't know how to get. 

[Furiz]

Yea the last part is the difficult one, I know it will roll to the 10lbs side for sure, but don't know the acceleration, or should FWB system correct that.

[Machalot]

One of the SMEs on here told me there's no auto roll trim in the FLCS.

I have a textbook with a partial F-16 dynamics model in it.  I googled it and found this paper that reproduces some of the quantities.  Check out page 5, moments of inertia!

https://archive.siam.org/books/dc11/f16/Model.pdf

 

Edited July 3, 2021 by Machalot
moi

[Furiz]

So i did a little test, same loadout as before, only removed 2 of those aim9x and their pylons,

but now I dropped a JDAM, the aircraft should now be rolling to the Maverick side since that side is heavier when JDAM (500lbs) is dropped, cause the weight difference was 10lbs with full loadout, but what happens is that the aircraft is still slightly rolling to the GBU-38 side.

 

F-16 roll issue.trk

[Machalot]

How confident are you in the weights you listed earlier? 

[Furiz]

Used calculator with mission editor,

every time I added something, like a Mav G + its rack for example I got its weight there, presuming those values are correct in the mission editor, on the GBU-38 label it says 500lbs, so if you drop one you just subtract 500 from 1263 (2x Gbu-38 + BRU-57) and you get the current weight on that station.

 

Then the difference of weight is on the AGM-65G side by 753lbs, the aircraft should then roll to the AGM-65G side, but it is not rolling there, it is rolling towards the GBU-38 side, slightly but its rolling.

 

I think that needs to be looked at, maybe even more weapons are having incorrect weight values.

 

Or I'm missing something with the weight calculations.

[Machalot]

Try reversing your loadout left to right and see if you get the same result? 

[Furiz]

I tried that, when I load 2x GBU-38 on the TGP side it is also rolling to the GBU-38 side (which is the heavier side), after I dropped one of the GBU-38 it slowed the roll rate but continued rolling to that side.

[Machalot]

So either the GBUs are heavier than advertised, or the Mavs are lighter, or there is an aerodynamic effect we aren't aware of. 

[TobiasA]

It is the distance from the center of lift. Store is further out -> more momentum. The momentum of the TGP is almost not there, don't use the whole weight of the TGP in this equation.
It is not the same if you carry the weight on the outside or inside.

 

Test the following: Mount a Mark 84 on station 3 and another one on station 6. Nothing else. Same weight left and right, but one bomb is further out. It will roll into the direction of the outer store since this one will induce a higher roll momentum.

 

This is correct.

[Frederf]

BRU+2xJDAM is 1552 lbs.

LAU+MaverickG is 1082 lbs.

 

That's including the suspension gear of 289 lbs. which they share in common to mount to the wing. Subtracting this the JDAM load adds 1263 lbs. and MaverickG 793 lbs.

[Machalot]

22 minutes ago, TobiasA said:

It is the distance from the center of lift. Store is further out -> more momentum. The momentum of the TGP is almost not there, don't use the whole weight of the TGP in this equation.
It is not the same if you carry the weight on the outside or inside.

 

Test the following: Mount a Mark 84 on station 3 and another one on station 6. Nothing else. Same weight left and right, but one bomb is further out. It will roll into the direction of the outer store since this one will induce a higher roll momentum.

 

This is correct.

We're good with the mass moments. The remaining issue is that the single GBU seems to be heavier than the Mav but maybe it shouldn't be according to the mission editor weight values. @Frederfhad some different numbers so @Furizneeds to recheck.

[TobiasA]

vor 3 Minuten schrieb Machalot:

We're good with the mass moments. The remaining issue is that the single GBU seems to be heavier than the Mav but maybe it shouldn't be according to the mission editor weight values. @Frederfhad some different numbers so @Furizneeds to recheck.

I don't think you understood what I said, but probably because I am no native speaker. The further out a store is, the more roll torque it will create. Since the TGP is on the inside, its weight does not count fully. 

You can't just compare weight, you need to compare torque. 

[Furiz]

4 minutes ago, TobiasA said:

I don't think you understood what I said, but probably because I am no native speaker. The further out a store is, the more roll torque it will create. Since the TGP is on the inside, its weight does not count fully. 

You can't just compare weight, you need to compare torque. 

 

Yes we understand that, what we don't understand is when you drop one of the GBU-38 you lose 500lbs on one side, why is it still rolling to that side, while the other side is now heavier. Even the Mav+LAU without TGP is heaveier than single GBU38+BRU

Edited July 4, 2021 by Furiz

[Furiz]

35 minutes ago, Machalot said:

We're good with the mass moments. The remaining issue is that the single GBU seems to be heavier than the Mav but maybe it shouldn't be according to the mission editor weight values. @Frederfhad some different numbers so @Furizneeds to recheck.

 

What I did was that I entered ME and Loaded my plane with Aim120c on each wingtip, don't ask me why:P With full fuel and chaff/flare and full gun ammo, pylons are all mounted, weight of the aircraft is 29635lbs.

 

Adding AGM-65G + LAU117 = 30428

                                                 -29635

                                                = 793 = AGM-65G + LAU117 weight

 

Adding 2xGBU-38 +BRU57 = 30898

                                                -29635

                                              = 1263 = 2xGBU-38 +BRU57 weight

 

Adding Litening TGP            = 30093

                                               - 29635

                                               = 458 = Litening weight

 

Those are the mission editor numbers.

 

 

AGM-65G + Litening = 1251

2x GBU-38                 = 1263

 

So that means it is heavier on the GBU side for 12lbs.

 

Yes the TGP is inside, and it should be rolling to the heavier side, I just thought it rolls too fast.

But what happens when you drop one of the GBU-38 (500lbs) is that the aircraft still rolls on the GBU-38 side even tho there is now 488lbs more weight on the AGM-65G side.

 

Or the weight of TGP is not creating that much of a lever as it is too close to the center, Frederf didn't even mention TGP.

Edited July 4, 2021 by Furiz

[Machalot]

35 minutes ago, Furiz said:

Or the weight of TGP is not creating that much of a lever as it is too close to the center, Frederf didn't even mention TGP.

Ok, I somehow thought you had removed the TGP to compare the reverse loadout.  If you were always counting the weight of the TGP then @TobiasAis right, and you/we have not properly calculated the mass moments.  The TGP probably has only a 10% contribution compared to the other stores due to its location. 

[Frederf]

From editor:

 

No weapons weight

With just station 3 pylon and MAU-12

With GBU-38x2 on station 3

With AGM-65G/LAU-117 on station 3

With TGP

You want to compare the lateral moments so each piece of hardware contributes according to its weight and lateral position. The station positions are in a document I don't have but you can take a picture of the F-16 and measure the positions in pixels compared to the overall wingspan.

 

Let's see, halfwingspan call 100%, station 3 is 65.6% of that, station 5R is about 14% of that. Let's call a pound times a % demiwingspan 1 unit

 

Arms:

station 3 (GBU-38x2) -1552*65.6

station 5R (TGP) +459*14

station 7 (Maverick G) +1082*65.6

 

-101811.2

+7686

+70979.2

total of 23146 units moment to the left

 

As percentage with the GBUs being 100%, Maverick is 70% and TGP is 7.5% in their respective directions.

Edited July 4, 2021 by Frederf

[Furiz]

Ok that is all correct, I only removed the pylon weight cause they are the same weight, it should be rolling on the GBU-38 side but why is it rolling to that side when you drop one of the GBUs, you lose 500lbs,

 

from Frederf calc

 

AGM-65G = 1082lbs

GBU-38x2 = 1552lbs

                     - 500lbs - 1 dropped GBU-38

                     = 1052lbs

 

So why is it still rolling to GBU-38 side? when now the AGM-65G side is heavier + the litening is on that side too, which adds a little bit as well.

[TobiasA]

vor 12 Stunden schrieb Furiz:

Ok that is all correct, I only removed the pylon weight cause they are the same weight, it should be rolling on the GBU-38 side but why is it rolling to that side when you drop one of the GBUs, you lose 500lbs,

 

from Frederf calc

 

AGM-65G = 1082lbs

GBU-38x2 = 1552lbs

                     - 500lbs - 1 dropped GBU-38

                     = 1052lbs

 

So why is it still rolling to GBU-38 side? when now the AGM-65G side is heavier + the litening is on that side too, which adds a little bit as well.

Frederf explained it so nice.
You are not taking the distance from the center of the lift (read: middle axis of the plane) and the station itself into account, you are just putting weight into the equation.

It is not about the weight, it is about momentum or torque, whatever is easier to understand. 500 pounds on the outer station create more torque than 500 pounds on the inner station.

[Furiz]

23 minutes ago, TobiasA said:

Frederf explained it so nice.
You are not taking the distance from the center of the lift (read: middle axis of the plane) and the station itself into account, you are just putting weight into the equation.

It is not about the weight, it is about momentum or torque, whatever is easier to understand. 500 pounds on the outer station create more torque than 500 pounds on the inner station.

 

are you even reading the post?

 

Mav on Sta 7 is at the exact oposite spot from Sta 3 where GBU-38 x2 are

when you drop 1 of GBU-38s you lose the weight so that station now holds 1052lbs while Mav holds 1082 lbs, so you tell me which side is heavier now?

[Frederf]

Pylon, MAU-12 289 lbs

Previous plus 1x GBU-38 820 lbs (+531)

Previous plus 2x GBU-38 plus BRU-57 1552 lbs. (+531 +531 +201)

 

1082 lbs vs (1552-531=1021) suggests that the station 7 side is heavier but notice that GBU-38 does not fall from the middle of station. With Mav/7, GBU/3 the first GBU falls from the inboard station. This leaves the outboard bomb which contributes more to the roll moment than the inboard bomb did. Actual analysis would require measurement of the arm lengths down to the detailed level of the inboard and outboard sides of the BRU-57. Unfortunately for testing purposes the reverse config Mav/3, GBU/7 doesn't drop the outboard bomb first so I can't compare that way. I don't seem to be able to S-J or otherwise expend the other bomb first.

 

The difference of 61 lbs is 6% of the average of the two weights. Is the difference between the middle of station 3 and the center of outboard GBU-38 more or less than 6%? I don't know.

Edited July 5, 2021 by Frederf

[TobiasA]

vor 57 Minuten schrieb Furiz:

 

are you even reading the post?

 

Mav on Sta 7 is at the exact oposite spot from Sta 3 where GBU-38 x2 are

when you drop 1 of GBU-38s you lose the weight so that station now holds 1052lbs while Mav holds 1082 lbs, so you tell me which side is heavier now?

You forget that the GBU-38 is about 590 lbs since it is not only the warhead but also guidance. The TGP is about 450lb, but only about 1/5th of the distance to the center, so it can be accounted with 90lb. This would in total give a slight advantage to the mav side, but the inner JDAM is released first so the outer JDAM wins.
 

Edited July 5, 2021 by TobiasA
typo