I am trying to replicate the fly-by-wire (FBW) system of the Super Hornet for a game called SimplePlanes. I was wondering if it is maintaining a certain $g$ number, turn rate, angle of attack, or a mix of everything. And, does anyone an equation that can replicate this?
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$\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$– CommunityCommented Jul 30, 2022 at 18:14
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$\begingroup$ I’m voting to close this question because this is a question for the Aviation. $\endgroup$– StephenG - Help UkraineCommented Jul 30, 2022 at 18:25
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1$\begingroup$ In your simulation, does the pilot have any input? $\endgroup$– Michael HallCommented Jul 31, 2022 at 14:24
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1$\begingroup$ @DeltaLima You are right, it depends on the mode. Pretty much all aircrafts equipped with FBW (except for these with an unstable airframe) have a least a direct mode in which the pilot directly commands the deflections of the control surfaces. The next higher mode up is most of the times one which I describe in my answer. If tuned correctly, such a mode is employed throughout the envelop of the aircraft. The next higher mode up are autopilot functionalities... So to answer your question, in most cases it is rate feedback, or nz feedback or a mixture. $\endgroup$– U_flowCommented Aug 2, 2022 at 12:51
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2$\begingroup$ @MichaelHall You have a valid counterpoint, but still, there is a difference between no dynamics at all and something designed to handle nicely. You would be suprised of how good handling qualities contribute to a nice "playability" of simulators. $\endgroup$– U_flowCommented Aug 2, 2022 at 17:41
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2 Answers
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The F/A-18 relies heavily on computerized control to achieve a number of objectives. These are (without claiming completeness):
- Good handling qualities
- Spin resistance
- Acceptable High Angle-of-Attack flight and maneuvering characteristics
- Avoidance of Adverse yaw
- Avoidance of Proverse yaw
- ...
How these objective are achieved is described in a pretty detailed paper about the F/A-18 Flight control system available online.
Some key (and pretty standard) concepts are the following:
Longitudinal (Pitch) Control
A very standard way to treat the pitch control is to use vertical acceleration feedback ($n_z$) for high-speed and pitch rate $q$ feedback for low-speeds. This is done as pilots judge how much their aircraft pitches at low speed by how much the horizont moves, and at high speed by how much acceleration they feel (remember at higher velocities you feel much more acceleration then you see the horizont move...) The formula would be $$ u_x = K_\text{p} e(t) + K_\text{i} \int_0^t e(\tau)$$ whereby $e(t)=d_x - n_z$ ($d_x$ is the pilot input) for high speed and $e(t) = d_x - q$ for low speed. In the intermediate speed range you have a blending of both of these errors. The exact gains for $K_p$ and $K_i$ are also speed dependent and certainly not publicized. They also include special considerations for high AoA which was important at that time, however for a game called simple planes, that perhaps makes no difference...
Lateral (yaw) control
For lateral control, normally you aim at $\beta=0$ whereby the pedals control how much $\beta$ you build up. Again that is goverened over PI-controllers whereby the gains are scheduled on basis of the velocity. Again the paper describes special spin recovery schemes and anti-adverse-yaw implementations. You will have to decide yourself if you want to implement something like this
Roll control
Here, I would simply govern the roll rate $p$, again with a speed-scheduled PI controller
It is worth mentioning that the implementation of a realistic controller relies heavily on a realistic flight model. Therefore if your flight model is simplistic, then your FCS implementation can also be simplistic, but do not expect a high level of fidelity of the overall system. However a realistic FC can be a lot of work to tune, implement and test, so keep that in mind.
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...if it is maintaining a certain g number, turn rate, angle of attack, or a mix of everything...
A mix of everything. The computers adjust the active flight controls for:
Providing artificial stability of the airframe. An aircraft is most manoeuvrable if it wants to change direction by itself already, which would require constant small inputs from the pilot in order to maintain flight direction. Not a good situation for human handling, the computer has no problems with it though and can artificially create the stable feel for the human.
Limiting the forces on the structure to the maximum design limits. Above image is a still from this video, depicting the limiting manoeuvres of an F-16. Active fly-by-wire limits the maximum g-load on the airframe, without having to limit the all-moving-tail area or its maximum deflection angle.
And, does anyone an equation that can replicate this?
A single equation I don't have unfortunately. There would be at least 6 required for each degree-of-freedom of the aeroplane, each one with quite a few coupled inputs. And the constants in the equations vary from aeroplane to aeroplane and I don't reckon that are published anywhere.
