Altimeters

Finally, a place for sport pilot instructors and/or wannabees to talk about instructing.

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drseti
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Postby drseti » Thu Jun 21, 2012 9:50 am

designrs wrote:let's say it's 30.00 for a given airport which is at 000 MSL. On the same day, at the same time, above the same airport, is a baro setting of 30.00 correct at 10,000 feet?


Short answer: yes.

30.00 is 30.00 is 30.00, regardless of your altitude. (Exception: flying above 18,000 feet, by convention, you'd set your altimeter to 29.92, regardless of actual barometric pressure. That's called flying by pressure altitude rather than actual altitude, and you'd be flying at a given "flight level" rather than a particular altitude.)
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
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jnmeade
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Postby jnmeade » Thu Jun 21, 2012 10:02 am

The "correction" has nothing to do with yesterday's reading. The "correction" is the change necessary to make the altimeter give an indicated altitude that is in acceptable agreement (won't often be the same) as true altitude.
Again, the altimeter was built to think that when at seal level it should read zero because it thinks that air weights 29.92 in mg there. But if it is a very hot day, air won't weight that much. It might weight 29.52. Where does the altimeter indicate it is? 500' in the air. That can't be right. Suppose it is a bitter cold day with a high pressure ridge the column of air is denser so the altimeter indicates it is -500 feet. Also not right. Both are dangerous.
A "sensitive" altimeter lets us input a correction which makes the altimeter indicate approximately true altitude. We usually just crank the new pressure setting in the Kollsman window. But, FAA tests make us think in terms of corrections. Otherwise it's not a term we very often use.

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Postby jnmeade » Thu Jun 21, 2012 10:16 am

drseti wrote:
designrs wrote:does the baro setting effect the airspeed indicator?


Yes, it does. [quote/]

Everything Paul said is right. From my own perspective, just be sure that you don't think that setting the altimeter to a different number will change the indicated air speed on your ASI. I dont' think you meant that, but just in case you did.....


The only reason to care about TAS is in computing time enroute and fuel burn (and when bragging about your aircraft's performance).


we do want to add one caveat. In many - maybe even most - airplanes never exceed speed Vne is a true airspeed number. But not all - check your POH.

TAS goes up about 2 % /1000 feet (roughly).

Let's say your CTSW flew 120 kias at sea level WOT. Your TAS is pretty close to 120 k. You took it to 10,000 and flew it at 120 kias (OK, maybe you were in a dive). Your TAS is now about 144 knots. Vne on the CTSW is about 145 knots. You're pretty close. Now, all it takes is a good bit of turbulence and you can exceed Vne. So, check for your airplane and see if Vne is TAS or IAS and keep that in mind as you look at the ASI at altitude.
Temperature and other factors can enter as well.

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designrs
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Postby designrs » Thu Jun 21, 2012 11:04 am

Awesome discussion!
POH says Vne on PiperSport is 138 knots (IAS)
So if indicated airspeed says you're good... you are good... in smooth air at or just below 138 knots?

On paper that's great, but realistically that's probably pushing it?
Especially if the plane is at significant altitude, knowing that TAS is actually higher?

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PHAK

Postby bryancobb » Thu Jun 21, 2012 1:35 pm

A beautiful New Book that reminds me of WorldBook Encyclopedia.
Free from the FAA.

Image

Here's Chapter 7 "Flight Instruments"

Pages 7-3 through 7-7 covers the ALTIMETER and has a great explanation of the different TYPES of altitude and how they are obtained, on the bottom of 7-6 and onto the top of 7-7.

http://www.faa.gov/library/manuals/avia ... r%2007.pdf
Bryan Cobb
Sport Pilot CFI
Commercial/Instrument Airplane
Commercial Rotorcraft Helicopter
Cartersville, Ga
bryandcobb@att.net

jnmeade
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Postby jnmeade » Fri Jun 22, 2012 11:18 am

designrs wrote:Awesome discussion!
POH says Vne on PiperSport is 138 knots (IAS)
So if indicated airspeed says you're good... you are good... in smooth air at or just below 138 knots?

On paper that's great, but realistically that's probably pushing it?
Especially if the plane is at significant altitude, knowing that TAS is actually higher?


As you allude to, since Vne is basically a TAS speed, when an IAS speed is given we really need to know the altitude the IAS speed was determined at. TAS will be different at different alititudes for the same IAS.

"VNE—the speed which should never be exceeded. If flight is attempted above this speed, structural damage or structural failure may result."

Doesn't say it will, but the warning is clear. One of the main reasons for Vne as I understand it is flutter.

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Postby designrs » Fri Jun 22, 2012 11:40 am

So this is probably one of those areas where it really helps to have an in-depth understanding of the aircraft, the manufacturer, and how the published limits are established.

My guess is that there are probably some very solid planes, extensively tested, with conservative published limit numbers... and there are probably some planes that barely live up to the published specs.

Of course it is always best be on the side of caution!

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Postby drseti » Sat Jun 23, 2012 7:48 am

Vne is, in my experience, generally specified in terms of IAS, not TAS. This makes good sense if we're worried about such things as flutter and structural damage, which are related to dynamic air pressure. If the air is less dense, it is less damaging to the airframe, as well as less impacting on the pitot tube. So, logically, it is IAS which we need to be concerned with. (But, as mentioned, check your AOI to be sure.)
The opinions posted are those of one CFI, and do not necessarily represent the FAA or its lawyers.
Prof H Paul Shuch
PhD CFII DPE LSRM-A/GL/WS/PPC iRMT
AvSport LLC, KLHV
fly@AvSport.org
AvSport.org
facebook.com/SportFlying
SportPilotExaminer.US

jnmeade
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Postby jnmeade » Sun Jun 24, 2012 11:33 am

I researched and wrote this long treatise. Lucky for all of you, I did a preview and in the process lost it. I'll simply my long windedness.

The FAA says all design speed will be in EAS equivalent airspeed. In a perfect world, EAS=TAS=CAS=IAS at seal level on a standard day. EAS is TAS adjusted for density. At low altitudes (under 10k) and low speeds (200kts), EAS is close enough to IAS for practical work.

From what I can tell, Vne can be derived either from structural limitations which are usually defined in design dive speed and from flutter limitations. The Vd is accurate based on IAS.

Flutter is a vibration, often of a control surface but sometimes of a wing http://www.youtube.com/watch?v=kQI3AWpTWhM that is not dependent on how many air molecules hit the surface but on their speed, thus while the discussions I saw spoke of it in terms of IAS and TAS, one can see why TAS seems a natural response.

My suggestions that Vne was a TAS function was based on my readings about flutter and my experience flying jets where our IAS also had a TAS readout and where we had varying red lines that changed with altitude. I was part right but not all right. Paul's observation that Vne seems usually associated with IAS is in line with most light plane experiences we have.

In the event, Paul and I both agree that the POH for the specific plane is the source we should abide by. Paul is right that in most cases the reference will be in IAS. I hope this clarifies any confusion I may have generated. I have to admit, researching this topic is pretty obtuse.


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