The Fokker F-27 has pneumatics (at 3000psi) to operate landing gear extension and many other things. I read doing so saved 100 lbs. But almost every plane uses hydraulics much more than pneumatics.

- Are there any planes with where pneumatics operate the control surfaces? I imagine (pre computer control) pneumatics make fine control more difficult since you're working with a compressible medium.

- Are there any planes that have pneumatics and hydraulics as backups for one another (rather than two different hydraulic systems. The only example I know of is the Su-17, which had hydraulic flaps but pneumatics for emergency flap operations.

- Now that computer control is cheap, is there any chance that the use of pneumatics will increase?

- Are there any advantages for hydraulics over pneumatics,beyond the easier fine control and the ease of detecting leaks? Because air is lighter weight and less toxic than hydraulic fluid.

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

Starlionblue wrote:

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

Interesting you say that as that's what SpaceX is doing with the Starship - Tesla electric motors powering the grid fins on the booster (That's the goal I believe) and also powering the maneuvering flaperons on the upper stage. Much will depend on weight, if an equivalent electrical landing gear actuator is same/less weight but also less maintenance intensive, this would be taken up. Could we see all hydraulic reticulation based systems removed in future in favor of local hyd/elec actuators?

I see scope in following two systems in the years to come:

Heavy Duty Electrical Actuators.
High Pressure(upto 5000psi) Hyd Actuators.

77west wrote:

Starlionblue wrote:

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

Interesting you say that as that's what SpaceX is doing with the Starship - Tesla electric motors powering the grid fins on the booster (That's the goal I believe) and also powering the maneuvering flaperons on the upper stage. Much will depend on weight, if an equivalent electrical landing gear actuator is same/less weight but also less maintenance intensive, this would be taken up. Could we see all hydraulic reticulation based systems removed in future in favor of local hyd/elec actuators?

Maybe. Depends where the technology goes.

Of course it isn't just the weight of the actuators. It's also the weight of the power generation. EDPs vs EDGs.

Starlionblue wrote:

77west wrote:

Starlionblue wrote:

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

Interesting you say that as that's what SpaceX is doing with the Starship - Tesla electric motors powering the grid fins on the booster (That's the goal I believe) and also powering the maneuvering flaperons on the upper stage. Much will depend on weight, if an equivalent electrical landing gear actuator is same/less weight but also less maintenance intensive, this would be taken up. Could we see all hydraulic reticulation based systems removed in future in favor of local hyd/elec actuators?

Maybe. Depends where the technology goes.

Of course it isn't just the weight of the actuators. It's also the weight of the power generation. EDPs vs EDGs.

Ah yes, very true. I had not thought about that but it makes sense. It will be the overall system, actuators, generators et al.

77west wrote:

Starlionblue wrote:

77west wrote:

Interesting you say that as that's what SpaceX is doing with the Starship - Tesla electric motors powering the grid fins on the booster (That's the goal I believe) and also powering the maneuvering flaperons on the upper stage. Much will depend on weight, if an equivalent electrical landing gear actuator is same/less weight but also less maintenance intensive, this would be taken up. Could we see all hydraulic reticulation based systems removed in future in favor of local hyd/elec actuators?

Maybe. Depends where the technology goes.

Of course it isn't just the weight of the actuators. It's also the weight of the power generation. EDPs vs EDGs.

Ah yes, very true. I had not thought about that but it makes sense. It will be the overall system, actuators, generators et al.

And distribution system. electric systems evolved to power bus and local switches controlled by the bus commands. And tubes for high pressure are heavy...

Starlionblue wrote:

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

The 787 eliminated pneumatics for the cabin conditioning systems. I'm not talking about that at all. I'm talking about using pneumatics to move things, like a hydraulic system might.
In this context, I don't know that pneumatics would weigh more than hydraulics, if they are working at the same pressure.

One advantage of pneumatics is that the system can easily store energy. A small tank under high pressure stores tons of energy, and therefore the pump can be sized to average load and not momentary worst load. Hydraulics can have an accumulator, but they don't store as much energy for the same weight.

Smaller pump ... less weight.

767-300s use pneumatics for thrust reversers. They work very well.

Pneumatics are not used for any flight control system in the F-27 (Fokker, Fairchild, Fairchild-Hiller). Landing gear extension/retraction, brakes and nosewheel steering on some. Flaps are electric. All axis flight controls are conventional cables and pushrods.

In the 747-200 and 400, all four engine driven hydraulic systems were supplemented and backed up by four pneumatic air driven hydraulic pumps. They would frequently be intermittently activated under heavy hydraulic load (gear/flap ops) when the system dropped below a certain pressure.

Lpbri wrote:

767-300s use pneumatics for thrust reversers. They work very well.

Interesting, the CRJ200 uses pneumatics for thrust reverser actuation, and they don't work for crap.

Lpbri wrote:

767-300s use pneumatics for thrust reversers. They work very well.

The 767-400 uses pneumatics for the reversers. The -300 is regular old hydraulic power.

Nope. Dunno what your airline you are from, but 767-200 are hyd t/r all else pnuematic

DualQual wrote:

The 767-400 uses pneumatics for the reversers. The -300 is regular old hydraulic power.

Our -300 are all pneumatic.

Lpbri wrote:

767-300s use pneumatics for thrust reversers. They work very well.

Ya think? Chap 78 has always been an exception driver for us.

The Fokker F-27 pneumatics were an endless source of problems in cold weather operations on the type.

fr8mech wrote:

DualQual wrote:

The 767-400 uses pneumatics for the reversers. The -300 is regular old hydraulic power.

Our -300 are all pneumatic.

Lpbri wrote:

767-300s use pneumatics for thrust reversers. They work very well.

Ya think? Chap 78 has always been an exception driver for us.

Must be an engine difference then. Ours have hydraulic T/Rs.

Everyone is correct about the 767 Thrust Reverser. For GE engines they are pneumatic. For PW they are hydraulic just like the 757.

arcticcruiser wrote:

The Fokker F-27 pneumatics were an endless source of problems in cold weather operations on the type.

Yes indeed. Especially if the mechanical air compressors were worn out. I spent a January trying to operate a Fairchild out of Denver.

RetiredWeasel wrote:

In the 747-200 and 400, all four engine driven hydraulic systems were supplemented and backed up by four pneumatic air driven hydraulic pumps. They would frequently be intermittently activated under heavy hydraulic load (gear/flap ops) when the system dropped below a certain pressure.

That’s aircraft specific. Some birds, System 2 & 3 hydraulic demand pumps are electrically driven…

bigb wrote:

RetiredWeasel wrote:

In the 747-200 and 400, all four engine driven hydraulic systems were supplemented and backed up by four pneumatic air driven hydraulic pumps. They would frequently be intermittently activated under heavy hydraulic load (gear/flap ops) when the system dropped below a certain pressure.

That’s aircraft specific. Some birds, System 2 & 3 hydraulic demand pumps are electrically driven…

You're correct on the 400. Only systems 1 and 4 had demand air pumps in addition to the EDPs. The 100s and 200s had ADP demand pumps on all four at least for the NW birds. There was an electrical pump (#3 I think) but was only used for ground ops.

Dalmd88 wrote:

Everyone is correct about the 767 Thrust Reverser. For GE engines they are pneumatic. For PW they are hydraulic just like the 757.

Yes that's the beauty with the Boeing birds from that era - so many variations with different airlines and engine combination.

RetiredWeasel wrote:

bigb wrote:

RetiredWeasel wrote:

In the 747-200 and 400, all four engine driven hydraulic systems were supplemented and backed up by four pneumatic air driven hydraulic pumps. They would frequently be intermittently activated under heavy hydraulic load (gear/flap ops) when the system dropped below a certain pressure.

That’s aircraft specific. Some birds, System 2 & 3 hydraulic demand pumps are electrically driven…

You're correct on the 400. Only systems 1 and 4 had demand air pumps in addition to the EDPs. The 100s and 200s had ADP demand pumps on all four at least for the NW birds. There was an electrical pump (#3 I think) but was only used for ground ops.

Just went on the 744 system description and I am faced with this... (effectivity code edited out of course )

EFF ALL
(1) Each of the systems has an engine-driven pump (EDP) that is used as the primary power source. Each system also has a demand pump that is used during operations that require large hydraulic flow. The demand pump is also used if the EDP does not operate correctly.

EFF AAA, BBB
(2) Systems 1 and 4 have air-driven pumps (ADP).

EFF CCC
(3) Each of the systems have air-driven pumps (ADP).

EFF AAA, BBB
(4) Systems 2 and 3 have ac motor-driven pumps (ACMP).

EFF BBB
(5) System 4 also has an auxiliary ac motor-driven pump that operates only when the airplane is on the ground.

EFF AAA, CCC
(6) Systems 1 and 4 also have auxiliary ac motor-driven pumps that operate only when the airplane is on the ground.

and that's the combination available with just one airline....

kitplane01 wrote:

Starlionblue wrote:

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

The 787 eliminated pneumatics for the cabin conditioning systems. I'm not talking about that at all. I'm talking about using pneumatics to move things, like a hydraulic system might.
In this context, I don't know that pneumatics would weigh more than hydraulics, if they are working at the same pressure.

One advantage of pneumatics is that the system can easily store energy. A small tank under high pressure stores tons of energy, and therefore the pump can be sized to average load and not momentary worst load. Hydraulics can have an accumulator, but they don't store as much energy for the same weight.

Smaller pump ... less weight.

pneumatics are too slow for all the things that should follow inputs. As Starlionblue pointed out: gas can be compressed, liquids, for all intents and purposes, not.

best regards
Thomas

My take on one vs the other is that the differences are due mainly down to the compressibility (or otherwise) of the working fluid.
Pnue gives a good storage of energy and can react fast because of the storage nature of compressed air however because of the compressibility there is little fine positional control. If you want to move something from one stop to another quickly then pneu is good, if you need variable positions then Hyd is better.

In terms of pressure I would expect that Pneu would be generally lower due to the dangers posed by compressed gasses. I would assume that aicraft would also use multiple hydraulic accumulators between pumps and some sort of PRV to keep pressure fluctuations down at peak loads.

Fred

flipdewaf wrote:

If you want to move something from one stop to another quickly then pneu is good, if you need variable positions then Hyd is better.

the working gas in the pneumatic line also allows for a lot of movement of the controlled surface, it would be like like replacing conventional control cables with "control springs". Hence it is only good for stop to stop, if those positions also lock.

best regards
Thomas

Eons ago, while I was in the USAF, I flew the B-52G and D model. Flying the D was like going back to the Jurassic era! The hydraulics and electric were all pneumatic driven. We had 4 alternators and 4 hydraulic packs. From a pilot's perspective, the operation of those applicable systems was no faster or slower than you would see on a G model. However, the actual pneumatic system was prone to leaks. I would imagine that was more a function of age and low-level flying than a design deficiency on the pneumatic system. I can't remember having any issues with the hydraulics or electrics on the D compared to the G. The flight controls were all cable operated so I won't even go there!!!

tommy1808 wrote:

flipdewaf wrote:

If you want to move something from one stop to another quickly then pneu is good, if you need variable positions then Hyd is better.

the working gas in the pneumatic line also allows for a lot of movement of the controlled surface, it would be like like replacing conventional control cables with "control springs". Hence it is only good for stop to stop, if those positions also lock.

best regards
Thomas

I think that USED to be true, but in an era of very cheap computer controls you can make a pneumatic servo reliably go to any desired position.

tommy1808 wrote:

kitplane01 wrote:

Starlionblue wrote:

Pneumatics are not suitable for large control surfaces. As you say the medium is compressible.

- Pneumatics can be made very compact, which saves space and weight.
- Air may be less dense than hydraulic fluid, but the weight of hydraulic fluid in an airliner is a rounding error in terms of total weight.

I predict we won't see more pneumatics. The 787 is an example of the elimination of pneumatics. Electric actuators are a more likely development.

The 787 eliminated pneumatics for the cabin conditioning systems. I'm not talking about that at all. I'm talking about using pneumatics to move things, like a hydraulic system might.
In this context, I don't know that pneumatics would weigh more than hydraulics, if they are working at the same pressure.

One advantage of pneumatics is that the system can easily store energy. A small tank under high pressure stores tons of energy, and therefore the pump can be sized to average load and not momentary worst load. Hydraulics can have an accumulator, but they don't store as much energy for the same weight.

Smaller pump ... less weight.

pneumatics are too slow for all the things that should follow inputs. As Starlionblue pointed out: gas can be compressed, liquids, for all intents and purposes, not.

best regards
Thomas

I'm not an expert, but my strong impression is that with modern computer controls this is no longer a problem. You can position the actuator to any spot you want, with precision and reliably.

Question: Which is more reliable and less prone to leaks .. a hydraulic or pneumatic system running at the same pressure.
It might be a hard question to answer since few pneumatic systems run at high pressure.

kitplane01 wrote:

tommy1808 wrote:

kitplane01 wrote:

The 787 eliminated pneumatics for the cabin conditioning systems. I'm not talking about that at all. I'm talking about using pneumatics to move things, like a hydraulic system might.
In this context, I don't know that pneumatics would weigh more than hydraulics, if they are working at the same pressure.

One advantage of pneumatics is that the system can easily store energy. A small tank under high pressure stores tons of energy, and therefore the pump can be sized to average load and not momentary worst load. Hydraulics can have an accumulator, but they don't store as much energy for the same weight.

Smaller pump ... less weight.

pneumatics are too slow for all the things that should follow inputs. As Starlionblue pointed out: gas can be compressed, liquids, for all intents and purposes, not.

best regards
Thomas

I'm not an expert, but my strong impression is that with modern computer controls this is no longer a problem. You can position the actuator to any spot you want, with precision and reliably.

as long as you know exactly what forces work on whatever you are controlling, something you don´t know for control surfaces. And there is no way to get the "spingyness" out of a pneumatic system.

kitplane01 wrote:

Question: Which is more reliable and less prone to leaks .. a hydraulic or pneumatic system running at the same pressure.
It might be a hard question to answer since few pneumatic systems run at high pressure.

from a molecular perspective my money is on pneumatic being more prone to leak. The molecules are smaller and gases don´t stick together.

best regards
Thomas

tommy1808 wrote:

kitplane01 wrote:

Question: Which is more reliable and less prone to leaks .. a hydraulic or pneumatic system running at the same pressure.
It might be a hard question to answer since few pneumatic systems run at high pressure.

from a molecular perspective my money is on pneumatic being more prone to leak. The molecules are smaller and gases don´t stick together.

best regards
Thomas

Pneumatics are built to handle leaks. They have an infinite supply of gas whereas hydraulics must be refilled manually. Which is why pneumatics are really popular for systems made up of several parts that are prone to leaks, like trains where each car has a pneumatic brake but only the locomotive has a pressure generator.

mxaxai wrote:

tommy1808 wrote:

kitplane01 wrote:

Question: Which is more reliable and less prone to leaks .. a hydraulic or pneumatic system running at the same pressure.
It might be a hard question to answer since few pneumatic systems run at high pressure.

from a molecular perspective my money is on pneumatic being more prone to leak. The molecules are smaller and gases don´t stick together.

best regards
Thomas

Pneumatics are built to handle leaks. They have an infinite supply of gas whereas hydraulics must be refilled manually. Which is why pneumatics are really popular for systems made up of several parts that are prone to leaks, like trains where each car has a pneumatic brake but only the locomotive has a pressure generator.

Industrial pneumatics is often leaky by design, when high side pressure is released into atmosphere when actuator switches.

tommy1808 wrote:

kitplane01 wrote:

tommy1808 wrote:

pneumatics are too slow for all the things that should follow inputs. As Starlionblue pointed out: gas can be compressed, liquids, for all intents and purposes, not.

best regards
Thomas

I'm not an expert, but my strong impression is that with modern computer controls this is no longer a problem. You can position the actuator to any spot you want, with precision and reliably.

as long as you know exactly what forces work on whatever you are controlling, something you don´t know for control surfaces. And there is no way to get the "spingyness" out of a pneumatic system.

Sure there is .. feedback loops.

You tell the controller where you want the actuator. It measures the current position, does math, and adjusts valves. All of this is easy, and can be done faster than the actual control surface will ever move. I believe the hydraulic systems already do this, so the sensors and valves and computers are already there. We did this sort of stuff on high school robotics teams. (My point is not that aero control is high school easy, but that sensors and actuators in a feedback loop are very common and not hard in general.)

kitplane01 wrote:

I believe the hydraulic systems already do this, so the sensors and valves and computers are already there.

yes, but they work like a push rod, not like a spring, and transfer force instantaniously (well, at the speed of sound ~1400m/s).

best regards
Thomas

Pneumatic systems I have worked on tend to be 125 PSI design pressure devices, equipment wanting 80 psi such as valve actuators, shop impact wrenches, air cylinders, and air motors, no return just exhaust to ambient. Only if the process needs higher force will a system be say 300 PSI, however the hydro test on air systems is typically water as a rupture under air pressure can be explosive with the gas expansion. Only when really compact storage is high pressure gases of any kind used. HP Air on submarines is 4,500 PSI operating, Class 5,000 PSI fittings, piping must be super clean, will ignite any organics present. Air is great for controls in explosion proof spaces.

Hydraulics typical system operating pressure of 3,000 PSI, 5,000 PSI is common, the Enerpac hydraulic jack systems utilize 10,000 PSI so comparison of delivered force to a 100 psi cylinder is 30, 50 or 100x, far more compact with hydraulics. Anything with forces over 1,000 lb on a plane would be done with hydraulic.

Electrical linear actuators are replacing a lot of hydraulic systems. For example, full motion simulators used with the Navy & I believe all aircraft ones, are changing out to electric linear actuators for faster motion, far reduced maintenance, environmental issues with hydraulic fluids. Similarly, electric motors are replacing hydraulic motors except for high density applications. A 100 HP hydraulic motor is like 1/10th the size of a 100 HP electric motor.

DiamondFlyer wrote:

Lpbri wrote:

767-300s use pneumatics for thrust reversers. They work very well.

Interesting, the CRJ200 uses pneumatics for thrust reverser actuation, and they don't work for crap.

There are a lot of systems you could substitute in this sentence and it still be accurate.