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> <... How do the Mass Air requirements
change with altitude ...>
> They don't, except as the heat rejection
requirements change.
> You get the same mass airflow at 180 kias at sea
level or 15 k ft. Of course
> your TRUE airspeed at 15k is a lot higher
and that's what pumps more of the
> thinner air through the system.
Also, to the extent that fuel flow is less at
> 15 k ft, so you have less
heat rejection requirements and therefore less mass
> flow
requirements.
>
> <... if the fuel burn was 8.5gph and altitude
was 25K ...>
> .... you'd be making about 300 kts TAS and would no
doubt have the same mass
> airflow as at SL 8.5 gph.
>
> Mass
airflow is a function of indicated airspeed which is dynamic pressure
which
> is how many molecules are going past a point in a unit of
time.
>
> Just a theory ... Jim S.
>
One common formula for air mass flow is as
follows:
W = p*V*A
Where W is air mass flow either in Slugs/Unit time
or pounds mass (lbm) or kilograms/unit time, etc. p is the density of the
air mass, V the velocity of the air stream and A the area of the duct (or what
ever) you have the air flowing through.
A formula for Dynamic pressure on the other hand is
Pd = 1/2pV^2
which results in units of
pressure (lbf - pound force per square foot, square inch, square CM or other
such area units. So Dynamic pressure and air mass flow are two separate
things. However, density and velocity of the air are a factor in
both.
You can have high dynamic pressure and low mass
flow or vise versa. Velocity (V^2) is the dominate factor in dynamic
pressure whereas volume (V*A) is the dominate factor in dynamic pressure.
FWIW
Ed Anderson
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