Density Altittude

LearningToFly

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If flying in high density area how would this affect my Indicated airspeed? I have heard that airspeed will read the same because there are still the same amount of air molecules rushing threw the pitot tube. I have also heard that since it is a high density area there are less molecules in the air and will over all lower indicated airspeed.
 

Way2Broke

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The higher your altitude (Density or otherwise), the higher your true airspeed. However, indicated airspeed remains the same.
 

midlifeflyer

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I'm not sure, but you may be mixing up a couple of concepts. I'm not sure what "airspeed will read the same" means. Yes, if the ASI reads 90 knots at both sea level stand 10,000' then IAS is the same at both sea level and 10,000'.

The relationship between TAS and IAS an high density altitude is this:

Because of lower molecular density of the air at altitude, if you have the same IAS at altitude as at sea level, the TAS at altitude will be higher.

But, especially for cruise speeds, you might not be able to =get= the same IAS at altitude as at sea level because the engine has less power (about 3% loss per 1000') and your prop is less efficient (less to "bite" into).
 

westwind

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Indicated airspeeds are the same. While landing, the ground will be whizzing by much faster than what you might be used to, so rely on the airspeed indicator, not how fast (or slow) it looks like you are going compared to your home field. You can chase down some good articles from AOPA's website. or Sparky Imeson's website www.mountainflying.com/ What is your home field elevation and what elevation is the field you plan on visiting?
 
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Singlecoil

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It sounds to me like you might be a little confused on some terms.

We refer to pressure altitude to describe the pressure of an airmass in terms of feet above sea level. As you go up, pressure goes down. If a low pressure weather system blows into town and drops the pressure, your aircraft will perform like it is already a certain number of feet above where it is sitting on the ground.
Temperature also has an effect. We correct pressure altitude for non-standard temperature to get density altitude.

Here is where I think you are confused. When we refer to a high density altitude airport, we are actually talking about a situation in which the air mass has a LOWER density than normal. It is easy to see the words high density altitude and think the air is more dense, but the term actually refers to high density altitude. Higher altitude, lower density air.
 

Donsa320

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LearningToFly said:
If flying in high density area how would this affect my Indicated airspeed? I have heard that airspeed will read the same because there are still the same amount of air molecules rushing threw the pitot tube. I have also heard that since it is a high density area there are less molecules in the air and will over all lower indicated airspeed.

And another thing, there are no air molecules rushing through the pitot tube. Just pressure, no real flow, unless you have leaks. <grin>

DC
 

LearningToFly

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First off thanks for all the input. So back to Density altitude. We agree that the higher the density altitude the air is really thin. Does this effect lift on the airplane? I understand lift is created by airflow over the wings. Now if the air is thin there wont be as much air molecules going over the wing. Would this affect lift?
 

MauleSkinner

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LearningToFly said:
First off thanks for all the input. So back to Density altitude. We agree that the higher the density altitude the air is really thin. Does this effect lift on the airplane? I understand lift is created by airflow over the wings. Now if the air is thin there wont be as much air molecules going over the wing. Would this affect lift?

For the same indicated airspeed, the airplane will handle exactly the same. The difference being the true airspeed will be higher, resulting in lower rates and higher radii of turns for the same bank angle.

If you're able to achieve normal power settings for the configuration and/or flight segment that you're on (i.e., you can still make your "24 square" for cruise), your indicated airspeed will be approximately the same as it would at lower altitudes, but you'll see the increase in TAS. If you're not able to maintain the same power settings, your indicated airspeed will be lower, and angles of attack will be higher for a given flight segment.

Fly safe!

David
 

NYCPilot

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Both lift and power are adversely affected by an increase in density altitude. This holds true for a normally aspirated engine or after a turbocharged engine reaches its critical altitude with the wastegate fully closed.

At higher altitudes the density of the air is decreased due to adiabatic expansion of the air. This basically means that the air at altitude is less compressed due to the ability of the air molecules to "spread out." As air descends it is compressed into a smaller area and becomes more dense.

Due to a decrease in air molecules at altitude, lift is decreased. Since there are less air molecules available to travel over the wings surface to produce lift, the aircraft requires a faster true airspeed to compensate. This means you need to be moving faste through the air in order to generate the same amount of lift at a lower altitude. This applies when holding the angle of attack constant and below the critical angle of attack.

As you might have noticed, your propeller is shaped like a wing and is also refered to as an airfoil. This decrease in air density also decreases the amount of thrust produced by the propeller as well as decrease in lift from the wing. Both are airfoils, one produces "lift" and the other "thurst." Basically its the same concept with different names.

The decrease in air density also affects the engine operation as there are less oxygen molecules to mix with the fuel at high altitudes. This is why you will lean out the fuel mixture when you are usually above 3,000'. an engine normally requires a ratio of 14:1 - 14 parts oxygen to 1 part fuel. This decrease in the fuel/air mixture accounts for a decrease in the amount of power the engine is able to develop at higher altitudes.

Higher density altitudes will:

Negatives
decrease the wings ability to produce lift
decrease the propellers abilty to create thrust
decrease the amount of power the engine can produce

Positives
decreased fuel consumption
faster true airspeed
 

midlifeflyer

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LearningToFly said:
First off thanks for all the input. So back to Density altitude. We agree that the higher the density altitude the air is really thin. Does this effect lift on the airplane? I understand lift is created by airflow over the wings. Now if the air is thin there wont be as much air molecules going over the wing. Would this affect lift?
You've got it. Density altitude affects just about everything. Engine power output (which is helped by turbocharging) and both wing and propeller efficiency (which isn't helped by turbocharging).

This might help illustrate it in your mind:

There's a rule that you fly an approach to landing at the same indicated airspeed at a high D-Alt airport as at a sea level one even though we know that the resulting airspeed and groundspeed will be higher. Think about why. Based on what you figured out already, you should be able to answer this.
 
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