On my commute yesterday it occurred to me that I did not know the mechanical design of an airplane’s joystick. I got to thinking: Suppose I wanted to design an airplane completely from scratch. How would I construct the control?
The joystick moves fore and aft to control the elevators, and from side to side to control the ailerons. Either one of those functions is easy. But putting them together in the same control presents a couple of problems. Let’s assume for the sake of simplicity that I have a long stick with a ball near the bottom, and the ball fits into a socket so that the stick can move in any direction*. By putting a pushrod at the bottom of the stock, the elevators could be controlled. Now put lateral pushrods on the bottom to control the ailerons. (Of course there might/will be cables and bellcranks for ‘plumbing’ the controls, but we’re keeping it simple.) When the stick is moved longitudinally, the aileron pushrods move longitudinally. Would this tend to raise or lower both ailerons collectively (i.e., both up or both down)? When the stick is moved laterally, the elevator pushrod is moved laterally. Would this cause up- or down movement in the elevators? Or is there naturally enough slop in aircraft controls that any movement would be so slight as to not matter? I can’t seem to find pics of the mechanical end of an aircraft’s joystick.
*If I were really building one, I wouldn’t use a ball-and-socket. Instead I’d have a longitudinally-mounted ‘axle’ that would rotate left and right for the ailerons, with the joystick mounted to a pivot point in the middle so that the stick could be moved fore and aft for the elevators.
Are you specifically curious about an airplane, or just joystick mechanics in general? I remember pulling the boot off my stick shift a few times and was surprised at how simple it was. The stick moved in two dimensions. Forward and backwards, left and right.
Hopefully I can explain this.
There was a shaft that went, right to left, through the stick. The stick pivoted back and forth on this. Down at the bottom there was a control that transferred the back and forth movements to the transmission. Make sense so far?
Now, that shaft that we talked about? It was fixed to the stick so when you moved the stick right and left that shaft transmitted those movements down to the transmission.
Now, it was a bit more complicated then that, but not much. There was less “shafts going through sticks” and more ball joints involved, but that was the basic idea. I don’t know if that’s the way an airplane joystick worked, but it seemed pretty elegant for cramped place.
ETA, come to think of it. As I’m remembering more about this, I think, what was actually going on was that the stick was connected at central location via a ball joint. Then there was two shafts connected to it also via ball joints. When went to the right (left and right movement) and one when towards the front of the car (back and forth movement).
There are some plans available. I have some from a plans-built helicopter. Basically the stick would have a hinge joint on a rotating rod. So your stick is a vertical shaft that would move forward or back for pitch, and the horizontal rod it attaches to can rotate the whole thing left of right for roll. The vertical shaft extends below the horizonatal rod, and the pitch control wires are attached to the shaft at equal distances above and below the horizontal rod. Then control horns mounted on the horizontal rod attach to the roll control wire. The length of the horns and the shaft extension determine the mechanical advantage.
At this point I’m really just guessing, but I did come across this image. From here, I think it’s self explanatory to see what happens when you move the stick back and forth. The shaft that runs under the pilot’s feet would also move back and forth. Now, if you look at it, you can also tilt the control stick left and right which would translate into rotational movement of the shaft that runs to the back of the plane. I would assume that triangular piece is then attached to whatever is controlled by left and right movement of the control stick.
Furthermore, if that triangular piece were on (greased) splines, it would be able to remain stationary (front to back) while the shaft moved back and forth through it in response to back and forth movement of the control stick and that it would need to ignore.
That one looks like it has a pushrod on the pitch control, not sure what the roll control actuators would be. I’ve seen a couple of designs that used pushrods and rotating rods for the roll control, both more complicated than using wires. All the pitch controls I’ve seen on small planes used wires for the pitch. For a center stick, the pitch wires run straight back toward the tail. Some designs have differential mechanisms for roll, allowing airlerons to have different range of movements up and down.
Gobboldygook. I had a feeling this would get out of my league very quickly. See, I have no problem visualizing different ways to get a joystick to control to control two different things without them interfering with each other. It’s all the airplane terminology that’s getting lost on me.
That’s what I tried to describe in the OP. What I was having trouble with is that the way I tried to describe it, the pitch linkage would move left and right when the ailerons are used. In that case, it seems there would be some pitch deflection as the stick is moved laterally.
Yes, that’s why the pitch wires are located above and below the roll rod. The wires are a pair and would twist so the length would stay the same for both wires. The total range of motion is short enough to for the mechanism to take it up, but that’s a really simplistic system. For the helicopter, the actuators are rods, carrying both the forward and sideways motion back to a triangular piece that tilts on double levers arms in different directions. Anyway, I should have added what several guys told me, “it’s worth it to buy the plans or the parts or the whole unit because somebody figured it out already”.
Also, I think there are a newer designs using electric servos, and you use a high quality joystick for electronic control.
Indeed. If I were going to build an airplane I’d buy a kit. Why design something myself when someone else has already done it better? This is just one of those things that occurs to me on my long commutes.
I’ve seen and worked on these in a number of gliders and light aircraft.
They are pretty simple. Typically there is a fore-aft axle shaft that pivots on the fuselage, this operates the ailerons via cables or rods. The stick is attached to this by a pivot axle that runs right to left when the ailerons are at neutral. So moving the stick fore and aft moves only the stick, and moving it side-side rotates the aileron shaft.
The elevator is linked to the stick either by rods or cables that run straight back from the stick, so there is little/no movement induced in the elevator when the ailerons are operated. A slight amount is tolerable, as the pilot has to make pitch compensations when banking for a turn anyway, so a little more or less is virtually not noticeable.
That is mechanical sticks. I have also worked with a fly-by-wire stick, specifically the one that “flies” the refueling boom on the KC-10 aerial tanker. This was quite interesting in that it used a four bar linkage so that there was a virtual pivot for pitch inside the boom operators wrist joint, and the “roll” (for the boom it was actually yaw) axis was just below the armrest so the operators arm could roll on the arm rest and not need to slide.
The stick used RVDTs to signal the boom control computer (based on a Sperry 18/19b bitslice processor…three boards just for the processor) and had torquer motors to provide artificial feel for tactile feedback. And 25 years later that is about all I remember about that.