@Peter wrote: ….. but this would be subterfuge by the gas shock supplier – prob90 some obscure French company catering for the homebuilt market, now long ago bankrupt – rather than Socata who would have not known anything about it.
You often write about obscure French companies pr some guy in his garage etc etc.
But The Daher Aviator Marketplace does keep stocks of legacy aircraft parts for Rallyes and TBs. I think they also have parts made.
AIUI they also do the maintenance manuals etc. Or at least that’s where a guy near here, basically rebuilding his aerobatic Rallye has got all his stuff.
Or am I missing your point.
The point is that Socata publish a CMM (component maintenance manual) for this gas shock which shows two o-rings.
I think Vic is correct in general but it remains a mystery. There must be some other factor in operation.
Note also a groove for another o-ring, unused. An o-ring in that position would not have exerted pressure on the wiper seal.
Ok I see things don’t always end up as originally designed.
at all. I would not want another o-ring in there, with that kind of high air pressure the strut would get a bit stiff in action. In industrial hydraulics you would not find o-rings in same operation conditions, instead polyurethane lipped seals. But sure, in aircraft there are constraints in available space
for that type of seal, so o-ring it must be.
It is debatable if use of a feltring is a great idea, in fact a matter of trust in the effectiveness of the wiper seal. Provided it can keep grit out the oiled felt would be kind to the wiper seal for long time, comparable to teleforks on m.bikes. When that effectiveness degrades the felt will collect dirt that may work its way in and wear bushes. But then that strut gets no real sideloads from wheel forces like in teleforks.
For sure, but legally Socata are on the hook for publishing a CMM like that.
Also, FWIW, I have not previously heard of the wiper seals popping out.
My plane had one gas shock with 1 o-ring and 1 gas shock with 2 o-rings. And since mine was one of the last TBs made (basically, TBs from 2002 and 2003 – the last ones – were assembled largely from a pile of bits made in 2000-2001) it would seem probable that if indeed the “one o-ring case” was deliberate, practically all TB20/TB21 planes made would have had 2 o-rings. And all of them should have popped out.
Or, hey, practically all TB20/TB21 planes made would have had 1 o-ring 
But that’s unlikely since the CMM mentions 2.
More likely IMHO the person assembling one of mine had a bit much red wine 
Yes, I have been carefully wiping my wiper seals before flight, and I think that is why my two gas shocks have made 21 years which is much longer than most others have managed to get.
I wondered whether that wider groove was intended for a rectangular cross-section seal, replacing both o-rings.
or groove roughness. When same imperfections are present on the places of upper o-ring, no sufficient pressure build-up will happen for popping out the wiper by its upper o-ring. So it depends on individual struts and finish to face that pop.
Anyway, I would not want to risk a second o-ring on standard place pressing out the wiper, too much work of replacing seals in a disassembling party.
This is also applicable.
I went to work out how much pressure would be needed to gas the unit up in situ. 35mm diameter, and assuming 1/3 of the 900kg empty weight is sitting on it, it comes out at 31kg/cm2.
I make it just 439psi, which seems wrong. Way too little. That’s because it is… That is what is needed to just lift the shock off the bottom. In reality you need to achieve 439psi with the unit about half extended, so what is the volume ratio? One would need to measure that, and measure it with a fill of the specified volume of Fluid 41. My guess is that it is of the order of 5:1, leading to about 2000psi.
You would need to push this past the schrader valve. Any opinions?
I think you need exactly the same pressure.
For simpler numbers, let’s assume the strut is EXACTLY 1 sqare inch internal diameter, and the weight that needs to be supported is EXACTLY 500 pounds, so you need 500 psi pressure to counteract the downward force of the aircraft. This force requirement is completely independent from how far the strut is expanded (as long as it is off the stops, and ignoring the minimal “seesaw” effect from the change of aircraft angle).
So when you inflate a nearly-flat strut at 500 PSI to 501 PSI, the strut will expand (1 pound more upward force from the strut than downward force from the aircraft) and lift the aircraft a little until it is again at the exact pressure required to hold up the aircraft – 500 psi.
Of course the bottle you use to fill it will need pressure werll above 500 psi to contain enough gas to fill the strut before it drops down to 500 PSI; so a bottle with 10x the volume required to fill the strut to half extension and 1,000 psi pressure can only fill up the strut from flat 5 times, or 15 times if it is at 2,000 psi.
I should just defer to people of a superior IQ, every time
Of course you are right, and I – without realising it – discovered this yesterday, when I connected my refill rig to the strut, no jacking up yet, and it has a gauge on the output port, which was showing about 500psi.
So no need for jacks at all. Bloody hell. All that nonsense written in the MM, the CMM for the strut, US Socata forum… Makes my life far easier!
Thank you!
