F-14B Fuel Consumption Model – Part II: AoA, Flaps and Consistency

Before getting to the actual model and its considerations, a few words about some related topics.

Degrees of AoA and Units of AoA

We are familiar with what the AoA is (in case you are not, here is a quick link to Wikipedia) but the F-14’s Angle of Attack indicator works in Units of AoA.
In this model I am reporting both the Units of AoA and the Degrees of AoA (UAoA and °AoA from now on), the latter is taken from the bar displayed in the external views, along other information (RPM, Pitch, IAS, etc).
I haven’t found many sources describing the relation between UAoA and °AoA, then I run into this Jabbers‘ comment (source):
fuel-model-results-2-jabbers-youtube

In the following example, the °AoA reported by DCS is 3.2°; the UAoA provided by DCS-BIOS is 6.82 and the UAoA calculated by means of the formula above is 7.53.
This is the in-game AoA Indicator:
fuel-model-results-2-aoa-indicator
The fact that the °AoA values do not come from the avionics itself may explain why they do not match the equation. That being said, whenever you find a UAoA value in this model, it comes from DCS-BIOS.

The UAoA is a very important indicator because, according to the manual:

The indicator has markers on the right for climb (5), cruise (8.5), and stall (29), and a reference bar for on-speed approach (15).

This model can show if the conditions for optimal cruise match, depending on the payload, DI and so on.

Sharpening the tools

fuel-model-results-2-tft

In order to make my life easier, the data-gathering faster and more precise, I added a couple more features to my DCS-BIOS TFT, namely the AoA but also the Inner and Outer needles. The raw values are a good means to understand if the speed is constant or changing. These makes the reading more precise and much faster.
On top of that, these values eliminate the parallax error, a problem familiar to anyone used to analogue measuring tools.

Challenging the Consistency

Before collecting all the values I needed, I decided to test how reliable my modus operandi is. I decided to work at an altitude of 20,000ft.
I gathered the values in three ways:

  1. 20000 P: “Polling”, by spawning at a certain speed, letting Iceman stabilize and annotating the values;
  2. 20000 U1: “Upwards”, by spawning at 250kts, setting the altitude autopilot and collecting data every +25 kts (with unlimited fuel);
  3. 20000 D1: “Downwards”, by spawning at 550kts, setting the altitude autopilot and collecting data every -25 kts (with unlimited fuel).

NOTE: I haven’t collected all these values in one go, but for the majority of times, a wind from W, speed 003 was reported in the F-14 display. The aircraft heading is 291°. Odd since I didn’t set any wind in the mission.

fuel-model-results-2-consistency-fuel-flow
fuel-model-results-2-consistency-fuel-flow-chart

°AoA, Pitch and RPM are taken from the F2 view. I am not sure if Pitch and RPM have a purpose in this model but better have more data than less, isn’t it?
fuel-model-results-2-consistency-deg-aoa
fuel-model-results-2-consistency-chart-deg-aoa

fuel-model-results-2-consistency-pitch
fuel-model-results-2-consistency-chart-pitch

fuel-model-results-2-consistency-rpm
fuel-model-results-2-consistency-chart-rpm

The UAoA from the in-cockpit AoA Indicator is obtained by means of DCS-BIOS.
fuel-model-results-2-consistency-dcs-bios-uaoa
fuel-model-results-2-consistency-chart-dcs-bios-uaoa

This is the ratio between UAoA and °AoA.
fuel-model-results-2-consistency-uaoa-vs-aoa
fuel-model-results-2-consistency-chart-uaoa-vs-aoa

The values look very consistent, especially at lower speeds. At higher speeds instead, a movement of the throttle in the order of a few tenths of millimetres can swing hundreds of lbs/h. There is only one, very visible difference at 325kts.

Further testing and observations

The series 20000 D2 is an attempt to check why, at 325kts, one of the values spikes in such way. Flying with the stick hidden I didn’t notice it in the beginning, but eventually I saw it quickly moving from a heavily forward-trimmed position to neutral. I did some other tests and finally I pictured the behaviour of the trimmer by means of the Controls Indicator.

fuel-model-results-2-observation-trim
Behaviour of the Trim with Alt AP activated

I plotted the values of TRIM, UAoA and the Fuel Flow:
fuel-model-results-2-observation-data

fuel-model-results-2-observation-tft-last

I decided to go deeper into the problem (some of you Pilots may have already understood what’s going on here) so I expanded my TFT again to check Wing Sweep, Left and Right Stabilizers, Manoeuvring Flaps and Slats.

I spawned at GS 350kts, reached 560kts then went back to 340kts and slowly finished at 250kts.
The following is the resulting table, reordered by GS:

fuel-model-results-2-observation-new-data
New data gathered: 20,000ft, GS between 350kts and 560kts and 340kts to 250kts.

The reason of the sharp change in the fuel and UAoA trends is the toggling of the manoeuvring flaps. It is also interesting to note that the manoeuvring flaps are actuated at different speeds in different test runs, meaning there are other conditions involved.

The following is the chart of the Fuel Flow. It is consistent with the previous tests, yet it is much more detailed.
fuel-model-results-2-observation-new-fuel-flow

fuel-model-results-2-observation-new-uAoA

fuel-model-results-2-observation-new-wings-sweep

fuel-model-results-2-observation-stabs

NOTE: The difference between the Left and Right stabs is probably due to the mentioned wind reported on the HSD.

Do the effect of the manoeuvring flaps affect the model? No, in my opinion. Since the goal of the model is providing a reference consumption value and the amount of parameters that affect the fuel usage are so many, such difference is quite marginal. 300lbs/h per engine = 600lbs per hour considering both the F110-GE-400.
Moreover, I can use the results from other tests whenever I run into this “trim-oddity”, using the most frequent occurrence of the data.
Finally, a mission is hardly flown by means of constant Alt AP and therefore the aircraft is handled by the pilot. He can adjust the parameters to optimize the endurance, if necessary.

That being said, if you are flying at constant speed and notice a high UAoA and high fuel consumption, try playing with the trim and power settings a little. You may improve your fuel endurance.

fuel-model-results-2-manoeuvring-flaps

Manual testing from T/O to 20,000ft

Now that the model is completed for the altitude of 20,000ft, the last test consists in departing from the ground (I did it hot from the runway), get to 20,000ft and accelerate and decelerate to certain speeds and see what happens. If the values match (a certain tolerance must be taken into account) the data previously collected, then it means that the modus operandi works, the model is consistent and I can proceed and apply the same methodology to different loadouts and DI.

fuel-model-results-2-manual-test
Values gathered from T/O to 20000ft

The following chart shows the comparison between the manually gathered values and the model. The values haven’t been adjusted for the altitude, and the “No AP” data were collected without using any Autopilot, therefore they may be not entirely correct since it is hard to keep a non-FBW perfectly level at a defined speed.
fuel-model-results-2-manual-chart

Clapping wings

Out of curiosity, I also gathered some data whilst the wing sweep lever was manually set to 20° and 68°. These are the results:

fuel-model-results-2-wing-sweep-20-deg
20° Manual Wing Sweep

fuel-model-results-2-wing-sweep-68-deg
68° Manual Wing Sweep

And here are a couple charts to better understand the behaviour:

fuel-model-results-2-wing-sweep-compare-chart
Fuel consumption for Auto sweep, 20° and 68°.

fuel-model-results-2-wing-sweep-uaoa-compare-chart
Units of AoA for Auto sweep, 20° and 68°.
fuel-model-results-2-wing-sweep-chart
Wing sweep for Auto sweep, 20° and 68°.

As expected, the Wing sweep 20° (WS20°) matches closely the Auto Sweep until the GS gets above Mach .70. At that point, the wings create so much drag that the performance crumbles. The Wing sweep 68° (WS68°) instead is suboptimal at low speed but it gets better as the speed increases. Eventually, I assume it overlaps the Auto sweep performance curve.
The Units of AoA are more interesting as they show the automatic manoeuvring flaps toggled at three different moments.


The next part will get into the details of the model, starting with the same loadout used in these tests (CAP).

One comment

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.