Hydraulics Use

[Dean_Dolph]

A thread on hydraulic pre-rotators reminded me that hydraulics is an interesting subject and I've often wondered what the negatives are in using them in homebuilt aircraft control systems and etc.

The routing of tubing would certainly be easier than the control tubes or push/pull cables we use now. I recognize that there would be tube anchoring details to work out and the sizing of the supply and drive cylinders would present a new challenge but one that shouldn't be hard to determine. Cleanliness and purging the system of air would be paramount in maintaining the system.

The toughest design problem would probably be the linkage of the controlled point with the hydraulic drive cylinder. Whether there is a weight penalty or not would have to be determined. Also, I'm not sure what kind of confidence level we can have in a hydraulic cylinder. Obviously commercial aircraft have confidence in hydraulics. Any one have thoughts on this subject?

[Caribean_gyro]

My system squeks a lot. I am in the process of changing the 3 drive belt. any way I can improve RPM. I feel that on a heavy wind the drag slow down my blades and bearly get 100 rpm on a stand still.

ChuckP

[pwendell]

Dean,

I've got the feeling that the push tubes are one of those things that, even if they aren't ideal, are just so cheap, simple and reliable that it would be hard to find a compelling substitute.

In addition to the issues you mentioned with hydraulics, I would add the increased weight, expense and complication of redundancy. Surely we would require at least two hydraulic systems that don't share a common point of potential failure, since loss of cyclic control in flight would most likely be fatal.

[Screw]

Screw-In

Why would anyone need a hydraulic system for cyclic control?

Screw-Out

[Dean_Dolph]

Peter, without going thru a design/construction process I don't know for sure but I suspect that the weight issue would be a trade off, certainly not be much of a penalty. After discarding everything except the joy stick and replacing the parts with small tubing and two double acting hydraulic cylinders I doubt that there would be much difference. The hydraulic cylinders for this application shouldn't be very big and would be a low pressue application. Remember, this is a closed system like the hydraulic brakes on your car that uses a supply cylinder (the master) and the control cylinder (think brake cylinder). You are probably right about increased expense. However, push/pull cables aren't cheap and that hasn't stopped them from being used.

Redundancy, yeah that could be an issue if there isn't any confidence in the system. We have confidence in control tubes because we haven't had many, if any, failures. Any possible failures apparently have been avoided by pre-flights such as the rod ends that Ken J. has referenced from time to time. Ron Herron designed in redundant push/pull cables in the Little Wing but others that have used them haven't. Go figure! I don't know if Ron used redundancy because of lack of confidence or for some other reason. He is the only one that is using them for rotor control that I know of. But in this case, is hydraulic redundancy really needed? You do have confidence in you car's braking system don't you?

Appreciate your thoughts and maybe R. Herron will chime in and give his since I believe he is also an A&P.

Anyone have any idea how much force is being applied to gyro controls? Knowing this would allow sizing of the cylinders.

[Dean_Dolph]

John, I know I'm not very succinct but if you read my first post again you will find out why.

The routing of control tubes has always been a hassel or once the design has been worked out and the control system has been debugged then routing other 'stuff' around the control system becomes a problem. Running small diameter metal tubing can present a lot of work in the bending but it takes up less space and can be run around/thru 'stuff' without affecting operation.

Then there is the fact that if a hydraulic control system is considered reliable then we can forget about the maintenance issues with a mechanical system. This includes primarily the rod ends and other pivot points.

There has been at least one fatal incident when Leroy Hardee went down when a bolt in his control system was displaced and locked the controls. I don't recall all the details so don't know if it was a design issue that prevented it from being spotted during a pre-flight or not.

I'm not sold on a hydraulic system but I do believe it has possibilities to solve and simplify control system design. I'm just throwing this out to see what holes can be shot in the thought.

[pwendell]

But in this case, is hydraulic redundancy really needed? You do have confidence in you car's braking system don't you?


Dean,

Yes and no. Most, if not all cars, have two separate brake systems, one controlling the right front and left rear brakes and the other the opposite pair. A blown seal or punctured line in one system will not affect the other. These systems do share a common master cylinder, however. Cars also have an emergency brake and a transmission to help slow down.

I think redundancy is necessary because, even though hydraulic systems are very reliable, they can fail suddenly and completely. A thorough preflight wouldn't be able to detect a seal on the verge of failing. If a seal failed in flight, the entire hydraulic system would become useless in seconds, and, without a backup system, the gyro would probably crash.

[Mike Hook]

Hydralics are cool, there so much you can do with them. If you had two double acting master cylinders sending to two slave double acting cylinders and set them up as a closed system with a small reservoir.

The fourmula for a cylinder is pressure in force is equal to the applied pressure in PSI x the area in square inches.

Now that applies to the open end of the cylinder, the rod end you must subtract the anular area of the rod from the cylinder area to figure the applied force. In all by having different dia.'s of master and slave cylinder you can have different lengths of travel and different ratios of applied pressure.

with a master cylinder remember that it will take more pressure one way than the other because of the anular area of the rod in the master. You can over come that by reversing the slave so that the opposite end of the slave gets the pressure. Kinda of a trade off the make it even.

Mike

[Dean_Dolph]

Peter, anything is possible so, sure, it certainly is possible that a cylinder could fail. A tubing puncture isn't likely but I suppose it could happen. Failure at a tubing fitting is more likely but 40 years in chemical R&D convinces me that can be avoided. A catastrophic cylinder failure where there isn't any advance evidence, while possible, also doesn't sound likely. But if the confidence level isn't there then hydraulics, or for that matter any other new device in any application, shouldn't be used.

I'm going to include something in this note that addresses a misconception I see in other's responses. That is if I'm interpreting them correctly. And that is the fact that in a hydraulic cyclic control system there wouldn't be any pump! There is just two double acting cylinders tied together by high pressure tubing. One cylinder is the pump/reservoir and the other is the controlled device. The pilot supplys the pump cylinder's energy. The system might have a separate reservoir for expansion and system filling, I haven't thought that far into the design.

If I ever find out what the control forces are then that might end any thought of hydraulics. If they are so high that a practical size for the cylinders isn't possible then that will end it. If it turns out that the forces are reasonable then it might be possible to use redundant systems and still not suffer a weight penalty.

[Brent_Brown]

I know the Nolan brothers use hydraulic in the coaxial helicopter they made. the system looked light. Some good photos of them are in an PRA mag some time age.

[Dean_Dolph]

Yeah, Brent, I believe you are right! I forgot about them. I'll have to go back and look at the pictures I took of their machine the first time they brought it to a PRA Convention.