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Altitude calculation based on Air pressure
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| Гость | Дата: Воскресенье, 14.08.2011, 06:01 | Сообщение # 1 |
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| Does anyone knows how to calculate Altitude based on Air pressure , sorry guys i don't remember it.
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| IROman | Дата: Воскресенье, 14.08.2011, 06:05 | Сообщение # 2 |
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| I registrated already!
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:06 | Сообщение # 3 |
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| Altitude... as in, feet above sea-level, measured by ambient air-pressure at an altitude; is realtive to current barometric pressure... and why frequent altimeter settings are important.
A reasonably accurate ratio between a change in ambient air-pressure, to a change in altitude, is 0.1" = 100ft .. and you can confirm this by adjusting the Kollsman Window number (altimeter setting).
Barometric pressure is what the ambient pressure would be at sea-level, regardless of your altitude. IOW.. On a 'standard day', the ambient pressure at sea-level, is 29.92" .. and barometers are calibrated to display sea-level pressure, regardless of the ambient-pressure on the ground, because like in Denver, the ground is some 5,000ft above sea-level, but nearby pilots need barometric pressure, not ambient pressure. Denver's standard day barometric pressure is 29.92, even though it's ambient pressure is aproximately 24.92" (and why on a standard day at Denver; a normally-aspirated piston engine can only see ~24" of manifold pressure at full throttle)..
So.. to calculate altitude by air pressure, you need to know the barometric pressure... then apply the 0.1" = 100ft
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| IROman | Дата: Воскресенье, 14.08.2011, 06:06 | Сообщение # 4 |
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| Quote (@737@) Altitude... as in, feet above sea-level, measured by ambient air-pressure at an altitude; is realtive to current barometric pressure... and why frequent altimeter settings are important.
A reasonably accurate ratio between a change in ambient air-pressure, to a change in altitude, is 0.1" = 100ft .. and you can confirm this by adjusting the Kollsman Window number (altimeter setting).
Barometric pressure is what the ambient pressure would be at sea-level, regardless of your altitude. IOW.. On a 'standard day', the ambient pressure at sea-level, is 29.92" .. and barometers are calibrated to display sea-level pressure, regardless of the ambient-pressure on the ground, because like in Denver, the ground is some 5,000ft above sea-level, but nearby pilots need barometric pressure, not ambient pressure. Denver's standard day barometric pressure is 29.92, even though it's ambient pressure is aproximately 24.92" (and why on a standard day at Denver; a normally-aspirated piston engine can only see ~24" of manifold pressure at full throttle)..
So.. to calculate altitude by air pressure, you need to know the barometric pressure... then apply the 0.1" = 100ft
I am scratching my head now, my instructor taught me the calculation but now it's been almost a year since I left the training. Yep he told me about the pressure setting and stuff.
Kollsman Window I just know about the windows in my house :).
He actually showed me a very simple way of doing it which I had grasped very quickly, hmm looks like I will have to dig up my notes and dig deep.
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:07 | Сообщение # 5 |
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| I get a little wordy.. lol
The formula is .. 0.1" = 100ft.. or .. [29.92 - (a reading at altitude)] X 1000, is you pressure altitude.. adjust that for current barometric pressure and you have your actual altitude.
The Kollsman window is the setting numbers on an altimeter... what you 'dial-in' when setting the altimeter.
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| IROman | Дата: Воскресенье, 14.08.2011, 06:07 | Сообщение # 6 |
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| :) sorry still don't get it.
The following is the WCA
Windspeed/(TAS/60) * sin wind angle ' WCA
TAS - (windspeed * COS angle) = Speed
he showed me something similar.
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:07 | Сообщение # 7 |
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| Your first post asked about altitude.. not airspeed..
What you just posted is how to determine your ground-speed TRACK, when compensating for winds aloft..
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| IROman | Дата: Воскресенье, 14.08.2011, 06:07 | Сообщение # 8 |
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| Quote (@737@) Your first post asked about altitude.. not airspeed..
What you just posted is how to determine your ground-speed TRACK, when compensating for winds aloft..
Yep the post is regarding Altitude, what I posted about WCA was just an example of what I was showed, it was some kind of a calculation for altitude* pressure/ xxxxx which I am not able to get.
My apologies for any confusion.
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:07 | Сообщение # 9 |
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| Ah.. I think you meant 'like'.. as in a step-by-step thing.. ?
If so..
1) Ambient-pressure at altitude, minus 29.92 = pressure difference between your altitude, and sea-level (standard day).
2) Pressure difference (absolute value, no negative number) X 1000 = pressure-altitude
3) Pressure altitude adjusted for barometric-pressure = actual altitude
Maybe it would help if you put this in context.. Like, when/why would you caluculate altitude by ambient-pressure (and what instrument would you be using to measure ambient-pressure)
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| IROman | Дата: Воскресенье, 14.08.2011, 06:07 | Сообщение # 10 |
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| Ya ya kinda step by step. I actually never reached the instrument flying as I was VFR (He said doing VFR look at the instruments only for reference) doing airmanship , EP's, Level turns/climbs and stuff. So I guess I would not know why/when and what instruments should be used.
Ah now I have to redo everything all over again :(.
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:08 | Сообщение # 11 |
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| Just keep that ratio in mind.. for every 100ft change in altitude, ambient-pressure changes by 0.1"
Now, what is it that you have to do all over again... maybe that will shed some light onto the question you want to ask ?
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:08 | Сообщение # 12 |
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| For reference.. the first thing that comes to mind for me, is wanting to know at which actual altitude, I'd reach something critical; as in, available manifold pressure, or when I had to be on oxygen (both change as barometric pressure changes.. ie.. a different weather system)
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| IROman | Дата: Воскресенье, 14.08.2011, 06:08 | Сообщение # 13 |
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| Well when I started training and moved from a beginner to intermediate I used to do distance*speed/60 - deviation - WCA - head/tail winds all this during a flight ,which involved as I mentioned earlier was Climb - SLF - descent / 360 / 30AOB / 45AOB (used to calculate LIFT during 30/45 AOB's) almost everything during a VFR flight all this I got to do all over as FS busy collecting dust since a year. Forgot that damn trim bad bad bad.
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:08 | Сообщение # 14 |
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| Quote (IROman) Well when I started training and moved from a beginner to intermediate I used to do distance*speed/60 - deviation - WCA - head/tail winds all this during a flight ,which involved as I mentioned earlier was Climb - SLF - descent / 360 / 30AOB / 45AOB (used to calculate LIFT during 30/45 AOB's) almost everything during a VFR flight all this I got to do all over as FS busy collecting dust since a year. Forgot that damn trim bad bad bad.
Important note:
A 'Standard day" is not only a Barometric-pressure deal.. Temperatures are a factor too. A standard day is when barometric-pressure is 29.92, AND the temperature is ~15C ..
The ultimate number is 'Density-altitude'... it takes into consideration barometric-pressure and temperature..
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| IROman | Дата: Воскресенье, 14.08.2011, 06:09 | Сообщение # 15 |
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| Just remembered we did do the ISA thing but that's what I don't remember. The whole training was fantastic.
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:09 | Сообщение # 16 |
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| I appreciate mental gymnastics more than most.. they help you understand these aviation things, instinctually, not just by memory.
Like the hoops you have to jump through in training.. planning a flight-leg by; True-course, to magnetic-course, to heading adjusted for magnetic-deviation/variation.. and then adjust it for winds-aloft.. and then calculate your duration per winds aloft.. and so on...
But never forget, that winds are never exactly as forecast (direction nor velocity), and there aint a pilot alive who can hold a heading +/- a degree .. and compasses aren't functionally that accurate.. and lines of declination are not linear.. and DGs are never exactly synched. Those gymnastics are to drill the practical applications into your sub-conscious. So again..I don't discourage it... but they don't apply to 'everday' flight-planning.
Sure, if a leg might push your fuel.. you'll want more information, and do your best to be precise, but again.. your experience for interpreting winds-aloft trump a two-hour session with the E6B. And your instinctual awareness about AoB and lift, is realistically limited to just knowing that bank-angles raise stall speeds (ie..you won't be staring at the attitude-indicator, and then try to mentally super-impose a new stall-speed line on the airspeed-indicator).. or your instinctual awareness about how density-altitude effects climbs (or takeoff rolls, for that matter) is.. well.. instinctual. Sure, it's a good isea to break out takeoff performance charts.. but if you're litterally making a go/nogo descision, on a hot day with a low-pressure system moving in, because it feels bad, instinctually, and bumps the limits on those charts... you best not be taking off at all.
As for calculating lift for a climb.. a climb/lift thing worth calculating (above and beyond clearing an obstacle..see 'instincts'), would be for a flight at significant altitude.. You'll pass through barometric-pressure changes, and temperature changes (see 'lapse-rate') that amount to enough to render exactness, moot.
SOOooo , in other words.. the gymnastics are great.. just keep context in mind.
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| IROman | Дата: Воскресенье, 14.08.2011, 06:09 | Сообщение # 17 |
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| He also explained that the HSI is a few degrees here and there due avionics interference , But on a small plane does anyone use an E6B ?
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| САА | Дата: Воскресенье, 14.08.2011, 06:10 | Сообщение # 18 |
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| 
That's what I use. Seems to work most the time.
С уважением, А.А.Степанов.
http://avsim.pp.ru/blog/
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:11 | Сообщение # 19 |
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| Small airplanes are where they're used most, if for no other reason than that's when their use is learned.
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| IROman | Дата: Воскресенье, 14.08.2011, 06:12 | Сообщение # 20 |
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| Quote (САА)
That's what I use. Seems to work most the time.
Hey that's an Altimeter not an E6B
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| @737@ | Дата: Воскресенье, 14.08.2011, 06:12 | Сообщение # 21 |
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| Well.. look back at your first post... An altimeter measures air pressure, and diaplays it as altitude.. right ?
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| IROman | Дата: Воскресенье, 14.08.2011, 06:12 | Сообщение # 22 |
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| Absolutely
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| GR4Jockey | Дата: Воскресенье, 14.08.2011, 06:12 | Сообщение # 23 |
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| Except the aircraft's altimeter could be off of actual altitude by several hundred feet. These day's, GPS with WAAS is more accurate for clearing a mountain peak. Yet, we need to be at the same "pressure" altitude for aircraft seperation.
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| GR4Jockey | Дата: Воскресенье, 14.08.2011, 06:14 | Сообщение # 24 |
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| Up to the stratosphere (11000m) the equations are;
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| IROman | Дата: Воскресенье, 14.08.2011, 06:15 | Сообщение # 25 |
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| Hey guys I am very bad at maths :) would not understand the P=12xxx /xxx -xxxx :) , it was just a very simple calculation which was given to me by my instructor and it was very easy too. That's what I have been trying to search & ask but not been able to find it anywhere even in my notes.
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