On most retractables this doesn’t happen – because the hydraulic fluid follows the path of least resistance so if there is even a few % more stiction in one leg than in the other(s) that one will move first and the other(s) will start to move only when the free-est one hits the rubber stop.
Socata mention a mod kit which uses some spring loaded valves to equalise the pressure and make the 2 main gear legs go up at the same time.
Would there be a good reason to do this? The only issue I see with the present system is that you get a lot of momentary yaw.
Electric retract motor. Problem solved :-)
Peter wrote:
Would there be a good reason to do this?
It’s much more sexy to watch!
I’ve never seen the gear retract on an aeroplane that I was flying, so it doesn’t bother me. Anyway, Spitfires retract one leg at a time, so that’s good enough
It’s much more sexy to watch!
Obviously, and that’s worth, what do you reckon, €100k? 
Electric retract motor. Problem solved :-)
So, why does everybody use hydraulics? It’s a good Q. I reckon one reason is the light weight of the whole system, and it has massive power. The TB20 uses ~ 2000psi. That is about 1 ton of force, and you get this with a ~5kg pump. Even “tiny” homebuilts use those pumps. This indirectly delivers high reliability because, no matter what one does maintenance-wise, there is always a risk of something sticking.
Peter wrote:
So, why does everybody use hydraulics?
Beech Bonanza/Baron/Travel-air are electromechanical, not hydraulic, so not everyone! And of course Mooney for some time used the Armstrong method.
I wonder if the reliability of Bonanza gear is different to hydraulic gear retraction.
Peter wrote:
everybody use hydraulics?
Mooney’s (1967+) landing gear is electrically actuated with one single motor.
The ST-10’s too, but I’m aware that 55 examples built 45 years ago don’t count too much.
Peter wrote:
I reckon one reason is the light weight of the whole system
And how is the hydraulic pressure generated in your TB20? With a handcrank?
Presumably it is generated using an electric motor.
While I haven’t weighed all the components of the gear retract system, I cannot believe it is that lightweight, with all the hydraulic lines, pistons, valves etc.
Peter wrote:
That is about 1 ton of force,
You can generate a lot of force with a small electric motor as well, if you have suitable reduction gears. Gear retract doesn’t have to happen very quickly.
I am also puzzled why even recently designed aircraft would still come with a electro-hydraulic landing gear. Electric seems to be the name of the game to replace other actuation methods (not only hydraulics but also pneumatics) in many industrial applications. Nice example is motion systems for simulators, even the high payload ones are now electric. So many advantages in terms of reliability/maintenance. So, we got rid of the pneumatically driven stand-by instruments in modern aircraft, and why not completely go electric? The only thing I can think of is the ease of emergency extension of a hydraulic landing gear (manually dump the pressure and the gear extends by gravity). So there is probably more at play here because designers are not dumb. The question does not only apply to landing gear btw, if you look at recently designed larger civil and military aircraft, still plenty of hydraulic actuators on board, next to electric ones, AFAIK.
