We figured we’d write-up a full and detailed breakdown of the latest DCS: F-14 flight modelling changes, our assumptions, methodologies and some of the theory behind the why and how. We love to share and learn together as a community and enthusiasts, and considering these changes impact the way you fly and fight the F-14; why not have you as part of this last step of our development journey.
We’ve spent a considerable amount of time recently perfecting and tuning the last parts of the flight model for our F-14. First, we’ve focused entirely on the F-14A; the TF-30 powered version of the aircraft. To give you a better overview of the how and why, let’s do a quick deep dive and give you some background.
Enjoy the read and get in touch if you have questions!
At Heatblur; we have a number of tools to help us verify and test our flight models, and over the past few years we’ve built an entirely new suite, which combines all of our experience over the past few years. However, we do still spend plenty of time testing the flight models in-game, as ultimately, this is where end-users will experience flying the aircraft. It’s more time consuming and less efficient than external automatic tools, but it acts as an extra buffer of safety, to ensure that any external factors inherent to the simulator itself do not skew the results.
An advantage of using an in-game method of testing perfectly accounts for DCS’ calculated stores drag, which is an achilles heel of the F-14 NATOPS performance data (there’s no data for a clean aircraft!). However, there are limitations to testing this way; the main one being that tests mostly have to be run at stabilised flight conditions, otherwise known as “trimmed” states. The easiest and most useful flight conditions that can be tested in-game are constant altitude and speed cases as they give a stable flight condition to evaluate. Kinematic states cannot be overridden like they would in an external testing tool. Despite these limitations, in-game testing still provides a wide range of useful test cases.
Once the stores drag update was set in stone a while back, it enabled more detailed testing of the flight model itself against the F-14A NATOPS performance manual data. First, level flight time-to-mach tests were run to investigate acceleration performance in level non-accelerated (1G) flight.
Note that Max Mach occurs at 36,000 feet at 2.15, and at Sea Level tops out at 1.14. The “dip” in Ps between approximately 0.9 and 1.15 TMN is caused by transonic drag, which explains the reduction in acceleration as the aircraft passes through the transonic region.
Note that a 62,000lb aircraft requires five minutes to accelerate in level flight (no unload) from max endurance 0.59 IMN to 1.8 IMN. It takes approximately two additional minutes for the aircraft to accelerate from 1.80 IMN to the NATOPS limit of 1.88 IMN.
The 1G flight envelope testing and resultant tuning assured that excess power to accelerate was roughly correct across the entire speed and altitude range of the aircraft. This enabled accelerated flight beyond 1G, in essence turn performance, to be tuned.
Turn performance is easiest to evaluate at a constant altitude for several reasons, the main one being that turn performance data is given at fixed altitudes in NATOPS. Hand-flying a perfectly level turn while maintaining speed within 1 knot while recording data is extraordinarily difficult. These tests could be flown manually by you all, but flying with enough precision to have repeatable results to tune against is impossible. Therefore, some sort of automated and precise way of flying is required. Outside of our custom flyout and benchmarking tools, in-game scripting tools can provide much useful data and are available to the community at large.
One specific script uses pitch and roll controls to maintain a target speed and altitude while banking in a turn, with pedal input to centre the ball (aka zero sideslip, coordinated turn). Power is set to Zone 5 AB. The result is a perfect level flight turn at max power.
Note the constant altitude and decrementing speed as more aft stick is added, as well as the relationship between G, turn rate, AOA, and speed.
Below is the equation for specific excess power:
It’s easy to see that increasing weight and drag will cause excess power to decrease. When thrust and drag are equal, Ps=0. Another way to view excess power is in this form:
Where dh/dt is change in altitude over time (climb rate), and dV/dt is change in speed over time (acceleration). The script above t is designed to hold altitude, meaning dh/dt = 0. Then it uses the pitch control to increase G and drag while targeting a set airspeed so that dV/dt = 0 at the target speed, and therefore Ps=0.
Increased G means increased AOA and increased turn rate, changing airflow around the aircraft that increases drag and power required to maintain speed or energy state. Because the amount of thrust is nearly constant, the speed and/or acceleration decreases until all forces are balanced and an energy state equilibrium point is reached; this is known as having zero specific excess power. Adding more G and turn rate to the airplane will require more AOA, resulting in more drag and more power required, causing a loss of speed or altitude, which is a negative excess power state. Decreasing G load and turn rate will result in the opposite; less power required, an increase in speed or altitude. A positive excess power state.
The moment you can no longer sustain a turn’s speed and altitude, you are in a negative excess power state. When you have a positive energy state, it means you have extra power available to either climb or accelerate in the turn. A zero excess power state also exists at the level flight (1G) maximum speed point for any altitude, which is given in the performance manual and shown above. This is very helpful information to remember in a combat situation.
Keep in mind that as AOA increases, engine inlet performance and therefore thrust generally decrease (less power available), which will affect the sustained turn rate. AOA is increased in turns with higher G loads and/or decreasing airspeed, which unfortunately tends to be the exact scenario when you need more thrust.
As the aircraft is maintaining both altitude and speed, it’s riding along the zero specific excess power line (Ps=0) you always see highlighted on the energy charts. Before diving into this topic, keep in mind these charts are estimated from flight testing. The data in them may not be perfect to the real world. These charts are also known as “doghouse plots”. Here’s one below:
On these types of charts you will see a family of Ps lines, typically measured in ft/sec or m/sec. These lines show the energy state of the aircraft at a particular flight condition. Along the -200ft/sec line for example, the F-14A doesn’t have enough thrust to maintain speed or altitude while in a turn, and in order to maintain speed the aircraft would need to be descending at 200 ft/sec or 12,000 ft/min. In order to maintain altitude, the aircraft would be losing speed rapidly, causing G available to change rather quickly. The areas away from the Ps=0 line are very difficult to test in-game due to the fact the aircraft needs to be descending or climbing rapidly, and very quickly would no longer be at the chart’s altitude of 10000 ft. For these kinds of tests we use our in-house custom windtunnel tools.
At 53873 lbs, the F-14A’s highest documented sustained turn rate is about 15.5 deg/sec at 0.55 Mach, with the maneuvering slats and flaps out at 5000 ft. Peak sustained G is just around 6.5 G and 11.7 deg/sec near M0.85 at 5000 ft, but the turn rate at this speed is lower due to the turn’s speed/radius and airframe’s lift and thrust available. Why is that? In order to sustain 15.5 deg/sec at M0.85, we would need more lift (about 8 G’s worth) and a huge amount of thrust, or way less drag to sustain the turn.
This is the performance that can be achieved with eight (!) total missiles and 8400 lbs of gas on the airframe, adding up to 53873 lbs. Weight is a significant factor in turn performance. This is the lightest gross weight we have data for, and therefore the best performance numbers we have. Clean turn performance will be better (less drag). Lighter weight turn performance will be better (less lift needed meaning less induced drag). Lower altitude turn performance will be better (better engine performance).
Turn rate performance below 225 KIAS with the flaps out will be better, but climb performance will suffer. Recall the 225 KIAS airspeed and 2G load limit with the landing flaps extended.
As you study the performance charts, keep in mind that they often don’t take into consideration other aircraft limitations. For example, at the aforementioned gross weight in the previous paragraph, the F14A/B are both limited to 5.8 G, with Max G of 6.5 only available at a gross weight below 49,548 lbs. Maximum rolling G is even less, starting at 5.2 G at max “fighting weight”.
The Ps performance charts specifically state they are designed to be used for aircraft comparison, and the depicted aircraft performance often exceeds the NATOPS speed limitations for the missile loadout shown in the chart. Below is the NATOPS speed limit for the 4×4 configuration (2B1), notice that it is more restrictive than the Specific Excess Power Diagram (the first chart in the article).
For reference, a lightweight F-16C at 26’000 lbs with a minimal combat load and some gas peaks at 8.8G, 18 deg/sec, near M0.8 at sea level. At 10000 ft the Viper is down to 6.9 G, 13.5 deg/sec. At this same altitude the F-14A can turn at 13.5 deg/sec with 8 missiles and 8400 lbs of gas, but at a lower speed of M0.65. If you can get the Viper down to M0.65, it’s possible to out-rate them in a turning fight using your advantage in lift. With fewer missiles or weight the F-14A’s situation further improves. Being able to take advantage of this fact is a matter of pilot skill. Too bad the F-14 can’t carry the AIM-9X!
Also note that G, turn rate, turn radius, and bank angle all have a direct relationship no matter what aircraft is being discussed. The only question is how much excess power any aircraft has to maintain a turn at a given speed.
Now that we’ve gone over some of the theory and fundamental workflow, let’s dive into what the latest tuning pass actually means for our F-14 and for your flying, specifically.
Theory is great, but the results are what matter and how they influence your flying in and out of combat.
If testing results show the F-14 flight model can’t achieve the turn rate at the Ps=0 line for a given speed, the model must be tuned so the aircraft can maintain the chart’s given turn rate at speed.
Below is the real F-14A’s Ps=0 line for 5’000 ft, 10’000 ft, and 15’000 ft, along with the pre-update baseline Heatblur F-14A and the post-tuning result:
Before this round of tuning the turn performance was already pretty close, with the most deficiencies in the transonic region. Those issues have now been corrected, and we’re really approaching a very high degree of accuracy.
To show some additional in-game flight test results, here is level flight max speed in a lightweight clean configuration for MIL and Zone 5 AB:
|Zone 5 AB Max Speed, 1G clean
A clean F-14A can be very fast
|MIL Max Speed, 1G clean
As a max performance experiment, a clean config, sea level, minimum weight sustained turn test was run to show the theoretical maximum possible turn performance of the F-14A. Because the weight is drastically reduced, available G is much higher (G = Lift Force / Weight). Remember the aerodynamic performance (lift generated) is the same, the only difference here is weight and stores drag. Notice how significant the effects of altitude, weight, and drag are on the turn performance:
|STR: Sea Level 47000lbs, clean
Peak sustained turn rate jumps up to 20.6 deg/sec near M0.53. Peak sustained G goes up to 9 near M0.83. The lighter and cleaner the aircraft, the better it will perform. The tradeoff is loss of range (less fuel weight) and no useful payloads or loadout (less drag and weight).
These sustained turn numbers align with anecdotes from F-14 airshow demo pilots running lightweight clean configurations, who had to immediately put Gs on the airplane after takeoff to prevent excessive acceleration and turn radii too large for the demo area.
Ironically, the highest possible sustained turn rate in any config will exist at the moment fuel weight hits 0. Hopefully your fight ended directly above the airfield! For those concerned with having the cleanest fighting configuration possible, the “slickest” missiles will be the tunnel mounted Sparrows.
Because this tuning has been done across all speeds and altitudes, the Heatblur F-14A is now even more accurate in how it maintains energy states across the whole flight envelope, meaning more accurate performance while manoeuvring.
In a flight model, AOA is the most critical parameter to get right. It’s the main determinant of both lift and drag, as well as most other flight characteristics, including BFM performance. Being at the correct AOA for a given speed means the model’s lift generated is accurate. If drag data is available or the L/D ratio is known, drag vs AOA can also be determined.
Combining the sustained turn performance results with the knowledge that Lift/Drag vs speed are accurate across the flight envelope (see below), we can assume that deviations off the Ps=0 line, such as common dogfighting manoeuvres resulting in negative Ps states are also accurate. Think manoeuvres such as high instantaneous turn rate, high AOA, and high G pulls trying to get the nose around on an opponent when energy (altitude or speed) is being lost.
From an aerodynamics perspective, the F-14 is basically a transformer. At each degree of wing sweep it behaves like a different aircraft. Then when you add the myriad number of moving surfaces and loadout configurations, the permutation of test cases, configurations and potential pitfalls starts to become mind-boggling. Only a masochist should try to model an aircraft like this. We hope the time investment and commitment to deliver a faithful recreation of the F-14’s flight characteristics is apparent to the readers here and helps you enjoy flying this legendary aircraft.
It should be noted the above chart shows the automatic wing sweep is programmed to deliver Max L/D possible at all speeds, and the sweep mechanism can handle rapid speed changes. Keeping wings in auto will be better than manually sweeping the wings to hopefully gain some perceived “advantage”.
To see how some other DCS aircraft FMs stack up against the Heatblur F-14, check out this website created by a friend of ours: https://dcs.silver.ru/Diagram/F14B. Compare the charts to see how other aircraft in DCS perform vs their real life counterparts.
With this process nearing completion, the F-14A flight model is nearly in a fully finished state. The journey has been long, initially focusing on handling qualities with our very dedicated pilot SME, Victory205. We wanted to get those out of the way first to make sure handling was correct, with extra time spent focusing on handling qualities around the boat to ensure a very authentic carrier trapping experience. In the near future we will update the following FM items:
Of course the biggest of the items above is to finish a similar pass to the above for the F-14B; also fine-tuning the performance to perfectly match the available data and plots. We’ll chime in with another flight modelling update focusing on the F-14B results once we’re ready!
Thanks for reading, and for your support – we hope you enjoyed this deep dive, especially as a precursor to similar articles for the DCS: F-4E, DCS: A-6E and Eurofighter aircraft.