hydraulics / pneumatics 2

[golfpin]

G,day to all,
any thoughts on why hydraulic systems were preferred over pneumatic .........have just recently been looking over a YAK 3 and been apprised of some of its features, simplicity,yes, but that the systems are entirely pneumatic. Taking into consideration that it is designed with the Russian weather in mind it seems paradoxical to me that any other could be envisaged considering the fire hazard of hydraulics. Also that once a hydraulic leak occurs the loss of fluid ends the function, but with air, within reason as long as the pump[air supply] keeps going you still would get some function, or am I being too simplistic.? Comments thoughts appreciated.
Golfpin

 

[MikeHalloran]

Hydraulics do not present a fire hazard per se.
Oil hydraulics that leak in the presence of an ignition source do present the possibility of a hazard.

Pneumatics run from a bone-dry supply of gas should be quite reliable. Real pneumatics, run from a compressor and an inadequate dryer, do get corroded internally, and do freeze when the weather gets cold, all because of the water that came in with the air as humidity.

I have no idea what sort of defenses the Russians use against pneumatic system problems. According to "Ice Road Truckers", Canadians carry neat alcohol, and pour in a slug when a pneumatic system gets cranky. Upon reflection, that seems like a Russian solution, except, well you can finish the obvious joke.




Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

I seem to recall that the B17 used a pneumatic system with a 120 bar 4 cylinder compressor.
B.E.

You are judged not by what you know, but by what you can do.

 

[golfpin]

Thanks Mike and berkshire knew there had to be another use for vodka, antifreeze! Joking aside would it be a reasonble assumption that electric drive is the way designers have gone or does the size of the aircraft start to dictate what is used.
lived in Canada awhile Mike and definitely go with the alternate use of Vodka especially where I live know and tell the locals about -20 here is

















+25

 

[MikeHalloran]

Airplanes, even big ones, are starting to go 'fly by wire', complete with all-electric actuators.

I personally would prefer mechanical cables with a hydraulic assist, but nobody asked me.



Mike Halloran
Pembroke Pines, FL, USA

 

[WKTaylor]

Hope this quick draft makes sense...

Hydraulic systems.
Very Compact, high pressure.
Generally hydraulic systems are made redundant [multiple-path/supply system] to ensure they work with context of (1) or more system failures.
Fire retardant hydraulic oils, and directed [safe] internal fluid leak/drain paths, tend to make these systems fire/explosion-safe.
System failure deflates pressure instantaneously.... and fluid will leak-out until pumps are no longer effective. Pump overheating [fluid starvation] can be a serious problem.

Pneumatic systems.
Pneumatic systems tend to be hot due to on-board air compression.
For the first reason, pneumatics systems are rarely, if ever, made redundant.
Ducting tends to be very large and bulky, for low pressure/hot air. Insulation is generally required. High pressure pneumatic tubing can be made much smaller, without insulation. However, all Pneumatic system failures [punctures, etc] tend to be explosive due to sudden expansion of gas. This tendency can be far worse with larger pressure differentials, large [expanded] volume of air available [especially the higher-the-pressure/temperature systems]; and the worse the explosive/air-chemical reactive forces can be. Warning: Provisions to bleed-off sudden pressurized/hot air due to duct failure are mandatory, or structural failure [pressure blow-out, crushed low pressure fluid tubing, etc] can occur.
Hot pressurized air leaks have been known to be highly reactive with [all] oils, fuels, electrical wire insulation jackets, magnesium, titanium, etc. Pneumatic duct failures that breach adjacent electrical wiring or fluid lines can be catastrophic.

I have investigated many mishaps where high pressure, hot air, has been released suddenly [due to clamp, duct wall, weld, etc failures] causing all sorts of major damage.

Although I have dealt with low pressure/hot pneumatic systems for decades in aerospace, they still make me very nervous.


Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

Unfortunately, in science what You 'believe' is irrelevant – "Orion"

 

[golfpin]

Thanks for a very erudite explanation Will, to conclude my interrupted post a few lines back, my wife agrees a shot of vodka is good for my systems when I get cranky.
Golfpin

 

[tbuelna]

Where precision and/or high frequency actuation is required, hydraulics have the advantage over pneumatics in that the working fluid is essentially incompressible. The main drawbacks of distributed hydraulic systems on aircraft have been addressed with EHAs (Electro Hydrostatic Actuators). These actuators have a self-contained, sealed hydraulic system powered by electric motors. They don't have any external hydraulic lines, so they are less prone to leakage and more damage tolerant. Being self-contained they are also able to safely run higher fluid pressures.

 

[debodine]

This is strictly a guess on my part, but I would think perhaps the Russians had a supply problem with obtaining or manufacturing their own hydraulic fluids, thus designing with pneumatic systems eased their support needs. I tend to think this way because the purges of the high level military, political and economic leaders practiced by Stalin in the decades before WWII greatly damaged the ability of the Russian economy to build infrastructure for producing things like hydraulic fluids.

However, I am open to correction by people with facts that trump my guess.

 

[btrueblood]

According to wikipedia,

The pneumatic system for actuating landing gear, flaps and brakes, typical for all Yakovlev fighters of the time, was problematic. Though less reliable than hydraulic or electrical alternatives, the pneumatic system was preferred owing to significant weight savings.

The system was likely "fed" by bleeding air from the supercharger, so no additional pump was necessary.

 

[KENAT]

I'll back up what Will says about compressibility of the pneumatics and the issues that can cause - especially since almost any significant failure becomes explosive as he says.

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[golfpin]

Thanks for the input chaps, my limited knowledge re turbochargers and burnt fingers from playing with an airline should have tipped me off re temp and pressure and air, but in mitigation I was told by the owner of the said Yak that the main reason for use of air was because of the Russian weather, and having lived in Canada and having experienced oils turning into "jelly" I was inclined to accept that.
Thanks again Golfpin

 

[tbuelna]

debodine makes a great point. The Yak-3 designer's choice of pneumatics over hydraulics may have had more to do with economics than "weight savings".

Even thought the Yak-3 was put into service late in the war (1944?) the Soviets probably did not have the resources to develop and produce the synthetic hydraulic fluids the Germans and other Allied countries had, and thus were still reliant on mineral-type fluids. The older mineral-type hydraulic fluids had relatively poor low temp fluidity and the Soviet designers may have seen this to be a problem for this aircraft. The cost of hydraulic actuation systems may have also been a concern. High pressure hydraulic system components like pumps and valves require high-precision machining, which is costly and requires specialized equipment. The Yak-3 was produced in large numbers over a short time period (1944 to 1946) and the Soviets had very limited financial resources during that time.

Interesting topic. As a mechanical designer, I love looking at the details of existing designs and then try to figure out why the designer did what they did. Sometimes the reason becomes apparent after thinking about it for a while. But other times I can only speculate what the reason may have been.

 

[golfpin]

very nicely put chaps, yes cost etc must have been a large factor.
Have to go with tbulena on that point of "why did they do it that way".
Was very fortunate to go on board the static displays of the Concord and the Tupolev counterpart, well the difference even to the untrained eye, [my wife] she commented that looking at the cockpit of the tupolev compared to the concord was looking at something that came out of ww2.. I wonder if this has a lot to do with the number of accidents the russian craft are involvrd in, or is that an unfair statement?
Haven,t been offered a ride in the Yak but would probaly decline but did get a ride in a Tiger Moth on the occaison of my 70 th. what a blast, and it had no brakes! nothing like simplicity so said Da Vinci.
We lost a privately owned Yak awhile back, not sure of the circumstances but "finger" problem is becoming a major issue as us oldtimers move off into the sunset, poor training...I wonder if this is world wide? Hope have not moved too far of he point.
Thanks for the input.
Golfpin

 

[tbuelna]

golfpin-

One might also ask why the designers of the Mosquito bomber chose to build the airframe from laminated plywood or chose to use rubber springs in the landing gear. They had very sound reasons for doing so, and it is quite impressive that they got these designs to work adequately for the application.

 

[btrueblood]

"the Soviets probably did not have the resources to develop and produce the synthetic hydraulic fluids the Germans and other Allied countries had"

I'm about 90% certain that most A/C in WW2 had plain old mineral oil hydraulic systems, not the "fireproof" synthetics (Skydrol did not become available until 1948, according to that company's web page). Lots of stories of hydraulic fluid fires in the B17 and B24. The Soviet Union was capable of refining aviation fuel, so there's no reason that they would not have light petroleum oils available as well. While the synthetics were certainly possible to formulate in WW2, the seals they require were not.

"High pressure hydraulic system components like pumps and valves require high-precision machining"

Calling BS on this also. Tank cannons, aircraft engines, machine guns, etc. etc. also require precision machining. A hydraulic pump is pretty simple in comparison to a supercharged V12, of which the Russians had plenty. And "financial difficulties" are irrelevant in a command economy ("Stalin says build more Yaks. We build more Yaks").

Yes, it's fun to speculate. But getting the word from the old timer's mouth, however diluted down the years as the story is passed from historian to historian, or edited by wikipedians, is far better than guesswork. I think the weight issue was predominant in somebody's mind within that Soviet design bureau, and once the pneumatic system had been designed into the airframe it was difficult to design back out.

 

[MikeHalloran]

I'm not aware that Skydrol was ever used on any military aircraft. It certainly was not used on the ones I studied for simulation purposes in the early 70s.

I did the installation for one military flight simulator that used Skydrol. The simulated airplane did not, but some desk jockey inserted it in a spec, and it got cast in concrete. Some time after startup, a leak developed, and it took all the paint off the shed that housed the hydraulic supply. I was told it was quite a mess, and not welcome on a base where enlisted men scrubbed the decorative gravel with toothbrushes.

We fought tooth and nail to get Skydrol removed from the next simulator spec we saw, because it was such a pain to work with.


Skydrol is not fireproof, either; it's just harder to ignite than mineral fluids.


Mike Halloran
Pembroke Pines, FL, USA

 

[berkshire]

MikeHalloran (Mechanical)
Skydrol is also much more efficient at removing skin than other hydraulic fluids.
B.E.

You are judged not by what you know, but by what you can do.

 

[golfpin]

Wow guys this is unreal stuff, hope the site moderators don,t come down on us like the proverbial..
. Tbuelna you have touched on a subject that is so very dear to my heart, curiosity prodded my fading brain partly because of my involvement for some 8 years in F1 motor racing and the move in design format, composites and the like.

Being a flying fiend/fanatic/fool I tried to get some info on how and what the Mosquito was built of, came up with too little except for concrete molds and a few titbits tried to get more info on how and what the "sandwich" was that made up the aircraft, not much forthcoming on the internet. Truly a marvel [I think] of engineering how did they do the stress analysis in 1940 no computers no FEA!!!!! and from what I have read it was a success from day one in terms of handling flying behaviour etc,

interesting bit of flying history written by Karel Birkby relates the story of a South African manned Mosquito that encountered and filmed the first recordered contact with a ME 262 in WW2 over the Black forest area, they made it back, but that is a a story in itself, the damage that the wooden wonder sustained and still got back, badly shot up, is incredible.

Btrueblood very nicely worded comment thanks, got to go with the thought on weight, re the yak and pneumatics this was brought to the front with the Jap zero, light and extremely agile yes, but fragile. I suppose then design is fraught with compromise.
Interesting points you make re the availability of lubricants that were not affected by extreme temperatures, equally that the German technique of converting coal to fuel is the exact same process that started the fuel/chemical giant Sasol here in South Africa. Politics ? who knows.
Thanks for your input,
Cheers Golfpin

 

[edmeister]

In the '80s, the Mig-25 was nicknamed the "Flying Restaurant" because crews would drain and drink its alcohol-based hydraulic fluid. There are numerous stories of aircraft squadrons being grounded due to drained hydraulics & storeroom supplies being broken into & robbed. I believe that implementing pneumatics was based more on "practicality for the times" rather than engineering logic.

BTW .. it's not just a Russian military thing. American sailors in WWII drank "Torpedo Juice". which was the alcohol that fuel torpedoes of that era (also hazardous to health). Wikipedia has an interesting article about Torpedo Juice.