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psid
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LimoDriver
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A hydraulic actuator may have one thousand pounds of force applied to it. Opposing that force is a buildup of three thousand pounds of force. PSID is two thousand pounds of force on the high side, even though the system pressure is three thousand. It's the differential pressure.
From a practical application, where you will see it applied is in aircraft pressurization. To set cabin altitude, a certain amount of pressure inside the cabin must be applied. The higher you go, the lower the outside pressure, and the higher the differential pressure in the cabin.
At sea level, zero diferential pressure is required to hold a cabin altitude of sea level. At five thousand feet, however, 2.9 inches or so of differential pressure will hold sea level in the cabin. Or at ten thousand MSL, a 2.0 differential will hold five thousand feet, and so on. The numbers vary with the aircraft and the tightness of the fuselage...or the cabin porosity...how much air it leaks when pressurized outside of what the outflow valve can handle.
The exact numbers of PSID are relatively meaningless. For most aircraft, you merely set the values you want on the pressure controller, and the aircraft does the rest. If the aircraft doesn't do the rest, either abnormal or emergency proceedures apply...but the specific values are relative, and all you need to worry about is that they merely represent a value in the pressure difference between what's acting on the inside of the aircraft, and on the outside. A two PSID value means that two pounds per square inch pressure more are acting on the inside of the airplane pushing out, than outside pushing in.
Excessive PSID, or in other words, too much pressure inside the airplane, can damage it. I don't have it, but some sobering pictures are available of a KC-135 just a few years ago that got blown apart on the ground during maintenance, when the ground crew dummied the outflow valve and overpressured the pressure vessel. Too much pressure is just as critical as not enough.
Any time you see the "d" on PSI (eg, PSID), know that the author is attempting to communicate a difference in pressure between two sources or values, rather than merely an absolute pressure value. In this case, it isn't the pressure that's significance, but the difference between the two pressures that's doing the work, that has meaning, and which must be measured.
From a practical application, where you will see it applied is in aircraft pressurization. To set cabin altitude, a certain amount of pressure inside the cabin must be applied. The higher you go, the lower the outside pressure, and the higher the differential pressure in the cabin.
At sea level, zero diferential pressure is required to hold a cabin altitude of sea level. At five thousand feet, however, 2.9 inches or so of differential pressure will hold sea level in the cabin. Or at ten thousand MSL, a 2.0 differential will hold five thousand feet, and so on. The numbers vary with the aircraft and the tightness of the fuselage...or the cabin porosity...how much air it leaks when pressurized outside of what the outflow valve can handle.
The exact numbers of PSID are relatively meaningless. For most aircraft, you merely set the values you want on the pressure controller, and the aircraft does the rest. If the aircraft doesn't do the rest, either abnormal or emergency proceedures apply...but the specific values are relative, and all you need to worry about is that they merely represent a value in the pressure difference between what's acting on the inside of the aircraft, and on the outside. A two PSID value means that two pounds per square inch pressure more are acting on the inside of the airplane pushing out, than outside pushing in.
Excessive PSID, or in other words, too much pressure inside the airplane, can damage it. I don't have it, but some sobering pictures are available of a KC-135 just a few years ago that got blown apart on the ground during maintenance, when the ground crew dummied the outflow valve and overpressured the pressure vessel. Too much pressure is just as critical as not enough.
Any time you see the "d" on PSI (eg, PSID), know that the author is attempting to communicate a difference in pressure between two sources or values, rather than merely an absolute pressure value. In this case, it isn't the pressure that's significance, but the difference between the two pressures that's doing the work, that has meaning, and which must be measured.
Thanks for the reply. In a practical sense, how would psid be related to the landing gear in a Piper Seminole? The Seminole system is hydraulic and electric (the pumps). If the gear is up, hydraulic pressure is holding it up, but if the gear is down, then hydraulic pressure is released in the reservoir. What are we comparing the differential pressure to? Also, in the POH I know that my psi for gear up should be 1800psi +/- 100. It doesn't ever say psid.
Thanks for the help
Thanks for the help
rumpletumbler
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banned username 2
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rumpletumbler
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Hemispheres
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There is a published VMC ground for the CE-500, as I recall. I think it's 55k. That's below v1, so you'd go to idle thrust, anyway, but VMC ground relates to rudder authority in holding runway centerline with one engine at full thrust, where nosewheel steering alone won't do it. That's my recollection, anyway. I agree with avbug on this one. Psid concept came up only once, in an interview. And it related to pressurization--if exceeding max cabin pressure differential, what should you do? I said, "climb the cabin", which was apparently a correct response. Try that in the Seminole.
rumpletumbler
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rumpletumbler
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Hung Start
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Not to be a nitpicker,,,,, ah, yes I am!, But,,, Kate, there is still 1800 psi on the system in Piper gears. There is 1800 pounds holding the gear down also. True, it is designed to free fall, but it still uses hydraulic pressure to push it down in normal operations.
I have allways disagreed with the need to know numbers like that. If the pressure was only 1600lbs,, how would you know? And what would you do about it? That is an issue for the taleted folks that keep us in the air.
We have better things to worry about! In my humble opinion.
By the way, congrats on the checkride.
I have allways disagreed with the need to know numbers like that. If the pressure was only 1600lbs,, how would you know? And what would you do about it? That is an issue for the taleted folks that keep us in the air.
We have better things to worry about! In my humble opinion.
By the way, congrats on the checkride.
To: rumpletumbler, et. al. :
By "he", I was referring to the interviewer who asked me the pressurization question, not to "Kate". "Kate" did not interview me! In general terms, the original poster ("Kate") asked about "psid". In response, Avbug mentioned pressurization. I was not commenting directly on the relevance of the "psid" question posed by the examiner in the Seminole multi checkride, nor was I seeking to disparage "Kate's" examiner. That would not be instructive or responsive to "Kate's" original inquiry. In any event, "Kate" presumably now knows what "psid" is, and in what context it might relevant to a Q &A situation, be it an oral or an interview.
I think some confusion occurred here when "rumpletumbler" posted "It was a stupid question" immediately after my post about an interview question regarding presurization. I then responded, based on the pressurization/interview question, not the Seminole examiner's question. That is, in part, because my motivation in posting in this thread was not to judge or to disparage Kate's examiner, but to offer some input on "psid" and to relate an experience in which I had been asked a question involving pressure differential. As to whether "Kate's" examiner asked a "stupid question", I leave that inquiry to others apparently more eager to engage in such commentary. Adios to this thread. Best wishes to "Kate" in her future aviation endeavors.
By "he", I was referring to the interviewer who asked me the pressurization question, not to "Kate". "Kate" did not interview me! In general terms, the original poster ("Kate") asked about "psid". In response, Avbug mentioned pressurization. I was not commenting directly on the relevance of the "psid" question posed by the examiner in the Seminole multi checkride, nor was I seeking to disparage "Kate's" examiner. That would not be instructive or responsive to "Kate's" original inquiry. In any event, "Kate" presumably now knows what "psid" is, and in what context it might relevant to a Q &A situation, be it an oral or an interview.
I think some confusion occurred here when "rumpletumbler" posted "It was a stupid question" immediately after my post about an interview question regarding presurization. I then responded, based on the pressurization/interview question, not the Seminole examiner's question. That is, in part, because my motivation in posting in this thread was not to judge or to disparage Kate's examiner, but to offer some input on "psid" and to relate an experience in which I had been asked a question involving pressure differential. As to whether "Kate's" examiner asked a "stupid question", I leave that inquiry to others apparently more eager to engage in such commentary. Adios to this thread. Best wishes to "Kate" in her future aviation endeavors.
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rumpletumbler
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