Hey Fellas, (sorry about the first post, I hit the wrong button)

I am haviong a retract problem...Not jet related as it is with a p-51, but I thought you guys would be the best help.

Problem.... Retracts operate somewhat jerky. I am planning on removing them and using some white lithium grease on all friction areas. I have a flow control on EACH retract for best control. I have NOT flown the plane with these retracts yet, but I am thinking that the motor vibration may actually help.These are brand new ROBART retracts. I am also going to be changing the wheels that I have to a lighter foam wheel.

What do you guys think? Are they just going to be this way, or should I be able to get them to operate smoothly. Seems to me that most of them on the jets either snap up quickly or are very smooth looking.

Thanks,

Hey what?????

Good idea greasing the moving parts!

Have you tried to open up the flow restrictors a bit giving the struts a bit more speed on the way in?
Are they also ''jerky'' on the way out?

Where are the restrictors located in the circuit?

On the pneumatics we use at work, most of the restrictors are 1-way to avoid this problem, only acting to restrict flow out of the cylinder. If you're feeding pressurized air into a cylinder, you can limit is speed on either end. Problem is, when the cylinder gets 'sticky', an inlet-side restrictor can cause the pressure to build up, then overcome the stiction, then the supply of pressure is not enough again and it stops part way through the stroke. The pressure then builds up again, breaking the piston loose again and the cycle repeats itself until it hits the end of stroke...

If the restrictors were placed on the outlet side of the circuit, the disadvantage is that the cycle will begin rather fast, then slow down once the outlet pressure builds up behind the outlet orifice. It should come to a comfortable medium and flow evenly from there right to the end of stroke though. I think this is how the Robart valves work although I'm not sure of it... I think Yellow Aircraft the retracts in my Stingray are the same.

Regardless, I think your best bet is to start with relubricating the cylinders and joints. I did this with a syringe and air tool oil. You won't need much... Grease may be too thick for the application. Remove the retract from the model and cycle it each way a few times after putting a drop of oil on the cylinder rod to make sure the seal is wet. (first wipe off any dirt of coarse) Next cycle the cylinder a few times each way, adding a few drops of air tool oil in each nipple while its on the suction stroke. You should feel it loosen up noticeably if that was the problem. Next keep cycling it with a rag near the output to catch the excess oil, making sure the nipple is at the lowest point so it won't pool inside the cylinder. Keep doing this till the excess oil stops flowing. Wipe off the nipple with alcohol and reinstall the hose with a wire tie or one of those Robart retainer rings and it should be good to go.

Hope this helps.

Kelly

Staale,
If I open them up they start "snapping" to position. Yes they are a little jerky in both directions, worse on the way down. I am assuming that this is because they have to LIFT the weight of the wheels and wire struts on the way up.

Kelly,
I am using the ROBART 167 variable rate flow control valves. These do meter the exhaust air, allowing full pressure full volume to operate the cylinder. I do not think that lubricating the friction points is going to help much.... I may pull the sleeve off of the cylinders to make sure they (ROBART) didn't really goober the grease on the O-rings. I may also try a couple of new O-rings (even though these are new)

I must say.... I have been an industrial maintenance technician for most of my life and I am very familiar with hydraulics and pneumatics... Unless I have the airlines pinched somewhere, I think that the issue could benefit from larger tubing and fitting sizes. I believe that some of the retracts probably do work fine with the current setup, but I really think that more inlet air volume would help. When you are dealing with smaller volumes of air, it is harder to meter accurately.

Still open to ideas.....

Never experienced the problem as you describe my self.
Sounds like there may be something wrong with the retract units. If they are relatively new and not worn out, you shouldn't have problems like this... Are they still under warranty? If so I would try to exchange them for a new set....

Disconnect the air lines from the cylinders and operate the retract units manually to see if they bind. I had something similar happen to my YA F-18 when I replaced all of the airlines and Tees. I used all metal tees and come to find out, that the tees I used wouldn't flow enough air to operate the gear properly.

Just a thought.

Thanks Gary.... I have manually operated the gear and they don't feel bad at all...... It is possible though that the small plastic tees are a bit restrictive. I have 2 tees between the tank and the Variable Rate valves, but nothing between the valves and the cylinders.

sounds like a pressure problem.