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True Airspeed?

Old 10-18-2009, 06:41 AM
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Default True Airspeed?

I've been reviewing the basics lately, and realized i'm having a hard time understanding "true airspeed." This is what I do understand about airspeed:

Indicated Airspeed- The number read directly from your airspeed indicator

Calibrated airspeed- Indicated airspeed, corrected for instrument/position and installation errors

Groundspeed- The speed you are traveling over the ground, which involves using an E6B to factor in any head/tail/cross winds you may be encountering

So now my question is, how does the term true airspeed play into all that and what would be the easiest way of calculating it?
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Old 10-18-2009, 07:23 AM
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Then you have:

Equivalent Airspeed: Corrected for compressibility (not really a factor under 300kts)

True Airspeed: Corrected for non standard temperature / pressure / density.
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Old 10-18-2009, 08:19 AM
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The easiest way to explain it is that the pitot tube is calibrated at sea level pressure and 15 degrees celsius. At these conditions IAS will equal TAS. As you climb the ambient pressure and temperature (standard day) will decrease. The pitot tube is not accurate at different pressure altitudes and temperatures. Remember the airspeed indicator measures ram air pressure. If the pressure of the ambient air decreases the pitot tube will not "capture" the ram air pressure correctly and thus not accurately display the airplane's speed. Best way to calculate TAS correctly is with an E6B. If you have a GPS on board it may do the calculation as well.

Also remember that regardless of the true speed through the air, aircraft limitations are still based on IAS.
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Old 10-18-2009, 09:46 AM
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You'll want to try to understand true airspeed before you worry about how to calculate it.

It can be said that true airspeed is the actual speed of the aircraft through the air. When the density of air decreases we will go faster with any given power setting. This is due to a decrease in air resistence (parasite drag) due to less dense air. With that being said an increase in altitude will increase your true airspeed do to the decrease in air density.

It is also true that hot days (high denisty altitudes) will also cause higher speeds, because the air density will also decrease.

But now your wondering why the airspeed indicator stays the same.

we can say that Indicated airspeed is only correct during ISA at sea level. as we increase altitude airspeed (TAS) will increase but even though we are flying faster through the air the decrease in air denisty results in less molecules entering the pitot tube. So even though we are flying faster, there are the same ammount of molecules entering the pitot tube due to the loss in air density.

Now once you understand that you simply calculate TAS using a flight computer with a simple altitude to temp conversion.

Hope I could help
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Old 10-18-2009, 11:54 AM
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Whats the point of calibrated?
Has anyone ever used it?
I havent.
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Old 10-18-2009, 12:04 PM
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Thanks, its making sense now. It seems like i've had alot of information thrown at me over the past year and sometimes its tough to really absorb it all.
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Old 10-18-2009, 12:09 PM
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Originally Posted by slipped View Post
Whats the point of calibrated?
Has anyone ever used it?
I havent.
For calculations. Before you find TAS at altitude you should find CAS and use that to determine your true airspeed. True airspeed is your actual speed through the air found by IAS > CAS > TAS. If you go straight from IAS to TAS you will miss some information and have a slightly wrong answer.

Further, if you are in situations where EAS is an issue than you will convert TAS > EAS for the most accurate answer.
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Old 10-19-2009, 05:49 AM
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TAS is the most important value when flight planning, because this is the "Speed" at which you are traveling if there are Zero winds aloft.

Lots of typ-a-holics here, I'll try and keep it simple. Three step plan to getting GS. I avoid CAS calibrations merely on the fact they are minimal at best, and your performance graphs in the POH are already set-up so it won't be detrimental to the cause.

IAS: Basis for all of your information.
TAS: Built off of IAS and Altitude(corrected as mentioned above){POH will have a TAS value for your given altitude and power setting}
GS: Taken from your TAS and corrected for direction of flight vs. winds aloft
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Old 10-20-2009, 06:56 PM
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Now that you know what TAS is a good rule of thumb is to take your indicated airspeed and add 2% for every thousand feet climbed.

So if you are indicating 150 knots at 10,000 feet then your TAS is roughly 180 Knots.
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Old 10-20-2009, 07:16 PM
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There is another way to figure out your TAS aloft:

Remember that knob on the airspeed indicator and that little scale on the top of it? By rotating that knob the scale at the top will move. Line up the outside air temperature from your thermometer with your current altitude. Look at the pointer on the airspeed indicator and the bottom scale (for most the black numbers on a white background) and there's your true airspeed.
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