Pressure, Density and Density Altitude...

[Guest deleted@50488]

Many lines have been written on this subject at this BoS forums, either because of the sometimes weird behavior of the aircraft in terms of control authority and response, drag ( or absence of ... ), speeds at different regimes, turbocharger stages, etc...

 

There are certainly many sources over the Internet where we can find info on this subjects, but probably one of the best available, including some very handy calculators is the one in this link, which I thoroughly recommend:

 

An Introduction to Air Density
and 
Density Altitude Calculations

 

I would also particularly emphasize this sentence, taken from that site:

 

In general, if you really want to be precise and consistent, it will be best to focus attention on the actual air density, not density altitude.

Density altitude has been a convenient yardstick for pilots to compare the performance of aircraft at various altitudes, but it is in fact the air density which is the fundamentally important quantity, and density altitude is simply one way to express the air density. Actually, it would be far more meaningful, useful and precise if we would simply use the actual air density in kg/m³, and if the data in aircraft pilot's handbooks were also expressed in actual air density.

Hopefully, someday all of the aircraft performance tables/charts and weather reporting systems will simply use the actual air density and thereby avoid this arcane concept of density altitude... but, for now, we're stuck with "density altitude"

Edited February 10, 2015 by jcomm

[Crump]

if the data in aircraft pilot's handbooks were also expressed in actual air density.

 

 

Unfortunately it is not expressed that way.  It is converted from one atmospheric condition to Equivalent Density Altitude (Pressure Altitude) in a standard atmosphere.  

 

Using that it can be converted to whatever atmospheric density the engineer or pilots decides.

 

I will post more later.  

[Crump]

jcomm,

 

I think many people are having a hard time understanding some this because they do not understand what these ratios mean.  Some people here can calculate simple coefficients of pressure such as Coefficient of Lift.

 

It might be useful to bridge the understanding by going from known to the unknown.

 

Coefficient of Lift for example is a dimensionless ratio that represents a specific airfoils angle of attack in any condition of flight.

 

Atmospheric ratios are the same.  The define a relationship in any atmospheric condition.

 

Pressure Ratio represents a specific pressure altitude in an infinite number of atmospheric combination.

 

Temperature Ratio represents a specific altitude temperature in an infinite number of atmospheric combinations

 

Density Ratio represents a specific density altitude in an infinite number of atmospheric conditions.

 

Why use these ratio?

 

Ratios are used rather than actual values because they compare the value of a quantity directly with the sea level standard value.

 

That sets everything to the baseline performance conditions that aircraft performance is based upon....Standard Atmospheric Conditions.  That allows us to see the performance effects of changes in atmospheric conditions.

[69th_chuter]

Unnecessary, really.  All pilots/operators are concerned about is performance.  They don't care about weather or atmospheric pressure except as at affects performance.  Pressure altitude is perfect for that as aircraft performance charts are written with reference to that.  It'll never change (I'm kinda sure, sort of). 

[Crump]

 

 

Pressure altitude is perfect for that as aircraft performance charts are written with reference to that.

 

Pressure altitude by definition is equivalent density altitude on a standard day. 

 

The aircraft does not care about pressure altitude in non-standard conditions because the aircraft's performance is based on density altitude only.