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Simple hydraulic control linkage?

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Jeremy

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I'm trying to work around a elevon control linkage problem, one where I want to be able to rotate the wing for transport/storage without disconnecting the controls.

The obvious way around this is to use Teleflex cables, but they are still pretty stiff and quite heavy. The maximum control system forces are very low, around 100lbsf at the control horn.

It occurred to me that using hydraulic actuators might be an option, as these would allow a pretty simple installation. I'm not talking about a powered system, just something similar in principle to a hydraulic brake set-up, but with double acting cylinders.

The system pressure would be pretty low, most probably no more than 100 - 150psi at the very most, so I was sort of thinking about using lightweight pneumatic actuators operating with a suitable mineral oil. My guess is that I could build a light system this way, particularly if it were plumbed with lightweight Teflon hose, but I will admit straight off that I have absolutely no experience of hydraulics!

The advantages are primarily that the system could be really very simple, with actuator cylinders directly driving the elevons and the driven cylinders connected directly to the stick.

The potential disadvantages might be a small weight penalty plus my own uncertainty about such a systems reliability. Hydraulic brake systems seem to be extremely reliable, so perhaps my worries about reliability are unfounded - maybe it's just fear of using something unusual.

Does anyone know if this has been tried before? I've searched the archives here and not found a direct mention of such an idea.

Jeremy
 

orion

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It does sound like a good idea doesn't it? I've though about this a number of times over the years but each time keep running up against the same issues. There are three primary ones: Centering in flight, seepage and flutter.

The first issue is really more that of friction. Although the fluid (moving through an orifice) and the moving parts are one source, it's really the seals within the system that introduce the highest magnitude. As such, you'd either have to relax some of the sealing requirements (unrealistic since that would make a poor control system) or determine whether the amount of friction is detrimental or not. The net result of too much friction is simply that your controls will not center automatically in flight and as a corollary, it will be somewhat difficult to come up with a functional trim system.

The second issue also deals with centering. Even the best hydraulic systems have a bit of seepage. Over a period of time that seepage will cause the controls to go off center. That would include the stick or the surfaces themselves. As such, a part of your preflight would have to be the adjustment of some sort of centering circuit that would allow you to tweak every hydraulic cylinder within the system to get it back to center. Originally I thought this could be accomplished just by adjusting the rod end or clevis (which ever is on the end of your cylinders) but eventually came to the conclusion that that is not enough. For best centering there would have to be some sort of valved bypass circuit on each cylinder that would allow it to be moved to center without affecting any other part of the system.

The last issue deals with the integrity of the hydraulic system. Obviously for best control and stability you need a very tight circuit that has a displaced bleed port for the lines and a separate port for both ends of each cylinder. If the system allows the introduction of air anywhere within the lines (or failure to remove all of it), the integrity and rigidity are lost. Even a small air bubble could introduce a certain amount of flexibility, which could result in control surface instability.

Every now an then I come back to mulling this over since I do think it would be an ideal solution to running cables or linkages. But so far I haven't really had the time to come up with anything brilliant.
 

Jeremy

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Thanks Orion, the one thing I'd missed was the loss of centring. I did spend another couple of hours last night trying to work through a way around this, with no success. What's really needed is a way to make sure that the system compensates for creep without adding too much complexity. I think this could be done, but the light weight and simplicity would be lost.

Following on from a previous thread, I did take a look at electric actuators. It seems that affordable, commercially available, fast and accurate servos are available, but I doubt that they are reliable enough for a primary control system. The idea of using big model servos for trim systems works OK; I used a high torque model servo for an elevator trim system some years ago and found it to work well. The idea of using model servos for a crude autopilot certainly seems workable for a very light aircraft and worthy of further exploration.

Too many ideas, too little time...............

Jeremy
 

wally

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southwest TN.
loss of centering.
That reminds me of a farm tractor I once drove. when I turned and tried to hold the steering wheel all the way against the left or right stop, the wheel could still be turned slowly. It didn't seem to effect the overall operation - like trying to steer it straight but each time I came back to neutral, the steering wheel spokes were in a different orentation!
Wally
 

orion

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I don't know the specifics of tractor design but in some hydraulic steering systems (common in marine applications) there is often a check valve installed at the end of the steering wheel's range of rotation. As you reach the steering stop, a check valve opens, circulating the fluid within the pump rather than over pressurizing your lines and your steering cylinder. The steering gets to be very stiff at that point (the check valve bleed orifice is very small) but you can continue turning the wheel even though you're at the stop. Unless you had a very old tractor with a worn out system, that type of bleed valve wold most likely account for your ability to turn the wheel even past the end, and so see a different position on the wheel spokes each time as you straightened out the wheels.
 

Topaz

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Huh. Am I missing something about centering? Why would each cylinder need its own centering circuit? Couldn't you put a 'bridging' line and valve for each pair of cylinders, as below. Each cylinder then has a 'return' circuit when the valve is open that doesn't require movement of the other.



Taken further, couldn't you then tie all the 'centering' circuits through one master centering valve? I count four bridging lines (two for the ailerons, one for the elevator and one for the rudder), and I know there are four-circuit hydraulic valves out there. Since that valve is a single point of failure for the entire control system, maybe two 'master' valves in series where the crossover happens?

This way, you could open the master centering valve, put/leave the (external) gust locks on the airplane (thus mechanically centering each control surface), and then have a simple indicator on the stick to show when it's centered. Move the stick (and rudder peddles) to center, shut the centering valve(s) and the whole system is then centered.

Just thinking out loud here, but that seems like a very simple way to lick the 'leakage' centering problem. Unless I missed the obvious. Which happens from time to time. ;)

'Course, this does nothing for the friction or flutter issues... :ermm:
 
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Dana

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But what if it starts to drift in flight?

I looked into such a system awhile ago for a throttle control on a paramotor (replacing the push-pull cable). Seemed logical as I work with automation related pneumatics and hydraulics every day. I talked to an engineer at the cylinder company. He said somebody had tried such a system for a one man submersible, but during a relatively short dive, the controls drifted so far off center as to be pretty much useless.

Unpowered hydraulic actuation (without pump and control valves) works well only in one sided applications, where the system is open to a reservoir at one end, like brakes or clutch in your car. The reservoir not only takes care of any seepage, but also handles thermal expansion in the system.

Personally, I would not want to fly any aircraft with only a single hydraulic primary control system without mechanical backup... which would defeat your purpose of simplicity.

-Dana

If email had been around before the telephone was invented, people would have said, "Hey, forget email! With this new telephone invention I can actually talk to people!"
 

Craig

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On race cars, we use two brake master cylinders, connected with a threaded rod. The brake pedal pushrod then connects to an internally-threaded piece located on the threaded rod. We adjust fore/aft brake bias by turning a knob on the dash that adjusts (via flex cable) the left/right position of the pdeal pushrod.
Could not something like this be used to adjust the centering position? OR perhaps two of them - one for ailerons, one for elevator?
Just a thought.
 

orion

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One of the problems with the centering circuit drawn above is that the volume rates on either side of the piston within the cylinder are different due to the size of the push-rod. Since hydraulic fluid is incompressible, you essentially have a case of hydraulic lock. As such, this centering diagram wouldn't work since it would require that the stick moved in order to get any motion on the control surface, and the motion could only be in one direction. Essentially the stick cylinder motion would make up for the volume differential.

In order for this to work you'd need some sort of vented reservoir that would allow the differential volumes to take and deposit what they need independently. The reservoir could also then be part of the bleed system. I like the idea of doing this centrally but the system would still need an application specific design that would minimize the drift. But for high mounted actuators you'd have to figure out how to install the reservoir or those lines would just drain away.

Furthermore, in case of emergency, it would also be a good idea to somehow be able to do the centering and adjustment in flight, but now that may be getting a bit on the complicated side.

One suggestion I ran across a while back that might limit the drift would be to pressurize the system using something like a pressure accumulator within the circuit, or maybe even something simple like a CO2 cartridge. Under pressure the seals do a better job of maintaining pressure against the inner surfaces. But for this to work you do need a special seal configuration since just simple pressure on either side of a standard seal would do very little.
 

Topaz

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But what if it starts to drift in flight?...
Then very cheap cylinders have been used, cylinders have been used outside their spec, or incompatible hydraulic fluid has eroded the seals. Using pneumatic cylinders in place of proper hydraulic cylinders would be an example of a bad application, to my mind. Seepage past a well-designed system used at the rated pressures of the components should be very slow and minimal, at worst. A simple accumulator in the system could pressurise it to the rated pressures of the cylinders, if the seals require some system pressure to set properly.

I'm not saying this kind of control system is necessarily a great idea, but it does solve certain problems, and as such, is worth exploring.
 

Topaz

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One of the problems with the centering circuit drawn above is that the volume rates on either side of the piston within the cylinder are different due to the size of the push-rod. Since hydraulic fluid is incompressible, you essentially have a case of hydraulic lock....
The drawing was meant to be schematic, but, since you bring it up: Double-ended, double-acting cylinders. Let the free end simply float, unattached. Heavier, but simpler overall. The friction in the system goes up a bit compared to a 'conventional' cylinder, though, because of the extra seal introduced at the second rod end. It strikes me that since one would want to use very small cylinders for weight reasons anyway, the smaller seals would provide less friction in the first place?



Each cylinder is supported on a pivot in the middle, and the 'unused' pushrod end can be covered with a simple rubber boot to keep out dirt and such.​

Said rubber boot can be obtained very inexpensively in any drug or grocery store.​

In the contraceptive section... :emb:​

("But honey, I need them for the airplane! The aaiiiiiirplannnne!") :gig:​
 
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Jeremy

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Feb 23, 2003
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Salisbury, England
"Said rubber boot can be obtained very inexpensively in any drug or grocery store."

Indeed.......... Years ago I needed to seal some hydrophone connectors that were prone to occasional leaks. Condoms filled with castor oil, slipped over the whole hydrophone and cable, then tied off around the cable with light cord, proved to be ideal. The expression on the face of the young girl behind the counter when I went into the store to buy two gross (288) packs of condoms to take away on an extended sea trial remains with me to this day.............

Jeremy
 
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