Interesting idea, but the problem with that proposal would be the the rotational joints needed, and precision of movement (since the fluid is compressible and behaves like a spring), not to mention lower efficiency. High pressure hydraulics not only require proper rigid tubing, but also a motor capable of sustaining working pressure even when no motion is being executed. The only advantage I see in using hydraulics is higher reliability, if the system is well designed. Considering all those factors, I'd say that electric actuators would be more practical to use.... in fact, next generation aircraft are beginning to drop hydraulics and are implementing electric actuators instead...

A few things, precision of movement is typically not a problem, people typically use temposonic rods and can get down to mm accuracy. Fluids are compressible, but not noticeable and rarely taken into account, using pneumatics this is the case and your argument is correct. Lower efficiency is correct... as typically you are using a positive displacement pump (99% of the time) and the fluid needs to flow somewhere... a special valve is used, this "Cracks" and "shorts" the running fluid to the tanks... then cracking pressure*flow is the power dissipated (citation needed) but I know that's where power is lost.

All in all electric actuators are faster, but have a smaller "power to weight/side" ratio to hydraulics (making hydraulics stronger), valving and tubing is far more expensive and honestly just not what I'm looking for... but really good discussion!

Regarding planes: Just asked my father (777 pilot) all the flight controls are "hydraulic but electrically actuated"... basically this means the controls send an electric signal, this energizes a solenoid which moves a hydraulic valve allowing the working fluid to flow.

 

All in all electric actuators are faster, but have a smaller "power to weight/side" ratio to hydraulics (making hydraulics stronger)

Electric actuators do have smaller power to weight ratio. But I'm sure that only comes into play when you have several to many actuators in your system. For just a few actuators, hydraulics would be heavier since you'd need the system's electric motor, power pump, relief valve (the "cracking" and "shorting" one that you mentioned), tubing, oil deposit, etc... I still think my comment about hydraulic compressibility is valid for systems requiring pressures higher than 4 Kpsi, in the sense that a system working on those parameters would be hard to control with a high degree of precision, even with the addition of positional sensors, since it might experience serious hysteresis side effects. BUT.... of course it would all depend on what the final application is... and the load and speed required to do the job... Interesting discussion indeed.
Anyway, what are your thoughts regarding the design of a special electric generator directly coupled to the nitro engine? Like I said, I know nothing about that subject in particular, and I wonder if it would be feasible to build one that worked efficiently at 10,000 rpm's or more...

 

Regarding planes: Just asked my father (777 pilot) all the flight controls are "hydraulic but electrically actuated"... basically this means the controls send an electric signal, this energizes a solenoid which moves a hydraulic valve allowing the working fluid to flow.

There are undoubtedly many aircraft surfaces controlled by hydraulics, but according to one of the Airline maintenance forums the 737 introduced in 1967 had ballscrew actuated rudder and tail surfaces.
Stress was placed on regular greasing of the screw during regular maintenance.
Max.

 

There are undoubtedly many aircraft surfaces controlled by hydraulics, but according to one of the Airline maintenance forums the 737 introduced in 1967 had ballscrew actuated rudder and tail surfaces.
Stress was placed on regular greasing of the screw during regular maintenance.
Max.

From what you're saying, my guess then is that today's planes have a mix of hydraulic and electric actuators in them... with the electric ones being more numerous...

 

This is all very interesting:

Power-by-wire flight control actuators have been used for the first time in civil aviation to back up primary hydraulic actuators. Also, during certain manoeuvres they augment the primary actuators.[160] They have self-contained hydraulic and electrical power supplies. Electro-hydrostatic actuators (EHA) are used in the aileron and elevator, electric and hydraulic motors to drive the slats as well as electrical backup hydrostatic actuators (EBHA) for the rudder and some spoilers.[161]

The A380's 350 bar (35 MPa or 5,000 psi) hydraulic system is a significant difference from the typical 210 bar (21 MPa or 3,000 psi) hydraulics used on most commercial aircraft since the 1940s.[162][163] First used in military aircraft, high-pressure hydraulics reduce the weight and size of pipelines, actuators and related components. The 350 bar pressure is generated by eight de-clutchable hydraulic pumps.[163][164] The hydraulic lines are typically made from titanium; the system features both fuel- and air-cooled heat exchangers. Self-contained electrically powered hydraulic power packs serve as backups for the primary systems, instead of a secondary hydraulic system, saving weight and reducing maintenance.[165]

 

Seasons greetings to all,
cmartinez, All the fluids(hydraulic controls) I am aware of are NOT compressible to any easily measurable extant so do not behave "like a spring". I'm sure you have used a hydraulic on your car. Gases, on the other hand are highly compressible, such as the air in your car tyres.
As far as precision goes, there are many videos showing operators performing precision feats of manipulation using large hydraulic machinery (back hoes, dozers etc).

 

All the fluids(hydraulic controls) I am aware of are NOT compressible to any easily measurable extant so do not behave "like a spring"

That's also what I thought until a few years ago, when I designed a water jet cutting system that worked at 55 Kpsi. At that pressure water is compressed by 11%, and that pressure energy is stored in device called an attenuator, which is nothing more than a highly resistant vessel whose function is to stabilize the flow of water when the intensifier's plungers change direction (think of it as a simple capacitor). This couldn't be possible if water was not elastic to a certain degree. In fact, water's compressibility becomes extremely important when doing geological surveys involving phreatic layers and in seismographic studies.

Also, this link shows that:
"Petroleum fluids are relatively incompressible, but volume reductions can be approximately 0.5 percent for pressures ranging from 6900 kPa (1000 lb/sq in) up to 27,600 kPa (4000 lb/sq in). Compressibility increases with pressure and temperature and has significant effects on high-pressure fluid systems."

Of course 0.5% might not be much for ordinary hydraulic applications (construction equipment and the like, although I've observed up to 2% compressibility in the real world), but it means that higher pressure hydraulics are not that easy to use when very high precision is required, and hysteresis begins to a be problem that needs to be addressed.
To tell you the truth, I know nothing about aircraft hydraulic control systems... yet I wonder if this very slight compressibility property of the fluid becomes an issue when designing such systems... maybe that's one of the reasons why there are several pumps in the Airbus aircraft... so as to avoid lengthy hydraulic lines in which compressibility affects precision... other than an overall increase in the system's reliability and robustness, of course.

 

very high precision is required

The feedback from a temposonic probe is directly proportional to the extension of the cylinder, therefor the volume change is accounted for in the feedback and you can get exact positioning. If you're moving your working fluid from a high pressure medium to a low pressure medium (and trying to find the change in volume in the low pressure medium) and taking the flow rate/pressure at the high pressure medium, you are exactly correct, you're going to get an incorrect reading. Volume change will be an issue in the application.

It's all about where you're putting your transducers and what you're trying to control.

As I said before... pneumatic systems are compete shitshows when it comes to precise positioning from my experiences, because the energy required to break the static friction (cylinder or motor) is often far more then what is needed to compress the air significantly... I'll give an example because I'm bored.

1. Cylinder fully retracted, you want to push some load
2. You start filling the cap end of the cylinder with air... you can't break static friction, potential energy starts building up
3. Eventually you build up enough potential energy to break static friction... you now have a plethora of energy to accelerate you along because dynamic friction is much low
4. You overshoot
5. Repeat...

Regarding positioning of pneumatic cylinders... I've been brainingstorming some sort of control loop method that can quantify the breakaway pressure of extension and retraction of cylinders...as long as you have a two way valve, pressure tranducers on both ends of the cylinder, a temposonic, and constant load... My feeling is it's possible, but of course with most systems you're going to have a dynamic load...

But as far as I can see with my nitro engine power source, the biggest problem is: it's only going to run happily at one mechanical load/speed (with a little wiggle room), this is the same as a car engine, thus the transmission was introduced, I'm not overly interested in a mechanical solution as I often find them expensive and not very progressive. On the same note here is the characteristic of a DC motor...

So as you can see, if you apply rated voltage to the contacts of a DC motor, it's going to happily run at "no load speed" then once you apply a mechanical load, you're going to apply an electrical load as well, current will flow (please note that current is directly proportional to torque on the "y" axis related with constant Kphi-motor characteristic) This is the same characteristic backwards, as a generator (citation needed but i'm fairly sure)

So lets say our "no load speed" (motor characteristic) and out "happy nitro engine speed" are the same. We couple them together, fire up our nitro engine... awesome! we're seeing rated voltage at the DC motor contacts.... now we start electrically loading those contacts (drawing current) ... this may work for an amp or two... but eventually we're going stall the nitro engine... This is our problem, and now i'm going to propose a solution and maybe someone will call me crazy... lets go. NB: There might be large holes in my theory because it involves running devices outside operating ranges, but I've seen people do amazing things with motors, as they are amazing devices...

Lets call ... "happy nitro engine speed" Vn
And our dc motor "no load speed" Vm

if Vn = 10*Vm, our happy nitro speed is 10 times greater then our DC motor no load speed(very possible). If we couple our nitro engine to our DC motor, we put a meter on the DC motor leads... we're going to theoretically 10*Vm volts on the leads...

Now if we short the motor leads (and somehow the nitro engine doesn't stall, which it will) we will burn the shit out of the motor... because way more then rated current will flow and more then allowable power dissipation will partake in the motor wingdings. But we're not going to short the motor leads... because that would be silly. No, we're going to chop up DC signal and put it through a 10:1 transformer, then rectify and regulate the signal back into DC... So now we should see Vm at the output of our regulator circuit (rectifier in parallel with a big cap)

I'm thinking now we can load the regulated circuit, this will cause minimal current the flow in the primary wingdings of the system (less then what will burn the motor) and hopefully we will not be outside the "wiggle room of the nitro engine"

...

My thought process is all very jumbled... and I'm trying to poke holes in my own theory (as I'm sure there are many)... but has anyone else heard of any else doing something like this? an "electric gearbox" of sorts...

 

For a given load, the current will reduce with rpm due to the generation effect of the motor opposing the applied voltage.
The rpm will increase until the applied motor voltage - motor generated voltage produces a current to sustain a given load.
Max.

 

... but has anyone else heard of any else doing something like this? an "electric gearbox" of sorts...

Gosh... no offense, but I'd like to see you bored more often, that was rather good explanation that you gave ... no objections from this side...
So, from what I understood about your idea ... it was like, go from DC to AC and then back to DC?
I'm guessing that even with optimal circuit design you're looking at 0.85 x 0.85 = 72% efficiency, not considering losses in friction and the like... maybe your proposed system in the end won't perform better than 60% ... but I could be grossly underestimating things here, of course.... on the other hand, I'm not even sure if a mechanical coupling would perform better than that... although it would almost certainly be costlier

 

Gosh... no offense, but I'd like to see you bored more often, that was rather good explanation that you gave ... no objections from this side...
So, from what I understood about your idea ... it was like, go from DC to AC and then back to DC?
I'm guessing that even with optimal circuit design you're looking at 0.85 x 0.85 = 72% efficiency, not considering losses in friction and the like... maybe your proposed system in the end won't perform better than 60% ... but I could be grossly underestimating things here, of course.... on the other hand, I'm not even sure if a mechanical coupling would perform better than that... although it would almost certainly be costlier

Yes, that's exactly what I am proposing, it's actually very common, and is the theory of most DC-DC converters... to get a high efficiency, or to switch from a lower DC voltage to a higher DC voltage. Typically it's switching a DC signal and putting it though a transformer...

I'm wondering if maybe an "off the shelf" part may be my solution...

 

If it's any help, here's a link to a thread that I started a few months ago, in which members of this forum helped me design a transformerless voltage booster that went from 12VDC to 75VDC

 

If it's any help, here's a link to a thread that I started a few months ago, in which members of this forum helped me design a transformerless voltage booster that went from 12VDC to 75VDC

Oh wow that's really interesting... If i'm not mistaken it's using the characteristic of an inductor to appose change in current, then essentially "forcing" the current over that diode by cutting current flow through that FET... cool thanks for the link, I'll probably do a little more reading on that later...

 

Oh wow that's really interesting... If i'm not mistaken it's using the characteristic of an inductor to appose change in current, then essentially "forcing" the current over that diode by cutting current flow through that FET... cool thanks for the link, I'll probably do a little more reading on that later...

Yes, that's more or less what it's doing... it's actually "pumping" current through the diode (which is preferably of the fast-acting type) and then holding it on the other side through the capacitors. The circuit stabilizes when it reaches 75V, but the interesting thing is that it has to have a minimum load for it to work properly.

 

If you are wanting a fair amount of electrical generating capacity in small size that works very well at high RPM's I would suggest looking at a stripped down string trimmer or smaller chainsaw engine spinning a off the shelf higher wattage brushless DC motor, like what's used in the larger RC helicopters and planes, using it as a high speed permanent magnet alternator of sorts.

With that combo pushing several hundred watts off of a mini high speed genset for longer duration run times should not be a problem.

 

And the fuel is much more off the shelf.

 

Awesome thanks guys... This is all very interesting... Something like this might fit the bill well

http://www.ebay.com/itm/NEW-PREDATO...297?pt=LH_DefaultDomain_0&hash=item35e39fe009

This would probably be an expensive project of mine... most likely in the order of $1k+ but I'm hoping sometime this year I will get it going

 

Us something more reliable like a battery and electric motor.

 

More reliable than an internal combustion engine? Don't think so.

 

Interesting idea, but the problem with that proposal would be the the rotational joints needed, and precision of movement (since the fluid is compressible and behaves like a spring), not to mention lower efficiency. High pressure hydraulics not only require proper rigid tubing, but also a motor capable of sustaining working pressure even when no motion is being executed. The only advantage I see in using hydraulics is higher reliability, if the system is well designed. Considering all those factors, I'd say that electric actuators would be more practical to use.... in fact, next generation aircraft are beginning to drop hydraulics and are implementing electric actuators instead...

this is my reaction as well, but nevertheless a very cute idea.

maybe it will not work, but it is certainly worth exploring.