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Hydraulics & engine failure


Echo38

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Knowing little about engines & such, a question about hydraulics in real aircraft: when a WWII fighter's engine quits, is all hydraulic pressure immediately lost? Or can you (for example) lower your flaps before it "runs out"?

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Does the pressure deplete over time, regardless of whether you're operating things? Or does it only deplete while you're e.g. lowering/raising your flaps/gear?

 

Do jets work similarly to props in these matters?

 

In an airplane with hydraulically-boosted ailerons, such as the P-38L, would there be any way to save the hydraulic pressure for flaps, beyond simply not deflecting the ailerons?

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It all depends on how the lines in the system are interconnected. One of the important characteristics of fluids is their low compressibility. They do not preserve pressure like a gas. In a basic hydraulic circuit with a pump, directional valve and a cylinder, the pressure will remain the same after the pump has stopped working, but it will immediately be lost if there is any demand on the system and you try to set the piston in movement. If you want to preserve pressure you need a hydro pneumatic accumulator in which a gas is compressed by hydraulic pressure and thereby creates kind of a reserve for pressure.

Without such an accumulator the pressure should not deplete over time but it will be completely lost in the second you try to move anything.

 

More modern aircraft often have an accumulator for emergency gear extension and brake operation, can't speak for warbirds. Most small and older aircraft only have the option to unlock the gear and let it extend by gravity.

And in an airframe where hydraulic power is needed to operate the flight controls, why would worry about unimportant things like flaps?

 

Regarding jets, what do you want to know? The basic principles remain the same whether it is an aircraft or an excavator. Please specify your question.


Edited by Drotik
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That pretty much answers my question(s), thank you. Skimming through the P-47 and P-80 manuals also helped cast light on the subject. For example, the P-47 had a hand pump which could supply emergency hydraulic pressure to both gear or flaps. The P-80's landing flaps were electrically operated, so no engine power needed there.

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Since you asked about the P38, the ailerons were partially hydraulically boosted and partially powered by the pilot. If there was a hydraulic failure the pilot could still fly the aircraft, albeit with a lot more force involved. Systems back then were a lot lower pressure than they are in today's aircraft. Back then the pressures were around 1100-1600 Psi vs today on most aircraft the systems are at 3000 Psi. But it was a triple redundant system on the P38, a normal system, a auxiliary system, and an emergency system. Normal used the engine driven hydraulic pumps, if they went dead you had a hand pump in the cockpit that you could manipulate a valve to then power the entire system with the hand pump, and in a really bad situation you switched to the emergency system that had a different reservoir and lines but was pressurized by the hand pump. The weird part about the system is the brakes are actually only pressurized with the hand pump so pilots had to constantly be pumping away on the hand pump while taxiing to have brake pressure.

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The weird part about the system is the brakes are actually only pressurized with the hand pump so pilots had to constantly be pumping away on the hand pump while taxiing to have brake pressure.

 

Are you sure you're thinking of the right airplane? I've never come across anything like this during all of the reading I've done on the P-38, and I did a lot back in the day.

 

You got me curious enough to investigate this a bit now; both of the P-38 manuals I have say that the brakes use an entirely different hydraulic system from the rest, and mention nothing about needing to be pumped with the hand pump (and they do mention the hand pump for the backups for the main system).

 

Is it possible that you've misread something, or did that info come from another source, rather than the "Pilot Training Manual for the P-38 Lightning" or the "Pilots Manual for Lockheed P-38 Lightning" (sic)?

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