DanielBednar
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High pressure hydraulics vs bleed air

Fri Aug 02, 2019 5:49 pm

We always talk about how the bleed air we use is VERY high pressure, but it’s nothing compared to the 3,000 psi hydraulic systems. Would it be possible to have a pneumatic system that is pressurized to 3,000 like the hydraulics. I assume there wouldn’t be any benefits and there’s no way engines could provide such high pressure air. But what if they did, would it damage pneumatic ducts? And if it does damage stuff, why would the hydraulics not damage their piping too? This is a very weird question, I’ll admit
 
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Starlionblue
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Re: High pressure hydraulics vs bleed air

Fri Aug 02, 2019 7:09 pm

Fundamental difference: Air is compressible. Hydraulic fluid is not. So even if you could get up to 3000PSI with your bleed air it would still... compress... Which would lead to many issues with control surface positioning.

Having said that, I believe the Fokker F27 was all pneumatic with no hydraulics. There's always an exception in aircraft design. But it doesn't really scale too well to larger aircraft

Not sure if extremely high-pressure air would damage the ducts, but I imagine there'd be some moisture in there. And that's not good.

BTW hydraulics are up to 5000PSI in newere aircraft.
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mmo
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Re: High pressure hydraulics vs bleed air

Fri Aug 02, 2019 8:17 pm

Believe me, you don't want a pneumatic aircraft. I flew the B-52D which had pneumatic packs, hydraulics and pneumatic alternators; it was a nightmare. Bleed air leaks were a nightmare. Granted, it was 1940 technology, but having a hydraulic aircraft is great.
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DanielBednar
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Re: High pressure hydraulics vs bleed air

Fri Aug 02, 2019 8:52 pm

Thanks guys.

Starlionblue wrote:
BTW hydraulics are up to 5000PSI in newere aircraft.


What difference would it make? Does 5,000 as opposed to 3,000 have major performance differences?
 
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fr8mech
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Re: High pressure hydraulics vs bleed air

Fri Aug 02, 2019 10:15 pm

Simply, the plumbing is designed and rated for 3000 psi...actually a lot higher than that. The pneumatic ducting is rated for much lower pressure...probably in the 200-300 psi range at the engine and lower as the air progresses through the airframe.

Locations where high pressure air is used, e.g. accumulator charge lines, have plumbing similar in strength and design to hydraulic systems.
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Starlionblue
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Re: High pressure hydraulics vs bleed air

Sat Aug 03, 2019 3:46 am

DanielBednar wrote:
Thanks guys.

Starlionblue wrote:
BTW hydraulics are up to 5000PSI in newere aircraft.


What difference would it make? Does 5,000 as opposed to 3,000 have major performance differences?


As I understand it, with higher pressure the hydraulic lines can be smaller, which saves weight.
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Francoflier
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Re: High pressure hydraulics vs bleed air

Sat Aug 03, 2019 5:14 am

The F-27's pneumatic system was a pain in the a$$ to operate and maintain.
I believe it was a 3000 psi system, but it was a while ago so don't take my word.

It leaked everywhere, and depending on how much it leaked, you had to wait for a while before you could take off for the pressure to build up enough in the accumulators to raise the gear. And the noise, the horrible, strident, ear-piercing noise of those air compressors... as if the Darts needed help generating more noise.

On the plus side, it did that cool hissing sound when you shut down, like you just drove in a semi truck... :cool2:
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VC-10
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Re: High pressure hydraulics vs bleed air

Sat Aug 03, 2019 7:06 pm

DanielBednar wrote:
We always talk about how the bleed air we use is VERY high pressure, t


All the aircraft I have worked on ranging from the BAC One-Eleven through to the A350 have a pneumatic system that runs a 45/50 psi. I don't call that very high
 
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fr8mech
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Re: High pressure hydraulics vs bleed air

Sat Aug 03, 2019 7:37 pm

VC-10 wrote:
All the aircraft I have worked on ranging from the BAC One-Eleven through to the A350 have a pneumatic system that runs a 45/50 psi. I don't call that very high


I think we had this discussion before. From a maintenance point-of-view, chapter 36 air is indeed low pressure. High pressure, to the maintenance folks, exists in accumulators, struts and the like. But, from an operator’s view, there is low pressure, and that is conditioned air from an air-conditioning unit.
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kitplane01
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Re: High pressure hydraulics vs bleed air

Sun Aug 04, 2019 6:26 am

DanielBednar wrote:
We always talk about how the bleed air we use is VERY high pressure, but it’s nothing compared to the 3,000 psi hydraulic systems. Would it be possible to have a pneumatic system that is pressurized to 3,000 like the hydraulics. I assume there wouldn’t be any benefits and there’s no way engines could provide such high pressure air. But what if they did, would it damage pneumatic ducts? And if it does damage stuff, why would the hydraulics not damage their piping too? This is a very weird question, I’ll admit


Of course you can pressurize air to 3,000 psi. Not with bleed air, but with high pressure pumps.

But air, unlike hydraulic fluid, is compressible. For fluid, if you inject a fixed amount of fluid into an actuator you always get a given amount of travel (example: adding 1 cubic foot of air will always move the actuator 4 inches). With air, the amount the actuator moves varies depending on how much force resists the actuator's movement. To get a constant amount of travel in a pneumatic system requires a control system with position detector that can detect a range of motion (not just in or out, but how far), and controllable valves. Basically, each actuator needs a electrical power supply, and very small CPU. Such systems used to be expensive and unreliable, but now they can be cheap(er). I believe most actuators on large aircraft already have these things.

You might think that a pneumatic system would tolerate leaks. Large leaks cause the system to depressurize, so that's bad. Small leaks can be noisy, which is annoying but allows the system to function. Also, it's harder to find a leak in a pneumatic system (no puddles).

My undereducated guess is that if we were starting from scratch today with modern control system, a pneumatic system might be slightly more reliable and lighter weight than a hydraulic system. But I don't think it's obvious either way, and probably depends on things I don't know about.
 
StTim
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Re: High pressure hydraulics vs bleed air

Sun Aug 04, 2019 7:40 am

There is also the issue of failure in a pneumatic vs hydraulic system. As air is compressible should say the accumulation break when at high pressure you have an explosion. With hydraulic systems a similar situation results in a leak only.
 
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Spacepope
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Re: High pressure hydraulics vs bleed air

Mon Aug 05, 2019 3:50 am

There is also the issue with pressure drop when air is used. This can cause massive cooling and if the air supply isn’t cleaned well enough, icing from moisture in the system.
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JayinKitsap
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Re: High pressure hydraulics vs bleed air

Mon Aug 05, 2019 5:18 pm

High pressure air (>2,000 psi) has super strict cleanliness requirements. Any oil can cause an explosion, it is basically a diesel engine if a fuel is present. It is compressible (as others noted), expansion because of the volume change can cause intense cooling (as others noted). I've done test equipment of HP compressors and bleeding off from 4,500 to atmospheric can cause a 200F temperature drop.

Hydro tests are rarely done with air, imagine knocking the valve off of a compressed gas cylinder will send a rocket thru walls, if liquid a pop and that is about it.
 
Redbellyguppy
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Re: High pressure hydraulics vs bleed air

Thu Aug 15, 2019 6:37 am

The crj I once flew used pneumatics to actuate the thrust reversers...
 
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fr8mech
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Re: High pressure hydraulics vs bleed air

Thu Aug 15, 2019 7:46 am

Redbellyguppy wrote:
The crj I once flew used pneumatics to actuate the thrust reversers...


The B727, B747-100/200/400, B767 and A300 (PW4000) all have pneumatic reversers. I'm sure there are (were) many others.
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WIederling
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Re: High pressure hydraulics vs bleed air

Thu Aug 15, 2019 8:27 am

Francoflier wrote:
The F-27's pneumatic system was a pain in the a$$ to operate and maintain.
I believe it was a 3000 psi system, but it was a while ago so don't take my word.

isn't the pneumatic system invariably fed by bleed air?

How would you get 3000 psi (~200bar ) compressed air in an airplane?

adiabatic heating is enough of an issue for bleed air takeoff in the 100...300psi range.
( for bleed air you can't get beyond "outside pressure" times "pressure ratio")

compressible high pressure fluids are dangerous.
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