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Control via Hydraulics

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Pilot-34

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I wasn’t thinking of hydraulic boost so much as control
Hydraulic line doesn’t really seem to care much if you bend it 28 different ways to get it into something where a cable system would require some thing at each corner
 

D Hillberg

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It's easier to name un boosted controlled helicopters
Enstrom
Hughes
Brantley
Robinson R-22
All the rest have hydraulically boosted controls...
some have both cable controlled tail rotor or pp tubes - fewer have teleflex
 

cluttonfred

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Now that hydraulic brake systems are commonplace in bicycles, I wonder if you could adapt such components to aircraft use? If not the primary flight controls then maybe flaps or spoilers.
 

Pilot-34

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The thing I like about hydraulics is they run in a very hard steel tube and if some piece of freight moves around inside the plane and lodges against them it will not impinge on your control function.
 

rdj

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Now that hydraulic brake systems are commonplace in bicycles, I wonder if you could adapt such components to aircraft use? If not the primary flight controls then maybe flaps or spoilers.
I hope not. I have to bleed the brakes on my mountain bikes on a regular basis, get even a little of that mineral oil on the pads and they squeal like banshees, and they're almost impossible to repair in the field. Between hydraulic brakes and 1X drive-trains bicycles have been going backwards recently. I like my airplanes foolproof and functional, and besides the mechanical linkages exercise those flabby arm and leg muscles.
 

Pilot-34

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A brake caliper moves about .002". An elevator is 3" in a half second. Too much fluid friction.
I do not see how your first two statements would relate in anyway to the third.
Could you explain what you mean?
 

BBerson

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There is bunch of seal friction and fluid friction in moving two pistons 3 inches. That's why it isn't done on a one seater. Also, I hate hydraulic maintenance like the hydraulic clutch on my Toyota pickup. Much prefer cables.
 

Pilot-34

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I like my airplanes foolproof and functional, and besides the mechanical linkages exercise those flabby arm and leg muscles.
Lol FLY for YOUR HEALTH!
I hope you can make your point to the next nationwide Fad, so my healthcare prescription will include buy an airplane!

Seriously though the amount of effort required to do something doesn’t really relate to the linkage. It’s just carrier forces and should have negligible friction and inertia inputs.

And I’m not familiar with bicycle hydraulics but if you’ll think about the brakes on your car few things are more reliable and maintenance free than the hydraulic system.
 

Pilot-34

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There is bunch of seal friction and fluid friction in moving two pistons 3 inches. That's why it isn't done on a one seater. Also, I hate hydraulic maintenance like the hydraulic clutch on my Toyota pickup. Much prefer cables.
Oh I see where you’re going now.
Remember we’re talking about a hydraulic system here there’s no reason for anything To move about 3 inches.
There’s also no reason for any significant amount of fluid friction for the same reason.
I don’t think I would use the same fluid for flaps that I would for brakes.
 

Hot Wings

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There’s also no reason for any significant amount of fluid friction for the same reason.
Depends on what you call significant.
Also remember that we have to keep the weight down. By the time you the cylinders with a fluid filled line of similar weight to an aircraft cable the tube you end up with is quite small. This is pretty much how a hydraulic damper works - forcing a fluid through a small opening........and you still have the added weight of the cylinder and it's fluid volume.

I'm not anti hydraulic. I built a system for a gyro a long time ago that was simpler and weighed less than the conventional Brock mechanism but after the Nylaflow tubing shattered while uncoiling it*, I abandoned the idea.

* about a year old purchase from AS+S.
 

Pilot-34

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Remember this started out as a way to deal with a lot of bends in a line.
What’s it take to rig a dozen pulleys ? I was thinking of the simplicity versus weight trade off in that situation.
Also I am thinking different technology different ways of doing things.
The hydraulic cylinder for an aileron for example could be half an inch long and as big around as a small syringe.
That might also be really handy if your space underneath where your mounting your control stick is is quite small.
Perhaps small enough to mount it above the pivot point instead of below?
And of course the viscosity of the fluid used could be different Than we use on brakes and backhoes.
Could we get by with using a single line for two way control ?
At one time I had a model railroad that used old syringes and aquarium tubing to control the track turnouts (Switchs) as much as 20 feet away.
Obviously not much force or speed was needed.
That’s one of the reasons I am reluctant to say how the system would work for sure.
 

wsimpso1

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I do not see how your first two statements would relate in anyway to the third.
Could you explain what you mean?
Easy. In any system, you have to put in all of the energy to make the force on the control through its travel plus the friction forces through its travel. The control surface energy is fixed. Drag on a few pulleys or bellcrank bearings and spherical joints can be quite small and thus the energy nearly negligible. Go to hydraulics and the seal drag on both ends times the travel of both cylinders is significant. You have to supply that extra energy with your muscles. That is for big control movements.

When we consider that much of our flying is at small movements and small forces on the controls, you find another disavantage to them. The friction drag of seals on cylinders and rods is about the same no matter how hard you are using them. This significant threshold force to move the cylinders at all is something you have to go through for every little correction. and then you can feel what the airplane is telling you until the forces being made at the control surfaces exceeds the seal drag... It can be flown, but it makes it more difficult and distinctly removes the fine feel many of us prefer. It makes the airplane harder to fly well and unpleasant to operate. Who needs THAT crap?

Why do the work well on bikes and motorcycles and cars. The piston movement is tiny between rest and applied positions. The seal mostly deforms to allow this movement - If you imagine the seal cross section as a rectangle, with one side in a seal groove and the other running on the piston or bore, travel to apply the brake just deforms the seal elastically into a bit of a parallelogram. Release the brake, and the seal rebounds back to a rectangle, pulling the piston back a tiny distance, but enough to relive pressure on the pads. The only time the seal slides significantly in disc brakes is when you have pushed the pistons back to do service work - you cycle the lever a couple times to push the piston back in contact, and then the seals slide only microscopically on each brake apply - the amount of pad wear.

Unboosted Hydraulics = OK for brakes, Unboosted Hydraulics = Bad for control surfaces.

Billski
 

gtae07

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I've worked with hydraulic flight controls in various setups for almost my whole career. I'd never consider using such a system in a light aircraft. Too complicated, too heavy, too many things to break, and no real need. Cables and/or pushrods are sufficient.

Large aircraft with mechanical (non-FBW) hydraulic controls generally run cables/pushrods to the actuator, where they control a servovalve which ports high-pressure (~3000psi usually) fluid to the piston. Even FBW aircraft have servovalves at or next to the actuator; they just control those electronically.

What you don't see is valves at the stick porting fluid down long runs to the flight controls; that would be lots of extra tubing and have very poor control response. We experimented with locating the valves well away from the actuator on a ground test article and performance was marginal going to hilariously awful as we cooled the system down (to represent fluid characteristics at altitude).

Valves located remotely and porting fluid over long runs works fine for things like landing gear deployment or thrust reverser actuation, or even brakes (since as Billski notes, they don't move far). Not a good idea for things that need fast response.
 

Pilot-34

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Easy. In any system, you have to put in all of the energy to make the force on the control through its travel plus the friction forces through its travel. The control surface energy is fixed. Drag on a few pulleys or bellcrank bearings and spherical joints can be quite small and thus the energy nearly negligible. Go to hydraulics and the seal drag on both ends times the travel of both cylinders is significant. You have to supply that extra energy with your muscles. That is for big control movements.

When we consider that much of our flying is at small movements and small forces on the controls, you find another disavantage to them. The friction drag of seals on cylinders and rods is about the same no matter how hard you are using them. This significant threshold force to move the cylinders at all is something you have to go through for every little correction. and then you can feel what the airplane is telling you until the forces being made at the control surfaces exceeds the seal drag... It can be flown, but it makes it more difficult and distinctly removes the fine feel many of us prefer. It makes the airplane harder to fly well and unpleasant to operate. Who needs THAT crap?

Why do the work well on bikes and motorcycles and cars. The piston movement is tiny between rest and applied positions. The seal mostly deforms to allow this movement - If you imagine the seal cross section as a rectangle, with one side in a seal groove and the other running on the piston or bore, travel to apply the brake just deforms the seal elastically into a bit of a parallelogram. Release the brake, and the seal rebounds back to a rectangle, pulling the piston back a tiny distance, but enough to relive pressure on the pads. The only time the seal slides significantly in disc brakes is when you have pushed the pistons back to do service work - you cycle the lever a couple times to push the piston back in contact, and then the seals slide only microscopically on each brake apply - the amount of pad wear.

Unboosted Hydraulics = OK for brakes, Unboosted Hydraulics = Bad for control surfaces.

Billski
Bill ski you say that those forces make an aircraft unpleasant to operate .
Have you ever operated one? My curiosity has me desiring to seek one out just to see what they would be like.
 

Pilot-34

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I've worked with hydraulic flight controls in various setups for almost my whole career. I'd never consider using such a system in a light aircraft. Too complicated, too heavy, too many things to break, and no real need. Cables and/or pushrods are sufficient.

Large aircraft with mechanical (non-FBW) hydraulic controls generally run cables/pushrods to the actuator, where they control a servovalve which ports high-pressure (~3000psi usually) fluid to the piston. Even FBW aircraft have servovalves at or next to the actuator; they just control those electronically.

What you don't see is valves at the stick porting fluid down long runs to the flight controls; that would be lots of extra tubing and have very poor control response. We experimented with locating the valves well away from the actuator on a ground test article and performance was marginal going to hilariously awful as we cooled the system down (to represent fluid characteristics at altitude).

Valves located remotely and porting fluid over long runs works fine for things like landing gear deployment or thrust reverser actuation, or even brakes (since as Billski notes, they don't move far). Not a good idea for things that need fast response.
You sound like you have quite a bit of experience in hydraulic systems.
Do you think a small plane system would need any valves at all?
Do you know if there are hydraulic fluid’s with very thin viscosity?
I don’t think my small planes would have significant temperature differences between the ground and flight levels that they use do you think a very thin fluid could be used perhaps in the winter time when temperatures might get quite cold?
 

Hot Wings

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Bill ski you say that those forces make an aircraft unpleasant to operate .
<< >>
Have you ever operated one?
Not answering for wsimpso1 but:

It doesn't have to be an airplane to make this kind of control stiction unpleasant. It is a problem in any kind of equipment where the operator expects linear response. Move a control X% of the throw and you should expect X% of movement. For example if all you want is 3% of movement but there is 2% of stiction in the system from rest then you have to move the control 3% before you get any response at all and then the system is likely to overshoot to maybe 5% movement. Works the same in reverse when you try to correct the overshoot.
The problem is even more pronounced on a system with light control forces where we tend to think in terms of pressure rather than actual movement.
(automotive example: My Sprite vs a manual steer truck with 3+ turns lock to lock)

<< >>
Never operated an airplane with this problem, but I do know that there are some out there with "dead bands" due to aerodynamic effects. Same problem. different reason.
 

gtae07

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You sound like you have quite a bit of experience in hydraulic systems.
Do you think a small plane system would need any valves at all?
Every system I have worked with uses a high-pressure supply (powered by an engine-driven or electrically-driven pump). Crack the valve and you get movement. A system like this will tolerate some small level of internal leakage, and even a small level of external leakage over a finite time, without affecting functionality. And with a pressurized source you need valves, and a pump, and if you want real control feedback (usually considered a good thing on light airplanes) you need cables and/or pushrods to the control surface anyway. By that point on a light airplane, there's no reason to have an actuator, pump, tubing, valves, and so on.

I'm having a hard time picturing how you could make a usable hydraulic-powered flight control system without a pressurized source. I can see it with a one-way actuation system (e.g. a spoiler or airbrake that is forced closed by the slipstream) but I can't see a two-way control that is only pressurized by pilot pressure on master cylinders that doesn't run into problems with even a tiny bit of leakage, much less the breakout force/hysteresis issues Hot Wings and Billski mention.

Large aircraft added hydraulic boost not because it made routing easier, but because the control forces were unreasonably high without it.

A good cable system or pushrod linkage with good bearings will have really low internal friction and won't be nearly as messy or fussy to maintain.

Do you know if there are hydraulic fluid’s with very thin viscosity?
I don’t think my small planes would have significant temperature differences between the ground and flight levels that they use do you think a very thin fluid could be used perhaps in the winter time when temperatures might get quite cold?
Well, we were testing at -40F/-40C (they're the same!) and below. Probably not dealing with those temperatures in a small airplane. But still, it's really not worth the effort.

It is a problem in any kind of equipment where the operator expects linear response. Move a control X% of the throw and you should expect X% of movement. For example if all you want is 3% of movement but there is 2% of stiction in the system from rest then you have to move the control 3% before you get any response at all and then the system is likely to overshoot to maybe 5% movement. Works the same in reverse when you try to correct the overshoot.
The problem is even more pronounced on a system with light control forces where we tend to think in terms of pressure rather than actual movement.
On a different note that might illustrate a problem with controls that have no feedback, I once had the chance at work to fly a simple PC sim setup with a center stick that had no centering springs (long story there that I can't talk about)--it just flopped about all over the place. It was horrible--with no force feedback of any kind you just porpoised all over the place. I had better luck flying the statically-unstable aircraft sim in my controls class.


In my personal and professional opinion, hydraulic-powered flight controls are a solution in search of a problem, in terms of light aircraft. If routing a pushrod or cable setup is just truly impossible and for some reason you can't redesign things to make it possible, I personally think you'd be better off attempting a simple linear servo-based system or something. But try redesign first.
 
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Pilot-34

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Personally I think Everybody is trying to overthink the hydraulic control issue. And creating stawmen to beat up.

But lastly I don’t have much real experience with this type of a system either so I’m going to have to go do some experiments.
 
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