F-35 Flight Control System, Part One
In this article, we’ll discuss FBW, generally, and focus on some features of the F-35’s control laws (CLAW) from the pilot’s perspective. In the next article, we’ll get into some engineering details and see what’s so innovative – and historically significant – about the F-35’s approach to FBW.
Why Fly-by-Wire?
The F-35, in most of its flight envelope, is unstable in pitch and neutrally stable in yaw. What that means is that if there were a nose-up or nose-down disturbance that the stabs didn’t immediately react to counter, the disturbance would grow. RAPIDLY. At normal cruise speeds, the time for an angle of attack (AOA) disturbance to double, if not corrected, would be about a quarter of a second. This instability makes the airplane agile and highly efficient aerodynamically, but it would also make it unflyable were it not for the flight control system – doggedly, eighty times per second – positioning the stabs to keep the nose pointing into the wind. So, as golden-armed as we F-35 pilots are, if we were responsible for positioning the control surfaces ourselves, the airplane would be out of control in seconds.
Static stability isn’t the only thing artificially created in a FBW airplane. The dynamic response – the way the airplane responds to our control inputs – is also created artificially. That response can, in fact, be just about anything we want, since it’s determined by software…not nature.
What? We Don’t Like Nature?
Have you ever known someone who did exactly what you asked? (Okay, me neither, but work with me here.) FBW airplanes are a lot like that guy. Their response is, in a way, too perfect: they do exactly what we tell them. As a result, we have to un-learn some of the compensation we thought was “just part of flying.”
For example, when we want a snappy roll in a mechanically controlled airplane, we have to overdrive the stick to get the roll going, then apply a check in the opposite direction to stop it. Not so in our computer-controlled machine. The F-35, as most FBW airplanes, sees our lateral input not as a command to move a surface but as a command to provide a roll rate: it overdrives the surfaces to get the roll going, then backs them off to maintain the rate we’ve commanded. When we remove the command, it drives the control surfaces against the roll to bring it to a crisp stop. If we check, as we did with basic airplanes, the airplane obediently performs a quick head-fake in the direction of the check. Most of us experienced that in our first flight in a FBW airplane, but the tendency went away quickly as we learned the new response.
Another example is turn coordination, which relates to the amount of sideslip we get during rolls and turns. Automatic coordination isn’t unique to FBW: we’ve had aileron-rudder interconnects (ARIs) for years, and even the Wright Flyer had one
. But turn coordination in FBW airplanes can be very sophisticated. Generally, the F-35 tries to keep sideslip near zero, but in some cases it intentionally creates adverse or proverse yaw as necessary to control roll and yaw rates. We’ll talk about the use of pedals at high AOA in a later article, but, for general flying around, the best coordination we’ll get is with our feet on the floor.
The point is: When we move the stick and pedals, FBW gives us what we actually want – or what the control engineers want us to have – while suppressing the extraneous things nature has always tossed in along with it, things we previously had to compensate for or just learn to live with.
But Wait, There’s More!
FBW does more than just stabilize the airplane and clean up its response. It determines the very nature of the response itself. That response can be programmed to be whatever we want, as a function of the airplane’s configuration, speed, or whether it’s in the air or on the ground. For example, if we make a lateral stick input in CTOL mode, we get a roll rate. But in jetborne mode, we get a bank angle. At high speed, a pitch stick input commands a normal acceleration (“g”); at low speed with the gear up it commands a pitch rate; at low speed with the gear down, it commands an AOA; and in the hover, it commands a rate of climb or descent.
The ability to tailor the airplane’s response as a function of its configuration and flight regime is the beauty – and potential curse – of FBW. If control engineers get it right – if they define the modes properly, put the transitions in the right places, and give the pilot the right feedback – then control is intuitive. But if they make the various modes too complicated, or the feedback (visual or tactile) isn’t compelling, then modal confusion can set in and bad things can happen.
Some mode changes occur without our knowing, which is fine as long as we don’t have to change our control strategy. An example is the blend from pitch rate command at low speed to g-command at high speed. This transition is seamless from the pilot’s perspective.
Other changes require us to change our technique, which is okay if we command the changes ourselves and they’re accompanied by a compelling change in symbology. Examples are the transitions from gear-up (UA) to gear-down (PA), and from CTOL to STOVL.
There are few areas, though, where a mode change is important but not obvious, which is where pilot discipline and training come in. For example, the CV airplane has three different approach modes, easily selected using buttons on the stick and throttle. Two of these modes – APC and DFP
– are autothrottle modes, indicated by a three-letter label on the left side of the HUD. The third mode – manual throttle – is indicated by the absence of a label…arguably not the most compelling indication that you’re responsible for the throttle. This interface will probably evolve; in the meantime, we need to be disciplined and to make doubly sure we’ve got APC engaged before we turn throttle control over to George.
Another area is STOVL landing. The difference between what the power lever (a.k.a. throttle) does on the ground and what it does in the air is profound. On the ground, it acts like a normal throttle: pulling it full aft commands idle thrust. In air, it commands accel/decel rate: pulling it full aft commands a maximum decel. There’s plenty of redundancy in the weight-on-wheels sensors, but if the airplane ever thought it was still airborne after a vertical landing, and you pulled the throttle full aft, the airplane would go charging backward. This would be “untidy” (as our British friends say), especially on the ship. So we take every STOVL landing to a firm touchdown, and let the airplane itself set the throttle to idle when it determines it’s on the ground.
