proxlamus© Posted November 13, 2009 Share Posted November 13, 2009 Apparently I've had a three day brain fart.. and I cant seem to wrap my head around this... I've checked numerous different books and reference articles... Generally, on non-pressurized aircraft, the alternate static source is in the cabin. When used, this source introduces some error in the instruments because the cabin air pressure is lower than outside air pressure due to airflow over the cabin. Airspeeds and altitudes read higher than normal. The vertical airspeed indicator shows a momentary climb as the alternate static source is opened, followed by stabilization and normal readings thereafter. Now I must really really really not be thinking clearly.. but I thought that high velocity, fast flowing air around the fuselage was LOW pressure.. due to Bernoulli's Principle.. the fairly stationary, stagnant air inside the cockpit was HIGH pressure.. Example.. when driving in a car at a fairly high speed, cracking the window open would cause the "high" pressure air inside the cockpit to "blow" out or get "sucked" out to the high velocity, low pressure air outside the car. You can normally feel the air vents displacing more air since the low pressure outside is "sucking" out the high pressure air in the cockpit. *here is an article that backs up my statement.. http://www.physlink.com/education/askexperts/ae522.cfm * Someone help me get my head on straight.. even the FAA instrument written exam has this question. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 13, 2009 Share Posted November 13, 2009 I think they mean STATIONARY outside air pressure is higher than the pressure in the cabin, because the cabin is ostensibly having some of its pressure sucked out by the low pressure MOVING air around the fuselage. So if the air pressure is 11 ft/lbs at altitude and because of some air pressure being lost the inside the cabin is 10.5 ft/lbs, then your altimeter reads high etc. Quote Link to comment Share on other sites More sharing options...
proxlamus© Posted November 13, 2009 Author Share Posted November 13, 2009 that might make sense!! Im assuming now that the normal static pressure ports and altimeter/VSI have an instrument correction to compensate for the low pressure flowing around the aircraft.. to give you the stationary atmospheric pressure around the aircraft. ah ha Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 14, 2009 Share Posted November 14, 2009 The operative word is "generally" because it depends on the aircraft, if the vents are open, altitude, and and what is in the POH states. If the plane has ram air to the cabin (like most) then it's reversed. If I pull back the canopy 11" for fresh air, it's different again. If VFR, the problem is irrelevant. When IFR it's a tad more interesting but not much so long as you recognize it. The error is marginal actually within the +/- errors allotted for IFR anyway. Last thing is that it never happens during a flight due to design location. If anything the problem is always upstream (pitot blocked) and the vents remain open. Now THAT does get interesting when IFR and then you're VSI and RPM get much more scan time than normal because you put a post-it over the Airspeed Indicator anyway... Quote Link to comment Share on other sites More sharing options...
waddiejohn Posted November 14, 2009 Share Posted November 14, 2009 The high pressure air outside the cabin is supplied through the pitot tube which gets full impact air pressure when the aircraft is in motion and going fast enough to be effective. The VSI, Airspeed Indicator and Altimeter all get feed from the pitot head. If it gets pluged the pressure is supplied from at static pressure port. Check out AC 65-15A. In other words, don't take my word for it, I've been known to be wrong sometimes. Waddie Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 15, 2009 Share Posted November 15, 2009 As I said, the reason you don't want ALT static in the cockpit is that you can have relative vac OR pressure there depending how things are configured at any moment and speed and altitude. I can go from a ram air relative pressure to a partial vacuum just by plaing with the cabin heat or cracking open the canopy. What you need to know is that the effect is relatively minor (a few knots or a few feet). Go try it some time. Just pull the Alt Static and see what happens. Basically, not much... Quote Link to comment Share on other sites More sharing options...
waddiejohn Posted November 15, 2009 Share Posted November 15, 2009 Whether you want your alternate static source in the cockpit or not isn't relative to his question I don't think, although that is where they are sometimes placed. As I understand PrOxLamus' confusion it had to do with the reference to high pressure outside the cabin. That would, I think, come from the pitot tube which is placed in and area of normally clean air movement away from the low pressure area of the aircraft skin and facing into the relative wind. It sounds like one of those FAA questions that may be technically misstated and you can only chose the "Most Nearly Correct" answer. Or am I missing something? Reading my own previous post I see that I made it sound as if all three instruments get high pressure air from the Pitot tube when it is only the air speed indicator that gets full impact air pressure. I probably should post earlier in the day when my brain is working better. Waddie Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 15, 2009 Share Posted November 15, 2009 ...As I understand PrOxLamus' confusion it had to do with the reference to high pressure outside the cabin. Actually, his confusion was understandable because of the way the question was asked "cabin air pressure is lower than outside air pressure due to airflow over the cabin". That wording is opposite of the Bernoulli effect. Bernoulli says that slower moving air (like in a cabin) has HIGHER pressure than fast moving air (outside the cabin). That's a real head scratched how they came up with lower pressure inside while at the same time invoking the Bernoull's principal. Quote Link to comment Share on other sites More sharing options...
proxlamus© Posted November 15, 2009 Author Share Posted November 15, 2009 Rolling parts your correct why I've been scratching my head.. it just seems backwards. The FAA needs to specify their explanation a bit more Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 15, 2009 Share Posted November 15, 2009 Actually, his confusion was understandable because of the way the question was asked "cabin air pressure is lower than outside air pressure due to airflow over the cabin". That wording is opposite of the Bernoulli effect. Bernoulli says that slower moving air (like in a cabin) has HIGHER pressure than fast moving air (outside the cabin). That's a real head scratched how they came up with lower pressure inside while at the same time invoking the Bernoull's principal. I still don't see the contradiction here. I'm not a pilot, but it seems obvious to me that you would have lower cabin pressure in a non-pressurized cabin because of the Bernoulli effect. It's the same way that Bernoulli causes the gas to be sucked out of a carburetor. The airflow through the venturi creates vacuum in the ports and sucks the gas out, in the same way that the high speed airflow over the cockpit causes air to get sucked out of whatever opening there is in the cabin, again assuming that you don't have a vent facing forward for a ram air effect. I think I just figured it out. The altimeter readings are not fed through the pitot tube. That means that using Bernoulli to suck air out of the cabin will create a false high altitude reading. This description of a sensitive altimeter makes no mention of the pitot tube but does say that the altimeter's reading is based on the air pressure vs spring pressure inside the gauge. http://www.auf.asn.au/groundschool/umodule3.html The sensitive altimeter is the cockpit instrument that indicates the aircraft's altitude. The instrument is a refined aneroid barometer with a dial indicating height above a pre-set level rather than atmospheric pressure. The main component of such an instrument is a small, flexible, corrugated metal capsule from which the air has been partially evacuated — fitted with a metal closure or diaphragm. There is a spring within the capsule that applies a constant force to the bottom of the diaphragm, while atmospheric static pressure applies a counter force to the top, so that the diaphragm moves as atmospheric pressure changes. The movement of the pressure-sensing capsule is transferred and magnified — via a mechanical linkage or piezo-quartz component — to a dial pointer or pointers, or a digital display, which indicate the altitude reading. The static pressure is drawn from the aircraft's static vent, which may induce slight position errors due to aerodynamic effects around the vent. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 15, 2009 Share Posted November 15, 2009 The airflow through the venturi creates vacuum in the ports and sucks the gas out, in the same way that the high speed airflow over the cockpit causes air to get sucked out of whatever opening there is in the cabin.. Air is SUCKED OUT of the cabin in the example? That means that low pressure is OUTSIDE the cabin is doing the sucking. That means that the air pressure outside the cabin is less. That means that the air pressure inside the cabin is more. That's Bernoulli. That's why when the question was phrased using Bernoulli, it was incorrectly stated. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 15, 2009 Share Posted November 15, 2009 Air is SUCKED OUT of the cabin in the example?That means that low pressure is OUTSIDE the cabin is doing the sucking. That means that the air pressure outside the cabin is less. That means that the air pressure inside the cabin is more. That's Bernoulli. That's why when the question was phrased using Bernoulli, it was incorrectly stated. The air pressure we're comparing is not the air pressure inside the cabin and the low pressure air traveling over the surface of the plane. The comparison is the air pressure at a given altitude (outside air pressure) vs the air pressure in the cabin. The cabin pressure changes because of the air flow over the cabin. The question doesn't mention Bernoulli by name, and says "outside air pressure". It seems obvious that this is referring to air pressure at the altitude the plane happens to be at, not the air pressure in immediate vicinity of the aircraft. Here is the original quote again: Generally, on non-pressurized aircraft, the alternate static source is in the cabin. When used, this source introduces some error in the instruments because the cabin air pressure is lower than outside air pressure due to airflow over the cabin. Airspeeds and altitudes read higher than normal. The vertical airspeed indicator shows a momentary climb as the alternate static source is opened, followed by stabilization and normal readings thereafter. Poor wording perhaps, but it seems pretty clear to me what they are saying and what the reason is, and why. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 16, 2009 Share Posted November 16, 2009 Original wording: "Airspeeds and altitudes read higher than normal" C172RG POH: (With Alt Static) with the windows closed and the heater and defroster full on, the airspeed indicator may typically read as much as 3 knots slower and the altimeter 35 feet lower in cruise. That's WHY I went a bit BEYOND the question because the FAA is making a huge assumption that is often not reality. The Cessna operating manual is reality and it states the opposite. I fly a Tiger and I can use Alt Static and make airspeed and altimeter read high or low depending on configuration of vents, using cabin heat, or opening the canopy. Their entire assumption is invalid in aircraft that I fly and have flown. I can only state what I've seen flying over the last 35 years in unpressurized aircraft. What's the next question on the exam, trying to use Bernoulli to say that planes fly, even those with symmetrical airfoils? More hogwash. Quote Link to comment Share on other sites More sharing options...
waddiejohn Posted November 16, 2009 Share Posted November 16, 2009 Doesn't it depend on where exactly the presure reading is taken? Immediatly adjacent to the skin the pressure is low and due to leaks in the structure I would think pressure inside the cabin would drop in an effort to equalize making cabin pressure slightly lower than atmosphere. As we get further from the skin we go through a gradient until pressure reaches atmosphere and I would think that pressure would peak at slightly higher than atmosphere just before that point is reached. Case in point is the pitot tube mounted in many cases under the wing three or so inches from the skin. Pressure is definately higher there than in the cabin. ?????????? Waddie Quote Link to comment Share on other sites More sharing options...
waddiejohn Posted November 16, 2009 Share Posted November 16, 2009 Rolling Parts, I forgot to ask. I've been out of aviation for a while now so forgive my ignorance. Is it a Cessna Tiger your flying??? A friend of mine that I used to go to the Reno Air Races with had a Grumman Tiger and it was a neat little plane. I'm wondering if Cessna bought the rights or something. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 16, 2009 Share Posted November 16, 2009 Rolling Parts, I forgot to ask. I've been out of aviation for a while now so forgive my ignorance. Is it a Cessna Tiger your flying??? A friend of mine that I used to go to the Reno Air Races with had a Grumman Tiger and it was a neat little plane. I'm wondering if Cessna bought the rights or something. No problemo, It's even hard to follow from the inside! the Tiger production is pretty hard to follow ( American Aviation, Grumman, AGAC, Tiger, now rights were sold to an individual. I have a Grumman American Tiger from the late 70's. Actually Cessna bought Columbia not that long ago. I just used the 172RG POH because it was first at hand and illustrates the problem with the original statement fairly clearly. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 16, 2009 Share Posted November 16, 2009 Original wording: "Airspeeds and altitudes read higher than normal" I was interested in the airspeed thing and apparently it works the same way as the altimeter, with a diaphragm comparing the pressure in the pitot tube to the pressure in the cabin, so this would again explain why the gauge would read high if there were lower pressure in the cabin than there would be without some loss of pressure due to Bernoulli's Principle. I get what you're saying about YOUR plane, but I still think the idea is pretty straightforward and all that needs to be changed to make the statement accurate is to say that the altitude and airspeed may VARY from reality based on the difference between cockpit pressure and the still air pressure at your altitude where you are flying, or they could say "most unpressurized cabins read high" or something like that. Link describing how the air speed gauge works: http://www.aerospaceweb.org/question/instruments/q0251.shtml Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 16, 2009 Share Posted November 16, 2009 I was interested in the airspeed thing and apparently it works the same way as the altimeter, with a diaphragm comparing the pressure in the pitot tube to the pressure in the cabin Actually, the pitot "differential" pressure ONLY connects to the Airspeed. ONLY the airspeed indicator cares about the pitot tube ram air differential or if the pitot is even connected or not. The altimeter is completely sealed on one side and open on the other. Where it's open to will affect it somewhat (but not that much). It's important to know how YOUR plane reacts to changes in the primary instruments because in an emergency remembering such generalities can often be flat wrong (i.e. dangerous). Remembering contorted logic questions is doubly dangerous when IFR and having to mentally figure out a problem. Leave it to the FAA to make things harder than they need to be! Quote Link to comment Share on other sites More sharing options...
JMortensen Posted November 16, 2009 Share Posted November 16, 2009 Actually, the pitot "differential" pressure ONLY connects to the Airspeed.ONLY the airspeed indicator cares about the pitot tube ram air differential or if the pitot is even connected or not. Bad wording on my part. I was trying to say that it is similar in that the other side of the diaphragm is affected by the pressure in the cabin, not say that they are both connected to the pitot tube. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted November 16, 2009 Share Posted November 16, 2009 Their wording is even more strange. You only pull Alt Static if the readings are already really wrong. There is no "warning" light to tell you it's block and you need to pull the Alt Static. You deduce that the screwy readings are caused by a blocked system so pulling it actually indicates less screwy when you pull Alt Static source! The progression (in real life) is that you don't have a real reading when you pull the Alt Static. Pulling it will "normalize" the readings and make instruments read more right, not more wrong... EDIT: I guess in simple terms, you don't start with "normal readings" in that situation so their entire explanation of what you will see is way off. Quote Link to comment Share on other sites More sharing options...
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