The most common cause of loss of control, particularly in amateur-built airplanes, is an unintentional stall. When that happens at low altitude the results are usually fatal. I think one of the most effective ways to reduce the number of those accidents would be to improve control stick force.
Until the end of World War II there wasn’t much investigation into what made an airplane easy and predictable to fly. Airplanes tended to be individuals each with often very different feel on the controls, and response to control inputs.
After the war the NACA–forerunner of NASA–and the military conducted testing to determine what flying qualities made an airplane easy and predictable for pilots to fly. A more predictable airplane lowers pilot workload which is a good thing for the military who wants its pilots to concentrate on the enemy, and for civilians who want to carry passengers as safely as possible.
One of the major discoveries of the research was that stick force–how much force a pilot must apply to the controls to maneuver the airplane–is critical. Stick force is a subliminal cue from the airplane to the pilot to communicate what the pilot is asking the airplane to do, and how the airplane will respond.
Control force and feel is not unique to the airplane-pilot interface. Control input “feel” is important for just about any man-machine operation. For example, remember the power steering systems in cars of the 1950s and early 60s? In most you could spin the wheel with one finger when stopped, or cruising at 80 mph. The controls were totally numb. A driver got no feedback to let him know how the car was responding to his control inputs. Car steering systems have come a long way since those days, but not all airplanes have.
Testing in airplanes showed that the best flying qualities come when force on the ailerons is lightest, pitch force is greater and finally rudder displacement requires the most force. The rule of thumb is that if aileron force is 2, pitch stick force should be 4 and rudder force 6. This ratio is difficult to achieve, but it is a goal and the best flying airplanes come close.
When it comes to controllability, particularly at lower airspeeds, stick force in pitch, and particularly the stick force gradient, are most important for safety, and the hardest to achieve. The pitch force gradient is the increase in force needed to pull or push the airplane further away from its trimmed airspeed. In the best airplane the stick force gradient is linear, meaning the force builds in a steady predictable manner as the pilot deviates from trimmed airspeed.
The reason stick force and a positive force gradient is important to safety is that the force, the feedback if you will, from the controls instantly reminds the pilot he is changing airspeed and wing loading. With a strong positive stick force you know instantly that you are changing the flight profile. You inherently know that when pulling harder while flying at slower airspeed, such as in the traffic pattern, you are treading on thin ice and are loading the wing. When it takes a lot of pull to stall the wing you are more aware of what you’re doing then if you can stall with only light forces on the stick.
Ideally stick force is created by air loads on the control surfaces. But that can be tricky. For many reasons from the design of the control surface to the actual control mechanism stick force gradient is seldom linear, and maybe not even positive. And in small light airplanes the force may be so light that even if it is positive it doesn’t give the pilot the feedback he should have.
In certified airplanes the FAA became ever more demanding of good positive stick force in pitch as the years went by. In an airplane with much of a CG range it is almost impossible to meet the stick force rules without springs to pull against the controls, or a bob weight to resist your effort as the wing loads up.
Early in fly-by-wire control development designers tried sticks that sensed only force, but not displacement. Early F-16s are an example. Most pilots hated that and found it hard to fly. So sticks were designed with spring systems so that as the stick is displaced the springs provide the force gradient even though only an electronic command, not control cable movement, is being created.
Springs, bob weights and other devices to add stick force are missing from most, if not all, homebuilt designs. In fact I almost always hear descriptions of how feather light the control force of a design is and how that makes it fly like a fighter. When I hear that I think yes, like a fighter on the losing side.
Even in extreme aerobatic flying or other precision maneuvering many pilots intentionally fly with the airplane significantly out of trim so they are always holding stick force in pitch. Without the force it would be like driving that 1958 Chrysler.
Angle of attack display systems can help increase awareness of stall margin in light airplanes, but stick force can be even more effective because it is always there and you don’t need to look at a display to comprehend what the airplane is telling you. There is no way to decree how a homebuilt should handle and what its stick force gradients should be, but based on the experience of the standard certified airplanes more positive stick force could help. Finger tip flying is not better and more controllable than finger tip driving was in those cars years ago. Positive stick force makes for precise and predictable flying.