F-5E Center and Aft fuel cell capacity

[nairb121]

According to the -1 (https://docdro.id/51orrvC), the fuel system has 3 internal cells - the Left system is served by the Forward fuel cell, while the Right system is served by the Center and Aft fuel cells. The manual lists the capacities of each system, but I'm not able to find the individual capacities of the center and aft cells  - does anyone have information on this?

[Bucic]

Center and Aft are treated as one section, system-wise. My understanding is that this piece of information is not present because it's not useful. I'd love to lay my hands on AMM for the F-5E. They are rather absent for a grandpa like this.

On the other hand AMM's are not freely available for anything. Maybe it's by some governmental restrictions?

I've got plenty of helicopter maintenance documentation from my previous job but then again when I was on the road and needed some info I wasn't able to find any docs. Even though the helicopters were 100% civilian variants.

Edited August 30, 2023 by Bucic

[nairb121]

Weight and balance data would be good too (even better really). The reason I'm looking for this is that I believe there's an error in the modeling of the fuel consumption between the center and aft cells; Figure 2-2 in the -1 shows CG travel due to fuel consumption (also present in higher quality in the NATOPS):

However, based on my observations and testing, the CG movement with the system balanced is not consistent with the travel shown. Instead it travels much farther back, then rapidly back forward. I determined values for tank capacity and locations from the figure; however it would be much easier and more accurate if there were known true values I could use.

To match the travel shown in the figure, the center cell drains simultaneously with the aft cell, but empties first; this is consistent with the "CENTER CELL EMPTY" indicator shown. However, CG travel in DCS is consistent with a model in which the center cell drains completely before the aft cell begins to be used. I'm hoping to do some testing this weekend to produce clear data on this, so weight and balance data for the fuel cells (and the aircraft as a whole) would be extremely valuable.

[Bucic]

I don't think the capacity is stated anywhere. In fact the entire FM chapter on fuel system starting from 1-37 doesn't even mention center cell.

May I ask how exactly would you use such data (with FM page if possible)?

[nairb121]

The center cell is mentioned on 1-41 and shown in the figure 1-34 on page 1-42:

Using the known system capacities and the CG travel (in the previous post), I was able to determine approximate values for the tank positions and capacities, and produce a plot to confirm these value match the manual:

My previous testing was based on determining AOA for straight and level flight at the same stick position for various fuel states (set via mission editor and with unlimited fuel) - a tail-heavy aircraft would hold a higher AOA and lower airspeed, and a nose-heavy aircraft the opposite. In theory the graph of AOA vs. fuel state should follow the same trend as the CG shift shown above - but the trend was quite clearly different:

It offsets to the right similarly to the unbalanced trend; however, the fuel was not unbalanced (since it was unlimited and set by the editor, it couldn't be), and the point of the farthest aft CG was at a much higher fuel state. However, after some trial and error I discovered that it matches closely the plot of the CG if the center cell depletes completely before any consumption from the aft cell (the CG is significantly forward of the previous plot as the tests included ammunition):

My plan for followup testing this weekend is to do some takeoff roll tests - with neutral stick, the speed at rotation will be a function of the CG and the total weight of the aircraft.

[Bucic]

4 hours ago, nairb121 said:

It offsets to the right similarly

It's not clear to me what offsets to what. Just making sure you're not comparing a %MAC(fuel) graph with the AOA(fuel) graph.

Quote

The center cell is mentioned on 1-41 and shown in the figure 1-34 on page 1-42:

That may well be, but it isn't meant in any way that is actually significant to the pilot. Which coincides with lack of center cell specific indications in the cockpit. You may also take a look at the figure 1-34 to see that the center cell lacks any significant components. To all my knowledge it's "just a cavity." I'd love to learn about a possible deeper story behind it but for now I'm not convinced there's anything to it. Again - from the pilot's point of view.

Could you please remind me the location of the flight preparation charts in the FM where you go from loadout to a confirmation of safe mission CG location range? I'm on mobile lately. I assume that is your purpose in digging into balance changes. Is this correct?

 

Edited August 31, 2023 by Bucic

[nairb121]

1 hour ago, Bucic said:

It's not clear to me what offsets to what. Just making sure you're not comparing a %MAC(fuel) graph with the AOA(fuel) graph.

I was just referring to the way the CG/AOA trend toward the far right side of their respective graphs. And I am indeed comparing, or at least relating, the %MAC and the AOA graphs - the AOA in stabilized level flight at a given stabilator angle is purely a function of the CG location.

1 hour ago, Bucic said:

That may well be, but it isn't meant in any way that is actually significant to the pilot. Which coincides with lack of center cell specific indications in the cockpit. You may also take a look at the figure 1-34 to see that the center cell lacks any significant components. To all my knowledge it's "just a cavity." I'd love to learn about a possible deeper story behind it but for now I'm not convinced there's anything to it. Again - from the pilot's point of view.

 

Correct, it's just a tank that feeds by gravity alone into the aft tank. My only concern with it is its effect on the CG.

1 hour ago, Bucic said:

Could you please remind me the location of the flight preparation charts in the FM where you go from loadout to a confirmation of safe mission CG location range? I'm on mobile lately. I assume that is your purpose in digging into balance changes. Is this correct?

Unfortunately the manual refers to other documentation for CG calculations, which I have not been able to find. This would be much easier if I could.

My purpose in this is to confirm whether the modeling is correct, and provide solid evidence for a bug report if it's not. It's relevant because, if my hypothesis is correct and the tanks aren't being used in the right proportions, it causes the CG to be up to 3% MAC aft of where it should be, causing incorrect flight behavior.

[=475FG= Dawger]

Your supposition that aft CG will produce a lower airspeed is incorrect. 
 

Aft CG results in less tail downforce required, reducing the amount of induced drag produced by the horizontal stabilizer. 
 

The result is increased speed. 
 

CG does not determine AoA required for level flight. That is a function of weight and speed. 
 

CG affects the aerodynamic forces required to maintain the desired AoA. 

Edited September 1, 2023 by =475FG= Dawger

[nairb121]

4 hours ago, =475FG= Dawger said:

Your supposition that aft CG will produce a lower airspeed is incorrect. 
 

Aft CG results in less tail downforce required, reducing the amount of induced drag produced by the horizontal stabilizer. 
 

The result is increased speed. 
 

CG does not determine AoA required for level flight. That is a function of weight and speed. 
 

CG affects the aerodynamic forces required to maintain the desired AoA. 

The critical factor in this testing was the fixed position of the stabilators. As you said, the aft CG results in less tail downforce required in level flight at a given speed; however, with the tailplanes maintained in their position, an aft CG results in a nose-up moment for this reason, with the result of stabilizing at a higher AOA and lower speed. With the stabilator position removed as a variable, and in steady state, the pitch moment equation becomes a relation only of AOA/speed and CG position. 

However, this is the reason that I'm seeking to perform further, and more straightforward, testing to confirm before submitting a bug report. My intent is to test this based on rotation speed with neutral stick; V^2 will be proportional to Weight * CG distance forward of the wheels.