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Best way to get more oomph in pneumatic retracts

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Best way to get more oomph in pneumatic retracts

Old 02-25-2019, 07:41 AM
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Auburn02
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Default Best way to get more oomph in pneumatic retracts

When working with heavy retracts, what's the secret to getting more bang for your psi? Multiple valves, i.e. one for main and another for nose gear or even one valve per wheel? Larger diameter tubing? Smaller tubing? Which seems counter intuitive, but I know BVM uses that tiny 2mm tube and it still works on large models like the 1/5th F-16 and F/A-18.

I don't want the gear to slam up and down, but I do need a little more power to raise some stout gear. And I've seen some larger planes with heavier gear operate flawlessly with a pneumatic system so I know it can be done, but what are the best practices when working with such a setup?
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Old 02-25-2019, 08:10 AM
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Increasing the volume not pressure always seems to be the key.

Bigger diameter cylinders will get you more actuation force if you have the ability to modify the design.

Paul
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Old 02-25-2019, 08:46 AM
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Originally Posted by Auburn02 View Post
When working with heavy retracts, what's the secret to getting more bang for your psi? Multiple valves, i.e. one for main and another for nose gear or even one valve per wheel? Larger diameter tubing? Smaller tubing? Which seems counter intuitive, but I know BVM uses that tiny 2mm tube and it still works on large models like the 1/5th F-16 and F/A-18.

I don't want the gear to slam up and down, but I do need a little more power to raise some stout gear. And I've seen some larger planes with heavier gear operate flawlessly with a pneumatic system so I know it can be done, but what are the best practices when working with such a setup?
My first question would be have you ever seen the system working properly, in the air with air loads present. Or, alternatively, do you see folks always messing up with them because they don't work properly. If it's the second, it's probably a bad design period. I would not throw more money at it and try to convert to electrical if you can.

If you must use air, it takes a combination of both volume and pressure. Most mechanical units can take a good 150 psi, but electrical valves may fail. If you think you need more than 150 psi......then the unit is a bad design. I'm afraid the slamming of the gear may be unavoidable, but the doors can be throttled using inline restrictors.

Good luck!
David
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Old 02-25-2019, 10:18 AM
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Originally Posted by Auburn02 View Post
When working with heavy retracts, what's the secret to getting more bang for your psi? Multiple valves, i.e. one for main and another for nose gear or even one valve per wheel? Larger diameter tubing? Smaller tubing? Which seems counter intuitive, but I know BVM uses that tiny 2mm tube and it still works on large models like the 1/5th F-16 and F/A-18.

I don't want the gear to slam up and down, but I do need a little more power to raise some stout gear. And I've seen some larger planes with heavier gear operate flawlessly with a pneumatic system so I know it can be done, but what are the best practices when working with such a setup?

Try braking when switching "gear up".
Sometimes the gyroscopic effect fights the gear from retracting.
A logical sw of "gear up" and "t. o. flaps", can activate the brakes.
Works perfect for me.

Jack
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Old 02-25-2019, 10:53 AM
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Extra valve or valve per unit is our usual solution to undersize/poor mechanical leverage

Dave

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Old 02-25-2019, 11:11 AM
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Try braking when switching "gear up".
I use a sequencer (Jeti) to pulse the brakes on/off when I select gear up to stop any wheel rotation. I named it 'Autobrake' as a function.

Paul
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Old 02-25-2019, 11:58 AM
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To get more power the only 2 choices are more piston area in the retract cylinder, or higher air pressure. Number of valves and/r tubing size has nothing to do with lifting power, only speed. So bigger tubing will get you gear that slams up or down. smaller tubing will slow the rate of movement. After all that is what a restrictor is all about, slowing down the flow of the air to make the gear move slower. So pressure in PSI is force and volume is speed. A bigger diameter cylinder will give you more square inches of piston area so that means there will be more force to haul up that heavy gear.
It also matters a lot, especially on the nose gear if it retracts forward lifting against the airflow or does it retract to the rear moving with the airflow. Remember that if the airflow is helping the gear to retract then you are fighting it to come back down, but then you also have gravity helping to lower the gear. Coming down the available pressure in the system will be less than what was available to raise the gear.When the gear is fighting the wind, it is best to make that move at the lowest possible airspeed so that less power is needed.

The design of the gear plays a big part in how much weight it can lift. Just what is the mechanical advantage being provided by the distance from the cylinder mount to the fulcrum of the pivot. Is the cylinder extending to raise the gear or retracting to raise the gear. The air cylinder has more power available to extend the piston than it does to retract the piston.

If you are right on the edge, as mentioned it may help to blop the brakes to stop wheel spin will make it a bit easier to lift that gear.
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Old 02-25-2019, 12:17 PM
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Originally Posted by JSF-TC View Post
I use a sequencer (Jeti) to pulse the brakes on/off when I select gear up to stop any wheel rotation. I named it 'Autobrake' as a function.

Paul
I need to try that on the JSF. Thanks for the tip!
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Old 02-25-2019, 12:26 PM
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Originally Posted by Dave Wilshere View Post
Extra valve or valve per unit is our usual solution to undersize/poor mechanical leverage

Dave

That is what I did on the CARF Su-27 build as well, and this one is for an older CARF MiG-29 (with Matrix air gear, not hydraulic). I think I will set it up the same as I did on the Su-27, with one electronic valve on the nose and one for the mains. It is outfit with 4mm tubing currently, but I may replace with 3mm to try and reduce the speed as long as it doesn't cost me any force (as mentioned earlier, we add restrictors all the time to accomplish this, seems the oversize airline is not needed). I'm set up now with a single air tank but have plenty of room for another and essentially no weight penalty so I will add that for peace of mind.

Good tips about the brakes too, I may set that up as well.

Last edited by Auburn02; 02-25-2019 at 12:28 PM.
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Old 02-25-2019, 12:43 PM
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I mix my brakes with gear and use a curve. The reason I do this is to keep dirt and grime, flung by the wheel, out of the wheel wells. I never thought of the gyro effect so that certainly is a side benefit.
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Old 02-25-2019, 01:13 PM
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Originally Posted by flyinfool1 View Post
To get more power the only 2 choices are more piston area in the retract cylinder, or higher air pressure. Number of valves and/r tubing size has nothing to do with lifting power, only speed. So bigger tubing will get you gear that slams up or down. smaller tubing will slow the rate of movement. After all that is what a restrictor is all about, slowing down the flow of the air to make the gear move slower. So pressure in PSI is force and volume is speed. A bigger diameter cylinder will give you more square inches of piston area so that means there will be more force to haul up that heavy gear.
It also matters a lot, especially on the nose gear if it retracts forward lifting against the airflow or does it retract to the rear moving with the airflow. Remember that if the airflow is helping the gear to retract then you are fighting it to come back down, but then you also have gravity helping to lower the gear. Coming down the available pressure in the system will be less than what was available to raise the gear.When the gear is fighting the wind, it is best to make that move at the lowest possible airspeed so that less power is needed.

The design of the gear plays a big part in how much weight it can lift. Just what is the mechanical advantage being provided by the distance from the cylinder mount to the fulcrum of the pivot. Is the cylinder extending to raise the gear or retracting to raise the gear. The air cylinder has more power available to extend the piston than it does to retract the piston.

If you are right on the edge, as mentioned it may help to blop the brakes to stop wheel spin will make it a bit easier to lift that gear.
The one thing you're not taking into account with speed is the inertia of the gear. When the gear first starts to retract, its just swinging sideways, the weight of the gear/wheels/brakes isn't yet a factor. Having the gear move fast at that point means that it will have more inertia to overcome the weight and any "friction or sticktion" in the mechanism as they approach full up. This can mean the difference between the gear fully retracting and locking up, and it not.

I've had many gear (most gear actually) that would retract (and extend) reliably and positively when setup for maximum speed, but when slowed down, would not be reliable. I know that seeing the gear slowly retract as in a full scale is cool, but on takeoff, I generally can't see it anyway as the wings are in the way, and a jet with the unlocked gear hanging down (and the air pissing out) is worse looking that gear that "slams" up and down.

Also, gear that retracts as fast as possible has less aerodynamic drag on it as the jet is generally still accelerating and getting the gear up before the first turn out also prevents G loading from making retraction harder as well. I setup my pneumatic gear to move as fast as possible and its generally much more reliable that way. The doors I generally slow down and/or put effective limiters in to keep from breaking hinges, cylinder mounts, etc.

Bob
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Old 02-25-2019, 01:57 PM
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I have thought about using helium instead of air.
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Old 02-25-2019, 02:18 PM
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Originally Posted by rhklenke View Post
I've had many gear (most gear actually) that would retract (and extend) reliably and positively when setup for maximum speed, but when slowed down, would not be reliable.
Yes exactly. You can always try restrictors later, you have got to get the gear moving properly first. Then you can tell if you are going backwards when you start restricting/slowing things down. If you start slow then encounter problems you will never know. I love the brake trick before retraction BTW. Clever

Last edited by mr_matt; 02-25-2019 at 02:53 PM.
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Old 02-25-2019, 02:42 PM
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Sit at the lights and flap your arm up and down with your hand flat out of the window, then get to 50-60mph and do it.
Ive never restricted my gear and funny enough all my models have super reliable retract operation. On the Mephisto the forward retracting ER50 units have a speed limit...or they just stop moving.
The extra valve gets rid of extra joints and tubing length if you plumb it right, and you are not dividing the supplied air by three, it works on set ups that are ‘just on the edge’
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Old 02-25-2019, 03:44 PM
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Originally Posted by jetspud View Post
I have thought about using helium instead of air.
Wont do a thing .....in fact it will cause greater issues due to parasitic leakage , helium will even "leach" thru teflon
Electric is the way - program ability
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Old 02-25-2019, 03:45 PM
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Originally Posted by jetspud View Post
I have thought about using helium instead of air.
Wont do a thing .....in fact it will cause greater issues due to parasitic leakage , helium will even "leach" thru teflon
Electric is the way - program ability
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Old 02-25-2019, 03:55 PM
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Originally Posted by rhklenke View Post
The one thing you're not taking into account with speed is the inertia of the gear. When the gear first starts to retract, its just swinging sideways, the weight of the gear/wheels/brakes isn't yet a factor. Having the gear move fast at that point means that it will have more inertia to overcome the weight and any "friction or sticktion" in the mechanism as they approach full up. This can mean the difference between the gear fully retracting and locking up, and it not.

I've had many gear (most gear actually) that would retract (and extend) reliably and positively when setup for maximum speed, but when slowed down, would not be reliable. I know that seeing the gear slowly retract as in a full scale is cool, but on takeoff, I generally can't see it anyway as the wings are in the way, and a jet with the unlocked gear hanging down (and the air pissing out) is worse looking that gear that "slams" up and down.

Also, gear that retracts as fast as possible has less aerodynamic drag on it as the jet is generally still accelerating and getting the gear up before the first turn out also prevents G loading from making retraction harder as well. I setup my pneumatic gear to move as fast as possible and its generally much more reliable that way. The doors I generally slow down and/or put effective limiters in to keep from breaking hinges, cylinder mounts, etc.

Bob
+2

And any advice given by Uncle (Tio!) Jack is pure gold!
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Old 02-25-2019, 04:10 PM
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I have been using a combination of large air volume, 80-100psi and restrictors on Robart/Yellow gear for years and many cycles with zero failures. For valves I use Robart, BVM and UP3, all with perfect operational records. I add a couple drops of silicone oil each season and check the systems. So far so good. Keeping it simple has been the key for me.
By Large air volume I mean each installation has at least a one large tank or two plus any medium or small tank I can squeeze in. In one of the new builds I have 4x large tanks in another I have 2x large and 2x medium tanks so I practice large volume installations due to their success.

Doors and air cylinders and hinge geometry is another topic but generally you want larger diameter well placed cylinders for more force and stability. The little diameter cylinders can be overcome by certain size doors or poor geometry or both.

Last edited by Chris Nicastro; 02-25-2019 at 04:13 PM.
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Old 02-25-2019, 04:26 PM
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I agree with Chris as well, use multiple tanks at lower pressure. I have seen blow by on some operating cylinders with very high pressure (over 110 psi)
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Old 02-25-2019, 05:24 PM
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Great thread, I have this problem with my F-105 gear. Huge mains and tires, retract forward in wind, big gear door that rotates to be like a drag chute speed brake when retracting. That was my plan also to use huge volume and as high pressure as possible, and retract before accelerating. Also the nose gear retracts forward into the wind too.
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Old 02-25-2019, 05:59 PM
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These SMC flow control valves are good to slow down on one direction
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Old 02-26-2019, 12:54 AM
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You guys with air operated retract systems on large plane should look at Hydraulic systems. I tried everything on my Arado 555 for retracts and installed huge air piston cylinders, large throughput valves and big tanks. They failed to close the doors in the air. I changed to electric retracts which were in their infancy at the time but again they did not work in the air making landing with partly extended legs interesting.

Retracts are modified Behotec C50 units with Festo fittings and high pressure Nylon lines with the pressure relief valve set at 120 psi. magomhrc.com do a thinner pipe that is held with metal sleeves that will be easier to install.

Boulton Paul system based on Magomhrc.com units

Final version of the system for the Gotha using simplified Magomhrc.com units

Mk 1 system as a trial rig for my Gotha P60 before the Magomhrc units had become know about.

This may be of interest to those that have not seen it before apologies for those that have seen it before

John
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Old 02-26-2019, 05:44 AM
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Jon, can you indicate the specific valve you will recommend for the typical 2-mains /1- nose retracts units?
Seems to be 1-way Valve M3 V2 , plus a bunch of Festo 4 way to distribute to each retract.

Thanks for sharing
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Old 02-26-2019, 06:03 AM
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Originally Posted by Edgar Perez View Post
Jon, can you indicate the specific valve you will recommend for the typical 2-mains /1- nose retracts units?
Seems to be 1-way Valve M3 V2 , plus a bunch of Festo 4 way to distribute to each retract.

Thanks for sharing
Edgar

The valve I used was the M5 1 way valve and I unscrewed the fittings and replaced them with M5 Festo ones. Simply tee off the lines for sending the Hydraulic to each retract in the usual way. The Festo ones I used were the smooth ones that have an internal Hex to use for screwing down. This will do the trick for both the nose and mains legs. You can vary the speed of the retract by using the speed controller but in practice I found that a small 2 cell Lipo was fine for power. They now do both brushless or brushed so the choice is yours. With the retracts I used M3 to 4mm pipe Festo fittings where space was a bit tight. The retracts will need to be taken apart to drill and tap them for the Festos and make sure they are clean of any swarf.

John
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Old 02-26-2019, 02:28 PM
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Originally Posted by Jgwright View Post
Edgar

The valve I used was the M5 1 way valve and I unscrewed the fittings and replaced them with M5 Festo ones. Simply tee off the lines for sending the Hydraulic to each retract in the usual way. The Festo ones I used were the smooth ones that have an internal Hex to use for screwing down. This will do the trick for both the nose and mains legs. You can vary the speed of the retract by using the speed controller but in practice I found that a small 2 cell Lipo was fine for power. They now do both brushless or brushed so the choice is yours. With the retracts I used M3 to 4mm pipe Festo fittings where space was a bit tight. The retracts will need to be taken apart to drill and tap them for the Festos and make sure they are clean of any swarf.

John
Have you found any pneumatic cylinders that wont work with hydraulics? Thanks for this info.
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