Hydraulic Retracts
 

http://www.youtube.com/watch?v=0CSChvsn6us&feature=youtu.be For some time I have wanted to convert a set of retracts to hydraulic. I have the pleasure of being on the flying field with Mick Burrell quite often and he has demonstrated his hydraulic retracts to me several times. He unlike many never seems to have retract problems on the flying field despite having several large models.


Mick has posted on RCU before about how he went about it and it looked straightforward enough. I had a set of Behotec C50 retracts that were spare and I was planning to use them in my next plane. My first job was to modify the retracts for the new plane so the mains have a 20 degree forward rake to match the original design. They also needed to twist and turn and I made up a ball link with a positive lock so they hopefully will not wobble. The mains are 300mm long from pivot to bottom of wheel and the nose leg is 400mm long.


The nose leg was much more of a problem as ideally I wanted 20 degrees forward rake but decided to go for 105 degrees. This involved changing the pivot point and making a new trunnion. Thank goodness for CAD!


I used Behotec oleos and lengthened the nose leg. The wheels and brakes and hubs are made by me.


The retracts need 4mm festo fittings and I discovered that the cylinder was easily removed and the end cover unscrewed with a tool I made with 2 pins in it. Drilling the end cap was easy work as was tapping M5 thread for the Festo fitting.


The nipple on the body of the retracts was different and has an airway drilled at a slanted angle into the M3 tapped hole. I was concerned about breaking through the aluminium, but went ahead and drilled it out to 1.3mm from the 1mm it was originally. I used a M4 Festo which has a M3 thread so only a very short section was at small bore.


The cylinder I used as the accumulator was an old Robart cylinder I happened to have lying around. I went through my springs and selected one that would give 120 psi air pressure when inserted inside before bottoming out. As it turned out the retracts seem to operate comfortably at 60 psi so there is plenty in hand and I can change the spring for a stiffer one to raise the pressure.


The micro switch I positioned so that it was at 95 psi when switching off with the piston of the Robart cylinder. Drilling and tapping the robart cylinder was straightforward after unscrewing the end and removing the piston.


The final part of the jigsaw is the servo operated Battleswitch. I had already found the system retained pressure over many hours with no leaks so I would have confidence in having the accumulator piston activated all the flight. This can be turned on manually before the flight with a servo controller to save using up a channel on the radio.


The Eurokit valve work great and is ideal for the job with a simple servo operation. The pump I used is an Xicoy brushed fuel pump and seems powerful enough for the job. I use a small 2 cell A123 battery and that copes with many cycles.


Hope this is all of help to anyone thinking of having a go. This is not my idea or system I just followed Mick’s directions, thanks Mick!!




John

Very clever! thanks for sharing.

Liked the detail of using a robart piston with a microswitch as acumulator... maybe this solution as a whole is better than electric retracts?

Looks great, but how do you keep air out of the system (or bleed out the air that does get in) on a plane with removable wings?

You can get quick release fittings that do not put air into the system. Sprint cars use these all the time for a quick change of there front end.

Looks an excellent solution.

has anyone tried this kit ?

http://www.hobbyking.com/hobbyking/s...arch=Hydraulic

Yes there are a number of modellers who have used the UMS system and it works very well when set up rite.

Erbroens

I tried electric actuators on these retracts and gave up on them, and I think that the hydraulic retracts could be better as the pressure can easily be increased by changing the spring pressure.

Dennis

Yes I looked at the UMS system that HK sell. They were out of stock when I last looked. I contacted UMS in India directly and have a pump coming so I can see how it compares to the one I am using. The benefit of Mick's system is that it does not need electronic pressure sensing, it is a simple mechanical actuation. Difficult to see how anything could go wrong.

Kevin

The air bleeding is not difficult. The volume of the cylinders is greater than the pipework so it bleeds out into the tank. Mine is completely free of bubbles. It will work with air in it but they appear 'spongy' as they retract. The retracts these will be used in are for a plane with a centre section that contains the 3 retracts so I will not have to disassemble them. Mick has several planes that need wings removing and does not have a problem, once the pressure is released the Festos can be undone and bungs put in place to save any drips. After assembling the plane the retracts can be cycled a few times to get rid of the air. Alternatively quick release fittings can be used.

Exactly, the acumulator acting as a pressure regulator... I am thinking in a machined piston with a adjustment screw to fine set the spring force.

John
don't give up on the UMS controller, it works just fine. I know Mick got stuck with some from a faulty batch in the day but I have installed this standard system in very many planes now with countless cycles. My original JL Mig conversion is still going strong many years later and have never had a controller or motor fail.

The latest installs I am doing at the moment is in a 1:5 Scale Jets F16 with main cylinder being replaced by 2 x SMC 16mm air cylinders as well as my own FEJ 1:4.6 F16 and my SM 1:7.5 F14.
As for disconnecting wings, I have this system in my JL little T45 and simply fit plastic twist type connectors on the 3mm lines, the fuse has 1 x male and 1 x female and the same on the wing. Because the UMS system does not hold constant pressure you simply break the connections and join the male/female fittings together on the fuse and the same on the wing. Due to the high surface tension of oil, you get zero leakage when you break the lines, it is even hard to shake the oil out of them.
the other big trick on systems that don't run traditional trunnion locks is to use the Festo pilot operated check valves to lock the oil in the cylinders. This is a must for the Migs.

I just tried the stiffest spring I have and it needs 10 bar of pressure 145 psi to compress it. The little Xicoy pump is able to easily compress it. This gives much more pressure and more of a safety margin.

Bart
Yes I fitted a non return valve otherwise the pressure would be lost and it would switch the pump back on. The system holds pressure overnight in both up and down directions without any leaks.

I did not give up on the UMS system, I just was not able to buy it so made one up from largely scrap parts I had to hand.

Incidentally I did not mention that I used LHM hydraulic fluid.

John

I don't have much to add to all that except the accumulator system does retain pressure as with my SM A10 the gear is locked down by pressure, nothing else.

m

Do you have a link to those check valves?

Hey... what type of oil have you used?

here´s my Hydraulic setup

http://www.youtube.com/watch?v=0rZmV1jgBMA F-16

Thomas

try this link.

http://www.globaljetclub.com/index.p...product_id=848


john

Oil used in UMS system is the Singer oil which is a sewing machine oil.
Valves are Festo HGL MS-Q54 don't have a link but that the number on the valve. Hopefully Aussie Bart will chime in and give you more detail as he is the Hydraulic engineer by trade and knows what he is talking about.

well i have a problem when flying in cold weather about freezing... then the oil is to thick, and action is very slow.

I used LHM hydraulic fluid.

Kaspar, I was very interested in your video , nice job. I also tried a brushless speed controller to adjust the speed. I left this for the moment until i fit out in the plane. I shall be using a Powerbox Cockpit SRS. This has a built in sequencer and has some useful looking tools for doors and retracts. Did you not use the electronic box? The servos on the doors looks like a nice solution, but there must be a lot of them!


John

http://www.citroen-ds-id.com/gen/Hyd..._Fluids_3.html


Have a look at this page which discusses the problems of low temperature affesting viscosity.

John

John,

Thanks - this is just what I'm looking for!

Bob

the box from UMS was crap.. the motor ran for a long time.. the pump got hot... and no speed adjustment.

my next hyd gear i would use gear sensor for the doors instead of a timer, and the same for the pump, but still use a speed regulator, or maybe something analog dail in the model

about the citroen fluid... i did not become any wiser... is the viscosity for LHM also great in about freezing? can you link to a bottle of it?

Don't know if any of you know Captainbob on here but he finished 3rd at Toledo last year with a Skyhawk jet that had a custom made hydraulic system that ran his retracts, steering, slats and speed brake. I spent some time talking to him about his system and design. You may want to PM him...he has a wealth of knowledge.

The viscosity of the sewing machine oil is around 10 cst, it's variation from hot to cold is the lowest of all the grades. Automatic transmission fluid (ATF) has a much flatter viscosity change however it is 32 cst and too viscous for the size lines and nipples we are using.

if the viscosity of your oil is too high, the pump works hard just pushing the oil through the lines. These are little gear pumps with fixed clearances and the oil is designed to "slide" past the gears from the pressure side back to the suction side when you get to around 10 bar, without this function and no relief valve your system will fail.

The other great benefit of sewing machine oil is that it has no additives, it is the additives that fail in the extreme cold.

As for speed, you simply set this with battery voltage. I run anything from 2s lipo on the bigger gear right down to 1s lipo for the small gear. My 1:5 sterner vampire with Sierra retracts runs extremely well on 1s lipo and has done for over 5 years, my 1:5 SM Cougar runs on 2s life (6.6 volt). It's that simple.

One last bit of information is that the pump motor like all others has a finite amount of power. This power is consumed with a combination of flow & pressure. At low flows you can have higher pressure and conversely at high flows you are limited by the amount of pressure you can generate before the max motor power is reached.

If you are only using 3 mm lines, you already have a high pressure drop so you need to consider dropping the flow by selecting battery voltage. I use 4mm lines in order to minimize the pressure drop.
if you get this balance right, then the controller is not seeing cut off current during the whole cycle but instead only at the end of the cycle.

If you have a look at this link, http://m.youtube.com/watch?v=nAG7ean...g&feature=plpp you will see some testing I did when I modified the Scale Jets gear. The first 1:45 is using the MTH system that is supplied with the Carf migs, I really do not like this system as it uses an accumulator charged with air pressure and a micro switch to operate the pump, the air leaks from the accumulator, once the air leaks out you can't generate any pressure.

After this I changed to the UMS system although I ended up almost flattening the 2s lipo while playing with the flow controls. I fitted another battery at the 4:30 mark. This gear is fitted with the Festo pilot check valves on each cylinder to lock the oil in as well as flow controls to balance the speeds. You will notice various items hanging off the gear such as hammers and then some dedicated weights. The weight sets are 1 kg each. At the end of it you will see the gear parked halfway with the weights on them, they sat like this for over a week and had zero leakage thanks to the festo valves.

i guess one last thing to point out, it can be clearly seen just how many cycles were done one after another with no controllers burning out and no pump/motors failing. Again if setup correctly they worked reliably and for years.
Shane Bartlett

John, pm. me your new address and I will send you an UMS system for your evaluation. I described it in RCJI some time ago having bought it on Johnis' recommendation. The motor did get hot on tests. The other hydraulic system and one which works well is the Hausl system, very powerful pump, this is installed in my FC Mig 29.

I was very impressed with Aussie Bart's system operation when I saw it at Wangaratta.

Still like Mick's simple approach which seems to work very well indeed.

David.

Kaspar

This is the fluid I used.

http://www.ebay.co.uk/itm/like/28101...ff14=95&ff19=0

As you can see it is used on Citroen and Rolls Royce cars.

I have found several viscosity figures for 100C to -40C. This is a different manufacturer but same product.
http://www.opieoils.co.uk/pdfs/MOTUL...3-%28GB%29.pdf

The citroen specification is Citroen Standard B71 2710


John