Trouble-Shooting pnuematic retracts
 

Have a set of Robart Airs that worked flawlessly for 2 years. Now they only partially retract or won't drop when time to land and they are out of air when checked on the ground. Have bench tested numerous times and they hold their air for hours and cycle perfectly. :Put the plane in the air and they don't function properly.

My next step is to replace the air valve but anyone with similar experiences, please respond.

Also, I'm not interested in an electric vs. air debate. If that's your game, please start another thread.

Thanks in advance for your reply's.

Hello,
Just been through a similar scenerio. Only my right gear was collapsing on rollout when landing. Found that the air pressure dropped 30 or so psi during flight. Retracts alone held pressure and also held pressure with small leakage when retracted while on the ground. Tested each line separately and eventually found a brass T for airlines was scored from using a hobby knife to remove tubing thereby causing a leak. I would recommend testing each line separately to find the leak using a half split method of troubleshooting.
John

I'd also redo the air line connections at all the air cylinders. Just cut off the tubing at the air cylinder nipple and reattach. I had one like yours before and that's what it turned out to be. A small break in the air line tubing. The air valve is a good candidate as well. Could be many things. It sounds like the vibration when the engine is running is exposing the air leak.

Good day Lifer; they hold air for hours, is that only with the gear extended for hours? Have you tested with the gear retracted for several hours to see if they leaked air then?

Roger

They held air in both positions. The only time it leaks out is when the engine is running and I am flying it.

But think how good you're getting at belly landing! :D

Radfordrc had the pleasure of seeing me belly-land twice at the "Warbirds over Missouri" meet last week.

I have since re-attached the lines at the cylinders per an earlier suggestion. I am awaiting a replacement air valve coming in from HobbyKing. I have used their valves a replacements before and been pleased.

As a test, I attached the airlines that go to the wing to themselves so as to create a closed loop without the retracts attached. The tailwheel remained in the loop. When I pressurized the system, the tailwheel immediately popped into position and there was no air hissing sounds. When I moved the airvalve into the other position, the tailwheel moved to the other position but there was a pronounced hissing so I think I have found the problem except I don't know why that didn't happen when the wing-mounted retracts were in the loop.

Could it be the smaller total volume without the wing retracts attached made the leak more apparent?

Hi,

This is probably not your problem (or a likely problem for anyone), but a funny little problem I once found. I had a tiny knick in the line coming off of the fill valve, though I don't know how the knick got there. Anyway, I kept losing pressure in flight even though the plane would hold pressure on the ground for hours. As I was trouble-shooting, I accidentally found the problem when my palm pushed down on the line at the valve. In flight, the air would leak because any high-G maneuver (which was practically the whole flight with this bird) would push down just enough on the line (which had a bunch of metal tees right after the valve) to expose/open the knick. Check your lines!

They hissing is coming from the air valve and is normal. When you hook the air lines in a closed loop air is metered from the inlet side of the valve to the outlet side. If there wasn't an outlet side the piston in the retract unit wouldn't move.

Thanks to all. Shaun's advice is one I hadn't heard of but it makes sense and I'll try that next.

I'll report my findings.

get some soapy water and apply around all connections to the valve and to cylinders. I had a hairline crack in the nipple of one cylinder recently. Would hold pressure but the vibration of flight would cause the leak.

I only found it by moving lines back and forth when I suddenly heard the hiss and tracked it down.

the soapy water will show where the leak is as it will bubble.

Best of luck!

Yesterday I re-attached the airlines to the main gear after trimming off the first 1/2 inch thereby making the connection to the air nipples with fresh line. I re-pressurized the system and the gear seemed to have a little more push when I held my hand against the retract motion in an attempt to duplicate resistance the airspeed might offer. It was slightly stronger which might lead me to believe the fresh connection did help.

I then re-connected the lines that normally go to the wing to each other as discussed earlier. With the retract valve in one position there was no air hissing but in the other position the hissing returned and was more pronounced. It was definitely coming from the central nipple that connects to the air tank. I believe the valve is the most likely culprit but the increased volume of the hissing also tells me the main gear connections were weak. When I get the new valve I will install it and repeat these steps. If no hissing, then I will test fly. Hopefully no more skidding in on the belly pan.

I'll post as soon as I get more results.

Well, the HK valve came in. I installed it and pressurized the system the the two lines that normally go to the wing connected to themselves. With the old valve it was quiet in one position and hissed like crazy in the other. With the closed loop described here I believe the valve is pressurized from both sides simultaneously and this reveals valve defects.
The new valve had a barely audible hiss in both positions but I really mean barely. I then reattached the wing lines and aired the system to 90 p.s.i. The system is holding pressure for about 1/2 hour with no movement on the pressure needle. Good enough for me.

Now that it was out of the plane I gave the old Robart valve a good looking over. One side of the shaft is smooth but the opposite side has a lot of what appear to be machining marks parallel to the direction of travel. I smeared some of Robarts valve lube on the marks and they became indistinguishable. I wiped the lube off completely and the marks were visible again and you could detect them with a fingernail.

The valve has 2 years service and that may or may not be a good lifespan, but I am going to chalk this one up to poor quality control and the use of a lot of grease to make it serviceable.

I'll fly it sometime next week and report on the results with the new Hobbyking valve..

Sounds like you have a leak in the retract unit itself. Put the gear in the position that causes the hiss. Then clamp off the lines to the retract unite one at a time. When you hear the hiss stop you have isolated the guilty retract unit. It may need cleaning or some oring lube.

I'll try that. Thanks!

Yesterday I flew the plane with the new valve and all the other changes previously discussed. The plane took off, I hit the retract switch and watched the gear go up. I did a smooth left-hand turn and one of the gears was already down by the time the turn was complete. I hit the switch and had enough air to push it down and then landed. It was a 45 second flight. Took off with 95 psi in the tank and landed with 25 psi. Barely enough to get back....in a 45 second flight.

It held air on the ground for over 5 minutes, in both positions, on the pre-flight test. It dumped it's air during the retraction cycle. I'm thinking the seals in the air cylinders in the main retracts.

Any thoughts?

Wow that's a major loss of air. Definitely not a slow leak. Maybe the air fill jack or bad servo not closing the valve completely under vibration? Nipple on the air tank? At this point I would completly replace the airlines and air fill jack. Sorry can't be more help but it's really shooting in the dark. Sure would be nice if you could run the plane on the ground with the gear up to help isolate the problem. Otherwise it sounds like a belly landing is coming up.

New servo, new airvalve. Yeah, I've had a lot of practice at belly landings. Next step is to lube the retract cylinders. Last step is rebuild those cylinders. After that, drinking.

The Next Chapter:

Threw in the towel and called Robart. Described the situation to a nice guy named Eric and he said he had never heard of that before. Great. So, rather than imitate a dog chasing it's own tail, I decided to send it to them for inspection and repair. Price seemed reasonable (the estimate) so it was mailed Friday. I'll update when I have more.

Funny thing, I flew a Robart-equipped Giant Scale P-40 this morning that I have never touched in 3 years and it worked perfectly....

If anybody is still following this thread, I'd like to submit my latest findings:

The mains were sent to Robart and they tested and found one leaked and the other was fine. They replaced the defective one, cleaned the other and water tested both. Total charge was about $40 which I consider very reasonable and I had them back in less than a week, including my mailing time. If anybody wants to complain about Robart, call me.

I test flew the next day and the gear retracted normally but just like the last time, they were coming back down by the time I completed the first turn. I hit the retract switch and surprisingly, they came down and locked. Upon landing they still held about 60 of the 100 psi they took off with. The leak was slowed, but not stopped. They had tested fine on the ground but failed to go up and lock. They were losing air only at the almost locked point.

The only thing left was the tailwheel cylinder. It came in 4 days and once the old one was out I plugged the air nipples one at a time and pushed the rod. On the gear-down position, it held pressure just fine. On the gear-up position, it held absolutely no pressure at all. Pre-flight testing didn't reveal any of this but with the gear fighting wind, trying to retract after takeoff, the seal simply vented out the pressure.

Will install the new cylinder and test fly soon but I suspect this year-long chapter is concluded. I hope somebody else can benefit from this thread.I'll post the flight-test results at that time.

Glad you solved the problem.

radfordc,

Appreciate the thought but I won't consider it solved til I land on the wheels with pressure still in the tank. Club meeting tonight so I'll leave at 3:30 and mow then fly before the meeting. I'll post the results.

Final Report:

Test flew before the club meeting tonight. All retract functions normal. Nobody, not me, anyone here, not even Robart considered the tailwheel retract air cylinder as a suspect. In retrospect, it should be considered just as any other air cylinder as they are all looped in together when pressurized.
Hope this helps someone in the future. I gained a lot of experience in the retract systems and hopefully it will be a lot easier to diagnose in the future.

Thanks to all who chimed in!