Tech question on inflation of main legs….

Pressures for the Miles Gemini oleo are – unloaded 400 psi and fully compressed (1900 lb wheel load) is 1885 psi.

227 times normal atmospheric pressure (roughly) sounds pretty dangerous to me, being used to blowing my car’s tyres up to 2.2 bar.

I only mentioned the blow down bottle as a comparison between Bars and PSI as a unit/ measure of pressure Mike – 227 bar does not sound very dangerous whereas 3300 psi does 🙂

rgds baz

The blow-down bottle mentioned earlier is only charged to that high pressure because it needs to discharge a relatively small amount of air into a larger system and extend some oleos. Once it is discharged the pressure rapidly drops off as the gas leaves the bottle and enters the system, the pressure reducing to a more normal level as it does so.

Just think of an analogy of a small CO2 bottle being used to inflate a normal sized balloon. The bottle pressure needs to be that much higher to contain enough gas to “operate” the system, i.e. inflate the balloon, which, once inflated is at a much lower pressure.

Anon.

I rebuilt one that crashed…. replaced the leg etc, the new brake caliper provided by the Poles caused all sorts of problems, I would bleed the brakes, it would be soft the next day, It went on for weeks, and everyone from the owner down was questioning my abilty to bleed the system, in desperation I took the new caliper off and dropped it in a coffee jar of brake fluid and lo and behold it was actually gassing…. no matter what I did to remove the air in the system, the caliper was overnight adding air back into it!!! Gawd knows what they made it from, a second item cured it. I have flown in them BTW.

It was doing a PFL and in the aircraft behind awaiting to take off, the instructor with a trial lesson explained what was happening, anyway it came down hard, tore off the leg and wheel, landed on the wing tip and went skating off the side of the runway, the trial lesson turned to the instructor and said… “they don’t half make it realistic” 😀

Tony -they are incredible fun to fly in the big engined versions as long as you dont
think about how they are built!

Brain fade lol, either way it should have been burnt at build.

The Koliber which is a polish built French Robin had a similar pressure if I remember correctly, similar strut too.

Polish-built Rallye, Tony! 🙂

The Koliber which is a polish built French Rallye had a similar pressure if I remember correctly.

just checked it was 333 psi

http://www.morane.de/technik/12-eram-stossdaempfer

The Spanhoe Norviege is getting the same treatment – give them a ring.

Just to clarify it is a flier, the situation is our machine is like no other so a look in the book is not possible! Having said that the later production machines are based on ours so lots of info can be drawn from their manuals! I suspect our main legs are from a production machine as they have high serial numbers so in theory I could inflate them to the quoted fig (as per photo on the leg ) BUT a production 856 is 305kg heavier so…….. My main question was really that in my experience this seems a really high PSI for a light aircraft leg so does anyone else have any experience of high pressure oleo’s on GA/Vintage aircraft??

Didn’t realise it was a non flyer – good advice from FB about possibly using oleo collars.
If that is not possible I would wait until the weather warms up and put in a much lower pressure in than the oleo placard,I doubt you would need full pressure for a static a/c.

rgds baz

Sounds like this isn’t a flier. My advice, if in doubt don’t do it – pressures are dangerous as you know and there’s a manual process for a reason. If I were you, I’d lift it up and collar the oleo until you can get the right book and the right figures.

FB

Hey Jon – If I was in your shoes I would double,triple and quadruple check before I inflated the damn thing 🙂 .
Might be worth jacking up,taking a pressure reading and then trying a smaller pressure increase and see what the extension looks like.
Alternatively – wait for warmer weather when the pressures will go up a bit and see what she looks like then 🙂 (that is a half serious suggestion btw)

rgds baz

Thank you for the replies and we had about arrived at what you say, so its nice to have others say the same! Just sounds a high pressure on such a little bird! Maybe I should just man up and charge it to the correct figure but must say it sounds mighty high!

62.5 kgcm2 = 890psi which still seems quite high for a lightish a/c.
I always used to try to charge oleo legs at max extension (weight off wheels) as it is easier to get the pressure exactly correct,otherwise you usually have to use a pressure/extension graph.And of course the extension will only be correct if the oil level is correct 🙂

One of the reasons I hate metric pressure units – they are so large.

What sounds more dangerous 61 Bar or 880 psi ?

One of the a/c I used to work on had an u/c blow down bottle charged to 227 bar (which = 3300 psi)

Detendu = relaxed

So it will be 62.5 kgcm2 if the weight is off the strut (ie a/c jacked up with strut leg at full extension)
Sometimes known as ‘base’ pressure.

The higher pressure will be with weight ‘on’