Hydraulic Systems

[uwochris]

Hey guys,

I have kind of a "dumb" question to ask in regards to hydraulic systems.

When does it become necessary for the system to require a hydrualic pump (either electric or engine driven) to pressurize the fluid in order to power the system?

For example, on my 152, I know there is no need for a hydraulic pump to power the brakes. Is this because the brake lines do not cover a large distance (i.e. they're not that long), and thus there will not be much friction loss when I pump the brake pedals to transmit the force?

Now, look at an airliner, like a 747. I'd imagine that because this a/c is so large, the hydraulic lines on this plane will be very long. In this case, something will be required to pressurize the fluid in order for it to transmit a force to the output side.

Is my thinking correct? Does the length of the hydraulic lines determine whether or not a pump is required, or is there something else (i.e. required output force)?

Thanks in advance,

Chris.

 

[FN FAL]

Hey guys,

I have kind of a "dumb" question to ask in regards to hydraulic systems.

When does it become necessary for the system to require a hydrualic pump (either electric or engine driven) to pressurize the fluid in order to power the system?

For example, on my 152, I know there is no need for a hydraulic pump to power the brakes. Is this because the brake lines do not cover a large distance (i.e. they're not that long), and thus there will not be much friction loss when I pump the brake pedals to transmit the force?

Now, look at an airliner, like a 747. I'd imagine that because this a/c is so large, the hydraulic lines on this plane will be very long. In this case, something will be required to pressurize the fluid in order for it to transmit a force to the output side.

Is my thinking correct? Does the length of the hydraulic lines determine whether or not a pump is required, or is there something else (i.e. required output force)?

Thanks in advance,

Chris.

it's something else...take a look at Pascal's law...then take a look at those little baby feet inside those slip on shoes and penny loafers the 747 drivers wear. Then take a look at the size of the brake calipers on a 747 and think about how much force will be required in order to clamp down on a rolling 747.

When you squeeze one end of a tube of toothpaste out the other end, you are watching Pascal’s principle in action. The principle was first stated clearly in 1652 by Blaise Pascal (for who the unit of pressure is named):

A change in the pressure applied to an enclosed incompressible fluid is transmitted undiminished to every portion of the fluid to the walls of its container.

Pascal’s principle put more simply, basically means that an incompressible fluid transmits pressure. This is the basis to hydraulic lever. In a hydraulic lever, for example, you apply a force to the left-hand piston over a given area, this force is then transformed in to a pressure which is transmitted through the hydraulic fluid or oil. This pressure then transforms back in to an output force over another given area for the right-hand piston.

 

[guido411]

In essence, there IS a hydraulic pump in a 152 brake system, it's YOU. Your feet push the pedals, which pluges down a piston inside the cylinder attached to the pedal, moving the fluid against the brake pucks in the calipers. A hyraulic system works on MOVEMENT of fluid, there's no way to move fluid without a pump of some kind. Just a technicality.

 

[Hugh Jorgan]

Guido,
I'd be interested to hear the story behind your avitar if you know it.

 

[guido411]

Hugh,

It was taken about a month ago in SLC. We taxied in at night and saw all the airport rescue trucks lined up with lights flashing outside Salt Lake Jet Center. I got some pretty good night shots of it too, but the shutter speed being so slow they turned out sort of blurred. Turns out the next day this airplane was still in the same position so got some more day shots of it.

The rest of the story goes about like this. Airplane loaded with prisoners for transport to unkown destination. Preflight walk around reveals a flat tire. Deplane pax for tire change. The asphalt the 727 was parked on was not stressed for the weight which it was about to receive. When they jacked the airplane up off the left side, the jack sunk into the pavement. Somehow (I'm unclear as to exactly the fatal sequence of events went) as they were trying to raise the jack from out of the sunken asphalt the balance shifted enough rearward that the airplane tipped backward against the rear airstair. The airstair began folding up so the plane came to rest on the tail stand with the stairs partially collapsed. Got some good pics of the detail from the rear. All in all sounds like a bad day by all, but on the bright side, some prisoners got to spend a little less time in a jail cell than they otherwise would have had to.

 

[TurboS7]

All hydraulic systems require a pump to operate. In the case of the brakes on your 152 the pump happens to be your foot.

 

[GravityHater]

I think he is asking 'when' or at what point do we move from human actuated to hydraulic pump actuated systems....
C150 human
C425 human
Citation human
Convair ??
G-IV ??
727 boosted
C130 boosted
etc etc.

At what weight (or other factors in the a/c design or specs is a human no longer able to reasonably stop an aircraft without a boost system.

 

[avbug]

There is no point at which a motor driven pump is necessary; any hydraulic system may be actuated by a man driven pump. However, there comes a point at which a man driven pump becomes very impractical.

Think about what you're accomplishing with hydraulic actuation. You're using the effects of leverage through a fluid medium to move something. That can be with a direct line between something as simple as a brake caliper or piston and a foot pedal, such as in a simple hydraulic brake, or it can be an open center system in which fluid is constantly flowing but not under pressure until a system is actuating.

In the case of a direct system, a pump need not be running. A pump isn't even necessary to produce the pressure; that can be accomplished by means of mechanical advantage by properly utilizing differing size pistons, and lines...that's what using hydraulic actuation is all about. It can also be accomplished through the use of an accumulator to "store" hydraulic pressure (and therefore energy) that's been built up little by little using a hand pump, foot pump, or other means.

I've hand pumped down flaps and gear in large four engine airplanes. But I can tell you from first hand experience that it's exhausting, and a lot more work than fun.

An open center system uses constant circulation of hydraulic fluid by a pump; generally an engine driven pump as utilization of an electrical pump might exceed possible duty cycles. Fluid is contantly circulated, without impediment, until a hydraulically actuated item is selected. When an item is selected, a valve is moved that blocks the free flow of hydraulic fluid, diverting it to that item. Once the item is done moving, mechanical sequencing takes place under hydraulic pressure to return the actuating valve to the open center position, allowing pressure to drop to zero, and fluid to circulate freely again.

Such a system makes constantly pumping by a person impractical. An engine driven pump which is constantly turning means that the fluid is constantly circulating, waiting to be used. This type of system requiresa pump in use all the time.

The nature of what pump or driving device is used depends on the system, how it is to be used, and what is desired in the operation of the system.

 

[TonyC]

Does the length of the hydraulic lines determine whether or not a pump is required, or is there something else (i.e. required output force)?

The latter.

The brakes on the large airplane you referenced are not directly actuated by the pilots' rudder pedals. The pedals are connected to actuators that direct hydraulic fluid under pressure (nominally 3000 psi) to the brakes.

Theoretically, there is no pressure loss over distance (since the lines aren't perfectly rigid, reality sets in). Pumps are used to produce pressures that humans can't feasibly produce. "[N]o point at which a motor driven pump is necessary"?!?! I beg to differ. When it's time to raise the gear and retract the slats and flaps, pumps are necessary.

 

[avbug]

Necessary, vs. really nice to have. Any hydraulic action could be accomplished by hand; as I stated, I've worked many of the major operations on large airplanes by hand in emergencies; it certainly can be done...even on high pressure systems.

There is no magical point at which a motor driven pump must be used, but there are many applications in which it is very desirable. Including many 3,000 psi systems.

The question posed by owochris asked at what point an electric or engine driven pump becomes necessary to actuate a system. At no point does it become necessary, only very desirable.

Does a pressure drop occur over long distances? Yes, internal line friction leads to inefficiency. However, this is accounted for in engineering the system. Line flex is negligable, and most hydraulic line is hard line, and sees no appreciable expansion with application of hydraulic pressure. Hydraulics are only one way to move a surface; the same thing can be accomplished with mechanical linkages and electrical motors. Hydraulics can save weight, and may be tapped for multiple systems, and have additional advantages in rigidity.

Hydraulics also have a number of disadvantages. Anybody who has ever had pneumonia following inhalation of H5606 (or worse, skydrol) after getting a cockpit or cabin misted knows about that...or who has had to replenish a rapidly draining resorvoir with jerry cans in severe turbulence...electrical controls become a giddy pipe dream about then.

 

[TonyC]

I'd like to know where the hand pump is on the MD-11... or the 727...



I'm curious about some other airplanes, but those two are important to me - - I might need to use them.

 

[FN FAL]

I'd like to know where the hand pump is on the MD-11... or the 727...



I'm curious about some other airplanes, but those two are important to me - - I might need to use them.

Maybe the penny loafers and slip on shoes analogy confused him...so he started thinking about pumps instead.

 

[avbug]

I never stated that all systems use a hand pump, only that it is possible to move any system with a hand pump. Many systems don't have one installed. Putting on in the system would make it possible.

During maintenance, generally an electrical pump is used externally, and hooked into the system; we call it a hydraulic mule. Also available for use, although as previously stated, much less desirable, is a hand pump.

Simply because the system doesn't incorporate a hand pump doesn't mean that it isn't possible to use a hand pump to accomplish the same thing.

The orginal poster, owochris, asked at what point one becomes necessary to use an electrical pump or engine driven pump in a hydraulic system. At no point does it become necessary. Only desirable. The only difference between what can be done by hand and what cries out for a bigger pump? Volume. The same pressures may be reached either way, and the same loads may be moved either way. One takes a lot longer, one does not.

 

[A Squared]

Ok, Tony and Avbug, since y'all (I vacationed in Texas recently) are being pedantic, I'll join in. Yes, sometimes engine driven, or electrically driven hydraulic pumps *are* necessary. In many cases, especially recip aircraft, meeting performance criteria like second segment climb gradient depends upon the landing gear being retracted within a certian time. Meeting this time limit would be impossible with a hand pump, no matter how industrious your first officer is. (remember that an importan part of being a captain is knowing when to delegate)

An example is a gross weight increase which is available for Convairs (I forget which model it affects) anyway, one of the required modifications is replacing a fixed displacement hydraulic pump with a variable displacement pump, which speeds gear retraction, and enables the airplane to meet the required gradient at the higher takeoff weight.

 

[redd]

Geez, give it a rest Tonyc, you might learn something if you just listen once in a while. Take a chill pill. You're really starting to sound quite petty, like you have on the other threads lately.

 

[Uncle Sparky]

....hey...... a party !!

......... Meeting this time limit would be impossibele with a hand pump, no matter how industrious your first officer is. (remember that an importan part of being a captain is knowing when to delagate)

An example is a gross weight increase which is availabel for Convairs (I forget which model it affects) anyway, one of the required modifications is replacing a fixed displacement hydraulic pump with a variable displacement pump, which speeds gear retraction, and enables the airplane to meet the required gradient at the higher takeoff weight.

Why would a variable displacement pump be preferred over just a higher volume pump?

Hey guys,

I have kind of a "dumb" question to ask in regards to hydraulic systems.

When does it become necessary for the system to require a hydrualic pump (either electric or engine driven) to pressurize the fluid in order to power the system?

For example, on my 152, I know there is no need for a hydraulic pump to power the brakes. Is this because the brake lines do not cover a large distance (i.e. they're not that long), and thus there will not be much friction loss when I pump the brake pedals to transmit the force?

Now, look at an airliner, like a 747. I'd imagine that because this a/c is so large, the hydraulic lines on this plane will be very long. In this case, something will be required to pressurize the fluid in order for it to transmit a force to the output side.

Is my thinking correct? Does the length of the hydraulic lines determine whether or not a pump is required, or is there something else (i.e. required output force)?

Thanks in advance,

Chris.

If I were to grab a number out of the air , I would say 10,000bow. As far as the "convenience" issue, I would say that applies to Mooneys and Arrows.......perhaps something as large as a 208 or Navajo and that's about it. Line length is not really a factor. Hydraulic fluid is incompressible. Basically, that volume of fluid that runs through the line from the cockpit to the brake can be thought of as one unbroken, nearly frictionless, solid piece because of that fact.
Area and weight are the two terms you really need to be concerned with. Overall weight of the aircraft, wheel diameter, brake diameter and shoe or pad area would be the biggest factors to consider when deciding pressure supply system requirements, in my opinion.

 

[A Squared]

....hey...... a party !! Why would a variable displacement pump be preferred over just a higher volume pump?

I don't know for sure.

possibility: an engine driven pump is running all the time the engine is turning, there has to be a bypass system capable of continuously handling the flow. It could be that installing a fixed displacement pump with sufficient displacement to move the gear in the required time, would require installing a new bypass system in order to handle the increased flow, which might be more expensive than using the variable displacelent pump.

Maybe it's more economical to use a variable pump which can supply higher flow/lower pressure when needed, and lower flow/higher pressure when not needed, than it is to use a pump which can at all times met the max flow requirements *and* the max pressure requirements (which don't necessarily happen at the same time)

LIke I said, I'm not sure, but the point is that pumping ability has an affect on performance numbers and therefore on certification standards. Next time I see my buddy who owns the Convairs, I'll ask him about the modification.

 

[Uncle Sparky]

my own thinking was...."why a variable pump on a system already designed to relieve a fixed pump?" ....but now that you mention it, the relief circuit was designed for the volume of the original fixed pump. A larger volume fixed pump would be great during the gear cycle but any other time would be loading up the relief circuit. So now, it's either all new high pressure relief plumbing or ....a hanger!?. .......I think I answered my own question.

Maybe it's more economical to use a variable pump which can supply higher flow/lower pressure when needed, and lower flow/higher pressure when not needed, than it is to use a pump which can at all times met the max flow requirements *and* the max pressure requirements (which don't necessarily happen at the same time)

.....seems logical as well.....

 

[100LL... Again!]

Several factors are at work:

#1 The brake pads in your 152 only need to move a few millimeters to get the job done. Notice that your brake pedal moves much more - kind of like using a lever. You apply some force to the long arm of the lever and get more force at the short arm. Leverage, as it is sometimes called, or force multiplication.

#2 You could use a small footpump to raise the gear on a 747 if one of the following conditions were true - either the gear was a lot lighter OR the gear only needed to move a short distance to retract. You could even pump it up and down as it normally is if you had a pump handle many tens of feet long, or were willing to pump thousands of strokes.

To answer your question, what determines when a pump is required?

Basically, when the force required to operate the brake/gear/flap/whatever is great enough that:

1) It would take too much hand/foot FORCE to work the pump to operate the device in a timely fashion,

OR

2) It would take too much TIME to work the pump to operate the device with a pump that could be manually operated.


You could raise the gear on a 747 using a pump the size of a computer mouse that you could operate with your little finger. It would take weeks to raise the gear though, just like you can empty a lake with a spoon.

 

[FN FAL]

Doesn't the Beaver use hand operated hydraulic pump for operating the flaps?