REFLECTOR: Hydraulic system pressure

[David Rene Dugas]

Since over center was mentioned the health of ones nitrogen over center cylinder is important in function and safety when gear is down although not in up position. Without it the bounce could easily negate the over center linkage. I was adjusting my linkage yesterday and was curious if the nose would collapse if the gear was retracted 6 inches with occupants but no landing violence?

René Dugas
Apple the REALLY smart phone. 

On Sep 5, 2013, at 8:14 PM, Don Johnston <numa at comcast.net> wrote:

> That assumes the system is closed. Which means that as the nose tries to retract, pressure builds which will prevent it from fully retracting.
> 
> I was addressing the previous comment "When the nose gear is over center, pressure from the nose strut actually pulls on the cylinder rather than pushes, so the hydraulic pressure has no benefit there"
> 
> My contention is that pressure in the system (either static or dynamic) is needed to prevent the nose gear from retracting.
> 
> 
> 
> 
> On Thu, Sep 5, 2013 at 7:39 PM, John Dibble <aminetech at bluefrog.com> wrote:
>> When I was fitting a new nose cylinder, I opened the dump valve when I wanted to retract the gear by hand to check clearances.  Sometimes I would forget to open the dump valve.  In that case I could still push the linkage to under center, but I could not budge the wheel when I tried to pull it up, so I don't see how it can retract, even when under center, if the valve is closed.  The next time I have the canard off, I'll bleed off the pressure, push the linkage under center and see if I can push enough to compress the shock spring and see what happens.  I'll be surprised if it retracts.
>> 
>> 
>> On 9/5/2013 6:28 PM, Don Johnston wrote:
>>> I would be willing to bet that if removed the hydraulic pressure from the system and had a slightly worn gas strut, that in a dozen landings or so you would get enough bounce in the system to have the nose gear come back off overcenter enough to have an uncommanded retraction.
>>> 
>>> But I'm not going to be the one to test that theory.  ;-)
>>> 
>>> 
>>> 
>>> On Thursday, September 5, 2013, Scott Baker wrote:
>>>> On 9/5/2013 4:25 PM, John Dibble wrote:
>>>>> On 9/5/2013 3:08 PM, Scott Baker wrote:
>>>>>> Hello John,
>>>>>> The recommended gear DOWN hydraulic pressure of 550psi is already significantly lower than the recommended gear UP pressure (something in the neighborhood of 1150psi).
>>>>>> While true, the main landing gear does not benefit from hydraulic pressure when in the gear DOWN and LOCKED position - the nose gear linkage is aided by hydraulic pressure to keep it in the over-center (Down/Locked) position.
>>>>>> Somewhere in the Reflector archives is a video focusing on the nose gear mono shock at the moment of landing.  When the nose wheel touches the runway, the top of the nose strut wants to move rearward - and in fact, it does move a small distance, which is counteracted by the mono-shock.  Without the benefit of hydraulic pressure to help keep the nose gear linkage in the over-center position, the nose gear could easily collapse at the nose wheel touches down.
>>>>> I respectfully disagree.  With the linkage in over center position, rearward strut motion will try to push the linkage even more over center, but the cylinder won't allow that because it's extended as far as it can go.
>>>>> 
>>>>> John
>>>> 
>>>> Theory and reality sometimes do not match.
>>>> When considering "What gear-down hydraulic pressure is the best?" ... Since the hydraulic system is already in place - and since the factory recommended 550psi is a really low pressure for hydraulic systems that does not cause undue stress on hydraulic seals (and is proven to work) - why not err on the side of what works?
>>>> Fixing an abraded nose section following a nose gear collapse is a major inconvenience.  Why invite trouble?
>>>> Best,
>>>> Scott B.
>>>> 
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