Why do helicopters have two pedals (as shown in the picture); why is one not enough? As we know, the pedals provide directional control through the tail rotor, to yaw left or right. As we also know, the tail rotor is required to overcome the torque of the main rotor, which will otherwise cause the helicopter to spin. So, in my opinion, one pedal is enough: increase the tail rotor's pitch to yaw against the main rotor torque, and decrease it to yaw following the torque direction. My question is then, why do helicopters need two pedals? Is one not enough? And how exactly does the pilot control the helicopter in yaw?
Two pedal control is arranged such, that, in helicopters and other aircraft as well, the pedal forces are balaced (by default or by trimming) during straight and level flight.
The arrangement suggested in question would mean that the pilot would need to apply positive force on the single pedal all the time. This will quicly become strenuous, and maintaining a constant force for a long time is not as easy as one might think.
Physical strain is not the only downside. One pedal arrangement cannot be trimmed for neutral condition, as this would remove the ability to use full control envelope. This leads to extra "mental" load, as maintaining directional stability needs constant focus.
In addition to ease of use, the two pedal arrangement assures the control forces will not get excessive. With two pedals the range of the requiered force can be described as, say, 0 to 100 units for either foot from one extreme to the opposite one. With one pedal, the range would be 0 to 200 respectively, or twice the travel of the pedal to compensate for this.
There are of course other ways you could control the tail rotor, for instance there is a control adaptation for disabled people where the tail rotor is controlled by hand rather than foot using a yoke on the stick rather than pedals.
A single pedal system is possible, but it would be either unsafe, uncomfortable to use, or both. There are two viable options for a single pedal system (there are more possibilities but none are viable in a direct linkage system where significant force is involved). It's not a problem of intuition, there's no reason people couldn't use a single pedal with precision, it's mostly to do with musculature.
Spring loaded: in this scenario there is a spring to force the return of the pedal. There are several problems with this:
- Safety: if the spring breaks then the pedal will not return, the torque will remain in the last setting, which is potentially fatal. I would not want to trust my life to a single spring
- Muscle fatigue: there's a reason cruise control was invented for cars, holding a pedal down for long periods of time is tiring. I've driven across the US twice in a car with no cruise control and I can tell you it was very uncomfortable, there were times I had to stop just to give my calf a break. That was in a modern car with sensor based throttle control and a light pedal, in a helicopter with direct linkages and no power assistance the forces involved would be higher, and much harder on the calf muscles. This could be mitigated somewhat with friction locks for cruise, trims and the like, however it would still be a significant factor
No spring: In this system the single pedal would be pushed down and pulled up by the foot, which would either be strapped to the pedal or more likely there would be some sort of hook over the toe. The overriding problem with this design is that the human leg isn't built for it - the calf muscles are far stronger than the muscles on the front of the leg. Try lifting even a small weight with your foot and you'll find it's very hard to do, and your muscles will quickly tire.
There are other possibilities, for instance side to side motion, however those muscles are even weaker, you would not be able to use that in a helicopter with a direct linkage.
We have two directions in yaw, and we have two feet, so we can afford to have two pedals - one for each direction.
One pedal? Theoretically, yes. You can spring load that pedal to make it serve the purposes of both - press to yaw left, release to yaw right, partially press to maintain heading. But I can imagine myself getting confused all the time - if I gotta turn left, do I press or release? - not convenient when you need to make a sudden input. With two pedals, you don't have to think - you simply press the pedal corresponding to your intended yaw direction. Single pedal creates more problems than it solves, and it solves none.
It's true that yaw control is only one axis, so a single (1-axis) control lever, as in 1 pedal, would theoretically be enough. In standard helicopter design, the two pedals in a helicopter also only actuate 1 axis (yaw), and sometimes (usually?) they are even fixed to a bar that rotates around a single center pivot.
While I'm not a helicopter controls designer, I assume the main reasons for that are:
History - Helicopters became airworthy after airplanes had been common for a while. Autogyros date to 1923, and some helicopter development seems to have taken place in the late 1920s to the late 1930s, with the first mass produced helicopter being Sikorsky R-4, in 1944 (https://en.wikipedia.org/wiki/Helicopter#First_practical_rotorcraft). Fixed wing airplanes were a lot earlier, e.g. the Blériot XIII dates to 1908. It was this airplane that was using the yoke + rudder pedals controls, and in airplanes, this type of control was to stay. It's not very far fetched for a new design, to use proven controls that pilots are used to - especially in the early testing phases.
Controls precision - There is a reason we use our hands to write, or to operate machinery. A kettledrum may be operated per pedal, but a violinist usually doesn't stomp on the strings (unless angry?). Most humans are quite capable of doing precise complex movements with their hands, and a lot less with their legs/feet. Feet-operated controls are usually the ones that require a lot of force but are not super delicate, like a brake pedal in an old car without power brake. Using both feet to pivot a single bar for yaw makes precise control much easier.
force symmetry: in case of maneuver loads (say, a +3g turn), in a two pedal design both pedals get the same force, and they cancel each other out. In a single pedal design, whether push-pull or whatever, it's the pilot who needs to counter-act those forces by muscle strength if no control input is desired