A single servo autopilot can control bank and yaw (you need both to make a turn). That would be 2-axis. A 3-axis autopilot also controls pitch.
That is one way of seeing it. Given that 3-axis sounds better than 2-axis, people like to claim they have 3-axis. 
A yaw damper is not really an autopilot axis, usually a separate system with the goal of increasing passenger comfort.
You got me confused…
With one servo you can control just roll. There are no pitch-only APs AFAIK. So the AP would do
When you add a pitch servo (plus optionally but highly desirably a pitch trim servo) you can have
plus goodies like altitude capture (ALT+ARM), IAS hold, IAS limit, stall protection, etc.
You got me confused… With one servo you can control just roll.
Exactly. Yaw will follow roll on its own by “positive roll-yaw coupling” aerodynamically built into stable and controllable aeroplanes – this is why you can keep your feet on the floor in those aircraft except for takeoff and landing. Where that works less well, the controls in yaw in roll can be connected by springs, like in most twin engined Cessnas (piston,turboprop and jet!). So in these aeroplanes, a single roll servo will move the rudder together with the ailerons.
That very much depends on the aircraft category and what the basic equipment looks like.
Precisely. In mine, the one great add-on would be a system to close ALL air draughts (though the adjustable openings in the side windows would have to stay, and also a limited amount of underpressure somewhere behind).
Then again of course the said double-platinum credit card might be useful too, especially if some kind soul would see to the debet side of it…
I’d generally categorise upgrades by 4 criteria: Efficiency, work-load reduction, comfort, safety.
Efficiency would include aerodynamic tweeks, Gami injectors, electronic ignition, engine monitoring, oxygen for altitude etc
Work-load would include autopilots, avionics, moving maps
Comfort/safety would include BRS, belt-airbags,ANR, seats/interior, A/C, draught seals maybe helmets and ’chutes if your flying is more adventurous etc.
Though if you’re into back country flying, you could also consider floats, tundra tyres, stone guards etc.
Where you best spend your money depends a lot on where you’re starting from and what your typical mission is.
But very little of that will add much to resale value, so make sure that you get the value from the flying that you do.
My preference is to make the aircraft close to the way it came from the factory, in both equipment and condition. Then the logical upgrade is to another plane and do it again, which is fun for me. With that in mind the upgrades that I like are safety related, and not expensive: a fuel totalizer and a portable moving map GPS both make cross-country flight easier and safer. The totalizer was $500 and I do like the $300 iPad Mini running $7/month Foreflight.
There is one project I’d like to do, and that is an STC’d aileron spade kit for Luscombes. I’d develop it myself, sell the kits and make money
In the single pilot IFR thread ‘what next’ said:
Here in Germany, at least pre-EASA (no idea if that has changed), in order to fly IFR, you either needed a second crewmember who had to have at least a RT license or a working 2-axis autopilot. Which to me always seemed to be a sensible rule.
I never know what people mean when they say 2-axis and 3-axis, so I always try to ask nicely
I assume that this particular ‘2-axis’ means HDG + ALT Hold, (at least)?
And that law (with the P2 RT alternative) seems surprisingly practical to me. The one time I really need an autopilot is listening out for ATC and writing down an ATIS at the same time.
ADL, best device on board.
I assume that this particular ‘2-axis’ means HDG + ALT Hold, (at least)?
It basically means that the aeroplane must be able to be flown hands-free for as long as the pilot has other tasks to perform. A basic single-axis autopilot like the ones installed in the old Cessna 172 that many of were trained in is not sufficient. It is able to hold a heading or track a navaid, but it can’t hold an altitude or a steady climb rate and therefore the pilot still needs one hand on the yoke.
S-Tec defines their systems as Single Axis if they have roll control ONLY, Dual axis if they have roll and pitch. The third axis would be yaw, something which hardly exists in small planes. Bendix King define their systems similarly but they used to call the addition of a yaw damper as the 3rd axis.
Some people define the electric trim as a separate axis, I disagree with that. Electric trim is an addon to the pitch axis.
And I agree, for an airplane designed to be used for travelling, a Roll/Pitch AP is one of the more useful things you can get.
