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Purging Air From Light Aircraft Hydraulic Systems

Purging Air From Light Aircraft Hydraulic Systems

Purging Air From Light Aircraft Hydraulic Systems


I'm working on a hydraulic system for light aircraft landing gear retraction/extension.
Everyone I know that operates similar systems says it is very rare (like essentially never) that they purge/bleed the thing.
(These are owner/operators, so maybe others are solving that problem for them.)

Hydraulic mechanics tell me similar things about larger construction hydraulics - you just cycle the things and the air ends up
in the tank, where it harmlessly bubbles away. (These sources are real maintainers and trustworthy in that context.)

My confusion is that since any volume in the cap end after retraction, as well as the volume of the line from the pump to the cap end, never
gets expelled to tank. How does that air get purged? Similar remarks regarding the rod end.


RE: Purging Air From Light Aircraft Hydraulic Systems

If there is air at start-up in a cylinder: it dissolves in the oil which comes from the pump when the pressure increases. Then when the oil returns to the reservoir the pressure in the fluid becomes zero. Then the air rises to the surface in the tank. Phenomenon of the coca cola bottle when the cap is removed.
If the volume in the cylinder is large compared to the volume consumed at each cycle and the assembly is unfavorable (for example cylinder higher than the tank): the solution is to sweep. We feed by a pipe with a check valve at the entrance of the cylinder and we come out by another pipe.

RE: Purging Air From Light Aircraft Hydraulic Systems

Thanks very much for the info. I was able to look up the specifics (solubility at temp for water in my case) and from that I can get enough of an idea to proceed with
prototype. In my case I'm probably going with something like dex-cool for the fluid and as it is a 100 psi working pressure system (hand pump) I even found tables on
line up to 100 psi for air.

The separate return (sweep as you mention) makes perfect sense and I'm hoping to avoid the complication.

Thanks again for the explanation. Air in the lines for the first proto was a real show stopper. Now I have hope.


RE: Purging Air From Light Aircraft Hydraulic Systems

"My confusion is that since any volume in the cap end after retraction, as well as the volume of the line from the pump to the cap end, never gets expelled to tank. How does that air get purged? Similar remarks regarding the rod end."

In general I agree, your typical cylinder will purge itself from air by just cycling it a number of times.

As I understand there are two mechanisms that removes the air in hydraulic systems:
1) Mixing of air and oil via turbulence
The air is physically mixed with the oil, typically where the oil flows into the cylinder. Like shaking a bottle of olive oil. While the air will separate out very soon again, it will still give time for at least some air to follow the oil out of the cylinder.

2) Absorption of air in oil
The same mechanism as in a soda maker. Even with moderate pressures, the oil can absorb a good amount of air. This air will come out as bubbles once the pressure is lowered.

However, there are a few traps/ things to watch out for:
1) Long hoses/small actuator
If the full cylinder volume can fit inside the hose it will be much harder to get the air out, if not impossible. The air bubbles will just go back an forth in the hose without ever reaching the tank. In that case active air bleeding is the only option.

2) Pilot hoses
Same reason as for 1). While the air will in theory transmit the same pressure as it is compressed, my personal experience is that air in pilot hoses for valves or pump regulators will just cause weird behaviour that defies logic.

3) Dieseling effect
Especially with larger cylinders. It is described here: https://www.powermotiontech.com/sensors-software/m...
Google responds well to dieseling hydraulic cylinder.

4) Tank design
A poorly designed tank will not separate out air well. If there's no middle baffle or if the return flow is close to the pump suction, the air that was purged out of the cylinder will go right back out into the system. Right after startup of a new system with lots of air everywhere all the oil in the tank can be aerated and it is then a good idea to let it rest for a half an hour or so to let the air reach the surface.

5) Load bearing cylinders and motors
A motor holding a load (such as a winch or a tracked vehicle in a steep incline) should always start by lifting/going uphill to avoid running air into the motor from the low pressure side and potentially lose the load. Typically a field repair situation.
A cylinder running a load overcenter (over the top) should be filled with oil on both sided. If not the load will be lost when the load changes direction and meets an empty cylinder chamber.

Please note that this is just my personal experience from the stuff I worked with. Other people working in other branches of hydraulics with different type of equipment may have other experiences.

RE: Purging Air From Light Aircraft Hydraulic Systems

Hey - good point on the pilot logic supply. I'm trying to rely on that for uplocks (load holding) so I guess the only hope there is gravity and tapping at install time.

I'm trying to use antifreeze as the fluid and as it's 100 psi I expect no diesel effects and a worse time of self-purge.

Thanks for all the info.

RE: Purging Air From Light Aircraft Hydraulic Systems

Before choosing antifreeze, take a look at the water-glycol fluids that are deliberately formulated for use as hydraulic oils ("HFC fluid" is a useful search term). The lubricity isn't great, but at least there is some.


RE: Purging Air From Light Aircraft Hydraulic Systems

Great info on the HFC of which I was unaware! Search worked well as you suggest.
Thanks very much, just the ticket.


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