pneumatic retracts problem need Help!!!
 

I am building a Tamjets F-16 and have had all sorts of issues related to the landing gear functioning properly.
To make a long story short i have 2 remaining problems:
1) Nose gear is giving me a hard time as it doesn't unlock unless you give it a little push. But the weirdest thing is that when air tank is almost empty it unlocks although the main legs have no sufficient air to unlock. I have tried to grease the axle and sides of the unit but this didn't seem to help.
2) Gear seem to open/close much easier while plane is in an inverted position then when it is upright... i can do several cycles on a same air tank while inverted then when plane is upright,,,
To make things even more complicated (while it is meant to make install easier, i have one main leg hit a switch valve when fully retracted to close the doors. here again as nose gear stays stuck, its door closes before hitting the extended leg...

Thanks for looking and appreciate suggestions to solve this issue,

Is the retrac leg and tire jammed up against the intake liner when retracted?? A picture of the nose retract install would be helpful.

From your description it sounds like when the gear is fully up, the cross bar on the end of the cylinder rod is jamming in the cutouts in the side frames at the end of stroke. The fact that the gear releases more easily when the pressure is low points to this as there isn’t so much force to push the cross bar into the jamming position.

There are a couple of reasons why this might be happening. One is that the cross bar reaches the end of one slot before it reached the end of the slot on the opposite side. Then what happens is the cylinder forces the cross bar until the end of the other slot is reached twisting the cross bar and jamming it. The way to check for this is to manually push the cross bar to the fully up position with no air in the system and check that the cross bar reaches exactly to the ends of both slots. If it doesn’t, it might be that the side frames and the cross pieces that join them are out of alignment. It’s worth loosening all the screws holding the side frames and seeing if you can realign them before tightening the screws. If this doesn’t fix the problem your next step is to find a round file that fits in the slot and carefully file the shorter slot until it matches the longer one until the cross bar reaches the ends of both slots exactly.

If you’ve checked and the cross bar is reaching the ends of both slots, then the problem is the shape or surface finish inside the ends of the slots. The fix here is to use a piece of say 400 grit wet or dry sandpaper to smooth the insides of the slots and very carefully ease the point where the cross bar goes into the lock position so that it can slide out when down is selected. A good application of grease can help.

Unfortunately a lot of the engineering in the model world leaves a lot to be desired and leads to this type of problem!

Hope this helps,

Malcolm

Just to add to the last post, sometimes the bearers where the retract unit bolts to are not completely level. When the retract is bolted down this twists the side frames causing jamming.

Before you do any rectification it’s worth unbolting the retract and see if this helps the situation.

Vincent,
nose leg is not in contact with the liner, in fact it was doing the same thing before installing the liner.

Malcom,
Thank you for taking the time to explain in detail. slide is moving correctly and reaching both ends at the same time.
however what you said about the twisting could be the problem in addition to having to smooth the edge where slider goes into lock position.
i hope i will get a reliable smooth movement at least on the ground as i am also expecting surprises while airborne especially with the door mechanism which is activated by one of the main legs, if door is not fully closed this might create a leak and empty air tank.

Follow up on the subject. when i loosened the mounting screws i felt that nose gear worked a little bit better but still needed somehow high pressure to close.
Another issue has now appeared as nose gear door is closing before the leg jamming into it. i tried to insert a restrictor on the out line of the door to slow it down but it didn't work.
This is quite frustrating, i haven't been in such an awkward problem before this installation.

I agree with everyone concerning the twist frame. I’ve used washers under some of the bolt holes to level.
The knuckles still might be too tight.
There might be by pass in the actuator, and not giving the retraction cycle enough power to bring the gear up. Of course as you said, jet inverted the gear easily retract, because gravity is helping. If you have a stethoscope, cheap at auto parts stores, to listen to the actuator. Or just go ahead and lube the actuator with O’ring oil.

While jet is inverted nose gear extends easily and operation is more consistent which is somehow weird and puzzling.
i have already dismantled/ cleaned and oiled all 3 actuators just in case.

Even if i get consistent movement on the ground i still cannot guarantee that it will work while airborne if air pressure will push the main doors open it would release the pressure on the doors switch valve....

So it looks like there is still a mechanical friction somewhere and with the model inverted gravity isn’t pushing the components in the direction where the friction is. Tight trunnions are still the most likely culprit. Won’t harm anything to remove the trunnion block and ease the trunnion bores with 400 wet or dry wrapped round a wooden dowel.

re the nose gear door issue, a fix would be to add a second switch valve triggered by the nose gear fully retracting. You would then pipe the two valves in series so that both mains and nose gear need to be retracted before doors start to close. A pain I know but you aren’t going to get enough delay with restrictors and one valve.

Malcom,

Thank you for your input. i will try to sand lightly trunnion bores and see what happens. Actually if i get a smooth movement of the gear the nose gear door will not be a problem anymore...

A video of the operation would help with diagnosing.

Why don't you move the door switch valve to the nose gear?

I see this all the time in industrial pneumatic designs where an understanding of how cylinders are properly powered with pressure and slowed with restrictors/check valves.

The applied pressure to the in side of a cylinder should not be restricted, and it is the exhausting side of the cylinder that is where you restrict it to control the speed.
The problem comes in that the out restricted side will be needed as an in requiring pressure and must not go thru the restrictor and now the other end becomes the out or exhausting air needing restricted

In industry a cylinder flow control restrictor has a built in check valve to bypass the restriction when going opposite direction. What you described is exactly why not doing it proper does not work,

Here is a diagram of the cylinder restrictor/check valve disregard the valve porting prior to cylinder

https://fluidpowerjournal.com/determ...ylinder-speed/

I use clippard mini restrictors with built in checks for smooth retracts. Some times you do get away with restrictors in the exhausting side for up retract and none on the down side because air pressure going into a restrictor for down generally needs little air volume as the weight of the wheel and gravity work in your favor snapping down is generally better than up.

Trying to download video without success...

Unfortunately with the inlet duct in the way there is no room for the door switch.

Buy another valve and use a sequencer. .

i am using the behotec valve (gear and brakes on the same valve) in addition to the gear door switch. 1 source for air supply.
Changing valve will necessarily mean changing the whole setup... Too late at this stage.