Technical question for a novice

Roy, Charlie ?
Switches

 

it would be the field elevation
the altimeter uses the differnce between the sea level pressure and current altiude pressure to calculate how high you are as u ascend there will be less air above you resulting a lower pressure. the altimeter has a capsule inside it measure the air pressure diffence between inside the capsule which is set by you in this case standard pressure would be 29.992(it rarely that) and the air pressure outside the capsule changes with the altitude that is how the altimeter works.
hope it help
-mr.charlie aka grammar police if im wrong correct me but not for spelling or gammar

 

Steve, Welcome to AviatorChat!

Standard pressure is always measured at Sea Level.
29.92 in/Hg at the temp of 59 deg f. ( 15 deg C)

But.... Remember this!
IT IS ONLY A REFERENCE POINT

Standard temperature and pressure, abbreviated STP, refers to nominal conditions in the atmosphere at sea level. This value is important to physicists, chemists, engineers, and pilots and navigators.

Standard temperature is defined as Fifteen degrees Celsius (15 deg C), which translates to 59 degrees Fahrenheit (59deg F) at sea level, in air at standard pressure. (29.92 inches Hg)

NOW I'LL SAY IT AGAIN, .... It is just a reference point.

Standard pressure supports 760 millimeters in a mercurial barometer (760 mmHg). This is about 29.9 inches of mercury, and represents approximately 14.7 pounds per inch (14.7 lb/in2). Imagine a column of air measuring one inch square, extending straight up into space beyond the atmosphere. The air in such a column would weigh about 14.7 pounds. at 18,000 feet it would weigh approximately half, or 7.35 psi

Standard Pressure will always be 29.92 inchesHg, it is a constant.
This is the basis for instrument calibrations, and performance data.

Now, change the temperature, and you not only change the density, you also change the relative equivilent altitude.


It can all be computed.

That is called figuring out your "Density altitude", or...how many molecules of air, are occupying a given space at a certain temp.

Imagine 100 ping pong balls in a room 10' x 10' x 10' .
That's one ball for every ten cubic foot.
Pressurize the space around the room so that the room is compressed to 1' x 1' x 1'.
Now the same 100 ping pong balls exist in 1/100th of the space.
edit: correction= 1/1000th
lots of balls in a smaller volume. This is equivalent to colder weather, and higher Barometer readings
More molecules of air in a given space. (think of it as heavier air)
Warm air is lighter, it rises. Cold air sinks... 'cause it's heavier. (more dense)

If your airports altitude is at sea level ( zero feet above sea level) at 59 degrees,
when the temp soars to, oh... say 80 degrees f., the "relative" or density altitude"
will be the same as if your airport was located 1000 ft above sea level.

Now go to the reverse... say it's colder than standard temp.
say it's 32 degrees f. the "relative" or density altitude"
will be the same as if your airport was located 1000 ft below sea level.
(Numbers are for demonstration/ teaching purposes only! Actual density altitudes will be different)


This will become clearer as you get into the climb performance of your aircraft, and how much runway you will need at different temperatures.
Yep! It changes alright !

Hope this helps you understand the subject somewhat better.

Charlie

 

Ahh Grasshopper, you are with the big boys now,


There are no free rides... no "give-ya's"
Life is tough. Get used to it.

Do it right, and there won't be any correcting needed.

Charlie

 

Yes Stephen,
You'll grasp the concept. Don't worry about it too much.
It's great that you are interested enough to study EXTRA,
so that you, can become a much better pilot.
Don't cram too much all at once though,
you'll need time to absorb the information.
It'll come about all by itself. One day you'll say.....

AHA!!! That's it! ......and it will all be totally clear.

Good luck!
..and feel free to ask any questions at all about aeronautics.


Charlie

 

By George, I think He's got it!


edit: Almost...

Just noticed an error...
If a low pressure area has moved in to the airport area, then the reference point will NOT be 29.92, but will be a lower number which will be the latest barometric pressure as corrected to sea level.

The reference point is a constant 29.92 ALWAYS. The density altitude is interpolated from there.

Water boils at 212 deg f. at sea level.
In Denver, (alt approx 1 mile up), it boils at a lower temp,
due to density altitude being higher (by approx 5 thousand feet).

but the standard boiling point of water remains at 212deg.




Charlie

 

Stephen,

Here is a very good article that explains it more thoroughly

http://www.usatoday.com/weather/wdenalt.htm

Charlie

 

........and your Instructor will be alot happier!

I think you'll do just fine. This is the tough part...

Charlie

 

Sorry - getting into this thread a bit late - but I do commend you on your effort to understand the science of the subject. All too often learning to fly becomes focused on the hands and feet aspect of aircraft control, rather than the much bigger picture of what you might call "airmanship". It's like learning to hit golf balls off a tee at the driving range. Even if you get where you can hit the ball long and straight, you won't score very well on the course because there is a lot more to the game.

An aside (for instructors) - I didn't usually use "golf" analogies while instructing because I think people who approach learning to fly like learning to play golf are wasting their time or worse - you won't usually hurt yourself or others by being a bad golfer (FORE!!!!!!!!!!!!) but you just can't afford to take that attitude about flying.

 

I don't understand how boiling point get lower at higher altitude.

 

Atmospheric pressure stops liquid from becoming vapor. Ask any astronaut what happens if they spring a leak in the pressure suit while out on a space walk.

 

This phase diagram may help you. It's for water. Pressure (y-axis) versus temperature (x-axis). Pick a temperature and look at what happens if you reduce the pressure.
Image Unavailable, Please Login

 

That also explains the phenomenon of sublimation.

 

.... and why your father's car's cooling system is pressurized.









" The problem with ignorance is, that you don't know what, you don't know"
............................................................................attributed to Ben Franklin








............................................................................

 

Charlie gave you quite a comprehensive and correct technical dissertation of the what and how of Standard Temperature and Pressure (STP)

Perhaps saying things in a different order may help you a little. (or it just may confuse you more)

As Charlie mentioned STP is simply a reference point from where to start. It almost never actually exists in the real world.

When we say that STP at Mean Sea Level (MSL) is "DEFINED" as 29.92 inces of Mercury at a temperature of 15 deg. C, we can then extrapolate the pressure changes by changing first one parameter at a time and then multiple parameters to derive the rest.

With these standard data, we can extrapolate the temperatures and pressures at any altitude by tables which define the rate of [standard] temperature changes and pressure changes with every 1000 feet increase in altitude. As a rule of thumb, temperature decreases approx. 2 deg. C and pressure decreases approx. 1" Hg for every 1000 feet (remember, on a STANDARD day)

So at Denver, within this same mass of air, it will be about 10 deg. C cooler and about 5" HG lower. When I go out to my airplane I would set my altimeter to 29.92 and read about 5000 feet. If I set it to zero feet altitude (sea level) my altimeter setting would read approx. 25.00 in the setting window.

Now, since there is rarely any such thing as a standard day with a standard behaving mass of air, one needs to apply correction factors for all the non-standard conditions. What are those non-standard conditions? Most have already been mentioned but are: non-standard temperature (for whaever altitude you are at); non-standard pressure (since all air masses do not behave in a standard manner); and humidity (which has little effect).

By setting your altimeter while on the ground to the published elevation of where the aircraft is sitting, you will read the "corrected", for non-standard conditions, pressure in the altimeter setting window. This will be what the control tower, ATIS or ASOS broadcasts to you before you take flight. Since air masses do not behave in a consistant manner over vast expanses of the planet, while enroute you will need to reset your altimeter as you fly from one area to another. This information is readily available and is broadcast from local airports or obtained through an air traffic control facility while enroute. You will do the reverse of setting the elevation on the ground and reading the altimeter setting by obtaing the setting and reading the altitude. Since the setting you receive is already corrected for the local (non-standard) conditions, if you were to land at the place you recieved this altimeter setting then you altimeter would read the elevation fo the airport. It has to work!

In summary, remember the following simple points: 1 - The altimeter is nothing more than a precision pressure measuring device. 2 - Standard days only exist in textbooks. 3 - Altimeter settings whether derived at airport elevation or received enroute, are corrected for the non-standard temperature and air mass pressure deviations from standard.

And if you think that all these things are confusing, just wait until you start looking at all the different kinds of airspeed!

Planeflyr

 

True.. this would never happen... there is no "standard pressure day".
it is only a reference point from which to make calculations.

Just as "thirty lashes makes you angry" ... they (30 lashes) don't exist.
It is only a reference, to what makes you angry.
But you've got the idea..

I would however stop using the word "day" when speaking of standard pressure.
That's, what might be confusing...

There are high pressure "areas" and low pressure "areas". These pressures, are calculated from standard pressure.
*** No "DAYS" involved.***

You fly, from one pressure area, to another.
That is why the settings change.
Not whether the day, is a standard day or not. There is no standard day.

Yes.

See how your words read when I leave out the "Standard day" aspect:

When you leave the second airport to go to the third at the 5,000 feet elevation, then you will need to call the control
tower again to get the altimeter setting due to a high pressure area having moved into the
airport vicinity, then the control tower may broadcast a number of 30.1 in/Hg.

PERFECT!!!


Read again, what I wrote in the first reply....

"Standard pressure is always measured at Sea Level.
29.92 in/Hg at the temp of 59 deg f. ( 15 deg C)

But.... Remember this!
IT IS ONLY A REFERENCE POINT"

That means that whatever the atmospheric pressure exerts at sea level under those conditions, ....it is called "Standard Pressure".
It will always be 14.7 pounds of pressure per square inch ( 14.7 PSI)
Day, night, Sat., Sun., or Holidays!

If it is a bit warmer, then the pressure exerted will be slightly less.
or, less than Standard pressure.

If it is a bit colder, then the pressure exerted will be slightly more.
or, more than Standard pressure.

You are doing well. Keep hangin in there!
It will all come together, as soon as you
get some flight time under your belt.

Then watch the questions pour out.... ;-)


Charlie

 

Heh heh...
I just remembered how confused I was, when learning

"Excessive downforce on the elevator makes you climb....
lack of downforce on the elevator makes you descend..."

Wha??? HUH??? Downforce = climb??

Charlie

 

Anyone else - yes

 

Don't confuse the lad!

Excesive downforce on the elevator makes you stall - not climb!

Lack of downforce on the elevator can just as well result in negative static stabilty - which makes you descend certainly - but in pieces

All that is unnecessarily confusing to (most at least) novice pilots.