Undercarriage question

I don’t know much about about the rest of you but I reckon my question has been comprehensively answered. Many thanks for all your replies. There are a lot of things about aircraft systems that I don’t understand, as shown by my question, so I am always glad of the help I get from everyone here.

Of course – I hadn’t thought of that but it makes sense. Would not have been helped by the fact that to save weight they would have used the smallest, lightest pump possible.

Yes low capacity,low pressure pumps and using a cut out/in type system rather than the later variable delivery type pump.

Well, the further up the leg goes, the greater the force needed to keep moving it, as the wheel is getting further from the pivot-point horizontally, and thus is more affected by gravity.

Thus, while the hydraulic pressure is great enough to move both legs from the vertical alignment, the increasing combined force requirement soon exceeds the capacity of the hydraulic system, and one leg stops (or slows greatly) until the more easily-moved one reaches full retraction and the fluid stops flowing through that actuator.

Of course – I hadn’t thought of that but it makes sense. Would not have been helped by the fact that to save weight they would have used the smallest, lightest pump possible.

‘Sticktion’.

used to be a fairly well used term to describe the force needed to move (say) a hydraulic ram which has tight seals,certainly is not a term that i invented 😉

‘Sticktion’.

Well, the further up the leg goes, the greater the force needed to keep moving it, as the wheel is getting further from the pivot-point horizontally, and thus is more affected by gravity.

Thus, while the hydraulic pressure is great enough to move both legs from the vertical alignment, the increasing combined force requirement soon exceeds the capacity of the hydraulic system, and one leg stops (or slows greatly) until the more easily-moved one reaches full retraction and the fluid stops flowing through that actuator.

Thanks guys, I guessed it might be something along those lines though would that account for the odd case of both mains starting off together then one pausing half way until the other is retracted as occasionally happens? Actually I guess it would if one of them binds a bit somewhere in its travel.

hydraulics.

Having multiple legs retract at the same time without proper valves causes severe cavitation problems as the legs hydraulic fluids “fight’ each other on their way up.Have seen it myself on a homebuilt and it doesn,t look all that comfortable for the aircraft.:D

I would guess that everybody is correct!!;)
Some aircraft did appear to have a deliberate ”lock one up,then t’other” but other a/c i would say definitely fit the ”the leg with the least friction/stiction” will retract first followed by the other(or sometimes just a bit slower!!),hydraulic systems were very basic in the 40’s and i dont think many manufacturers would have fitted a sequence/priority valve for legs.
I also wonder if an (airborne) slip/skid might assist one leg if you have a low pressure system!!
Even on more modern a/c a slight imbalance between the port (L) and stbd(R) restrictor valve can make retraction assymetric;)

I thought it was more deliberate than that, i.e. a valve caused one leg to retract and then the other.

Spot on…..one hydraulic system retracts both (or all three) legs that way. Simplificating and adding lightness 😉

I thought it was more deliberate than that, i.e. a valve caused one leg to retract and then the other.

I believe that is the case also. Without adequate sequencing valves and flow limiters the undercarriage will retract unevenly – the pressure will move the leg which has less friction first. Under pressure, fluid will exerpt the same pressure throughout the system, so the part which moves more easily will move first. Does that make sence?

As above, it was to share the available hydraulic pressure.

Personally I had always assumed that this was only down to the fairly uncomplex hydraulics used, which gave rise to uneven retraction because of limited hydraulic pressure and the demand placed on the system…
I may be wrong, but it makes sense to me :confused: